roles of cloud physics in development of tropical cyclone

The main energy source of tropical cyclone (TC) is condensational heating due to water vapor. The ice phase processes such as freezing heating and melting/sublimation cooling modify the structure and development of tropical cyclone. The objective of this study is to clarify what influences have these processes in development of TC.

We use 2km-mesh nonhydrostatic model (JMA-NHM) on an f-plane with explicit cloud microphysics. The basic state of horizontally uniform temperature and humidity profile is given from averaging reanalysis data of European Centre for Medium Range Weather Forecast (ERA40) over subtropical region of the Western North Pacific in August for 5 years (1998-2002). SST is fixed at 303K. We give a axisymmetric vortex to the idealized environment. In control experiment, the vortex starts to evolve rapidly after 36h integration and the maximum wind speed reaches over 80m/s at 120h integration. The cloud-resolving model successfully reproduces typical TC structure such as warm core and eyewall cloud.

To understand the roles of ice phase processes, we make warm experiment without ice substances generation. In warm experiment, the development of TC starts earlier than the control experiment by 18h and the maximum wind speed reaches over 90 m/s. These are mainly caused by enhanced downdrafts. Melting and sublimation cooling due to ice substances enhances downdrafts. The downdrafts dry the subcloud layer and form cold outflows that reduce the low-level inflow.

In addition, we make two experiments to understand the roles of dry subcloud layer and cold outflow. When we fill the subcloud layer with water vapor, the timing of pressure falling is almost equal to control experiment but the maximum wind speed reaches as intense as warm experiment. When we diffuse the latent cooling due to melting and sublimation to weaken downdrafts, the timing of pressure falling is as early as warm experiment. The maximum wind speed is weaker than that of control experiment. This may be considered that weakened maximum intensity of TC associates with dry subcloud layer. In this presentation, we will further analyze to discuss more details.