Cloud Particle Size Distribution Research Articles

The cloud imaging and particle size experiment on the Aeronomy of Ice in the mesosphere mission: Instrument concept, design, calibration, and on-orbit performance

The Cloud Imaging and Particle Size Experiment (CIPS) is one of three instruments aboard the Aeronomy of Ice in the Mesosphere spacecraft. CIPS provides panoramic ultraviolet images of the atmosphere over a wide range of scattering angles in order to determine the presence of polar mesospheric clouds, measure their spatial morphology, and constrain the parameters of cloud particle size distribution. The AIM science objectives motivate the CIPS measurement approach and drive the instrument requirements and design, leading to a configuration of four wide-angle cameras arrayed in a ‘+’ arrangement that covers a 120° (along orbit track)×80° (across orbit track) field of view. CIPS began routine operations on May 24, 4 weeks after AIM was launched. It measures scattered radiances from PMCs near 83 km altitude to derive cloud morphology and particle size information by recording multiple exposures of individual clouds to derive PMC scattering phase functions and detect nadir horizontal spatial scales to approximately 3 km. This paper describes the instrument design, its prelaunch characterization and calibration, and flight operations. Flight observations and calibration activities confirm performance inferred during ground test, verifying that CIPS exceeds its measurement requirements and goals. These results are illustrated with example flight images that demonstrate the instrument measurement performance.

Validation of the Aura MLS cloud ice water content measurements

Retrieval and validation of upper tropospheric ice water content (IWC) measurements with the Aura Microwave Limb Sounder (MLS) are described. The MLS version 2.2 (V2.2) IWC, derived from 240‐GHz cloud‐induced radiances (Tcir) at high tangent heights, is scientifically useful at 215–83 hPa. The V2.2 IWC represents a bulk cloud property averaged over a ∼300 × 7 × 4 km3 volume near the pointing tangent height. Precision, accuracy, and spatial resolution of the V2.2 IWC are determined through model simulations and comparisons with CloudSat observations. Comparisons of MLS V2.2 and CloudSat R03 IWCs are made for the months of January and July in terms of normalized probability density function (PDF). The differences between MLS and CloudSat IWC PDFs are generally less than 50% over the IWC range where the MLS technique is valid. At pressures <177 hPa and extratropical latitudes, the MLS V2.2 IWC exhibits a slightly low bias compared to CloudSat, part of which can be attributed to systematic errors in the MLS retrieval. Cloud inhomogeneity and particle size distribution are the leading sources of uncertainties in the V2.2 IWC.

The Relationship between Anvil Clouds and Convective Cells: A Case Study in South Florida during CRYSTAL-FACE

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.

Calibration of the Cloud Particle Imager Probes Using Calibration Beads and Ice Crystal Analogs: The Depth of Field

Abstract This paper explains and develops a correction algorithm for measurement of cloud particle size distributions with the Stratton Park Engineering Company, Inc., Cloud Particle Imager (CPI). Cloud particle sizes, when inferred from images taken with the CPI, will be oversized relative to their “true” size. Furthermore, particles will cease to be “accepted” in the image frame if they lie a distance greater than the depth of field from the object plane. By considering elements of the scalar theory for diffraction of light by an opaque circular disc, a calibration method is devised to overcome these two problems. The method reduces the error in inferring particle size from the CPI data and also enables the determination of the particles distance from the object plane and hence their depth of field. These two quantities are vital to enable quantitative measurements of cloud particle size distributions (histograms of particle size that are scaled to the total number concentration of particles) in the atmosphere with the CPI. By using both glass calibration beads and novel ice crystal analogs, these two problems for liquid drops and ice particles can be quantified. Analysis of the calibration method shows that 1) it reduces the oversizing of 15-μm beads (from 24.3 to 14.9 μm for the sample mean), 40-μm beads (from 50.0 to 41.4 μm for the sample mean), and 99.4-μm beads (from 103.7 to 99.8 μm for the sample mean); and 2) it accurately predicts the particles distance from the object plane (the relationship between measured and predicted distance shows strong positive correlation and gives an almost one-to-one relationship). Realistic ice crystal analogs were also used to assess the errors in sampling ice clouds and found that size and distance from the object plane could be accurately predicted for ice crystals by use of the particle roundness parameter (defined as the ratio of the projected area of the particle to the area of a circle with the same maximum length). While the results here are not directly applicable to every CPI, the methods are, as data taken from three separate CPIs fit the calibration model well (not shown).

