The vertical heat and moisture exchange in the convective boundary layer over mountainous terrain is investigated using large-eddy simulation. Both turbulent and advective transport mechanisms are evaluated over the simple orography of a quasi-two-dimensional, periodic valley with prescribed surface fluxes. For the analysis, the flow is decomposed into a local turbulent part, a local mean circulation, and a large-scale part. It is found that thermal upslope winds are important for the moisture export from the valley to the mountain tops. Even a relatively shallow orography, possibly unresolved in existing numericalweather prediction models, modifies the domain-averaged moisture and temperature profiles. An analysis of the turbulent kinetic energy and turbulent heat and moisture flux budgets shows that the thermal circulation significantly contributes to the vertical transport. This transport depends on the horizontal heterogeneity of the transported variable. Therefore, the thermal circulation has a stronger impact on the moisture budget and a weaker impact on the temperature budget. If an upper-level wind is present, it interacts with the thermal circulation. This weakens the vertical transport of moisture and thus reduces its export out of the valley. The heat transport is less affected by the upper-level wind because of its weaker dependence on the thermal circulation. These findings were corroborated in a more realistic experiment simulating the full diurnal cycle using radiation forcing and an interactive land-surface model.

<p>In the convective boundary layer over mountainous regions, the mean values and the fluxes of quantities like heat, mass, and momentum are strongly influenced by thermally induced flows. Several studies have pointed out that the enhanced warming of the air inside a valley can be explained by the valley-volume effect whereas the cross-valley circulation leads to a net export of heat to the free atmosphere. We are interested in the influence of an upper-level wind on the local circulations and the boundary-layer properties, both locally and in terms of the horizontal mean, as this aspect has not yet received much attention. LES are carried out over idealized, two-dimensional topographies using the CM1 numerical model. For the analysis, turbulent, mean-circulation, and large-scale contributions are systematically distinguished. Also, budget analyses are performed for the turbulence kinetic energy and the turbulent heat and mass flux. Based on the first results for periodic topographies, no crucial influence on the horizontally averaged heat-flux and temperature profile can be observed, even though the flow pattern of the thermal wind is qualitatively changed. In addition to that, the impact on moisture transport will be evaluated and simulations over different topographies as well as for different atmospheric conditions and surface properties will be presented.</p>

The formation of low stratus cloud over idealized hills is investigated using numerical model simulations. The main driver for the cloud formation is radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, the advection terms in the prognostic equations for heat and moisture produce vertical mixing across the upper cloud edge, leading to a loss of cloud water content. This behavior is depicted via a budget analysis. More precisely, this spurious mixing is caused by the diffusive error of the advection scheme in regions where the sloping surfaces of the terrain‐following vertical coordinate intersect the cloud top. This study shows that the intensity of the (spurious) numerical diffusion depends strongly on the horizontal resolution, the order of the advection schemes, and the choice of scalar advection scheme. A large‐eddy simulation with 4‐normalm$$ \mathrm{m} $$ horizontal resolution serves as a reference. For horizontal resolutions of a few hundred meters and simulations carried out with a model setup as used in numerical weather prediction, a strong reduction of the simulated liquid‐water path is observed. In order to keep the (spurious) numerical diffusion at coarser resolutions small, at least a fifth‐order advection scheme should be used. In the present case, a weighted essentially nonoscillatory scalar advection scheme turns out to increase the numerical diffusion along a sharp cloud edge compared with an upwind scheme. Furthermore, the choice of vertical coordinate has a strong impact on the simulated liquid‐water path over orography. With a modified definition of the sigma coordinate, it is possible to produce cloud water where the classical sigma coordinate does not allow any cloud formation.

<p>Orography induced gravity waves are investigated in a Multi Scale Gravity Wave Model (MS-GWaM) over idealized topography. MS-GWaM is a prognostic gravity-wave model, which parametrizes both the propagation and dissipation of subgrid-scale GWs. It is a Lagrangian ray-tracer model, which applies WKB-theory and calculates the propagation of ray volumes in spectral space. Its novelty is that not only the dissipative effect, but also the non-dissipative effects due to direct wave-mean flow interaction are captured. In our conceptual studies we investigate mountain wave generation, which is induced via a time-dependent large-scale wind encountering a prescribed topography. The framework used in our experiments is the PincFloit model, which integrates the pseudo-incompressible equations. We use it both in low resolution with MS-GWaM and in high resolution LES mode as a wave resolving reference. In the reference LES simulations the idealized topography, a mountain chain, is represented with an immersed boundary method. In the MS-GWaM experiments there is no resolved topography, but its effect is modelled as a lower boundary condition. The lower boundary condition is represented by initializing ray volumes with wave number and wave action density depending on the mountain characteristics and the large scale wind speed, based on the assumption that the flow follows the terrain. In the wave resolving reference experiments the flow does not strictly follow the terrain, but other instabilities (rotor formation, boundary layer separation) arise around the mountains. These processes decrease the available momentum transported by GWs, which was initially not accounted for in MS-GWaM. Thus an overestimation of wind deceleration was found in the MS-GWaM parametrization compared to the wave resolving simulation. To correct for this overestimation, an effective mountain height is introduced into Ms-GWaM, which is calculated by a scaling function between mountain height and flow properties using the Froude number.</p>

<p>Local thermal circulations developing over heated valley slopes strongly influence the convective boundary layer (CBL) over mountainous terrain. For this study, large-eddy simulations are carried out both over idealized valleys and semi-idealized complex terrain. The flow is decomposed into a turbulent part, a local mean circulation capturing the slope winds, and a large-scale (upper-level) wind. This allows a detailed budget analysis for heat and moisture. The temperature distribution is horizontally fairly uniform inside each valley due to the homogenizing effect of the thermally-induced circulations. In contrast to that, the slope winds contribute strongly to the transport of moisture up to the ridges. The entrainment of dry air by the recirculation leads to a horizontally non-uniform moisture distribution. Consequently, a large-scale, upper-level wind hardly affects the horizontally homogeneous temperature distribution while it can considerably reduce the vertical moisture transport: a horizontal wind mixes the moisture from the slope-wind layer into the dryer regions of each valley. Single updrafts mark the small-scale end of coherent motions in the CBL over complex terrain. A conditional sampling method is applied in order to identify the thermal plumes using a passive tracer. In the mixed layer, the plumes are moving upslope with the slope wind. In order to quantify the contribution of the plumes to the vertical transport of heat and moisture, the joint probability density functions of the turbulent fluxes are calculated and decomposed into a local and a coherent part. From this perspective, the turbulence statistics is analyzed at different heights in the CBL and compared to the statistics over flat terrain. In general, the plumes turn out to dominate the vertical fluxes in the valleys and in the lower part of the boundary layer. Especially at ridge height, where the updrafts are few but almost stationary, the statistics are fairly different.</p>