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1
Gettelman, A., T. Birner, V. Eyring, H. Akiyoshi, D. A. Plummer, M. Dameris, S. Bekki et al. "The Tropical Tropopause Layer 1960–2100". Atmospheric Chemistry and Physics Discussions 8, n. 1 (29 gennaio 2008): 1367–413. http://dx.doi.org/10.5194/acpd-8-1367-2008.
Abstract (sommario):
Abstract. The representation of the Tropical Tropopause Layer in 13 different Chemistry Climate Models designed to represent the stratosphere is analyzed. Simulations for 1960–present and 1980–2100 are analyzed and compared to reanalysis model output. Results indicate that the models are able to reproduce the basic structure of the TTL. There is a large spread in cold point tropopause temperatures that may be linked to variation in TTL ozone values. The models are generally able to reproduce historical trends in tropopause pressure obtained from reanalysis products. Simulated historical trends in cold point tropopause temperatures and in the meridional extent of the TTL are not consistent across models. The pressure of both the tropical tropopause and the level of main convective outflow appear to be decreasing (increasing altitude) in historical runs. Similar trends are seen in the future. Models consistently predict decreasing tropopause and convective outflow pressure, by several hPa/decade. Tropical cold point temperatures increase by 0.2 K/decade. This indicates that tropospheric warming dominates stratospheric cooling at the tropical tropopause. Stratospheric water vapor at 100 hPa increases by up to 0.5–1 ppmv by 2100. This is less than implied directly by the temperature and methane increases, highlighting the correlation of tropopause temperatures with stratospheric water vapor, but also the complex nature of TTL transport.
2
Lei, Siliang, Xijuan Zhu, Yuxiang Ling, Shiwen Teng e Bin Yao. "Tropical Tropopause Layer Cloud Properties from Spaceborne Active Observations". Remote Sensing 15, n. 5 (22 febbraio 2023): 1223. http://dx.doi.org/10.3390/rs15051223.
Abstract (sommario):
A significant part of clouds in the tropics appears over the tropopause due to intense convections and in situ condensation activity. These tropical tropopause layer (TTL) clouds not only play an important role in the radiation budget over the tropics, but also in water vapor and other chemical material transport from the troposphere to the stratosphere. This study quantifies and analyzes the properties of TTL clouds based on spaceborne active observations, which provide one of the most reliable sources of information on cloud vertical distributions. We use four years (2007–2010) of observations from the joint Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat and consider all cloudy pixels with top height above the tropopause as TTL clouds. The occurrence frequency of TTL clouds during the nighttime is found to be almost 13% and can reach ~50–60% in areas with frequent convections. The annual averages of tropical tropopause height, tropopause temperature, and cloud top height are 16.2 km, −80.7 °C, and 16.6 km, respectively, and the average cloud top exceeds tropopause by approximately 500 m. More importantly, the presence of TTL clouds causes tropopause temperature to be ~3–4 °C colder than in the all-sky condition. It also lifts the tropopause heights ~160 m during the nighttime and lowers the heights ~84 m during the daytime. From a cloud type aspect, ~91% and ~4% of the TTL clouds are high clouds and altostratus, and only ~5% of them are associated with convections (i.e., nimbostratus and deep convective clouds). Approximately 30% of the TTL clouds are single-layer clouds, and multi-layer clouds are dominated by those with 2–3 separated layers.
3
Tegtmeier, Susann, James Anstey, Sean Davis, Rossana Dragani, Yayoi Harada, Ioana Ivanciu, Robin Pilch Kedzierski et al. "Temperature and tropopause characteristics from reanalyses data in the tropical tropopause layer". Atmospheric Chemistry and Physics 20, n. 2 (22 gennaio 2020): 753–70. http://dx.doi.org/10.5194/acp-20-753-2020.
Abstract (sommario):
Abstract. The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere with air masses showing chemical and dynamical properties of both regions. The representation of the TTL in meteorological reanalysis data sets is important for studying the complex interactions of circulation, convection, trace gases, clouds, and radiation. In this paper, we present the evaluation of climatological and long-term TTL temperature and tropopause characteristics in the reanalysis data sets ERA-Interim, ERA5, JRA-25, JRA-55, MERRA, MERRA-2, NCEP-NCAR (R1), and CFSR. The evaluation has been performed as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The most recent atmospheric reanalysis data sets (ERA-Interim, ERA5, JRA-55, MERRA-2, and CFSR) all provide realistic representations of the major characteristics of the temperature structure within the TTL. There is good agreement between reanalysis estimates of tropical mean temperatures and radio occultation data, with relatively small cold biases for most data sets. Temperatures at the cold point and lapse rate tropopause levels, on the other hand, show warm biases in reanalyses when compared to observations. This tropopause-level warm bias is related to the vertical resolution of the reanalysis data, with the smallest bias found for data sets with the highest vertical resolution around the tropopause. Differences in the cold point temperature maximize over equatorial Africa, related to Kelvin wave activity and associated disturbances in TTL temperatures. Interannual variability in reanalysis temperatures is best constrained in the upper TTL, with larger differences at levels below the cold point. The reanalyses reproduce the temperature responses to major dynamical and radiative signals such as volcanic eruptions and the quasi-biennial oscillation (QBO). Long-term reanalysis trends in temperature in the upper TTL show good agreement with trends derived from adjusted radiosonde data sets indicating significant stratospheric cooling of around −0.5 to −1 K per decade. At 100 hPa and the cold point, most of the reanalyses suggest small but significant cooling trends of −0.3 to −0.6 K per decade that are statistically consistent with trends based on the adjusted radiosonde data sets. Advances of the reanalysis and observational systems over the last decades have led to a clear improvement in the TTL reanalysis products over time. Biases of the temperature profiles and differences in interannual variability clearly decreased in 2006, when densely sampled radio occultation data started being assimilated by the reanalyses. While there is an overall good agreement, different reanalyses offer different advantages in the TTL such as realistic profile and cold point temperature, continuous time series, or a realistic representation of signals of interannual variability. Their use in model simulations and in comparisons with climate model output should be tailored to their specific strengths and weaknesses.
4
Dzambo, Andrew M., Matthew H. Hitchman e Kai-Wei Chang. "The Influence of Gravity Waves on Ice Saturation in the Tropical Tropopause Layer over Darwin, Australia". Atmosphere 10, n. 12 (5 dicembre 2019): 778. http://dx.doi.org/10.3390/atmos10120778.
Abstract (sommario):
Gravity waves (GWs) in the tropical tropopause layer (TTL) can help dehydrate the lower stratosphere through rapid cooling events, but observational studies of GWs in the TTL are limited. Using a long-term, high-resolution radiosonde temperature dataset, an atmospheric state classification technique, and wavelet analysis, we characterize temperature perturbations generated by GWs in the TTL over Darwin, Australia across eight atmospheric states. We find a peak in GW power just above the tropical tropopause and a climatological maximum during peak monsoon season. While accounting for a chronic negative temperature bias near the tropical tropopause, we estimate that, in the upper troposphere, GWs impose a 2 K temperature perturbation during non-monsoon states and a 3 K temperature perturbation during the monsoon states, with corresponding values of 5 K and 6 K in the upper TTL. A 3 K negative temperature perturbation will lead to significant perturbations in relative humidity with respect to ice, which has implications for environmental ice number concentrations and TTL cloud fraction.
5
Gettelman, A., T. Birner, V. Eyring, H. Akiyoshi, S. Bekki, C. Brühl, M. Dameris et al. "The Tropical Tropopause Layer 1960–2100". Atmospheric Chemistry and Physics 9, n. 5 (4 marzo 2009): 1621–37. http://dx.doi.org/10.5194/acp-9-1621-2009.
Abstract (sommario):
Abstract. The representation of the Tropical Tropopause Layer (TTL) in 13 different Chemistry Climate Models (CCMs) designed to represent the stratosphere is analyzed. Simulations for 1960–2005 and 1980–2100 are analyzed. Simulations for 1960–2005 are compared to reanalysis model output. CCMs are able to reproduce the basic structure of the TTL. There is a large (10 K) spread in annual mean tropical cold point tropopause temperatures. CCMs are able to reproduce historical trends in tropopause pressure obtained from reanalysis products. Simulated historical trends in cold point tropopause temperatures are not consistent across models or reanalyses. The pressure of both the tropical tropopause and the level of main convective outflow appear to have decreased (increased altitude) in historical runs as well as in reanalyses. Decreasing pressure trends in the tropical tropopause and level of main convective outflow are also seen in the future. Models consistently predict decreasing tropopause and convective outflow pressure, by several hPa/decade. Tropical cold point temperatures are projected to increase by 0.09 K/decade. Tropopause anomalies are highly correlated with tropical surface temperature anomalies and with tropopause level ozone anomalies, less so with stratospheric temperature anomalies. Simulated stratospheric water vapor at 90 hPa increases by up to 0.5–1 ppmv by 2100. The result is consistent with the simulated increase in temperature, highlighting the correlation of tropopause temperatures with stratospheric water vapor.
6
Schiller, C., J. U. Grooß, P. Konopka, F. Plöger, F. H. Silva dos Santos e N. Spelten. "Hydration and dehydration at the tropical tropopause". Atmospheric Chemistry and Physics Discussions 9, n. 4 (24 agosto 2009): 17495–529. http://dx.doi.org/10.5194/acpd-9-17495-2009.
Abstract (sommario):
Abstract. High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H2O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.
