Process Studies of the Impact of Land-Surface Resolution on Convective Precipitation Based on High-Resolution ICON Simulations

This study investigated the relevant processes responsible for differences of convective precipitation caused by land-surface resolution. The simulations were performed with the ICOsahedral Nonhydrostatic model (ICON) with grid spacing of 156 m and Large Eddy Simulation physics. Regions of different orographic complexity, days with weak synoptic forcing and favourable convective conditions were selected. The resolution of land-surface properties (soil type, vegetation) and/or the orography was reduced from 156 to 5000 m. Analyses are based on backward trajectories (Lagrangian Analysis Tool (LAGRANTO)), heat budget and convective organisation potential (COP) calculations. On average, the relative difference of areal mean daily precipitation at 1250 and 5000 m land-surface resolutions compared to 156 m were 6% and 15%, respectively. No consistent dependency of precipitation on orography or land-surface properties was found. Both factors impact convective initiation over areas with embedded mesoscale-sized land-surface heterogeneities. The position of convective precipitation was often influenced by the resolution of orography. Coarsening from 156 to 5000 m considerably changed the location of wind convergence and associated convection initiation. It also affects the onset times of clouds (<20 min) and precipitation (≈1 h). Cloud aggregation and microphysical processes proved to be important for further development towards convective precipitation

Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

We performed a high-resolution numerical simulation to study the development of extensive low-level clouds that frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Savè in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere caused by horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to test the hypotheses for cloud formation resulting from this study

Detection of structures in the horizontal wind field over complex terrain using coplanar Doppler lidar scans

Coplanar scans from three Doppler lidars are used to retrieve the horizontal wind field in a horizontal plane of about 5 km × 5 km in size above the city of Stuttgart in south-western Germany. Stuttgart is located in moderate mountainous terrain that is characterized by a basin-shaped valley (Stuttgart basin) which opens into the larger Neckar Valley. Using the retrieved horizontal wind field, which is available on 22 days with a temporal resolution of 1 min and a horizontal resolution of 100 m, we investigate the mesoscale structure of the horizontal flow in the valleys with respect to time of the day, stratification and wind above the mean ridge height, and determine how fast the cells in the convective boundary layer move downstream, i.e. we estimate the convection velocity. The measurements reveal a large spatial and temporal variability of the flow. During stable conditions, the flow below the mean ridge height is decoupled from the flow aloft and downvalley wind dominates in the valleys. At the opening of the Stuttgart basin into the Neckar Valley outflow dominates during nighttime, whereas inflow into the basin prevails in the early morning. During thermally unstable conditions the flow in the valleys is mainly coupled to the flow aloft with a preference for upvalley wind direction. Convective cells moving downstream are detected in the horizontal wind field and a method to estimate the convection velocity from the horizontal wind field measurements is presented. The mean convection velocity is found to be higher by 24 % than the mean horizontal wind speed at the same height and about similar to the wind speed 100 m further up

Spatio-temporal Structure of the Boundary Layer under the Impact of Mountain Waves

During the Hydrological cycle in the Mediterranean Experiment (HyMeX) in autumn 2012 intensive measurements were conducted in the Tavignano Valley, which extends from the centre to the coast of the island of Corsica. On the investigated day, the atmospheric boundary layer (ABL) in the valley showed a distinctive spatio-temporal variability, which resulted from the interaction and superposition of mesoscale dynamically- and thermally-driven processes and dry convection. Based on the observations, not all of the observed ABL characteristics could be explained and hypotheses on the involved processes were formulated in a previous study. To close the observational gaps and to test the hypotheses, high-resolution simulations with the COSMO (Consortium for Small-scale Modeling) model were now performed. The model was able to reproduce the main ABL characteristics and could hence be used to address the processes affecting the ABL. The main features were: in the upper part of the valley, the stable nocturnal ABL was eroded from top and bottom alike by shear-generated turbulent mixing in the vicinity of a mountain wave and buoyancy- and shear-driven surface-based turbulent mixing, leading to a very abrupt increase of the daytime ABL depth. In the lower part of the valley, the ABL remained rather shallow and was dominated by a superimposed thermally-driven sea breeze and upvalley wind. In the afternoon, the formerly deep ABL in the upper part of the valley rapidly decreased when the combined sea breeze and upvalley wind moved up the valley. While the ABL depth was rather horizontally homogeneous in the lower part of the valley and near the coast, it showed a considerable variability in the valley\u27s upper part on scales of a few kilometres due to the varying dominance of the different processes. The local ABL depth also varied considerably in time depending on which influence dominated, i.e. of surface heating, mountain wave or sea breeze and upvalley wind. As the simulated sea breeze strongly depended on the sea-surface temperature, the results were sensitive to the chosen value in the model

