|Abstract:||Our understanding of tropical cyclogenesis, one of the greatest mysteries in tropical meteorology, has advanced significantly in the past few decades, partly owing to the availability of satellite data and high-resolution numerical model simulations. This research investigates the coupling among the primary circulation, secondary circulation and precipitation during tropical cyclogenesis through analyses of satellite data and cloud-representing numerical model simulations.
Previous studies have emphasized the importance of the positive feedback between the ocean surface heat fluxes and the primary circulation, but our analyses based on high-resolution numerical model simulations suggest that the local evaporation and its positive interaction with the primary circulation may not be as important as generally appreciated for tropical cyclone development. In fact, we demonstrate that an increase in the fractional contribution by the inward moisture flux with the storm intensification implies the importance of the positive feedback among the primary circulation, the secondary circulation, and convection for tropical cyclone development. Convection near the storm center feeds off moisture converging from large radii, and the release of latent heat concentrated near the circulation center then drives the overturning secondary circulation, increases the low-level moisture convergence, and intensifies the tangential circulation.
We further examine the precipitation evolution during tropical cyclone formation and explored the role of different types of precipitation. Specifically, we quantified the frequency occurrence of stratiform precipitation, shallow convection, mid-level convection (i.e., congestus) and deep convection as well as their contributions to the total precipitation using precipitation and cloud-type retrievals from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Precipitation increases substantially within 36 h before genesis. Stratiform clouds, mid-level convection, and deep convection increase individually and all thereby contribute to the increasing precipitation. The contribution by stratiform precipitation is due to its increasing areal coverage, while its pixel rain rate changes little from Day -3 to Day +1. The contribution by mid-level convection and deep convections results from their increasing areal coverage and intensifying rain rates. Among the three types of convection, deep convection has the largest pixel rain rate, but mid-level convection occurs most frequently and makes the largest contribution to the total precipitation. The overall contribution by convective clouds, despite their low areal coverage, is comparable to that by stratiform precipitation.
Spectral latent heating profiles derived from the TRMM PR are also examined and compared to diabatic heating profiles from a high-resolution numerical model simulation. The total areal mean heating results entirely from the diabatic heating associated with deep convection, congestus, and upper-tropospheric stratiform precipitation processes. The contribution by stratiform precipitation to the upper-level heating is comparable to deep convection, especially before genesis. Overall, the total areal mean heating is shown to occur over a deep layer owing to contributions from both convective and stratiform processes, not by convection alone. The areal mean heating associated with stratiform precipitation in the upper-troposphere leads to the development of a deep diabatic heating profile, which may aid in generating the warm core in the upper-troposphere and thus, help to hydrostatically reduce the surface pressure. Lastly, similarities that exist between the conditional mean heating profiles from the model and TRMM suggest WRF likely overestimates the frequency of occurrence of deep convection. This study suggests that tropical cyclone formation is an outcome of the collective contribution by different types of precipitation, instead of the result of a few intense, deep convective clouds.