7
Schiller, C., J. U. Grooß, P. Konopka, F. Plöger, F. H. Silva dos Santos e N. Spelten. "Hydration and dehydration at the tropical tropopause". Atmospheric Chemistry and Physics 9, n. 24 (23 dicembre 2009): 9647–60. http://dx.doi.org/10.5194/acp-9-9647-2009.
Abstract (sommario):
Abstract. High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H2O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.
8
Ryu, Jung-Hee, e Sukyoung Lee. "Effect of Tropical Waves on the Tropical Tropopause Transition Layer Upwelling". Journal of the Atmospheric Sciences 67, n. 10 (1 ottobre 2010): 3130–48. http://dx.doi.org/10.1175/2010jas3434.1.
Abstract (sommario):
Abstract An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed “cold trap”) occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.
9
Lin, Pu, David Paynter, Yi Ming e V. Ramaswamy. "Changes of the Tropical Tropopause Layer under Global Warming". Journal of Climate 30, n. 4 (1 febbraio 2017): 1245–58. http://dx.doi.org/10.1175/jcli-d-16-0457.1.
Abstract (sommario):
Abstract This paper investigates changes in the tropical tropopause layer (TTL) in response to carbon dioxide increase and surface warming separately in an atmospheric general circulation model, finding that both effects lead to a warmer tropical tropopause. Surface warming also results in an upward shift of the tropopause. A detailed heat budget analysis is performed to quantify the contributions from different radiative and dynamic processes to changes in the TTL temperature. When carbon dioxide increases with fixed surface temperature, a warmer TTL mainly results from the direct radiative effect of carbon dioxide increase. With surface warming, the largest contribution to the TTL warming comes from the radiative effect of the warmer troposphere, which is partly canceled by the radiative effect of the moistening at the TTL. Strengthening of the stratospheric circulation following surface warming cools the lower stratosphere dynamically and radiatively via changes in ozone. These two effects are of comparable magnitudes. This circulation change is the main cause of temperature changes near 63 hPa but is weak near 100 hPa. Contributions from changes in convection and clouds are also quantified. These results illustrate the heat budget analysis as a useful tool to disentangle the radiative–dynamical–chemical–convective coupling at the TTL and to facilitate an understanding of intermodel difference.
10
Dessler, A. E., e S. C. Sherwood. "A model of HDO in the tropical tropopause layer". Atmospheric Chemistry and Physics Discussions 3, n. 4 (29 agosto 2003): 4489–513. http://dx.doi.org/10.5194/acpd-3-4489-2003.
Abstract (sommario):
Abstract. Any theory of water vapor in the tropical tropopause layer (TTL) must explain both the abundance and isotopic composition of water there. In a previous paper, we presented a model of the TTL that simulated the abundance of water vapor as well as the details of the vertical profile. That model included the effects of "overshooting" convection, which injects dry air directly into the TTL. Here, we present results for the model after modifying it to include water's stable isotopologue HDO (where D represents deuterium, 2H). We find that the model predicts a nearly uniform HDO depletion throughout the TTL, in agreement with recent measurements. This occurs because the model dehydrates by dilution, which does not fractionate, instead of by condensation. Our model shows that this dehydration by dilution is consistent with other physical constraints on the system. We also show the key role that lofted ice plays in determining the abundance of HDO in the TTL. Such lofted ice requires a complementary source of dry air in the TTL; without that, the TTL will rapidly saturate and the lofted ice will not evaporate.
11
Dessler, A. E., e S. C. Sherwood. "A model of HDO in the tropical tropopause layer". Atmospheric Chemistry and Physics 3, n. 6 (8 dicembre 2003): 2173–81. http://dx.doi.org/10.5194/acp-3-2173-2003.
Abstract (sommario):
Abstract. Any theory of water vapor in the tropical tropopause layer (TTL) must explain both the abundance and isotopic composition of water there. In previous papers, we presented a model of the TTL that simulated the abundance of water vapor as well as the details of the vertical profile. That model included the effects of "overshooting" convection, which injects dry air directly into the TTL. Here, we present results for the model after modifying it to include water's stable isotopologue HDO (where D represents deuterium, 2H). We find that the model predicts a nearly uniform HDO depletion throughout the TTL, in agreement with recent measurements. This occurs because the model dehydrates by dilution, which does not fractionate, instead of by condensation. Our model shows that this dehydration by dilution is consistent with other physical constraints on the system. We also show the key role that lofted ice plays in determining the abundance of HDO in the TTL. Such lofted ice requires a complementary source of dry air in the TTL; without that, the TTL will rapidly saturate and the lofted ice will not evaporate.
12
Froyd, K. D., D. M. Murphy, T. J. Sanford, D. S. Thomson, J. C. Wilson, L. Pfister e L. Lait. "Aerosol composition of the tropical upper troposphere". Atmospheric Chemistry and Physics Discussions 9, n. 2 (9 aprile 2009): 9399–456. http://dx.doi.org/10.5194/acpd-9-9399-2009.
Abstract (sommario):
Abstract. Aerosol composition was measured by the NOAA single particle mass spectrometer (PALMS) aboard the NASA WB-57 high altitude aircraft platform during two Aura Validation Experiment (AVE) campaigns based in Costa Rica in 2004 and 2006. These studies yielded the most complete set of aerosol composition measurements to date throughout the tropical tropopause layer (TTL) and tropical lower stratosphere. We describe the aerosol properties of the tropical atmosphere and use composition tracers to examine particle sources, the role of recent convection, and cirrus-forming potential in the TTL. Tropical dynamics and regional air sources played principal roles in dictating tropospheric aerosol properties. There was a sharp change in aerosol chemical composition at about 12 km altitude coincident with a change in convective influence. Below this level, maritime convection lofted condensable material that generated acidic, sulfate-rich aerosol. These particles contained significant amounts of methanesulfonic acid (MSA) and showed evidence of cloud processes. In contrast, continental convection injected particles and precursors directly into the TTL, yielding a population of neutralized, organic-rich aerosol. The organics were often highly oxidized and particles with oxidized organics also contained nitrate. Above the tropopause, chemical composition gradually changed toward sulfuric acid particles but neutralized particles were still abundant 2 km above the tropopause. Deep continental convection, though sporadic and geographically localized, may strongly influence TTL aerosol properties on a global scale. The abundance of organic-rich aerosol may inhibit ice nucleation and formation of tropopause level cirrus.
13
Froyd, K. D., D. M. Murphy, T. J. Sanford, D. S. Thomson, J. C. Wilson, L. Pfister e L. Lait. "Aerosol composition of the tropical upper troposphere". Atmospheric Chemistry and Physics 9, n. 13 (7 luglio 2009): 4363–85. http://dx.doi.org/10.5194/acp-9-4363-2009.
Abstract (sommario):
Abstract. Aerosol composition was measured by the NOAA single-particle mass spectrometer (PALMS) aboard the NASA WB-57 high altitude aircraft platform during two Aura Validation Experiment (AVE) campaigns based in Costa Rica in 2004 and 2006. These studies yielded the most complete set of aerosol composition measurements to date throughout the tropical tropopause layer (TTL) and tropical lower stratosphere. We describe the aerosol properties of the tropical atmosphere and use composition tracers to examine particle sources, the role of recent convection, and cirrus-forming potential in the TTL. Tropical dynamics and regional air sources played principal roles in dictating tropospheric aerosol properties. There was a sharp change in aerosol chemical composition at about 12 km altitude coincident with a change in convective influence. Below this level, maritime convection lofted condensable material that generated acidic, sulfate-rich aerosol. These particles contained significant amounts of methanesulfonic acid (MSA) and showed evidence of cloud processes. In contrast, continental convection injected particles and precursors directly into the TTL, yielding a population of neutralized, organic-rich aerosol. The organics were often highly oxidized and particles with oxidized organics also contained nitrate. Above the tropopause, chemical composition gradually changed toward sulfuric acid particles but neutralized particles were still abundant 2 km above the tropopause. Deep continental convection, though sporadic and geographically localized, may strongly influence TTL aerosol properties on a global scale. The abundance of organic-rich aerosol may inhibit ice nucleation and formation of tropopause level cirrus.
14
Ravindra Babu, S., M. Venkat Ratnam, Ghouse Basha, B. V. Krishnamurthy e B. Venkateswara Rao. "Effect of tropical cyclones on the tropical tropopause parameters observed using COSMIC GPS RO data". Atmospheric Chemistry and Physics Discussions 15, n. 9 (5 maggio 2015): 13043–71. http://dx.doi.org/10.5194/acpd-15-13043-2015.
Abstract (sommario):
Abstract. Tropical cyclones (TCs) are deep convective synoptic scale systems and play an important role in modifying the thermal structure, tropical tropopause parameters and hence stratosphere–troposphere exchange (STE) processes. In the present study, high vertical resolution and high accuracy measurements from COSMIC Global Positioning System (GPS) Radio Occultation (RO) measurements are used to investigate and quantify the effect of tropical cyclones that occurred over Bay of Bengal and Arabian Sea in last decade on the tropical tropopause parameters. The tropopause parameters include cold point tropopause altitude (CPH) and temperature (CPT), lapse rate tropopause altitude (LRH) and temperature (LRT) and the thickness of the tropical tropopause layer (TTL), that is defined as the layer between convective outflow level (COH) and CPH, obtained from GPS RO data. From all the TCs events, we generate the mean cyclone-centered composite structure for the tropopause parameters and removed from climatological mean obtained from averaging the GPS RO data from 2002–2013. Since the TCs include eye, eye walls and deep convective bands, we obtained the tropopause parameters based on radial distance from cyclone eye. In general, decrease in the CPH in the eye is noticed as expected. However, as the distance from cyclone eye increases by 3, 4, and 5° an enhancement in CPH (CPT), LRH (LRT) are observed. Lowering of CPH (0.6 km) and LRH (0.4 km) values with coldest CPT and LRT (2–3 K) within the 500 km radius from the TC centre is noticed. Higher (2 km) COH leading to the lowering of TTL thickness (2–3 km) is clearly observed. There exists multiple tropopause structures in the profiles of temperature obtained within 1° from centre of TC. These changes in the tropopause parameters are expected to influence the water vapour transport from troposphere to lower stratosphere and ozone from lower stratosphere to the upper troposphere and hence STE processes.
