Simulations of a typical midlatitude squall line were used to investigate a mechanism for discrete propagation, defined as convective initiation ahead of an existing squall line leading to a faster propagation speed for the storm complex. Radar imagery often shows new cells appearing in advance of squall lines, suggesting a causal relationship and prompting the search for an “action-at-a-distance” mechanism to explain the phenomenon. In the simulations presented, the identified mechanism involves gravity waves of both low and high frequency generated in response to the latent heating, which subsequently propagate out ahead of the storm. The net result of the low-frequency response, combined with surface fluxes and radiative processes, was a cooler and more moist lower troposphere, establishing a shallow cloud deck extending ahead of the storm. High-frequency gravity waves, excited in response to fluctuations in convective activity in the main storm, were subsequently ducted by the storm’s own upper-tropospheric forward anvil outflow. These waves helped positively buoyant cumulus clouds to occasionally form in the deck. A fraction of these clouds persisted long enough to merge with the main line, invigorating the parent storm. Discrete propagation occurred when clouds developed into deep convection prior to merger, weakening the parent storm. The ducting conditions, as diagnosed with the Scorer parameter, are shown to be sensitive to vertical wind shear and radiation, but not to the microphysical parameterization or simulation geometry.

[1] A three-dimensional cloud-resolving model is used to simulate the transport of lowertropospheric passive tracers into the lowermost stratosphere via midlatitude convection. In previous studies of troposphere-to-stratosphere convective transport the extent of irreversible transport is unclear because the tropopause location is difficult to determine in the highly perturbed environment directly above an active storm. To determine the irreversibility of cross-tropopause transport in this study, 10-hour simulations are carried out to cover the growth and decay cycles of the storm. After the decay of convection, isentropes relax to quasi-flat surfaces, and the position of the tropopause becomes much easier to establish. Air parcels containing boundary layer tracers were able to penetrate the stable stratosphere because diabatic processes increased the parcel's potential temperature sufficiently to make the parcel neutrally buoyant at stratospheric altitudes. The boundary layer tracer was carried upward in the core of the updraft whereas tracers originating from higher levels were lifted on the flanks of the updraft and therefore underwent less transport into the stratosphere. Three different cases were simulated: a prototypical supercell, a prototypical multicell, and a supercell observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field campaign. In the prototypical supercell simulation, at 1 km above the tropopause the maximum concentration of boundary layer tracer is diluted to 26% of its original concentration; the maximum concentration of the tracer originating in the layer between 1 and 4 km is diluted to 23% of its original concentration. Simulation of the STEPS storm showed similar irreversible transport in a less idealized case. Both supercell storms produced more transport than the prototypical multicell storm.

[ 1 ] Previo us simulat ions with a two-di mensional cloud-resol ving model have shown that gravity wave s generat ed by tropospheri c convect ion can propaga te into the meso sphere , where they break and produce local heati ng and local acceler ations. The forci ng associ ated with this wave breaki ng excit es seconda ry g ravity waves that propaga te upward and down ward away from the wavebreaki ng regio n. Typical h orizontal and vertical scales of indi vidual center s of forcing caused by wave breaki ng are $25 km and 10 -20 km, respec tively. Groups of indi vidual forci ng centers tend to be aligned along phase line s of the prima ry wave s. In the region east of the stor m center the forci ng groups move eastward and downwar d with the primary wave ph ase line s, whi le indi vidual forci ng centers move upwa rd. To a first approxi mation, the seconda ry gravity wave s are a line ar respon se to the local ized mom entum and thermal forci ng associated with wave breaki ng, though the forci ng is its elf generat ed by the nonlinear wave -break ing proces s. The seconda ry wave s can be deriv ed from know ledge of the tem poral and spatial variability of the forcing.

[1] More observations of vertical mass transport in deep convection are needed to improve dynamical understanding of detrainment processes and for verification of transport models. A methodology for using radar reflectivity as a direct observation of vertical transport of mass from the boundary layer to the upper troposphere and lower stratosphere is investigated, and the ''level of maximum detrainment'' (LMD) is proposed. The case investigated is the 26 January 1999 squall line from the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere field campaign. Echo top heights and dual-Doppler derived divergence profiles are used to define the mass detrainment range. Over 10% of anvil echo tops occurred above the sounding-derived level of neutral buoyancy of 15.4 km during the mature stage of the storm, and convective tops reached above 18 km. Anvil ice water content, with a simple correction for ice fall speed, is found to be a good proxy for both the LMD, which for the storm analyzed is 11.25 km, and for the detrainment range of 6 to 17 km. More cases need to be analyzed to confirm the strength of this methodology, but the case study presented shows a strong correlation between anvil properties determined from radar reflectivity and the mass detrainment profile. Thus, radar reflectivity can be used as an indicator of the LMD to test model convective and transport parameterizations.