An Examination of Essential Ingredients for Heavy Orographic Rainfall Associated with Typhoon Morakot (2009) over the Taiwan Central Mountain Range

The contributions of the essential ingredients to the heavy orographic precipitation associated with the passage of Typhoon Morakot (2009) over the Taiwan Central Mountain Range (CMR) are evaluated using the Advanced Research Weather Research and Forecasting (WRF) model. Four simulations were run with different initialization data to select a control (CNTL) case to verify the track, composite reflectivity, maximum wind intensity, precipitation pattern and maximum rainfall with observational data. The WRF model is able to reproduce the observed fields. The common ingredients for orographic rain were also tested over an averaged area with local maximum rainfall. The results showed high precipitation efficiencies, strong low level jet (LLJ), steep CMR, high moist flow upstream, low-level convergence, and slower movement of the Typhoon Morakot are essential ingredients for producing heavy orographic rainfall. These essential ingredients are also examined by running a series of cases with lower and no topography (NT) and compared with the CNTL case. The relative contribution of these essential ingredients will be investigated by a statistical model. Similarly, the results showed that during landfall, the rainfall associated with eyewall and rainbands is enhanced by deep convection on the windward side of CMR and this is seen in both simulated cases. Due to the predominant southwesterly flow, a sounding data was extracted from the upstream wind flow to run an idealized simulation to test the orographic rain index (ORI) proposed in a previous study. The results showed that, there is a good correlation between ORI and observed rainfall rate.