15
Jensen, Eric J., Leonhard Pfister, David E. Jordan, Thaopaul V. Bui, Rei Ueyama, Hanwant B. Singh, Troy D. Thornberry et al. "The NASA Airborne Tropical Tropopause Experiment: High-Altitude Aircraft Measurements in the Tropical Western Pacific". Bulletin of the American Meteorological Society 98, n. 1 (1 gennaio 2017): 129–43. http://dx.doi.org/10.1175/bams-d-14-00263.1.
Abstract (sommario):
Abstract The February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX data are publicly available online (at https://espoarchive.nasa.gov/).
16
Ravindra Babu, S., M. Venkat Ratnam, G. Basha, B. V. Krishnamurthy e B. Venkateswararao. "Effect of tropical cyclones on the tropical tropopause parameters observed using COSMIC GPS RO data". Atmospheric Chemistry and Physics 15, n. 18 (16 settembre 2015): 10239–49. http://dx.doi.org/10.5194/acp-15-10239-2015.
Abstract (sommario):
Abstract. Tropical cyclones (TCs) are deep convective synoptic-scale systems that play an important role in modifying the thermal structure, tropical tropopause parameters and hence also modify stratosphere–troposphere exchange (STE) processes. In the present study, high vertical resolution and high accuracy measurements from COSMIC Global Positioning System (GPS) radio occultation (RO) measurements are used to investigate and quantify the effect of tropical cyclones that occurred over Bay of Bengal and Arabian Sea in the last decade on the tropical tropopause parameters. The tropopause parameters include cold-point tropopause altitude (CPH) and temperature (CPT), lapse-rate tropopause altitude (LRH) and temperature (LRT) and the thickness of the tropical tropopause layer (TTL), that is defined as the layer between convective outflow level (COH) and CPH, obtained from GPS RO data. From all the TC events, we generate the mean cyclone-centred composite structure for the tropopause parameters and removed it from the climatological mean obtained from averaging the GPS RO data from 2002 to 2013. Since the TCs include eye, eye walls and deep convective bands, we obtained the tropopause parameters based on radial distance from the cyclone eye. In general, decrease in the CPH in the eye is noticed as expected. However, as the distance from the cyclone eye increases by 300, 400, and 500 km, an enhancement in CPH (CPT) and LRH (LRT) is observed. Lowering of CPH (0.6 km) and LRH (0.4 km) values with coldest CPT and LRT (2–3 K) within a 500 km radius of the TC centre is noticed. Higher (2 km) COH leading to the lowering of TTL thickness (2–3 km) is clearly observed. There are multiple tropopause structures in the profiles of temperature obtained within 100 km from the centre of the TC. These changes in the tropopause parameters are expected to influence the water vapour transport from the troposphere to the lower stratosphere, and ozone from the lower stratosphere to the upper troposphere, hence influencing STE processes.
17
Kuang, Zhiming, e Christopher S. Bretherton. "Convective Influence on the Heat Balance of the Tropical Tropopause Layer: A Cloud-Resolving Model Study". Journal of the Atmospheric Sciences 61, n. 23 (1 dicembre 2004): 2919–27. http://dx.doi.org/10.1175/jas-3306.1.
Abstract (sommario):
Abstract The tropical tropopause layer (TTL), and in particular the cold point tropopause, has been previously suggested as a feature decoupled from convection. Using a cloud-resolving model, the authors demonstrate that convection, in fact, has a cooling effect in the TTL that significantly affects its thermal structure. In particular, the cold point is found to be strongly tied to the convective cooling maximum. The authors interpret these as natural features of an entrainment layer such as the TTL. The recognition that the cold point tropopause is strongly tied to, rather than decoupled from, convection suggests that dehydration processes at the cold point cannot be assumed as gradual and the effect of convection may not be ignored.
18
Voigt, C., H. Schlager, A. Roiger, A. Stenke, M. de Reus, S. Borrmann, E. Jensen, C. Schiller, P. Konopka e N. Sitnikov. "Detection of reactive nitrogen containing particles in the tropopause region – evidence for a tropical nitric acid trihydrate (NAT) belt". Atmospheric Chemistry and Physics Discussions 8, n. 4 (23 luglio 2008): 14145–68. http://dx.doi.org/10.5194/acpd-8-14145-2008.
Abstract (sommario):
Abstract. The detection of nitric acid trihydrate (NAT, HNO3×3H2O) particles in the tropical transition layer (TTL) harmonizes our understanding of polar stratospheric cloud formation. Large reactive nitrogen (NOy) containing particles were observed on 8 August 2006 by instruments onboard the high altitude research aircraft M55-Geophysica near and below the tropical tropopause. The particles, most likely NAT, have diameters less than 6 μm and concentrations below 10−4 cm−3. The NAT particle layer was repeatedly detected at altitudes between 15.1 and 17.5 km over extended areas of 9.5 to 17.2° N and 1.5° W to 2.7° E above the African continent. Satellite observations suggest that the NAT particles could have nucleated on ice fed by convective activity. Once nucleated, the NAT particles can slowly grow within the TTL for days, while being transported over long distances. Their in-situ detection combined with global model simulations of the NAT supersaturation near the tropical tropopause indicate the potential for a tropical tropopause NAT particle belt.
19
Voigt, C., H. Schlager, A. Roiger, A. Stenke, M. de Reus, S. Borrmann, E. Jensen, C. Schiller, P. Konopka e N. Sitnikov. "Detection of reactive nitrogen containing particles in the tropopause region – evidence for a tropical nitric acid trihydrate (NAT) belt". Atmospheric Chemistry and Physics 8, n. 24 (15 dicembre 2008): 7421–30. http://dx.doi.org/10.5194/acp-8-7421-2008.
Abstract (sommario):
Abstract. The detection of nitric acid trihydrate (NAT, HNO3×3H2O) particles in the tropical transition layer (TTL) harmonizes our understanding of polar stratospheric cloud formation. Large reactive nitrogen (NOy) containing particles were observed on 8 August 2006 by instruments onboard the high altitude research aircraft M55-Geophysica near and below the tropical tropopause. The particles, most likely NAT, have diameters less than 6 μm and concentrations below 10-4 cm−3. The NAT particle layer was repeatedly detected at altitudes between 15.1 and 17.5 km over extended areas of 9.5 to 17.2° N and 1.5° W to 2.7° E above the African continent. Satellite observations suggest that the NAT particles could have nucleated on ice fed by convective activity. Once nucleated, the NAT particles can slowly grow within the TTL for days, while being transported over long distances. Their in-situ detection combined with global model simulations of the NAT supersaturation near the tropical tropopause indicate the potential for a tropical tropopause NAT particle belt.
20
Ashfold, M. J., N. R. P. Harris, E. L. Atlas, A. J. Manning e J. A. Pyle. "Transport of short-lived species into the Tropical Tropopause Layer". Atmospheric Chemistry and Physics 12, n. 14 (19 luglio 2012): 6309–22. http://dx.doi.org/10.5194/acp-12-6309-2012.
Abstract (sommario):
Abstract. We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We focus on Southeast Asia and the Western Pacific which appears to be a particularly important source of air that enters the TTL. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ~15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ~16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Niño (2006) and La Niña (2007) conditions and find year-to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ~15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short-lived tracer travelling from the BL to the TTL above Borneo. These calculations were performed as a prelude to the SHIVA field campaign, which took place in Borneo during November 2011. So finally, to validate our approach, we consider measurements made in two previous campaigns. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft now capable of making such measurements.
21
Ashfold, M. J., N. R. P. Harris, E. L. Atlas, A. J. Manning e J. A. Pyle. "Transport of short-lived species into the Tropical Tropopause Layer". Atmospheric Chemistry and Physics Discussions 12, n. 1 (6 gennaio 2012): 441–78. http://dx.doi.org/10.5194/acpd-12-441-2012.
Abstract (sommario):
Abstract. We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We investigate the conditions which might occur during one such campaign, SHIVA, which takes place in Borneo during November 2011. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ~15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ~16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Niño (2006) and La Niña (2007) conditions and find year-to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ~15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short-lived tracer travelling from the BL to the TTL above Borneo. Finally, we consider measurements made in two previous campaigns in order to validate our approach. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft capable of making such measurements.
22
Duncan, B. N., S. E. Strahan e Y. Yoshida. "Model study of the cross-tropopause transport of biomass burning pollution". Atmospheric Chemistry and Physics Discussions 7, n. 1 (15 febbraio 2007): 2197–248. http://dx.doi.org/10.5194/acpd-7-2197-2007.
