Dynamics and predictability of Hurricane Nadine (2012) evaluated through convection-permitting ensemble analysis and forecasts with NASA HS3 field campaign observations

The predictability and dynamics of long-lived Hurricane Nadine (2012) are explored through analyzing both deterministic and ensemble simulations generated by the WRF-based ensemble Kalman filter (WRF-EnKF) hurricane analysis and forecast system developed at the Pennsylvania State University (PSU). Besides assimilating all non-radiance standard observations, these simulations also ingest dropsonde data collected by the Global Hawk aircraft during the 2012 campaign of NASA's Hurricane and Severe Storm Sentinel (HS3). Nadine was an extremely long-lived tropical cyclone (TC), particularly for the Atlantic Basin, spending a total of nearly 21 days as a hurricane or tropical storm. In addition, for a significant portion of its lifetime, Nadine was able to maintain tropical characteristics under seemingly harsh environmental conditions of strong vertical wind shear, environmental dry air in the vicinity of the storm along with relatively cold sea surface temperatures. Simulations during this period of persistence of Nadine, initialized at 0000 UTC 20 September 2012 and 0000 UTC 22 September 2012, are utilized to investigate potential factors such as dry air entrainment and outflow boundaries that allowed for the tropical cyclone to persevere. Given the complicated relationship between strong vertical wind shear and dry air entrainment, the dynamics that permitted this perseverance are analyzed and an assessment of Nadine's structure is performed with regards to the evolution of dry air in the area of the storm.

In addition, the predictability of track and intensity forecasts at the 0000 UTC 20 September 2012 initialization time is also explored through the utilization of ensemble sensitivity techniques. At this time, an approaching mid-latitude trough created significant track divergence in both the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the Global Forecast System (GFS) track forecasts. Many ensemble members indicated that the approaching trough would absorb Nadine and subsequently cause the TC to turn eastward, while other simulations forecasted Nadine to turn southwestward ahead of the approaching trough. Influences on the forward motion of both the approaching mid-latitude trough and Nadine are currently being identified, and the impacts on the associated intensity forecasts are being explored.