A NUMERICAL STUDY OF NONLINEAR NONHYDROSTATIC CONDITIONAL SYMMETRICALINSTABILITY IN A CONVECTIVELY UNSTABLE ATMOSPHERE

Nonlinear nonhydrostatic conditional symmetric instability (CSI) is st udied as an initial value problem using a two-dimensional (y, z) nonli near, nonhydrostatic numerical mesoscale/cloud model. The initial atmo sphere for the rotating, baroclinic (BCF) simulation contains large co nvective available potential energy (CAPE). Analytical theory, various model output diagnostics, and a companion nonrotating barotropic (BTN F) simulation are used to interpret the results from the BCF simulatio n. A single warm moist thermal initiates convection for the two 8-h si mulations. The BCF simulation exhibited a very intricate life cycle. F ollowing the initial convection, a series of discrete convective cells developed within a growing mesoscale circulation. Between hours 4 and 8, the circulation grew upscale into a structure resembling that of a squall-line mesoscale convective system (MCS). The mesoscale updrafts were nearly vertical and the circulation was strongest on the barocli nically cool side of the initial convection, as predicted by a two-dim ensional Lagrangian parcel model of CSI with CAPE. The cool-side mesos cale circulation grew nearly exponentially over the last 5 h as it slo wly propagated toward the warm air. Significant vertical transport of zonal momentum occurred in the (multicellular) convection that develop ed, resulting in local subgeostrophic zonal wind anomalies aloft. Over time, geostrophic adjustment acted to balance these anomalies. The sy stem became warm core, with mesohigh pressure aloft and mesolow pressu re at the surface. A positive zonal wind anomaly also formed downstrea m from the mesohigh. Analysis of the BCF simulation showed that convec tive momentum transport played a key role in the evolution of the simu lated MCS, in that it fostered the development of the nonlinear CSI on mesoscale time scales. The vertical momentum transport in the initial deep convection generated a subgeostrophic zonal momentum anomaly alo ft; the resulting imbalance in pressure gradient and Coriolis forces a ccelerated the meridional outflow toward the baroclinically cool side, transporting zonal momentum horizontally. The vertical (horizontal) m omentum transport occurred on a convective (inertial) time scale. Take n together, the sloping convective updraft/cool side outflow represent s the release of the CSI in the convectively unstable atmosphere. Furt her diagnostics showed that mass transports in the horizontal outflow branch ventilated the upper levels of the system, with enhanced mesosc ale lifting in the core and on the leading edge of the MCS, which assi sted in convective redevelopments on mesoscale time scales. Geostrophi c adjustment acted to balance the convectively generated zonal momentu m anomalies, thereby limiting the strength of the meridional outflow p redicted by CSI theory. Circulation tendency diagnostics showed that t he mesoscale circulation developed in response to thermal wind imbalan ces generated by the deep convection. Comparison of the BCF and BTNF s imulations showed that baroclinicity enhanced mesoscale circulation gr owth. The BTNF circulation was more transient on mesoscale time and sp ace scales. Overall, the BCF system produced more rainfall than the BT NF. Based on the present and past work in CSI theory, a new definition for the term ''slantwise convection'' is proposed.