Latent Heating and Cooling Rates in Developing and Non-developing Tropical Disturbances during TCS-08: Radar-equivalent Retrievals from Mesoscale Numerical Model Simulations and ELDORA Observations

Earlier studies of tropical cyclone formation in the western North Pacific have focused on such synoptic disturbances as the monsoon depression, monsoon trough, easterly wave, or tropical upper-tropospheric trough cells. In the tropical cyclone formation component of the recent Tropical Cyclone Structure (TCS-08) field experiment during August-September 2008, the primary focus was more on the contribution of mesoscale convective systems (MCSs) and the associated mesoscale convective vortices that are embedded in the synoptic circulation. Increased computing capability has enabled numerical simulations of the TCS-08 cases to be performed at deep convection-permitting resolution. The objective of this study is to examine latent heating and cooling rates associated with convection in two representative mesoscale numerical models in comparison with the convection with the rates from the Doppler radar observation of convection in two developing cases and two non-developing circulations during the TCS-08.

The instantaneous values of latent heating and cooling rates in numerical simulations (e.g., ~ 5 s with a 1 km grid) are available every time step through prognostic equations of cloud physics, which are difficult to obtain from the airborne radar measurement system. Accordingly, the three-dimensional ELDORA-derived latent heating and cooling rates are estimated through the thermodynamic retrieval including stationary assumption. The radar-equivalent retrievals of latent heating and cooling rates are obtained by applying the radar thermodynamic retrieval algorithm with the numerically simulated three-dimensional wind and reflectivity values on a 1 km grid. Since the magnitude and vertical structure of the radar-equivalent latent heating rates obtained with the thermodynamic retrieval agree reasonably well with the temporal averages (~30 min) of the model time step latent heating rates, it is concluded that the non-stationarity effect primarily accounts for individual time step latent heating and cooling rates being two to three times larger than values from the ELDORA retrievals.

Contoured frequency altitude diagrams and vertical profiles of the net latent heating and cooling rates from the model simulations are compared with the ELDORA-retrieved rates in similar cloud cluster regions relative to the center of circulation. In both the developing and non-developing cases, the radar-equivalent retrievals from the two models tend to over-estimate heating rates for less-frequently occurring, intense convective cells that contribute to positive vorticity generation and spin-up in the lower troposphere. The model maximum cooling rates are consistently smaller in magnitude than the heating rate maxima for the non-developing cases as well as the developing cases. Whereas these model cooling rates are predominantly associated with melting processes, the effects of evaporative cooling are under-estimated in convective downdraft regions and at upper levels. Due to the net warming of the columns, the models tend to over-intensify the lower-tropospheric circulations if these intense convective cells are close to the circulation center and vertical wind shear is favorable. Further improvements in model physical processes are required to realistically represent the evaporative cooling effects.