A Numerical Study of the Sensitivity of Typhoon Track and Convection Structure to Cloud Microphysics.

Typhoon Saola affected Taiwan from July 30, 2012, to August 03, 2012. It started in a northwestward track, and it was weakening to Category 1 storm and had a sharp cyclonic deflection track when it was about to made landfall in Taiwan on August 02. After the deflection, the typhoon moved northward and then northwestward across northern Taiwan. The cyclonic southward deflection and the northward motion constituted 12 h of looping motion which increase both the typhoon duration time and the rainfall in northeastern Taiwan. This research is to study the looping motion by the Weather Research and Forecasting model for this very weak typhoon. The model is able to reproduce the looping motion similar to that of observation. The model shows a strong asymmetry of precipitation and cross‐mountain downslope wind near the typhoon center while the typhoon track is looping toward the southwest. The potential vorticity (PV) tendency diagnosis confirms that diabatic heating and downslope wind vertical stretching are the major contributors to tropical cyclone southwestward motion, rather than the commonly referred channeling effect. The asymmetric rainfall disappears when the tropical cyclone starts moving northward in its end‐stage of looping. The northward motion is dominated by horizontal advection of PV tendency. The results highlight the influence of convection and downslope wind near steep terrain on the initial southward motion in the looping track. The model sensitivity study indicates that a large part of track uncertainty resides in the interaction of model internal dynamics and model cloud microphysics. Plain Language Summary: Typhoon Saola, a weak Category one storm, affected Taiwan from July 30, 2012, to August 03, 2012. This northwestward moving typhoon experience a sharp cyclonic deflection motion when it was about to made landfall, and then move northwestward across northern Taiwan. The cyclonic southward deflection and the northward motion constituted 12 h of looping motion which increase both the typhoon duration time and the rainfall in northeastern Taiwan. Our numerical simulation reproduces the looping motion similar to that of observation. The model simulates, in general agreement with the observations, a strong asymmetry of precipitation and cross‐mountain downslope wind near the typhoon center while the typhoon track is looping toward the southwest. The asymmetric rainfall disappears when the tropical cyclone starts moving northward in its end‐stage of looping. We demonstrate that diabatic heating and downslope wind vertical stretching are the major contributors to tropical cyclone southwestward motion, rather than the commonly referred channeling effect. The horizontal advection of potential vorticity tendency then dominates for the northward motion in the end‐stage. The model sensitivity experiments indicate that a large part of track uncertainty resides in the interaction of model internal dynamics and model cloud microphysics. Key Points: The looping track of a weak Typhoon Saola near the terrain island of Taiwan is simulated with Weather Research and Forecasting modelThe PV tendency analysis highlights the influence of convection and downslope wind near Taiwan on the southward motion of looping trackA large part of looping track uncertainty resides in model cloud microphysics [ABSTRACT FROM AUTHOR]