A Comparison of Circulations in Transverse and Longitudinal Planes in an Unstable Planetary Boundary Layer

Abstract

Circulations in the vertical-lateral and vertical-longitudinal planes in an unstable planetary boundary layer are compared through the use of tetroon trajectories. On the average, the circulation in the transverse plane is 40% greater than in the longitudinal, but in the afternoon the transverse circulation is twice as great, providing evidence for the existence of longitudinal roll vortices at this time. The absolute magnitude of the transverse circulation increases uniformly with increase in wind speed and increase in the depth of the well-mixed layer, but the longitudinal circulation does not. The tetroon-derived stress increases with increase in wind speed and increase in absolute transverse circulation, suggesting that longitudinal roll vortices represent an efficient mechanism for the earthward transport of momentum. Comparisons are made between these tetroon results and results obtained by Deardorff from a three-dimensional numerical model of the unstable planetary boundary layer.