Cloud Optical Thickness Retrievals Research Articles

Uncertainties Associated With the Surface Texture of Ice Particles in Satellite-Based Retrieval of Cirrus Clouds: Part II—Effect of Particle Surface Roughness on Retrieved Cloud Optical Thickness and Effective Particle Size

The simplified ray-tracing technique reported in Part I of this paper is employed to compute the single-scattering properties of hexagonal columns with maximum dimensions ranging from 2 to 3500 mum with a size-bin resolution of 2 mum at wavelengths of 0.86 and 2.13 mum. For small ice crystals, the current treatment of surface roughness may not be adequate because the applicability of the principles of geometric optics breaks down for small roughness scale. However, for ice crystals smaller than 40 mum, the aspect ratios of these particles are close to one, and the effect of surface roughness is quite small. In this paper, the diffraction is accounted for in the same way as in the case of smooth particles. It is essentially unfeasible to incorporate the effect of surface roughness into the numerical computation of the diffraction contribution. The scattering properties of individual ice crystals are then averaged over 18 particle size distributions whose effective particle radii (re) range from 5 to 90 mum. The single-scattering properties of ice clouds are strongly sensitive to surface roughness condition. Lookup tables that are built for the correlation between the bidirectional reflectances at wavelengths of 0.86 and 2.13 mum with different roughness conditions are used to retrieve ice cloud optical thickness and effective particle size over oceans. Pronounced differences are noticed for the retrieved cirrus cloud optical thickness and effective particle sizes in conjunction with different surface roughness conditions. The values of the retrieved cirrus cloud optical thickness in the case of the rough surface are generally smaller than their counterparts associated with smooth surface conditions. The effect of surface roughness on the retrieved effective particle radii is not pronounced for slight and moderate roughness conditions. However, when the surfaces of ice crystals are substantially rough, the retrieved effective radii associated with roughened particles are larger and smaller than their smooth surface counterparts forlarge (re>50 mum) and small (re<35 mum) ice crystals, respectively, whereas the effect of surface roughness on the retrieved effective radii shows a nonmonotonic feature for moderate particle sizes (35 mum<re<50 mum). In general, the dominant effect of surface roughness on cloud property retrievals is to decrease the retrieved optical thickness and to increase the retrieved effective particle size in comparison with their counterparts in the case of smooth ice particles.

Retrieval of cloud properties from near‐ultraviolet, visible, and near‐infrared satellite‐based Earth reflectivity spectra: A comparative study

In this study, we investigate the capability to retrieve cloud parameters from near‐ultraviolet, visible, and near‐infrared satellite‐based reflectivity measurements. These parameters are essential to enable trace gas retrievals for cloud‐contaminated satellite scenes. We compare the retrieval of cloud top pressure, cloud fraction, and cloud optical thickness from simulated reflectivity measurements in three wavelength ranges: (1) 350–400 nm, which includes pronounced Ring effect structures; (2) 460–490 nm; and (3) 755–775 nm, which contain absorption features of O2‐O2 and O2, respectively. Retrieval simulations are performed for both a typical noise level of present‐day space‐borne spectrometers and additional random‐like measurement biases. Furthermore, we investigated the importance of the spectral continuum for the retrieval of cloud properties. It is found that reflectivity measurements in the wavelength ranges 350–400 and 755–775 nm provide complementary information on cloud properties. Both spectral windows provide more information on clouds than the 460–490 nm window. The best results are obtained for the combination of the 350–400 and 755–775 nm windows. In this case all three cloud parameters can be retrieved independently, and a high robustness is obtained with respect to random‐like measurement biases. Here it is not required to resolve the Ring effect structures in the near‐ultraviolet window. For this combination of spectral windows the error on retrieved NO2 columns is reduced considerably.

