The Use of Helicity in Parametrization of Planetary Boundary Layer Structure: Case Study on Tropical Cyclone

To enhance the simulation of tropical cyclones, we consider a new parametrization scheme for planetary boundary-layer (PBL) structure based on helicity, intended to improve the description of the overall structures of the tropical cyclone helical PBL simulated in a numerical model. This scheme was preliminarily tested in the Yonsei University (YSU) PBL scheme integrated within the National Center for Atmospheric Research Weather Research and Forecasting model. Based on verification of track simulations for tropical cyclones that made landfall over China, tropical cyclone Morakot (2009) was selected for further evaluation of the new scheme. Compared with the original scheme based on the Richardson number, the new scheme elevated the PBL height associated with intense convection, which is consistent with observation. Importantly, the new scheme improved the numerical simulation of intense rainfall by modulating the PBL environment for convection evolution. Furthermore, the PBL height and 2-m temperature over land at night, which are frequently overestimated by the original YSU scheme, were improved using the new scheme. Because of its effects on PBL structures and convection evolution, the simulation of tropical cyclone Morakot intensity was improved by the new scheme.