Abstract (sommario):
Abstract. We present a modeling study of the troposphere-to-stratosphere transport (TST) of pollution from major biomass burning regions to the tropical tropopause layer (TTL) and lower stratosphere (LS). We show that biomass burning pollution regularly and significantly impacts the composition of the TTL/LS. TST occurs through 1) slow ascent in the TTL and 2) quasi-horizontal exchange in the regions of the subtropical jets; we find both pathways to be important. The seasonal oscillation in CO in the TTL/LS (i.e., the CO "tape recorder") is caused largely by seasonal changes in biomass burning. Another contributing factor is the long-range transport of northern hemispheric pollution (e.g., biofuels and fossil fuels) to the northern tropics in boreal winter. Other tropical sources of CO (e.g., methane oxidation) have insignificant seasonal variation, contributing little to the tape recorder. Interannual variation of CO in the TTL/LS is caused by year-to-year variations in biomass burning and the strength, frequency, and locations of deep convection, which lofts pollution to the upper troposphere. During our study period, 1994–1998, we find that the highest concentrations of CO in the TTL/LS occur during the strong 1997/98 El Niño event for two reasons: i.~tropical deep convection was stronger and ii.~emissions were higher. This extreme event can be seen as an upper bound on the impact of biomass burning pollution on the TTL/LS. We estimate that the 1997 Indonesian wildfires increased CO in the entire TTL and tropical LS (<60 mb) by more than 40% and 10%, respectively, for several months. Zonal mean ozone increased and the hydroxyl radical decreased by as much as 20%, increasing the lifetimes and, subsequently TST, of trace gases. Our results indicate that the impact of biomass burning pollution on the TTL/LS is likely greatest during an El Niño event due to favorable dynamics and historically higher burning rates.
23
Fu, Qiang, Maxwell Smith e Qiong Yang. "The Impact of Cloud Radiative Effects on the Tropical Tropopause Layer Temperatures". Atmosphere 9, n. 10 (28 settembre 2018): 377. http://dx.doi.org/10.3390/atmos9100377.
Abstract (sommario):
A single-column radiative-convective model (RCM) is a useful tool to investigate the physical processes that determine the tropical tropopause layer (TTL) temperature structures. Previous studies on the TTL using the RCMs, however, omitted the cloud radiative effects. In this study, we examine the impact of cloud radiative effects on the simulated TTL temperatures using an RCM. We derive the cloud radiative effects based on satellite observations, which show heating rates in the troposphere but cooling rates in the stratosphere. We find that the cloud radiative effect warms the TTL by as much as 2 K but cools the lower stratosphere by as much as −1.5 K, resulting in a thicker TTL. With (without) considering cloud radiative effects, we obtain a convection top of ≈167 hPa (≈150 hPa) with a temperature of ≈213 K (≈209 K), and a cold point at ≈87 hPa (≈94 hPa) with a temperature of ≈204 K (≈204 K). Therefore, the cloud radiative effects widen the TTL by both lowering the convection-top height and enhancing the cold-point height. We also examine the impact of TTL cirrus radiative effects on the RCM-simulated temperatures. We find that the TTL cirrus warms the TTL with a maximum temperature increase of ≈1.3 K near 110 hPa.
24
Levine, J. G., P. Braesicke, N. R. P. Harris e J. A. Pyle. "Seasonal and inter-annual variations in troposphere-to-stratosphere transport from the tropical tropopause layer". Atmospheric Chemistry and Physics 8, n. 13 (10 luglio 2008): 3689–703. http://dx.doi.org/10.5194/acp-8-3689-2008.
Abstract (sommario):
Abstract. In an earlier study of troposphere-to-stratosphere transport (TST) via the tropical tropopause layer (TTL), we found that the vast majority of air parcels undergoing TST from the base of the TTL enter the extratropical lowermost stratosphere quasi-horizontally and show little or no regional preference with regards to origin in the TTL or entry into the stratosphere. We have since repeated the trajectory calculations – originally limited to a single Northern Hemisphere winter period – in a variety of months and years to assess how robust our earlier findings are to change of timing. To first order, we find that the main conclusions hold, irrespective of the season, year and phase of the El Niño Southern Oscillation (ENSO). We also explore: the distribution of TST between the Northern and Southern Hemispheres; the sensitivity of modelled TST to the definition of the tropopause; and the routes by which air parcels undergo transport exclusively to the stratospheric overworld. Subject to a dynamical definition of the tropopause, we identify a strong bias towards TST in the Southern Hemisphere, particularly during the Northern Hemisphere summer. The thermal tropopause, defined according to the World Meteorological Organization, lies above the dynamical tropopause throughout the extratropics. Inevitably, on switching to the thermal definition, we calculate much less transport across the tropopause, particularly in the subtropics, which could be important with regards to interpretation of processes affecting ozone chemistry in the extratropical lowermost stratosphere (ELS). In contrast to the rather homogeneous nature of TST into the ELS, we find that transport to the overworld takes place from relatively well-defined regions of the TTL, predominantly above the West Pacific and Indonesia, except for an El Niño period in which most transport takes place from regions above the East Pacific and South America.
25
Virts, Katrina S., e John M. Wallace. "Annual, Interannual, and Intraseasonal Variability of Tropical Tropopause Transition Layer Cirrus". Journal of the Atmospheric Sciences 67, n. 10 (1 ottobre 2010): 3097–112. http://dx.doi.org/10.1175/2010jas3413.1.
Abstract (sommario):
Abstract Cloud fields based on the first three years of data from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission are used to investigate the relationship between cirrus within the tropical tropopause transition layer (TTL) and the Madden–Julian oscillation (MJO), the annual cycle, and El Niño–Southern Oscillation (ENSO). The TTL cirrus signature observed in association with the MJO resembles convectively induced, mixed Kelvin–Rossby wave solutions above the Pacific warm pool region. This signature is centered to the east of the peak convection and propagates eastward more rapidly than the convection; it exhibits a pronounced eastward tilt with height, suggestive of downward phase propagation and upward energy dispersion. A cirrus maximum is observed over equatorial Africa and South America when the enhanced MJO-related convection enters the western Pacific. Tropical-mean TTL cirrus is modulated by the MJO, with more than twice as much TTL cirrus fractional coverage equatorward of 10° latitude when the enhanced convection enters the Pacific than a few weeks earlier, when the convection is over the Indian Ocean. The annual cycle in cirrus clouds around the base of the TTL is equatorially asymmetric, with more cirrus observed in the summer hemisphere. Higher in the TTL, the annual cycle in cirrus clouds is more equatorially symmetric, with a maximum in the boreal winter throughout most of the tropics. The ENSO signature in TTL cirrus is marked by a zonal shift of the peak cloudiness toward the central Pacific during El Niño and toward the Maritime Continent during La Niña.
26
Immler, F., K. Krüger, M. Fujiwara, G. Verver, M. Rex e O. Schrems. "Correlation between equatorial Kelvin waves and the occurrence of extremely thin ice clouds at the tropical tropopause". Atmospheric Chemistry and Physics Discussions 8, n. 1 (13 febbraio 2008): 2849–62. http://dx.doi.org/10.5194/acpd-8-2849-2008.
Abstract (sommario):
Abstract. A number of field-campaigns in the tropics have been conducted in the recent years with two different LIDAR systems at Paramaribo in Suriname (5.8° N, 55.2° W). The lidars detect particles in the atmosphere with high vertical and temporal resolution and are capable of detecting extremely thin cloud layers which frequently occur in the tropical tropopause layer (TTL). Radiosonde as well as operational ECMWF analysis show that temperature anomalies caused by equatorial Kelvin waves propagate downward, well below the cold point tropopause (CPT). We find a clear correlation between the temperature anomalies introduced by these waves and the occurrence of thin cirrus in the TTL. In particular we found that extremely thin ice clouds form regularly where cold anomalies shift the tropopause to high altitudes. This finding suggests an influence of Kelvin wave activity on the dehydration in the TTL and thus on the global stratospheric water vapour concentration.
27
Palazzi, E., F. Fierli, F. Cairo, C. Cagnazzo, G. Di Donfrancesco, E. Manzini, F. Ravegnani, C. Schiller, F. D'Amato e C. M. Volk. "Diagnostics of the Tropical Tropopause Layer from in-situ observations and CCM data". Atmospheric Chemistry and Physics Discussions 9, n. 3 (12 maggio 2009): 11659–98. http://dx.doi.org/10.5194/acpd-9-11659-2009.
Abstract (sommario):
Abstract. A suite of diagnostics is applied to in-situ aircraft measurements and one Chemistry-Climate Model (CCM) data to characterize the vertical structure of the Tropical Tropopause Layer (TTL). The diagnostics are based on the vertical tracers profiles, relative vertical tracers gradients, and tracer-tracer relationships in the tropical Upper Troposphere/Lower Stratosphere (UT/LS), using tropopause coordinates. Observations come from the four tropical campaigns performed from 1998 to 2006 with the research aircraft Geophysica and have been directly compared to the output of the ECHAM5/MESSy CCM. The model vertical resolution in the TTL allows for appropriate comparison with high-resolution aircraft observations and the diagnostics used highlight common TTL features between the model and the observational data. The analysis of the vertical profiles of water vapour, ozone, and nitrous oxide, in both the observations and the model, shows that concentration mixing ratios exhibit a strong gradient change across the tropical tropopause, due to the role of this latter as a transport barrier and that transition between the tropospheric and stratospheric regimes occurs within a finite layer. The use of relative vertical ozone gradients, in addition to the vertical profiles, helps to highlight the region where this transition occurs and allows to give an estimate of its thickness. The analysis of the CO-O3 and H2O-O3 scatter plots and of the Probability Distribution Function (PDF) of the H2O-O3 pair completes this picture as it allows to better distinguish tropospheric and stratospheric regimes that can be identified, first, by their differing chemical composition. The joint analysis and comparison of observed and modelled data allows us to evaluate the capability of the model in reproducing the observed vertical structure of the TTL and its variability, and also to assess whether observations from particular regions on a monthly timescale can be representative of the fine scale mean structure of the Tropical Tropopause Layer.
