18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001: 8:20 AM
On 4D-Var and the use of pressure tendency data to analyse the intensification-rate of baroclinic storms
Erik H. Andersson, ECMWF, Reading, Berks., United Kingdom
Surface pressure tendency observations contain dynamically important information on the evolution of mid-latitude baroclinic storm systems. Modern four-dimensional variational (4D-Var) data assimilation schemes can use such data in a dynamically correct way as the forecast model itself is used as a constraint for the assimilation.

In this paper we present results with a 4D-Var scheme that makes effective use of tendency information in the form of time series of hourly surface pressure data from synops and ships. We show that enhanced observation frequency improves the short-range forecast of rapidly developing small scale synoptic systems.

Single-observation experiments showed that a pressure time-sequence from a single surface station can intensify the analysed development of a mid-latitude baroclinic system, that was underestimated in the background. The structure of the analysis increment is indicative of increased baroclinicity, through sharpened frontal temperature gradients, lowered tropopause and increased vertical motion. Thus is far from what could be expected from the background term alone, but rather a demonstration of the strong dynamical influence on the 4D-Var increments.

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