Cloud Particle Effective Radius Research Articles

Shortwave radiative fluxes from MODIS: Model development and implementation

A forward inference scheme driven with satellite‐based information is developed to derive top of the atmosphere, surface, and atmospheric spectral shortwave radiative fluxes at global scale. The model takes into account all major atmospheric constituents, the characteristics of water and ice clouds, and the variation of cloud particle effective radius. The multilayered structure accounts for surface elevation effects and for the vertical distribution of the radiative fluxes. Spectral fluxes such as photosynthetically active radiation and near‐infrared radiation can also be estimated. The model is implemented with products from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor both on Terra and Aqua at 1° spatial resolution for a 3‐year period (2003–2005) and at global scale. The derived surface fluxes are evaluated against the globally distributed Baseline Surface Radiation Network (BSRN) measurements and compared with products from independent sources. It is demonstrated that the twice a day MODIS‐based estimates of daily radiative fluxes adjusted for the diurnal cycle and the ground observations have a correlation coefficient greater than 0.96, a relative root‐mean‐square error less than 18%, and a relative bias between 0 and 6% for BSRN sites located in the Antarctic, the Atlantic (islands), North America, Europe, and other continental sites.

Satellite detection of severe convective storms by their retrieved vertical profiles of cloud particle effective radius and thermodynamic phase

A new conceptual model that facilitates the inference of the vigor of severe convective storms, producing tornadoes and large hail, by using satellite‐retrieved vertical profiles of cloud top temperature (T)–particle effective radius (re) relations is presented and tested. The driving force of these severe weather phenomena is the high updraft speed, which can sustain the growth of large hailstones and provide the upward motion that is necessary to evacuate the violently converging air of a tornado. Stronger updrafts are revealed by the delayed growth of re to greater heights and lower T, because there is less time for the cloud and raindrops to grow by coalescence. The strong updrafts also delay the development of a mixed phase cloud and its eventual glaciation to colder temperatures. Analysis of case studies making use of these and related criteria show that they can be used to identify clouds that possess a significant risk of large hail and tornadoes. Although the strength and direction of the wind shear are major modulating factors, it appears that they are manifested in the updraft intensity and cloud shapes and hence in the T‐re profiles. It is observed that the severe storm T‐re signature is an extensive property of the clouds that develop ahead in space and time of the actual hail or tornadic storm, suggesting that the probabilities of large hail and tornadoes can be obtained at substantial lead times. Analysis of geostationary satellite time series indicates lead times of up to 2 h.

Influence of inhomogeneous cloud fields on optical properties retrieved from satellite observations

Analyses of solar radiation exchanges between the atmosphere and clouds are vital for the understanding of climate processes and cycles. Comparisons of satellite‐to‐satellite or satellite‐to‐ground‐truth observations aiming at, elucidating the radiative behavior of atmospheric components (clouds, aerosols, gas, etc.), or validating data of a particular satellite are a common practice in global radiation investigations. In order to assess the quality of cloud optical properties derived from Geostationary Meteorological Satellite‐5/Stretched Visible Infrared Spin Scan Radiometer (GMS‐5/SVISSR), the former procedure (satellite‐to‐satellite comparison) was used. Data derived from GMS‐5/SVISSR satellite were compared with those from the polar‐orbiting Terra‐Moderate Resolution Imaging Spectroradiometer (Terra‐MODIS) satellite. This comparison showed serious discrepancies between cloud optical depth (COD) data retrieved from the two satellites' observations. GMS‐5/SVISSR‐retrieved COD appeared mostly lower than that of Terra‐MODIS. To understand the origin of such differences, an identification procedure of the major factors likely to affect these data is conducted. Some of these factors were the satellite viewing and solar conditions, the cloud thermodynamic phase differentiation and particle effective radius, and the cloud inhomogeneity. Then emphasis was put on the examination of the latter effect (i.e., the cloud inhomogeneity). The analysis procedure was as follows: First, data having close‐viewing geometries between both satellites were selected and used to understand the effects of the remaining factors. Among these, the cloud thermodynamic phase appeared to play the major role as analyses showed that most of the COD differences between both satellites were confined within ice clouds while warm clouds had the least discrepancies. This would suggest that the choice of a water cloud particle radiative transfer model to analyze a 2‐phase cloud radiation data, as used here, may produce large uncertainties in ice COD retrievals from at least one of the satellites. To avoid the cloud phase problem, a more restrictive data set comprising only water clouds (besides close‐viewing geometries between both satellites) was selected, and the impact of the degree of cloud inhomogeneity on the COD retrievals was evaluated. The study reveals that the 3‐D radiative effects deriving from the external cloud inhomogeneity, i.e., cloud asymmetry and structured sides, were the most influencing properties here. The GMS‐5/SVISSR interpretation of inhomogeneous cloud optical properties showed larger uncertainties than that of Terra‐MODIS. Furthermore, COD values of GMS‐5/SVISSR were systematically lower than those of Terra‐MODIS for the pixels at shadow sides of the cloud, while at illuminated sides they often showed higher values. For gentle or near‐plane‐parallel cloud surfaces, fewer discrepancies were noticed (the best agreement between both satellites' retrievals). At steep slopes of the shadow and illuminated cloud sides, GMS‐5/SVISSR average COD data were respectively under‐ and overestimated compared to those of Terra‐MODIS. COD differences between the two satellites could be sometimes higher than 30% for slopes steeper than 0.5 K/km.