28
Grise, Kevin M., e David W. J. Thompson. "On the Signatures of Equatorial and Extratropical Wave Forcing in Tropical Tropopause Layer Temperatures". Journal of the Atmospheric Sciences 70, n. 4 (1 aprile 2013): 1084–102. http://dx.doi.org/10.1175/jas-d-12-0163.1.
Abstract (sommario):
Abstract Temperatures in the tropical tropopause layer (TTL) play an important role in stratosphere–troposphere exchange and in the formation and maintenance of thin cirrus clouds. Many previous studies have examined the contributions of extratropical and equatorial waves to the TTL using coarse-vertical-resolution satellite and reanalysis data. In this study, the authors provide new insight into the role of extratropical and equatorial waves in the TTL using high-vertical-resolution GPS radio occultation data. The results examine the influence of four different wave forcings on the TTL: extratropical waves that propagate vertically into the stratosphere, extratropical waves that propagate meridionally into the subtropical stratosphere, extratropical waves that propagate meridionally into the subtropical troposphere, and the equatorial planetary waves. The vertically and meridionally propagating extratropical stratospheric waves are associated with deep, zonally symmetric temperature anomalies that extend and amplify with height throughout the lower-to-middle tropical stratosphere. In contrast, the extratropical tropospheric waves and the equatorial planetary waves are associated with tropical temperature anomalies that are confined below 20-km altitude. The equatorial planetary waves dominate the zonally asymmetric component of the TTL temperature field, and both the equatorial planetary waves and the extratropical tropospheric waves are linked to large temperature variability in a 1–2-km-deep layer near the tropical tropopause. The fine vertical scale of the TTL temperature features associated with the equatorial planetary waves and the extratropical tropospheric waves is only readily apparent in high-vertical-resolution data.
29
Rivoire, Louis, Thomas Birner, John A. Knaff e Natalie Tourville. "Quantifying the Radiative Impact of Clouds on Tropopause Layer Cooling in Tropical Cyclones". Journal of Climate 33, n. 15 (1 agosto 2020): 5527–42. http://dx.doi.org/10.1175/jcli-d-19-0813.1.
Abstract (sommario):
AbstractA ubiquitous cold signal near the tropopause, here called “tropopause layer cooling” (TLC), has been documented in deep convective regions such as tropical cyclones (TCs). Temperature retrievals from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) reveal cooling of order 0.1–1 K day−1 on spatial scales of order 1000 km above TCs. Data from the Cloud Profiling Radar (onboard CloudSat) and from the Cloud–Aerosol Lidar with Orthogonal Polarization [onboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)] are used to analyze cloud distributions associated with TCs. Evidence is found that convective clouds within TCs reach the upper part of the tropical tropopause layer (TTL) more frequently than do convective clouds outside TCs, raising the possibility that convective clouds within TCs and associated cirrus clouds modulate TLC. The contribution of clouds to radiative heating rates is then quantified using the CloudSat and CALIPSO datasets: in the lower TTL (below the tropopause), clouds produce longwave cooling of order 0.1–1 K day−1 inside the TC main convective region, and longwave warming of order 0.01–0.1 K day−1 outside; in the upper TTL (near and above the tropopause), clouds produce longwave cooling of the same order as TLC inside the TC main convective region, and up to one order of magnitude smaller outside. Considering that clouds also produce shortwave warming, it is suggested that cloud radiative effects inside and outside TCs only explain modest amounts of TLC while other processes must provide the remaining cooling.
30
Hosking, J. S., M. R. Russo, P. Braesicke e J. A. Pyle. "Modelling deep convection and its impacts on the tropical tropopause layer". Atmospheric Chemistry and Physics Discussions 10, n. 8 (26 agosto 2010): 20267–302. http://dx.doi.org/10.5194/acpd-10-20267-2010.
Abstract (sommario):
Abstract. The UK Met Office's Unified Model is used at a high global resolution (N216, ~0.83° × ~0.56°, ~60 km) to assess the impact of deep tropical convection on the structure of the tropical tropopause layer (TTL). We focus on the potential for rapid transport of short-lived ozone depleting species to the stratosphere by rapid convective uplift. The modelled horizontal structure of organised convection is shown to match closely with signatures found in the OLR satellite data. In the model, deep convective elevators rapidly lift air from 4–5 km up to 12–14 km. The influx of tropospheric air entering the TTL (11–12 km) is similar for all tropical regions with most convection stopping below ~14 km. The tropical tropopause is coldest and driest between November and February, coinciding with the greatest upwelling over the tropical warm pool. As this deep convection is co-located with bromine-rich biogenic coastal emissions, this period and location could potentially be the preferential gateway for stratospheric bromine.
31
Hosking, J. S., M. R. Russo, P. Braesicke e J. A. Pyle. "Modelling deep convection and its impacts on the tropical tropopause layer". Atmospheric Chemistry and Physics 10, n. 22 (26 novembre 2010): 11175–88. http://dx.doi.org/10.5194/acp-10-11175-2010.
Abstract (sommario):
Abstract. The UK Met Office's Unified Model is used at a climate resolution (N216, ~0.83°×~0.56°, ~60 km) to assess the impact of deep tropical convection on the structure of the tropical tropopause layer (TTL). We focus on the potential for rapid transport of short-lived ozone depleting species to the stratosphere by rapid convective uplift. The modelled horizontal structure of organised convection is shown to match closely with signatures found in the OLR satellite data. In the model, deep convective elevators rapidly lift air from 4–5 km up to 12–14 km. The influx of tropospheric air entering the TTL (11–12 km) is similar for all tropical regions with most convection stopping below ~14 km. The tropical tropopause is coldest and driest between November and February, coinciding with the greatest upwelling over the tropical warm pool. As this deep convection is co-located with bromine-rich biogenic coastal emissions, this period and location could potentially be the preferential gateway for stratospheric bromine.
32
Paulik, L. C., e T. Birner. "Quantifying the deep convective temperature signal within the tropical tropopause layer (TTL)". Atmospheric Chemistry and Physics Discussions 12, n. 8 (7 agosto 2012): 19617–47. http://dx.doi.org/10.5194/acpd-12-19617-2012.
Abstract (sommario):
Abstract. Dynamics on a vast range of spatial and temporal scales, from individual convective plumes to planetary-scale circulations, play a role in driving the temperature variability in the tropical tropopause layer (TTL). Here, we aim to better quantify the deep convective temperature signal within the TTL using multiple datasets. First, we investigate the link between ozone and temperature in the TTL using the Southern Hemisphere Additional Ozonesondes (SHADOZ) dataset. Low ozone concentrations in the TTL are indicative of deep convective transport from the boundary layer. We confirm the usefulness of ozone as an indicator of deep convection by identifying a typical temperature signal associated with reduced ozone events: mid and upper tropospheric warming and TTL cooling. We quantify these temperature signals using two diagnostics: (1) the "ozone minimum" diagnostic, which has been used in previous studies and identifies the upper tropospheric minimum ozone concentration as a proxy for the level of main convective outflow; and (2) the "ozone mixing height", which we introduce in order to identify the maximum altitude in a vertical ozone profile up to which reduced ozone concentrations, typical of transport from the boundary layer are observed. Results indicate that the ozone mixing height diagnostic better separates profiles with convective influence than the ozone minimum diagnostic. Next, we collocate deep convective clouds identified by CloudSat 2B-CLDCLASS with COSMIC GPS temperature profiles. We find a robust large-scale deep convective TTL temperature signal, that is persistent in time. However, it is only the convective events that penetrate into the upper half of the TTL that have a significant impact on TTL temperature. A distinct seasonal difference in the spatial scale and the persistence of the temperature signal is identified. Deep-convective cloud top heights are found to be well described by the level of neutral buoyancy.
33
Bourguet, Stephen, e Marianna Linz. "Weakening of the tropical tropopause layer cold trap with global warming". Atmospheric Chemistry and Physics 23, n. 13 (6 luglio 2023): 7447–60. http://dx.doi.org/10.5194/acp-23-7447-2023.
Abstract (sommario):
Abstract. Lagrangian trajectories have previously been used to reconstruct water vapor variability in the lower stratosphere, where the sensitivity of surface radiation to changes in the water vapor concentration is strongest, by obtaining temperature histories of air parcels that ascend from the troposphere to the stratosphere through the tropical tropopause layer (TTL). Models and theory predict an acceleration of the Brewer–Dobson circulation (BDC) and deceleration of the Walker circulation with surface warming, and both of these will drive future changes to transport across the TTL. Here, we examine the response of TTL transport during boreal winter to idealized changes in the BDC and Walker circulation by comparing the temperature histories of trajectories computed with ERA5 data to those calculated using the same data but with altered vertical and zonal wind velocities. We find that lower-stratospheric water vapor mixing ratios calculated from trajectories' cold point temperatures can increase by about 1.6 ppmv (about 50 %) when only zonal winds are slowed, while changes to vertical winds have a negligible impact on water vapor concentrations. This change follows from a decrease in zonal sampling of the temperature field by trajectories, which weakens the “cold trap” mechanism of dehydration as TTL transport evolves. As the zonal winds of the TTL decrease, the fraction of air that passes through the cold trap while ascending to the stratosphere will decrease and the coldest average temperature experienced by parcels will increase. Future changes to TTL temperatures can be applied as an offset to these temperature histories, including enhanced warming of the cold trap due to El Niño-like warming, which has a secondary impact on the fraction of air that is dehydrated by the cold trap. Some of the resultant moistening may be negated by a decreased rate of temperature change following the cold point, which will allow more ice to gravitationally settle before sublimating outside of the cold trap. This result presents a mechanism for a stratospheric water vapor feedback that can exist without changes to TTL temperatures.
