On the Role of Large-Scale Updrafts and Downdrafts in Deviations From Monin–Obukhov Similarity Theory in Free Convection

Abstract

We investigate by means of direct numerical simulation how large-scale circulations produce deviations from Monin-Obukhov similarity theory (MOST) in the limit of free convection, disentangling the role of large-scale downdrafts from updrafts using conditional analysis. We compare the convective boundary layer to two other free-convective flows: Rayleigh-Bénard convection with an adiabatic top lid and classical Rayleigh-Bénard convection. This serves a dual purpose: firstly, to ascertain how changes in the upper boundary conditions and thereby in the large-scale circulations modify the near-surface behaviour and secondly, to assess to what extent we can extrapolate results from idealized systems to the unstable atmospheric surface layer. Using a low-pass filter to define the large scales we find that, whilst deviations from MOST occur within large-scale downdraft regions, strong deviations also occur within large-scale updraft regions. The deviations within updrafts are independent of the filter length scale used to define the large-scale circulations, independent of whether updrafts are defined as ascending air, or as air that is both ascending and positively buoyant, and are not due to changes with height of the updraft area fraction. This suggests that even updraft properties are not just determined locally, but also by outer scales. Cold, strong downdrafts in classical Rayleigh-Bénard convection notably modify the near-surface behaviour compared to the other two systems. For the moderate Reynolds numbers considered, Rayleigh-Bénard convection with an adiabatic top lid thus seems more appropriate than classical Rayleigh-Bénard convection for studying the unstable atmospheric surface layer in the limit of free convection.

New Insights into Wind Shear Effects on Entrainment in Convective Boundary Layers Using Conditional Analysis

“…4). Moreover, a smaller mixed-layer turbulent area fraction in the shear-free CBL is consistent with the observation that free-tropospheric air can be carried deep into the boundary layer by largescale downdrafts without too much mixing (Lohou et al 2010;van de Boer et al 2014;Fodor et al 2019). This effect is more prominent in the shear-free CBL, where the descending branches of the convection cells are much less rotational than the ascending turbulent plumes (Fig.…”

Steady-State Large-Eddy Simulations of Convective and Stable Urban Boundary Layers

“…2.2.1). For example, Inagaki et al (2012) highlight that coherent structures in the convective mixed layer significantly impact the nature of flow within the UCL and Fodor et al (2019) illustrate that large-scale updrafts and downdrafts cause deviations from Monin-Obukhov similarity theory within the freely convective surface layer and quantify this effect for different top boundary conditions. The applicability of this assumption is analyzed in detail in Sects 4.2 and 4.3.…”

“…The main differences are found to be in the upper half of the flow where the warmer air entrained down into the boundary layer alters the form of the updrafts and downdrafts [see Sorbjan (1996) for details]. A recent study by Fodor et al (2019) also investigates the impact of different top boundary conditions (capped and uncapped) on the CBL (in the limit of free convection) and similarly identifies differences in the upper half of the flow (the surface layer is shown to be unaffected unless downdrafts are sufficiently cold and strong). It was shown in Sect.…”

Revisiting the role of intermittent heat transport towards Reynolds stress anisotropy in convective turbulence

“…The previous works on the atmospheric surface layer (ASL) plumes have focused on the following: (a) deducing their detailed structures and dynamics (Wilczak 1984); (b) identifying the coupling between the surface and air temperatures (Garai & Kleissl 2011, 2013); and (c) investigating the difference in the Monin–Obukhov similarity functions by conditioning on the updraft and downdraft motions (Li et al 2018; Fodor, Mellado & Wilczek 2019). However, some early investigators noted that in an unstable ASL there were certain intermittent bursts in the upward heat flux, persisting for approximately 10–20 s duration, which were associated with large downward momentum transport (Kaimal & Businger 1970; Haugen, Kaimal & Bradley 1971).…”