A Global Study of Synoptic-Scale Changes in Tropical Cyclone Structure and the Relationship to Large-Scale Moisture

Enhanced mid-level tropospheric moisture is among the necessary ingredients for tropical cyclone (TC) genesis. However, the role of large-scale environmental moisture in modulating the intensity and structure of mature TCs is less understood. Recent research on this topic has focused on the influence of environmental humidity on the structure of TCs over the open ocean or in idealized modeling environments. Relatively few studies have investigated the influence of large-scale moisture on the evolution of precipitation in the global record of historical TCs.  In this study, I utilize a shape metric methodology to evaluate patterns of TC precipitation during 1998-2014 using version 7 of the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis.  To focus on the tropical portion of the TC lifecycle and avoid baroclinic and extratropical influences, this study confines the “best track" data to tropical storms (1-min sustained winds > 18 m s^-1) and to positions located within the 30°N to 30°S band surrounding the equator.  First, to diagnose synoptic-scale structural changes in precipitation for each of the resulting n=1415 TCs, I construct a 3-hourly time series of compactness measures that encompass the characteristic geometries of TCs, ranging from weak storms with poorly organized convection to stronger storms with highly circular and compact precipitation patterns. As designed, these three compactness measures span from zero (highly circular) to one (asymmetric, fragmented, and dispersed from the TC center, respectively). I then apply a moving Mann-Whitney U test to determine significant (p < 0.05) precipitation restructuring time windows.  The results present a historical perspective on preferred geographic regions for evolving TC precipitation structure.  For example, within the North Atlantic basin, increasing compactness is observed across the main development region, within the Caribbean Sea, and in the southern and eastern Gulf of Mexico, whereas decreasing compactness is observed in the western Gulf of Mexico and in the vicinity of Greater Antilles.  Second, I investigate the environmental conditions associated with the observed TC precipitation pattern changes using atmospheric and oceanic fields derived from the Climate Forecast System Reanalysis (CFSR).  This study examines both dynamic (low- and upper-level divergence, low-level vertical vorticity, deep- and mid-level vertical wind shear) and thermodynamic (low-level and mid-level relative humidity, sea surface temperature) parameters but places a particular emphasis on the influence of environmental moisture, which exerts a significant control on the evolving precipitation patterns.  Other significant environmental parameters include sea surface temperature and vertical wind shear. To summarize the results, I develop a conceptual model of TC synoptic-scale precipitation evolution based on large-scale moisture availability in low and high wind shear environments, with separate but related illustrations for the northern and southern hemispheres.