34
Evan, Stephanie, Jerome Brioude, Karen Rosenlof, Sean M. Davis, Holger Vömel, Damien Héron, Françoise Posny et al. "Effect of deep convection on the tropical tropopause layer composition over the southwest Indian Ocean during austral summer". Atmospheric Chemistry and Physics 20, n. 17 (10 settembre 2020): 10565–86. http://dx.doi.org/10.5194/acp-20-10565-2020.
Abstract (sommario):
Abstract. Balloon-borne measurements of cryogenic frost-point hygrometer (CFH) water vapor, ozone and temperature and water vapor lidar measurements from the Maïdo Observatory on Réunion Island in the southwest Indian Ocean (SWIO) were used to study tropical cyclones' influence on tropical tropopause layer (TTL) composition. The balloon launches were specifically planned using a Lagrangian model and Meteosat-7 infrared images to sample the convective outflow from tropical storm (TS) Corentin on 25 January 2016 and tropical cyclone (TC) Enawo on 3 March 2017. Comparing the CFH profile to Aura's Microwave Limb Sounder's (MLS) monthly climatologies, water vapor anomalies were identified. Positive anomalies of water vapor and temperature, and negative anomalies of ozone between 12 and 15 km in altitude (247 to 121 hPa), originated from convectively active regions of TS Corentin and TC Enawo 1 d before the planned balloon launches according to the Lagrangian trajectories. Near the tropopause region, air masses on 25 January 2016 were anomalously dry around 100 hPa and were traced back to TS Corentin's active convective region where cirrus clouds and deep convective clouds may have dried the layer. An anomalously wet layer around 68 hPa was traced back to the southeast Indian Ocean where a monthly water vapor anomaly of 0.5 ppmv was observed. In contrast, no water vapor anomaly was found near or above the tropopause region on 3 March 2017 over Maïdo as the tropopause region was not downwind of TC Enawo. This study compares and contrasts the impact of two tropical cyclones on the humidification of the TTL over the SWIO. It also demonstrates the need for accurate balloon-borne measurements of water vapor, ozone and aerosols in regions where TTL in situ observations are sparse.
35
Paulik, L. C., e T. Birner. "Quantifying the deep convective temperature signal within the tropical tropopause layer (TTL)". Atmospheric Chemistry and Physics 12, n. 24 (21 dicembre 2012): 12183–95. http://dx.doi.org/10.5194/acp-12-12183-2012.
Abstract (sommario):
Abstract. Dynamics on a vast range of spatial and temporal scales, from individual convective plumes to planetary-scale circulations, play a role in driving the temperature variability in the tropical tropopause layer (TTL). Here, we aim to better quantify the deep convective temperature signal within the TTL using multiple datasets. First, we investigate the link between ozone and temperature in the TTL using the Southern Hemisphere Additional Ozonesondes (SHADOZ) dataset. Low ozone concentrations in the TTL are indicative of deep convective transport from the boundary layer. We confirm the usefulness of ozone as an indicator of deep convection by identifying a typical temperature signal associated with reduced ozone events: an anomalously warm mid to upper troposphere and an anomalously cold upper TTL. We quantify these temperature signals using two diagnostics: (1) the "ozone minimum" diagnostic, which has been used in previous studies and identifies the upper tropospheric minimum ozone concentration as a proxy for the level of main convective outflow; and (2) the "ozone mixing height", which we introduce in order to identify the maximum altitude in a vertical ozone profile up to which reduced ozone concentrations, typical of transport from the boundary layer are observed. Results indicate that the ozone mixing height diagnostic better separates profiles with convective influence than the ozone minimum diagnostic. Next, we collocate deep convective clouds identified by CloudSat 2B-CLDCLASS with temperature profiles based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Global Position System (GPS) radio occultations. We find a robust large-scale deep convective TTL temperature signal, that is persistent in time. However, it is only the convective events that penetrate into the upper half of the TTL that have a significant impact on TTL temperature. A distinct seasonal difference in the spatial scale and the persistence of the temperature signal is identified. Deep-convective cloud top heights are on average found to be well described by the level of neutral buoyancy.
36
Wang, W., K. Matthes e T. Schmidt. "Quantifying contributions to the recent temperature variability in the tropical tropopause layer". Atmospheric Chemistry and Physics 15, n. 10 (26 maggio 2015): 5815–26. http://dx.doi.org/10.5194/acp-15-5815-2015.
Abstract (sommario):
Abstract. The recently observed variability in the tropical tropopause layer (TTL), which features a warming of 0.9 K over the past decade (2001–2011), is investigated with a number of sensitivity experiments from simulations with NCAR's CESM-WACCM chemistry–climate model. The experiments have been designed to specifically quantify the contributions from natural as well as anthropogenic factors, such as solar variability (Solar), sea surface temperatures (SSTs), the quasi-biennial oscillation (QBO), stratospheric aerosols (Aerosol), greenhouse gases (GHGs) and the dependence on the vertical resolution in the model. The results show that, in the TTL from 2001 through 2011, a cooling in tropical SSTs leads to a weakening of tropical upwelling around the tropical tropopause and hence relative downwelling and adiabatic warming of 0.3 K decade-1; stronger QBO westerlies result in a 0.2 K decade-1 warming; increasing aerosols in the lower stratosphere lead to a 0.2 K decade-1 warming; a prolonged solar minimum contributes about 0.2 K decade-1 to a cooling; and increased GHGs have no significant influence. Considering all the factors mentioned above, we compute a net 0.5 K decade-1 warming, which is less than the observed 0.9 K decade-1 warming over the past decade in the TTL. Two simulations with different vertical resolution show that, with higher vertical resolution, an extra 0.8 K decade-1 warming can be simulated through the last decade compared with results from the "standard" low vertical resolution simulation. Model results indicate that the recent warming in the TTL is partly caused by stratospheric aerosols and mainly due to internal variability, i.e. the QBO and tropical SSTs. The vertical resolution can also strongly influence the TTL temperature response in addition to variability in the QBO and SSTs.
37
Chen, Jiong, Zhe Li, Zhanshan Ma, Yong Su e Qijun Liu. "Sensitivity of Tropical Tropopause Layer Cirrus Prediction in GRAPES Global Forecast System". Monthly Weather Review 149, n. 11 (novembre 2021): 3609–25. http://dx.doi.org/10.1175/mwr-d-21-0025.1.
Abstract (sommario):
Abstract A warm bias with a maximum value of over 4 K in the tropical tropopause layer (TTL) is detected in day-5 operational forecasts of the Global/Regional Assimilation and Prediction System (GRAPES) for global medium-range numerical weather prediction (GRAPES_GFS). In this study, the predicted temperature changes caused by different processes are examined, and the predicted cloud fractions are compared with the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis data. It is found that the overprediction of the TTL cirrus fraction contributes to the warm bias due to cloud-radiative heating. The interactions among the ice nucleation, deposition/sublimation, and the large-scale condensation together determine the results of the TTL ice crystal content prediction. Moreover, a range of sensitivity experiments show that the TTL ice crystal content prediction is sensitive to the threshold relative humidity over ice (RHi) in the ice nucleation process. Then the uncertainties of the formulas for saturation vapor pressure over ice at very low temperatures are discussed. The RHi calculated based on the Magnus–Tetens formula is up to 10% higher than that based on the Goff–Gratch formula. As the Goff–Gratch formula is applicable over a broader range of 184–273 K, it is more suitable for the cold TTL. When the Goff–Gratch formula rather than the Magnus–Tetens formula is used in the microphysics scheme, the TTL cirrus forecasts are improved greatly, and the warm bias disappears completely. After investigating the interplay of the dynamical, microphysical, and radiative processes, we find a positive feedback mechanism that exacerbates the TTL cirrus prediction error.
38
Jensen, E. J., L. Pfister, T. P. Bui, P. Lawson e D. Baumgardner. "Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus". Atmospheric Chemistry and Physics Discussions 9, n. 5 (1 ottobre 2009): 20631–75. http://dx.doi.org/10.5194/acpd-9-20631-2009.
Abstract (sommario):
Abstract. In past modeling studies, it has generally been assumed that the predominant mechanism for nucleation of ice in the uppermost troposphere is homogeneous freezing of aqueous aerosols. However, recent in situ and remote-sensing measurements of the properties of cirrus clouds at very low temperatures in the tropical tropopause layer (TTL) are broadly inconsistent with theoretial predictions based on the homogeneous freezing assumption. The nearly ubiquitous occurence of gravity waves in the TTL makes the predictions from homogeneous nucleation theory particularly difficult to reconcile with measurements. These measured properties include ice number concentrations, which are much lower than theory predicts; ice crystal size distributions, which are much broader than theory predicts; and cloud extinctions, which are much lower than theory predicts. Although other explanations are possible, one way to limit ice concentrations is to have on the order of 50 L−1 effective ice nuclei (IN) that could nucleate ice at relatively low supersaturations. We suggest that ammonium sulfate particles, which would be dry much of the time in the cold TTL, are a potential IN candidate for TTL cirrus. Possible implications of the observed cloud microphysical properties for ice sedimentation, dehydration, and cloud persistence are also discussed.
