Parameterized mesoscale forcing mechanisms for initiating numerically-simulated isolated multicellular convection

Numerical simulations of deep moist convection rely heavily on the insertion of artificial perturbations into the background environment as a means to generate sustained vertical motion, clouds and convective storms. The effects of larger scale forcing on both the simulated storm and its environment are often neglected. It is proposed that prolonged mesoscale forcing is a key contributor to the regeneration of convective updrafts frequently observed in isolated multicellular convection. A new method to represent parameterized mesoscale forcing in a three-dimensional cloud model via near boundary layer momentum flux is presented and evaluated against commonly used convective initiation techniques in idealized simulations of non-precipitating deep moist convection.

The momentum flux technique formulated for use in this study incorporates a low-level convergence field to produce initial upward vertical motion on the order of 10's of cm s^-1, out of which deep moist convection is initiated. The sensitivity of the convective response is examined for varying convergent layer geometries and strengths, and the momentum flux technique is compared with two other initiating mechanisms: an ‘initiating bubble' and constant heat flux. The developmental processes, cloud structures, and updraft intensity of the convection are dependent on the characteristics of the forcing mechanisms. Regeneration of intense convective updrafts is observed in several simulations using the constant heat and momentum fluxes, while only one short-lived updraft occurs when the initiating bubble technique is applied. Implications on the future use of the flux methods to study isolated multicellular deep moist convection are also presented.