Breakdown and recovery of the eyewall of Super Typhoon Rai (2021) crossing the Philippine Islands

To advance the comprehensive knowledge of the breakdown and recovery processes of tropical cyclones (TCs) that cross the Philippine Islands, we highlighted Super Typhoon Rai (2021)and performed a control (CTL) run and two sensitivity runs in which the topography of the islands was modified, using a regional atmospheric model. The two sensitivity runs consisted of the Flat Land (FL) run, in which the mountain elevation was modified to 0 m, and the Ocean (OC) run, in which the islands were replaced by the ocean. Rai attenuated rapidly in the CTL and FL runs during landfall, and its weakening rate was mostly the same between the two runs. In the two runs, similar decreases in the diabatic heating rate, equivalent potential temperature in the planetary boundary layer (PBL), tangential wind speed, and PBL inflow characterized the eyewall breakdown, suggesting that land surface friction and less surface evaporation facilitate the breakdown process. In contrast, the eyewall recovery was earlier in the FL run than in the CTL run. The wind-terrain interaction due to mountainous terrain may cause the differences in the recovery process between the two runs. When Rai entered the South China Sea, its intensity in the two runs was reinforced rapidly and became comparable to that in the OC run. It was also found that a distinctive change in the observed eye size before and after Rai passes through the islands is not due to the modification forced by the terrain because the eye size in the OC run expands continuously. At the second peak phase of Rai's intensity, the TC sizes in the CTL and FL runs were smaller than that in the OC run, implying that the presence and absence of the eyewall breakdown lead to such a difference. The expansion of the TC size was inhibited even though the eyewall recovered completely after crossing the islands.