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Structure and Evolution of Rainfall in Modeled Hurricanes at Landfall

Sytske K. Kimball (skimball@usouthal.edu) University of South Alabama

As was made clear by recent U.S. landfalling hurricanes, the distribution and intensity of rainfall can vary greatly within an individual storm, but also from case to case. The factors controlling these differences are very complex and inter-dependent. This study simplifies the problem by investigating just the impacts of the nature of the land surface. Six identical, idealized hurricanes are forced to make landfall on a straight, east-west oriented, flat coast with different roughness lengths and moisture availabilities. Results are compared to a control experiment without land.

Before landfall, the storms display a left-right asymmetry in rainfall distribution, with most of the rain falling in the right half of the storm. Subtle differences occur between the different cases initially, but as the storms approach land these differences become more pronounced. Cases with drier land surfaces display a larger degree of asymmetry. Thorough investigation of the 3- dimensional equivalent potential temperature field reveals that the rainfall asymmetry before landfall is driven by dry air intrusion. Dry air intrusion also occurs in the case without land with relatively dry air entering the right-rear quadrant below 650 hPa. The cyclonic winds cause the dry air to rotate around to the front of the vortex. Below 850 hPa the air experiences outflow in the front half of the vortex, makes 1 complete revolution while rising, and re-enters the eyewall in the right-rear quadrant at around 750-700 hPa. This destabilizes the right side of the storm. Meanwhile the portion of the entrained air below 850 hPa, experiences inflow in the front of the vortex and enters the left side of the eyewall at lower levels, stabilizing that side of the storm. This explains the slight left-right asymmetry in convection and rainfall seen in the control case. In the land cases, additional dry air from the land side enters the vortex in the left front quadrant, enhancing the stabilization on that side. At the same time, dry land air approaches the vortex from the left rear, rises, and reinforces the environmental dry air on the right side at 700-750 hPa. This additional dry air entrainment, enhances the degree of left-right asymmetry in the landfalling cases, especially for cases with a drier land use category.

During landfall, the asymmetry shifts abruptly with a rainfall maximum occurring to the left of the storm track. Cases with larger roughness lengths in combination with high or average moisture availability produce the most rainfall. A case with a low roughness length in combination with large moisture availability ranks next. Cases with low roughness length and medium to low moisture produce the lowest rainfall maxima at landfall. This rainfall peak is driven by low-level convergence forced by differential friction over land and water. In the right- front quadrant, low-level convergence is forced by speed convergence of the tangential wind, while in the left-front quadrant directional convergence of the radial wind drives the low-level convergence. Convection forms slightly downwind from the convergence maxima, and rain is swept even further downstream, into the left side of the vortex, by the stronger low-level tangential winds.

After landfall, the eyewall structure slowly collapses as the rainfall coverage broadens and becomes more stratiform. Rainfall rates fall off in all cases. More rainfall is observed in the moister surface cases, as expected.

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