Direct 4D-Var assimilation of all-sky radiances. Part II: Assessment

TitleDirect 4D-Var assimilation of all-sky radiances. Part II: Assessment
Publication TypeTechnical memorandum
Date Published04/2010
Secondary TitleECMWF Technical Memoranda
AuthorsGeer, AJ, Bauer, P, Lopez, P

This paper assesses the performance of an all-sky (i.e. clear, cloudy and precipitating) direct assimilation of microwave imagers that was introduced operationally in the four-dimensional variational (4D-Var) assimilation system of the European Centre forMedium-RangeWeather Forecasts (ECMWF) in March 2009. Results are based on data-denial experiments and are quantified using forecast verification scores and comparisons to observations. The reliability of forecast verification scores is examined. Humidity-based scores can be misleading in the initial three days of forecasts, but scores based on pressure and wind fields are more reliable, because they are less sensitive to the choice of verifying analysis. The new approach is compared with the previous rain and cloud-affected assimilation, which used a 1D+4D-Var technique. One downside of the old technique is that it ‘recycles' the first guess as new information, which causes misleading ‘improvements' in forecast scores. The new approach avoids this problem. It also does not require any cloud detection, so unlike the old approach used in clear-skies, it is not susceptible to biases caused by undetected cloud. However the new system is implemented with a cautious quality control and large observation errors. Hence, the analysed and forecast tropical humidity fields are constrained only about half as much in the new system as in the old. However, the new approach is comparable in terms of its positive impact on dynamical fields (pressure and vector wind) in the tropical lower troposphere.

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