Effects of the Assimilation of Relative Humidity Reproduced From T-PARCII and Himawari-8 Satellite Imagery Using Dynamical Initialization and Ocean Coupled Model: A Case Study of Typhoon Trami (2018)

This study investigated rapid intensification (RI, +30 kt in 24 h) and rapid weakening (RW, -20 kt in 24 h) for Typhoon Trami (2018) using the Weather Research and Forecasting V4.2 with the three-dimensional Price-Weller- Pinkel ocean model. As with the previous Typhoon Lan (2017) case study, the three-dimensional relative humidity field reproduced from Tropical Cyclones-Pacific Asian Research Campaign for the Improvement of Intensity Estimations/Forecasts dropsonde data and Himawari-8 satellite imagery was assimilated during every tropical cyclone dynamical initialization process. Specifically, dropsonde data obtained from two aircraft campaigns for Lan and Trami is used. Numerical results showed that compared to without this special data assimilation, Trami’s RI and RW simulations were better improved with this special data assimilation with respect to track and intensity forecasts. Around the RI period, vertical wind shear noticeably decreased and convective bursts (vertical velocity ≥ 3 m s^-1 with 30 dBZ at 2 km height) significantly increased during the RI period. With these favorable ambient and storm inner-core environments, Trami quickly formed an eye structure. After RI and slow intensification periods, Trami eventually reached the Category 5 Saffir-Simpson hurricane scale. This maximum intensity was almost maintained until it had turned northwards. After that, as its translation speed significantly decreased, RW occurred with substantial upwelling. This upwelling caused a stable boundary layer and made significant asymmetry of surface heat fluxes and convective clouds. During this significant sea surface cooling period, deep convective cells were significantly suppressed in the eyewall area. As a result, Trami underwent RW during this period. To sum up, Trami’s RI may be associated with the reduction of negative dynamic forcing around the RI period, whereas Trami’s RW may be related to negative thermodynamic forcing by ocean cooling with a very slow translation speed during the RW period. More numerical results and detailed analyses of Trami’s RI and RW will be shown in the 2023 EGU General Assembly.

 

Keywords: dropsonde data assimilation, tropical cyclone dynamical initialization, rapid intensification, rapid weakening, WRF atmosphere and ocean coupled model

 

Acknowledgment

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-00410.