Validation of liquid cloud property retrievals from SEVIRI using ground‐based observations

Partly due to aerosol effects stratocumulus clouds vary considerably in liquid water path (LWP), geometrical thickness (h) and droplet number concentration (Nc). Cloud models have been developed to simulate h and Nc using satellite retrieved cloud optical thickness (τ) and effective radius (re) values. In this paper we examine the consistency between LWP and h values inferred from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard METEOSAT‐8. The use of METEOSAT‐8 data means that time series of LWP and h can be validated at a 15‐minute resolution, and used for examining the first indirect aerosol effect. For single‐layered stratocumulus clouds the LWP and h retrievals from SEVIRI are compared to corresponding ground‐based observations at two Cloudnet sites. A study on the sensitivity of the cloud model to the uncertainties in SEVIRI retrievals of τ and re reveals that h and Nc simulations are only accurate for clouds with effective radii larger than 5 μm. The SEVIRI and ground‐based retrievals of LWP and h show very good agreement, with accuracies of about 15 g m−2 and 20 m, respectively. This agreement could only be achieved by assuming sub‐adiabatic profiles of droplet concentration and liquid water path in the cloud model. The degree of adiabaticity for single‐layered stratocumulus clouds could be quantified by simultaneous analysis of SEVIRI and ground‐based LWP and h values, which suggests that stratocumulus clouds over North Western Europe deviate, on average, from adiabatic clouds.

Use of MISR measurements to study the radiative transfer of an isolated convective cloud: Implications for cloud optical thickness retrieval

Radiative transfer simulations of an isolated deep convective cloud reconstructed with stereo‐techniques from the Multiangle Imaging Spectroradiometer (MISR) are compared with the reflectances measured at the nine MISR viewing angles. The simulations were done using a three dimensional Monte Carlo model, in which ocean reflectance, aerosol and Rayleigh scattering were prescribed to match the surrounding clear‐sky MISR measurements. Making reasonable assumptions regarding the vertical and horizontal distribution of the volume extinction coefficient, we were able to reproduce the MISR measurements with the 3D radiative calculations. While the uniqueness of the these distributions cannot be proven, they all lead to retrievals of much larger cloud optical thickness and cloud water content than for a 1D retrieval. Averaged over the cloud, the difference was a factor of about 3, rising to 9 locally. This is a consequence of horizontal photon transport that serves to highlight the inadequacy of 1D retrievals for the case of deep convective cloud. Concerning the internal cloud properties, we noticed the angular distribution of modeled radiances did not match the measured radiances when an ice crystal phase function was applied. Better estimates of the optical depths and water contents of deep convective clouds appear to be obtainable by integrating an estimate of the extinction coefficient over the vertical cloud extent (when this can assessed) than by attempting to invert the radiance measured from a single‐angle view using 1D theory.

View angle dependence of cloud optical thicknesses retrieved by Moderate Resolution Imaging Spectroradiometer (MODIS)

This study examines whether cloud inhomogeneity influences the view angle dependence of MODIS cloud optical thickness retrieval results. The degree of cloud inhomogeneity is characterized through the local gradient in 11 μm brightness temperature. The analysis of liquid phase clouds in a 1 year long global data set of Collection 4 MODIS data reveals that while optical thickness retrievals give remarkably consistent results for all view directions if clouds are homogeneous, they give much higher τ‐values for oblique views than for overhead views if clouds are inhomogeneous and the Sun is fairly oblique. The mean optical thickness retrieved for the most inhomogeneous third of cloudy pixels is more than 40% higher for oblique views than for overhead views if the solar zenith angle exceeds 60°. After considering a variety of possible scenarios, the paper concludes that the most likely reason for the increase lies in three‐dimensional radiative interactions that are not considered in current, one‐dimensional retrieval algorithms. Namely, the radiative effect of cloud sides viewed at oblique angles seems to contribute most to the enhanced optical thickness values. The results presented here will help understand and estimate cloud retrieval uncertainties related to cloud inhomogeneity and may eventually help correct for the observed view angle‐dependent biases.

