Impacts of the Lowest Model Level Height on Tropical Cyclone Intensity and Structure

Variable thicknesses in the lowest half- model level (LML) are often used in atmospheric models to compute surface diagnostic fields such as surface latent and sensible heat fluxes. The effects of the LML on simulated tropical cyclone (TC) evolution were investigated in this study using the Weather Research and Forecasting (WRF) model. The results demonstrated notable influences of the LML on TC evolution when the LML was placed below 12 m. The TC intensification rate decreased progressively with a lowering of the LML, but its ultimate intensity change was relatively small. The maximum 10-m winds showed different behavior to minimum sea level pressure and azimuthally-averaged tangential winds, and thus the wind-pressure relationship was changed accordingly by varying the LML. The TC circulation was more contracted in association with a higher LML. Surface latent heat fluxes were enhanced greatly by elevating the LML, wherein the wind speed at the LML played a dominant role. The changes in the wind speed at the LML were dependent not only on their profile differences, but also the different heights they were taken from. Due to the enhanced surface heat fluxes, more intense latent heat release occurred in the eyewall, which boosted the storm's intensification. A higher LML tended to produce a stronger storm, and therefore the surface friction was reinforced, which in turn induced stronger boundary layer inflow together with increased diabatic heating.