Heterogeneity Effects in Optical Cloud Liquid Water Path Retrievals

We investigated heterogeneity effects in plane-parallel optical cloud liquid water paths (CLWPs), comparing Moderate Resolution Imaging Spectroradiometer (MODIS) estimates with those from Advanced Microwave Scanning Radiometer (AMSR-E), which are less susceptible to such effects. Consistency of optical and microwave retrievals was analyzed in one year of observations as a function of scene heterogeneity and solar/view geometry using two different heterogeneity metrics, measuring either cloud top variability or horizontal liquid water variability. While both heterogeneity metrics showed similar geographical and seasonal variations, cloud top variability proved less effective in identifying scenes with significant 3D effects in quarter-degree domain mean CLWPs. Microwave and optical CLWPs showed consistently larger correlations and smaller rms differences for homogeneous than for heterogeneous scenes. Zonal means were in relatively good agreement in the summer hemisphere; however, in the winter hemisphere, MODIS CLWPs sharply increased toward the poles in contrast to AMSR-E. This discrepancy was mainly due to 3D effects in MODIS retrievals over heterogeneous scenes at large solar zenith angles. Microwave retrievals in all scenes, and optical retrievals in homogeneous scenes showed similar and small solar zenith angle dependence, and were remarkably consistent at all view directions. In heterogeneous scenes at low sun, however, MODIS CLWPs increased by 40% on average, and were also considerably larger for oblique views than for overhead views due to increased viewing of illuminated cloud sides. We note that the heterogeneity parameters themselves also exhibited similar U-shaped view angle dependence. The results presented may help create more consistent microwave and optical CLWP climatologies.