Cloud optical thickness retrievals from ground‐based pyranometer measurements

A method is developed to retrieve total cloud optical thickness (COT) from global solar radiation (GSR) detected by ground‐based pyranometer, and approaches to input aerosol/molecular/gas parameters for COT retrievals are presented. On the basis of numerical simulations and comparative tests, main error factors of COT retrievals are analyzed, which include radiation data error, cloud inhomogeneity, uncertainties of aerosol optical parameters, and surface albedo. The retrieved COT error, caused by a –5% or 5% systematic error of the GSR measurement, is within ±0.6 and ±5.0 for COT ranges of 0–5.0 and 5–100, respectively. The AOT, the aerosol single scatter albedo (SSA), and the surface albedo are three significant parameters affecting COT retrieval accuracy. The mean SSA in the pyranometer spectral response range and the broadband surface albedo are suitably used in the retrievals. If uncertainties of AOT, SSA, and surface albedo are within ±0.1, ±0.05, and ±0.05, respectively, the retrieval accuracy is accepted for most applications. Furthermore, COTs (τPyr) from pyranometer data at two meteorological observatories are compared with COTs (τISCCP) from ISCCP and COTs (τMODIS) from MODIS. The relative standard deviations between monthly mean τPyr and τMODIS, or τPyr and τISCCP, are all less than 45.4% for both sites. The agreement among the yearly mean τPyr,τMODIS, and τISCCP is satisfactory. The absolute (relative) deviations between the yearly mean τPyr and τMODIS are within ±1.55 (8%) for both sites, and the deviations between the τPyr and τISCCP are within ±1.94 (25%). The yearly mean τPyr also agrees considerably well with τISCCP in the broken cloud case.

Estimate of satellite‐derived cloud optical thickness and effective radius errors and their effect on computed domain‐averaged irradiances

The process of retrieving cloud optical thickness and effective radius from radiances measured by satellite instruments is simulated to determine the error in both the retrieved properties and in the irradiances computed with them. The radiances at 0.64 μm and 3.7 μm are computed for three cloud fields (stratus, stratocumulus, and cumulus) generated by large eddy simulation models. When overcast pixels are assumed and the horizontal flux is neglected in the retrieval process, the error in the domain‐averaged retrieved optical thickness from nadir is 1% to −32% (1% to −27%) and the error in the retrieved effective radius is 0% to 67% (0% to 63%) for the solar zenith angle of 30° (50°). Using the radiance averaged over a 1 km size pixel also introduces error in the optical thickness because of the nonlinear relation between the reflected radiance and optical thickness. Both optical thickness and effective radius errors increase with increasing horizontal inhomogeneity. When the 0.64 μm albedo is computed with the independent column approximation using retrieved properties from nadir (oblique) view for a solar zenith angle of 50°, the error is −0.3% to 14% (−5% to −30%) relative to the albedo from 3‐D radiative transfer computations with the true cloud properties. The albedo error occurs even though the radiance at one angle is forced to agree because a plane parallel cloud with a single value of optical thickness and effective radius cannot consistently match the radiance angular distribution. In addition, the error in the retrieved cloud properties contributes to the albedo error. When albedos computed with cloud properties derived from nadir and oblique views are averaged, the albedo error can partially cancel. The absolute error in the narrowband 0.64 μm (3.7 μm) albedo averaged over a 1° × 1° domain is less than 1.5% (0.6%), 5.0% (4.1%), and 7.1% (11%) in order of increasing inhomogeneity, when albedos computed with cloud properties derived from viewing zenith angles between 0° and 60° are averaged and when the solar zenith angle is between 10° and 50°. When the solar zenith angle is 70°, the error increases to up to +24% (+37%) for all three scenes.

Impact of three‐dimensional radiative effects on satellite retrievals of cloud droplet sizes

There are several dozen papers that study the effects of cloud horizontal inhomogeneity on the retrievals of cloud optical thickness, but only a few of them deal with cloud droplet sizes. This paper is one of the first comprehensive attempts to fill this gap: It takes a close theoretical look at the radiative effects of cloud 3‐D structure in retrievals of droplet effective radii. Under some general assumptions, it was found that ignoring subpixel (unresolved) variability produces a negative bias in the retrieved effective radius, while ignoring cloud inhomogeneity at scales larger than a pixel scale (resolved variability), on the contrary, leads to overestimation of the domain average droplet size. The theoretical results are illustrated with examples from Large Eddy Simulations (LES) of cumulus (Cu) and stratocumulus (Sc) cloud fields. The analysis of cloud drop size distributions retrieved from both LES fields confirms that ignoring shadowing in 1‐D retrievals results in substantial overestimation of effective radii which is more pronounced for broken Cu than for Sc clouds. Collocated measurements of broken Cu clouds by Moderate Resolution Imaging Spectrometer (MODIS) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) are used to check simulations and theory with observations. The analysis of ASTER and MODIS data and associated derived products recommends against blindly using retrieved effective radii for broken cloud fields, especially if one wants to relate aerosol amounts to cloud droplet sizes.

