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. Issue 24 (23rd December 2022)

• Record Type: Journal Article
• Authors: Williams, Ian N.; Qiu, Shaoyue
• Is Part Of: Journal of geophysical research. Volume 127:Issue 24(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-23
• Subjects: convective boundary layer -- vertical velocity spectra -- atmospheric stability -- turbulence scale and structure -- Doppler lidar; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2022JD037137
• Languages: English
• ISSNs: 2169-897X

Abstract

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 ∗ > 0.5 m s − 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. Key Points: Turbulence spectra show wide variation in energy content in energy production and mesoscale regimes relevant to boundary layer clouds The portion of energy in the largest scales varies with surface friction velocity, implying shear instability in secondary circulations Friction velocity masks the role of surface buoyancy flux in transitions from intense narrow updrafts to roll and wave‐like structures