Doppler Radar Data Assimilation for Short-Term Forecasting of Typhoon Meranti (2010) at Landfall

The impact of high-resolution radar data on the initialization and prediction of Meranti, the 10th typhoon to hit China in 2010, is examined. Radial velocity (Vr) and reflectivity (Z) data from eight coastal operational radars in mainland China and Taiwan are assimilated over a 6-hour period before Meranti landfall, using the ARPS 3DVAR and cloud analysis package through 60-min assimilation cycles. All experiments that assimilate radar data enhance the typhoon vortex with the maximum surface winds closer to those from the best-track data, compared to the corresponding NCEP GFS analysis. Although the ARPS3DVAR doesn't update the pressure directly, the minimum sea-level pressures can drop quickly in response to the stronger vortex at the end of the analysis cycle through the model forecast. It is found that the assimilation of radar data in the first cycle has largest impact on the analysis of the vortex structure with the horizontal wind increment having a well-organized structure of cyclonic rotation, while the subsequent assimilation cycles only adjust the storm-scale structure.

With the improved initial conditions, the subsequent 12-hour forecasts of typhoon structure, intensity and track is greatly improved. Associated with the improvement of the structure and track forecasts, the heavy rainfall induced by the interaction of the typhoon circulation and complex terrain in Fujian province is well captured. Assimilating both Vr and Z data leads to the best forecasts. The Vr data help improve the intensity and track most, suggesting its dominant role in typhoon forecasts. The additional Z data, however, help produce a better eyewall and rainband structure. A set of sensitivity experiments show that analyzed vortex is related to the assimilation frequency. Higher assimilation frequency can lead to a better analysis and prediction. Assimilating radar data together with the MSLP from the best track can produce a stronger vortex in the analysis than assimilating radar data alone, but such improvement in the intensity can't last for more than one hour in forecasts. Assimilating the single Doppler radar data with a good sampling of the typhoon inner-core region, contributes mainly to the improvement of the prediction.