Two Oddities Related to the Transition in Atmospheric Turbulence Structure from External Forcing

Abstract

During the daytime, the surface of the earth bounds the lower troposphere, a highly turbulent flow region with three-dimensional eddies at the “microscale” (~10 m to ~1000 m) that is forced from below by solar heating and above by nonsteady quasi-two-dimensional turbulent winds at the “mesoscale” (~50 km - 1000 km). In this brief 30-minute presentation I shall describe two oddities we have discovered that result from this mesoscale forcing. The first results when extremely low levels of surface heating from the sun (e.g., in the morning) forces an otherwise “neutral” atmospheric boundary layer and causes a surprising and dramatic change in turbulence structure. The second results from nonsteady forcing of the atmospheric boundary layer from above by a change in direction of mesoscale winds, for example associated with the passage of a weather front. If the underlying boundary layer is near neutral, the change in the wind direction can also cause a sudden change in turbulence structure, but for a very different reason than that due to solar heating.

JAMES BRASSEUR

University of Colorado Boulder