Displays

Cities largely affect boundary-layer climates due to complex surface structures, pollutant emissions, and anthropogenic heat release. As urban populations are expanding worldwide, insight is required into the urban surface radiation and energy balance and urban greenhouse gas fluxes. However, little long-term flux measurement records are available for dense city centres. We present one year (June 2018 - May 2019) of flux observations taken at a 40-meters tower in the city centre of Amsterdam. We analyse the diurnal and seasonal variation of the turbulent and greenhouse gas fluxes, and we estimate the flux footprint to gain insight in flux variation with wind direction. Also, anthropogenic heat flux and storage fluxes are estimated from emission inventories and the objective hysteresis model respectively. This analysis shows that, especially during the winter, the sum of the sensible and latent heat flux exceeds the net radiation. Thus, the storage flux and anthropogenic heat flux are significant energy providers. Also, we find a surprisingly good surface energy balance closure, especially during summer. To achieve annual energy closure, the sensible heat and latent heat flux require an increase of 13%. Moreover, we find that the measured carbon dioxide flux (45 kg CO₂ m^-2 y^-1) is close to bottom-up source quantification (47 kg CO₂ m^-2 y^-1). For some wind directions, the agreement is better than for others. In addition, we show that the annual methane emission is slightly higher than the emission found in Florence and London. Yet the methane source partitioning in Amsterdam remains open for more research.

How to cite: Steeneveld, G.-J., van der Horst, S., and Heusinkveld, B.: Observing the surface radiation and energy balance, carbon dioxide and methane fluxes over the city centre of Amsterdam, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1547, https://doi.org/10.5194/egusphere-egu2020-1547, 2020.

More than half of the world population lives in cities. It is imperative to improve our predictive understanding of the urban boundary layer. In particular, considerable knowledge gaps still exist in turbulent transport of scalars (temperature, moisture and air pollutants) over urban rough surfaces, especially in the urban roughness sublayers. Using obstacle-resolving large eddy simulations, we first compare and contrast momentum and passive scalar transport over large, three-dimensional roughness elements. Dispersive scalar fluxes are shown to be a significant fraction of the total fluxes within the roughness sublayers. Strong dissimilarity is also noted between the dispersive momentum and scalar fluxes. The results highlight the need for distinct parameterizations of the turbulent and dispersive fluxes, as well as the importance of considering the contrasts between momentum and scalar transport for flows over very rough surfaces. In addition, the links between momentum and scalar roughness lengths (z_0m and z_0s) are explored by developing a conceptual framework that considers z_0m and z_0s at two distinct scales, namely micro and macro scales. Using a surface renewal theory for macro-scale roughness lengths, a log(z_0m/z_os) scaling with Re_*^1/2 is predicted and is supported by LES results. Overall, these results underline the potential of using wall-modeled, large-obstacle resolving LES to improve our process-based understanding, as well as to identify and represent the missing first-order physical processes in the ABL.   

How to cite: Li, Q.: The impacts of large bluff roughness elements on turbulent transport of momentum and scalar in the urban boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1722, https://doi.org/10.5194/egusphere-egu2020-1722, 2020.

Pulsed Doppler wind lidars (PDWL) have been extensively used in order to study the atmospheric turbulence. Their ability to scan large areas in a short period of time is a substantial advantage over in-situ measurements. Furthermore, PDWL are capable to scan horizontally as well as vertically thus providing observations throughout the atmospheric boundary layer (ABL). By analysing PDWL observations it is possible to identify large turbulent structures in the ABL such as thermals, rolls and streaks. Even though several studies have been carried out to analyse such turbulent structures, these studies examine peculiar cases spanning over short periods of time.

For this study we analysed the turbulent structures (thermals, rolls, streaks) over Paris during a two-months period (4 September – 6 October 2014, VEGILOT campaign) observed with a PDWL installed on a 70 m tower in Paris city centre. The turbulent radial wind field was reconstructed from the radial wind field of the horizontal surface scans (1° elevation angle) by using the velocity azimuth display method. The VEGILOT campaign provided 4577 horizontal surface scans, hence for the classification of the turbulent structures we developed an automatic method based on texture analysis and machine learning of the turbulent radial wind fields. Thirty characteristic cases of each turbulent structure types were selected at the learning step after an extensive examination of the meteorological parameters. Rolls cases were selected at the same time that cloud streets were visible on satellite images, streaks cases were selected during high wind shear development near the surface and thermals case were selected when solar radiation measurements in the area were high. In addition, sixty cases of “others”, representing any other type of turbulence, were added to the training ensemble. The analysis of errors estimated by the cross-validation shows that the K-nearest neighbours’ algorithm was able to classify accurately 96.3% of these 150 cases. Subsequently the algorithm was applied to the whole dataset of 4577 scans. The results show 52% of the scans classified as containing turbulent structures with 33% being coherent turbulent structures (22% streaks, 11% rolls).

Based on this classification, the physical parameters associated with the different types of turbulent structures were determined, e.g. structure size, ABL height, synoptic wind speed, vertical wind speed. Range height indicator and line of sight scans provided vertical observations that illustrate the presence of vertical motions during the observation of turbulent structures. The structure sizes were retrieved from the spectral analysis in the transverse direction relative to the synoptic wind, and are in agreement with the commonly observed sizes (a few 100 m for streaks, a few km for rolls).

How to cite: Cheliotis, I., Dieudonné, E., Delbarre, H., Sokolov, A., Dmitriev, E., Augustin, P., Fourmentin, M., Ravetta, F., and Pelon, J.: Statistical study of coherent turbulent structures properties observed by a Doppler lidar over Paris during two months, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7156, https://doi.org/10.5194/egusphere-egu2020-7156, 2020.

Lately, researchers, policy makers and governments have focused their attention mainly on air quality in urban areas. The issue of quality of air in rural areas remains neglected, although studies (Largeron & Staquet, 2016 and the reference therein) show that especially in hilly/mountainous regions on the countryside, air pollution can be a serious problem.

Our aim is to quantify the influence of ground temperature inversions on spatiotemporal variability of equivalent black carbon (eBC) and Particulate Matter (PM) mass concentrations in mountainous regions, example of which is the model region Loški Potok, Slovenia.

Simultaneous mobile measurements with two instrumented backpacks (AE51, MA200, OPSS 3330, temperature sensor) (Alas et al., 2018) were performed along the woody karst hollow with the village Retje at the bottom. Mobile measurements were performed in winter 2017/18, three times a day (in the morning, at noon, and in the evening) with a 10- and 20-minute intercomparison with the reference instruments (AE-33, TROPOS and TSI MPSS) at the station on top of the hill and in the village. The regression slope between two AE51 microaethalometers was 1. Fixed instruments showed good agreement with the mobile instruments during inter-comparison periods (Alas et al., 2020).

The mean value of eBC and PM_2.5 mass concentrations for the whole relief depression during temperature inversion episodes was 4 µg/m³ of eBC and 39.7 µg/m³ of PM_2.5. During periods with mixed atmosphere mean eBC and PM_2.5 mass concentrations were 0.9 and 11.4 µg/m³. The eBC and PM_2.5 mass concentrations between 17:00 and 19:00 CET in the village reached 16–20 µg/m³ of eBC and 170–250 µg/m³ of PM_2.5, yet on the top of the hill Tabor concentrations were 2–3.5 µg/m³ of eBC and 10–15 µg/m³ of PM_2.5.

