Abstract
The role of wind shear in the decay of the convective boundary layer (CBL) is systematically investigated using a series of large-eddy simulations. Nine CBLs with weak, intermediate, and strong wind shear are simulated, and their decays after stopping surface heat flux are investigated. After the surface heat flux is stopped, the boundary-layer-averaged turbulent kinetic energy (TKE) stays constant for almost one convective time scale and then decreases following a power law. While the decrease persists until the end of the simulation in the buoyancy-dominated (weak-shear) cases, the TKE in the other cases decreases slowly or even increases to a level which can be maintained by wind shear. In the buoyancy-dominated cases, convective cells occur, and they decay and oscillate over time. The oscillation of vertical velocity is not distinct in the other cases, possibly because wind shear disturbs the reversal of vertical circulations. The oscillations are detected again in the profiles of vertical turbulent heat flux in the buoyancy-dominated cases. In the strong-shear cases, mechanical turbulent eddies are generated, which transport heat downward in the lower boundary layers when convective turbulence decays significantly. The time series of vertical velocity skewness demonstrates the shear-dependent flow characteristics of decaying CBLs.
Highlights
The characteristics of the convective boundary layer (CBL), a type of planetary boundary layer (PBL) in which surface buoyancy flux drives turbulence, exhibit a strong dependency on surface and atmospheric forcings
The convective time scale t∗ in this study is defined as h/wm, where wm is the convective velocity scale considering the impact of surface wind shear [29,30]
In the buoyancy-dominated B1–3 cases, the boundary-layer-averaged turbulent kinetic energy (TKE) stays constant for about a period of one t∗ and decays following a power law
Summary
The characteristics of the convective boundary layer (CBL), a type of planetary boundary layer (PBL) in which surface buoyancy flux drives turbulence, exhibit a strong dependency on surface and atmospheric forcings. When surface buoyancy flux is large relative to wind shear, convective cells composed of central downdrafts and narrow circumferential updrafts appear like the Rayleigh–Bénard convection [1,2]. Khanna and Brasseur [7] simulated five CBLs in a range of −h/L from 0.44 to 730, where h and L are the PBL depth and the Obukhov length, respectively. The stability parameter −h/L indicates the relative roles of shear and buoyancy in the production/consumption of turbulent kinetic energy (TKE) in the PBL [8], and Khanna and Brasseur [7] demonstrated that various convective flow structures appear depending on the stability. Salesky et al [6] simulated CBLs in a wider range of −h/L and found that the transition between the convective rolls and cells occurs gradually but a significant change in their organization
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