Doppler Radar Observations of the Development of a Boundary-Layer Nocturnal Jet

Abstract

A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s?1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical velocities were significantly downward during the late afternoon and upward at night.