On the effect of relative timing of diurnal and large-scale forcing on summer extreme rainfall characteristics over the central United States

Impacts of diurnal radiative forcing on flow and rainfall patterns during summer flood and drought conditions (1993 and 1988, respectively) in the central United States were investigated using a regional climate model. The modeling approach, which included evaluation of sensitivity to modification in the solar hour, enabled evaluation of the impact on an event-by-event basis. The effect of the solar hour forward shift of 12 h on boundary layer wind speed over north-central Texas, which is often related to rainfall in the central United States through northward moisture advection, followed almost exactly the shift in solar hour. Domain-averaged daily rainfall in the central United States simulated with 12-h solar shift frequently showed in the flood year a backward or forward time shift of;12 h in the timing of its peak, an increase or decrease of rainfall rate, and on a few occasions noticeable formation of short-lived rainfall events. This pattern suggests relatively high sensitivity to the timing of the diurnal radiative forcing with respect to the large-scale perturbations. In contrast, in the drought year 12-h solar shifted simulations these modifications were weaker. The climatological domain-average diurnal cycle of rainfall showed for the flood year a well-defined 12-h shift when comparing the control and perturbed simulations. In contrast, in the drought year such a shift was not well defined.