What determines the differences found in forest edge flow between physical models and atmospheric measurements? - An les study

A recent study has shown that Doppler lidar is a state-of-the-art method to obtain spatially and temporally resolved flow fields in forest edge flow regimes. In that study, the general flow features observed by lidar were found to be similar to those detected above a physical tree model in a wind tunnel. But in pivotal details, for example regarding the absolute height and the inner structure of the internal boundary layer (IBL), significant differences were detected. The main objectives of this Large-Eddy Simulation (LES) study are to analyze these differences and to associate them to the meteorological and physical differences between the set-ups of the wind tunnel and the atmospheric measurement. This enables on the one hand a model evaluation for the LES and the physical model respectively, and on the other hand a better understanding of the results from the lidar measurements. Results from an LES with neutral stratification and without Coriolis force show a similar IBL structure as in the wind tunnel and represent well-known characteristics of forest edge flow. A variation of the forest density only marginally affects the IBL structure. The presence of a finite forest clearing as observed at the lidar site increases the turbulence level of the IBL, compared to a set-up with a quasi-infinite clearing like in the wind tunnel. Including Coriolis force further enhances the turbulence levels to values observed by lidar. An increasing thermal instability results in even higher turbulence levels. Hence, differences between wind tunnel and atmospheric measurements are mainly traced back to differences in the flow forcing and in the onflow conditions upstream of the forest edge. Furthermore, a statistical analysis reveals that insufficient averaging of the lidar data also contributes to the observed deviations from the wind tunnel results. Based on this analysis, we suggest that at least two and a half hours of measurements during equivalent atmospheric conditions are necessary to obtain a statistically representative mean IBL structure

Nocturnal Low-level Jet Evolution in a Broad Valley Observed by Dual Doppler Lidar

The temporal evolution of a nocturnal low-level jet (LLJ) in the 40km$40\,\text{km}$ broad Rhine Valley near Karlsruhe is studied, in the framework of a case study, with two heterodyne detection Doppler lidars using the new scan concept of “virtual towers”. For validation of this measuring technique, we performed comparative case studies with a tethered balloon and the highly instrumented 200m$200\,\text{m}$ KIT tower. The findings show capabilities of the virtual tower technique for wind measurements. Virtual towers can be placed at all locations within the range of Lidar measurements. Associated with nocturnal stable stratification, the LLJ, a wind speed maximum of about 9ms-1$9\,\text{m}\,\text{s}^{-1}$, develops at 100m$100\,\text{m}$ to 150m$150\,\text{m}$ agl, but the wind does not show the typical clockwise wind direction change that is reported in many other studies. This is attributed to the channeling effect occurring in broad valleys like the Rhine Valley when the boundary layer is stably stratified. Such channeling means a significant deviation of the wind direction from the Ekman spiral so that low-altitude winds turn into valley-parallel direction

The impact of convergence zones on the initiation of deep convection: A case study from COPS

During the ‘Convective and Orographically-induced Precipitation Study’ (COPS) performed in summer 2007, deep convection developed on July 15, although convective available potential energy was only moderate and convective inhibition was high. Convection was restricted to an area east of the Black Forest crest. Data analysis revealed that the convection was triggered by different mechanisms. Due to a surface high which was situated east of the Black Forest and a surface low which approached the investigation area from the west, a mesoscale convergence zone was established between the two regions and moved eastwards. Secondly, high insolation favoured the development of slope and valley winds and high evapotranspiration resulted in an increase of moisture in the planetary boundary layer (PBL). The thermally driven circulation systems formed a convergence zone along the mountain crest. When the synoptically induced mesoscale convergence zone reached the Black Forest, the different convergence zones superimposed optimally, such that strong updraughts were observed above the mountain. These updraughts penetrated the PBL-capping inversion and nearly reached the level of free convection. About 15 min after the convergence zone had passed the Black Forest crest, first clouds developed east of it. While moving further eastwards, the convergence zone intensified and became visible as a north-south oriented cloud line in the satellite images. Some deep convective cells with precipitation formed within the cloud line. The dense COPS network allowed the capture of the position and characteristics of the convergence zone and explains why convection developed in some restricted areas only

The dependence of convection-related parameters on surface and boundary-layer conditions over complex terrain

The field campaign ‘Convective and Orographically-induced Precipitation Study’ (COPS) was performed in south-western Germany and eastern France in summer 2007. Within the COPS context this study focused on the process chain of soil moisture, surface fluxes, conditions of the convective boundary layer (CBL), and convection-related parameters. The results were different for valley and mountain sites. Only in the Rhine valley did the ratios of sensible and latent heat to the net radiation at the surface, H0/Q0 and E0/Q0 respectively, reveal a weak dependence on soil moisture. H0/Q0 was lower and E0/Q0 was higher at higher soil moisture. The correlation of the diurnal increase of the equivalent potential temperature,e, with the energy supplied by H0 and E0 was found to be lower for higher surface inhomogeneity. Furthermore, only a weak dependence of the CBL depth on the sensible surface heat flux was found for valley sites and was non-existent for the mountain crest. The convective indices in the whole COPS domain were found to depend on e in the CBL. The absolute values of conditional and potential instability are not necessarily the decisive parameters for convection to occur, because highest instabilitywas observed in the Rhine valley while convection was preferably initiated over the mountains. Convective inhibition (CIN) was positively correlated with the capping strength and negatively with the CBL height: the higher the CBL, the lower the upper threshold of CIN. The frequency of low CIN was higher in the Black Forest mountains than in the Rhine valley, which facilitates convection initiation over the mountain sites

Observation of convection initiation processes with a suite of state-of-the-art research instruments during COPS IOP 8b

International audienceIn the afternoon of 15 July 2007, a thunderstorm was initiated within a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographically-induced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary-layer structure, lids and the water-vapour field in the pre-convective environment of the event are discussed. For this purpose, we investigated measurements of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated-water-vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large-scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent-heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area in which CI took place