39
Immler, F., K. Krüger, M. Fujiwara, G. Verver, M. Rex e O. Schrems. "Correlation between equatorial Kelvin waves and the occurrence of extremely thin ice clouds at the tropical tropopause". Atmospheric Chemistry and Physics 8, n. 14 (25 luglio 2008): 4019–26. http://dx.doi.org/10.5194/acp-8-4019-2008.
Abstract (sommario):
Abstract. A number of field-campaigns in the tropics have been conducted in recent years with two different LIDAR systems at Paramaribo (5.8° N, 55.2° W), Suriname. The lidars detect particles in the atmosphere with high vertical and temporal resolution and are capable of detecting extremely thin cloud layers which frequently occur in the tropical tropopause layer (TTL). Radiosonde as well as operational ECMWF analysis showed that equatorial Kelvin waves propagated in the TTL and greatly modulated its temperature structure. We found a clear correlation between the temperature anomalies introduced by these waves and the occurrence of thin cirrus in the TTL. In particular we found that extremely thin ice clouds form regularly where cold anomalies shift the tropopause to high altitudes. These findings suggest an influence of Kelvin wave activity on the dehydration in the TTL and thus on the global stratospheric water vapour concentration.
40
Virts, Katrina S., John M. Wallace, Qiang Fu e Thomas P. Ackerman. "Tropical Tropopause Transition Layer Cirrus as Represented by CALIPSO Lidar Observations". Journal of the Atmospheric Sciences 67, n. 10 (1 ottobre 2010): 3113–29. http://dx.doi.org/10.1175/2010jas3412.1.
Abstract (sommario):
Abstract The spatial and temporal variability of cirrus cloud fraction within the tropical tropopause transition layer (TTL) is investigated based on three years of data from the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, analyzed in conjunction with fields from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim and temperature profiles from radiosondes launched at Manus Island, Papua New Guinea (2°S, 147°E). TTL cirrus is found to be mainly confined to the rising branch of the Hadley cell within ∼15° of the equator, with maximum cloud fraction between 14 and 15 km. The time-varying spatial pattern of cloud fraction within this belt does not resemble the pattern of cloud fraction in the layer below, as would be expected if the TTL cirrus were formed by the spreading of the anvils of convective clouds. On the contrary, within the stably stratified layer above ∼13 km, cirrus cloud fraction and temperature both appear to be modulated by the planetary-scale vertical velocity field. The time-varying spatial patterns are reminiscent of the vertical-propagating Kelvin wave response to an equatorial heat source, with the coldest, cloudiest air in the TTL centered approximately 30° of longitude to the east of the strongest heating.
41
Carminati, F., P. Ricaud, J. P. Pommereau, E. Rivière, S. Khaykin, J. L. Attié e J. Warner. "Impact of tropical land convection on the water vapour budget in the tropical tropopause layer". Atmospheric Chemistry and Physics 14, n. 12 (23 giugno 2014): 6195–211. http://dx.doi.org/10.5194/acp-14-6195-2014.
Abstract (sommario):
Abstract. The tropical deep overshooting convection is known to be most intense above continental areas such as South America, Africa, and the maritime continent. However, its impact on the tropical tropopause layer (TTL) at global scale remains debated. In our analysis, we use the 8-year Microwave Limb Sounder (MLS) water vapour (H2O), cloud ice-water content (IWC), and temperature data sets from 2005 to date, to highlight the interplays between these parameters and their role in the water vapour variability in the TTL, and separately in the northern and southern tropics. In the tropical upper troposphere (177 hPa), continents, including the maritime continent, present the night-time (01:30 local time, LT) peak in the water vapour mixing ratio characteristic of the H2O diurnal cycle above tropical land. The western Pacific region, governed by the tropical oceanic diurnal cycle, has a daytime maximum (13:30 LT). In the TTL (100 hPa) and tropical lower stratosphere (56 hPa), South America and Africa differ from the maritime continent and western Pacific displaying a daytime maximum of H2O. In addition, the relative amplitude between day and night is found to be systematically higher by 5–10% in the southern tropical upper troposphere and 1–3% in the TTL than in the northern tropics during their respective summer, indicative of a larger impact of the convection on H2O in the southern tropics. Using a regional-scale approach, we investigate how mechanisms linked to the H2O variability differ in function of the geography. In summary, the MLS water vapour and cloud ice-water observations demonstrate a clear contribution to the TTL moistening by ice crystals overshooting over tropical land regions. The process is found to be much more effective in the southern tropics. Deep convection is responsible for the diurnal temperature variability in the same geographical areas in the lowermost stratosphere, which in turn drives the variability of H2O.
42
Emanuel, Kerry, Susan Solomon, Doris Folini, Sean Davis e Chiara Cagnazzo. "Influence of Tropical Tropopause Layer Cooling on Atlantic Hurricane Activity". Journal of Climate 26, n. 7 (1 aprile 2013): 2288–301. http://dx.doi.org/10.1175/jcli-d-12-00242.1.
Abstract (sommario):
Abstract Virtually all metrics of Atlantic tropical cyclone activity show substantial increases over the past two decades. It is argued here that cooling near the tropical tropopause and the associated decrease in tropical cyclone outflow temperature contributed to the observed increase in tropical cyclone potential intensity over this period. Quantitative uncertainties in the magnitude of the cooling are important, but a broad range of observations supports some cooling. Downscalings of the output of atmospheric general circulation models (AGCMs) that are driven by observed sea surface temperatures and sea ice cover produce little if any increase in Atlantic tropical cyclone metrics over the past two decades, even though observed variability before roughly 1970 is well simulated by some of the models. Part of this shortcoming is traced to the failure of the AGCMs examined to reproduce the observed cooling of the lower stratosphere and tropical tropopause layer (TTL) over the past few decades. The authors caution against using sea surface temperature or proxies based on it to make projections of tropical cyclone activity as there can be significant contributions from other variables such as the outflow temperature. The proposed mechanisms of TTL cooling (e.g., ozone depletion and stratospheric circulation changes) are reviewed, and the need for improved representations of these processes in global models in order to improve projections of future tropical cyclone activity is emphasized.
43
Fernandez, R. P., R. J. Salawitch, D. E. Kinnison, J. F. Lamarque e A. Saiz-Lopez. "Bromine partitioning in the tropical tropopause layer: implications for stratospheric injection". Atmospheric Chemistry and Physics Discussions 14, n. 12 (3 luglio 2014): 17857–905. http://dx.doi.org/10.5194/acpd-14-17857-2014.
Abstract (sommario):
Abstract. Very short-lived (VSL) bromocarbons are produced at a prodigious rate by ocean biology and these source compounds (SGVSL), together with their degradation inorganic products (PGVSL), are lofted by vigorous convection to the tropical tropopause layer (TTL). Using a state-of-the-art photochemical mechanism within a global model, we calculate annual average stratospheric injection of total bromine due to VSL sources to be 5 pptv, with ~3 pptv entering the stratosphere as PGVSL and ~2 pptv as SGVSL. The geographic distribution and partitioning of VSL bromine within the TTL, and its consequent stratospheric injection, is highly dependent on the oceanic flux, the strength of convection and the occurrence of heterogeneous recycling reactions. Our calculations indicate atomic Br should be the dominant inorganic species in large regions of the TTL during daytime, due to the low ozone and cold conditions of this region. We propose the existence of a "tropical ring of atomic bromine" located approximately between 15 and 19 km and 30° N to 30° S. Daytime Br / BrO ratios of up to ~4 are predicted within the Br ring in regions of highly convective transport, such as the tropical Western Pacific. Then, we suggest experimental programs designed to quantify the bromine budget of the TTL and the stratospheric injection of VSL biogenic bromocarbons should include a strategy for the measurement of atomic Br during daytime and HOBr or BrCl during nighttime.
44
Fernandez, R. P., R. J. Salawitch, D. E. Kinnison, J. F. Lamarque e A. Saiz-Lopez. "Bromine partitioning in the tropical tropopause layer: implications for stratospheric injection". Atmospheric Chemistry and Physics 14, n. 24 (16 dicembre 2014): 13391–410. http://dx.doi.org/10.5194/acp-14-13391-2014.
Abstract (sommario):
Abstract. Very short-lived (VSL) bromocarbons are produced at a prodigious rate by ocean biology and these source compounds (SGVSL), together with their inorganic degradation products (PGVSL), are lofted by vigorous convection to the tropical tropopause layer (TTL). Using a state-of-the-art photochemical mechanism within a global model, we calculate annual average stratospheric injection of total bromine due to VSL sources to be 5 pptv (parts per trillion by volume), with ~ 3 pptv entering the stratosphere as PGVSL and ~ 2 pptv as SGVSL. The geographic distribution and partitioning of VSL bromine within the TTL, and its consequent stratospheric injection, is highly dependent on the oceanic flux, the strength of convection and the occurrence of heterogeneous recycling reactions. Our calculations indicate atomic Br should be the dominant inorganic species in large regions of the TTL during daytime, due to the low ozone and cold conditions of this region. We propose the existence of a "tropical ring of atomic bromine" located approximately between 15 and 19 km and between 30° N and 30° S. Daytime Br / BrO ratios of up to ~ 4 are predicted within this inhomogeneous ring in regions of highly convective transport, such as the tropical Western Pacific. Therefore, we suggest that experimental programs designed to quantify the bromine budget of the TTL and the stratospheric injection of VSL biogenic bromocarbons should include a strategy for the measurement of atomic Br during daytime as well as HOBr and BrCl during nighttime.
