Balanced and Unbalanced Circulations in a Primitive Equation Simulation of a Midlatitude MCC. Part I: The Numerical Simulation

Abstract

A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23-24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nestedgrid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically reported in the mature and decaying stages of the life cycle. Several hours later, a second MCV formed near the opposite end of the convective line, and by the mature phase of the MCC, these MCVs were embedded within a more complex system-wide vortical flow in the lower troposphere. Analysis of the first MCV during its incipient phase indicates that the vortex initially formed near the surface by convergence/stretching of the large low-level ambient vertical vorticity in this region. Vertical advection appeared largely responsible for the upward extension of this MCV to about 3.5 km above the surface, with tilting of horizontal vorticity playing a secondary role. This mechanism of MCV formation is in contrast to recent idealized high-resolution squall line simulations, where MCVs were found to result from the tilting into the vertical of storm-induced horizontal vorticity formed near the top of the cold pool. Another interesting aspect of the simulation was the development of a banded vorticity structure at midtropospheric levels. These bands were found to be due to the apparent vertical transport of zonal momentum by the descending rear-to-front circulation, or rear-inflow jet. An equivalent alternative viewpoint of this process, deformation of horizontal vorticity filaments by the convective updrafts and rear-inflow jet, is discussed. Part II of this work presents a complementary approach to the analysis presented here, demonstrating that the circulations seen in this MCC simulation are, to a large degree, contained within the nonlinear balance approximation, the related balanced omega equation, and the PV as analyzed from the PE model results.

A Subtropical Oceanic Mesoscale Convective Vortex Observed during SoWMEX/TiMREX

“…8a. This is similar to the role of the enhanced nocturnal low-level jet (LLJ) associated with continental MCVs and their heavy precipitation (Cotton et al 1989;Bartels and Maddox 1991;Olsson and Cotton 1997;Davis and Trier 2002;Trier and Davis 2002;Schumacher and Johnson 2009); such cases tend to exhibit an asymmetric precipitation structure with mesoscale rotation. 15a), transports high u e air northward in the lower troposphere, thereby increasing potential instability ahead of the MCV center (Fig.…”

“…The easterly flow in the present case had subsided, thereby becoming warm and dry. In contrast, a typical midlatitude MCV forms within the stratiform region or at the end of the convective line (Cotton et al 1989;Olsson and Cotton 1997;Chen and Frank 1993), and the midtropospheric warming is usually associated with condensation heating in the stratiform region (Zhang and Fritsch 1988). 6b,e).…”

Evaluations of NAM Forecasts on Midtropospheric Perturbation-Induced Convective Storms over the U.S. Northern Plains

“…4a), as indicated by the comma-shaped, rotating structure of the composite radar reflectivity (cf. Olsson and Cotton 1997) near the Minnesota-Iowa border. The MCV occurred slightly to the east of MP 2's vorticity center, and continued to grow and move eastward, while the MP propagated southeastward (Fig.…”

Observational Analysis of the Predictability of Mesoscale Convective Systems

“…The values of mid-level PV (i.e., 320 K) are not necessarily what one would expect for the different conditions. A positive PV anomaly usually develops in the middle troposphere due to the heating profile within the stratiform region ofMCSs (Hertenstein and Schubert 1991;Olsson and Cotton 1997). It is theorized that the MCS-generated positive PV anomaly can help lift low-level air to sustain the system (Raymond and Jiang 1990).…”