An LES-based airborne Doppler lidar simulator and its application to wind profiling in inhomogeneous flow conditions

Wind profiling by Doppler lidar is common practice and highly useful in a wide range of applications. Airborne Doppler lidar can provide additional insights relative to ground-based systems by allowing for spatially distributed and targeted measurements. Providing a link between theory and measurement, a first large eddy simulation (LES)-based airborne Doppler lidar simulator (ADLS) has been developed. Simulated measurements are conducted based on LES wind fields, considering the coordinate and geometric transformations applicable to real-world measurements. The ADLS provides added value as the input truth used to create the measurements is known exactly, which is nearly impossible in real-world situations. Thus, valuable insight can be gained into measurement system characteristics as well as retrieval strategies. As an example application, airborne Doppler lidar wind profiling is investigated using the ADLS. For commonly used airborne velocity azimuth display (AVAD) techniques, flow homogeneity is assumed throughout the retrieval volume, a condition which is violated in turbulent boundary layer flow. Assuming an ideal measurement system, the ADLS allows to isolate and evaluate the error in wind profiling which occurs due to the violation of the flow homogeneity assumption. Overall, the ADLS demonstrates that wind profiling is possible in turbulent wind field conditions with reasonable errors (root mean squared error of 0.36 m s−1 for wind speed when using a commonly used system setup and retrieval strategy for the conditions investigated). Nevertheless, flow inhomogeneity, e.g., due to boundary layer turbulence, can cause an important contribution to wind profiling error and is non-negligible. Results suggest that airborne Doppler lidar wind profiling at low wind speeds (<5ms −1) can be biased, if conducted in regions of inhomogeneous flow conditions

Soil moisture impacts on convective indices and precipitation over complex terrain

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields

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

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. View Full-Tex

Predictability of convective precipitation for West Africa: verification of convection-permitting and global ensemble simulations

Within the framework of this investigation, convection-permitting (CP) ensemble forecasts were generated for West Africa by combining different initial and lateral boundary conditions (IBCs) with perturbations that address the uncertainty of land-surface atmosphere interactions (land-surface perturbations). For a multi-analysis setup, IBCs were taken from model analyses of different global models; for a single-model setup, they were selected from the ensemble system of the European Centre for Medium-range Weather Forecasts (ECMWF). The different ensemble setups were assessed using common probabilistic scores as well as by spatial forecast verification of precipitation generated mainly by convective systems during the West African monsoon season. Additionally, it was investigated whether the CP ensemble forecasts were superior to the ECMWF ensemble forecasts.Probabilistic scores were higher for the single-model ensemble than for the multi-analysis setup, but the latter displayed a larger dispersion and more extreme scenarios. From this, it is concluded that the different model analyses can differ strongly from each other. The land-surface perturbations were able to generate sufficient complementary spread. While the CP simulations showed a stronger negative precipitation bias in the southernmost region near the Guinean coast, the ECMWF simulations exhibited a negative bias further north in the Sahel region, where larger convective systems occur less frequently. Not in all cases did the CP ensemble versions produce better probabilistic scores than the global ensemble forecasts, but they yielded larger spread and less underdispersion. Rank histograms, though, were also influenced by the different structure of the precipitation patterns of the global and CP forecasts. Scores improved when using a later version of the CP model as well as with the skill of the global ensemble forecasts used as IBCs. Altogether, the proposed realization of CP ensemble forecasts is found to be suited for the prediction of convective precipitation in West Africa