The Three-Dimensional Structure of Convection in the Atmospheric Surface Layer

Abstract

During April 1978, a field experiment was undertaken at the Boulder Atmospheric Observatory (BAO), near Boulder, Colorado, to investigate convective plumes in the atmospheric surface layer. The plume translational velocities are determined for a wide range of stabilities, using an array of 16 temperature sensors, spanning a 100 m ? 40 m area at a height of 4 m, and a three-dimensional sonic anemometer. The translational velocities are calculated from the phase information of the temperature cross spectrum, and by measuring the transit times of the plumes between sensors aligned in a direction parallel to the wind. Velocities obtained by the two methods are shown to be in rough agreement. Individual plume velocities are found to vary in proportion to the plume height. The three-dimensional plume structure is investigated using both the horizontal array and the 300 m BAO tower. Under slightly unstable, high wind speed conditions, the majority of the plumes are distinctly elongated in the mean wind direction, with typical longitudinal and lateral dimensions of several hundreds and several tens of meters, respectively. As the instability increases the plume dimensions are found to decrease in the downwind direction, and to increase in the crosswind direction. For very unstable, low wind speed conditions, the plumes are most often observed to be elongated in the lateral direction, although occasionally the isotherm patterns display a meandering behavior, with the trailing edge of one plume becoming the leading edge of the next. The plume tilt is found to become more nearly vertical as ?z/L increases, due to the surface shear layer becoming small in comparison with the height of the measurements. In addition, the plumes are found to have greatly varying vertical extents, and often several small plumes will merge together to form a larger plume at a higher level.