Tropical Cyclone Outflow Jets Observed during the 2012-13 Hurricane and Severe Storm Sentinel (HS3) Field Campaign

A new observing strategy utilizing dropsondes deployed from a high-altitude Global hawk UAV was employed for the first time in 2012 over Atlantic Hurricanes Leslie and Nadine during the NASA Hurricane and Severe Storms Sentinel (HS3) campaign which allows detailed vertical structure of hurricane outflow layers to be observed. Additional outflow layer observations were obtained in 2013 during Tropical Storm Gabrielle's pre-depression stage. We hypothesize that TC intensity change may be associated with either active or passive evolution of the outflow layer, i.e. that either the outflow layer changes force TC intensity change or are a result of TC intensity change. The objective of these new observations is to establish the relative importance of these competing hypotheses and to establish the pattern of outflow layer jet structures with which hurricane intensity change may be associated. The dropsondes were deployed from altitudes ranging from 55-60 K ft. which allows for detailed vertical profiles of hurricane outflow structures to be observed. In addition, use of 3-hourly satellite-derived upper layer (150-300 mb) atmospheric motion vectors (AMVs) derived by UW-CIMSS, and 6-hourly COAMPS-TC modeled outflow allows for the dropsonde observations to be placed in the context of evolving spatial and temporal interaction of environmental features and the hurricane outflow. High-resolution cirrus cloud structures observed concurrently by the Cloud Physics Lidar (CPL) are suggestive of small-scale turbulent features. Observations made along the outflow jet wind maxima in Leslie and Nadine reveal a sharp outflow maximum just below the top of the cirrus cloud layer which is coincident with the tropopause, defined by the dropsonde temperature observations. Multiple constant wind layers are observed below the wind max within the outflow layer.

Observations made within Leslie's outflow jet show a southeasterly-directed outflow feature at the periphery of the outflow layer while observations from Nadine (2012) and pre-Gabrielle (2013) reveal eastward-directed outflow jet features emanating from super-cells within the inner core of these systems. The former may be an indication of an active role of TC outflow jets in outflow layer development while the latter may be suggestive of a passive role. However, while the Global Hawk mini-sondes define the instantaneous structure and location of the outflow jet, inferences from the time series of CIMSS AMVs suggest that the broader outflow jet structures appear to have preceded super-cell development, providing circumstantial evidence that even in the super-cell situations, the outflow jets may play an active role in outflow layer development and subsequent TC intensity change.