An Investigation into the Thermodynamics of Overland Tropical Cyclone Intensity Change in Weakly/Non-Baroclinic Environments

There are two leading theories regarding how tropical cyclones maintain or increase their intensity over land in weakly baroclinic environments. In the first, tropical cyclones are maintained overland by enhanced upward surface enthalpy fluxes facilitated by the tropical cyclone’s rains, whereas in the second, tropical cyclones are maintained by enhanced enthalpy fluxes under inflowing trajectories at larger radii from the cyclone’s center. Both theories assume that the tropical cyclone is within a moist lower to midtropospheric environment. These theories have not been rigorously tested, however. Accordingly, this study uses a quasi-idealized version of the Weather Research and Forecasting model lacking parameterized radiation to test the sensitivity of overland tropical cyclone intensity in weakly baroclinic environments to the underlying land surface’s characteristics. In these simulations, the coldest initial land surfaces result in the strongest simulated tropical cyclones after landfall as they are associated with a downward-directed surface sensible heat flux that increases boundary-layer stability. Initial soil moisture content plays a secondary role, with higher initial soil moisture facilitating greater surface evaporation that cools and moistens the near-surface air. The increased boundary-layer stability in both cases suppresses tropical cyclone outer rainband activity, preventing low midtropospheric equivalent potential temperature air from being mixed into the inflowing near-surface trajectories by convective downdrafts. The results do not fully support the first of the two leading hypotheses but do partly support the second hypothesis. Further study is warranted to extend this study’s findings to more-realistic physical conditions. Perhaps “Initial soil moisture content plays a secondary role, with higher initial soil moisture facilitating greater surface evaporation that cools and moistens the near-surface air.”