A CASE OF DOWNSTREAM BAROCLINIC DEVELOPMENT OVER WESTERN NORTH-AMERICA

Numerical simulations have been made of the initiation of a strong rid ge-trough system over western North America and the eastern Pacific (t he terminus of the Pacific storm track), with the objective of determi ning the extent to which downstream development contributed to its gro wth, 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 heat fluxes, and latent heating were removed not only reprod uced the primary features of the ridge-trough system-permitting a clea rer interpretation of the factors contributing to its growth-but actua lly generated a stronger system, suggesting that these effects as a wh ole inhibited storm development. Application of an energy budget that distinguishes between energy generation via baroclinic processes and g eneration via the convergence of geopotential fluxes revealed that ear ly growth of the system was dominated by flux convergence. These findi ngs are in agreement with the results of previous studies that have sh own that eddies near the downstream end of a storm track grow, at leas t initially, primarily through the convergence of downstream energy fl uxes. Baroclinic conversion, mostly in the form of cold advection, bec ame the primary energy source only after the development was well unde r way. This sequence of initial energy growth via flux convergence fol lowed 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 ene rgy 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 sto rm in both simulations was mainly the result of flux divergence after the storm reached maturity, although this process was somewhat delayed in the control case because of larger fluxes resulting from the dispe rsion of additional kinetic energy generated by latent heat release up stream from the system. It is believed that the techniques employed he re could represent a valuable new tool in the study of the development of such baroclinic systems and the diagnosis of model deficiencies.