Cloud drop effective radius for trade wind cumuli observed during RICO by aircraft and MODIS

Moderate Resolution Imaging Spectroradiometer (MODIS);effective radius;liquid water content;Rain In shallow Cumulus over the Ocean (RICO);aircraft

Haney, Conor ; Di Girolamo ; Larry

关键词: Moderate Resolution Imaging Spectroradiometer (MODIS);    effective radius;    liquid water content;    Rain In shallow Cumulus over the Ocean (RICO);    aircraft;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/44474/Conor_Haney.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Warm, tropical boundary clouds represent one of the biggest sources of uncertainty in climate sensitivity to anthropogenic forcing. Information useful for gaining a better understanding about such clouds is relatively plentiful from satellites. But warm tropical clouds, such as those found in the trades, are often horizontally heterogeneous over the pixel scales of meteorological satellite imagers; the clouds may even be sub-pixel in size. Due to the 1-D nature of the radiative transfer calculations used in retrieving cloud drop effective radius and cloud optical depth from satellites, systematic errors in the satellite retrievals can be introduced due to the 3-D radiative transfer through heterogeneous clouds found in nature. To date, validation for these satellite retrievals of cloud properties have been for stratiform clouds, not for cumulus. Here, we use in-situ aircraft measurements from RICO (“Rain In shallow Cumulus over the Ocean”) for these cumuli, and investigate how they compare to satellite retrievals from MODIS (“Moderate Resolution Imaging Spectroradiometer”). In doing so, the vertical structure of the cloud drop effective radius and liquid water content of these trade-wind cumuli are also investigated from the FSSP-100 and 2D-C cloud probes aboard the C-130 aircraft flown during RICO.The constructed profiles show a general increase in cloud drop effective radius with increasing height up to around 2000 m altitude, followed by a decrease with height beyond that point, though with less samples. The liquid water content profile shows similar behavior. When using the aircraft data to compare to MODIS 2.1 μm band retrievals, the FSSP re mean is 10.8 μm, much smaller than the MODIS Terra mean re(2.1) of 23.6 μm and Aqua mean re(2.1) of 20.7 μm. The MODIS 3.7 μm band shows an re distribution closer to that of the aircraft, where the Terra mean re(3.7) is 14.1 μm and the Aqua mean re(3.7) is 15.0 μm. Analysis of the spatialiiiheterogeneity metric, Hσ, shows that the difference between the 2.1 μm and 3.7 μm band MODIS re retrievals show an increasingly large disparity as Hσ increases, where re(2.1) is much larger than re(3.7). The MODIS 2.1 μm band retrievals for more homogeneous clouds have a lower mean re of 17.7 μm compared to heterogeneous retrievals, while cloud optical depth retrievals for homogeneous clouds have a higher mean τ of 14.4, compared to 5.7 for heterogeneous clouds. This overestimation of effective radius and underestimation of optical depth is in line with what is expected when 3-D radiative transfer effects play a major role. The results of this study reinforce the fact that analysts should use caution when using such satellite retrievals.