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The atmospheric large‐scale environment determines the occurrence of local extreme precipitation, and it is unclear how climate change affects this relationship. Here we investigate the present‐day relationship between large‐scale circulation types (CTs) and daily precipitation extremes over Scandinavia and its future change. A 50‐member EC‐Earth3 large ensemble is used to assess future changes against internal variability. We show that CTs are related to extreme precipitation over the entire domain. The intensity of extreme daily precipitation increases in all seasons in the future climate, generally following the strength of warming in the six different future scenarios considered. However, no significant future change is found in the relationship between extreme precipitation and the CTs in any season or scenario. The results have important implications for applications that rely on the stability of this relationship, such as statistical and event‐based dynamical downscaling of future weather and climate predictions and long‐term climate projections. Plain Language Summary The occurrence of local devastating extreme precipitation events is influenced by the large‐scale flow of the atmosphere, that is, high‐ or low‐pressure systems and winds from different directions. Understanding this connection helps us to predict precipitation extremes using more readily available information about the large‐scale flow. However, it is not known if the relationship that we observe in the present climate will hold under future climate conditions. Here we study the present‐day relationship between the large‐scale flow and local precipitation events over Scandinavia and analyze how it will change toward the end of the 21st century. We find that extreme precipitation events become more intense over entire Scandinavia in the future climate, but their connection to the large‐scale environment remains unchanged. Key Points Large‐scale circulation types (CTs) can be used as precursors of local extreme precipitation events over Scandinavia The intensity of extreme precipitation events over Scandinavia increases with the warming strength in the future climate The relationship between extreme precipitation and CTs remains unchanged over Scandinavia in the future climate

This study examines the spatial variability of the nocturnal wind field using eight networks of surface observations ranging in horizontal width from 500 m to 65 km. The wind field is partitioned into small-scale variability (submeso motions) and the spatially-averaged wind vector. The vector-averaged wind is analogous to the wind resolved by a numerical model, posed here in terms of the wind that is vector averaged over an observational network. The small-scale variability represents the unresolved subgrid (sub-network) variation estimated in terms of the spatial variation of the wind vector within the observational domain. The bulk formula for the spatially-averaged heat flux is modified to account for the subgrid variation of the wind field. Investigation of the spatial variability of the wind field is also motivated by the need to estimate the representativeness of observations of the wind vector at an individual measurement site with respect to the wind field over the surrounding landscape. The small-scale variability of the observed wind field is contrasted between the networks as a function of the spatially-averaged wind vector, stratification, size of the network, and the topography. A strong dependence on topography emerges in spite of different instrumentation, deployment strategy, and processing for each network. Even weak topography can be important. A better design for future observational networks is briefly discussed.

We show that the geometry of motions in atmospheric boundary‐layer time series exhibits considerable independence from scale in spite of changing physics. The scale‐independence of structure shapes is shown by using a simple technique to extract basic shapes from the time series for timescales between 3 s and 2 h. A set of predefined basic shapes is chosen subjectively as those that occur most frequently in the time series: sine, step, ramp‐cliff and cliff‐ramp. The frequency of occurrence of shapes changes with the timescale, with a pronounced minimum at scales between 2 and 10 min depending on the stability and the shape function. This is in accordance with the minimum of kinetic energy between turbulence and mesoscales. However, the ratios of occurrences between different shapes are approximately scale‐independent. What shapes are preferred depends only on the variable examined. The physics of different shapes and scales is examined from characteristics of individual shapes. Steep edges of shapes seem to be predominantly related to downward transport of heat and momentum, which weakens with increasing scale. Sine shapes on the other hand seem to be related to turbulent eddies and shear instability at small scales, and to internal gravity waves at larger scales with stable stratification. Therefore, the physics of individual shapes is shown to change with scale, while the geometry seems to remain approximately scale‐independent. Key Points Organized structures are recognized as different geometrical shapes The physics of the flow changes for different shapes and time scales The geometry of the flow does not depend on time scale