45
Jensen, E. J., L. Pfister, T. P. Bui, P. Lawson e D. Baumgardner. "Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus". Atmospheric Chemistry and Physics 10, n. 3 (5 febbraio 2010): 1369–84. http://dx.doi.org/10.5194/acp-10-1369-2010.
Abstract (sommario):
Abstract. In past modeling studies, it has generally been assumed that the predominant mechanism for nucleation of ice in the uppermost troposphere is homogeneous freezing of aqueous aerosols. However, recent in situ and remote-sensing measurements of the properties of cirrus clouds at very low temperatures in the tropical tropopause layer (TTL) are broadly inconsistent with theoretial predictions based on the homogeneous freezing assumption. The nearly ubiquitous occurence of gravity waves in the TTL makes the predictions from homogeneous nucleation theory particularly difficult to reconcile with measurements. These measured properties include ice number concentrations, which are much lower than theory predicts; ice crystal size distributions, which are much broader than theory predicts; and cloud extinctions, which are much lower than theory predicts. Although other explanations are possible, one way to limit ice concentrations is to have on the order of 50 L−1 effective ice nuclei (IN) that could nucleate ice at relatively low supersaturations. We suggest that ammonium sulfate particles, which would be dry much of the time in the cold TTL, are a potential IN candidate for TTL cirrus. However, this mechanism remains to be fully quantified for the size distribution of ammonium sulfate (possibly internally mixed with organics) actually present in the upper troposphere. Possible implications of the observed cloud microphysical properties for ice sedimentation, dehydration, and cloud persistence are also discussed.
46
Jucker, M., e E. P. Gerber. "Untangling the Annual Cycle of the Tropical Tropopause Layer with an Idealized Moist Model". Journal of Climate 30, n. 18 (17 agosto 2017): 7339–58. http://dx.doi.org/10.1175/jcli-d-17-0127.1.
Abstract (sommario):
Abstract The processes regulating the climatology and annual cycle of the tropical tropopause layer (TTL) and cold point are not fully understood. Three main drivers have been identified: planetary-scale equatorial waves excited by tropical convection, planetary-scale extratropical waves associated with the deep Brewer–Dobson circulation, and synoptic-scale waves associated with the midlatitude storm tracks. In both observations and comprehensive atmospheric models, all three coexist, making it difficult to separate their contributions. Here, a new intermediate-complexity atmospheric model is developed. Simple modification of the model’s lower boundary allows detailed study of the three processes key to the TTL, both in isolation and together. The model shows that tropical planetary waves are most critical for regulating the mean TTL, setting the depth and temperature of the cold point. The annual cycle of the TTL, which is coldest (warmest) in boreal winter (summer), however, depends critically on the strong annual variation in baroclinicity of the Northern Hemisphere relative to that of the Southern Hemisphere. Planetary-scale waves excited from either the tropics or extratropics then double the impact of baroclinicity on the TTL annual cycle. The remarkably generic response of TTL temperatures over a range of configurations suggests that the details of the wave forcing are unimportant, provided there is sufficient variation in the upward extent of westerly winds over the annual cycle. Westerly winds enable the propagation of stationary Rossby waves, and weakening of the subtropical jet in boreal summer inhibits their propagation into the lower stratosphere, warming the TTL.
47
Carminati, F., P. Ricaud, J. P. Pommereau, E. Rivière, S. Khaykin, J. L. Attié e J. Warner. "Impact of tropical land convection on the water vapour budget in the Tropical Tropopause Layer". Atmospheric Chemistry and Physics Discussions 13, n. 12 (18 dicembre 2013): 33055–87. http://dx.doi.org/10.5194/acpd-13-33055-2013.
Abstract (sommario):
Abstract. The tropical deep overshooting convection is known to be most intense above continental areas such as South America, Africa and the maritime continent. However, its impact on the Tropical Tropopause Layer (TTL) at global scale remains debated. In our analysis, we use the 8 yr Microwave Limb Sounder (MLS) water vapour (H2O), cloud ice water content (IWC) and temperature datasets from 2005 to date, to highlight the interplays between these parameters and their role in the water vapour variability in the TTL, separately in the northern and southern tropics. The water vapour concentration is displaying a systematic diurnal cycle with a night-time peak in the tropical Upper Troposphere (pressure ≥146 hPa) and the opposite in the TTL (121 to 68 hPa) and the tropical Lower Stratosphere (pressure ≤56 hPa), of larger amplitude above continents than continental-oceanic areas such as the maritime continent or full oceanic areas such as the Western Pacific. In addition, the amplitude of the diurnal cycle is found systematically larger (5–10%) in the southern than in the northern tropics during their respective summer, indicative of a more vigorous convective intensity in the south. Using a regional scale approach, we investigate the geographical variations of mechanisms linked to the H2O variability. In summary, the MLS water vapour, ice water cloud and temperature observations are demonstrating a clear contribution of TTL and lower stratosphere moistening by ice crystals overshooting updrafts over land tropical regions and the much greater efficiency of the process in the Southern Hemisphere.
48
Durran, Dale R., Tra Dinh, Marie Ammerman e Thomas Ackerman. "The Mesoscale Dynamics of Thin Tropical Tropopause Cirrus". Journal of the Atmospheric Sciences 66, n. 9 (1 settembre 2009): 2859–73. http://dx.doi.org/10.1175/2009jas3046.1.
Abstract (sommario):
Abstract Thin cirrus clouds in the tropical tropopause layer (TTL) are warmed through the absorption of infrared radiation. The response of the cloud and the surrounding atmosphere to this thermal forcing is investigated through linear theory and nonlinear numerical simulation. Linear solutions for the circulations forced by a fixed heat source representative of TTL cirrus clouds show ascent in the region of the heating, accompanied by horizontal flow toward the heat source at the base of the heated layer and horizontal outflow at the top of the layer. Gravity waves propagate positive temperature perturbations well beyond the lateral edges of the heated region. Cool layers that also spread horizontally are produced immediately above and below the heated region. Numerical simulations with a cloud-resolving model allow the radiative heating to change in response to the redistribution of the cloud by the evolving velocity field. The basic atmospheric response in the numerical simulations is nevertheless similar to that generated by the fixed heat source. In the numerical simulations, the advection of ice crystals by the radiatively forced velocity field also lofts the cloud, while horizontally spreading its top and narrowing its base. Ice crystal sedimentation is neglected in these calculations, but it appears that the radiatively induced upward vertical velocities are likely strong enough to maintain clouds consisting of very small crystals (radii less than 4 μm) against sedimentation for many hours.
49
Levine, J. G., P. Braesicke, N. R. P. Harris e J. A. Pyle. "Seasonal and inter-annual variations in Troposphere-to-Stratosphere Transport from the Tropical Tropopause Layer". Atmospheric Chemistry and Physics Discussions 8, n. 1 (10 gennaio 2008): 489–520. http://dx.doi.org/10.5194/acpd-8-489-2008.
Abstract (sommario):
Abstract. In an earlier study of troposphere-to-stratosphere transport (TST) via the tropical tropopause layer (TTL), we found that the vast majority of air parcels undergoing TST from the base of the TTL enter the extratropical lowermost stratosphere quasi-horizontally and show little or no regional preference with regards to origin in the TTL or entry into the stratosphere. We have since repeated the trajectory calculations - originally limited to a single northern hemisphere winter period - in a variety of months and years to assess how robust our earlier findings are to change of timing. To first order, we find that the main conclusions hold, irrespective of the season, year and phase of the El Niño Southern Oscillation (ENSO). We also explore: the distribution of TST between the northern and southern hemispheres; the sensitivity of modelled TST to the definition of the tropopause; and the routes by which air parcels undergo transport exclusively to the stratospheric overworld. Subject to a dynamical definition of the tropopause, we identify a strong bias towards TST in the southern hemisphere, particularly during the northern hemisphere summer. The main difference on switching to the World Meteorological Organization's thermal tropopause definition is that much less TST is modelled in the subtropics and, relative to the dynamical definition, we calculate significantly less transport into the extratropical lowermost stratosphere (ELS) – an important region with regards to ozone chemistry. In contrast to the rather homogeneous nature of TST into the ELS, we find that transport to the overworld takes place from relatively well-defined regions of the TTL, predominantly above the West Pacific and Indonesia, except for an El Niño period in which most transport takes place from regions above the East Pacific and South America.
50
Ye, Hao, Andrew E. Dessler e Wandi Yu. "Effects of convective ice evaporation on interannual variability of tropical tropopause layer water vapor". Atmospheric Chemistry and Physics 18, n. 7 (3 aprile 2018): 4425–37. http://dx.doi.org/10.5194/acp-18-4425-2018.
Abstract (sommario):
Abstract. Water vapor interannual variability in the tropical tropopause layer (TTL) is investigated using satellite observations and model simulations. We break down the influences of the Brewer–Dobson circulation (BDC), the quasi-biennial oscillation (QBO), and the tropospheric temperature (ΔT) on TTL water vapor as a function of latitude and longitude using a two-dimensional multivariate linear regression. This allows us to examine the spatial distribution of the impact of each process on TTL water vapor. In agreement with expectations, we find that the impacts from the BDC and QBO act on TTL water vapor by changing TTL temperature. For ΔT, we find that TTL temperatures alone cannot explain the influence. We hypothesize a moistening role for the evaporation of convective ice from increased deep convection as the troposphere warms. Tests using a chemistry–climate model, the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM), support this hypothesis.