Добірка наукової літератури з теми "Intense Convective Clouds"

Abstract. Thermal structure associated with deep convective clouds is investigated using Global Positioning System (GPS) radio occultation measurements. GPS data are insensitive to the presence of clouds, and provide high vertical resolution and high accuracy measurements to identify associated temperature behavior. Deep convective systems are identified using International Satellite Cloud Climatology Project (ISCCP) satellite data, and cloud tops are accurately measured using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIPSO) lidar observations; we focus on 53 cases of near-coincident GPS occultations with CALIPSO profiles over deep convection. Results show a sharp spike in GPS bending angle highly correlated to the top of the clouds, corresponding to anomalously cold temperatures within the clouds. Above the clouds the temperatures return to background conditions, and there is a strong inversion at cloud top. For cloud tops below 14 km, the temperature lapse rate within the cloud often approaches a moist adiabat, consistent with rapid undiluted ascent within the convective systems.

Abstract. Thermal structure associated with deep convective clouds is investigated using Global Positioning System (GPS) radio occultation measurements. GPS data are insensitive to the presence of clouds, and provide high vertical resolution and high accuracy measurements to identify associated temperature behavior. Deep convective systems are identified using International Satellite Cloud Climatology Project (ISCCP) satellite data, and cloud tops are accurately measured using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIPSO) lidar observations; we focus on 53 cases of near-coincident GPS occultations with CALIPSO profiles over deep convection. Results show a sharp spike in GPS bending angle highly correlated to the top of the clouds, corresponding to anomalously cold temperatures within the clouds. Above the clouds the temperatures return to background conditions, and there is a strong inversion at cloud top. For cloud tops below 14 km, the temperature lapse rate within the cloud often approaches a moist adiabat, consistent with rapid undiluted ascent within the convective systems.

Tropical Rainfall Measuring Mission Precipitation Radar (TRMM-PR) based vertical structure in intense convective precipitation is presented here for Indian and Austral summer monsoon seasons. TRMM 2A23 data is used to identify the convective echoes in PR data. Two types of cloud cells are constructed here, namely intense convective cloud (ICC) and most intense convective cloud (MICC). ICC consists of PR radar beams having Ze>=40 dBZ above 1.5 km in convective precipitation area, whereas MICC, consists of maximum reflectivity at each altitude in convective precipitation area, with at least one radar pixel must be higher than 40 dBZ or more above 1.5 km within the selected areas. We have selected 20 locations across the tropics to see the regional differences in the vertical structure of convective clouds. One of the important findings of the present study is identical behavior in the average vertical profiles in intense convective precipitation in lower troposphere across the different areas. MICCs show the higher regional differences compared to ICCs between 5-12 km altitude. Land dominated areas show higher regional differences and Southeast south America (SESA) has the strongest vertical profile (higher Ze at higher altitude) followed by Indo-Gangetic plain (IGP), Africa, north Latin America whereas weakest vertical profile occurs over Australia. Overall SESA (41%) and IGP (36%) consist higher fraction of deep convective clouds (>10 km), whereas, among the tropical oceanic areas, Western (Eastern) equatorial Indian ocean consists higher fraction of low (high) level of convective clouds. Nearly identical average vertical profiles over the tropical oceanic areas, indicate the similarity in the development of intense convective clouds and useful while considering them in model studies.

Abstract The University of Wisconsin Convective Initiation (UWCI) algorithm utilizes geostationary IR satellite data to compute cloud-top cooling (UW-CTC) rates and assign CI nowcasts to vertically growing clouds. This study is motivated by National Weather Service (NWS) forecaster reviews of the algorithm output, which hypothesized that more intense cloud-top cooling corresponds to more vigorous short-term (0–60 min) convective development. An objective validation of UW-CTC rates using a satellite-based object-tracking methodology is presented, along with a prognostic evaluation of such cloud-top cooling rates for use in forecasting the growth and development of deep convection. In general, both a cloud object’s instantaneous and maximum cooling rate(s) are shown to be useful prognostic tools in predicting future radar intensification. UW-CTC rates are shown to be most skillful in detecting convective clouds that achieved intense radar signatures. The UW-CTC rate lead time ahead of the various radar fields is also shown, along with an illustration of the benefit of UW-CTC rates in operational forecasting. The results of this study suggest that convective clouds with the strongest UW-CTC rates are more likely to achieve significant near-term (0–60 min) radar signatures in such fields as composite reflectivity, vertically integrated liquid (VIL), and maximum estimated size of hail (MESH) compared to clouds that exhibit only weak UW-CTC rates.

