Estimates of cloud vertical structure and water amount over tropical oceans using VIRS and TMI data

A microwave, visible and infrared (MVI) method was applied to coincident, collocated Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and VIRS (Visible and Infrared Scanner) data collected from January to August 1998 to estimate the cloud vertical structure and water path over tropical oceans. The derived quantities include the liquid water path (LWP), ice water path (IWP), total water path (WP), the frequencies of single‐layer and overlapping nonprecipitating clouds, and the cloud top‐height, base‐height and thickness for single‐layered water clouds. LWP data retrieved from TMI measurements were compared with global and regional LWP retrieved from VIRS for warm nonprecipitating clouds for different seasons. The global mean difference between TMI LWP and VIRS LWP is less than 0.01 mm and varies with season. For cold clouds, the TMI LWP is only 25–30% of the VIRS WP. In these cases, the ice clouds observed by VIRS overlap liquid water clouds. This result is similar to that from an earlier study, however, the well‐matched TMI and VIRS data provide more accurate instantaneous estimates than were previously possible. The assumption, that the cloud is entirely ice, used to compute IWP introduces some errors in the WP because of cloud overlapping. However, the difference between IWP estimated for all TMI FOVs containing only ice‐phase VIRS pixels and the IWP estimated from the difference between the VIRS WP and TMI LWP is less than 10%. This result suggests that IWP is independent of the lower‐level clouds for nonprecipitating systems. For warm single‐layered clouds, the estimated low‐cloud‐base altitudes (∼800m) and thicknesses (between 800 m to 1000 m) are, on average, very close to low‐cloud results derived from combined surface and upper‐air observations. The frequency of overlapped nonprecipitating clouds over ocean was about 4%, 16% and 38% for low, middle, and high clouds, respectively. The results clearly demonstrate the potential for enhanced monitoring of maritime clouds using infrared, solar, and microwave imagers on the same platform.