Heatwaves (HWs) are high-impact phenomena stressing both societies and ecosystems. Their intensity and frequency are expected to increase in a warmer climate over many regions of the world. While these impacts can be wide-ranging, they are potentially influenced by local to regional features such as topography, land cover, and urbanization. Here, we leverage recent advances in the very high-resolution modelling required to elucidate the impacts of heatwaves at these fine scales. Further, we aim to understand how the new generation of km-scale regional climate models (RCMs) modulates the representation of heatwaves over a well-known climate change hot spot. We analyze an ensemble of 15 convection-permitting regional climate model (CPRCM, ~ 2–4 km grid spacing) simulations and their driving, convection-parameterized regional climate model (RCM, ~ 12–15 km grid spacing) simulations from the CORDEX Flagship Pilot Study on Convection. The focus is on the evaluation experiments (2000–2009) and three subdomains with a range of climatic characteristics. During HWs, and generally in the summer season, CPRCMs exhibit warmer and drier conditions than their driving RCMs. Higher maximum temperatures arise due to an altered heat flux partitioning, with daily peaks up to ~ 150 W/m 2 larger latent heat in RCMs compared to the CPRCMs. This is driven by a 5–25% lower soil moisture content in the CPRCMs, which is in turn related to longer dry spell length (up to double). It is challenging to ascertain whether these differences represent an improvement. However, a point-scale distribution-based maximum temperature evaluation, suggests that this CPRCMs warmer/drier tendency is likely more realistic compared to the RCMs, with ~ 70% of reference sites indicating an added value compared to the driving RCMs, increasing to 95% when only the distribution right tail is considered. Conversely, a CPRCMs slight detrimental effect is found according to the upscaled grid-to-grid approach over flat areas. Certainly, CPRCMs enhance dry conditions, with knock-on implications for summer season temperature overestimation. Whether this improved physical representation of HWs also has implications for future changes is under investigation.

Recent studies using convection-permitting (CP) climate simulations have demonstrated a step-change in the representation of heavy rainfall and rainfall characteristics (frequency-intensity) compared to coarser resolution Global and Regional climate models. The goal of this study is to better understand what explains the weaker frequency of precipitation in the CP ensemble by assessing the triggering process of precipitation in the different ensembles of regional climate simulations available over Europe. We focus on the statistical relationship between tropospheric temperature, humidity and precipitation to understand how the frequency of precipitation over Europe and the Mediterranean is impacted by model resolution and the representation of convection (parameterized vs. explicit). We employ a multi-model data-set with three different resolutions (0.44°, 0.11° and 0.0275°) produced in the context of the MED-CORDEX, EURO-CORDEX and the CORDEX Flagship Pilot Study \"Convective Phenomena over Europe and the Mediterranean\" (FPSCONV). The multi-variate approach is applied to all model ensembles, and to several surface stations where the integrated water vapor (IWV) is derived from Global Positioning System (GPS) measurements. The results show that all model ensembles capture the temperature dependence of the critical value of IWV (IWVcv), above which an increase in precipitation frequency occurs, but the differences between the models in terms of the value of IWVcv, and the probability of its being exceeded, can be large at higher temperatures. The lower frequency of precipitation in convection-permitting simulations is not only explained by higher temperatures but also by a higher IWVcv necessary to trigger precipitation at similar temperatures, and a lower probability to exceed this critical value. The spread between models in simulating IWVcv and the probability of exceeding IWVcv is reduced over land in the ensemble of models with explicit convection, especially at high temperatures, when the convective fraction of total precipitation becomes more important and the influence of the representation of entrainment in models thus becomes more important. Over lowlands, both model resolution and convection representation affect precipitation triggering while over mountainous areas, resolution has the highest impact due to orography-induced triggering processes. Over the sea, since lifting is produced by large-scale convergence, the probability to exceed IWVcv does not depend on temperature, and the model resolution does not have a clear impact on the results.

The advancement of computational resources has allowed researchers to run convection-permitting regional climate model (CPRCM) simulations. A pioneering effort promoting a multimodel ensemble of such simulations is the CORDEX Flagship Pilot Studies (FPS) on “Convective Phenomena over Europe and the Mediterranean” over an extended Alps region. In this study, the Distribution Added Value metric is used to determine the improvement of the representation of all available FPS hindcast simulations for the daily mean near-surface wind speed. The analysis is performed on normalized empirical probability distributions and considers station observation data as the reference. The use of a normalized metric allows for spatial comparison among the different regions (coast and inland), altitudes and seasons. This approach permits a direct assessment of the added value between the CPRCM simulations against their global driving reanalysis (ERA-Interim) and respective coarser resolution regional model counterparts. In general, the results show that CPRCMs add value to their global driving reanalysis or forcing regional model, due to better-resolved topography or through better representation of ocean-land contrasts. However, the nature and magnitude of the improvement in the wind speed representation vary depending on the model, the season, the altitude, or the region. Among seasons, the improvement is usually larger in summer than winter. CPRCMs generally display gains at low and medium-range altitudes. In addition, despite some shortcomings in comparison to ERA-Interim, which can be attributed to the assimilation of wind observations on the coast, the CPRCMs outperform the coarser regional climate models, both along the coast and inland.

