Long‐Term Observations of Turbulence Vertical Velocity Spectra in a Convective Mixed Layer: Dependence on Land‐Surface Forcing in the U.S. Southern Great Plains

Abstract

Doppler lidar vertical velocity retrievals were analyzed for the scale and structure of mixed‐layer turbulence over a 7‐year period, on fair‐weather warm season days (442 cases) in the U.S. Southern Great Plains. Based on the hypothesized influence of land‐surface forcing and updraft size on convective boundary layer clouds, a spectral analysis was performed to quantify effects of surface forcing (surface buoyancy flux, friction velocity, and evaporative fraction) on turbulence scale. Significant (order‐of‐magnitude) variations in spectral density were found in the energy‐production subrange and mesoscale regimes. Integral scale decreased with increasing buoyancy flux (and Monin‐Obukhov stability parameter), while spectral density in the energy‐production subrange increased, implying a transition to buoyancy‐driven cellular structures with narrower updrafts. The influence of stability parameter was limited to the neutral to convective transition, and could not explain the wide variation in spectral density in the mesoscale regime. However, high friction velocity (u∗>0.25em0.50.25emnormalm0.25ems−1 ${u}^{\ast } > \,0.5\,\mathrm{m}\,{\mathrm{s}}^{-1}$) was associated with larger integral scale (updraft width), and a greater portion of spectral density in the mesoscale regime. Lidar profiles and radar reflectivity for individual cases revealed evidence of roll and wave‐like structures contributing to mesoscale variability on high friction velocity days, suggesting shear instabilities on days with wetter land surface conditions and smaller buoyancy flux. The role of friction velocity emphasizes the multivariate nature of surface influences on turbulence, and implies that variance‐based turbulence closures may not be adequate for capturing effects of surface forcing on updraft size.