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

Abstract

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.

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

“…The origin of this systematic bias is a known characteristic of single-band optical thickness retrievals and is clearly demonstrated in Fig. 1 of Marshak et al (2006). The convexity of a single-band LUT curve produces low-biased retrievals for symmetrically distributed (or averaged) reflectances.…”

Impact of cloud horizontal inhomogeneity and directional sampling on the retrieval of cloud droplet size by the POLDER instrument

“…The effects of cloud horizontal homogeneity also make the retrieval more complex. Marshak et al (2006) found that ignoring the cloud variability at the sub-pixel scale results in underestimates of the CDR, while ignoring cloud inhomogeneity at scales exceeding the pixel scale can lead to overestimates. It is found that the vertical structure induced by drizzle and 3-D radiative effects operate together to cause dramatic differences between the 1.6, 2.1, and 3.7 µm retrievals (Zhang et al, 2012;Zhang, 2013;Nakajima et al, 2010a, b;Nagao et al, 2013).…”

Characteristics of cloud liquid water path from SEVIRI onboard the Meteosat Second Generation 2 satellite for several cloud types

“…Both effects tend to compensate each other when deriving LWP. But the magnitude of the effect depends strongly on the geometric formation of the cloud fields and the viewing conditions (see Marshak et al, 2006;Iwabuchi and Hayasaka, 2002;Barker et al, 1999). Therefore most studies consider the average effect of a small sample of test cases or have to limit the studies with certain constraints (e.g.…”

“…Schutgens and Roebeling (2009) limited their study to cloud fields that were at least 25 km contiguous but not patched fields). Marshak et al (2006) showed, that the sign of the differences in effective radius and cloud optical thickness retrieval depends on the cloud's structure and the viewing conditions (i.e. mainly the sun zenith angle and can be either positive or negative).…”