Onset of south west monsoon (SWM) over Kerala is associated with intense convection followed by heavy rainfall over the west-coast of India. The intense rainfall events are usually associated with meso-scale convective systems embedded in large scale synoptic system over the Arabian Sea. Such deep and intense cumulus convection can have an important effect on the dynamics and energetics of large-scale atmospheric systems, because of the large magnitudes of the energy transformations associated with changes of phase of water in precipitating cumulus clouds as well as the strong updrafts and downdrafts in the troposphere. The prime objective of this study is to understand the convective structure (active/suppressed) of the atmosphere over the west-coast of India during ARMEX-I (Arabian Sea Monsoon Experiment). This study uses an approach to obtain the average structure of a cloud cluster and its interaction with the environment that enables in distinguishing the variation of kinematic and convective parameters from suppressed to convectively active process. Upper air observations obtained from four coastal land stations viz., Bombay, Goa, Mangalore and Trivandrum, alongwith that obtained over ORV Sagar Kanya are used to calculate both the convective and the kinematic parameters at the centre of the polygon formed by these observation locations. Time averaged circulation kinematic parameters and vertical velocity during active and suppressed convective phases off the west coast of India were discussed. The apparent heating and the apparent moisture sink are also estimated through residuals of the thermodynamic equations during intense and weak phases of SWM.

AbstractThis study is based on the analysis of 10 years of data for radar reflectivity factor Ze as derived from the TRMM Precipitation Radar (PR) measurements. The vertical structure of active convective clouds at the PR pixel scale has been extracted by defining two types of convective cells. The first one is cumulonimbus tower (CbT), which contains Ze ≥ 20 dBZ at 12-km altitude and is at least 9 km deep. The other is intense convective cloud (ICC), which belongs to the top 5% of the population of the Ze distribution at a prescribed reference height. Here two reference heights (3 and 8 km) have been chosen. Regional differences in the vertical structure of convective cells have been explored by considering 16 locations distributed across the tropics and two locations in the subtropics. The choice of oceanic locations is based on the sea surface temperature; that of the land locations is based on propensity for intense convection. One of the main findings of the study is the close similarity in the average vertical profiles of CbTs and ICCs in the mid- and lower troposphere across the ocean basins whereas differences over land areas are larger and depend on the selected reference height. The foothills of the western Himalaya, southeastern South America, and the Indo-Gangetic Plain contain the most intense CbTs; equatorial Africa, the foothills of the western Himalaya, and equatorial South America contain the most intense ICCs. Close similarity among the oceanic profiles suggests that the development of vigorous convective cells over warm oceans is similar and that understanding gained in one region is extendable to other areas.

Abstract Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

AbstractThe use of geostationary satellites for monitoring the development of deep convective clouds has been recently well documented. One such approach, the University of Wisconsin Cloud-Top Cooling Rate (CTC) algorithm, utilizes frequent Geostationary Operational Environmental Satellite (GOES) observations to diagnose the vigor of developing convective clouds through monitoring cooling rates of infrared window brightness temperature imagery. The CTC algorithm was modified to include GOES visible optical depth retrievals for the purpose of identifying growing convective clouds in regions of thin cirrus clouds. An automated objective skill analysis of the two CTC versions (with and without the GOES visible optical depth) versus a variety of Next Generation Weather Radar (NEXRAD) fields was performed using a cloud-object tracking system developed at the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies. The skill analysis was performed in a manner consistent with a recent study employing the same cloud-object tracking system. The analysis indicates that the inclusion of GOES visible optical depth retrievals in the CTC algorithm increases probability of detection and critical success index scores for all NEXRAD fields studied and slightly decreases false alarm ratios for most NEXRAD thresholds. In addition to better identifying vertically growing storms in regions of thin cirrus clouds, the analysis further demonstrates that the strongest cooling rates associated with developing convection are more reliably detected with the inclusion of visible optical depth and that storms that achieve intense reflectivity and large radar-estimated hail exhibit strong cloud-top cooling rates in much higher proportions than they do without the inclusion of visible optical depth.

