Estimating the Regional Bias of MODIS-retrieved Cloud Droplet Effective Radius through MISR-MODIS Data Fusion

Listed as one of the Essential Climate Variables by the Global Climate Observing System, the effective radius (Re) of the cloud drop size distribution plays an important role in the energy and water cycles of the Earth system. Re is retrieved from several passive sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), based on a visible and near-infrared bi-spectral technique that had its foundation more than a quarter century ago. This technique makes a wide range of assumptions, including 1-D radiative transfer, assumed single-mode drop size distribution, and cloud horizontal and vertical homogeneity. It is well known that deviations from these assumptions lead to bias in the retrieved Re.

Recently, an effort to characterize the bias in MODIS-retrieved Re through MISR-MODIS data fusion revealed biases in the zonal-mean values of MODIS-retrieved Re that varied from 2 to 11 µm, depending on latitude (Liang et al., 2015). Here, in a push towards bias-correction of MODIS-retrieved Re, we further examine the bias with MISR-MODIS data fusion as it relates to other observed cloud properties, such as cloud horizontal heterogeneity, cloud optical depth, sun-view geometry, etc. Our results reveal that while Re bias do show a certain degree of dependence on some properties, no single property dominates the behavior in MODIS-retrieved Re bias. Through data stratification by observed cloud properties and latitude, we introduce a bias-correction approach for MODIS-retrieved Re at regional scales. We estimate the global distribution of bias-corrected MODIS-retrieved Re, and compare our results with previous studies on Re. While our results suggest that there are large differences between the original MODIS-retrieved Re and the bias-corrected Re, our bias estimates of MODIS-retrieved Re show good agreement with various field observations and model simulations. One curious finding seems to suggest that the vertical distribution of drop sizes for marine stratocumulus clouds are very different from other types of marine liquid water clouds.