Characteristics of aerosol and cloud particle size distributions in the tropical tropopause layer measured with optical particle counter and lidar

Abstract. An optical particle counter (OPC) is used in conjunction with lidar measurements to examine the characteristics of the particle size distribution in cirrus cloud in the tropical tropopause layer (TTL) over Thailand where the TTL is defined as the height at which temperature is lower than −75°C in this paper. Of 11 OPC launches, cirrus cloud was detected at 10–15 km high on 7 occasions, cirrus was detected in the TTL in 6 cases, and simultaneous OPC and lidar measurements were made on two occasions. Comparison of lidar and OPC measurements reveal that the cloud heights of cirrus in the TTL varies by several hundred meters over distances of tens kilometers; hence the height is not always horizontally uniform. The mode radii of particles constituting the clouds are estimated by lidar and OPC measurements to be less than approximately 10 μm. The regression lines of the particle size distribution with and without cirrus cloud exhibit similar features at equivalent radii of <0.8 μm. Enhancement in the integrated number concentration at radii greater than 0.8 μm indicates that liquid particles tend to be frozen at a radius of 0.8 μm, with cirrus clouds above 10 km exhibiting similar features. On the other hand, enhancement in the particle size distribution at radii greater than 0.9 μm and a peak at around 0.8 μm in the ratio of the standard deviation of count values to that of the Poisson distribution of the averaged count values are common features of cirrus clouds in the TTL, where the ratio shows the vertical homogeneity of the particle number. These typical features suggest that the transition from liquid, sulfuric acid aerosol, to ice is more observable in the TTL and the timing of freezing may vary with height in the TTL.

Estimating cirrus cloud properties from MIPAS data

High resolution mid‐infrared limb emission spectra observed by the spaceborne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) showing evidence of cloud interference are analyzed. Using the new line‐by‐line multiple scattering [Approximate] Spherical Atmospheric Radiative Transfer code (SARTre), a sensitivity study with respect to cirrus cloud parameters, e.g., optical thickness and particle size distribution, is performed. Cirrus properties are estimated by fitting spectra in three distinct microwindows between 8 and 12 μm. For a cirrus with extremely low ice water path (IWP = 0.1 g/m2) and small effective particle size (De = 10 μm) simulated spectra are in close agreement with observations in broadband signal and fine structures. We show that a multi‐microwindow technique enhances reliability of MIPAS cirrus retrievals compared to single microwindow methods.

Correlation Pattern between Effective Radius and Optical Thickness of Water Clouds Simulated by a Spectral Bin Microphysics Cloud Model

The cloud particle size distribution of water clouds was simulated by a non-hydrostatic spectral bin microphysics cloud model. The result shows two different correlation patterns in the simulated optical thickness and effective radius; positive and negative correlations for non-drizzling and drizzling clouds, respectively, similar to the observed features reported by past remote sensing works. The correlation pattern in a pristine condition is mainly composed by negative correlation with small fragment of positive correlation, similar to the satellite-observed characteristics over the FIRE region off California. In the case of polluted condition, on the contrary, only a positive correlation is simulated, similar to the satellite observation over the ASTEX region of North Atlantic Ocean. Satelliteobserved contrast of correlation patterns between FIRE and ASTEX regions can be explained by the difference in the aerosol burden of airmasses over the two regions.

Aircraft icing research flights in embedded convection

Results from in-cloud measurements with an instrumented aircraft from an icing research campaign in Southern Germany in March 1997 are presented. Measurements with conventional optical cloud probes and of the ice accretion on a cylinder exposed to the flow show the existence of supercooled large drops (SLD) in the size range up to 300 µm simultaneously with severe icing with ice-accretion rates of up to 3.5 mm min−1. Nearly all periods with icing, including the ones with severe icing, occurred in mixed-phase convective cells embedded in surrounding stratus clouds. The spatial scales of SLD occurrence, respectively severe icing, ranged between several hundred meters and some kilometers and correspond to the length of the transects through the embedded cells. SLD formed through the coalescence process and were found through the whole cloud depth pointing to a source region near cloud top, in line with the arguments of Rauber and Tokay (1991). No indication of ice-multiplication by the Hallet-Mossop process was found, despite of the favorable temperatures for that process. Comparisons of the measured amount of accreted ice with the observed cloud-particle size distributions quite surprisingly indicate that ice accretion is mostly caused by 10–30 µm sized drops rather than by SLD. The latter, therefore, appear to be a by-product of a hypothesized liquid water accumulation zone near cloud top which is also the primary cause of the observed severe ice accretion. The results confirm the importance of embedded convection and of mixed phase clouds with high amounts of liquid water and simultaneously occurring SLD.