Abstract In this study, we employ a cloud-resolving model to investigate how gravity influences convection and clouds in a small-domain (96 × 96 km) radiative–convective equilibrium. Our experiments are performed with a horizontal grid spacing of 1 km, which can resolve large (>1 km2) convective cells. We find that under a given stellar flux, sea surface temperature increases with decreasing gravity. This is because a lower-gravity planet has larger water vapor content and more clouds, resulting in a larger clear-sky greenhouse effect and a stronger cloud warming effect in the small domain. By increasing stellar flux under different gravity values, we find that the convection shifts from a quasi-steady state to an oscillatory state. In the oscillatory state, there are convection cycles with a period of several days, comprised of a short wet phase with intense surface precipitation and a dry phase with no surface precipitation. When convection shifts to the oscillatory state, the water vapor content and high-level cloud fraction increase substantially, resulting in rapid warming. After the transition to the oscillatory state, the cloud net positive radiative effect decreases with increasing stellar flux, which indicates a stabilizing climate effect. In the quasi-steady state, the atmospheric absorption features of CO2 are more detectable on lower-gravity planets because of their larger atmospheric heights. While in the oscillatory state, the high-level clouds mute almost all of the absorption features, making the atmospheric components hard to characterize.

One of the important goals of NASA’s Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) was to further the understanding of the evolution of tropical anvil clouds generated by deep convective systems. An important step toward understanding the radiative properties of convectively generated anvil clouds is to study their life cycle. Observations from ground-based radar, geostationary satellite radiometers, aircraft, and radiosondes during CRYSTAL-FACE provided a comprehensive look at the generation of anvil clouds by convective systems over South Florida during July 2002. This study focused on the relationship between convective rainfall and the evolution of the anvil cloud shield associated with convective systems over South Florida on 23 July 2002, during the CRYSTAL-FACE experiment. Anvil clouds emanating from convective cells grew downwind (to the southwest), reaching their maximum area at all temperature thresholds 1–2 h after the active convective cells collapsed. Radar reflectivity data revealed that precipitation-sized anvil particles extended downwind with the cloud tops. The time lag between maximum rainfall and maximum anvil cloud area increased with system size and rainfall. Observations from airborne radar and analysis of in situ cloud particle size distribution measurements in the anvil region suggested that gravitational size sorting of cloud particles dispersed downshear was a likely mechanism in the evolution of the anvil region. Linear regression analysis suggested a positive trend between this time lag and maximum convective rainfall for this case, as well as between the time lag and maximum system cloud cover. The injection of condensate into the anvil region by large areas of intense cells and dispersal in the upper-level winds was a likely explanation to cause the anvil cloud-top area to grow for 1–2 h after the surface convective rainfall began to weaken. In future work these relationships should be evaluated in differing regimes of shear, stability, or precipitation efficiency, such as over the tropical oceans, in order to generalize the results. The results of this study implied that for these cloud systems, the maximum in latent heating (proportional to rainfall) may precede the peak radiative forcing (related to anvil cloud height and area) by a lead time that was proportional to system size and strength. Mesoscale modeling simulations of convective systems on this day are under way to examine anvil evolution and growth mechanisms.

