Validation of Satellite-Derived Liquid Water Paths with Ground-Based Microwave Radiometers for Various Cloud Regimes

Liquid water path (LWP) of clouds is an important climatic variable, especially for use in GCM's (Global Climate Models). Satellites provide the advantage of widespread coverage of this and other cloud characteristics, while ground-based microwave radiometers (MWR) can only provide point measurements over small areas. To provide high temporal resolution coverage of large areas, the VISST (Visible Infrared Solar Split-Window Technique) is utilized to derive LWP from the suite of GOES satellites, providing coverage over the Pacific and Atlantic Oceans, and the continents of North America and Australia. These widespread areas are covered by clouds from many different continental and marine regimes, and confidently using one specific satellite retrieval technique to derive all of them requires validation in several regimes. VISST-determined LWP is from retrievals of cloud optical depth and droplet effective radius, based on matching visible reflectance and infrared radiance observations with theoretical model estimates of the same variables. This procedure uses an assumed droplet size distribution over a range of effective radii. To validate the derived LWP in different cloud regimes, comparisons are made to MWR retrievals of LWP from the Atmospheric Radiation Measurement (ARM) site at Pt. Reyes, California (mostly marine stratus), the convective regimes present over the Nauru, Manus, and Darwin ARM sites in the Tropical Western Pacific, midlatitude sites in Oklahoma, North Dakota, and France, and from ship observations in the Pacific Ocean. The sensitivity of the retrievals to the assumed droplet size distributions is also discussed.