Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals

Abstract

Abstract. Remote sensing of cloud effective particle size with passive sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) is an important tool for cloud microphysical studies. As a measure of the radiatively relevant droplet size, effective radius can be retrieved with different combinations of visible through shortwave and midwave infrared channels. In practice, retrieved effective radii from these combinations can be quite different. This difference is perhaps indicative of different penetration depths and path lengths for the spectral reflectances used. In addition, operational liquid water cloud retrievals are based on the assumption of a relatively narrow distribution of droplet sizes; the role of larger precipitation particles in these distributions is neglected. Therefore, possible explanations for the discrepancy in some MODIS spectral size retrievals could include 3-D radiative transport effects, including sub-pixel cloud inhomogeneity, and/or the impact of drizzle formation. For three cloud cases the possible factors of influence are isolated and investigated in detail by the use of simulated cloud scenes and synthetic satellite data: marine boundary layer cloud scenes from large eddy simulations (LES) with detailed microphysics are combined with Monte Carlo radiative transfer calculations that explicitly account for the detailed droplet size distributions as well as 3-D radiative transfer to simulate MODIS observations. The operational MODIS optical thickness and effective radius retrieval algorithm is applied to these and the results are compared to the given LES microphysics.We investigate two types of marine cloud situations each with and without drizzle from LES simulations: (1) a typical Correspondence to: T. Zinner (tobias.zinner@lmu.de) daytime stratocumulus deck at two times in the diurnal cycle and (2) one scene with scattered cumulus. Only small impact of drizzle formation on the retrieved domain average and on the differences between the three effective radius retrievals is noticed for both cloud scene types for different reasons. For our, presumably typical, overcast stratocumulus scenes with an optical thickness of 8 to 9 and rain rates at cloud bottom up to 0.05 mm/h clear drizzle impact on the retrievals can be excluded. The cumulus scene does not show much drizzle sensitivity either despite extended drizzle areas being directly visible from above (locally >1 mm/h), which is mainly due to technical characteristics of the standard retrieval approach. 3-D effects, on the other hand, produce large discrepancies between the 1.6 and 2.1 µm channel observations compared to 3.7 µm retrievals in the latter case. A general sensitivity of MODIS particle size data to drizzle formation is not corroborated by our case studies.

Evaluating MODIS cloud retrievals with in situ observations from VOCALS-REx

“…This uses a wavelength channel (0.86 µm over ocean) which is dominated by scattering and experiences negligible absorption by liquid water. The scattering properties of non-absorbing wavelengths remain relatively unchanged with the introduction of a drizzle mode to the size distribution (Zinner et al, 2010) and the retrieval of optical depth is largely invariant to changes in the vertical profile of droplet size. It is therefore unexpected that drizzle should influence the retrieval of τ c on the scale suggested by figure 4.…”

“…They tested these concepts by using cloud fields generated by a large eddy model and 3-D radiative transfer simulations to generate realistic synthetic measurements on which to perform cloud retrievals. Zinner et al (2010) found that for overcast stratocumulus scenes, biases induced by introducing a drizzle mode and 3-D radiative effects were typically only of the order of a few tenths of a micron.…”

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

“…In addition, the LUT is calculated for a plane-parallel cloud field using a 1-D radiative transfer model that does not consider the influence of the 3-D structure of clouds. However, Horváth (2004) found that no more than 17 % of the pixels containing marine liquid water clouds are plane-parallel objects, suggesting that the retrieval error arising from the solar-viewing geometry cannot be neglected (Zinner et al, 2010;Wolters et al, 2010;VantHull et al, 2007;Di Girolamo et al, 2010). The effects of cloud horizontal homogeneity also make the retrieval more complex.…”

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

“…For example, the polarimetric and bispectral methods have different sensitivity to the cloud vertical profile, and at the same time they are also both affected by cloud horizontal inhomogeneity (Zinner et al, 2010;Zhang et al, 2012Zhang et al, , 2016Miller et al, 2016). It is difficult, if not impossible, to disentangle these factors based on observations alone.…”