Experiences with 0–36 hour Explicit Convective Forecasting with the WRF–ARW Model

During the past three springs and summers, 36 hour realtime forecasts were conducted daily with WRF-ARW using a 4 km horizontal grid resolution and explicit convection over the central and eastern US, using the 00 UTC ETA model for the initial state and boundary conditions. These forecasts were then compared to guidance offered by the operational 12 km ETA, which employed convective parameterization. One of the overiding questions is whether there is enough ``value added'' from such high resolution simulations to support moving towards explicit convective forecasting operationally. Measuring such ``value-added'' is certainly a difficult task in itself. Still, using various subjective measures of forecast quality, we will present evidence that significant value-added does exist as regards forecasts of convective mode and propagation. Significant benefits may also be evident when considering the climatological aspects of convection, where representing convective processes ``explicitly'' seems to be critical to properly representing the diurnal cycle and convective episodes. However, value-added is less obvious when considering more general guidance as to timing, location and overall intensity of convective outbreaks. These forecasts also highlight current limitation to existing physics packages (i.e., pbl, microphysics, etc.) when applied at such high resolutions, as well as the need for a better representation of the initial state, which often includes active convection.