A Simulation Study On The Rapid Intensification Of Typhoon Meranti (2016) Based On The Assimilation Of ATOVS Data

The 6-hourly Global Forecast System (GFS) reanalysis data was chosen as the initial and boundary condition, the Typhoon Meranti was simulated using the Weather Research and Forecasting (WRF) model, which was called the control experiment. In order to understand the effect of the Advanced Television and Infrared Observation Satellite Operational Vertical Sounder (ATOVS) radiance data in typhoon forecasting, we designed the assimilation experiment. Based on the WRF data assimilation (WRFDA) system and its three-dimensional variational data assimilation (3DVAR) method, the radiance data of Advanced Microwave Sounding Unit A (AMSU-A) and High Resolution Infrared Sounder 4(HIRS-4) were assimilated into the improved WRF v3.7. In comparison to the experimental results,we found that (1) the assimilation of the radiance date improved the accuracy of Typhoon Meranti through adjusting the initial fields of typhoon including the temperature and moisture. (2) Compared to the observation, the track of the assimilation experiment was well simulated. The assimilation experiment reproduced the track deflection, the landfall location and time and the moving speed of Typhoon Meranti. (3) The prediction error of typhoon intensity in the assimilation experiment was reduced by about one time. (4) The assimilation of ATOVS data can significantly influence the convective structure in the eye area, leading to the improvement of typhoon simulation.Based on the high-resolution assimilation results,the vertical wind shear,convective burst (CB) and cloud microphysical processes as well as the rapid-intensification (RI) mechanisms of Typhoon Meranti were analyzed in detail.The main conclusions were as follows. (1) Typhoon Meranti experienced the RI process in relatively weak shear environments. It was associated with the burst of deep convection in its inner core. During the onset of RI, strong convective cells were found gathering in downshear eyewall quadrant or the left side of shear. They also indicated an asymmetric structure in the eyewall. As Meranti intensified rapidly,the strong convective activity occurred both in upshear and downshear quadrants,then the symmetrical structures developed well. (2) At 6h before RI onset, the large convective available potential energy (CAPE) caused the sudden increasement of CB numbers. As shear decreased, the onset of RI was triggered by CBs, which contributed to the upper-level warming by drawing down the stratospheric air. In the reduced shear environments, hydrometeors increased and the great latent heat released in the upper troposphere through the enhancement of CBs, which enhanced the upper-level warm core and caused the intensification of typhoon. In addition, the upper-level outflow layer coincided with the height of the warm core, helping protect the warmer air from ventilation by environmental flows, which was contributed to the intensification of Typhoon Meranti.