Coastal precipitation enhancement due to mesoscale features induced by a landfalling tropical cyclone

Landfalling tropical cyclones (TCs) present a great challenge to forecasters, especially when trying to locate regions of high wind, storm surge, and heavy precipitation. Coastal areas are especially vulnerable to tropical cyclones, because of the low topography -- often with significant hills or mountains further inland -- and the proximity to a large moisture source. Most studies focus on TC hazards near the eye of the storm; however, often overlooked is the effect the tropical cyclone has on regions distant from the storm center. At this distance, storm surge and extremely high winds pose little threat, but heavy precipitation regions attributable to the TC can sometimes be found 500 km from the storm center. Mesoscale features such as cold-air damming, coastal frontogenesis, and topographic modification can add to the complexity of a given storm's precipitation distribution. This presentation will examine how a TC can create and modify these three mesoscale phenomena and thereby enhance rainfall near the coast.

Atlantic Tropical Storm Marco (1990) presents a case of a weak TC which dropped more than 250 mm of rain in some locations over a four day period. In the regions of heaviest precipitation totals, most of the rain fell while the storm was over 300 km to the south. Marco induced a coastal front which combined with topographic influences, large-scale features, and antecedent conditions to cause this substantial precipitation much further away from the storm than would otherwise be expected. NCEP's Unified Precipitation Dataset (UPD), synoptic upper-air analyses from NCEP/NCAR's Global Reanalysis and the North American Regional Reanalysis, NHC best track data, and hourly high-resolution surface data from the USAF's DATSAV3, NCEP's ADP, and ICOADS will be used to highlight the important baroclinic zones, roughness effects, and interacting components of this case.