Abstract
Abstract. Deep convective clouds (DCCs) have been widely studied because of their association with heavy precipitation and severe weather events. Changes in the properties of DCCs are likely in a changing climate. Ten years of data collected by Atmospheric Infrared Sounder (AIRS) allow us to identify decadal trends in frequency of occurrence of DCCs over land and ocean. In the past, DCCs have been identified in the thermal infrared by three methods: (1) thresholds based on the absolute value of an atmospheric window channel brightness temperature; (2) thresholds based on the difference between the brightness temperature in an atmospheric window channel and the brightness temperature centered on a strong water vapor absorption line; and (3) a threshold using the difference between the window channel brightness temperature and the tropopause temperature based on climatology. Simultaneous observations of these infrared identified DCCs with the Advanced Microwave Sounding Unit–Humidity Sounder for Brazil (AMSU-HSB) using 183 GHz water channels provide a statistical correlation with microwave deep convection and overshooting convection. In the past 10 years, the frequency of occurrence of DCCs has decreased for the tropical ocean, while it has increased for tropical land. The area of the tropical zone associated with DCCs is typically much less than 1%. We find that the least frequent, more extreme DCCs show the largest trend in frequency of occurrence, increasing over land and decreasing over ocean. The trends for land and ocean closely balance, such that the DCC frequency changed at an insignificant rate for the entire tropical zone. This pattern of essentially zero trend for the tropical zone, but opposite land/ocean trends, is consistent with measurements of global precipitation. The changes in frequency of occurrence of the DCCs are correlated with the Niño34 index, which defines the sea surface temperature (SST) anomaly in the east-central Pacific. This is also consistent with patterns seen in global precipitation. This suggests that the observed changes in the frequency are part of a decadal variability characterized by shifts in the main tropical circulation patterns, which does not fully balance in the 10-year AIRS data record. The regional correlations and anti-correlations of the DCC frequency anomaly with the Multivariate ENSO Index (MEI) provide a new perspective for the regional analysis of past events, since the SST anomaly in the Nino34 region is available in the form of the extended MEI from 1871.
Highlights
Cold deep convective clouds (DCCs) are frequently associated with severe weather and heavy precipitation events
During September 2002, while AMSU-Humidity Sounder for Brazil (HSB) on EOS Aqua was in operation, 53 % of all DCCw0 identified using the infrared alone were identified as microwave deep convective, and 22 % as microwave overshooting convection
This fraction increased to 72 % for the DCCw4, with 39 % identified as microwave deep and overshooting convection
Summary
Cold deep convective clouds (DCCs) are frequently associated with severe weather and heavy precipitation events. A detection of a trend in the frequency of these clouds would have significant implication on our understanding of changes in the frequency of severe storms. While there are many papers on deep convective clouds, no studies of trends in frequency of deep convective clouds are found in the literature. The association between severe weather and extremely cold clouds was first noted in 11 μm thermal infrared images from Earth orbiting satellites three decades ago (Adler et al, 1979; Reynolds, 1980). The association between heavy rainfall and deep convective clouds has led to the development of a precipitation algorithm for GOES (Janowiak and Arkin, 1991; Joyce and Arkin, 1997). Cases where the brightness temperature at 6.7 μm is warmer than at 11 μm (e.g., Ackerman, 1996; Schmetz et al, 1997) were interpreted as Published by Copernicus Publications on behalf of the European Geosciences Union
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