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Capes
O objetivo deste estudo é analisar a variabilidade da precipitação em quatro municípios do planalto costeiro do Estado da Bahia: Salvador, Ilhéus, Eunapólis e Teixeira de Freitas. As séries horárias de precipitação obtidas do processamento dos dados brutos de estações automáticas de coleta de dados operadas pelo Instituto do Meio Ambiente e Recursos Hídricos (INEMA) formam a principal base de dados desta pesquisa. O período de estudo é de abril de 2000 a fevereiro de 2009. O ciclo diário da precipitação foi analisado através do total precipitado e da frequência de chuva (em dias) considerando os 24 intervalos de uma hora que constituem um dia. A técnica do percentil aplicada às séries temporais dos totais diários de precipitação possibilitou identificar eventos intensos. Chove durante todo o ano nos quatro municípios. De maneira geral, o número de eventos intensos e de dias com chuva é maior em anos com totais pluviométricos mais elevados. O ciclo diário da frequência de chuva tem aspectos semelhantes para Salvador e Ilhéus, como o mínimo vespertino e o máximo na madrugada (Ilhéus) e manhã (Ilhéus e Salvador). O ciclo diário de Eunápolis e Teixeira de Freitas apresenta dois períodos de máximo (início da manhã e início da tarde), e dois de mínimo, um breve, no meio da manhã, e outro prolongado, no período da noite. Num estudo de caso selecionado foi analisado um aglomerado convectivo que se desenvolveu na área de um cavado no escoamento de leste sobre o Estado da Bahia, na noite do dia 14 e madrugada do dia 15 de outubro de 2006.
The objective in this study is to analyze the precipitation variability in four municipalities of the coastal high plains of Bahia State: Salvador, Ilhéus, Eunapólis e Teixeira de Freitas. Hourly precipitation time series obtained by processing the raw data of automatic weather stations operated by the Institute for Environment and Water Resources (INEMA) is the main dataset used in the research. The period of study is from April 2000 to February 2009. The diurnal precipitation cycle was analyzed by means of the rainfall totals and frequencies (in days) considering the 24 one-hour intervals that constitute one day. Intense rainfall events were identified by applying the percentile technique to the daily 24-hour precipitation totals time series. It rains all year long in the four municipalities. In general, the number of intense events and raindays is higher in years with larger precipitation totals. The diurnal cycle of rainfall frequency for Salvador and Ilhéus has similar characteristics: an afternoon minimum and a maximum at dawn (Ilhéus) and morning (Ilhéus and Salvador). The daily cycle for Eunapólis and Teixeira de Freitas show two periods of maximum (early morning and early afternoon), and two minima, one short, in mid-morning, and a long-lasting one, in nighttime. A selected case study was performed focusing on a cloud cluster that developed in a trough area in the easterlies over Bahia State in October 2006, in the night of the 14th and dawn of the 15th.