As a result of human activities (residential wood burning) and the shallow thickness of an inversion layer temporal and spatial variability of pollutant concentrations in the study area is significant. During stable atmospheric conditions (temperature inversion) concentration levels strongly increase yet rapidly decrease with the temperature inversion break up.

Key words: carbonaceous aerosols, atmospheric stability, residential wood combustion (RWC), mobile measurements, rural areas

The research was supported by the Slovenian Research Agency, the COST Action CA16109 and the Municipality of Loški Potok.

Alas, H. et al. (2018). Aerosol and Air Quality Research, 18, 2301–2317.

Alas, H. et al. (2020). Manuscript in preparation.

Largeron, Y., & Staquet, C. (2016). Atmospheric Environment, 135, 92–108.

How to cite: Glojek, K., Močnik, G., C. Alas, H. D., Cuesta-Mosquera, A., Drinovec, L., Gregorič, A., Ogrin, M., Weinhold, K., Ježek, I., Müller, T., Rigler, M., Remškar, M., van Pinxteren, D., Herrmann, H., Ristorini, M., Merkel, M., Markelj, M., and Wiedensohler, A.: The impact of Temperature inversions on Black Carbon and Particle Mass Concentrations from Wood-burning in a Mountainous Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21310, https://doi.org/10.5194/egusphere-egu2020-21310, 2020.

The estimate and parameterization of mass and energy fluxes exchanged in the atmospheric surface-layer, between the biosphere and the atmosphere, plays a key role in many disciplines, e.g. meteorology and atmospheric sciences, ecology and precision agriculture.

In mountain environments, crests lines tend to decouple the atmospheric processes close to the ground from those in the upper layers and deeply affect the penetration of solar radiation on the floors and the sidewalls of valleys. The consequent differential heating of the surface allows the onset of many local phenomena, such as thermally-driven flows and temperature inversions, with impacts on the regime of the exchanges. Indeed, the low-wind conditions, the wind interaction with landforms and the atmospheric stability control the turbulence and the development of sub-meso motions, i.e. those phenomena responsible for the diffusion and transport of substances, respectively.

The Wheat Project is a 2-year-long project, started on November 2018 and funded by the CARITRO Foundation (“Cassa di Risparmio di Trento e Rovereto”, Italy), which aims at investigating the basic mechanisms responsible for biosphere-atmosphere exchanges in mountain areas, in order to improve their estimate and scaling.

Three datasets collected by long-term research infrastructures and composed of both biochemical and atmospheric quantities measured over different ecosystems were selected in the Trentino Alto Adige/South Tirol region (Italy). Data were measured at the research facility managed by the Free University of Bolzano/Bozen at Caldaro, over an apple orchard (220 m ASL, available period 2010-2018), and at the research facilities managed by the Fondazione Edmund Mach at Monte Lavarone, over a forest (1349 m ASL, available period 2000-2018) and at Viote del Monte Bondone, over an Alpine grassland (1553 m ASL, available period 2002-2018).

This contribution focuses on the pre-processing procedure adopted to identify representative periods for the analyses and on the methods implemented to retrieve turbulence parameters. In particular, the identification of the characteristic time-scales of small-scale turbulence is carried out through an application of the anisotropic analysis of turbulence, whereas the separation of the turbulence signal from low-frequency fluctuations is performed by implementing a recursive digital filter. Finally, some preliminary results regarding the estimate and the scaling of the turbulent fluxes of momentum, sensible heat, moisture and carbon dioxide are presented.

How to cite: Falocchi, M., Giovannini, L., Belelli Marchesini, L., Gianelle, D., Montagnani, L., and Zardi, D.: The estimate and scaling of mass and energy fluxes from three different ecosystems in the Trentino Alto Adige/South Tyrol (Italy) region: Preliminary results from the Wheat Project., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18022, https://doi.org/10.5194/egusphere-egu2020-18022, 2020.

The heterogeneity of the urban features, in addition to the inherent challenges added by highly complex terrain, has not allowed the scientific community to reach a complete understanding of the Atmospheric Boundary Layer (ABL) dynamics regarding the land-atmosphere interactions. The intricacies are higher when trying to simulate the observed interactions and their implications for air quality in a numerical modeling framework.

Over the last two decades, the ABL research community has dedicated several research efforts to study turbulent exchanges and ABL processes over complex terrain, and the implications of the particular features of these sites have on turbulence characteristics. A better knowledge of the ABL structure and dynamics is fundamental to understand processes such as air pollutant dispersion and disposal in the atmosphere, development and evolution of deep convection, and urban effects on meteorology. One of the aspects hindering our understanding is the lack of pertinent information from urbanized mountainous regions representative of the entire globe, useful to assess the different hypotheses and conceptual models of the Mountain Boundary Layer (MBL) dynamics. Most of the short- and long-term ABL field experiments in mountainous terrains have taken place over the high-latitude regions such as the Alps and the Rockies, and few over in the tropical Andes, where the Cordillera plays an essential role in controlling orographic rainfall intensification and the ventilation in inter-Andean valleys, resulting in knowledge gap regarding momentum, and latent and sensible heat flux exchanges over low-latitude, urban, complex terrain regions. In addition to a top-down approach, it is essential to follow a bottom-up strategy to study in detail the turbulent heat, mass, and momentum transfer in the Andean region.

The COMPLEX Experiment (COmplex terrain Measurement and modeling Project of Land-atmosphere Energy eXchanges) is a new effort focused on the long-term energy balance measurement campaign settled in the Aburrá Valley, a narrow highly complex mountainous-urban terrain located in the Colombian Andes. The primary purpose of this campaign is to identify the more relevant phenomenological features and processes responsible for ABL spatio-temporal variability, and land-atmosphere interactions in inter-Andean valleys. The long-term observational set-up includes eight sites equipped with turbulent flux sensors and net radiometers, in a cross-section of the valley, a microwave radiometer, a boundary layer radar, a scintillometer, and radiosonde intense observation periods (IOPs). We present the status of the COMPLEX experiment equipment deployment and preliminary results on the relationship of the transition between the stable boundary layer and the convective boundary layer and air quality in the region, and an exploration of the diurnal cycle of the different turbulent terms of the energy budget as a function of time and hill location.

How to cite: Herrera, L., Hoyos, C., and Urán, J.: The Complex Terrain Measurement and Modeling Project of Land–Atmosphere Energy Exchanges (COMPLEX) Experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21259, https://doi.org/10.5194/egusphere-egu2020-21259, 2020.

Sloping terrain of any inclination favour the development, under daytime heating, of thermally-driven organised flows, displaying peculiar boundary layer structures, and eventually triggering the development of atmospheric convection.

The ubiquitous occurrence of variously tilted surfaces - from gently sloping plains top steep cliffs, or valley sidewalls – makes the understanding of such flows of utmost importance in view of the appropriate forecasting of the associated boundary layer transport processes. These may display quite a different structure from those, much better known, occurring over horizontal plain surfaces [1]. Also, they display a highly conceptual relevance, as the simplest, prototypal situations for many other thermally driven-flows over complex terrain [2]. Finally, with the increasing resolution of operational model runs, a more accurate parameterisation of these processes is required for a realistic simulation of their development in space and time.   

