A Numerical Study of the Role of Humidity in the Updraft Driven by Moist Slantwise Convection

Abstract

A hydrostatic numerical model with simple microphysical parameterization is used to simulate moist slantwise convection (MSC) in an archetypal initial condition free of other kinds of instability. The numerical experiments are designed to explore the roles of rain evaporation and the spatial distribution of humidity on the evolution of the unstable motion. The simulations show that the necessary condition of instability qe < 0 (equivalent Ertel potential vorticity) becomes less negative in the region where the slanted updraft and downdraft develop. Potential upright instability is generated above the updraft and also below it in the colder sector. The updraft resembles an ageostrophic streak of 200 km perpendicular to the thermal wind vector and drifts slowly into the warmer sector with a velocity of 4.5 km h?1, while the parcels near the surface in this sector move toward the updraft. The results indicate the occurrence of a weak low-level jet ahead of the updraft along the thermal wind vector and also a strong high-level jet in the colder sector. It is found that the surface acts to decrease the value of qe in the colder region in a shallow layer, although this is not essential in the updraft maintenance. Instead, two conditions play important roles in its maintenance: 1) rain evaporation below the updraft and 2) moist atmosphere on the warm side of the updraft. The rainwater evaporation drives an unstable slantwise downdraft into the warm sector and increases the convergence at low levels, maintaining the slanted updraft active for many hours. The difficulty in triggering the unstable motion with a small initial perturbation suggests that MSC is perhaps unlikely to occur in nature without a coupled forcing, such as frontogenetical forcing or convective buoyant instability.