Idealized cloud-resolving simulations of land–atmosphere coupling over tropical islands

  • The diurnal amplitude and extreme precipitation are stronger with direct LAC.
  • The sensitivity of LAC is the strongest over the urban area.
  • The mismatch of cold pools and surface cooling causes the sensitivity of LAC.

This study investigates the impact of land-atmosphere coupling (LAC) over tropical islands using a Vector-Vorticity equation cloud-resolving Model (VVM). Two sets of idealized simulations are performed with three land surface conditions (Urban, Pasture, and Grass). The first set considers direct LAC in which VVM is fully coupled with LSM. The second set of experiments eliminates direct LAC by prescribing surface fluxes in VVM, in which the high spatiotemporal variabilities are preserved from the fully coupled VVM/LSM. We can interpret the difference in convective systems' temporal and spatial evolution between the two sets of experiments as the LAC's impact. The results show that direct LAC enhances the diurnal amplitude and extreme precipitation under all land surface conditions. The impact of direct LAC is profound on the diurnal amplitude in the Urban experiment in which the sensible heat fluxes dominate. It produces 71% more precipitation with direct LAC than without. The result is due to the strong cold pool intensity by convective systems with large updraft core clouds in the Urban experiment. The mismatch between surface cooling and the convection in the experiments without direct LAC, on the other hand, prohibits further development of convection. In the Grass experiment, in which the latent heat fluxes dominate, the cold pool effects become less critical and have weaker impacts on convection development. Sensitivity experiments on free atmosphere moisture show that the dependency still holds except that the precipitation intensity is higher in a moister environment.

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