A climatological study of ice cloud reflectivity

The utility of GOES channels 1 (visible), 3 (water vapor), and 4 (longwave infrared) has been well documented for a variety of applications. However, few studies have addressed the application of the shortwave infrared channel 2 (3.9 ìm) to storm top analysis. During the daytime, channel 2 has an emitted thermal component, but for cold thunderstorm tops, a reflected solar component makes a significant contribution to the total radiance. The amount of shortwave reflectance depends on the microphysical structure at cloud top; smaller ice crystals tend to reflect more incoming radiation. We therefore can make inferences about cloud-top microphysical structure based on channel 2 albedo values.

This study looks at the long-term distribution of cold cloud 3.9 ìm albedo values over the CONUS. A strong geographical pattern emerges: areas just downwind of major mountain ranges tend to promote significantly more reflective cloud tops, while most of the eastern US has relatively low values. These results suggest that cloud-top ice crystals tend to be much smaller and more numerous in mountain and high plains convection.

This presentation will display results of the climatological study, and discuss the possible implications. Clouds with numerous small ice crystals are much more effective reflectors of solar radiation, so this may have important consequences from a climatic standpoint. In addition, there may also be a link between thunderstorm updraft strength and small cloud-top ice crystals, suggesting that shortwave albedo may be used as a severe weather nowcasting parameter.