The Aviation Weather Center (AWC) generates High Level Significant Weather forecasts covering 2/3 of the globe which include areas of convective activity that will affect jet aircraft. Verification of these forecasts requires an objective method for global monitoring of convective activity. In addition a real time diagnostic of convective activity would be of great benefit to aircraft routing decisions. Traditional methods of automated convective cloud diagnostics have utilized the brightness and texture signatures of the various satellite bands. However these methods tend to overestimate the active areas of convection because of inability to distinguish cirrus from the thunderstorm's core. This problem is especially difficult during the night when only the 11 micron infrared channel is used.

A method to objectively determine areas of active convection using the temperature difference between the 11 micron IR channel and the 6.7 Water Vapor channel has been developed. These channels are available on all the geostationary satellites, and are available in global composite images (Mosher, 1996). The method relies on the assumption that in areas of active uplift the two channels will have the same temperature, but in areas of decaying or advecting cirrus the two channels will have different temperatures because the ice crystals will advect downwind of convective areas and evaporate. Since the ice crystals will have a fall velocity as they advect and the evaporated moisture will not, the net result should be that the IR temperature would be slightly warmer than the water vapor temperature for mature or decaying cirrus.

Global IR and Water Vapor data sets have been subtracted to create this satellite convective diagnostic along with several filtering processes to remove false identifications. In areas with little initial moisture, such as high latitude areas, the temperature difference is small, causing false identifications. Use of a filter of best lifted index more stable than +5 degree C from the AVN global model has eliminated these areas. Areas of cirrus generating cells associated with the subtropical jet are filtered through the use of AVN 250 hPa warm advection without adversely impacting the detection of convective cells. Areas of embedded convection in synoptic storms produce false identifications. These areas do not have lightning, but do have some radar echoes. At the moment these areas have not been removed from the product. Another area needing attention is a correction for limb darkening. Since limb darkening is slightly different for the two channels, the difference of the two channels has a bias toward the limbs of the earth's disk. A limb darkening correction will need to be applied to the global composite processing before the diagnostic can be used operationally for global convection detection. Preliminary results show promise in determining areas of active deep convection. Another area of future research is to investigate if the filtered non-thunderstorm uplift areas may be related to upper level in-cloud turbulence encounters by aircraft.

References: Mosher, Frederick R., 1996: Use of Derived Products from GOES-8 Data at the AWC. Preprints of the 8th Conference on Satellite Meteorology, Atlanta, Georgia. Amer. Meteor. Soc. P1.18.