Upper-Level Outflow, Predictability, and HS3 Observation Impact

The outflow layer is hypothesized to play a key role in tropical cyclone intensification and structural changes, and is investigated in a comprehensive manner using the innovative Global Hawk (from the AV-1 and AV-6 aircraft) and satellite observations from the Hurricane and Severe Storms Sentinel (HS3) field program in the North Atlantic during 2012-2013 and Navy's COAMPS-TC prediction system as well as it's adjoint modeling system. A COAMPS-TC ensemble Kalman filter (EnKF) data assimilation system is applied as well. As part of the HS3 research, several key issues are identified in relation to tropical cyclone outflow dynamics, processes, and evolution: i) the coupling of the TC outflow with the inner-core convection and its relationship to intensity changes, ii) the evolution of TC outflow during trough interaction and its importance for accurate intensity prediction, and iii) the impact of observations (HS3 and satellite) in sensitive regions in the TC environment (including outflow regions away from the TC inner core) on predictions of TC intensity and structure.

The morphology of upper-tropospheric outflow jets are analyzed using the observations taken by AV-6 in the 2012 and 2013 field campaigns. In particular, we were interested in analyzing the detailed vertical structure outflow jets, as resolved by the AV-6 dropsondes. Analysis pertaining to two AV-6 overflights of outflow jets, one from Hurricane Leslie and a second from Hurricane Nadine, highlights for both cases a strong shear zone in the lower stratosphere in which the Richardson number is below unity. The Richardson number is driven to such low values by the strong shear, overcoming the moderate static stability of the lower stratospheric vertical temperature gradient.

Numerical experiments have been carried out using COAMPS-TC to assess the impact of HS3 dropsonde observations on the forecasts. The experiments were run over a 9 day period from 19-28 September, which featured 3 HS3 flights into Hurricane Nadine. The assimilation of the HS3 dropsondes improves the track prediction by over 50 nm at the 120 h lead time. The HS3 dropsondes have a larger positive impact on the maximum wind error and central pressure error, relative to the track error. For example, assimilation of the HS3 dropsondes reduces the maximum wind error from 15 knots to less than 10 knots at 48 h, with similar wind error reductions at later lead times. The central pressure error is reduced considerable from 16 hPa to 10 hPa at 48 h. The adjoint model shows the strongest sensitivity near the entrance region of the upper-level jet. We have also carried out data denial experiments using the COAMPS-TC ensemble Kalman filter (EnKF). Inclusion of HS3 dropsondes leads to a reduction in the EnKF track error and reduced ensemble spread.