However, up-slope flows have received so far much less attention than downslope flows originating from cooling, which have been extensively investigated by means of theoretically analysis, field experiments and numerical simulations. Even the theoretical analysis on their onset and structure are rather limited (e.g. to gentle slopes: [3]). Analytical solutions, such as Prandtl’s [4], rely on severely restrictive assumptions (parallel flow, constant or slowly varying eddy viscosity and diffusivity, along-slope invariance of the ambient factors). Extensions of such solutions relaxing those restrictions are still limited [5]. Even extensive high-resolution numerical simulations are rare, and not much progress has been made after Schumann’s [6]. Further insight, especially on the conditions for flow separation, have been gained through laboratory-scale simulations [7], which however are limited to moderate flow situations.

The proposed presentation offers a comprehensive overview of our present understanding of these phenomena, ideas for scaling laws appropriate for these winds, and challenging open questions for future research.

References

1. Rotach, M. W., and D. Zardi, 2007: On the boundary layer structure over complex terrain: Key findings from MAP. Quart. J. Roy. Meteor. Soc., 133, 937-948.
2. Zardi, D. and C. D. Whiteman, 2013: Diurnal Mountain Wind Systems, Chapter 2 in “Mountain weather research and forecasting – Recent progress and current challenges” (Chow, F. K., S. F. J. De Wekker, and B. Snyder Editors), Springer Atmospheric Sciences, Springer, Berlin.
3. Hunt, J. C. R., H. J. S. Fernando, and M. Princevac, 2003: Unsteady thermally driven flows on gentle slopes. J. Atmos. Sci., 60, 2169-2182.
4. Prandtl L. 1942. Führer durch die strömungslehre, ch. V. Vieweg und Sohn [English translation: Prandtl, L., 1952: Mountain and Valley Winds in Stratified Air, in Essentials of Fluid Dynamics, Hafner Publishing Company, pp.422-425].
5. Zammett, R. J., and A. C. Fowler, 2007: Katabatic winds on ice sheets: A refinement of the Prandtl model. J. Atmos. Sci., 64, 2707–2716.
6. Schumann U. 1990. Large-eddy simulation of the up-slope boundary layer. Quart. J. Roy. Meteor. Soc. 116, 637–670.
7. Hilel Goldshmid, R.; Bardoel, S.L.; Hocut, C.M.; Zhong, Q.; Liberzon, D.; Fernando, H.J.S. Separation of Upslope Flow over a Plateau. Atmosphere 2018, 9, 165.

How to cite: Zardi, D.: Surface-layer scaling for thermally-driven up-slope flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22284, https://doi.org/10.5194/egusphere-egu2020-22284, 2020.

Height of the stable boundary layer (SBL) presents an important diagnostic used to describe the relevant processes governing the evolution and characteristics of SBL, and the extent to which the surface is communicating with the free atmosphere.  Here we investigate the SBL height over a gentle (1°) mesoscale slope on which relatively deep mid-latitude katabatic flows (with jet maxima between 20 and 50 m) develop during clear nights. We show that detecting the SBL top depends on the method used (Richardson number, flux- and anisotropy-profiles). The detected SBL depth, mostly deviates from the jet maximum height or the top of the near-surface inversion. The flat terrain formulations for the SBL height correlate well with the detected top of the SBL if instead of background stratification, near-surface stratification is used in their formulations, however, they mostly largely overestimate the SBL height. The difference to flat-terrain SBL is also shown through the dependence of size of the dominant eddy with height. In katabatic flows the eddy size is semi-constant with height throughout the SBL, whereas in flat terrain eddy size varies significantly with height.

How to cite: Stiperski, I., Holtslag, A. A. M., Lehner, M., and Whiteman, C. D.: Stable boundary layer height on a gentle slope, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15187, https://doi.org/10.5194/egusphere-egu2020-15187, 2020.

High-resolution large-eddy simulations of the Antarctic very stable boundary layer reveal a mechanism for systematic and periodic intermittent bursting. A non-bursting state with a boundary-layer height of just 3 m is alternated by a bursting state with a height of ≈5 m. The bursts result from unstable wave growth triggered by a shear-generated Kelvin-Helmholtz instability, as confirmed by linear stability analysis. The shear at the top of the boundary layer is built up by two processes. The upper, quasi-laminar layer accelerates due to the combined effect of the pressure force and rotation by the Coriolis force, while the lower layer decelerates by turbulent friction. During the burst, this shear is eroded and the initial cause of the instability is removed. Subsequently, the interfacial shear builds up again, causing the entire sequence to repeat itself with a timescale of 10 min. Despite the clear intermittent bursting, the overall change of the mean wind profile is remarkably small during the cycle. This enables such a fast erosion and recovery of the shear. This mechanism for cyclic bursting is remarkably similar to the mechanism hypothesized by Businger in 1973. In his proposed mechanism, the momentum in the upper layer is increased by the downward turbulent transport of high-momentum flow. From the results, it appears that such transfer is not possible as the turbulent activity above the base flow is negligible. Finally, it would be interesting to construct a climatology of shear-generated intermittency in relation to large-scale conditions to assess the generality of this Businger mechanism.

How to cite: van der Linden, S., van de Wiel, B., Petenko, I., van Heerwaarden, C., Baas, P., and Jonker, H.: A Businger Mechanism for Intermittent Bursting in the Stable Boundary Layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5227, https://doi.org/10.5194/egusphere-egu2020-5227, 2020.

Dust devils are convective vortices with a vertical axis of rotation mainly characterized by a local minimum in pressure and a local maximum in vertical vorticity within the vortex core. They are made visible by entrained dust particles. That's why they occur primarily in dry and hot areas. Currently, there is great uncertainty about the extent to which dust devils contribute to the atmospheric aerosol and heat transport and thereby influence earth's radiation budget as well as boundary layer properties. Past efforts to quantify the aerosol or heat transport and to study dust devils' formation, maintenance, and statistics using large-eddy simulation (LES) as well as direct numerical simulation (DNS) have been of limited success. Therefore, this study aims to provide better statistical information about dust devil-like structures and to extend, prove or disprove existing theories about the development and maintenance of dust devils. Especially, the vortex strength measured through the pressure drop in the vortex core is regarded, which is, in past LES simulations, almost one order of magnitude smaller compared to the observed range of several hundreds Pascals.
So far, we are able to reproduce observed core pressures with LES of the convective boundary layer by using a high spatial resolution of 2m while considering a domain of 4km x 4km x 2km, a model setup with moderate background wind and a spatially heterogeneous surface heat flux. It is found that vortices mainly appear at the vertices and branches of the cellular pattern and at lines of horizontal flow convergence above the centers of the strongly heated patches. The latter result is in contrast to some older observations in which vortices seemed to be created along the patch edges. Also further statistical properties, like lifetimes, diameters or frequency of occurrence, fit quite well in the observed range. Nevertheless, statistics of dust devils from LES face the general problem that they are highly influenced by the used grid spacing and thereby by the structures that can be explicitly resolved. For example, the near surface layer, which plays a major role for the vortex development, is poorly resolved and turbulent processes in this layer are highly parameterized. DNS would overcome this problem. Therefore, dust devil-like structures are also investigated with DNS by simulating laboratory-like Rayleigh-Bénard convection with Rayleigh numbers up to 10¹². Such high Rayleigh numbers have never been used in DNS studies of dust devils. The focus is on the vortex formation dependence on the used Rayleigh number and aspect ratio. First results of the laboratory-like Rayleigh-Bénard convection simulated with DNS confirm the existence of dust devil-like structures also on small scales with much lower Rayleigh numbers than in the atmosphere.
In a next step, detailed statistics of dust devil-like structures in Rayleigh-Bénard convection will be derived focusing on Rayleigh number and aspect ratio dependencies. Afterwards, results will be compared to LES simulations of dust devils and experimental data.

