A COMPARISON OF SSM I AND TOVS COLUMN WATER-VAPOR DATA OVER THE GLOBAL OCEANS/

This paper presents a comparison of column water vapor (CWV) informati on derived from both infrared measurements as part of the TIROS-N Oper ational Vertical Sounder (TOVS) and Special Sensor Microwave/Imager (S SM/I) in an attempt to assess the relative merits of each kind of data . From the analyses presented in this paper, it appears that both type s of satellite data closely reproduce the bulk climatological relation ships introduced in earlier studies using different data. This include s both the bulk relationship between CWV and the sea surface temperatu re and the annual variation of CWV over the world's oceans. The TOVS w ater vapor data tends to be systematically smaller than the SSM/I data and when averaged over the ocean covered regions of the globe this di fference is between 2-3 kgm-2. Using a cloud liquid water threshold te chnique to establish clear sky values of SSM/I water vapor, we conclud e that the differences between TOVS and SSM/I are largely a result of the clear sky bias in TOVS sampling except in the subsidence regions o f the subtropics. The clear sky bias is considerably smaller than prev iously reported and we attribute this improvement to the new physical retrieval scheme implemented by NOAA NESDIS. While there is considerab le agreement between the two types of satellite data, there are also i mportant differences. In regions where there is drying associated with large scale subsidence of the atmosphere, the TOVS CWV's are too mois t relative to both radiosonde and SSM/I data and this difference may e xceed 10 kgm-2. The explanation for this difference lies in the limita tions of infrared radiative transfer. By contrast, in regions of deep convection, such as in the ITCZ, TOVS CWV is systematically lower than the SSM/I CWV. Both TOVS and SSM/I data demonstrate similar kinds of gross effects of large scale circulation on the water vapor except in these subsidence regions where TOVS data leads to an underprediction o f the effects of subsidence drying.