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Large thunderstorms can be identified in the AIRS data as areas where the brightness temperature of the 1231 cm-1 atmospheric window channel in non-polar areas is less than 210 K. Each day about 6000 large thunderstorms are identified by this test, almost exclusively within 30 degrees of the equator. Since the size of the AIRS footprint at nadir is 13.5 km, a brightness temperature of less than 210 K indicates that the top of the anvil of the thunderstorm protrudes well into the tropopause. Such objects are commonly referred to as Deep Convective Clouds (DCC). Our interest in DCC was motivated by the question "Are severe weather events increasing due to global warming". Each DCC is a severe weather event, although not on the scale of the much less frequent hurricanes, which can be identified in the AIRS data as clusters of several hundred DCC. For the past four years the number of DCC per day has been fairly stable for all tropical oceans combined, but a significant increase can be seen day and night in the Atlantic Ocean. This increase may be related to the slowing of the Atlantic meridional overturning circulation. The most prominent features in brightness temperature spectra of DCC are due to stratospheric CO2, Ozone and Methane. In the channels with weighting functions below the stratosphere the brightness temperature is typically 205 K, with a characteristic 1 to 2.5 K drop between 1000 and 750 cm-1, equivalent to a 2-4 % drop in emissivity. This is likely due to the presence of cirrus (ice) particles. Firmer conclusions about the significance of the DCC count require a considerably longer data set than the currently available four years from AIRS. We plan to continue this analysis with AIRS data. Extension of the analysis to past operational sounders in polar orbit requires independent validation of the stability and absolute accuracy of the radiometric calibration at 210 K.
AIRS is an imaging hyperspectral grating-array spectrometer on the EOS Aqua spacecraft, launched on May 4, 2002.
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