Retrieval of Cloud Microphysical Properties from MODIS and AIRS

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS) measurements from the NASA Earth Observing System Aqua satellite enable global monitoring of the distribution of clouds during day and night. The MODIS is able to provide a high-spatial-resolution (1–5 km) cloud mask, cloud classification mask, cloud-phase mask, cloud-top pressure (CTP), and effective cloud amount during both the daytime and the nighttime, as well as cloud particle size (CPS) and cloud optical thickness (COT) at 0.55 μm during the daytime. The AIRS high-spectral-resolution measurements reveal cloud properties with coarser spatial resolution (13.5 km at nadir). Combined, MODIS and AIRS provide cloud microphysical properties during both the daytime and nighttime. A fast cloudy radiative transfer model for AIRS that accounts for cloud scattering and absorption is described in this paper. One-dimensional variational (1DVAR) and minimum-residual (MR) methods are used to retrieve the CPS and COT from AIRS longwave window region (790–970 cm−1 or 10.31–12.66 μm, and 1050–1130 cm−1 or 8.85–9.52 μm) cloudy radiance measurements. In both 1DVAR and MR procedures, the CTP is derived from the AIRS radiances of carbon dioxide channels while the cloud-phase information is derived from the collocated MODIS 1-km phase mask for AIRS CPS and COT retrievals. In addition, the collocated 1-km MODIS cloud mask refines the AIRS cloud detection in both 1DVAR and MR procedures. The atmospheric temperature profile, moisture profile, and surface skin temperature used in the AIRS cloud retrieval processing are from the European Centre for Medium-Range Weather Forecasts forecast analysis. The results from 1DVAR are compared with the operational MODIS products and MR cloud microphysical property retrieval. A Hurricane Isabel case study shows that 1DVAR retrievals have a high correlation with either the operational MODIS cloud products or MR cloud property retrievals. 1DVAR provides an efficient way for cloud microphysical property retrieval during the daytime, and MR provides the cloud microphysical property retrievals during both the daytime and nighttime.

Case study of inhomogeneous cloud parameter retrieval from MODIS data

Cloud parameter retrieval of inhomogeneous and fractional clouds is performed for a stratocumulus scene observed by MODIS at a solar zenith angle near 60°. The method is based on the use of neural network technique with multispectral and multiscale information. It allows to retrieve six cloud parameters, i.e. pixel means and standard deviations of optical thickness and effective radius, fractional cloud cover, and cloud top temperature. Retrieved cloud optical thickness and effective radius are compared to those retrieved with a classical method based on the homogeneous cloud assumption. Subpixel fractional cloud cover and optical thickness inhomogeneity are compared with their estimates obtained from 250m pixel observations; this comparison shows a fairly good agreement. The cloud top temperature appears also retrieved quite suitably.

An Algorithm for the Retrieval of Droplet Number Concentration and Geometrical Thickness of Stratiform Marine Boundary Layer Clouds Applied to MODIS Radiometric Observations

Abstract Algorithms are now currently used for the retrieval of cloud optical thickness and droplet effective radius from multispectral radiance measurements. This paper extends their application to the retrieval of cloud droplet number concentration, cloud geometrical thickness, and liquid water path in shallow convective clouds, using an algorithm that was previously tested with airborne measurements of cloud radiances and validated against in situ measurements of the same clouds. The retrieval is based on a stratified cloud model of liquid water content and droplet spectrum. Radiance measurements in visible and near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS), which is operated from the NASA platforms Terra and Aqua, are analyzed. Because of uncertainties in the simulation of the continental surface reflectance, the algorithm is presently limited to the monitoring of the microphysical structure of boundary layer clouds over the ocean. Two MODIS scenes of extended cloud fields over the North Atlantic Ocean trade wind region are processed. A transport and dispersion model (the Hybrid Single-Particle Lagrangian Integrated Trajectory Model, HYSPLIT4) is also used to characterize the origin of the air masses and hence their aerosol regimes. One cloud field formed in an air mass that was advected from southern Europe and North Africa. It shows high values of the droplet concentration when compared with the second cloud system, which developed in a more pristine environment. The more pristine case also exhibits a higher geometrical thickness and, thus, liquid water path, which counterbalances the expected cloud albedo increase of the polluted case. Estimates of cloud liquid water path are then compared with retrievals from the Special Sensor Microwave Imager (SSM/I). SSM/I-derived liquid water paths are in good agreement with the MODIS-derived values.

