Validation of Satellite Retrievals of Cloud Microphysics and Liquid Water Path Using Observations from FIRE

Abstract

Cloud effective radii (re) and cloud liquid water path (LWP) are derived from ISCCP spatially sampled satellite data and validated with ground-based pyranometer and microwave radiometer measurements taken on San Nicolas Island during the 1987 FIRE IFO. Values of re derived from the ISCCP data are also compared to values retrieved by a hybrid method that uses the combination of LWP derived from microwave measurement and optical thickness derived from GOES data. The results show that there is significant variability in cloud properties over a 100 km?80 km area and that the values at San Nicolas Island are not necessarily representative of the surrounding cloud field. On the other hand, even though there were large spatial variations in optical depth, the re values remained relatively constant (with σ≤2?3 µm in most cases) in the marine stratocumulus. Furthermore, values of re derived from the upper portion of the cloud generally are representative of the entire stratiform cloud. When LWP values are less than 100 g m?2 then LWP values derived from ISCCP data agree well with those values estimated from ground-based microwave measurements. In most cases LWP differences were less than 20 g m?2. However, when LWP values become large (e.g., ≥200 g m?2), then relative differences may be as large as 50%?100%. There are two reasons for this discrepancy in the large LWP clouds: 1) larger vertical inhomogeneities in precipitating clouds and 2) sampling errors on days of high spatial variability of cloud optical thicknesses. Variations of re in stratiform clouds may indicate drizzle: clouds with droplet sizes larger than 15 µm appear to be associated with drizzling, while those less than 10 µm are indicative of nonprecipitating clouds. Differences in re values between the GOES and ISCCP datasets are found to be 0.16±0.98 µm.