34th Conference on Radar Meteorology


Towards the assimilation of precipitation data from ground-based radars in ECMWF's 4D-Var system

Philippe Bernard Lopez, ECMWF, Reading, Berkshire, United Kingdom

The feasibility and potential benefits of the variational assimilation of ground-based radar rain rates in ECMWF's global 4D-Var system have recently been investigated.

As a first attempt, the two-step '1D+4D-Var' approach initially chosen by ECMWF to assimilate satellite cloudy/rainy microwave radiances operationally has been applied to assimilate hourly rain rates from the NCEP Stage IV combined radar-gauge analyses. In this method, rain data are first assimilated through a 1D-Var procedure to create pseudo-observations of total column water vapour, which are then assimilated in 4D-Var. More recently, the direct assimilation of NCEP Stage IV data in 4D-Var has also been tested.

Results from experiments using both methods will be presented and the pros and cons of '1D+4D-Var' versus direct 4D-Var will be discussed. The main issues, requirements and limitations of the variational approach will be summarized. These include uncertainties in observation and model error statistics, the effect of non-linearities, the screening of observations, the proper choice of observable (logarithmic transform) and the issue of non-rainy model background situations.

In addition, results from denial experiments in which radar observations were assimilated in the absence of all other competing moisture observations (from surface stations, radio-soundings or satellites) over the USA will demonstrate that radar observations alone can have a positive impact on the quality of 4D-Var analyses and subsequent forecasts.

Finally, the prospects and requirements for the operational assimilation of radar data at ECMWF will be discussed.

extended abstract  Extended Abstract (492K)

Session 13A, Data Assimilation I
Friday, 9 October 2009, 8:00 AM-10:00 AM, Auditorium

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