An investigation into the role of moist, vertical drafts in the rotational development of storms

A simple two-dimensional, quasi-steady-state model is used to investigate the effect of various parameters on producing convergence at the top of downdrafts. The results from this model indicate that convergence occurs within a narrow region of the storm cloud at the top of strong downdrafts within certain environments. The inflow region consistently occurred over a thickness of 1-2 km. This convergence roughly doubled the tangential velocities of a mesocyclone within one hour. The environment that produced the strongest downdraft convergence was characterized by low relative humidity, a dry-adiabatic lapse rate, a large concentration of small droplets and minimal horizontal airflow.; This paper details a sensitivity study that examined the effect of initial droplet size, initial relative humidity, environmental lapse rate, droplet drag, initial liquid water content and non-hydrostatic pressure gradients on downdraft convergence. The results show initial droplet size is a particularly important consideration followed by relative humidity, environmental lapse rates and droplet concentration. Non-hydrostatic pressure gradients accelerated the downdraft above the maximum inflow and decelerated it below.; This model is unlike other downdraft studies that have assumed a developed rain distribution and vertical velocity structure at the top of the downdraft. This study concentrates on the top of the downdraft where the vertical velocity is zero and droplet sizes are small. These conditions are representative of the cloud region within many large convective systems. Using a monodispersion of cloud-size droplets, a neutral stability atmosphere and an initial vertical velocity of -0.5 m/s, the model produced a horizontal inflow velocity of 2-3 m/s at a radius of 5 km.; Several results showed the sensitivity and the limitations of the model. The results indicate that the model was sensitive to the assumed downdraft radial velocity profile. The inflow thickness was related to the type of radial profile of vertical velocity assumed. The inflow thickness was a minimum using a top-hat velocity profile and a maximum with a triangular velocity profile. Droplets with radii larger than 0.5 mm were found to be too large to be moved horizontally by a 2-3 m/s wind and represented the limiting droplet size. The results from this study provide a better understanding of downdraft inflow structure and the role of downdrafts in the rotational development of storms.