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

Zinner, Tobias and Wind, G. and Platnick, S. and Ackerman, A.S. (2010) Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals. Atmospheric Chemistry and Physics, 10, pp. 9535-9549. DOI: 10.5194/acp-10-9535-2010.

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Official URL: http://www.atmos-chem-phys.net/10/9535/2010/

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 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.

Document Type:Article
Title:Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals
Authors:
AuthorsInstitution or Email of Authors
Zinner, TobiasDLR
Wind, G.NASA Goddard Space Flight Center, Greenbelt, USA
Platnick, S.NASA Goddard Space Flight Center, Greenbelt, USA
Ackerman, A.S.NASA Goddard Inst. for Space Studies, New York, USA
Date:2010
Journal or Publication Title:Atmospheric Chemistry and Physics
Refereed publication:Yes
In Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:10
DOI:10.5194/acp-10-9535-2010
Page Range:pp. 9535-9549
Status:Published
Keywords:drizzle, retrieval, effective radius
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EO - Erdbeobachtung
DLR - Research area:Space
DLR - Program:W EO - Erdbeobachtung
DLR - Research theme (Project):W - Vorhaben Atmosphären- und Klimaforschung (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Atmospheric Remote Sensing
Deposited By: Jana Freund
Deposited On:25 Oct 2010 18:04
Last Modified:12 Dec 2013 21:03

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