How to cite: Giersch, S. and Raasch, S.: Genesis and Features of Dust Devil-Like Vortices in Convective Boundary Layers – A Numerical Study Using LES and DNS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1579, https://doi.org/10.5194/egusphere-egu2020-1579, 2020.

As an attempt to find a way of evaluating the surface drag in global models, we have derived a climatology of the boundary-layer wind-turning angle over land (Lindvall and Svensson, 2019). It is based on radiosonde observations from 800 stations in the Integrated Global Radiosonde Archive (IGRA). The climatology and how the wind turning depend on a suite of parameters is analyzed. Results from previous studies indicating the importance of the planetary boundary layer (PBL) stratification for the angle of wind turning are confirmed. A clear increase in the wind-turning angle with wind speed, particularly for stratified conditions, is also evident. According to Rossby number similarity theory, the crossisobaric angle for a neutral and barotropic boundary layer decreases with the surface Rossby number, Ro. The IGRA observations indicate that this dependence on Ro might partly be linked to the dependence of the stratification on the wind speed, a dependence that seems to prevail even for the high wind speeds, a criterium that traditionally is used to approximate a neutral PBL. The vertical distribution of the turning of the wind is analyzed using the high resolution Stratospheric Processes And their Role in Climate (SPARC) data. For unstable cases, there is a maximum in the directional wind shear around the PBL top, whereas for the most stable class of cases there is a maximum near the surface. The midlatitude cross-isobaric mass transport is estimated using the IGRA data. The wind-turning angles from reanalysis fields and climate models are also presented, they generally underestimate the turning angle.

How to cite: Svensson, G., Lindvall, J., and Pyykkö, J.: Wind-turning over the atmospheric boundary layer in observations and models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11238, https://doi.org/10.5194/egusphere-egu2020-11238, 2020.

Fog strongly perturbs the aviation, marine and land transportation, leading to human losses and high financial costs. The primary objective of SOFOG3D is to advance our understanding of fog processes at the smallest scale to improve forecasts of fog events by numerical weather prediction (NWP) models.

Specifically, SOFOG3D conducts process studies on very well documented situations, using synergy between 3D high-resolution Large Eddy Simulation (LES) and unprecedented 3D detailed observations. SOFOG3D will particularly focus on the impact of surface heterogeneities (types of vegetation, rivers, orography) on the fog life cycle, on fog microphysics properties, on entrainment at fog top, on the surface energy budget, and on the impact of aerosols. SOFOG3D will also investigate how improving the initial conditions of NWP models can improve fog forecasts. To that end, data from a ground-based MWR network will be assimilated using an innovative ensemble-based variational data assimilation scheme.

A 6 months field experiment took place during wintertime 2019/2020 in the South-West of France to provide 3D mapping of the boundary layer during fog events. The observation strategy is to combine vertical profiles derived from new remote sensing instruments (microwave radiometer (MWR), Doppler cloud radar and Doppler lidars) and balloon-borne in-situ measurements, with local observations provided by a network of surface stations, and a fleet of Unmanned Aerial Vehicles (UAV) to explore fog spatial heterogeneities.

Three nested domains has been instrumented with increasing density to provide observations from regional scale (300x200 km) down to local scale on the super-site (10x10 km), thanks to Meteo France and U.K. Meteorological Office sensors. On the super site, meteorological conditions, visibility, aerosol optical, microphysical and hygroscopic properties, fog microphysics and liquid water content, water deposition, radiation budget, heat and momentum fluxes on flux-masts has been performed on different areas to investigate the impacts of surface heterogeneities on fog processes, as well as turbulence anisotropy. Combination of cloud radar and MWR measurements will allow optimal retrieval of temperature, humidity and liquid water content profiles.

We will present the instrumental set-up that has been deployed during this campaign and discuss the main objectives of the project. An overview of fog events that occurred during the 6 months experiment will be given, and preliminary analysis of data collected during IOPs with a tethered balloon and UAVs will be presented.

How to cite: Burnet, F., Lac, C., Martinet, P., Fourrié, N., Haeffelin, M., Delanoë, J., Price, J., Barrau, S., Canut, G., Cayez, G., Dabas, A., Denjean, C., Dupont, J.-C., Honnert, R., Mahfouf, J.-F., Montmerle, T., Roberts, G., Seity, Y., and Vié, B.: The SOuth west FOGs 3D experiment for processes study (SOFOG3D) project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17836, https://doi.org/10.5194/egusphere-egu2020-17836, 2020.

With the aim to simulate the exchange of energy and matter between air and vegetation, we applied the LES PALM to a typical Central European forest. The presentation shows how the level of detail within vegetation model and the orography alters the simulated flow.

The site of investigation is a managed mixed forest stand (mainly Picea abies, height 30 m; a long-term CarboEurope monitoring site) within the Tharandter Wald near Dresden, Germany. Terrestrial laser scans (TLS) provided the data basis for the high-resolution vegetation model of this forest stand and a nearby clearing (50x90 m) building the inner range of the model domain. To investigate orographic effects on the flow, we extended the domain for about 1.5 km to the west. This includes the S-Berg, which is about 40 m height and therefore the highest elevation on the windward side. We used information from airborne laser scanning (ALS) along with forest inventory data to build a vegetation model as well as a digital elevation model for the extended area (2 km in streamwise and 1.5 km in lateral direction) with a resolution of (2m)³. 

In a first step, we restricted all simulations to a neutral atmosphere to exclude effect of buoyancy.

Wind data from four measurement towers (from DFG SPP 1276 MetStröm) provided data for a validation of the simulations. They were located within the inner domain along a west-east transect over the clearing.

How to cite: Plorin, M., Grunicke, S., Bernhofer, C., and Queck, R.: Flow modeling within a Central European forest using large-eddy simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18600, https://doi.org/10.5194/egusphere-egu2020-18600, 2020.

The characterization of the structure of non-stationary, noisy fluctuations in a time series, e.g., the time series of the velocity components or temperature in turbulent flows, is a problem of central importance in fluid dynamics, nonequilibrium statistical mechanics, atmospheric physics and climate science. Over the past few decades, a variety of statistical techniques, like detrended fluctuation analysis (DFA), have been used to reveal intricate, multiscaling properties of such time series. We present an analysis of velocity and temperature time series, which have been obtained by measurements over the canopy of Hyytiälä Forest in Finland.
In our study we use DFA, its generalization, namely, multifractal detrended fluctuation analysis (MFDFA), and the recently developed multiscale multifractal analysis (MMA), which is an extension of MFDFA. These methods allow us to characterize the rich hierarchy or multi- fractality of the dynamics of the time series of the velocity components and the temperature. In particular, we can clearly distinguish these time series from white noise and the signals that display simple, monofractal, scaling with a single exponent (also called the Hurst exponent). It is useful to recall that monofractal scaling is predicted for fluid turbulence at the level of the Kolmogorov’s phenomenological approach of 1941 (K41); experiments and direct numerical simulations suggest that three-dimensional (3D) fluid turbulence must be characterised by a hierarchy of exponents for it is truly multifractal.

