The tropical atmospheric boundary layer: role in the formation and maintenance of hurricanes

Numerical models of hurricanes provide important guidance to understanding the role of the boundary layer in generating and maintaining the mature hurricane. Frictionally driven boundary layer convergence of moisture is crucial, as is the evaporation and advection of moisture. Sensible heat plays an apparently minor role in the model, suggesting changes in sea surface temperature have a small and delayed effect in models with more complex boundary layer parameterisation. Climatological studies confirm the crucial role of moisture in terms of evaporation, advection, and vertical transport to mid and upper troposphere. In non-deepening storms and in incipient storms, composite studies show much greater low tropospheric inflow than is assumed by models. The latter observation places the role of frictional convergence in doubt in the formative stages. Climatological studies show a marked difference in the wind field of tropical disturbances and the organised flow of the incipient storms. Neither model nor composite studies clarify the interactions between cumulus convection and the boundary layer. Observations of the hurricane boundary layer are few. They represent the cloud-free peripheral region of the storm and closely resemble undisturbed profiles. Precipitating convection shrinks, cools, and dries the mixed layer. Re-establishment of the mixed layer depends on wind speed, subsidence, and lapse rate. A critical threshold in wind speed is postulated before a tropical disturbance is likely to develop into a hurricane. It is in this development stage that the boundary layer structure and processes seem most important to the hurricane.