Retrieval of Raindrop and Cloud Particle Size Distributions with 14 GHz and 95 GHz Radars

We have developed a sequential algorithm to retrieve vertical profiles of cloud and rainfall microphysical information from radar reflectivities of a 14 GHz precipitation radar, and a 95 GHz cloud profiling radar. This sequential algorithm retrieves the number density NT, the mean volume diameter Dm and the liquid water content LWC from the nearest to the farthest ranges considering the significant attenuation and Mie scattering. To avoid the accumulation of numerical errors, the retrieval is computed in adjacent ranges assuming NT to be constant in each pair. We demonstrate that our method retrieves NT and Dm stably, even if the raindrop size distribution is different from our assumption, or even if some bias errors are appended to the radar constant. The method is applied to collocated ground-based observation by a 14 and a 95 GHz radar, and the retrieved results are evaluated by comparing them against disdrometer-derived size distributions, where the rainfall rate was from 0.1 to 4.6 mm/hr. The retrieved Dm, and the disdrometer-derived Dm, agree to within 30%. The retrieved log10 LWC, and the disdrometer-derived log10 LWC, also agree to within 30% when the rainfall rate is greater than 0.5 mm/hr.

On retrieving the microphysical properties of cirrus clouds using the moments of the millimeter‐wavelength Doppler spectrum

Techniques that use only measurements provided by millimeter‐wavelength Doppler radar for retrieving the properties of clouds provide certain practical advantages over techniques that require measurements from multiple instruments. We concentrate on the capacity of the moments of the Doppler spectrum to provide information suitable for retrieving the microphysical properties of cirrus clouds. Specifically, we present an algorithm that uses the zeroth and first moments of the Doppler spectrum (radar reflectivity and mean Doppler velocity) to retrieve the cloud particle size distribution. We also discuss a methodology for evaluating the second moment of the Doppler spectrum for information related to the width of the particle size distribution. The properties of observed cirrus particle spectra are first examined to ascertain what degree of complexity is required of assumed functional representations of particle spectra in cirrus volumes. We find that although particle spectra with bimodal tendencies are common, single‐mode functions are able to capture the essential characteristics of these spectra to within observational uncertainty for most cirrus. Furthermore, two‐parameter functions such as the exponential function are shown to be adequate representations of cirrus particle spectra in situations where the radar reflectivity factor is less than about −5 dBZe and the temperatures are colder than about 253 K. A technique for separating air motion contributions from the Doppler velocity is examined, and an approach for similarly processing the Doppler spectral width is introduced. In comparison to aircraft data, we find uncertainty in the derived quiet air velocity and spectrum width to be on the order of 20% and 40%, respectively. Using recent parameterizations of ice crystal terminal velocity, mass, and backscatter cross section, we present a straightforward algorithm for retrieving cirrus cloud microphysical properties from observations of radar reflectivity and Doppler velocity. A simulation experiment with variable particle habits and realistic observational error shows that the retrieval uncertainties are on the order of 60% and 40% for ice water content and mass median particle size, respectively. These uncertainty magnitudes are supported by comparison to aircraft data.

In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region

In situ aerosol measurements were performed in the Indian Ocean Intertropical Convergence Zone (ITCZ) region during the Airborne Polar Experiment‐Third European Stratospheric Experiment on Ozone (APE‐THESEO) field campaign based in Mahé, Seychelles between 24 February and 6 March 1999. These are measurements of particle size distributions with a laser optical particle counter of the Forward Scattering Spectrometer Probe (FSSP)‐300 type operated on the Russian M‐55 high‐altitude research aircraft Geophysica in the tropical upper troposphere and lower stratosphere up to altitudes of 21 km. On 24 and 27 February 1999, ultrathin layers of cirrus clouds were penetrated by Geophysica directly beneath the tropical tropopause at 17 km pressure altitude and temperatures below 190 K. These layers also were concurrently observed by the Ozone Lidar Experiment (OLEX) lidar operating on the lower‐flying German DLR Falcon research aircraft. The encountered ultrathin subvisual cloud layers can be characterized as (1) horizontally extending over several hundred kilometers, (2) persisting for at least 3 hours (but most likely much longer), and (3) having geometrical thicknesses of 100–400 m. These cloud layers belong to the geometrically and optically thinnest ever observed. In situ particle size distributions covering diameters between 0.4 and 23 μm obtained from these layers are juxtaposed with those obtained inside cloud veils around cumulonimbus (Cb) anvils and also with background aerosol measurements in the vicinity of the clouds. A significant number of particles with size diameters around 10 μm were detected inside these ultrathin subvisible cloud layers. The cloud particle size distribution closely resembles a background aerosol onto which a modal peak between 2 and 17 μm is superimposed. Measurements of particles with sizes above 23 μm could not be obtained since no suitable instrument was available on Geophysica. During the flight of 6 March 1999, upper tropospheric and lower stratospheric background aerosol was measured in the latitude band between 4°S and 19°S latitude. The resulting particle number densities along the 56th meridian exhibit very little latitudinal variation. The concentrations for particles with sizes above 0.5 μm encountered under these background conditions varied between 0.1 and 0.3 particles/cm3 of air in altitudes between 17 and 21 km.

