Thin Ice Clouds Research Articles

Retrievals of cloud ice water path by combining ground cloud radar and satellite high‐frequency microwave measurements near the ARM SGP site

A framework to retrieve ice water paths over a broad area by combining observations of surface cloud radar and satellite high‐frequency microwave measurements in a physically consistent way is presented. The surface cloud radar provides the statistics of vertical distribution of ice water content, while the satellite retrievals use those statistics to broaden a point measurement to an areal measurement. This study seeks to improve retrievals by adopting newly available ice microphysical properties from recent in situ observations, and by treating single scattering properties based on discrete dipole approximation simulations of realistic nonspherical ice particles. First, a new radar reflectivity‐ice water content relation is derived using backscattering cross sections calculated from six types of nonspherical ice particles. Ice water content profiles derived from radar reflectivity profiles are then used to calculate brightness temperatures at satellite observing frequencies by a radiative transfer model, which forms an a priori database for a Bayesian ice water path retrieval algorithm. Comparison between the satellite and surface radar retrievals shows that the two agree better for cases with ice water paths greater than 100 g m−2. For lower ice water paths, the difference between satellite and surface radar retrievals becomes large, presumably because of the insensitivity of the microwave scattering to optically thin ice clouds.

An Arctic Springtime Mixed-Phase Cloudy Boundary Layer Observed during SHEBA

Abstract The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m−2 with a diurnal amplitude of ∼20 W m−2. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.

Ice Cloud Optical Thickness and Extinction Estimates from Radar Measurements

A remote sensing method is proposed to derive vertical profiles of the visible extinction coefficients in ice clouds from measurements of the radar reflectivity and Doppler velocity taken by a vertically pointing 35-GHz cloud radar. The extinction coefficient and its vertical integral, optical thickness t, are among the fundamental cloud optical parameters that, to a large extent, determine the radiative impact of clouds. The results obtained with this method could be used as input for different climate and radiation models and for comparisons with parameterizations that relate cloud microphysical parameters and optical properties. An important advantage of the proposed method is its potential applicability to multicloud situations and mixed-phase conditions. In the latter case, it might be able to provide the information on the ice component of mixed-phase clouds if the radar moments are dominated by this component. The uncertainties of radar-based retrievals of cloud visible optical thickness are estimated by comparing retrieval results with optical thicknesses obtained independently from radiometric measurements during the yearlong Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. The radiometric measurements provide a robust way to estimate t but are applicable only to optically thin ice clouds without intervening liquid layers. The comparisons of cloud optical thicknesses retrieved from radar and from radiometer measurements indicate an uncertainty of about 77% and a bias of about 214% in the radar estimates of t relative to radiometric retrievals. One possible explanation of the negative bias is an inherently low sensitivity of radar measurements to smaller cloud particles that still contribute noticeably to the cloud extinction. This estimate of the uncertainty is in line with simple theoretical considerations, and the associated retrieval accuracy should be considered good for a nonoptical instrument, such as radar. This paper also presents relations between radar-derived characteristic cloud particle sizes and effective sizes used in models. An average relation among t, cloud ice water path, and the layer mean value of cloud particle characteristic size is also given. This relation is found to be in good agreement with in situ measurements. Despite a high uncertainty of radar estimates of extinction, this method is useful for many clouds where optical measurements are not available because of cloud multilayering or opaqueness.

Titan's atmospheric engine: an overview

Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH 4 and N 2 into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport.

Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds

In situ measurements of NOy, NOx, and temperature confirm that nitric acid trihydrate (NAT) particles could form at the tropical tropopause. The HNO3 mixing ratio near the tropical tropopause is typically no larger than about 0.2–0.3 ppbv, and the corresponding equilibrium mass of nitric acid trihydrate (NAT) is no larger than 0.3 μg m−3. Considerably larger NAT condensed masses are required to explain the HALOE extinctions; however, localized regions of enhanced HNO3 produced by oxidation of lightning‐generated NO might exist. NAT layers would only be identified as clouds by SAGE II if the particle diameters are in the optimum range of about 0.6 to 2 μm and the condensed NAT mass is larger than about 0.2 μg m−3. The SAGE II extinction ratio measurements (0.5 μm/1.0 μm) cannot distinguish NAT clouds from mixtures of optically thin ice clouds and background aerosols.

