Upper tropospheric humidity and thin cirrus

Upper tropospheric water vapor data from the UARS Microwave Limb Sounder (MLS) are compared with coincident optically thin (often subvisible) cirrus cloud measurements from the UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) at 147 hPa in the tropics. A strong correlation is found between mean relative humidity with respect to ice (RHI) and percent occurrence of cirrus clouds. The cloudiness - RHI relationship is established from the coincident CLAES and MLS observations (Oct. 1991-May 1993), and used with 1991-97 MLS data to find by proxy the percent occurrence of cirrus for 1993-97. Interannual variability of cloudiness is investigated with the extended (1991-97) cirrus data, and a clear El Nine signature is seen. The observed relationship of RHI and optically thin cirrus provides a potential diagnostic of climatological behavior of atmospheric models, in which cirrus is important for radiative balance, heterogeneous chemistry, and the near-tropopause water budget.