On upstream blocking over heated mountain ridges

Abstract

Nested large-eddy simulations of heated flows over idealized mesoscale mountain ridges are conducted to investigate the impacts of diabatic surface heating on orographic blocking. Three specific heating-related impacts are isolated and quantified: (i) destabilization of the impinging planetary boundary layer, (ii) upslope flow driven by elevated heating and (iii) erosion of blocked layers by buoyancy-driven mixing. The heating gives rise to a manyfold enhancement of simulated flow-over (the fraction of impinging momentum flux that surmounts the crest) in weaker flows over taller ridges, owing primarily to upslope flow driven by elevated heating. By contrast, the other two effects enhance flow-over by up to ∼ 50%. Unexpectedly, complete neutralization of the subcrest boundary layer does not put an immediate end to upstream blocking-once entrenched, blocking can persist for several hours due to inertial effects, particularly in weaker flows. A simplified scaling is developed to predict mechanically and thermally induced pressure perturbations induced by the ridge. The scaling, which captures the magnitudes and sensitivities of the simulated pressure perturbations reasonably well, is used to formulate two non-dimensional parameters, one the ratio of pressure-induced upwind deceleration to the impinging momentum flux (J) and the other the ratio of thermal to mechanical pressure perturbations (R). A regime diagram of flow-over, constructed in J -R parameter space, shows that blocking is minimized for small J (the mechanically unblocked regime) and/or large R (the thermally forced regime).

Moist Orographic Convection: Physical Mechanisms and Links to Surface-Exchange Processes

“…In practice, however, most studies applying M to such flows simply average N and U over the subcrest layer (e.g., [23]). Other complexities stem from diabatic processes, namely surface heating (e.g., [24][25][26]) and terrain-forced saturation and latent-heat release (e.g., [27,28]). Such heating locally reduces N (and, hence, M), which facilitates terrain-forced ascent.…”

“…Whether an orographic flow is driven by mechanical and/or thermal forcing is central to interpreting the physics of convective initiation. Various nondimensional parameters comparing thermal to mechanical forcing amplitudes have been derived using linear theory [25,31,32], thermodynamic heat-engine theory [32][33][34][35], or scaling the pressure perturbations arising from each forcing [26,36]. Although the assumptions underlying each derivation varies, they all broadly agree that the role of thermal forcing is maximized for light background winds and weak low-level stability.…”

“…Kirshbaum [26], however, cautioned that M is a flawed predictor of mountain flow regimes in the presence of thermal forcing. They found that a generalized M accounting for multi-layer flow (e.g., a convective ABL topped by an inversion), along with a thermal forcing parameter, were required to construct a regime diagram for diurnally heated orographic flows.…”

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…These should rely on a decomposition of the vertical transport into a turbulent component and an advective slope circulation component [85]. Second, the thermodynamic impact of thermally forced upslope circulations on the atmosphere above mountains can impact dynamical processes such as orographic blocking [86] and has implications towards convective preconditioning and convective initiation [51]. These two perspectives on the properties of upslope winds refer to largely distinct spatial scales, namely a single point along a slope in one case, and a whole mountain ridge in the other.…”

Does Strong Tropospheric Forcing Cause Large‐Amplitude Mesospheric Gravity Waves? A DEEPWAVE Case Study

“…ε values above ε c indicate blocking resulting in flow around the mountain, while ε < ε c suggests flow over the mountains which favors the excitation of mountain waves. Although the actual ε values have to be taken with great care (Reinecke and Durran () & Kirshbaum ()), it helps to classify the general flow regime and its transitional behavior during the event. In order to reflect this uncertainty, ε was calculated for different mountain heights h m ranging from 500 m to 2500 m as indicated by the error bars in Figure a.…”