Tropical Cyclone Outflow Structure Observed during the Hurricane and Severe Storms Sentinel (HS3) and Tropical Cyclone Intensity (TCI) Experiments (2012-2014)

Abstract:

Little is known about the detailed vertical and horizontal structure of the Tropical Cyclone (TC) outflow layer owing to lack of in-situ observations in this region over the years. We hypothesize that TC outflow structure change due either to external environmental interactions or internal dynamical changes are related to TC intensity changes, making the outflow layer an important region of study for improvement of TC predictability. Dropsonde profiles through TC outflow layers were obtained during the Hurricane and Severe Storms Sentinel (HS3) experiment (2012-2014) and the Tropical Cyclone Intensity (TCI) experiment (2014). Using mini-dropsondes deployed with the Airborne Vertical Atmospheric Profiling System (AVAPS) from a NASA Global Hawk and eXpendable Digital Dropsondes (XDDs) deployed with the High Definition Sounding System (HDSS) from a NASA WB-57F, new insights into the vertical structure of the TC outflow layer have been obtained.

Atmospheric Motion Vectors (AMVs) show that ‘far-field’ outflow jet dropsondes in Hurricane Leslie (2012) were obtained during a period of jet development and deformation in response the ‘pincer effect’ of an upper trough to the east and an upper cold low to west of Leslie. We speculate that the resulting deformation of the outflow layer and associated jet feature may have been responsible for limiting further development of Leslie. AMVs in Hurricane Nadine (2012) and pre-TC Gabrielle (2013) showed that outflow jets sampled by dropsondes developed over time scales of several hours. Wind profile observations in outflow jet ‘roots’ near ‘convective bursts’ showed that they were weaker and thicker near the convection and became thinner and stronger downstream as the ‘far-field’ region was sampled.

All dropsonde profiles showed that the outflow layer contained numerous thin isothermal layers and layers of enhanced vertical wind shear. These numerous thin unstable layers were characterized by a super-critical Richardson number in excess of ¼. The outflow layer therefore appears to contain multiple transient thin unstable layers generating turbulence that may play a role in the dynamics of the outflow layer and its impact on TC predictability.