Thunderstorms are a significant hazard to aircraft in flight. Over the United States, radar and remotely sensed lightning are the primary methods of detecting deep convective thunderstorms. However these detection methods do not extend significant distances over the oceans, and satellite based methods of detecting convection must be utilized. Traditional geostationary 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 the satellite sensors' inability to distinguish anvil cirrus from the active convection .

A new method has been developed to objectively determine areas of active convection using the temperature difference between the 11 micron IR channel and the 6.7 micron Water Vapor channel. The method relies on the assumption that in areas of active uplift, the two channels will have the same temperature since the deep convection will be at the top of the troposphere with no significant water vapor above the thunderstorm. In areas of a decaying or advecting cirrus anvil downstream, the two channels will have different temperatures. At the convective cloud top, cloud particles will evaporate and this water vapor will advect downwind, remaining at approximately the same level as the convective top. However, the ice crystals in the cirrus cloud will fall to a lower level such that the net result should be that the IR temperature would be slightly warmer than the water vapor temperature for cirrus downwind of active convection.

Since all the geostationary satellites around the globe have both infrared and water vapor channels, this technique can be applied globally. A global convective diagnostic is generated by first taking the temperature differences between the IR and Water Vapor channels, and then compositing the differences into a global composite. The diagnostic display shows the pixels where the IR-WV difference is less than or equal to 1 deg C. While convection is one of the primary methods to generate uplift at the top of the troposphere, other processes such as ageostrophic motions around jets and/or warm advection can also generate cirrusproducing cells causing false convective identifications. To eliminate these false identifications, the global composites are filtered for atmospheric stability using an AVN model stability parameter. Areas with a Lifted Index of +1 and greater are eliminated from the convective diagnostic. While this filtering eliminates the false cirrus, it sometimes eliminates convective areas with elevated convection. Other stability parameters are being investigated to improve this filtering.

Verification efforts have begun. The satellite based convective diagnostic is being verified over the contiguous 48 United States and adjacent coastal waters against the National Convective Weather Diagnostic (NCWD) which is generated from radar VIL data and lightning data over the United States (Magenhardt, et. al., 2000). Preliminary verification indicates a True Skill Statistic (TSS) of better than .4, but with a size bias of over 2 as compared to the NCWD.

References:

Megenhardt, Dan, C.K. Mueller, N. Rehak, and G. Cunning, 2000: Evaluation of the National Convective Weather Forecast Product. Preprints, 9th Conference on Aviation, Range, and Aerospace Meteorology, Orlando, FL. Amer. Meteor. Soc.,171-176.