Comparisons of Atlantic Tropical Cyclone Outflow Structure and Environmental Interaction as Functions of Intensity Change and Environmental Shear

Since the seminal work of Merrill (1988), there has been relatively little study of tropical cyclone (TC) outflow and the upper-level TC environment in the context of TC intensity change (Rappin et al. 2011 and Emanuel 2012 notable exceptions). This is in contrast to the extensive amount of research that has related intensity change to cyclonic structural changes in the TC. In light of the much larger and higher resolution datasets now available, we seek an improved understanding of the relationship between the structure of TC outflow, the TC environment, and TC intensity change.

ERA-Interim reanalysis is used to analyze the upper troposphere around all Atlantic TCs over warm sea surface temperature (SST) during 1979-2013, yielding 3509 six-hourly samples from 299 TCs. Raw composites derived from these samples show consistencies with the observations of Merrill (1988), who composited Atlantic hurricanes from 1977-1983. Specifically, intensifying TCs exhibit more divergent outflow streamlines within 500 km of the center and more anticyclonic flow extending along the zonal axis outward from the storm. Weakening TCs exhibit a more constricted and sheared outflow pattern, with air from the primary anticyclone recirculating over the storm core, and larger values of potential vorticity (PV) to the west and east of the anticyclone.

With the large available sample size, anomalies from spatially-dependent climatologies are developed in order to examine common features in strengthening and weakening TCs under both low and high vertical wind shear regimes, independent of the regional differences in the outflow climatology of Atlantic TCs. Early results suggest that cases of significant intensification under high shear are associated with a positive PV anomaly to the north of the TC, increasing radial outflow to the northeast and flux convergence of angular momentum within 1000 km. These results are also being examined in the context of Challa and Pfeffer (1981), McBride and Zehr (1981), Molinari and Vollaro (1990), DeMaria (1993), Challa and Pfeffer (1998), and Hanley et al. (2001). Ongoing analysis seeks to a) quantitatively and qualitatively define and describe the shape of TC outflow to determine if such measures can be indicators of future short-term intensity change, and b) to help clarify the subtle character of TC-trough interactions that induce TC intensification (e.g. Elena 1985; Opal 1995; Charley 2004) and those that cause weakening.