Very small fractional area (0.1%) occupied by the cumulonimbus (Cb) clouds belies their importance in Earths hydrological cycle and climate. For example, Riehl and Malkus (1958) estimated that the vertical transport of energy needed for the global energy balance can be accomplished by 1500 to 5000 active, undiluted Cb clouds (i.e., hot towers). Cb clouds feed hydrometeors to the anvil cloud region in mesoscale convective system (MCS). Applications such as the estimation of the vertical profile of latent heating, cumulus parameterizations, satellite rainfall retrievals, inferring the probability of lightening, etc., require information on the vertical distribution of hydrometeors in convective clouds (e.g., Xu and Zipser, 2012). Knowledge of the vertical structure of Cb clouds near individual cloud scale becomes necessary for validating cloud resolving model results. However, information on the vertical structure of convective clouds at horizontal scales comparable to that of a deep convective cloud is not available over most regions in the tropics. The PR provides an unprecedented long time series of data on the 3D structure of precipitating clouds in the tropics. The TRMM PR equivalent radar reflectivity factor (Ze) data product 2A25 version 6 is the main data used in the study. The present thesis work primarily focuses on the properties of convective clouds at the PR pixel scale. TRMM, operational since December 1997, is a non-sunsynchronous satellite with 350 inclination and samples the tropics several times a day (e.g., Kummerow et al., 1998, 2000). The PR works in Ku band (13.8 GHz or 2.2 cm wavelength), and its scan, consisting of 49 beams, had a width of 215 km when launched and _250 km after August 2001. The beam width is 0.710; nearby beams are separated by 0.710, giving a maximum scan angle of 170 (Kummerow et al., 1998). There are 80 levels in the vertical, each having 250 m resolution with the lowest level being the Earths ellipsoid. The height corresponding to different vertical levels in the 2A25 data set is the distance measured along the PR beam. Hence, corrections to pixel heights along different beams have been applied. Present thesis presents the vertical structure of radar reflectivity factor in tall cumulonimbus towers (CbTs) and intense convective clouds (ICCs) embedded in the South Asian monsoon systems and other tropical deep cloud systems. CbT is defined referring to a reflectivity threshold of 20 dBZ at 12 km altitude and is at least 9 km thick. ICCs are constructed referring to reflectivity thresholds at 8 km and 3 km altitudes. Cloud properties reported here are based on 10 year climatology. It is observed that the frequency of occurrence of CbTs is highest over the foothills of Himalayas, plains of northern India and Bangladesh, and minimum over the Arabian Sea and equatorial Indian Ocean west of 900E. The regional differences depend on the reference height selected, namely, small in the case of CbTs and prominent in 6􀀀13 km height range for ICCs. Land cells are more intense than the oceanic ones for convective cells defined using the reflectivity threshold at 3 km, whereas land versus ocean contrasts are not observed in the case of CbTs. Compared to cumulonimbus clouds elsewhere in the tropics, the South Asian counterparts have higher reflectivity values above 11 km altitude. One of the main findings of the present thesis is the close similarity in the average vertical profiles of CbTs and ICCs in the mid and lower troposphere across the ocean basins, while differences over land areas are larger and depend on the reference height selected. Foothills of the Western Himalayas, southeast South America and Indo-Gangetic Plain contain the most intense CbTs, while equatorial Africa, foothills of the Western Himalayas and equatorial South America contain the most intense ICCs. Close similarity among the oceanic cells suggests that the development of vigorous convective cells over warm oceans is similar and understanding gained in one region is extendable to other areas. South Asia contains several areas where the seasonal summer monsoon rainfall is influenced by the orography. One of the fundamental questions concerning the orographic rainfall is the nature of the associated precipitating clouds in the absence of synoptic forcing. It is believed that these are shallow and mid-level clouds, however, there is not much information in the literature on their vertical structure. Chapter 4 explores the vertical structure of active shallow (SC) and mid-level clouds (MLC) in Southeast Asia which are associated with the orographic features. Shallow and mid-level clouds have been defined such that their tops lie below 4.5 km and between 4.5 and 8 km, respectively. Only those TRMM PR passes are considered for active shallow and mid level cloud, which consists less than 5% deep cloud (_ 8 Km), compared to shallow cloud (_ 4.5 km) and mid level cloud (4.5 and _ 8 km). The reflectivity and height thresholds with constraint on percentage of deep clouds, ensure that we only captures the intense and isolated shallow and mid level clouds, away from deep cloud. The Western Ghats contains the highest fraction of the shallow clouds followed by the adjacent eastern Arabian Sea, while the Khasi hills in Meghalaya and Cardamom Mountains in Cambodia contain the least fraction of them. Average vertical profiles of shallow clouds are similar in different mountainous areas while that of mid-level clouds show some differences. Below 3 km, cloud liquid water content of the mid-level clouds is the highest over the Western Ghats and the eastern Arabian Sea. The average cloud liquid water content increases by 0.19 gm m􀀀3 for SCs between 3 km and 1.5 km, while the corresponding increase for MLCs is around 0.08 gm m􀀀3. MCS has a life cycle consisting of formative, intensifying, mature and dissipating stages. From the maximum projection of reflectivity on longitude and latitude plane from the 3D reflectivity fields, CS is defined as the common area of connected pixels with Ze _ 17 dBZ and polarized corrected temperature (PCT) _ 250 K, with atleast 500 km2. A CS is considered in subsequent analysis if its area detected in the Ze projection is at least 50% of its area seen in the PCT imagery. An algorithm is applied to obtain the phase of evolution. The algorithm is based on the average vertical profile of CSs and the reflectivity peak altitude (Hmax). An index namely reflectivity difference (RD) and Hmax is used to identify the phases of evolution. A close similarity has been observed during different phases in average vertical profiles as well as in CFAD. Growing or intensifying stage consists the highest reflectivity below the 2 km altitude. Mature phase does not show the much variation in Ze below the freezing level, whereas in the decaying stage, shows the largest regional differences in this layer of the atmosphere. Melting band signature is most pronounced in the decaying stage. Fraction of convective area decreases as CSs go through its life cycle, except over Atlantic Ocean during winter.

