Climatological Diurnal Cycles in Clear-Sky Brightness Temperatures from the High-Resolution Infrared Radiation Sounder (HIRS)

A climatology of the diurnal cycles of HIRS clear-sky brightness temperatures was developed based on measurements over the period 2002‐07. This was done by fitting a Fourier series to monthly gridded brightness temperatures of HIRS channels 1‐12. The results show a strong land‐sea contrast with stronger diurnalcyclesoverland,andextendingfromthesurfaceuptoHIRSchannel6or5,withregionalmaximaover the subtropics. Over seas, the diurnal cycles are generally small and therefore challenging to detect. A Monte Carlouncertaintyanalysisshowedthatmore robustresultsarereachedby aggregatingthe datazonallybefore applying the fit. The zonal fits indicate that small diurnal cycles do exist over sea. The results imply that for a long-lived satellite such as NOAA-14, drift in the overpass time can cause a diurnal sampling bias of more than 5 K for channel 8 (surface and lower troposphere).

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