The role of mass transfer in describing the dynamics of mesoscale convective systems

Abstract

AbstractWhen deep convection occurs in a rotating stratified fluid, potential vorticity (PV) anomalies form in response to the change in the mass field. This geostrophic adjustment process, and its importance in shaping the dynamics of long‐lived convective systems, is studied using a high‐resolution numerical model to simulate idealized precipitating and non‐precipitating convection.The first series of experiments considers the partitioning of energy between the balanced flow associated with the adjustment (EB), inertia‐gravity waves and turbulent dissipation and how this varies with convected mass (Mc). It is found, for two‐dimensional non‐precipitating convective plumes with a Coriolis parameter equal to 10−4 s−1, that, and, for the plumes which transport the largest quantities of mass, up to 60% of the energy is retained in the balanced flows – a much higher figure than previous estimates. For three‐dimensional (3–D) non‐precipitating plumes it is shown that EB α Mc2. It is suggested that these results indicate that the geostrophic adjustment to mass transfer is an important process in larger convective systems, such as Mesoscale Convective Systems, and that PV anomalies associated with such a system would be much stronger than the net anomaly associated with an ensemble of thunderstorms giving the same upward mass transfer. It is found, for the 3‐D runs, that inertia‐gravity waves comprise only 10% of the total energy released.A further series of experiments are carried out in which the effects of precipitation driven downdraughts are included. The downdraughts do not have the effect of simply reversing the mass transfer of the updraught, and the combination of the updraught and downdraught results in a complex PV anomaly structure with a mid‐level cyclonic vortex. Similar conclusions to those for the non‐precipitating convection case appear to follow for more realistic precipitating systems.