This paper reviews the current understanding of moist orographic convection and its regulation by surface-exchange processes. Such convection tends to develop when and where moist instability coincides with sufficient terrain-induced ascent to locally overcome convective inhibition. The terrain-induced ascent can be owing to mechanical (airflow over or around an obstacle) and/or thermal (differential heating over sloping terrain) forcing. For the former, the location of convective initiation depends on the dynamical flow regime. In "unblocked" flows that ascend the barrier, the convection tends to initiate over the windward slopes, while in "blocked" flows that detour around the barrier, the convection tends to initiate upstream and/or downstream of the high terrain where impinging flows split and rejoin, respectively. Processes that destabilize the upstream flow for mechanically forced moist convection include large-scale moistening and ascent, positive surface sensible and latent heat fluxes, and differential advection in baroclinic zones. For thermally forced flows, convective initiation is driven by thermally direct circulations with sharp updrafts over or downwind of the mountain crest (daytime) or foot (nighttime). Along with the larger-scale background flow, local evapotranspiration and transport of moisture, as well as thermodynamic heterogeneities over the complex terrain, regulate moist instability in such events. Longstanding limitations in the quantitative understanding of related processes, including both convective preconditioning and initiation, must be overcome to improve the prediction of this convection, and its collective effects, in weather and climate models.

Abstract:The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave-turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes.

Idealized, convection-resolving simulations of moist orographic flows are conducted to investigate the influence of temperature and moist stability on the drying ratio (DR), defined as the fraction of the impinging water mass removed as orographic precipitation. In flow past a long ridge, where most of the air rises over the barrier rather than detouring around it, DR decreases as the surface temperature (T s ) increases, even as the orographic cap cloud becomes statically unstable at higher T s and develops embedded convection. This behaviour is explained by a few physical principles: (1) the Clausius-Clapeyron equation dictates that the normalized condensation rate decreases as the flow gets warmer, (2) the replacement of ice-phase precipitation growth with warm-rain processes decreases the efficiency by which condensate is converted to precipitation, thereby lowering precipitation efficiency, and (3) embedded convection acts more to vertically redistribute moisture than to enhance precipitation. Over an isolated mountain, the effects of (1) and (2) are counteracted by moisture deflection around the barrier, which is stronger in the colder, more stable flows.

No abstract

Numerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed. A basic benchmark case and five other test cases are simulated in a two-dimensional framework using an identical initial state is based on conditions on 25 March 2006 during Intensive Observation Period (IOP) 6 of the Terrain-Induced Rotor Experiment (T-REX), in which intense mountain-wave activity was observed. All of the models use an identical horizontal resolution of 1 km and the same vertical resolution. The six simulated test cases use various terrain heights: a 100-m bell shaped hill, a 1000-m idealized ridge that is steeper on the lee slope, a 2500-m ridge, and a cross Sierra terrain profile. The models are tested with both free slip and no slip lower boundary conditions.The results indicate a surprisingly diverse spectrum of simulated mountain wave characteristics including lee waves, hydraulic-like jump features, and gravity wave breaking. The vertical velocity standard deviation is over a factor of two larger in the free slip experiments relative to the no slip simulations. Nevertheless, the no slip simulations exhibit considerable variations in the wave characteristics. The vertical flux of horizontal momentum profiles vary significantly among the models, particularly for the case with realistic Sierra terrain. The results imply relatively low predictability of key characteristics of topographically-forced flows such as the strength of downslope winds and stratospheric wave breaking. The vertical flux of horizontal momentum, which is a domain integrated quantity, exhibits considerable spread among the models, particularly for the experiments with the 2500-m ridge and Sierra terrain. The diversity among the various model simulations, all initialized with identical initial states, suggests that model dynamical cores may be an important component of diversity for the design of mesoscale ensemble systems for topographically-forced flows. The inter-model differences are significantly larger than sensitivity experiments within a single modeling system. IntroductionThe fundamental linear theory for the generation of inviscid mountain waves forced by stratified air flow over two-dimensional obstacles has been established for several decades (e.g., Queney et al. 1960;Smith 1979;Smith 1989). Vertically propagating mountain waves often amplify in the stratosphere due to the decrease of atmospheric density with altitude and nonlinear processes, which may lead to overturning and turbulent breakdown (e.g., Lindzen 1988;Bacmeister and Schoeberl 1989). Mountain waves can have an important impact on the atmosphere due to their role in downslope windstorms (Klemp and Lilly 1975), clear-air turbulence (Clark et al. 2000), vertical mixing of water vapor, aerosols, and chemical constituents in the stratosphere (Dörnbrack and Dürbeck 1998), potential vorticity generation (Schär and Durran 1997), and orographic drag influence on the general circulation (Bretherton 1969;Ólafsson and Bougeau...

