An Examination of the Tropical Cyclone Outflow Layer: NASA HS3 Observations and Comparison with COAMPS-TC Model Predictions

In 2012 and 2013, the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign collected unprecedented in-situ measurements of Atlantic basin tropical cyclones and their environment, via dropsondes released from a remotely-piloted Global Hawk aircraft. Because the Global Hawk flies in the lower stratosphere, its dropsondes are capable of profiling the full depth of the tropical cyclone outflow layer. Here, we examine HS3 outflow observations from Hurricane Leslie (2012), Hurricane Nadine (2012) and Tropical Storm Gabrielle (2013), focusing on the vertical structure and extent of the outflow layer and how these characteristics vary in the horizontal. Of particular interest is a layer of extreme vertical wind shear at the top of the outflow layer, which despite stratification typical of the lower stratosphere, has a low Richardson number indicative of turbulent vertical mixing. In the second part of the talk, we compare HS3 dropsonde observations of the outflow layer and satellite-derived atmospheric motion vectors to real-time COAMPS-TC predictions of the kinematic and thermodynamic state of the outflow layer. The purpose here is to characterize how well a state-of-the-science limited-area dynamical model can represent the tropical cyclone outflow layer and its evolution in time.