A Case Study of the Initialization of the Meteorological Fields Including Topography

Abstract

A forecast experiment has been conducted using an initialization scheme including the topography of the Himalayas. Based on an idea that the large scale atmospheric motion is quasi-horizontal and tends to detour the huge mountain, the scheme uses a manual modification of the initial mass field to result in airflow moving around the mountain rather than crossing over it. In this initialization procedure, the break of an isobaric surface in the lower atmosphere due to the lump of the mountain is taken into account. The initial wind field is assumed to be rotational based on the results of the numerical experiment conducted by Kondo (1978) for the divergent barotropic fluid flowing around a circular column. The balance equation is solved to derive the rotational component of wind taking account of the internal boundary condition along the edge of the break.
Two numerical time integrations have been carried out up to 48 hours with the initial data of 12 GMT 6 February 1978. One uses an excessively smoothed topography which is currently in operational use at the Japan Meteorological Agency and the other uses the more realistic topographical pattern of the Himalayas. The numerical prediction model adopted is the operational 4-level northern hemispheric primitive equation model (4L-NHM-2).
These two forecasts are compared with that of the operational NWP model which uses an initialization scheme without the topography. The forecast using the modified initial wind with the more realistic topography (Experiment 2) yields the best result in that the temperature field in the lower troposphere is the most accurate. The forecast using the modified initial wind with the excessively smoothed topography (Experiment 1) is the second and the operational forecast is the worst in that fictitious warming is conspicuous in the lee side of the mountain. It should be further remarked that the temperature pattern not only near the Himalayas but also far downstream near the Japanese Islands is improved considerably in the present experiment, particularly in Experiment 2.