Doppler Cloud Radar Derived Drop Size Distributions in Liquid Water Stratus Clouds

A cloud particle size retrieval algorithm that uses radar reflectivity factor and Doppler velocity obtained by a 35-GHz Doppler radar and liquid water path estimated from microwave radiometer radiance measurements is developed to infer the size distribution of stratus cloud particles. Assuming a constant, but unknown, number concentration with height, the algorithm retrieves the number concentration and vertical profiles of liquid water content and particle effective radius. A novel aspect of the retrieval is that it depends upon an estimated particle median radius vertical profile that is derived from a statistical model that relates size to variations in particle vertical velocity; the model posits that the median particle radius is proportional to the fourth root of the particle velocity variance if the radii of particles in a parcel of zero vertical velocity is neglected. The performance of the retrieval is evaluated using data from two stratus case study days 1.5 and 8.0 h in temporal extent. Aircraft in situ microphysical measurements were available on one of the two days and the retrieved number concentrations and effective radii are consistent with them. The retrieved liquid water content and effective radius increase with height for both stratus cases, which agree with earlier studies. Error analyses suggest that the error in the liquid water content vanishes and the magnitudes of the fractional error in the effective radius and shortwave extinction coefficient computed from retrieved cloud particle size distributions are half of the magnitudes of the fractional error in the estimated cloud particle median radius if the fractional error in the median radius is constant with height.

Cloud Droplet Residual Particle Microphysics in Marine Stratocumulus Clouds Observed during the Monterey Area Ship Track Experiment

Abstract The effect of marine boundary layer pollution level (as determined by the aerosol particle number concentration) on the size distribution of aerosol particles that formed cloud droplets in marine stratiform clouds is examined. In situ measurements of cloud droplet residual particles with a counterflow virtual impactor during the Monterey Area Ship Track experiment are presented. The variation in residual particle size distribution and number concentration in clouds that formed in marine boundary layers with different pollution levels are discussed. The measurements show a constant shape of the residual particle size distributions for the variety of pollution levels encountered, even though the aerosol number concentration varied by up to a factor of 6. The measurements indicate that particles smaller than 0.1-μm radius controlled cloud droplet number concentrations in the clouds investigated. From literature values of the maximum supersaturations encountered in stratocumulus clouds, one can estim...

A simple model of the ice particle size distribution in noctilucent clouds

A timescale analysis indicates that at noctilucent cloud (NLC) heights near 83 km, the growth and decay of ice particles and their transport by tide‐ and gravity‐wave‐associated vertical velocity components are the dominant processes that determine the particle size distribution. Other processes including sedimentation are of minor importance, so that the formation of NLCs should, in general, not depend on atmospheric conditions at the mesopause, in agreement with recent findings from simultaneous lidar and rocket experiments. Then a simple NLC model can be constructed consisting essentially of a partial differential equation which describes the temporal behavior of the particle size distribution in particle parcels moving with the wave‐associated air velocity. Contrary to previous ones, the present model yields a steady state oscillation within an integration time of 1 day because the optically active ice particles have lifetimes of a few hours only. NLC simulations yield particle concentrations, mean radii, and scatter ratios which are in good agreement with observational results. The model also predicts an occasional coexistence of small populations of optically active particles and large populations of microscopic particles, which may be of basic importance for the interpretation of recent observations indicating a more or less tight coupling between NLCs and simultaneously occurring polar mesosphere summer echoes.

