The Structure and Dynamics of Horizontal Roll Vortices in the Planetary Boundary Layer

Abstract

The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important. The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%. Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.