Prevalence of ice‐supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5×10−3 to 2×10−2 cm−3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m−2. If 0.02–0.1 cm−3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.

Observations of Martian ice clouds by the Mars Global Surveyor Thermal Emission Spectrometer: The first Martian year

Successful operation of the Mars Global Surveyor spacecraft, beginning in September 1997 (Ls = 184°), has permitted extensive observations over more than a Martian year. Initially, thin (normal optical depth <0.06 at 825 cm−1) ice clouds and hazes were widespread, showing a distinct latitudinal gradient. With the onset of a regional dust storm at Ls = 224°, ice clouds vanished in the southern hemisphere, to reappear gradually after the decay of the storm. The zonally averaged cloud opacities show little difference between the beginning and end of the first Martian year. A broad low‐latitude cloud belt with considerable longitudinal structure was present in early northern summer. Apparently characteristic of the northern summer season, it vanished between Ls = 140° and 150°. The latitudinal extent of this feature is apparently controlled by the ascending branch of the Hadley circulation. The most opaque clouds (optical depth ∼0.6) were found above the summits of major volcanic features; these showed spatial structure possibly associated with wave activity. Variety among low‐lying late morning clouds suggests localized differences in circulation and microclimates. Limb observations showed extensive optically thin (optical depth <0.04) stratiform clouds at altitudes up to 55 km. Considerable latitude and altitude variations were evident in ice clouds in early northern spring (Ls = 25°); near 30 km, thin clouds extended from just north of the equator to ∼45°N, nearly to the north polar vortex. A water ice haze was present in the north polar night (Ls = 30°) at altitudes up to 40 km. Because little dust was present this probably provided heterogeneous nucleation sites for the formation of CO2 clouds and snowfall at altitudes below ∼20 km, where atmospheric temperatures dropped to the CO2 condensation point. The relatively invariant spectral shape of the water ice cloud feature over space and time indicates that ice particle radii are generally between 1 and 4 μm.

Boundaries and internal structure of mixed phase clouds as deduced from ground-based 95-GHz radar and airborne lidar measurements

Knowledge on the occurrence of multiple cloud layers, the heights of their boundaries, their internal structure and phase is important for an accurate determination of radiative fluxes through the atmosphere. Multi-layer mixed phase clouds have simultaneously been profiled with a ground-based 95-GHz radar and Ne:YAG based airborne lidars. Due to their different attenuation and sensitivity to particle size and phase these instruments provide complementary information of cloud boundaries and cloud structure. Case studies from a field campaign over Southern UK are presented. In one case a narrow altostratus layer of mainly spherical, liquid particles was embedded in an ice cloud of larger vertical extent. The liquid layer was resolved in detail by the lidar but did not appear distinguishably in the radar profile, which was much more sensitive to the ice particles. Fallstreaks below the cloud base show a signature in the radar signals only. In another case the lidar beam was blocked in the upper part of a mid-level cloud layer with band like structure, while this cloud hardly appeared in the radar reflectivity which instead was dominated by another band 1 km below. Both instruments detected nearly the same cloud boundaries and structures when the liquid layer disappeared and an optically thin ice cloud remained. This study clearly reveals the tremendous information gain by a synergetic use of radar and lidar for cloud profiling.