We present an analysis of multifractality of velocity and temperature fields that have been measured, at different heights, over the canopy of Hyytiälä Forest in Finland. In particular, we carry out a detailed study of velocity and temperature time series by using MFDFA and MMA. Results from both these methods are consistent, as they must be; but, of course, the MMA results contain more information because they account for the dependence of the multifractality on the time intervals.

How to cite: Sahoo, G., Bhattacharjee, S., Vesala, T., and Pandit, R.: Multifractal analysis of velocity and temperature fluctuations in the atmospheric surface layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7220, https://doi.org/10.5194/egusphere-egu2020-7220, 2020.

Water isotopologues offers a direct constraint on the physical processes controlling surface fluxes.  A novel method is presented which enables in-situ measurements of the water vapour isotope flux between the snow surface of the Greenland Ice Sheet and the atmosphere.

These observations have become possible by combining a cavity ring-down laser absorption spectroscopy analyzer with high frequency latent heat flux eddy-covariance measurements.

This new method reveals an isotope flux driven by the diurnal cycle.
Water isotopes can thus act as a natural tracer giving information of the physical processes such as the influence of turbulent fluxes in the water cycle. This allows the assessment of sublimation and deposition processes in the low accumulation zone of the interior Greenland Ice Sheet.
Therefore, we can provide a strategy to benchmark the parameterizations of surface mass balance and surface fluxes in regional climate models.

How to cite: Wahl, S., Steen-Larsen, H. C., Zuhr, A., and Reuder, J.: A novel method for directly obtaining the water vapor isotope surface flux for evaluating snow-atmosphere exchange processes , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-450, https://doi.org/10.5194/egusphere-egu2020-450, 2020.

The evolution of micrometeorological measurements has been recently manifested by developments in methodological and analytical techniques using spatial surface brightness temperature captured by infrared cameras (Schumacher et al. 2019, Katurji and Zawar-Reza 2016). The Thermal Image Velocimetry (TIV) method can now produce accurate 2D advection-velocities using high speed (>20Hz) infrared imagery (Inagaki 2013, Schumacher 2019). However, to further develop TIV methods and achieve a novel micrometeorological measurement technique, all scales of motion within the boundary layer need to be captured.

Spatial observations of multi-frequency and multi-scale temperature perturbations are a result from the turbulent interaction of the overlying atmosphere and the surface. However, these surface signatures are connected to the larger scales of the atmospheric boundary layer (McNaughton 2002, Träumner 2015). When longer periods (a few hours to a few days) of spatial surface brightness temperatures are observed, the larger scale information needs to be accounted for to build a comprehensive understanding of surface-atmospheric spatial turbulent interactions. Additionally, the time-frequency decomposition of brightness temperature perturbations shows longer periods of 4-15 minutes superimposed over shorter periods of ~ 4–30 seconds. This suggests that that boundary layer dynamic scales (of longer periods) can influence brightness temperature perturbations on the local turbulent scale. An accurate TIV algorithm needs to account for all scales of motion when analysing the time-space variability of locally observed spatial brightness temperature patterns.

To analyse these propositions temporally high resolved geostationary satellite infrared data from the Himawari 8 satellite was compared to near-surface and high speed (20 Hz) measured air and brightness temperature using thermocouple measurements and infrared cameras. The satellite provides a temporal resolution of 10-minutes and a horizontal resolution of 2 by 2 km per pixel and therefore captures the atmospheric meso γ and micro α scale which signals are usually active for ~10 minutes to < 12 hours. Moreover, the Himawari 8 brightness temperature was used to create the near-surface mean velocity field using TIV. Afterwards, the velocity field was compared to the in-situ measured wind velocity over several days during January 2019.

The results show that the atmospheric forcing from the micro α scale to lower atmospheric scales has a major impact on the near-surface temperature over several minutes. A significant (p-value: 0.02) positive covariance between the Himawari 8 measurement and the local measured temperature 1.5 cm above the ground on a 10 minute average, specifically concerning cooling and heating patterns, has been found.

Further analysis demonstrates that the retrieved near-surface 2-D velocity field calculated from the Himawari 8 brightness temperature perturbations is correctly representing the mean velocity. This finding allows the classification of meso-scale atmospheric forcing and its direct connection to local scale turbulent 2-D velocity measurements. This extends the TIV algorithm by a multi-scale component which allows to address inter-scale boundary layer analysis from a new point of view. In respect to the current findings a new experiment will focus on the repeated induced local velocity patterns from large scale forcing which will be measured through the surface brightness temperature.

How to cite: Schumacher, B., Katurji, M., and Zhang, J.: The infrared measurement cascade: Connecting large scale meteorologically induced surface temperature perturbations to local spatial velocity structures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1106, https://doi.org/10.5194/egusphere-egu2020-1106, 2020.

Wind turbines operating in a stably stratified atmospheric boundary layer often interact with a veering wind, which is characterized by a clockwise wind direction change with height in the Northern Hemisphere. The rotational direction of the wind turbine rotor has a significant impact on the flow field in the wake in case of a veering wind, whereas it is of minor importance if the wind direction is the same over the whole rotor.

The impact of the rotational direction in a stably stratified atmospheric boundary layer results in contrasting rotational directions of the near and far wake in case of a common clockwise rotating rotor, whereas in case of a counterclockwise rotating rotor the rotational direction of the wake persists in the whole wake. The change of the rotational direction of the wake at a downstream location, which is related to the transition from the near wake to the far wake region, results in a larger streamwise wake elongation and a narrower spanwise wake width. In the lower and upper part, the wake deflection angle is also influenced by the rotational direction of the blades, resulting in a smaller wake deflection angle in case of a common clockwise rotating rotor in the Northern Hemisphere. In the Southern Hemisphere, the situation is reversed, an effect related to the Coriolis force impact on the Ekman spiral.

As the rotational direction impacts the inflow velocity, it effects the produced power of a downwind turbine and likewise the loads acting on a downwind turbine. For a hypothetical downwind turbine with a staggered spacing of 7 D, the power output difference would be up to 23% in idealized simulations, whereas the power output difference for a counterclockwise rotating rotor instead of a clockwise one also depends on atmospheric conditions like the strength of stratification, the strength of the veering wind, the rotor fraction impacted by a veering wind, and wind speed.

How to cite: Englberger, A., Dörnbrack, A., and Lundquist, J.: The impact of the rotational direction of a wind turbine on its wake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12606, https://doi.org/10.5194/egusphere-egu2020-12606, 2020.

