Processes Influencing Rain-Field Growth and Decay after Tropical Cyclone Landfall in the United States

This study measured rain-field sizes for tropical cyclones (TCs) after U.S. landfall and related changes in size to the diurnal cycle and extratropical transition (ET). For 45 TC landfalls, the spatial properties of the rain fields were calculated through an analysis of radar reflectivity returns within a geographic information system. Variables representing the conditions of the atmosphere and storm attributes were examined at three times and as changes over two time periods to account for lags between condition onset and change in raining-area sizes. Mann–Whitney U tests illustrated which of these variables had significantly different median values when the total raining area or high-reflectivity regions increased or decreased in areal extent over two 12-h periods after landfall. Results indicate that the diurnal cycle influenced changes in rain-field size. Rain-field growth occurred during the late morning and early afternoon, which is between the times for peak areal extent of oceanic- and land-based precipitation in the tropics. The rain fields of TCs completing an ET within 74 h of landfall increased in areal extent during the first 12 h after landfall and decayed during the second 12-h period as they neared the completion of ET. The availability of moisture, which was not related to either the diurnal cycle or processes associated with ET, was also important to rain-field growth or decay. In addition, it was discovered that, for the United States, landfall times have shifted from a peak before midnight during 1950–96 to after midnight during 1995–2008.