Satellite observations of convective storm tops in the 1.6, 3.7 and 3.9 μm spectral bands

Abstract

Spatial and temporal characteristics of convective storm tops observed in the 1.6, 3.7 or 3.9 μm and visible satellite spectral bands were examined. National Oceanic and Atmospheric Administration (NOAA)/Advanced Very High Resolution Radiometer (AVHRR) observations over Europe during the 1980s have shown that some storms exhibit a significant increase in the 3.7 μm cloud top reflectivity. Subsequent NOAA/AVHRR observations have shown that similar cloud top phenomena can be found as well for convective storms over the US Great Plains. The launch of the Geostationary Operational Environmental Satellite (GOES)-8, 9 and 10 has enabled the study of the evolution of storm cloud top structures in a similar band (3.9 μm) with high temporal resolution. This imagery shows that the smallest areas of increased 3.9 μm reflectivity (within or above storm tops) appear and fade on the scale of a few minutes, although larger ones can persist for tens of minutes to several hours. Occasionally, cloud top structures resembling plumes have been observed above some of the storms, apparently emanating from cores of these. Selected cases of convective storms exhibiting an increase in the 3.9 μm reflectivity have been studied with respect to internal storm structure as observed by NEXRAD Doppler radars. This revealed that the spots or areas with increased 3.9 μm reflectivity were typically found above relatively weak radar echo regions, though close to storm cores. However, a few of these “spots” have appeared above a mesocyclone near the time of associated tornado touchdown, suggesting that these spots might be the result of relatively small ice crystals present near the top of strong updrafts. One case of high 3.9 μm reflectivity over an entire storm top has been recorded simultaneously by GOES-8 and GOES-9 on 22–23 May 1996. Some aspects of bidirectional scattering are evident from the differences in 3.9 μm reflectivity observed from these two satellites. Finally, the appearance of storm tops is compared from observations in the 3.7 or 3.9 μm bands with those in the AVHRR/3 1.6 μm band, which has been recently implemented on NOAA-KLM polar orbiting satellites.