Energetics of an intensifying jet streak during the experiment on rapidly intensifying cyclones over the Atlantic (ERICA)

A characteristic life cycle of upper-tropospheric cyclogenetic precursors i nvolves the development of an elongated region of lower dynamic tropopause that forms in association with an intensifying midtropospheric jet/front. T ransverse divergent circulations associated with the jet/front steepen and depress the dynamic tropopause prior to the onset of lower-tropospheric cyc logenesis. A representative event that occurred during the second intensive observation period (IOP 2) of the Experiment on Rapidly Intensifying Cyclo nes over the Atlantic (ERICA, December 1988-February 1989) is analyzed from the perspective of local energetics. The goals of the analysis are (i) to document the evolution of the three-dimensional eddy kinetic energy (EKE) d istribution during this event and (ii) to identify the mechanisms leading t o EKE growth in the upper-tropospheric jet streak associated with the precu rsor disturbance prior to cyclogenesis, as well as in the developing lower- tropospheric cyclone. Computation of the local EKE budget during ERICA IOP 2 indicates that the R eynolds stress plays an important role in jet streak intensification over N orth America. Analysis of the Reynolds stress reveals that the contribution of this term is determined primarily by the relative orientation of the pe rturbation horizontal wind velocity and the dilatation axis of the time-mea n flow. In regions where the perturbation wind velocity is oriented within 45 degrees of normal to the dilatation axis of the time-mean Row, the contr ibution of the Reynolds stress to the EKE tendency is positive. The presenc e of a ridge over western North America favors jet streak intensification t hrough the Reynolds stress as northerly perturbation flow east of the ridge axis possesses a favorable orientation with respect to the dilatation axes of the time-mean flow over central North America. Local EKE increases acco mpany strengthening transverse divergent circulations, thus facilitating th e downward advection of stratospheric potential vorticity and eventually re sulting in the development of a mobile upper trough. This sequence is consi stent with the preference for mobile upper-trough genesis over central Nort h America in the presence of a northerly flow component, a finding document ed previously by Sanders.