SURFACE-BASED REMOTE-SENSING OF MARINE BOUNDARY-LAYER CLOUD PROPERTIES

Surface-based measurements are used to define some of the important ma crophysical and optical properties of marine clouds. These measurement s were taken during five different marine field programs. A progressio n is made from a midlatitude marine stratocumulus regime with an avera ge cloud fraction of 0.7 and a median cloud base of 460 m to a marine tropical regime with an average cloud fraction of 0.2 and a median clo ud base of 1050 m. Measurements of the solar transmission coefficient taken during the Atlantic Stratocumulus Transition Experiment (ASTEX) were used in a radiative transfer algorithm to produce values of albed o, absorption, and optical depth. A microwave radiometer provided meas urements of the liquid water path (LWP). For a given LWP, the ASTEX op tical depths averaged a factor of 2 smaller than the optical depths ob served during the marine stratocumulus phase of the First Internationa l Cloud Climatology Program Regional Experiment (FIRE) at San Nicolas Island, off the coast of southern California. The variability of bound ary-layer aerosol concentrations measured during ASTEX is sufficient t o produce a factor of 2 change in optical depth. Further evidence sugg ests that the cloud droplet effective radius was nearly a factor of 2 larger during ASTEX than during FIRE.