Over-ocean validation of the Global Convective Diagnostic

The Global Convective Diagnostic (GCD), a binary index of deep convection, operates on window infrared and water vapor bands of a geostationary satellite image. It is aimed at helping to meet the need of the aviation community for current information on hazardous deep convection. In this paper we describe a test of GCD performance over the tropical and sub-tropical northwestern Atlantic and northeastern Pacific Oceans. As ground truth, the test uses Height of Storm (HS) measurements from the Precipitation Radar (PR) on the Tropical Rainfall Measuring Mission. For a six-week period beginning in mid-May 2003, passes of the PR were matched in time and space to images from GOES-12. In eleven of the more-than-50 matches the time-space window of coincidence framed all or part of a convective system. Systems ranged from cloud clusters through easterly waves to squall lines. The subset of these systems so far analyzed yields the following results. First, performance varies considerably from system to system. Second, the GCD tends to perform better if the threshold temperature difference (infrared minus water vapor) is set to roughly zero (rather than plus one). Third, at this threshold the GCD quite consistently outperforms a single-threshold (215 K), infrared-only index. Fourth, the GCD falters in the discrimination of growing towers.