Retrieval of semitransparent ice cloud optical thickness from atmospheric infrared sounder (AIRS) measurements

An approach is developed to infer the optical thickness of semitransparent ice clouds (when optical thickness is less than 5) from Atmospheric Infrared Sounder (AIRS) high spectral resolution radiances. A fast cloud radiance model is developed and coupled with an AIRS clear-sky radiative transfer model for simulating AIRS radiances when ice clouds are present. Compared with more accurate calculations based on the discrete ordinates radiative transfer model, the accuracy of the fast cloud radiance model is within 0.5 K (root mean square) in terms of brightness temperature (BT) and runs three orders of magnitude faster. We investigate the sensitivity of AIRS spectral BTs and brightness temperature difference (BTD) values between pairs of wavenumbers to the cloud optical thickness. The spectral BTs for the atmospheric window channels within the region 1070-1135 cm/sup -1/ are sensitive to the ice cloud optical thickness, as is the BTD between 900.562 cm/sup -1/ (located in an atmospheric window) and 1558.692 cm/sup -1/ (located in a strong water vapor absorption band). Similarly, the BTD between a moderate absorption channel (1587.495 cm/sup -1/) and the strong water absorption channel (1558.692 cm/sup -1/) is sensitive to ice cloud optical thickness. Neither of the aforementioned BTDs is sensitive to the effective particle size. Thus, the optical thickness of semitransparent ice clouds can be retrieved reliably. We have developed a spectrum-based approach and a BTD-based method to retrieve the optical thickness of semitransparent ice clouds. The present retrieval methods are applied to a granule of AIRS data. The ice cloud optical thicknesses derived from the AIRS measurements are compared with those retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) 1.38and 0.645-/spl mu/m bands. The optical thicknesses inferred from the MODIS measurements are collocated and degraded to the AIRS spatial resolution. Results from the MODIS and AIRS retrievals are in reasonable agreement over a wide range of optical thicknesses.

Remote Sensing of Liquid Water and Ice Cloud Optical Thickness and Effective Radius in the Arctic: Application of Airborne Multispectral MAS Data

A multispectral scanning spectrometer was used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds, sea ice, snow, and tundra surfaces at 50 discrete wavelengths between 0.47 and 14.0 μm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) Arctic Clouds Experiment, conducted over a 1600 km × 500 km region of the north slope of Alaska and surrounding Beaufort and Chukchi Seas between 18 May and 6 June 1998. Multispectral images in eight distinct bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) were used to derive a confidence in clear sky (or alternatively the probability of cloud) over five different ecosystems. Based on the results of individual tests run as part of this cloud mask, an algorithm was developed to estimate the phase of the clouds (liquid water, ice, or undetermined phase). Finally, the cloud optical thickness and effective radius were derived for both water and ice clouds that were detected during one flight line on 4 June. This analysis shows that the cloud mask developed for operational use on MODIS, and tested using MAS data in Alaska, is quite capable of distinguishing clouds from bright sea ice surfaces during daytime conditions in the high Arctic. Results of individual tests, however, make it difficult to distinguish ice clouds over snow and sea ice surfaces, so additional tests were added to enhance the confidence in the thermodynamic phase of clouds over the Chukchi Sea. The cloud optical thickness and effective radius retrievals used three distinct bands of the MAS, with a recently developed 1.62- and 2.13-μm-band algorithm being used quite successfully over snow and sea ice surfaces. These results are contrasted with a MODIS-based algorithm that relies on spectral reflectance at 0.87 and 2.13 μm.

