A Lagrangian Study of Helical Circulations in the Planetary Boundary Layer

Abstract

During July 1966, nearly 100 tetroon flights were made at the National Reactor Testing Station (NRTS), Idaho Falls, with the primary purpose of verifying the existence of longitudinal roll-vortices, or helices, in the planetary boundary layer. The transponder-equipped constant volume ballons (tetroons) were ballasted to float 300 m above the ground and were tracked by two M-33 radars. One radar tracked two tetroons released simultaneously from sites 500 m apart (in a direction normal to the mean flow) and the other radar tracked two tetroons released simultaneously from the same sites about one-half hour later. In the flat, desert-like region of NRTS, there is evidence that counter-rotating helices of about 2 km diameter frequently exist during the afternoon. Basically, the helical motion appears to be one of solid rotation, with an average absolute value for the vorticity in the transverse plane of 4 × 10−3 sec−1, a magnitude similar to that derived from the vertical shear of the longitudinal wind. There is evidence that these helical structures move in a direction normal to the mean flow with a speed of about 1 m sec−1. During the afternoon, the average value of the tetroon-derived horizontal stress is nearly 6 dyn cm−2, and the average flux of kinetic energy from mean sheared flow to helix is nearly 6 cm2 sec−3. There is considerable agreement between the tetroon-derived data and the theoretical and laboratory work of Faller and Lilly on helical circulations, even though the evidence from this atmospheric experiment suggests that, during the afternoon, the longitudinal vortices are driven both by buoyancy and the vertical shear of the mean flow.