METEX21: Multiscale observations and simulations of plume behavior across the turbulence gray-zone in mountain-valley terrain

Multiscale numerical weather prediction models transition from mesoscale (Δ ≳ 1 km), where turbulence is fully parameterized, to microscale (Δ ≲ 100 m), where the majority of highly energetic scales of turbulence are resolved. This region, called the turbulence gray-zone, was intensively studied during a tracer release experiment called METEX21 in the mountainous U.S. southwest. Terrain-atmosphere interactions that influence local-scale or gray-zone (100’s m to < 5 km) plume transport and dispersion under varying atmospheric forcing conditions were of special interest. Plumes were generated using a smoke tracer released at various sites along horizontal and vertical transects. A full suite of meteorological instruments was deployed in the domain to gather wind, turbulence, thermodynamic and plume observations in the lower boundary layer. Three multiscale simulations which vary by the parameterization used for turbulence and mixing within the gray-zone were evaluated against the 9-days of field data. Here, we highlight significant plume behavior differences on synoptically-forced and locally-forced days and show evidence of how katabatic, anabatic, and mountain-valley diel wind reversals strongly influence plume behavior over the local-scale. We demonstrate that microscale predictions of transport and dispersion can be significantly influenced by the choice of turbulence and mixing parameterization in the terra incognita, particularly over regions of complex terrain and with strong local forcing. Lastly, we highlight the effectiveness of scanning lidars to measure 2-dimensional plume transport out to a 2–3 km distance; much farther than could be visibly observed. We hope that these results motivate future field campaigns involving controlled tracer releases and corresponding modeling studies of the turbulence gray-zone.