Example of Reduced Turbulence during Thunderstorm Outflow

View More View Less
  • 1 Atmospheric Release Advisory Capability, Lawrence Livermore National Laboratory, Livermore, California
© Get Permissions Rent on DeepDyve
Restricted access

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 1east 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 σθ and σϕ values decreased sharply throughout the outflow region, especially near the height of the wind speed maximum (120 m 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 AOL, 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.

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 1east 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 σθ and σϕ values decreased sharply throughout the outflow region, especially near the height of the wind speed maximum (120 m 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 AOL, 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.

Save