Cloud liquid water path comparisons from passive microwave and solar reflectance satellite measurements: Assessment of sub‐field‐of‐view cloud effects in microwave retrievals

Abstract

Satellite observations of the cloud liquid water path (LWP) are compared from special sensor microwave imager (SSM/I) measurements and GOES 8 imager solar reflectance (SR) measurements to ascertain the impact of sub‐field‐of‐view (FOV) cloud effects on SSM/I 37 GHz retrievals. The SR retrievals also incorporate estimates of the cloud droplet effective radius derived from the GOES 8 3.9‐μm channel. The comparisons consist of simultaneous collocated and full‐resolution measurements and are limited to nonprecipitating marine stratocumulus in the eastern Pacific for two days in October 1995. The retrievals from these independent methods are consistent for overcast SSM/I FOVs, with RMS differences as low as 0.030 kg m−2, although biases exist for clouds with more open spatial structure, where the RMS differences increase to 0.039 kg m−2. For broken cloudiness within the SSM/I FOV the average beam‐filling error (BFE) in the microwave retrievals is found to be about 22% (average cloud amount of 73%). This systematic error is comparable with the average random errors in the microwave retrievals. However, even larger BFEs can be expected for individual FOVs and for regions with less cloudiness. By scaling the microwave retrievals by the cloud amount within the FOV, the systematic BFE can be significantly reduced but with increased RMS differences of 0.046–0.058 kg m−2 when compared to the SR retrievals. The beam‐filling effects reported here are significant and are expected to impact directly upon studies that use instantaneous SSM/I measurements of cloud LWP, such as cloud classification studies and validation studies involving surface‐based or in situ data.