Collaborative Research: Understanding Downdrafts in Deep Convection

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Improving the prediction of thunderstorms, their impacts on global climate patterns, and their ability to produce severe weather relies on fundamental knowledge of the processes that contribute to vertical motions in these storms. Recently, attention has been focused on upward motions in storms, known as updrafts. Downward motions in storms, which are called downdrafts, have received comparatively little attention. This research will address the aforementioned knowledge gap related to downdrafts. The project targets understanding the influence of atmospheric properties (e.g., temperature, moisture, and changes in the wind direction and speed with height) on the forces that drive downdrafts, the height at which downdrafts originate, and how large they become. This will be accomplished through theoretical analysis and idealized simulations. The knowledge gained from this work will allow for improvement in how downdrafts are represented in weather and climate models. Thus, this study will benefit scientific communities and the general public by improving the fundamental understanding of thunderstorms, improving forecasting of severe thunderstorm and precipitation hazards, and improving climate prediction. The inclusion of undergraduate students, graduate students, and a postdoctoral scholar in this research will also have a direct impact on the development and training of future scientists. This project will provide foundational research for improvements in downdraft forecasting and parameterization. The project will deliver on this front in three ways: first, it will uncover the typical origin heights of downdrafts, which are not well understood at present; second, it will solidify the physical basis of downdraft conceptual models, and consequently improve predictions of downdraft accelerations; third, it will demonstrate the direct impact of our improved conceptual models for downdrafts on cumulus parameterization and global climate model performance. These goals will be accomplished through a large suite of idealized large-eddy simulations with varying, realistic base-state thermodynamic and kinematic profiles, analyzed with novel techniques for assessing downdraft properties including layered and targeted passive tracers as well established trajectory analysis techniques. The new Multiple Analytic Plume (MAP) cumulus parameterization will be improved through the addition of a downdraft parameterization that is informed by the idealized simulation findings. The project will also develop an improved sounding-derived parameter for downdraft intensity forecasting that accounts for deviations from parcel theory, including nonhydrostatic vertical perturbation pressure gradient accelerations and entrainment. The foundational research will inform sensitivity experiments in a widely used global climate model to understand the connection between downdrafts and large-scale climate state simulations. Combined, all lines of research will facilitate improved understanding of convective phenomena and forecasts on multiple atmospheric scales.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.