A Study on the Influences of Low-Frequency Vorticity on Tropical Cyclone Formation Based on Systematic Model Simulations

To understand the influences of the low-frequency mode of the environmental vorticity on tropical cyclone (TC) formation, all 52 TCs that formed in the western North Pacific in 2008-2009 are simulated using Weather Research and Forecasting Model (WRF; version 3.2). All simulations, using same model settings (including the most popular parameterization schemes), are repeated at four distinct initial times (120, 96, 72 and 48 hours before TC formation) and with two different initial data (ECMWF YOTC and NCEP FNL). All TCs are classified into two groups on the basis of environmental 850-hPa low-frequency vorticity (using a 10-day low-pass filter) during the period 24-48 hour before TC formation. The statistical results of all 416 simulations show that for TCs that formed in a more favorable environment characterized by larger low-frequency vorticity field (LTCs), i.e. monsoon-like systems, the model is more capable of simulating a TC formation, but with larger location bias. In contrast, the model is less capable of simulating a TC formation for TCs in less favorable environment or with smaller low-frequency vorticity (STCs), but with smaller location bias.

Fourteen selected TCs (7 LTCs and 7 STCs) are further simulated to examine the sensitivity of previous results to different cumulus parameterization schemes. Results show that for LTC case (such as Dujuan in 2009), the overall pattern of simulated convection is not sensitive to the cumulus schemes used in the model simulations. Consequently, the capability of the model to simulate a TC formation is not sensitive to the cumulus scheme used. However for STC (such as Nuri in 2008), the simulated convection pattern and location are very sensitive to the cumulus schemes used in the simulation. Therefore, whether the model can simulate properly the formation of a STC depends on the cumulus scheme used. Results of our analyses suggest that the convection process is not key factor for the formation of TC with large low-frequency vorticity, but very important for the formation of TC with smaller low-frequency vorticity.