THE COMPOSITE HORIZONTAL WIND-FIELD WITHIN CONVECTIVE STRUCTURES OF THE ATMOSPHERIC SURFACE-LAYER

The horizontal perturbation wind field within thermal structures encountered in the atmospheric surface layer was investigated. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Structures were selected on a typical ramplike appearance in the temperature time series. Ultimately, a set of 47 ''ramps'' was obtained. Conditional sampling and block averaging followed by a compositing technique were applied to construct ensemble averages of turbulent temperature and horizontal and vertical velocity. Properties of the horizontal velocity field were expressed in terms of ensemble averages of horizontal divergence, vertical vorticity, and deformation. The ensemble-averaged behavior at the five masts during passage of thermal activity was consistent. The convergent wind field within a ramp attains its maximum simultaneously with the maximum in vertical velocity. Both precede the temperature extreme. The air in the ramp is clearly decelerated, while it is accelerated in the succeeding downdraft. In the frame of reference moving with the ramp the average orientation of the wind vector in the accompanying downdrafts is always directed toward the position of the ramp. Within the ramp, dilatation of air is measured in the direction of the mean wind. Near the microfront, contraction of air occurs with a maximum in the succeeding downdraft. Vertical vorticity (of opposite sign) is measured in the right and left half of the ramps. Phenomena involved in the generation of this vorticity are discussed. The strength of the background wind might play a role in the generation of these rotations.