Predictability of the Overland Reintensification of North Atlantic Tropical Storm Erin (2007)

Tropical cyclones (TC) typically decrease in intensity upon interacting with land because of increased surface roughness and decreased surface evaporation. However, several studies have documented cases in which TCs maintain their intensity or even intensify over land within non- or weakly baroclinic environments. Yet, our understanding of the precise physical processes that support maintenance or intensification over land in non- or weakly baroclinic environments remains limited, and the predictive skill for these outcomes has yet to be quantified.

We begin this process by quantifying the predictive skill and forecast uncertainty of the overland intensification of North Atlantic Tropical Storm Erin in 2007 using a 50-member ensemble of free forecasts initialized from the output of an ensemble adjustment Kalman Filter-based cycled data assimilation system using the Data Assimilation Research Testbed software and Advanced Research Weather Research and Forecasting model. The ensemble outputs are then analyzed using ensemble sensitivity analysis (to provide meaningful physical insight into the relevant forecast sensitivities, even in environments where non-linear processes are important), ensemble subsetting (e.g., strong versus weak TCs), and others, to assess the sensitivity in overland intensity to finite-amplitude atmospheric variability. Additionally, simpler measures such as intensity variability across the ensemble are utilized as part of the analysis, which we compare to both over-water intensification cases and idealized simulations of overland intensity change.