A Case Study of Trough Merger In Split Westerly Flow

Abstract

Trough merger, defined as the amalgamation of two separate vorticity centers in distinct branches of westerlies into one coherent vorticity center, is studied for a case In November 1980. The analysis approach takes advantage of the failure of the operational Limited-Area Fine-Mesh Model (LFM) to simulate the mow process. A diagnostic analysis of the observed and forecast vorticity and thermodynamic structure is used to help isolate physical processes occurring during trough merger. During the period 17?19 November 1980 major cyclogenesis occurred along the coast of North America as a deepening 500 mb trough in the northern branch of the westerlies merged with a weaker trough in the southern branch of the westerlies. Cyclonic vorticity in the middle troposphere ahead of the northern trough was generated by: 1) horizontal convergence in the lower and middle troposphere and the upward advection of this vorticity and, 2) horizontal advection in the middle and upper troposphere. Frontogenesis along the 500 mb asymptote of confluence contributed to deep tropospheric ascent and enhanced the baroclinity. The overall baroclinic zone shifted northward ahead of the deepening northern trough as warming due to horizontal advection overwhelmed adiabatic cooling due to ascent. The LFM forecasts were deficient in all of these processes. In particular, the failure of the model to generate deep tropospheric ascent combined with an overprediction of the cooling due to adiabatic ascent precluded the model from shifting the principal baroclinic zone northward. A major quantitative precipitation forecast error resulted from the failure to predict trough merger.