Quantifying the Role of Tropospheric Relative Humidity on the Development of Tropical Cyclones

The sensitivity of tropical cyclone (TC) development to environmental moisture is studied using a two-dimensional, axisymmetric model. The model is initialized with tropospheric relative humidities (RH) ranging from 0% to 100%. Generally, the moister the initial environment, the shorter the amount of time it takes for a TC to develop. Moistening of the full troposphere in the TC inner core is a key ingredient for genesis to occur. Downdraft activity slows TC development by transporting drier, cooler air into the boundary layer. The precipitation efficiency, conversion of available potential energy efficiency and normalized gross moist stability (NGMS) are calculated to quantify the effects of the initial environmental moisture on the spin up process. Precipitation efficiency in the inner core increases with time, albeit more slowly the drier the initial condition, which reflects a gradual moistening of the troposphere and increasing net latent heating. In the drier simulations, available potential energy is used to moisten the troposphere rather than spin up the TCs' winds. Only when the TC inner core is nearly saturated does the conversion of available potential energy to kinetic energy become more efficient. Initially negative values of the NGMS in the drier simulations indicate bottom-heavy convection and a shallow overturning circulation. Comparison with three-dimensional forecasts with various moisture environments (RH ranging from about 45-65%) from a high-resolution, full-physics model shows that TC development occurs on a different time scale than in the two-dimensional simulations. Additionally, it is hypothesized that rainbands moisten the outer region and protect the inner core from entrainment of dry air.