On the role of vertical wind shear in AGCM-simulated tropical cyclones: Intensity change, Vertical structure and Precipitation distribution

Idealized simulation, usually called test case, is an important tool to evaluate atmospheric general circulation models (AGCMs) in representing a specific weather system such as tropical cyclones. However, vertical wind shear, a key environmental variable that controls the genesis and intensification of TC, is not available in current idealized tropical cyclone simulation of AGCMs. Here, we provide an analytic initialization technique to incorporate the background vertical wind shear and translational flow into idealized tropical cyclone simulations in AGCMs. The background wind and temperature fields are in exact thermal wind balance relationship. The role of vertical wind shear on the intensity change, vertical structure and precipitation distribution of AGCM-simulated tropical cyclones is investigated over 10 day period in half degree model simulations with the Community Atmosphere Model version 5.1.1 (CAM 5.1.1) developed at the National Center for Atmospheric Research (NCAR). It is found that westerly shear of less than 10 m/s could stimulate more rapid intensification and produce comparable intense storm as no shear does. 10 m/s is a critical threshold to inhibit the intensification of tropical cyclone. 15 m/s or larger can stop the tropical cyclone genesis process. Meanwhile, the easterly shear has considerably weaker effects on affecting TC development and cyclone genesis than the westerly shear does, especially for large wind shear such as 15 m/s. An extra mean flow causes minor impact on TC intensity. Generally, westerly mean flow has weaker effect than easterly flow does. This is more obvious when the imposed wind shear is large. The mean translation flow has little impact on the intensity of the tropical cyclone; however cause significant impact on the track and total precipitation rate of the finally simulated TCs.