Impact of SSM/I observations related to moisture, clouds and precipitation on global NWP forecast skill

TitleImpact of SSM/I observations related to moisture, clouds and precipitation on global NWP forecast skill
Publication TypeTechnical memorandum
Date Published08/2007
Secondary TitleECMWF Technical Memoranda
AuthorsKelly, GA, Bauer, P, Geer, AJ, Lopez, P, Thépaut, J-N

This paper presents the results from Observing System Experiments with the current ECMWF data assimilation and modeling system for quantifying the impact on both analysis and forecast quality of SSM/I observations sensitive to moisture and clouds as well as precipitation. SSM/I radiances have been assimilated operationally in clear-sky areas for many years and since June 2005 in cloud and rain-affected areas too. This paper examines experiments set up such that clear-sky and rain-affected observations were either added to a poor baseline observing system configuration or withdrawn from the full system. Experiment duration was 10 weeks of which the first 14 days were excluded from the evaluation to allow for model spin-up. The basic analysis impact evaluation demonstrates that both clear-sky and rain-affected observations account for the bulk correction of moisture in the ECMWF analysis. The rain-affected data exhibits a stronger interaction with vertical motion than the clear-sky data through convection and therefore affects tropical convergence and high cloud cover. SSM/I data adds 1 day of forecast skill over the first 48 hours when evaluated in addition to a poor observing system. In the Tropics, the rain-affected data contributes more skill to the moisture forecast than the clear-sky data at 700 hPa and above. In the Northern and Southern hemisphere, the effect is generally weaker and slightly in favour of clear-sky observations. A similar performance can be seen with respect to wind vector forecast skill that indicates the connection between moisture analysis and vertical motions.

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