A jet above the nocturnal boundary layer

E. Linacre and B. Geerts

1/'02

Wind profiler measurements in the Australian inland reveal a jet (i.e. a local maximum in the wind speed profile) developing during the night at around 500 metres, or higher in summer (1). It starts as a shallow disturbance and grows as an inertial response to the sudden removal of ground friction at sunset, as a consequence of the decoupling created by the ground inversion that arises during the night. The nocturnal jet is found over most of northern Australia, and in other flat areas under clear skies.

Nocturnal jets can at least partly be explained by frictional decoupling at night (Fig 1). Nocturnal radiation cooling makes the air adjacent to the flat land rather heavy, and this allows the air above the inversion to reach gradient wind speed.

Reference

(1) May, P.T. 1995. The Australian nocturnal jet and diurnal variations of boundary-layer winds over Mt Isa in north-eastern Australia. Quart. J. Roy. Meteor. Soc., 121, 987-1003.