The effects of aspect ratio and surface roughness on satellite retrievals of ice-cloud properties

This study investigates the effects of non-sphericity on satellite retrievals of ice-cloud properties including optical thickness and particle sizes. Ray-tracing technique is used to calculate single scattering phase function and single scattering albedo for both smooth and rough surfaces of hexagonal columns and plates at visible and near-infrared wavelengths. Two parameters, aspect ratio and distortion parameter, are used to simulate different, randomly oriented, ice crystal shapes and surface roughnesses in the ray-tracing process. A wide range of aspect ratio and distortion parameter is explored in the calculations. The resultant phase functions and single scattering albedos are used to compute bidirectional reflection functions by a radiative transfer model with adding–doubling technique. The results show that in a direct backscattering regima (Θ>150°), if no information of particle shape is available, the uncertainties in the retrieved optical thickness (a factor of more than ten) would make the retrieval meaningless. For images with viewing geometry outside of this region, the typical range of uncertainty of retrieved optical thickness is less than a factor of about two. Using averaged phase function will cut this uncertainty in half. That is, the uncertainty in the retrieved optical thickness is about 40%. Sensitivity tests show that aspect ratios are critical in reducing the uncertainties of the retrieved optical thickness using satellite data. The uncertainties in the retrieved ice particle sizes are also estimated in a similar way. It is found that using an averaged aspect ratio and roughness, the uncertainty of retrieved particle size is about 30% for small particles and 10% for large particles at λ=3.7 μ m. The corresponding uncertainties are about 70% for small particles and 35% for large particles if data at λ=1.6 or 2.2 μ m are used in the retrieval. However, at λ=1.6 and 2.2 μ m, retrieval of particle size may encounter even larger uncertainties for thin ice-clouds ( τ<3).

Ground based passive remote sensing of ice clouds with scattered solar radiation in the near infrared

Abstract The feasibility of ground based passive remote sensing in the near infrared for the estimation of optical properties of thin ice clouds is examined and confirmed. The radiative transfer through ice clouds is simulated as a function of crystal size, crystal shape and optical depth under the assumption of single scattering. A method is derived to estimate the effective radius r eff for optically thin ice clouds from radiances at wavelengths of 1046 and 1550 nm . The results are not dramatically different if one assumes hexagonal crystals or surface equivalent spheres. The estimation of the ice cloud optical depth depends strongly on particle shape, leading to a less reliable estimate. Ground based measurements of scattered near infrared radiances with the array spectrometer OVID have been used to verify and to apply the derivation technique. The evaluation of measurements for different ice cloud types and contrails mostly yields particle sizes between 30 and 200 μ m . Occasionally they are as small as 1.6 μ m or as large as 600 μ m . The known increase of particle size with cloud base temperature was confirmed. Contrails prove to contain small crystal sizes that increase with age to the values found in the surrounding cirrus clouds. A systematic size reduction in contrail clouds older than 30 minutes is indicated but cannot b e conclusively proven by the present data.

High-Resolution Satellite Analysis and Model Evaluation of Clouds and Radiation over the Mackenzie Basin Using AVHRR Data

Both the issues of high-resolution satellite analysis and model evaluation for a region centered on the Arctic Circle (60°–75°N) are addressed. Model cloud fraction, cloud height, and outgoing radiation are compared to corresponding satellite observations using a model-to-satellite approach (calculated radiances from model state). The dataset consists of forecasts run at 15-km resolution up to 30 h and nearly coincident Advanced Very High Resolution Radiometer (AVHRR) imagery during the Beaufort and Arctic Storm Experiment over the Mackenzie Basin for a monthly period in the fall of 1994. A cloud detection algorithm is designed for day and night application using the 11-μ channel of AVHRR along with available information on atmospheric and ground temperatures. The satellite and model estimates of cloud fraction are also compared to observations at 20 ground stations. A significant result of the validation is that the model has a higher frequency of low cloud tops and a lower frequency of midlevel cloud tops than the observations. On a monthly basis, the model 11-μ outgoing brightness temperature (TB) is consequently higher than observed by about 4.4 K at all forecast times, which corresponds to a deficit of 760 m in mean cloud-top height and about 10 W m−2 in outgoing flux at the top of the atmosphere. Possible errors in the parameterization of ice or water cloud emissivity are evaluated but ruled out as the dominant cause for the warm TB bias in the model. Rather, the problem is attributed to low clouds being trapped in the boundary layer, whereas high clouds appear to be reasonably well modeled. The role of thin ice clouds is further evaluated by comparing distributions of observed and modeled 11-μ minus 12-μ TB differences, DIF45 (channel 4 minus channel 5). The relationship between the true height of the clouds and the effective height observed by satellite is modeled from forecast outputs as a function of DIF45. The quality of daily estimates is evaluated from time series at various locations. The time series shows that there was a marked drop in DIF45 during the month, which is attributed to a decrease in the occurrence of cirrus clouds. Finally, the diurnal cycle of TB and cloud fraction is found to be relatively large with average monthly 0600–1800 UTC TB differences of both signs of the order of 4–8 K in broad sectors and cloud fraction differences of 10%–30%. Where low clouds prevail, the cloud fraction tends to decrease at night and TB increases. Overall, model–observation differences are dominated by differences in the vertical distribution of clouds. A reduction of this effect implies a modification of the “preferred” model climatology in terms of its vertical distribution of humidity and cloud water.

