On the Slowdown of Landfall Typhoon by Topography Phase Locked Convections

This paper examines the effect of the topography phase locked convections on the slowdown of the typhoons crossing the island of Taiwan. Data of 84 typhoons that made landfall at the east coast of Taiwan from 1960 to 2010 are analyzed. Our analysis suggest that 80% (70%) of slow (fast) translation typhoons, the typhoons with translation speed slower (faster) than one standard deviation of averaged land translation speed, made landfall at northern (southern) half of the Taiwan east coast. The slower cases are often with a slower pre-landfall speed and decelerate further during the landfall period. As a result, the average duration time and rainfall amount of the slow cases are much larger than that of the fast cases. Rainfall composite indicates that slow moving cases has larger rainfall amount at south-west part of Central Mountain Region (CMR), Taiwan. Our results further reveal that the average duration time for all the northern landfall cases is 2 hours longer than that of the southern landfall cases. This leads to our hypothesis that diabatic heating (DH) associated with rainfall phase locked by CMR may modify the potential vorticity (PV) tendency field and have a slowdown effect on the movement of storms. Idealized Weather Research and Forecasting Model experiments are studied and PV tendency diagnostic is applied. The net effect of vertical advection and DH term of PV tendency make an important contribution to TC motion and are responsible for the slowdown of typhoons. Our model results are in general agreement with observations; and suggest that the PV tendency due to the phase locked precipitation may have a positive feedback on the slowdown of typhoons for the typhoons made landfall north of 23.5 degrees latitude. Namely, the PV tendency due to the rainfall phase locked in the southwest slope of CMR may slow down the general northwest moving storm, and the slow-down of storm further produced more rainfall.