The momentum flux affects the energy exchange processes and thus may indirectly affect the water balance of agricultural fields. In wine vineyards, a high momentum flux between the vine rows may augment the evaporation and transpiration fluxes, and therefore decrease the water use efficiency. On the other hand, at night, high momentum fluxes may reduce (or even prevent) the formation of dew on the vine canopy, thus decrease the potential development of fungi and related diseases. We hypothesized that the wind direction relative to the row orientation in largely-spaced narrow hedge-rows characterizing wine vineyards greatly affects the turbulent structure and the momentum flux. This, in turn affects the vineyard microclimate, and ultimately, the grape quality. The objective of our research was to explore the effect of wine-vineyard row orientation on wind and temperature profiles below (and slightly above) the canopy and on the turbulence characteristics and eddy size.  The research was conducted in two adjacent vineyards in the Judean foothills in Israel (31°48'38.6"N 34°50'43.6"E and 31°48'37.1"N 34°50'24.0"E) having row orientations of NE-SW and SE-NW, respectively. With a NW prevailing wind direction, the wind is typically flowing perpendicularly to the former and in parallel to the latter. In each vineyard, 10 self-made type-T fine-wire thermocouples (0.08 mm diameter) were set on a pole places in the middle of the inter row, at heights above the ground of 5, 10, 20, 40, 80, 140, 220, 250, 300, and 400 cm. In addition, 4 fast-response 2D sonic anemometers were set at 10, 40, 140, and 250 cm above the ground. The measurements were conducted at 20 Hz.  Below canopy wind regime differed with orientation, mostly at heights lower than 2.5m. Higher wind speed below the canopy and smaller wind speed gradients were observed at the vineyard parallel to the prevailing wind direction.  Temperature gradients were mostly larger in the vineyard perpendicular to the prevailing wind direction.  Nevertheless, the power spectra were generally more uniform in height at the perpendicular vineyard.  

How to cite: Agam, N., Levi, Y., Alfieri, J., and Prueger, J.: The effect of row orientation on below-canopy turbulence characteristics in a wine-vineyard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1898, https://doi.org/10.5194/egusphere-egu2020-1898, 2020.

Dust devils are convective vortices able to lift sand and dust grains from the surface. They are common not only in the terrestrial deserts, but also on Mars, where they give a substantial contribution to the planetary dust budget (∼50%). Dust devils are characterized by a central pressure drop that generates the rotation and translate advected by the wind background.
One of the problems related to their monitoring is due to the impossibility to directly separate the translational and rotational motion components from the study of the wind speed and direction time series. This means that it is not possible to directly retrieve information on their translational speed and direction and on the maximum rotational wind speed using only a meteorological station. 
In addition, there are other fundamental parameters that are not directly measurable, such as the distance of passage of the vortex from the station, its sense of rotation and its diameter.
These limitations lead in general to a size/distance degeneracy of the results, i.e. the acquired meteorological signatures of a smaller dust devils passing near the station could not be distinguished from the ones of a bigger and farther event. This, in turn, leads to several problems in the study of their physics.
To avoid these issues, the monitoring meteorological station can be equipped with an imaging camera system with a sufficient acquisition rate, resolution and field of view. However, this is not always possible, in particular for the planetary space missions.
Here, we want to present two simple methods for the characterization of the wind speed and direction time series of dust devils that allow an easy solution for the measure of the vortex translational wind speed and direction, distance of passage and sense of rotation.
The knowledge of these parameters allows to completely characterize the measured dust devils encounter just using the meteorological station acquisition.
In order to test the methods, we performed a field campaign in the Sahara desert, deploying a fully equipped meteorological station coupled with a camera system. We compared the results of the meteorological analysis with the ones obtained from the images, confirming the effectiveness of our methodology.
This methodology can give a substantial improvement in the interpretation of the past and next martian dust devils data. For example, the ESA/Roscosmos ExoMars 2020 mission will host on its lander a meteorological station (METEO package) and the Dust Complex, a suite of specific instruments devoted to the study of the primary airborne dust. These instruments can be used in tandem for the characterization of the local dust devils activity.

How to cite: Franzese, G., Silvestro, S., Vaz, D., Popa, C., and Esposito, F.: Terrestrial and Martian Dust Devils: study of the translational motion and resolution of the size/distance degeneracy of the meteorological time series , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21323, https://doi.org/10.5194/egusphere-egu2020-21323, 2020.

How to cite: Grau Ferrer, A., Jiménez Cortés, M. A., and Cuxart Rodamilans, J.: Statistical characterization of the sea-breeze physical mechanisms through in-situ and satellite observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20503, https://doi.org/10.5194/egusphere-egu2020-20503, 2020.

For the purpose of understanding the detailed distribution of surface and air temperatures in a high-rise building block, a 3-dimensional Building-Block Meteorological observation EXperiment (BBMEX) campaign has been carried out over typical commercial area (Gwanghwamun) in Seoul Metropolitan Area, Korea during the heat-wave and tropical night periods (5-6 August) in 2019. Several types of fixed and mobile instruments were deployed in the experiment domain: A thermal infrared imager (TIR) monitored the surface temperature with 320×240 pixels including building wall, road, sidewalks at every 10 min; 6 automatic weather stations obtained air temperature and relative humidity, and wind speed and direction at every 1 min; a mobile weather vehicle (MOVE4) monitored road surface temperatures and 4-components of radiation at 1 s on roadway; a mobile cart for meteorological observation (MCMO) monitored surface, 0.5m, 1.5m, and 2.5m air temperatures at 1 s on the sidewalk and square. The TIR exhibited that east-face of a building was strongly heated during the morning time, while horizontal surface was strongly heated near noon. Air temperatures at 2 m high in 2×2 km² exhibited 1.5 ℃ temperature range at 06 LST, while 4.0 ℃ temperature range at 15 LST on 6 August 2019, depending on the location of site in building blocks. Air temperatures in Gwanghwamun Square were 1.5-1.7 ℃ and 0.1-2.2 ℃ higher than those observed at the Seoul synoptic station (1 km apart) in night and day, respectively. Surface and 0.5, 1,5, and 2.5m temperatures was 49.1 ℃, 38.7 ℃, 38.1 ℃, and 37.9 ℃, respectively, at 1500 LST on 6 August 2019, when the hottest air temperature in the year 2019 (36.9 ℃) was recorded at the Seoul station. Surface and air temperatures were found to be affected by many factors in a building-block such as shades, trees, building height and density, aspect ratio of building canyon, sky-view, ground-fountain, waterway, etc.

How to cite: Park, M.-S., Chae, J.-H., Min, J.-S., Kang, M., Jee, J.-B., Kim, S.-H., and Cho, C.-R.: A Building Block Urban Meteorological Observation Experiment (BBMEX) over Seoul City, Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6797, https://doi.org/10.5194/egusphere-egu2020-6797, 2020.

Horizontal distribution of building block scale meteorological information is important to understand the disastrous weather phenomena occurred at urban areas. Most meteorological models assume the same surface temperature, or an ideal surface temperature to simulate the high-resolution wind field in or above urban boundary-layer. This study aims to establish the basic foundation for producing the high-resolution and high-quality user-specific horizontal meteorological information at an urban building block in the Seoul Metropolitan Area. Therefore, the Mobile Cart for Meteorological Observation (MCMO) was developed and used in a meteorological experimental campaign during heat wave event days.
The MCMO includes 3 air temperature sensors, 1 weather transmitter, 1 infrared surface temperature sensor, 1 GPS (global positioning system), and video camera on the mobile cart. The MCMO measures the temperature at 4 altitudes (surface, 0.5m, 1.5m, and 2.5m), latitude, longitude, and surrounding environment condition of measurement site. The observation cycle is 1 second to produce pedestrian-friendly weather information. The meteorological experimental campaign was conducted in Gwanghwamun square in the Seoul, Korea. Gwanghwamun square is complex area which has high-rise building block, wide roads of heavy traffic, and green lung. Observation period was from 1200 LST 5 August 2019 to 2200 LST 6 August 2019 including the hottest day of the year. Through the meteorological experimental campaign, the MCMO shows the detail temperature change over time, location, and altitudes. The temperature was changed as the altitude of the sun changed. When the MCMO was move through the green lung or building block, also the temperature was changed. Temperature changes were the largest at surface temperature and tended to decrease as altitude increased. The MCMO can be used to understand high-resolution weather information and horizontal distribution of temperature in urban area. Additionally, another meteorological experimental campaign will be held in the summer of 2020.

