Potential for estimating cloud liquid water path over sea ice from airborne passive microwave measurements

In this paper we investigate the feasibility of determining cloud liquid water path from passive microwave measurements over sea ice. Simulations using a 32‐stream plane‐parallel microwave radiative transfer model indicate a consistent increase in brightness temperature attributable to cloud liquid water for conditions observed in the Arctic during the Surface Heat Budget of the Arctic (SHEBA) experiment. Uncertainties in brightness temperature simulations due to variations in surface emissivity, surface temperature, cloud temperature, and atmospheric water vapor are investigated. Surface emissivity variations are found to cause the largest uncertainties in top‐of‐atmosphere brightness temperature over the range of liquid water paths examined. An algorithm previously developed for estimating liquid water path over oceans from SSM/I data is adapted for retrievals over sea ice. The algorithm is applied to brightness temperature measurements from airborne microwave radiometers. Mean retrieved liquid water path (LWP) is compared to in situ measurements from airborne cloud microphysical probes. The correlation coefficient for the two data sets is found to be 0.989 with an RMS error of 14 g m −2 , although the retrieved LWP values show a high degree of variability. Accuracy is highest when LWP values are 100 g m −2 and above. Poorest accuracies are obtained for small LWP (<50 g m −2 ) where errors in the specification of surface emissivity have a large effect. These case studies provide evidence that liquid water clouds over sea ice produce detectable changes in microwave brightness temperature and suggest that interpretation of sea ice properties using certain microwave frequencies may be affected by the presence of liquid water clouds.