A case study of cirrus layers with variable 3.74 �m reflection properties in the first FIRE experiment, 2 November 1986

Remotely sensed scanning radiometer and lidar data on cirrus clouds were obtained during the cirrus FIRE IFO experiment in November 1986 from the ER-2 aircraft plat-form. Data were examined particularly on 2 November for an area in the vicinity of Wausau, Wisconsin where unusual effects were noticed in bispectral histograms from various channels in the scanner data. After calibration of the data in spectral channels of both the Scan Cloud Radiometer (SCR) and Multichannel Cloud Radiometer (MCR) instruments, including direct comparison between compatible channels in the two instruments, it was found that the 0.856 µm SCR channel gave good data, whereas the 0.665 µm and 0.74 µm SCR channels gave large offsets, when compared with the MCR 0.754 µm data. The latter channel was found to compare well in a second comparison with coincident AVHRR channel satellite data. Similarly, the SCR 11.17 µm data gave consistent results and the SCR 3.74 µm data were carefully calibrated. Bispectral histograms formed between 0.856 µm, 3.74 µm and 11.17 µm SCR channel data indicated that some coherent layers of cirrus clouds were giving enhanced solar reflectance at 3.74 µm, indicative of small (~ <25 µm radius) particles, whereas other neighbouring layers gave little reflectance. A comparison of 0.856 µm reflections with 11.17 µm absorption optical depth indicated that the small particles where probably ice crystals. A comparison of 3.74 µm solar albedo and 11.17 µm absorption optical depths of these layers with theoretical calculations for ice spheres indicated a mode radius of about 8 µm for the cloud particle size distribution. An estimate from similar recent calculations on hexagonal ice crystals indicated that the retrieved effective radius would be increased to 25 µm. The difference between the two retrieved radii was a measure of the uncertainty in the retrievals, considering also differences in the assumed size distributions. Qualitative comparison with ER-2 lidar data gave a tentative identification of the reflecting layers. The results demonstrate the power of the 3.74 µm channel for identification of small-particle layers in cirrus.

Microphysical examination of excess cloud absorption in the tropical atmosphere

To investigate the excess shortwave absorption by clouds, a numerical cloud generation model has been coupled to a plane‐parallel discrete ordinates radiative transfer model. The former was used in a time‐dependent fashion to generate a cumulonimbus turret and three types of cirrus anvil (precipitating, extended, detached) representing three stages of cloud evolution outward from the turret. The cloud particle size distributions, as a function of altitude, were used as input to the radiative transfer model using indices of refraction for pure water and pure ice and equivalent sphere Mie theory. The radiative transfer model was used to calculate the ratio of cloud forcing at the surface to cloud forcing at the top of the atmosphere, both for the broadband shortwave and as a function of wavelength. Recent empirical studies have placed this cloud forcing ratio at around 1.5, and our coupled model results approach this value for small solar zenith angles, when the cloud contains large (&gt;100 μm) ice particles that absorb significantly in the near infrared (primarily the 1.6‐μm window). However, the empirical studies are based on diurnal averages, and our plane‐parallel radiative transfer model yields an area and diurnally averaged cloud forcing ratio of only 1.18 for a tropical cumulonimbus and cirrus anvil system, primarily because of the rapid decrease of the ratio with solar zenith angle. The ratio decreases because of the increase in albedo with solar zenith angle, which is a characteristic feature of plane‐parallel clouds. Adding dust or aerosol to the cloud layers, to make them absorb at visible wavelengths, makes the instantaneous cloud forcing ratio larger for an overhead Sun but also makes the solar zenith angle dependence in the cloud forcing ratio more pronounced. These two effects cancel, eliminating interstitial aerosol as a possible explanation for the excess cloud absorption in plane‐parallel radiative transfer modeling. The strong dependence of the surface/top of the atmosphere cloud forcing ratio on solar zenith angle may be a fundamental defect with the plane‐parallel approach to solar radiative transfer in a cloudy atmosphere.

Visible spectra and chromaticity of noctilucent clouds

A computational scheme was developed to predict the visible spectra and chromaticity of noctilucent clouds (NLC) for arbitrary viewing geometries. The spectrum of a cloud in the visible is computed at 5‐nm intervals using the spherical shape of the stratified atmosphere, the measured solar irradiance spectrum, wavelength‐dependent extinction by air molecules, ozone, and aerosols, and scattering by a specified size distribution of cloud particles. The chromaticity calculated from each spectrum is used to evaluate the cloud's color. A comparison is made of the abilities of two proposed cloud particle size distributions to account for published spectra measured from the ground and from Earth orbit. A lognormal distribution of “small” particles with an effective radius of 42.6 nm is found to be better at reproducing measured spectra of NLC than a Junge (power law) distribution of “large” particles with an effective radius of 700 nm. Chromaticities of the small‐particle spectra show much better agreement with those of spectra measured from orbit and with the visual appearance of NLC viewed from the ground. Chromaticities are found to be weakly dependent on the atmospheric aerosol loading. However, NLC brightness is strongly dependent upon both tropospheric and stratospheric aerosols.

