GLOBAL OBSERVATIONS OF UPPER-TROPOSPHERIC WATER-VAPOR DERIVED FROM TOVS RADIANCE DATA

This paper describes a physically based method for the retrieval of up per-tropospheric humidity (UTH) and upper-tropospheric column water va por (UTCWV) based on the use of radiance data collected by the TIROS O perational Vertical Sounder (TOVS), principally channels 4 (14.2 mu m) , 6 (13.7 mu m), and 12 (6.7 mu m) of High-Resolution Infrared Radiati on Sounder; This paper demonstrates how TOVS radiance data, particular ly that of the upper-tropospheric water vapor channel 12, can be model ed usefully using a single band Malkmus model with parameters tuned to a particular sensor on a particular satellite. A significant uncertai nty arises from the treatment of continuum absorption, even in regions where line absorption is dominant. This uncertainty can introduce a b ias as large as 2 K, which in turn leads to an uncertainty of approxim ately 15%-20% in the retrieved UTH and UTCWV. The research described i n this paper points to the critical need for high-accuracy measurement s of upper-tropospheric water vapor to test retrievals such as the one described herein. The results suggest that the relative humidity of t he upper troposphere, especially over the domain of the Hadley circula tion taken to between 30 degrees N and 30 degrees S, undergoes a signi ficant seasonal change. This is contrary to the usual assumption of fi xed relative humidity adopted in simple climate feedback studies. Larg e seasonal changes in the region from 30 degrees N to 30 degrees S are possibly associated with the seasonal swings in the Hadley circulatio n. Similar seasonal changes in the 350-hPa overburden indicate that th ese swings in relative humidity occur as a result of significant seaso nal shifts in the upper-tropospheric water vapor content. In the regio n equatorward of 30 degrees latitude,;the Southern Hemisphere winter i s shown to be significantly drier than the Northern Hemisphere winter. This enhanced drying is consistent with the existence of more extensi ve regions of subsidence producing larger regions of dry upper-troposp heric air in the SH during winter than in the corresponding NH during winter, especially in the subtropical Eastern Hemisphere. Analyses of the data show the clear effects of moistening in the NH subtropics thr ough the monsoonal circulations over Asia and North America and little effect of monsoon circulation in the Southern Hemisphere.