n'è surprising's'elisttivity orme •wiñaTíeTcf, aerosol cónceñírafion", clo'ucf physicsrand precipitation over Dominica to trade-wind speed arises from a change from thermaliy to mechanicaliy triggered convection and from nonplunging to piunging airfiow. O rographie precipitation influences water resources, flooding and landslides, regional climates, and global water budgets. Furthermore, the study of mountain-induced lifting and precipitation serves as a prototype for the study of other types of airmass lifting in the atmosphere (e.g.,

Cloud-resolving numerical simulations of airflow over a diurnally heated mountain ridge are conducted to explore the mechanisms and sensitivities of convective initiation under high pressure conditions. The simulations are based on a well-observed convection event from the Convective and Orographically Induced Precipitation Study (COPS) during summer 2007, where an isolated afternoon thunderstorm developed over the Black Forest mountains of central Europe, but they are idealized to facilitate understanding and reduce computational expense.In the conditionally unstable but strongly inhibited flow under consideration, sharp horizontal convergence over the mountain acts to locally weaken the inhibition and moisten the dry midtroposphere through shallow cumulus detrainment. The onset of deep convection occurs not through the deep ascent of a single updraft but rather through a rapid succession of thermals that are vented through the mountain convergence zone into the deepening cloud mass. Emerging thermals rise through the saturated wakes of their predecessors, which diminishes the suppressive effects of entrainment and allows for rapid glaciation above the freezing level as supercooled cloud drops rime onto preexisting ice particles. These effects strongly enhance the midlevel cloud buoyancy and enable rapid ascent to the tropopause. The existence and vigor of the convection is highly sensitive to small changes in background wind speed U 0 , which controls the strength of the mountain convergence and the ability of midlevel moisture to accumulate above the mountain. Whereas vigorous deep convection develops for U 0 5 0 m s 21 , deep convection is completely eliminated for U 0 5 3 m s 21 . Although deep convection is able to develop under intermediate winds (U 0 5 1.5 m s 21 ), its formation is highly sensitive to small-amplitude perturbations in the initial flow.

The triggering of convective orographic rainbands by small-scale topographic features is investigated through observations of a banded precipitation event over the Oregon Coastal Range and simulations using a cloud-resolving numerical model. A quasi-idealized simulation of the observed event reproduces the bands in the radar observations, indicating the model's ability to capture the physics of the band-formation process. Additional idealized simulations reinforce that the bands are triggered by lee waves past small-scale topographic obstacles just upstream of the nominal leading edge of the orographic cloud. Whether a topographic obstacle in this region is able to trigger a strong rainband depends on the phase of its lee wave at cloud entry. Convective growth only occurs downstream of obstacles that give rise to lee-wave-induced displacements that create positive vertical velocity anomalies w c and nearly zero buoyancy anomalies b c as air parcels undergo saturation. This relationship is quantified through a simple analytic condition involving w c , b c , and the static stability N 2 m of the cloud mass. Once convection is triggered, horizontal buoyancy gradients in the cross-flow direction generate circulations that align the bands parallel to the flow direction.