Comparison of microwave and optical cloud water path estimates from TMI, MODIS, and MISR

This study investigated the consistency between microwave and optical water path estimates of oceanic clouds from Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI), Moderate Resolution Imaging Spectroradiometer (MODIS), and Multiangle Imaging Spectroradiometer (MISR). We used microwave estimates from the Wentz algorithm for warm, nonprecipitating clouds and both Wentz retrievals and standard TMI profiles for cold, precipitating clouds. Optical estimates were derived from cloud optical thickness and particle effective radius. For warm, nonprecipitating clouds the two methods showed good agreement at the 25‐km resolution of the microwave measurements, with liquid water path means being within 5–10%, an overall correlation of 0.85, and RMS difference of ∼25 g m−2. Multiangle optical retrievals showed only weak variations with view zenith angle, building further confidence in the results. An error analysis suggested that optical estimates were more certain than microwave ones, primarily because 1‐D plane‐parallel radiative transfer seemed to apply well at the coarser comparison scales. However, there appeared to be a slight but systematic dependence on cloud amount as microwave retrievals increasingly overestimated optical ones at cloud fractions below ∼65%. For cold, precipitating clouds we tested three common interpretations of optical retrievals: liquid water path, ice water path, and total water path. The relationship between microwave and optical estimates was weak in all cases, with correlations no more than 0.5, and RMS differences at least an order of magnitude larger than for warm clouds. The weakest correlation (0.37) was found when optical retrievals were interpreted as ice water paths. If anything, optical estimates appeared best correlated with microwave cloud liquid water paths.

Variability of clouds and water vapor in low latitudes: View from Moderate Resolution Imaging Spectroradiometer (MODIS)

This paper shows a small sampling of the atmospheric fields provided by the Moderate Resolution Imaging Spectroradiometer (MODIS), which is installed on both the Terra and Aqua satellites of the Earth Observing System. These fields include macroscale fields like cloud fraction, cloud top pressure, cloud optical depth, and column water vapor, and microscale fields like cloud particle effective radius. Most of the atmospheric fields resemble, in spatial pattern and temporal evolution, the cloud top temperature field; several, however, including effective radius and clear‐sky water vapor, have markedly different spatial patterns and temporal evolution, suggesting that large‐scale controls on tropical water vapor are more complicated than a simple connection with large‐scale convergence and convection would suggest. As with several previous studies, the cloud top temperature or pressure has a bimodal distribution with peaks in both the lower and upper troposphere. Our analysis also reveals a dependence of cloud effective particle size on sea surface temperature.

The time-space exchangeability of satellite retrieved relations between cloud top temperature and particle effective radius

Abstract. A 3-minute 3-km rapid scan of the METEOSAT Second Generation geostationary satellite over southern Africa was applied to tracking the evolution of cloud top temperature (T) and particle effective radius (re) of convective elements. The evolution of T-re relations showed little dependence on time, leaving re to depend almost exclusively on T. Furthermore, cloud elements that fully grew to large cumulonimbus stature had the same T-re relations as other clouds in the same area with limited development that decayed without ever becoming a cumulonimbus. Therefore, a snap shot of T-re relations over a cloud field provides the same relations as composed from tracking the time evolution of T and re of individual clouds, and then compositing them. This is the essence of exchangeability of time and space scales, i.e., ergodicity, of the T-re relations for convective clouds. This property has allowed inference of the microphysical evolution of convective clouds with a snap shot from a polar orbiter. The fundamental causes for the ergodicity are suggested to be the observed stability of re for a given height above cloud base in a convective cloud, and the constant renewal of growing cloud tops with cloud bubbles that replace the cloud tops with fresh cloud matter from below.