How to cite: Kang, M., Park, M.-S., Chae, J.-H., and Min, J.-S.: Horizontal distribution of temperature in a building-block using a Mobile Cart for Meteorological Observation (MCMO), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6668, https://doi.org/10.5194/egusphere-egu2020-6668, 2020.

       Higher and denser building groups are the most concentrated reflection of urbanization on the underlying surface reconstruction. With the continuous city expanding, urban wind field structure was changed, also the aerodynamic parameters dependent on. Based on observational data (slow-response) collected at 15 levels on Beijing 325m meteorological tower from 1991-2018, time and vertical trends of atmospheric stability, wind direction, wind speed, aerodynamic parameters were analyzed. Through Sen's slope, Mann-Kendall trend test and mutation analysis, we believe that urbanization has made a significant influence on local meteorological condition, and all the above variables mutated around the year of 1999. Before 1999, the proportion of neutral and unstable conditions declined with a trend of -0.63% and -2.0% per year respectively, and increased with a trend of +0.08% and +0.06% per year after 1999. As for wind direction, the dominant wind direction below 47m turned from southwest/northwest before 1999 to southeast after 1999, while above 47m remain unchanged as southeast, reflecting that the action range of urban impact is clearly distinguished from that of atmospheric background field. In terms of wind speed, the annual mean value trended to decrease at -0.0019m/s per year, and vertical wind speed trended to increased with height (per meter) at m/s per year, which reflected the continuous enhancement of attenuation effect of complex underlying on the near-ground wind speed. Furthermore, we found that although there was indeed a weaken tendency for wind speed in Beijing urban areas, but near neutral wind speed maintained a growth trend under 140m during 1999-2018. It was possible the deal with urban wake effect, wind field structure mutation or turbulence effect. Aerodynamic parameters  and d have undergone significant changes during the peak stage of urbanization, and tended to develop steadily with a 7-years fluctuations trend after that. In the past 28 years, d has increased from 1.34m in 1991 to 26.19m in 2018, while  has decreased from 2.75m to 1.02m. This is due to the fact that the increase of buildings average height is the result of roughness superposition. If the 7-year fluctuations trend continues, d of Beijing urban area will soon enter the next uplift period, during which the wind speed may increase slightly under nearly neutral conditions, and the cleaning effect on the pollution may be gradually enhanced.

How to cite: Liu, X.: The Change of Aerodynamic Parameters over Beijing Urban Area during 1991-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6228, https://doi.org/10.5194/egusphere-egu2020-6228, 2020.

Due to excessive anthropogenic emissions, heavy aerosol pollution episodes (HPEs) often occur during winter in the Beijing-Tianjin-Hebei (BTH) area of the North China Plain. Extensive observational studies have been carried out to understand the causes of HPEs; however, few measurements of vertical aerosol fluxes exist, despite them being the key to understanding vertical aerosol mixing, specifically during weak turbulence stages in HPEs. In the winter of 2016 and the spring of 2017 aerosol vertical mass fluxes were measured by combining large aperture scintillometer (LAS) observations, surface PM_2.5 and PM₁₀ mass concentrations, and meteorological observations, including temperature, relative humidity (RH), and visibility, at a rural site in Gucheng (GC), Hebei Province, and an urban site at the Chinese Academy of Meteorological Sciences (CAMS) in Beijing located 100 km to the northeast. These are based on the light propagation theory and surface-layer similarity theory. The near-ground aerosol mass flux was generally lower in winter than in spring and weaker in rural GC than in urban Beijing. This finding provides direct observational evidence for a weakened turbulence intensity and low vertical aerosol fluxes in winter and polluted areas such as GC. The HPEs included a transport stage (TS), an accumulative stage (AS), and a removal stage (RS). During the HPEs from 25 January 2017 to January 31, 2017, in Beijing, the mean mass flux decreased by 51% from 0.0049 mg m^-2s^-1 in RSs to 0.0024 mg m^-2s^-1 in the TSs. During the ASs, the mean mass flux decreased further to 0.00087 mg m^-2s^-1, accounting for approximately 1/3 of the flux in the TSs. A similar reduction from the TSs to ASs was observed in the HPE from 16 December 2016 to 22 December 2016 in GC. It can be seen that from the TS to the AS, the aerosol vertical turbulent flux decreased, but the aerosol particle concentration within surface layer increased, and it is inferred that in addition to the contribution of regional transport from upwind areas during the TS, suppression of vertical turbulence mixing confining aerosols to a shallow boundary layer increased accumulation.

How to cite: Yuan, R.: Understanding Aerosol Vertical Transport During Heavy Aerosol Pollution Episodes in the North China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4498, https://doi.org/10.5194/egusphere-egu2020-4498, 2020.

In this study, we investigated the effects of the anthropogenic heat caused by the energy usage on the air temperature distributions in an urban area using a CFD model. We calculated the anthropogenic heat fluxes using a top-down method with monthly and hourly allocation coefficients and the total amount of the yearly electrical energy usage of buildings. To construct the buildings and to estimate the anthropogenic heat fluxes in the CFD model for the target area, we used the land use and GIS data. We conducted the CFD simulations for the heatwave period (2018.08.02 ~ 2018.08.08) in a building-congested district around the Seoul ASOS (ASOS 108) to see how the anthropogenic heat fluxes affected the thermal environment in the target area. The target area is mostly composed of commercial and residential areas. The temperature increased near the roads and buildings. At the night time, the temperature increase near the buildings with high anthropogenic heat fluxes was more significant than the daytime. The comparison with the ASOS-observed temperatures showed that the inclusion of the anthropogenic heat fluxes improved the CFD simulations of temperatures.

How to cite: Mun, D.-S. and Kim, J.-J.: A Study on the Effects of the Anthropogenic Heat Fluxes on the Temperature Distribution in an Urban Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12118, https://doi.org/10.5194/egusphere-egu2020-12118, 2020.

The Kolmogorov constant is fundamental in stochastic models of turbulence, and significant in boundary layer meteorology especially. Though lots of experiments have been conducted to study Kolmogorov Constant, constant at high elevation and over urban surface was rarely researched. Therefore, in this paper, ultrasonic data at seven levels over an urban underlying surface were used to calculate the Kolmogorov constants of velocity. The results of Kolmogorov constants at the different floors indicated that the constants below 47m were smaller because of the influence of the urban canopy layer. Besides, the time-varying result showed that constants were universally independent of stability. Furthermore, Kolmogorov constant in this paper was close to the result determined by former experiments.

How to cite: Ding, W., Shi, Y., Zhang, Z., and Hu, F.: Vertical distribution of Kolmogorov Constants of Atmospheric Turbulence over an Urban Surface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21700, https://doi.org/10.5194/egusphere-egu2020-21700, 2020.

