A Case of Downstream Baroclinic Development over Western North America

Abstract

Numerical simulations have been made of the initiation of a strong ridge-trough system over western North America and the eastern Pacific (the terminus of the Pacific storm track), with the objective of determining the extent to which downstream development contributed to its growth, and the possible influence of topography on the energetics of the storm. While a control simulation demonstrated considerable skill in reproducing the storm, a ?simplified? simulation in which topography, surface that fluxes, and latent heating were removed not only reproduced the primary features of the ridge-trough system?permitting a clearer interpretation of the factors contributing to its growth?but actually generated a stronger system, suggesting that these effects as a whole inhibited storm development. Application of an energy budget that distinguishes between energy generation via baroclinic processes and generation via the convergence of geopotential fluxes revealed that early growth of studies that have shown that eddies near the downstream end of a storm track grow, at least initially, primarily through the convergence of downstream energy fluxes. Baroclinic conversion, mostly in the form of cold advection, became the primary energy source only after the development was well under way. This sequence of initial energy growth via flux convergence followed by additional contributions by lower-level baroclinic conversion comprise a process designated ?downstream baroclinic development? (DBD). A similar analysis of the control simulation showed that the energy budget was essentially the same, with the exception of baroclinic conversion, which was more significant early budget was essentially the same, with the exception of baroclinic conversion, which was more significant early in the eddy's development due to orographic lifting of warm westerly flow. The decay of the storm in both simulations was mainly the result of flux divergence after the storm reached the dispersion of additional kinetic energy generated by latent heat release upstream from the system. It is believed that the techniques employed here could represent a valuable new tool in the study of the development of such baroclinic systems and the diagnosis of model deficiencies.