Retrieval of microphysical, geometrical, and radiative properties of marine stratocumulus from remote sensing

Radiative transfer calculations in boundary layer stratocumulus are performed and used to retrieve cloud properties from remotely measured reflected radiances in the visible and near‐infrared. Calculations based on the vertically uniform plane‐parallel cloud model have been currently used for the retrieval of cloud optical thickness and droplet effective radius. They were extended in a previous work to the retrieval of cloud droplet number concentration and cloud geometrical thickness, by assuming a vertically stratified cloud model, with prescribed droplet spectrum. The retrieved values of cloud droplet number concentration though were substantially underestimated. Improvements are presented here with a more realistic parameterization of the droplet size distribution, based on the theory of droplet growth by vapor diffusion. The retrieval technique is applied to eight case studies of the second Aerosol Characterization Experiment and the retrieved values are compared to estimates derived from in situ cloud microphysical measurements. The technique provides accurate retrievals of cloud droplet number concentration, with no bias. The retrieval of cloud geometrical thickness though is systematically overestimated compared to that directly measured. This bias partly reflects the difference of cloud sampling between the two strategies. Remote sensing measurements, which are continuous, are capable of detecting the thickest cells of the cloud layer within the field of view of the radiometer, while discontinuous in situ vertical profiles provide a statistical average of the geometrical thickness. Part of the bias could also reflect three‐dimensional effects of the radiative transfer in spatially heterogeneous clouds, which are not accounted for by the plane‐parallel models. The data are then processed for deriving statistics of the retrieved values of cloud droplet number concentration, optical thickness, and liquid water path, for validation of large‐scale parameterizations of the aerosol indirect effect.

Effects of land use in Southwest Australia: 1. Observations of cumulus cloudiness and energy fluxes

The Southwest Australian region has large homogeneous tracts of differing vegetation types separated by a sharp transition called the vermin or bunny fence which runs for almost 750 km. Seasonal winter agriculture is found to the west of the fence, whereas to the east native perennial vegetation grows. Geostationary Meteorological Satellite‐5 imagery are used to construct monthly cumulus cloud frequency of occurrence maps for the region 0800 to 1500 LT in hourly increments for 1999 and 2000. Moderate Resolution Imaging Spectroradiometer (MODIS) imagery are used to retrieve regional values of surface temperature, albedo, Normalized Difference Vegetation Index, fractional soil moisture availability, sensible and latent heat fluxes. High spatial resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery are used to retrieve detailed values along the fence. MODIS imagery also is utilized to retrieve cloud optical thickness, droplet sizes, and liquid water paths. This study shows that higher soil moisture availability is found over agricultural areas during winter (September) and over native vegetation areas during summer (December). Latent heat fluxes are higher over native vegetation than over agricultural areas during summer, while sensible heat fluxes are lower. Cumulus clouds occur with higher frequency and have higher optical thicknesses, cloud liquid water contents, and effective radii over agricultural areas during the winter and over native perennial vegetation during the dry summer. This is due to higher latent heat fluxes and available energy over agriculture during winter and over native vegetation during summer. We conclude that land use differences result in differences in available soil moisture and surface energy fluxes, which in turn lead to the observed preferential enhancement of cumulus cloudiness and cumulus cloud properties.

Observations of Three-Dimensional Radiative Effects that Influence MODIS Cloud Optical Thickness Retrievals

Abstract When cloud properties are retrieved from satellite observations, current calculations apply one-dimensional (1D) theory to the three-dimensional (3D) world: they consider only vertical processes and ignore horizontal interactions. This paper proposes a novel approach that estimates 3D effects in cloud optical thickness retrievals. The proposed method combines visible and thermal infrared images to see whether 3D radiative effects make clouds appear asymmetric—that is, whether cloud surfaces tilted toward the sun are systematically brighter than surfaces tilted away from it. The observed asymmetries are then used to estimate 3D effects for 1-km-size pixels as well as 50-km-size areas. Initial results obtained for Moderate-Resolution Imaging Spectroradiometer (MODIS) images reveal that 3D effects cause abundant uncertainties in the 1-km-resolution 1D retrievals. Averaging over 50 km by 50 km areas greatly reduces the errors but does not remove them completely. Conservative estimates show that the m...

