Comparative Diagnostic Case Study of East Coast Secondary Cyclogenesis under Weak Versus Strong Synoptic-Scale Forcing

Abstract

Two cases of east coast cyclogenesis are compared. They occur under weak and strong synoptic scale forcing, respectively. A set of objective analyses with one degree latitude-longitude horizontal resolution and 100 mb vertical resolution is used to support the diagnostic analysis. A more limited subjectively prepared data set is used as a check against the objective analyses. Vertical motions are computed from a diagnostic ten-level nonlinear balance model. The vertical motions are partitioned into contributions from latent heat release, thermal advection and differential vorticity advection. Energetics, after Lorenz, are computed from the balanced vertical motions. One of the principal results is that thermal advection and latent heat release are both of nearly equal importance to low level convergence and vorticity production for the weak cyclone event, while differential cyclonic vorticity advection in addition is crucial to the deeper, more intense development. The kinematic analysis discloses that ascent peaks in the lower troposphere near 800 mb and again in the middle troposphere, between the mountains and the coast, for both storm developments. Convergence of in situ water vapor is the primary precipitation source. Some evidence is presented that there is a lower and upper tropospheric coupling during the passage of the weak cyclone south of New England. The stronger cyclone case is dominated by prominent synoptic-scale forcing in the upper troposphere. An energetics analysis reveals that the barotclinic and barotropic conversions are contributing to the development of eddy kinetic energy during both storm periods. However, baroclinic conversions and the generation term show peaks when the strong storm's secondary circulation is peaking, while for the weak secondary counterpart no such correspondence is observed-in fact, minima in contributions occur.