The contribution of tropical cyclones to the atmospheric branch of Middle America's hydrological cycle using observed and reanalysis tracks

Middle America is affected by tropical cyclones (TCs) from the Eastern Pacific and the North Atlantic Oceans. We characterize the regional climatology (1998-2016) of the TC contributions to the atmospheric branch of the hydrological cycle, from May to December. TC contributions to rainfall are quantified using Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) product 3B42 and TC tracks derived from three sources: the International Best Track Archive for Climate Stewardship (IBTrACS), and an objective feature tracking method applied to the Japanese 55-year and ERA-Interim reanalyses. From July to October, TCs contribute 10-30% of rainfall over the west and east coast of Mexico and central Mexico, with the largest monthly contribution during September over the Baja California Peninsula (up to 90%). TCs are associated with 40-60% of daily extreme rainfall (above the 95th percentile) over the coasts of Mexico. IBTrACS and reanalyses agree on TC contributions over the Atlantic Ocean but disagree over the Eastern Pacific Ocean and continent; differences over the continent are mainly attributed to discrepancies in TC tracks in proximity to the coast and TC lifetime. Reanalysis estimates of TC moisture transports show that TCs are an important moisture source for the regional water budget. TC vertically integrated moisture flux (VIMF) convergence can turn regions of weak VIMF divergence by the mean circulation into regions of weak VIMF convergence. We discuss deficiencies in the observed and reanalysis TC tracks, which limit our ability to quantify robustly the contribution of TCs to the regional hydrological cycle

Tropical cyclone flow asymmetries induced by a uniform flow revisited

The article of record as published may be found at: http://dx.doi.org10.1002/2015MS000477We investigate the hypothesized effects of a uniform flow on the structural evolution of a tropical cyclone using a simple idealized, three-dimensional, convection-permitting, numerical model. The study addresses three outstanding basic questions concerning the effects of moist convection on the azimuthal flow asymmetries and provides a bridge between the problem of tropical cyclone intensification in a quiescent environment and that in the vertical shear over a deep tropospheric layer. At any instant of time, explicit deep convection in the model generates flow asymmetries that ten to mask the induced flow asymmetries predicted by the dry, slab boundary layer of Shapiro, whose results are frequently invoked as a benchmark for characterizing the boundary layer-induced vertical motion for a translating storm. In sets of ensemble experiment in which the initial low-level moisture field is randomly perturbed, time-averaged ensemble mean fields in the mature stage show a coherent asymmetry in the vertical motion rising into the eyewall and in the total (horizontal) wind speed just above the boundary layer. The maximum ascent occurs about 45 degrees to the left of the vortex motion vector, broadly in support of Shapiro's results, in which it occurs ahead of the storm, and consistent with one earlier more complex numerical calculation by Frank and Ritchie. The total wind asymmetry just above the boundary layer has a maximum in the forward right sector, which is in contrast to the structure effectively prescribed by Shapiro based on an inviscid dry symmetric vortex translating in a uniform flow where, in an Earth-relative frame, the maximum in on the right.G.L.T. and R.K.S. were supported in part by grant SM 30/23-1 from the German Research Council (DFG). R.K.S. is supported also by the Office of Naval Research Global under grant N62909-15-1-N021. M.T.M. acknowledges the supports of NSF grants AGS-0733380 and NSF AGS-0851077 and NASA grants NNH09AK561 and NNG09HG031Approved for public release; distribution is unlimited