Ship-based liquid water path estimates in marine stratocumulus

Abstract

We examine liquid water paths (LWPs) derived from ship-based microwave radiometer brightness temperature (Tb) measurements collected within southeastern Pacific stratocumulus at 20°S, 85°W in October 2001. The boundary layer was typically well mixed and overcast. Three gaseous absorption models and two liquid dielectric models are evaluated. Total differences in retrieved LWP attributable to microwave absorption model differences are 10-25 g m-2, increasing with LWP. The most recent models produce the lowest LWPs. Most of the differences in the retrieved LWPs are caused by differences in the gaseous absorption models. Liquid dielectric model differences generate LWP differences of ∼6% of the total LWP. Radiosonde-calculated Tb using the most recent gaseous absorption model compare best to Tb measurements. The remaining LWP uncertainty due to model uncertainty is estimated at 6 g m-2. The pre-1995 gaseous and liquid absorption models in combination produce LWPs that exceed the calculated adiabatic values. For the 6-day best estimate LWP time series, the clouds attained LWPs close to the theoretical adiabatic limit for LWPs up to 150 g m-2, decreasing to ∼85% for LWPs of ∼250 g m-2. Such deductions also depend upon how the cloud boundaries, to which the adiabatic calculation is sensitive, are determined. Light drizzle, as inferred from cloud radar reflectivitiy measurements, is common even at low LWPs, but heavy drizzle (radar reflectivities >0 dBZ, equivalent to a cloud base drizzle rate of ∼2 mm d-1) is much less frequent, occurring <10% of the time even for LWPs of 200 g m-2. Copyright 2005 by the American Geophysical Union.