Feasibility study of derivation of cirrus information using polarimetric measurements from satellite

Monitoring techniques for deriving cirrus information over the ocean using polarimetric measurements from satellite are studied. Radiance and degree of linear polarization of the upwelling radiation at the top of a model atmosphere-ocean system are computed at the wavelength of 0.88 μm with the aid of the doubling-adding method, and where cirrus cloud is assumed to be composed of water droplets or randomly oriented hexagonal ice crystals (columns and plates). The computational results show, in particular, the following features. Degree of polarization observed at the Brewster angle in the solar plane is suitable to derive cloud optical thickness (τ) for large solar zenith angles, particularly in the case of τ < ∼ 5 while the radiance off the sunglint is suitable to infer τ in the range of τ < ∼ 30. However, for improvement of accuracy, cloud information such as thermodynamic phase, size distribution, and shape of cloud particles should be separately obtained, in particular for utilization of degree of polarization. The difference of degree of polarization appears between water particles and hexagonal ice crystals at the top of the atmosphere when the scattering angle is within 50–100°. Using this property, it should be possible to distinguish the thermodynamic phase of cloud particles. Furthermore, the feasibility of discrimination of the shape of the hexagonal ice crystals using the position of the neutral point is discussed, where phase angle is shown to shift to the backscattering direction at τ < 10. Hence, it is necessary to obtain τ separately.

Effects of radiation on clouds

The effects of radiation on the growth of individual cloud particles and on the structure of clouds are reviewed. It is shown that the rate of growth of both water drops and ice crystals may be changed significantly by effects of radiation either near the edges of cloud sheets or below cloud, although the overall effect on particle size depends on the period spent in such regions. Sub-cloud evaporation of ice particles may be reduced by radiative cooling of the crystals giving rise to persistent fall streaks. The role of radiative transfer in the formation of radiation fog is well established. It is shown, however, that the nature of the radiative process is crucial in explaining the observed distribution of liquid water in fogs. The way in which radiation fogs may be dissipated due to changes in the radiation balance are also described. Layer clouds contain regions of radiative flux divergence which give rise to localised heating or cooling. In low-level water clouds and in some higher-level ice clouds this gives rise to destabilisation of the cloud layer leading to the generation of turbulent kinetic energy. This may be enhanced by evaporation of cloud particles into air entrained into the cloud layer. The sensitivity of the cloud structure to the radiative processes may give rise to a pronounced diurnal variation in the structure and to a dependence of the structure on latitude. The turbulent motions generated, in part, by radiative processes feed back on the cloud particle size distribution either by increasing the time spent in the cloud by a small fraction of the particles or by modifying the coupling between those clouds regions in which particles form with those regions where growth occurs. The review brings together theoretical studies and observations. It is concluded that while the effects of radiation on the evolution of stratocumulus cloud sheets are comparatively well understood, some details remain unresolved. However, cirrus clouds are less fully understood owing to the limited observations and the greater coupling between radiative, dynamical and microphysical processes.

Modelling of cloud formation due to air-sea interactions in an energy-active zone

A mesoscale 3D numerical model is described, with which detailed calculations have been made of turbulence and wind characteristics in the atmospheric boundary layer (ABL), as well as cloud particle size distribution, longwave and solar radiation fluxes and flux divergences, and atmosphere-ocean heat exchange. Based on numerical experiments simulating winter conditions of the Newfoundland energy-active zone of the ocean (EAZO), atmosphere-ocean energy exchange is investigated. It is shown that the basic mechanisms for the EAZO formation involve the following processes: (i) at the hydrological front between cold and warm ocean currents, the fluxes of sensible and latent heat grow significantly; (ii) at this front, in a particular synoptic situation, overcast low-level cloudiness forms, screening solar radiation so that in winter, the radiation budget at the front is reduced, and the radiative flux into the ocean is less than the energy release to the atmosphere; (iii) frequent occurrence of such synoptic situations with cloudiness decreases the oceanic enthalpy and creates negative SST anomalies. The transport of these anomalies by currents to the western coasts of the continents causes anomalies of weather and climate.