A SAVANT case study: aerosol transport in drainage/converging flows at a shallow gully

Some studies have measured the effects of nocturnal converging and drainage flows on aerosol dispersion in mountain areas, however, few studies have examined this in less dramatic, but more common, topographic areas. The Stable Atmospheric Variability and Transport (SAVANT) project is awarded by NSF to study such nocturnal drainage flows and processes where these drainage flows converge in shallow gullies. The major objective of the project is to quantify, through a field measurement campaign, the effects of shallow cold air drainage on aerosol transport and dispersion. We hypothesize that converging flows, which we define as flow from cold-air drainage(s) merging into an area of relatively slower and less turbulent air (i.e. the background flow), will only occur under certain, identifiable, mesoscale synoptic conditions. We also hypothesize that the intense and sudden shear formed at the point of convergence will generate turbulence or gravity waves which in turn affect dispersion. While the measurement campaign has resulted in a rich dataset, the work proposed here will focus on answering the following questions:

1. Under what mesoscale conditions (i.e. cloud cover, threshold wind speed, surface cover, and stability regime) do converging flows exist?
2. What is the spatial and temporal scale of turbulence forced oscillations and/or gravity waves generated by converging flows, and do these oscillations/gravity waves follow the linear theory of internal gravity waves (IGW)?
3. How is aerosol dispersion and transport influenced by turbulence forced oscillations and/or gravity waves generated from a converging flow?

SAVANT experiments were conducted in a commercial farm at Mahomet, IL during September 15th to November 15th, 2018. Deployed instruments were the LAOF NCAR’s (10-20 m tall flux towers, SODAR, balloon radiosondes) and PI-provided instruments (smoker, Dustraks, two aerosol lidars and one Doppler lidar, CTEMPS, pressure sensors, surface temperature sensors, and drones-digital elevation measurements- DEM). Drainage flows were successfully observed on 13 nights. An exceptional case occurred on the night of October 17-18, when intermittent drainage flows were observed, smoke released near the top of the gully almost fully filled the gullies and the whole basin, and along-gully drainage flows oscillated in the crossing-gully direction.and.

This presentation will provide initial investigations and results from the October 17-18 case to answer the three project questions.