Effect of cold clouds on satellite measurements near 183 GHz

Near‐global analysis of 183 GHz measurements from the NOAA‐15 advanced microwave sounding unit (AMSU) B was conducted to investigate the impact of cold (< 240 K at 11 μm) clouds on upper tropospheric humidity (UTH) observations and in assessing the potential for deriving cloud microphysical properties. Collocated advanced very high resolution radiometer (AVHRR) data aided in identifying clouds and isolating the ice‐cloud‐scattering effect. This effect was determined by subtracting the measured AMSU‐B brightness temperature (Tb) from a background Tb estimated using AVHRR‐derived cloud optical depth data. Results for December 1999 over land and ocean show that nonprecipitating cold clouds have a measurable impact on 183 GHz Tbs although the average effect is rather weak (1.4 K). Cold clouds associated with precipitation had a much larger average effect (7 K); therefore only for these types of clouds is there sufficient information for potential quantitative estimation of cloud/precipitation physical properties. Nonprecipitating cold clouds bias estimates of UTH, on average, by 5% but can reach 20% for optically thick clouds. Precipitating clouds produce an 18% average bias. On the basis of these results it is recommended that UTH retrievals undergo filtering for precipitation (using combined microwave and infrared window channels) as well as for optically thick nonprecipitating cold clouds that fill a sensor's field of view, which may be screened using infrared split window techniques.