The Global Convective Diagnostic (GCD) is a technique originally developed as a tool for aviation interests to locate and avoid convective hazards. The technique uses a one degree C temperature difference between the infrared and water vapor channels on geostationary satellite images to identify active convection from cirrus outflow. The GCD assumes that areas of active convection reaching the tropopause will have the same temperature on the two channels. Downwind of active convection, there will be a slight difference between the infrared and water vapor temperature because passive, drifting clouds will have slight temperature difference since the ice crystalsof the clouds will be falling to lower, warmer levels, while the water vapor will remain at the original level. Mosher (2001) has shown that the GCD technique can be used to generate global mosaics of convective clouds because all the geostationary satellites have both the infrared and water vapor channels. There has been interest in the GCD as a monitoring tool for global convection for climate studies. For these applications the GCD's characteristics need to be more solidly established.

Verification studies of the GCD have been performed using radar and lightning data over the U.S. as ground truth. These verification studies have shown that the technique has skill in identification of convective areas, but that the identified areas are generally too larger. The GCD temperature difference threshold was established based on the fall of cirrus ice crystals to lower altitudes. Experiments are examining to determine the sensitivity of the GCD to varying channel temperature difference thresholds. Results will be presented showing how the skill of the GCD varies and improves with changes in channel temperature difference thresholds.

An additional question about the GCD has been how does it compare to traditional infrared temperature threshold techniques? Results will be presented showing comparison studies of the GCD and infrared threshold methods and how the GCD reduces the cirrus areas incorrectly identified as convection by temperature threshold techniques.

References

Mosher, Frederick R., 2001. A Satellite Diagnostic of Global Convection. Preprints, 11th Conference on Satellite Meteorology and Oceanography, Madison, Wi. Amer. Meteor. Soc. 416-419.