Upper-tropospheric humidity from MLS and ECMWF reanalyses

Abstract

This paper compares upper-tropospheric humidity from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite with European Centre for Medium-Range Weather Forecasts (ECMWF) data. MLS measurements are not included in the ECMWF analyses, and so a comparison of two independent datasets is possible. The focus of this paper is on the tropical region from 1991 to 1997 when MLS measurements are available and, to be more specific, from 1991 to 1994 during the ECMWF 15-yr reanalysis period (ERA-15). It is found that the contrast between moist and dry areas, or areas of convection and subsidence, is less pronounced in ECMWF data than in MLS measurements. This result applies on the scale of both global and regional circulations and results in a semiannual cycle of high and low correlations between the two datasets that is particularly pronounced over South America. Time series show that the impact of model changes to the absolute values of humidity make long-term comparison with MLS difficult.