Scaling the Vertical-Velocity Variance During the Very Late Afternoon Transition of the Convective Boundary Layer
Year: 2025
Type: article
Abstract: In the bulk of the convective boundary layer driven by surface heating, the vertical-velocity variance is known to scale with the convective velocity scale. This scaling relies on the quasi-equilibrium assumption that the surface heat flux (H, the water-vapor contribution to buoyancy is neglected) varies slowly compared to the adjustment time scale of the large scale convective eddies (or the eddy turnover time), a condition which over land is typically satisfied from the late morning to the early afternoon transition. Later on, when the surface heat flux starts decaying moderately compared to the eddy turnover time, a departure from the quasi-equilibrium regime is expected. A recent idealized large-eddy simulations (LES) study proposed a parameter for describing such departure during the late afternoon transition: $${\widetilde{r}} \equiv |H^{-1}dH/dt|^{-1}/t_*$$ r ~ ≡ | H - 1 d H / d t | - 1 / t ∗ , where $$t_*$$ t ∗ is the eddy turnover time. The quasi-equilibrium assumption applies when $${\widetilde{r}}$$ r ~ $$>>$$ > > 1, and breaks down when $${\widetilde{r}}$$ r ~ $$\sim $$ ∼ 1. Building on these results, we further investigate the scaling for the vertical-velocity variance during the very late afternoon transition, i.e. in the regime $${\widetilde{r}}$$ r ~ $$<<$$ < < 1 where the surface heat flux decays rapidly compared to the eddy turnover time. In a first step, we use LES to reveal that the regime $${\widetilde{r}}$$ r ~ $$<<$$ < < 1 is characterized by a new velocity scale $$w_{*r} \equiv ((g/\overline{\theta })|dH/dt| z_i^2)^{1/4}$$ w ∗ r ≡ ( ( g / θ ¯ ) | d H / d t | z i 2 ) 1 / 4 . Within the specific framework of this study using the parameter $${\widetilde{r}}$$ r ~ for identifying the out-of-equilibrium regime $${\widetilde{r}}$$
Source: Boundary-Layer Meteorology
Authors: Omar El Guernaoui, Dan Li, Joachim Reuder
Language: English
Subfield: Atmospheric Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $3,790
OpenAlex ID: W4407242568