Literature review

In their review of existing literature, [knupp_cotton_1985] identify four downdraft types based on the mechanisms that generate or maintain them, which informed the methods of this investigation: penetrative, cloud-edge, overshooting and precipitation-associated. The penetrative type arises when subsaturated environmental air is entrained (mixed) into saturated cloudy air, allowing evaporation of the excess liquid that creates negative buoyancy. The cloud-edge type, they note, is less understood and may result from evaporative cooling at the edges of clouds. An overshooting downdraft may be generated when the inertia of an updraft causes it to rise beyond its level of neutral buoyancy and subsequently sink. The precipitation-associated downdraft is generated by the evaporation of precipitation into subsaturated air beneath a cloud, with the cooling creating negative buoyancy. They note that 20 m/s is a typical upper limit on downdraft velocity in all cases. The model presented in this work most closely describes the precipitation-assiciated and penetrative types.

[knupp_cotton_1985] find that downdrafts often become subsaturated during descent, a conclusion reproduced by [thayer-calder_2013] in a Lagrangian (parcel-tracking) study of the output of a cloud-resolving model. The latter also found that downdrafts often descend past their neutral buoyancy levels. These findings are clearly reproduced in this work.

[market_et_al_2017] use case studies to investigate the dependence of downdraft penetration depth on quantities known as downdraft convective available potential energy (DCAPE) and downdraft convective inhibition (DCIN). The former is a measure of the maximum kinetic energy per unit mass a parcel in a given environment may gain, and the latter is a measure of the work necessary to bring the parcel to the ground once it passes its neutral buoyancy level, against the upward buoyant force. Both are calculated as integrals of the buoyant force with respect to height between chosen initial and final levels (bracketing regions of positive buoyancy for DCAPE and negative buoyancy for DCIN). They come to the conclusion that the smaller the DCIN relative to the DCAPE, the more likely downdrafts are to penetrate stable parts of the atmosphere and reach the ground. [sumrall_2020] conducts further case study investigations, with findings supporting a hypothesis that small ratios of \(|\mathrm{DCIN}/\mathrm{DCAPE}|\) are correlated with stronger and deeper downdraft activity and surface winds, and vice versa for large ratios. This work also reproduces the above findings.