A Numerical Simulation of the Landfall of Tropical Cyclones

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  • 1 Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University, Princeton, N.J. 08540
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Abstract

A GFDL tropical cyclone model was applied to simulate storm landfall. The numerical model is a three-dimensional, primitive equation model and has 11 vertical levels with four in the planetary boundary layer. The horizontal grid spacing is variable with finest resolution being 20 km near the center. This model was used successfully in the past to investigate the development of tropical cyclones over the ocean.

In the present experiments, a simple situation is assumed where a mature tropical cyclone drifts onto flat land. In such a case, the landfall can he simulated by changing the position of the coastline in the computational domain rather than by moving the storm. As the coastline moves with a specified speed, the surface boundary conditions are altered at the shore from those for the ocean to those for the land by increasing the surface roughness length and also by suppressing the evaporation.

Despite the simplicity and idealization of the experiments, the cyclone's filing rates are quite reasonable and a decay sequence is obtained. Notable asymmetries in the wind, moisture and precipitation fields exist relative to the coastline at the time of landfall. Roughness-induced, quasi-steady convergence and divergence zones am observed where onshore and offshore winds encounter the coastline. Spiral hands propagate and exist over the land area. A comparison of the energy and angular momentum budgets between ocean and land surface boundary conditions indicates a simultaneous broadening and weakening of the storm system in the decay process. The latent energy release through condensational processes is initially augmented over land by greater moisture convergence in the planetary boundary layer which counteracts the lack of evaporation from the land surface.

Supplementary experiments indicate that the suppression of evaporation is the most important factor in the decay of a storm upon Landfall. When the evaporation is suppressed, the storm eventually weakens whether the surface roughness is increased or not. An increased surface roughness, which causes increased inflow in the boundary layer, has little immediate negative impact on the storm intensity. Indeed, if the supply of latent energy is sufficient, a storm can deepen when encountering an increase in surface roughness. The decay rate in a later period well after landfall is influenced by the rate with which the water vapor of the storm system is depleted in the earlier period immediately after landfall.

Abstract

A GFDL tropical cyclone model was applied to simulate storm landfall. The numerical model is a three-dimensional, primitive equation model and has 11 vertical levels with four in the planetary boundary layer. The horizontal grid spacing is variable with finest resolution being 20 km near the center. This model was used successfully in the past to investigate the development of tropical cyclones over the ocean.

In the present experiments, a simple situation is assumed where a mature tropical cyclone drifts onto flat land. In such a case, the landfall can he simulated by changing the position of the coastline in the computational domain rather than by moving the storm. As the coastline moves with a specified speed, the surface boundary conditions are altered at the shore from those for the ocean to those for the land by increasing the surface roughness length and also by suppressing the evaporation.

Despite the simplicity and idealization of the experiments, the cyclone's filing rates are quite reasonable and a decay sequence is obtained. Notable asymmetries in the wind, moisture and precipitation fields exist relative to the coastline at the time of landfall. Roughness-induced, quasi-steady convergence and divergence zones am observed where onshore and offshore winds encounter the coastline. Spiral hands propagate and exist over the land area. A comparison of the energy and angular momentum budgets between ocean and land surface boundary conditions indicates a simultaneous broadening and weakening of the storm system in the decay process. The latent energy release through condensational processes is initially augmented over land by greater moisture convergence in the planetary boundary layer which counteracts the lack of evaporation from the land surface.

Supplementary experiments indicate that the suppression of evaporation is the most important factor in the decay of a storm upon Landfall. When the evaporation is suppressed, the storm eventually weakens whether the surface roughness is increased or not. An increased surface roughness, which causes increased inflow in the boundary layer, has little immediate negative impact on the storm intensity. Indeed, if the supply of latent energy is sufficient, a storm can deepen when encountering an increase in surface roughness. The decay rate in a later period well after landfall is influenced by the rate with which the water vapor of the storm system is depleted in the earlier period immediately after landfall.

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