The thermodynamic structure of the lower troposphere over the Southern Ocean is analyzed by employing over 16 years of high resolution upper air soundings from Macquarie Island (54.62°S, 158.85°E). The soundings are analyzed to develop an understanding of the structure of the boundary layer and wind shear occurring through the lower levels over this region, and to compare this to European Centre for Medium‐Range Weather Forecasts (ECMWF) model level reanalysis data for the Year of Tropical Convection (YOTC). A multiple layered structure is commonly observed in the high resolution soundings, and is also observed in YOTC, but with a lower frequency. The climatological mean and variability of a number of variables are calculated for both data sets, which reveals that YOTC performs well, but has weaknesses in modeling the observed moisture and wind fields, particularly evident in wind shear profiles. A distinction between a number of boundary layer types is made, and the frequency with which they occur is quantified for both data sets. This highlights important differences between the observations in the Macquarie Island soundings and the reanalysis product. Proxy cloud fields are constructed for the two data sets, and these suggest that clouds are commonly observed in a region between the top of the boundary layer and a secondary temperature inversion, i.e., a “buffer layer” in the words of Russell et al. (1998). The peak relative frequency of observing these clouds lies roughly around 912.5 hPa for both data sets. An examination of the wind shear across the cloud boundaries finds wind shear over cloud base occurs more frequently than cloud top, suggesting that the cloud fields are not embedded in a well‐mixed boundary layer. Key Points The structure of the boundary layer over the Southern Ocean is unique Important features of the ABL are poorly modeled in ECMWF models Clouds commonly reside above the well‐mixed ABL in a buffer layer/additional layer

Understanding interactions between wavelike disturbances and turbulence is critical for dealing with turbulence parameterization and improving numerical model performance under stable condi- tions. [...]reviewing our knowledge of gravity wave-turbulence interactions was the main objective of the symposium of Wave-Turbulence Interactions in the Stable Boundary Layer (WINABL). Because of the complexity of PBL gravity waves, it is difficult to isolate wave signatures from field observations especially when turbulence and wave frequency bands are indistinguishable. Spectral peaks associated with typical wavy time series are not found or are of questionable significance given the local nature of the wave packets.\\n Fiber optic measurements provide detailed vertical and horizontal structure of the temperature field and can be supplemented with corresponding hot wire measurements of the wind field over the same cross section.

The lack of adequate near-surface observations of the stable atmospheric boundary layer spatial structure motivated the development of an instrumented car for mobile turbulence measurements. The calibration and validation of the car measurements are performed using controlled field experiments and a comparison with an instrumented tower. The corrections required to remove the effects of the car motion are shown to be smaller and simpler than the corrections for research aircraft measurements. A car can therefore satisfactorily measure near-surface turbulence using relatively low-cost equipment. Other natural advantages of a car, such as the ability to drive on any road at any time of day or night and follow the terrain slope, as well as its low cost of operation, make it applicable to observations of a variety of flow regimes that cannot be achieved with the usual platforms, such as research aircraft or networks of flux towers.

A recently launched project under the auspices of the World Climate Research Program’s (WCRP) Coordinated Regional Downscaling Experiments Flagship Pilot Studies program (CORDEX-FPS) is presented. This initiative aims to build first-of-its-kind ensemble climate experiments of convection permitting models to investigate present and future convective processes and related extremes over Europe and the Mediterranean. In this manuscript the rationale, scientific aims and approaches are presented along with some preliminary results from the testing phase of the project. Three test cases were selected in order to obtain a first look at the ensemble performance. The test cases covered a summertime extreme precipitation event over Austria, a fall Foehn event over the Swiss Alps and an intensively documented fall event along the Mediterranean coast. The test cases were run in both “weather-like” (WL, initialized just before the event in question) and “climate” (CM, initialized 1 month before the event) modes. Ensembles of 18–21 members, representing six different modeling systems with different physics and modelling chain options, was generated for the test cases (27 modeling teams have committed to perform the longer climate simulations). Results indicate that, when run in WL mode, the ensemble captures all three events quite well with ensemble correlation skill scores of 0.67, 0.82 and 0.91. They suggest that the more the event is driven by large-scale conditions, the closer the agreement between the ensemble members. Even in climate mode the large-scale driven events over the Swiss Alps and the Mediterranean coasts are still captured (ensemble correlation skill scores of 0.90 and 0.62, respectively), but the inter-model spread increases as expected. In the case over Mediterranean the effects of local-scale interactions between flow and orography and land–ocean contrasts are readily apparent. However, there is a much larger, though not surprising, increase in the spread for the Austrian event, which was weakly forced by the large-scale flow. Though the ensemble correlation skill score is still quite high (0.80). The preliminary results illustrate both the promise and the challenges that convection permitting modeling faces and make a strong argument for an ensemble-based approach to investigating high impact convective processes.