Broken and inhomogeneous cloud impact on satellite cloud particle effective radius and cloud‐phase retrievals

The impact of sensor resolution on satellite‐derived cloud particle effective radius ( r e ) and cloud phase (CPH) for broken and overcast inhomogeneous clouds is investigated for the Cloud Physical Properties (CPP) retrieval algorithm used by the Climate Monitoring Satellite Application Facility. First, synthetic data sets of high‐resolution (1 × 1 km 2 ) and low‐resolution (3 × 3 km 2 ) radiances are used to illustrate the effect on the r e and cloud top temperature (CTT) retrieval, the cloud properties that are used for the CPH retrieval. It is shown that low‐resolution r e can be overestimated by up to 12 μ m and CTT by up to 20 K for thick broken and inhomogeneous overcast water clouds over ocean and land surfaces. The overestimation of r e may cause erroneous assignments of “ice” to water clouds. Second, 2 months of CPP retrievals on Moderate Resolution Imaging Spectroradiometer (MODIS) radiances are used to quantify the effect on r e and CPH over the Atlantic Ocean and central Europe. Over both areas, the low‐resolution r e is overestimated by up to +5 μ m for broken and up to 2 μ m for inhomogeneous overcast clouds. At low resolution, the fraction of water clouds is underestimated by 2.3% over the Atlantic Ocean and 0.6% over central Europe. The increase of CTT partly compensates for the increase in r e in the CPH retrievals at low resolution. If no CTT information were used, the underestimation of the water cloud fraction would be 3.5% and 2.2% for the Atlantic Ocean and central Europe, respectively. For inhomogeneous overcast clouds integrated over all inhomogeneity classes, the difference is −1.3% and −2.3% for central Europe and the Atlantic Ocean, respectively. Our results indicate that (1) the retrieval of r e in the CPP algorithm is sensitive to satellite sensor resolution in case of broken clouds and inhomogeneous overcast clouds and (2) despite this large r e sensitivity the CPH retrieval is much less sensitive to sensor resolution.