Surface Rainfall–Cold Cloud Fractional Coverage Relationship in TOGA COARE: A Function of Vertical Wind Shear

Thomas R. Saxen, Steven A. Rutledge
2000 Monthly Weather Review  
Shipboard radar-derived rain rates and satellite-observed IR brightness temperatures have been used to examine the relationship between cold cloud fractional coverage for brightness temperatures Ͻ235 K and areally averaged surface rainfall during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). A nearly linear relationship was observed with a ratio of mean rain rate to fraction cold cloud coverage of approximately 1 mm h Ϫ1 . This is in contrast to
more » ... the GOES precipitation index (GPI) methodology, which assumes a proportionality (GPI slope coefficient) of 3 mm h Ϫ1 . It was also observed that when considering 5-day timescales, the relationship between the cold cloud fractional coverage and surface rainfall exhibited considerable variability. This variability was in phase with the interseasonal oscillations (ISOs). During the convectively active phase of the ISOs, the deep vertical wind shear (700-150 mb) was strong and the convective organization was dominated by squall line type systems. Hence, the cold cloud fractional coverage tended to be greater than the area of rainfall, a large fraction of which was stratiform in nature. The GPI slope coefficient was typically less than 1 mm h Ϫ1 during these periods. During the suppressed phase of the ISOs, tropospheric shear was much weaker and the convective organization consisted primarily of isolated convective cells. The cold cloud fractional coverage was typically about equal to the raining area, which was split nearly evenly between convective and stratiform precipitation. This resulted in a significant increase in the GPI slope coefficient, typically greater than 2 mm h Ϫ1 during these suppressed phases. It is also shown that during COARE, the variability in the GPI slope coefficient can to a large extent be explained by variations in tropospheric wind shear.
doi:10.1175/1520-0493(2000)128<0407:srccfc>;2 fatcat:opcjew26qzeahdladefxlar444