A New Parameterization of Single Scattering Solar Radiative Properties for Tropical Anvils Using Observed Ice Crystal Size and Shape Distributions

Parameterizations of single scattering properties currently used in cloud resolving and general circulation models are somewhat limited in that they typically assume the presence of single particle habits, do not adequately account for the numbers of ice crystals with diameters smaller than 100 mm, and contain no information about the variance of parameterization coefficients. Here, new parameterizations of mean single scattering properties (e.g., single scatter albedo, asymmetry parameter, and extinction efficiency) for distributions of ice crystals in tropical anvils are developed. Using information about the size and shape of ice crystals acquired by a twodimensional cloud probe during the Central Equatorial Pacific Experiment (CEPEX), a self-organized neural network defines shape based on simulations of how the particle maximum dimension and area ratio (ratio of projected area to that of circumscribed circle with maximum dimension) vary for random orientations of different idealized shapes (i.e., columns, bullet rosettes, rough aggregates, and particles represented by Chebyshev polynomials). The size distributions for ice crystals smaller than 100 mm are based on parameterizations developed using representative samples of 11 633 crystals imaged by a video ice particle sampler (VIPS). The meanscattering properties for distributions of ice crystals are then determined by weighting the single scattering properties of individual ice crystals, determined using an improved geometric ray-tracing method, according to number concentration and scattering cross section. The featureless nature of the calculated phase function, averaged over all observed sizes and shapes of ice crystals, is similar to that obtained using other schemes designed to account for variations in sizes and shapes of ice crystals. The new parameterizations of single scatter albedo, asymmetry parameter, and extinction efficiency are then determined by functional fits in terms of cloud particle effective radius; there was no statistically significant dependence on either ice water content or temperature. Uncertainty estimates incorporated into the parameterization coefficients are based upon a Monte Carlo approach. Comparisons with previously used parameterizations and with parameterizations developed using single crystal habits are made to show that the determination of representative crystal habits is still a major unknown in the development of parameterization schemes.

New insights into Martian dust distribution and water‐ice cloud microphysics

In this paper we use a direct method which combines microphysics and photometric simulations of the Martian atmosphere. This approach allows us to reproduce vertical profiles of the light scattered by the haze at the limb of Mars. Subsequent results are compared to a multiple‐color image provided by the Viking Orbiter camera. The “free parameters” of our study are related to vertical transport and dust size distribution. A sensitivity study has been conducted, encompassing a relevant range of parameters, in order to obtain the most satisfying multispectral modeled profile with respect to the one inferred from data. Except for an unlikely distribution of submicron particles (reff ∼ 0.2 μm), no consistent fit can be obtained. This implies that size distributions of dust suggested by previous studies are not in agreement with the present analysis. This apparent discrepancy can be resolved by adding a separate peak of submicron particles to the size distribution previously extracted from Viking Lander images. Only in that case can spectral and vertical structures of haze be successfully matched. While a bimodal distribution would change the current picture of Martian dust, such distribution is a common representation of soil‐derived aerosol size function in Earth deserts. We also attempted to derive information from the cloud shown in the Viking limb image. Our estimates suggest that cloud particle effective radius is around 1.2–1.8 μm, while the cloud visible opacity is 0.02.

