To answer these questions we have examined three years (1998-2000) of Tropical Rainfall Measurement Mission (TRMM) Lightning Imaging Sensor (LIS) and Precipitation Radar (PR) data. PR reflectivity pixels were processed to provide ice water paths (IWP) over the global tropics by vertically integrating ice water contents upward from the altitude of the -10 C level (adjusted locally using NCEP Reanalysis data) to echo top. Ice water contents were computed for each PR pixel using exponential size distributions and particle ice densities adjusted as a function of precipitation type (convective or stratiform) and reflectivity magnitude. Individual pixel-level IWPs were subsequently averaged and gridded at a resolution of 0.5 x 0.5 degrees. LIS lightning flash densities (flashes/km^2/day) for pixels coincidentally observed by the PR were also computed and gridded at a resolution of 0.5 x 0.5 degrees. Finally, a statistical database of PR convective structure types was used to determine modes of regional convective variability as they relate to changes in the IWP-lightning relationship.
Preliminary results for the N. Hemisphere warm-season months of June-August, globally averaged over land, ocean and coastal regimes, suggest that the correlation between lightning and IWP is largest over land and neighboring coastal regions (R=0.7), where lightning flash rates and IWPs are the largest. The relative dearth of lightning observed over the tropical oceans results in a lower, but still statistically significant correlation of R=0.5. Importantly, while scatter about the linear least squares fit between IWP and flash rate is larger over the oceans than that of the land and coastal regimes, the basic linear relationships computed (e.g., slopes and intercepts) between these basic regime types are very similar. At first glance this suggests that the basic first order physics of non-inductive collision-based cloud charging and mixed phase precipitation development may be dominant enough to enable universal application of lightning data for retrieving precipitation ice water content- at least where lightning is occurring. However (and not-unexpectedly), there is also some spatial “fine structure” in the lightning-IWP relationship; this is especially evident over specific regions of each continent. A quick examination suggests that this fine structure is due to the systematic behavior of a few statistically partitioned and discrete convective system types which preferentially do or do not occur in each region. These results will be significantly expanded upon at the conference.