A solar reflectance method for retrieving the optical thickness and droplet size of liquid water clouds over snow and ice surfaces

Cloud optical thickness and droplet effective radius retrievals from solar reflectance measurements are traditionally implemented using a combination of spectral channels that are absorbing and nonabsorbing for water particles. Reflectances in nonabsorbing channels (e.g., 0.67, 0.86, 1.2 μm spectral window bands) are largely dependent on cloud optical thickness, while longer‐wavelength absorbing channels (1.6, 2.1, and 3.7 μm window bands) provide cloud particle size information. Cloud retrievals over ice and snow surfaces present serious difficulties. At the shorter wavelengths, ice is bright and highly variable, both characteristics acting to significantly increase cloud retrieval uncertainty. In contrast, reflectances at the longer wavelengths are relatively small and may be comparable to that of dark open water. A modification to the traditional cloud retrieval technique is presented. The new algorithm uses only a combination of absorbing spectral channels for which the snow/ice albedo is relatively small. Using this approach, retrievals have been made with the MODIS airborne simulator (MAS) imager flown aboard the high‐altitude NASA ER‐2 from May to June 1998 during the Arctic FIRE‐ACE field deployment. Data from several coordinated ER‐2 and in situ University of Washington Convair‐580 aircraft observations of liquid water stratus clouds are examined. MAS retrievals of optical thickness, droplet effective radius, and liquid water path are shown to be in good agreement with in situ measurements. The initial success of the technique has implications for future operational satellite cloud retrieval algorithms in polar and wintertime regions.

Microphysical and radiative properties of stratocumulus clouds: the EUCREX mission 206 case study

In this conclusion paper, remote sensing retrievals of cloud optical thickness performed during the EUCREX mission 206 are analyzed. The comparison with estimates derived from in situ measurements demonstrates that the adiabatic model of cloud microphysics is more realistic than the vertically uniform plane parallel model (VUPPM) for parameterization of optical thickness. The analysis of the frequency distributions of optical thickness in the cloud layer then shows that the adiabatic model provides a good prediction when the cloud layer is thick and homogeneous, while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect.

Cloud optical thickness and albedo retrievals from bidirectional reflectance measurements of POLDER instruments during ACE-2

The POLDER instrument is devoted to global observations of the solar radiation reflected by the Earth–atmosphere system. The airborne version of the instrument was operated during the ACE-2 experiment, more particularly as a component of the CLOUDYCOLUMN project of ACE-2 that was conducted in summer 1997 over the subtropical northeastern Atlantic ocean. CLOUDYCOLUMN is a coordinated project specifically dedicated to the study of the indirect effect of aerosols. In this context, the airborne POLDER was assigned to remote measurements of the cloud optical and radiative properties, namely the cloud optical thickness and the cloud albedo. This paper presents the retrievals of those 2 cloud parameters for 2 golden days of the campaign 26 June and 9 July 1997. Coincident spaceborne ADEOS-POLDER data from 2 orbits over the ACE-2 area on 26 June are also analyzed. 26 June corresponds to a pure air marine case and 9 July is a polluted air case. The multidirectional viewing capability of airborne POLDER is here demonstrated to be very useful to estimate the effective radius of cloud droplet that characterizes the observed stratocumulus clouds. A 12 μm cloud droplet size distribution appears to be a suitable cloud droplet model in the pure marine cloud case study. For the polluted case the mean retrieved effective droplet radius is of the order of 6–10 μm. This only preliminary result can be interpreted as a confirmation of the indirect effect of aerosols. It is consistent with the significant increase in droplet concentration measured in polluted marine clouds compared to clean marine ones. Further investigations and comparisons to in-situ microphysical measurements are now needed.

First results of the POLDER "Earth Radiation Budget and Clouds" operational algorithm

The POLDER instrument is devoted to global observations of the solar radiation reflected by the Earth-atmosphere system. Algorithms of the "Earth Radiation Budget and Clouds" processing line implemented at the French Space Center are applied to ADEOS-POLDER data. First results on derived cloud properties are presented from POLDER level 2 data of 10 November 1996 and level 3 products of June 1997. A good correlation is observed between the POLDER cloud detection algorithm and the Dynamical Clustering Method applied to METEOSAT data. The multidirectional capability of POLDER appears useful to check schemes of cloud optical thickness retrieval. As expected, a water droplet model is suitable for liquid water clouds and inadequate for ice clouds. That indirectly validates the algorithm of cloud phase recognition. An apparent pressure is derived from O/sub 2/-absorption measurements and a Rayleigh cloud pressure from polarization observations. For overcast conditions, the apparent pressure is larger (by more than 100 hPa) than the Rayleigh pressure chiefly due to the photon penetration effect. For partly cloudy conditions, it can be larger or weaker depending on the surface reflectivity. Preliminary comparisons between POLDER and ISCCP monthly mean products outline some differences resulting in part from the original characteristics of POLDER.