Track prediction of very severe cyclone ‘Nargis’ using high resolution weather research forecasting (WRF) model

Abstract

The recent very severe cyclonic storm (VSCS) ‘Nargis’ over the Bay of Bengal caused widespread destruction over Myanmar after hitting the coast on 2 May 2008. The real time forecasting of the VSCS ‘Nargis’ was a very difficult task as it did not follow the normal westerly/northwesterly track. In the present study, a detailed diagnostic analysis of the system ‘Nargis’ is carried out initially to investigate the features associated with this unusual movement and subsequently the real time forecast of VSCS ‘Nargis’ using high resolution advanced version weather research forecasting (WRF) model is presented. The advanced research WRF model was run for 72 h at 27 km and 20 km resolutions with 28, 29, 30 April and 1 May as the initial conditions. The diagnostic study indicates that the recurvature of the system ‘Nargis’ was mainly associated with: • upper level southerly/southwesterly steering wind at 200 hPa level associated with anticyclonic circulation over southeastern sector of the centre of the system • higher SST (29°C or more) with higher positive anomalies over the region to the northeast of the cyclone centre, and • large negative 24 h pressure changes and large vorticity maximum to the east of the system. The real time track forecast using the WRF model run at 27 km and 20 km resolution based on the initial conditions of 28 April (when the system was only 550 km away from the Indian coast) indicated that the system had a northeasterly forecast track and was not expected to cross the Indian coast. Similarly, based on 29 April initial condition the system showed east/east-northeasterly movement towards the Myanmar coast. The east/east-northeasterly movement of the ‘Nargis’ was persisting in the forecast based on 30 April and 1 May initial conditions with respective landfall errors of 85 km and 50 km with 27 km resolution, which reduces to 30 km and 40 km respectively with 20 km resolution, however, with a landfall time delay of about 10 h. Improvement of mean forecast errors at different forecast hours is noticed in WRF model run at higher resolution compared to that run at lower resolution. Thus, it is very clear that the advanced version WRF model had captured movement of the system reasonably well almost 3 days in advance. Consistence with the diagnostic analysis the WRF model forecast also indicates southerly/southwesterly strong steering wind at 200 hPa level and maximum pressure fall to the east of the system.