Stochastic Exchange Coefficients on Ensemble Tropical Cyclone Intensity

Tropical Cyclone (TC) intensity forecasts are impacted by errors in atmosphere and ocean initial conditions and the model formulation, which motivates using an ensemble approach. This study evaluates the impact of uncertainty in atmospheric and oceanic initial conditions, as well as stochastic representations of the drag (Cd) and enthalphy (Ck) exchange coefficients on ensemble Advanced Hurricane WRF (AHW) TC intensity forecasts via a series of experiments that consist of 35 initialization times over 20 Atlantic TCs from 2008-2011. Each ensemble experiment consists of different combinations of having deterministic, or ensemble atmospheric and/or oceanic initial conditions, as well as fixed, or stochastic representations of Cd or Ck. Among those experiments with a single uncertainty source, atmospheric uncertainty produces the largest standard deviation in TC intensity. While ocean uncertainty leads to continuous growth in ensemble standard deviation, the ensemble standard deviation in the experiments with Cd and Ck uncertainty levels off by 48 h. Combining atmospheric and oceanic uncertainty leads to larger intensity standard deviation than atmosphere or ocean uncertainty alone and preferentially adds variability outside of the TC core. By contrast, combining Cd or Ck uncertainty with any other source leads to negligible increases in standard deviation. All of the ensemble experiments are deficient in ensemble standard deviation; however, the experiments with combinations of uncertainty sources generally have an ensemble standard deviation closer to the ensemble-mean errors. 10