The Universality of the Normalized Vertical Velocity Variance in Contrast to the Horizontal Velocity Variance in the Convective Boundary Layer

Abstract

The vertical turbulent velocity variance normalized by the convective velocity squared as a function of the boundary layer depth–normalized height [i.e., ] in the convective boundary layer (CBL) over a homogeneous surface exhibits a near-universal profile, as demonstrated by field observations, laboratory experiments, and numerical simulations. The profile holds over a wide CBL stability range set by the friction velocity, CBL depth, and surface heating. In contrast, the normalized horizontal turbulent velocity variance increases monotonically with decreasing stability. This study investigates the independence of the profile to changes in CBL stability, or more precisely, wind shear. Large-eddy simulations of several convective and neutral cases are performed by varying surface heating and geostrophic winds. Analysis of the turbulent kinetic energy budgets reveals that the conversion term between and depends almost entirely on buoyancy. This explains why does not vary with shear, which is a source to only. Further analysis through rotational and divergent decomposition suggests that the near-universal profile of is fundamentally related to the dynamics and interactions of local and nonlocal CBL turbulence. Specifically, the preferential interactions between local wavenumbers and the downscale energy cascade of CBL turbulence offer plausible explanations to the universal profile of .

Further Improvement of Surface Flux Estimation in the Unstable Surface Layer Based on Large‐Eddy Simulation Data

“…The estimation for the modest background wind speed conditions (W4 and W6) agrees quite well with the reference (Figures 6a and 6b). But for the high background wind speed (Zhou et al, 2018(Zhou et al, , 2019 and those estimated according to different empirical schemes. and 6d).…”

“…Figure 8. (a) Surface friction velocity and (b) surface heat flux simulated by LES(Zhou et al, 2018(Zhou et al, , 2019 and those estimated according to different empirical schemes.…”

Theoretical Understanding of the Linear Relationship between Convective Updrafts and Cloud-Base Height for Shallow Cumulus Clouds. Part II: Continental Conditions

“…The value of C is derived from Lenschow et al's (1980) classical formula of the profile of vertical velocity variance that is empirically determined from aircraft measurements (see appendix C for detail derivation). Earlier studies have confirmed the good performance of this formula in both clear and cloudy conditions (Berg et al 2017;Brown et al 2002;Hogan et al 2009;Lareau et al 2018;Zheng et al 2015), and a recent work offers theoretical support for the universality of this formula (Zhou et al 2019).…”

Improved Length Scales for Turbulence Kinetic Energy–Based Planetary Boundary Layer Scheme for the Convective Atmospheric Boundary Layer

“…Five idealized CBL cases are simulated and a neutral boundary layer (NBL) case from a previous study (Zhou et al 2019) is adopted for this work. A list of model parameters is presented in Table 1.…”

“…A list of model parameters is presented in Table 1. The CBL cases have identical model setup as those in Zhou et al (2019), but with 4 times the horizontal domain size to include more samples (i.e., the organized convective structures) for averaging. Briefly, the simulations are driven by constant surface heat fluxes of 0.2, 0.05, and 0 K m s 21 for the B, S, and N cases, respectively.…”