High-Resolution Satellite Analysis and Model Evaluation of Clouds and Radiation over the Mackenzie Basin Using AVHRR Data

Abstract Both the issues of high-resolution satellite analysis and model evaluation for a region centered on the Arctic Circle (60°–75°N) are addressed. Model cloud fraction, cloud height, and outgoing radiation are compared to corresponding satellite observations using a model-to-satellite approach (calculated radiances from model state). The dataset consists of forecasts run at 15-km resolution up to 30 h and nearly coincident Advanced Very High Resolution Radiometer (AVHRR) imagery during the Beaufort and Arctic Storm Experiment over the Mackenzie Basin for a monthly period in the fall of 1994. A cloud detection algorithm is designed for day and night application using the 11-μ channel of AVHRR along with available information on atmospheric and ground temperatures. The satellite and model estimates of cloud fraction are also compared to observations at 20 ground stations. A significant result of the validation is that the model has a higher frequency of low cloud tops and a lower frequency of midlevel cloud tops than the observations. On a monthly basis, the model 11-μ outgoing brightness temperature (TB) is consequently higher than observed by about 4.4 K at all forecast times, which corresponds to a deficit of 760 m in mean cloud-top height and about 10 W m−2 in outgoing flux at the top of the atmosphere. Possible errors in the parameterization of ice or water cloud emissivity are evaluated but ruled out as the dominant cause for the warm TB bias in the model. Rather, the problem is attributed to low clouds being trapped in the boundary layer, whereas high clouds appear to be reasonably well modeled. The role of thin ice clouds is further evaluated by comparing distributions of observed and modeled 11-μ minus 12-μ TB differences, DIF45 (channel 4 minus channel 5). The relationship between the true height of the clouds and the effective height observed by satellite is modeled from forecast outputs as a function of DIF45. The quality of daily estimates is evaluated from time series at various locations. The time series shows that there was a marked drop in DIF45 during the month, which is attributed to a decrease in the occurrence of cirrus clouds. Finally, the diurnal cycle of TB and cloud fraction is found to be relatively large with average monthly 0600–1800 UTC TB differences of both signs of the order of 4–8 K in broad sectors and cloud fraction differences of 10%–30%. Where low clouds prevail, the cloud fraction tends to decrease at night and TB increases. Overall, model–observation differences are dominated by differences in the vertical distribution of clouds. A reduction of this effect implies a modification of the “preferred” model climatology in terms of its vertical distribution of humidity and cloud water.

The impact of subvisible cirrus clouds near the tropical tropopause on stratospheric water vapor

The radiative impact of subvisible cirrus ice clouds at and just below the tropical tropopause has been studied using a zonally averaged interactive chemistry‐radiation‐dynamics model. Model runs have been performed with and without the inclusion of the radiative heating of these thin ice clouds, and with and without sedimentation. Near‐infrared optical depths of 0.005–0.08 were computed for assumed log‐normal size distributions of spherical particles having mode radii of 2–10 µm. Particles with 6 µm mode radii have computed scattering ratios of 3–15 at 603 nm, in good agreement with lidar observations. The increased radiative heating of these clouds, 0.1–0.2 K/day, results in temperature increases of 1–2 K and vertical velocity increases of 0.02–0.04 mm/s. As a consequence of the warmer tropopause, lower stratosphere water vapor increases by as much as 1 ppmv. The dehydration resulting from sedimentation was found to be a much smaller effect.

