Detection of cirrus overlapping low-level clouds

A multispectral method is proposed to detect two-layer cloud systems with an optically thin (tau < 1.0) upper level (Pc < 600 mbar) cloud layer over a lower (600 mbar < Pc less than or equal to 900 mbar) cloud layer of tau > 1.0, with at least 100-mbar separation. The method uses the results from different high-resolution infrared radiometer sounder (HIRS) channel combinations assuming a single cloud layer in the CO2-Slicing technique and is valid over both land and ocean. Two months of HIRS data (July 1989 and January 1990) have been analyzed with this method. Globally (excluding regions poleward of 60 degrees) for HIRS field of view (FOV) (about 17 km at nadir) the fraction of such two-layer cloudiness is 25.5% over land and 32.0% over ocean for July 1989 and is 17.8% over land and 25.6% over ocean for January 1990 (this is not the total fraction of two-layer systems). The global distribution patterns of two-layer cloudiness for these 2 months are also presented: the fraction of two-layer cloudiness is larger over ocean than over land; it is larger in the tropics and midlatitude storm zones and smaller in the subtropical zones and marine stratiform cloud regions; over land it is larger in the summer than in the winter at all latitudes; over ocean it is larger in the summer for the northern hemispheric eastern Pacific-region while is larger in the winter for all other regions. Qualitative comparisons to surface and upper air observations are also presented and are very encouraging.