The Retrieval of Stratus Cloud Droplet Effective Radius with Cloud Radars

In situ samples of cloud droplets by aircraft in Oklahoma in 1997, the Surface Heat Budget of the Arctic Ocean (SHEBA)/First ISCCP Regional Experiment (FIRE)-Arctic Cloud Experiment (ACE) in 1998, and various other locations around the world were used to evaluate a ground-based remote sensing technique for retrieving profiles of cloud droplet effective radius. The technique is based on vertically pointing measurements from high-sensitivity millimeter-wavelength radar and produces height-resolved estimates of cloud particle effective radius. Although most meteorological radars lack the sensitivity to detect small cloud droplets, millimeter-wavelength cloud radars provide opportunities for remotely monitoring the properties of nonprecipitating clouds. These high-sensitivity radars reveal detailed reflectivity structure of most clouds that are within several kilometers range. In order to turn reflectivity into usable microphysical quantities, relationships between the measured quantities and the desired quantities must be developed. This can be done through theoretical analysis, modeling, or empirical measurements. Then the uncertainty of each procedure must be determined in order to know which ones to use. In this study, two related techniques are examined for the retrieval of the effective radius. One method uses both radar reflectivity and integrated liquid water through the clouds obtained from a microwave radiometer; the second uses the radar reflectivity and an assumption that continental stratus clouds have a concentration of 200 drops per cubic centimeter and marine stratus 100 cm−3. Using in situ measurements of marine and continental stratus, the error analysis herein shows that the error in these techniques would be about 15%. In comparing the techniques with in situ aircraft measurements of effective radius, it is found that the radar radiometer retrieval was not quite as good as the technique using radar reflectivity alone. The radar reflectivity alone gave a 13% standard deviation with the in situ comparison, while the radar–radiometer retrieval gave a 19% standard deviation.

Combined radar and lidar cloud remote sensing: Comparison with IR radiometer and in-situ measurements

A new combined radar and lidar inversion procedure has been developed for cloud particle effective radius and water content retrievals. The algorithm uses the radar reflectivity and an effective particle size to parameterize the extinction at the lidar wavelength while treating the derived sizes and lidar multiple scattering effects in a consistent fashion. This procedure has been applied to data from the CLouds And RAdiation experiment (CLARA) and the Cloud Lidar and Radar Experiment (CLARE' 98) campaigns. The results compare well with simultaneous IR-radiometer cloud measurements as well as measurements made in ice-clouds using aircraft mounted 2D probe particle-sizing instruments. In addition, the reflectivities calculated from the in-situ measurements alone are seen to consistent with the observed reflectivities.

Extinction coefficient (1 μm) properties of high‐altitude clouds from solar occultation measurements (1985–1990): Evidence of volcanic aerosol effect

The properties of the l‐μm volume extinction coefficient of two geographically different high‐altitude cloud systems have been examined for the posteruption period (1985–1990) of the April 1982 El Chichón volcanic event with emphasis on the effect of volcanic aerosols on clouds. These two high‐altitude cloud systems are the tropical clouds in the tropopause region observed by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the polar stratospheric clouds (PSCs) sighted by the Stratospheric Aerosol Measurement (SAM) II. The results indicate that volcanic aerosols alter the frequency distributions of these high‐altitude clouds in such a manner that the occurrence of clouds having high extinction coefficients (6×10−3 – 2×10−2 km−1) is suppressed, while that of clouds having low extinction coefficients (2×10−3 – 6×10−2 km−1) is enhanced. This influence of the volcanic aerosols appears to be opposite to the increase in the extinction coefficient of optically thick clouds observed by the Earth Radiation Budget Experiment (ERBE) during the initial posteruption period of the June 1991 Pinatubo eruption. A plausible explanation of this difference, based on the Mie theory, is presented. The Mie calculation indicates that there are two possible types of response of cloud extinction coefficient to changes in aerosol concentration depending on the primary effective radius (re) of cloud systems observed by the instrument. These two types of response are separated by the cloud particle effective radius of about 0.8 μm. When re is smaller than 0.8 μm, the cloud extinction coefficient decreases in response to increases of aerosol concentration, and when re is greater than 0.8 μm, the opposite happens. As a consequence, the effective radius of most, if not all, of the high‐altitude clouds, measured by the SAGE series of satellite instruments must be less than about 0.8 μm. This mean cloud particle size implied by the satellite extinction‐coefficient data at a single wavelength (1 μm) is further substantiated by the particle size analysis based on cloud extinction coefficient at two wavelengths (0.525 and 1.02 μm) obtained by the SAGE II observations. Most of the radiation measured by ERBE is reflected by cloud systems comprised of particles having effective radii much greater than 1 μm. A reduction in the effective radius of these clouds due to volcanic aerosols is expected to increase their extinction‐coefficient values, opposite the effect observed by SAGE II and SAM II. This work further illustrates the capability of the solar occultation satellite sensor to provide particulate extinction‐coefficient measurements important to the study of the aerosol‐cloud interactions. Finally, the June 1991 Mount Pinatubo major eruption put 3 times more material into the stratosphere than that of the 1982 El Chichón volcanic event. It is important to examine the variations of the extinction coefficient of these two high‐altitude cloud systems for the posteruption years of the Pinatubo volcanic event for further evidence of the impact of volcanic aerosols on high‐altitude clouds.