This chapter examines rainfall and associated phenomena and their possible relationship to solar activity. Rainfall can be measured directly using rain gauges or estimated by monitoring lake levels and river flows. Satellite and radar rainfall measurements have become increasingly important. Historical documentation on drought, or the absence of rain, also reveals empirical relationships. Both rainfall and evaporation show marked variations with latitude and geography. First, we examine these rainfall-associated variations and estimate how they might change with solar activity. Second, we cover empirical studies of rainfall, lake levels, river flows, and droughts. The sun bathes the Earth’s equator with enormous amounts of surface energy. Much of this absorbed radiant energy evaporates water, causes atmospheric convection, and is later released to space as thermal radiation. Steady-state energy escapes, so tropical temperatures do not rise without limit. Some absorbed energy is transported poleward by winds from the point of absorption. Intense convection near the equator leads to a large updraft known as the intratropical convergence zone (ITCZ), a band of lofty, high-precipitation clouds producing the largest rainfall of any region on Earth. Solar energy in the ITCZ is carried to high elevations where it diverges and moves poleward. It is unable to travel all the way to the poles, so instead creates a large atmospheric circulation cell known as the Hadley cell. The Hadley cell has an upward motion near the equator and downward motions at about 30° north and south latitude. These downflow regions produce clear air with few clouds and create areas of minimum rainfall called deserts. These regions of upflow and downflow are connected by poleward flows in the upper atmosphere and equatorward flows in the lower atmosphere, forming a complete circulation pattern. Outside the Hadley cell are temperate and polar regions. The temperate regions have more rainfall than the deserts, while the cold polar regions have even less precipitation. Figure 6.1 shows the three regions with relative maximum rainfall. The mean evaporation has a much simpler latitudinal variation that tends to follow the surface temperature. Figure 6.1 shows this variation as a parabolicshaped dotted line.

In this work a physical model of the natural convection heat transfer mechanism, molecular-quantum in nature, is proposed. On the surface of the solid there are a lot of chemical defects (atoms of different chemical elements) and geometrical ones (steps, kinks, terraces, dislocations) at microscopic and nanoscopic scale. All these defects make the surface of the wall to be not an equipotential surface. On the other hand, the existence of a gradient of temperature in a metal wall, which is involved in a heat transfer process, generates a gradient of conduction electrons. On the cool face of the wall there are more electrons as a result of Pe´ltier-Thomson effect. Because of surface’s defects the electrons are not uniformly distributed, on a high defect there are more electrons than on a depth defect and the electrical field is more intense on the high defect. The molecules of the fluid are adsorbed on the surface, and become polar molecules, as a result of the polarization by influence. The absorbed molecules form a multilayer in which take place more elementary processes, molecular-quantum in nature. These elementary processes are: the overlap between the electronic orbital of the solid and fluid, electron clouds perturbation, solid-fluid electron exchange by quantum tunneling effect, the motion under action of the Helmann-Feynman force between adsorbed molecules and a high defect of the wall, the absorption of the phonons from the surface’s atoms and rejection of the molecules from the surface. In this way natural convection is generated. The proposed model needs directly experimental confirmation.