Maximization of the precipitation from tropical cyclones over a target area through physically based storm transposition

Abstract. Certain methods of Probable Maximum Precipitation (PMP) estimation such as the generalized estimation method involve the transposition to the target area of intense storms that occurred in adjacent areas. This storm transposition step is based on the assumption that it is possible to delineate a meteorologically homogeneous region (World Meteorological Organization, 2009) surrounding the target area so that the precipitation field of a storm that occurred within this region may be transposed over the target area. Differences between conditions at the storm site and those at the project basin, such as differences in topography and in the proximity to the moisture source, are accounted for by transposition adjustments. In this article, a new method for the transposition of tropical cyclones (TCs) is presented. This method is fully physically based as it uses a regional atmospheric model (RAM) to reconstruct the intense precipitation field from a TC, thus crucially conserving the mass, momentum and energy in the system. In this transposition method, the initial vortex in the simulation initial conditions is first shifted spatially. More precisely, the TC at the simulation start date is first separated from its background environment, then shifted, and finally recombined with the background environment. Then, the RAM is run as usual to simulate the TC and its precipitation field. The new transposition method was applied to four hurricanes which spawned torrential precipitation in the United States, namely Hurricanes Floyd (1999), Frances (2004), Ivan (2004), and Isaac (2012), in order to maximize the 72-h accumulated precipitation depth over the drainage basin of the city of Asheville, N.C. It was observed that the precipitation fields changed in both structure and intensity after transposition. Besides, the tracks of the hurricanes were generally very sensitive to changes in the initial conditions, which is expected for a storm system whose dynamics is strongly nonlinear. In particular, a small change in the location of the initial vortex may result in a very different track, allowing the TC to go over the target area.