Collaborative Proposal: Using ARM Observations and Large Eddy Simulations to Understand Downdrafts in Deep Convection

Downward moving currents of air in thunderstorms are called downdrafts. Downdrafts are an integral part of our global climate system, and an essential weather and climate forecast models. Various aspects of downdrafts remain poorly understood, particularly relative to the upward moving currents of air in thunderstorms, which have received comparatively more research attention in recent decades. This insufficient understanding limits our ability to adequately forecast deep convection

convection

The rising of warm air and the sinking of cool air. Convection can cause local breezes, winds, and thunderstorms.

Source: NASA earth observatory Source

and represent it in global models. To address this uncertainty, we propose to address the following science questions:
What acceleration mechanisms (negative thermal buoyancy, condensate loading, pressure gradient) primarily drive downdraft mass flux?
What are the contributions from the different downdraft acceleration mechanisms to cold pool formation?
How are downdrafts spatially distributed relative to updrafts?
How do the answers to Q1–3 depend on updraft and environmental characteristics?

We will address our science questions using ARM observations, simulations, and theory. First, we will probe the extensive ARM Radar Wind Profiler (RWP) database. Using these observations, we will establish the vertical distribution of downdrafts, the vertical velocity within downdraft cores, relationships between downdrafts and updrafts (which are also observed by RWPs), and relationships between downdrafts and their environments. This observational analysis will address questions (2), (3), and (4).

We will comprehensively analyze downdraft behavior in cases simulated by the Large eddy simulation (LES) ARM Symbiotic Simulation and Observation project (LASSO) to augment our observational analysis. For our LASSO analysis, we will analyze downdrafts using novel procedures to quantify downdraft source heights, vertical accelerations, and mass flux,

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

addressing questions (1-3). We will also run and analyze additional large-domain and small-domain idealized LESs using Cloud Model 1 (CM1), combined with parcel trajectories and passive tracers, for extensive parameter space sensitivity exploration and quantification of downdraft behavior addressing all four primary research questions.

Finally, we will use the knowledge gained from our observational analyses and LES to guide a set of single column and global climate model

global climate model (GCM)

A complex mathematical representation of the major climate system components (atmosphere, land surface, ocean, and sea ice), and their interactions.

Source: NOAA Geophysical Fluid Dynamics Laboratory Source

experiments using the NASA Goddard Institute for Space Studies (GISS) ModelE. These simulations will explore sensitivity to changes in downdraft-related parameters in the ModelE cumulus parameterization. Parameter changes will be informed by the observational and LES results addressing the primary science questions above. This will allow us to quantify the influence of improved downdraft representation on the global mean state, convection occurrence, and regional rainfall distributions.