Example of reduced turbulence during thunderstorm outflow

Abstract

The vertical structures of turbulence, winds, and temperatures are analyzed from a 92-m instrumented tower and a collocated acoustic sodar during an outflow episode from a weak thunderstorm over sloping terrain in northern New Mexico. Prior to the onset of the outflow, strong insolation and light winds caused unstable conditions during the middle part of a June day, as evidenced by the large values of horizontal and vertical turbulence coefficients (σθ and σφ, respectively) extending from the surface up to at least 750 m above ground level (AGL). There was a dramatic change in wind direction and speed as the gust front passed during the early afternoon. The outflow was a well-defined jet, with its core reaching a maximum average of 16 m s-1 at 120 m AGL. The σθand σφ values decreased sharply throughout the outflow region, especially near the height of the wind speed maximum (120m AGL), where σφ reached a value of only 2°. Consequently, horizontal and vertical dispersion of a hypothetical pollutant could each decrease by about 55% at 12 m AGL to 87% at 120 m AGL up to several kilometers downwind. In turn, this could increase plume centerline concentrations by factors of 1.5 and 14 for releases at 12 and 120 m AGL, respectively. As a result of intensified winds and reduced turbulence in the outflow layer, elevated pollutant concentrations would rapidly be transported downwind before fumigation could lead to elevated pollutant levels at ground level.