Validation of automated cloud top phase algorithms: distinguishing between cirrus clouds and snow in &lt;italic&gt;a priori&lt;/italic&gt; analyses of AVHRR imagery

Quantitative assessments on the performance of automated cloud analysis algorithms require the creation of highly accurate, manual cloud, no cloud (CNC) images from multispectral meteorological satellite data. In general, the methodology to create these a priori analyses for the evaluation of cloud detection algorithms is relatively straightforward, although the task becomes more complicated when little spectral signature is evident between a cloud and its background, as appears to be the case in advanced very high resolution radiometer (AVHRR) imagery when thin cirrus is present over snow-covered surfaces. In addition, complex procedures are needed to help the analyst distinguish between water and ice cloud tops to construct the manual cloud top phase analyses and to ensure that inaccuracies in automated cloud detection are not propagated into the results of the cloud classification algorithm. Procedures are described that enhance the researcher’s ability to (1) distinguish between thin cirrus clouds and snow-covered surfaces in daytime AVHRR imagery, (2) construct accurate a priori cloud top phase manual analyses, and (3) quantitatively validate the performance of both automated cloud detection and cloud top phase classification algorithms. The methodology uses all AVHRR spectral bands, including a band derived from the daytime 3.7-?m channel, which has proven most valuable for discriminating between thin cirrus clouds and snow. It is concluded that while the 1.6-?m band is needed to distinguish between snow and water clouds in daytime data, the 3.7-?m channel remains essential during the daytime to differentiate between thin ice clouds and snow. Unfortunately this capability that may be lost if the 3.7-?m data switches to a nighttimeonly transmission with the launch of future National Oceanographic and Atmospheric Administration (NOAA) satellites.

Nonsphericity of aerosol particles and their contribution to radiative forcing

Observations of combined spectral aerosol optical thickness and sky brightness in the almucantar for clear sky conditions indicate a discrepancy between the measured and modeled radiation field in certain air mass situations [maritime polar air (mP) and particular continental subtropic air (cS)], which cannot be closed using the commonly made assumption of sphericity of the aerosol particles or explained by measurement uncertainties. In these cases, mostly associated with flat spectral slopes of the aerosol optical thickness (caused by large particles, e.g. at marine and desert aerosols as well as thin ice clouds) increased side scattering is observed in the sky brightness. A Coupled Inversion Radiation Transfer program (CIRATRA) is used for the determination of climate-relevant aerosol parameters assuming spherical as well as nonspherical particle shapes. The observed increased side scattering for the marine and desert aerosols only can be explained by the use of scattering phase functions produced by nonspherical particles and consequently smaller asymmetry parameters. In these cases the best fit to the measured sky brightnesses (within the observable range of scattering angles (3–150 deg)) could be obtained by the use of the semi-empirical scattering theory of Pollack and Cuzzi 1 with less asymmetric phase functions and smaller asymmetry parameters than Mie theory. Conclusions regarding radiative forcing by aerosols are made for different values of aerosol optical thickness. The cooling effect by aerosols in cases of nonsphericity reaches up to a factor of 2 compared with the cooling of spherical particles.

In situ water vapour measurement, and a case study of the drying mechanism of the tropical lower stratosphere

During an experiment performed in tropical regions, water vapour content was measured in the lower stratosphere, using long duration balloons. A set of profiles show a vertical structure consistent with what has been named the tape-recorder effect. One of the results, when the balloon descends to 100 hPa, shows a value of the water-vapour mixing ratio as low as 2.8 parts per million by volume, in a layer approximately 1 km thick at about 80 hPa. This layer is delimited by two minima of air temperature, suggesting that the drying is a consequence of a vigorous convective process. This kind of process can be so strong that the convective cloud overshoots the tropopause and forms an anvil of ice crystals which grow and finally fall under gravity. Some of the characteristics of the METEOSAT cloud imagery and of the ECMWF analyses could be interpreted as the signature of this type of convective process. Nevertheless one cannot discard other mechanisms, such as the generation of stratospheric buoyancy waves by tropical convection. These waves can cross the tropopause and form thin ice clouds in the lower stratosphere. In this hypothesis, also, the ice falls and transports moisture downwards.

