A method for simulating effects of quasi-stationary wave anomalies on regional climate

A combined observational and numerical modeling methodology was employed to examine the interaction of large-scale quasi-stationary waves with regiona l forcing in the drought of 1988 and flood of 1993 in the north-central Uni ted States. The study implements and tests a new lateral boundary condition closure approach enabling regional climate modeling sensitivity studies to account for anomalies external to the simulated domain. Thirty-day regional simulations of the continental United States using the MM5 regional model successfully reproduced the anomalous drought and flood conditions. In an additional set of simulations the observed large-scale qu asi-stationary wave anomalies in the dynamic and thermodynamic fields were filtered our through adjustment of the observationally based lateral bounda ry conditions. In these filtered simulations the simulated meteorological f ields over the drought/flood areas for both years tended toward those for n ormal years, suggesting that the proposed filtering technique can be used a s a tool to rest the regional response to external forcing. Comparisons of the control and filtered simulations illustrated the significant influence of large-scale anomalies on the strength and geographical distribution of t he low-level jet and related impacts on rainfall in the central United Stat es.