Characteristics of Vertical Turbulent Velocities in the Urban Convective Boundary Layer

摘要

Time series of vertical velocity fluctuations from horizontal aircraft flight segments made in the lower part of and near the middle of the convective boundary layer (CBL) over St. Louis, Missouri, and surrounding region were subjected to various statistical and objective analyses. The higher order vertical velocity moments, and positive and negative velocity statistics, were computed. The horizontal dimensions of updrafts and downdrafts, and related properties of these turbulent eddies were derived by conditional sampling analysis. Statistical results show that the means and standard deviations of positive and negative velocity fluctuations were greater over the urban area. The urban vertical velocity variance was 50% greater than rural values, and power spectra revealed greater production of vertical turbulent energy in the urban area over a wide frequency range. Convective similarity scaling parameters appeared to order both the urban and rural measurements.