In situwater vapour measurement, and a case study of the drying mechanism of the tropical lower stratosphere

AbstractDuring an experiment performed in tropical regions, water vapour content was measured in the lower stratosphere, using long duration balloons. A set of profiles show a vertical structure consistent with what has been named the tape‐recorder effect. One of the results, when the balloon descends to 100 hPa, shows a value of the water‐vapour mixing ratio as low as 2.8 parts per million by volume, in a layer approximately 1 km thick at about 80 hPa. This layer is delimited by two minima of air temperature, suggesting that the drying is a consequence of a vigorous convective process. This kind of process can be so strong that the convective cloud overshoots the tropopause and forms an anvil of ice crystals which grow and finally fall under gravity. Some of the characteristics of the METEOSAT cloud imagery and of the ECMWF analyses could be interpreted as the signature of this type of convective process. Nevertheless one cannot discard other mechanisms, such as the generation of stratospheric buoyancy waves by tropical convection. These waves can cross the tropopause and form thin ice clouds in the lower stratosphere. In this hypothesis, also, the ice falls and transports moisture downwards.

The Role of Monsoon Convection in the Dehydration of the Lower Tropical Stratosphere

Abstract Observations by radiosondes, satellites, and aircraft have shown that a minimum in water vapor mixing ratio in the lower tropical stratosphere (typically around 19-km altitude) is a climatological feature of the global water vapor distribution. The processes responsible for the formation and maintenance of this minimum are examined with the aid of a mesoscale dynamical model of tropical convection that includes bulk ice microphysics, radiative transfer, and surface processes. Model results suggest that convectively generated buoyancy waves may play an important role in dehydrating the tropical lower stratosphere. Vertical parcel displacements produced by buoyancy waves promote the formation of thin ice clouds in the lower stratosphere. Such clouds formed upwind of the convective region in all simulations where ice was allowed to form. When ice was allowed to precipitate, there was a decrease of ≈0.3 ppmm in total water mixing ratio at 19 km after a 30-h simulation. It is concluded that thin cirru...

On the limitation of using asymmetry factor for radiative transfer involving cirrus clouds

We investigate the limitation of using the asymmetry factor through the Henyey-Greenstein function in representing the scattering phase function for radiative transfer in ice clouds containing hexagonal ice crystals. Relative accuracy checks for solar fluxes are carried out using three cirrus cloud size distributions. The flux results computed from the Henyey-Greenstein phase function, based on the asymmetry factor, differ significantly from those computed from the phase function for hexagonal ice crystals. For optically thin ice clouds, the relative error can be as large as 70%. The maximum absolute error in solar reflected fluxes is 27 Wm −2. The present flux calculations demonstrate that the phase functions for ice crystals which contain strong forward scattering peak and specific halos and back scattering features cannot be adequately represented by a single parameterization in terms of the asymmetry factor.

Deep Optically Thin Cirrus Clouds in the Polar Regions. Part I: Infrared Extinction Characteristics

The spectral data obtained by the infrared interferometer Spectrometer (IRIS) flown on Nimbus 4 satellite in 1970 indicated the existence of optically thin ice clouds in the upper troposphere that probably extended into lower stratosphere, in the polar regions, during winter and early spring. The spectral features of these clouds differ somewhat from that of the optically thin cirrus clouds in the tropics. From theoretical simulation of the infrared spectra in the 8-25 micron region, it is inferred that these polar clouds have a vertical stratification in particle size, with larger particles (about 12 microns) in the bottom of the cloud and smaller ones (less than 1 micron) aloft. Radiative transfer calculations also suggest that the equivalent ice-water content of these polar clouds is of the order of 1 mg/sq cm.