Turbulent diffusion efficiently transports momentum, heat, and matter and affects their transfers between the surface and the atmosphere. As an important parameter in describing turbulent diffusion, turbulent heat diffusivity K_H has scarcely been studied in the context of frequent urban pollution in recent years. In this study, K_H under urban pollution conditions was directly calculated based on the K-theory. We found an obvious diurnal variation in K_H and its varying vertical distributions for each case and with time. Interestingly, the height of negative K_H rises gradually after sunrise, peaks at noon, and falls near sunset. Negative K_H is unusually significant at sunrise and sunset and approximately 140 m during most of the night. The magnitude and fluctuation in K_H are smaller in the pollutant accumulation stage (CS) at all levels than in the pollutant transport stage (TS) and pollutant removal stage (RS). Turbulent diffusion may greatly affect PM_2.5 concentration at the CS because of the negative correlation between PM_2.5 concentration and the absolute value of K_H at the CS accompanied by weak wind speed. The applicability of the K-theory is not very good during either day or at night. Note that these problems are inherent in K-theory when characterizing complex systems, such as turbulent diffusion, and require new frameworks or parameterization schemes. These findings may provide valuable insights for improving or establishing a new parameterization scheme for K_H and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.

How to cite: Zhang, Z., Shi, Y., Sun, H., Liu, L., and Hu, F.: The distribution of positive and negative turbulent heat diffusivity under urban pollution conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2632, https://doi.org/10.5194/egusphere-egu2020-2632, 2020.

The use of Numerical Weather Prediction (NWP) models is ubiquitous in our daily lives, whether to decide what to wear, to plan for the weekend, invest on wind turbines, decide strategies for food security or to forecast atmosphere-driven natural disasters, to name a few. Currently, intrinsic to most NWP models is the assumption of spatial homogeneity at kilometer to sub-kilometer scales when, for example, classic similarity scaling relationships are applied to account for unresolved near-surface momentum, heat and mass exchanges. While advances in computation (and computing) are enabling finer grid resolutions in NWP, representing land-atmosphere exchange processes at the lower boundary remains a challenge (regardless of the numerical resolution but not independent from it). This is partially a result of the fact that land-surface heterogeneity exists at all spatial scales and its variability does not ‘average’ out with decreasing scales. Such variability need not rapidly blend away from the boundary and thereby impacts the spatial distribution of fluxes throughout the near-surface region of the atmosphere.

     While, the effects of spatial surface heterogeneities have long been minimized under the assumption of an existing blending length-scale, in this work evidence is presented of the consequential effect of such surface heterogeneities. Specifically, canonical experiments based on in-situ measurements and high-resolution numerical simulations quantify the effect of surface thermal heterogeneities on an otherwise homogeneous planar surface. Therefore, such near-canonical case describes inhomogeneous scalar transport in an otherwise planar homogeneous flow when thermal stratification is weak or absent. In this work, the interaction between the characteristic length scales of the surface heterogeneities, and the scales of resolved fluid dynamics transport is further unraveled. Dispersive fluxes naturally appear as a means to account for unresolved, and time-lasting advection fluxes generated by a-priori unresolved spatial thermal heterogeneities. Results illustrate that dispersive fluxes can represent as much as 40% of the total resolved advection flux under weak wind conditions, and remain relevant under strong winds. Furthermore, results of this work appear not to only be relevant in the treatment of unresolved heterogeneities in NWP models, but also in understanding the unresolved problem of surface energy budget closure.

How to cite: Calaf, M., Morrison, T., Margairaz, F., Perelet, A., W. Higgins, C., A. Drake, S., and R. Pardyjak, E.: Surface thermal heterogeneities, dispersive fluxes and the conundrum of unaccounted statistical spatial inhomogeneities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13388, https://doi.org/10.5194/egusphere-egu2020-13388, 2020.

Do we get a better picture of the world around us if we simultaneously observe many aspects instead of a few? Dense sensing networks are an elaborate way to validate our representation of land surface boundary layer processes commonly derived from single point monitoring stations or a three-dimensional model world. More samples promise unique insights into interactions that occur at different scales, separated in space and time.

We present a combination of techniques that purvey a) observations of the temperature and wind field in high detail and b) the extraction of information about dynamic interactions near the surface. A field experiment was conducted in complex terrain, in which landscape features dramatically modulate local flow patterns and the atmospheric stability during summer days rapidly transitions on a diurnal scale and between locations. Wind and temperature were simultaneously observed using a network of Doppler lidar, sonic anemometer, fiber-optic temperature sensing (DTS) and thermal imaging velocimetry (TIV) instrumentation, centered around the TERENO/ICOS preAlpine grassland observatory station Fendt, Germany, during the ScaleX Campaigns (https://scalex.imk-ifu.kit.edu). Data analyses relied on signal decomposition and statistical clustering, aimed at the characterization of (non-)turbulent motions and their feedback on turbulent mixing near the surface. The combination of methods offered multiple levels of detail about the development and impact of organized structures in the atmospheric boundary layer.

The study shows that the exploration of novel micrometeorological and data sciences techniques helps advance our knowledge of fundamental aspects of atmospheric turbulence, and provides new avenues for theoretical and numerical studies of the atmospheric boundary layer.

How to cite: Zeeman, M., Katurji, M., and Banerjee, T.: High-resolution temperature and wind field observations in the atmospheric boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13324, https://doi.org/10.5194/egusphere-egu2020-13324, 2020.

Katabatic winds are gravity flows that develop over sloping terrain due to radiative cooling at the surface. They have been extensively studied, but experimental works have generally been performed over gentle slopes. Some recent papers (eg. [3]) focused on the combined effect of surface angle and buoyancy on turbulence over steep slopes. In such configurations, the vertical component of the turbulent sensible heat flux may differ a lot from the slope-normal component, suggesting that buoyancy may act on turbulent quantities in an unusual way when katabatic jets develop over steep slopes. Such behavior seems to affect stability parameters used in Monin-Obukhov similarity theory applied in most meteorological models.

We study the buoyancy production term in the continuity of the work from [3], drawing on temperature and wind speed measurements acquired during 10 nights in November 2012 [1]. In situ measurements were performed under stable anticyclonic conditions, over an alpine slope of around 21° (French Alps) on a 4 level mast up to 6.5m, at a frequency sampling of 10 to 20Hz.

We conclude that turbulent kinetic energy and turbulent momentum flux are damped below the maximum wind speed height as expected from stably stratified atmospheric boundary layer. Conversely, turbulent kinetic energy can be locally reinforced by buoyancy in the external part of the katabatic jet, which confirms the results from [3]. Buoyancy may also produce turbulent momentum flux around the maximum wind speed due to the asymmetry of the jet. Results compare well with recent numerical modeling of a katabatic jet along a curved alpine slope under similar meteorological conditions [2].

Another field experiment took place during 16 nights in February 2019, over a snow-covered slope of around 34° in a similar location. The 11 wind speed levels and 17 temperature levels up to 12m, associated with a change of the surface level due to packing and melting of the snow, widen the range of analysis of the vertical profile. These data are associated with meteorological measurements and with a tethered balloon up to 50-100m above the ground surface.

Wind velocity measurements with a multi-hole pressure probe (cobra type) close to the ground provided more information than the previous dataset at a high frequency sampling of 1250 Hz. We show that the classical turbulent boundary layer wind speed profile applies well to the inner-layer region of katabatic jets, in spite of the presence of a maximum on the vertical streamwise velocity profiles. We find no significative changes caused by buoyancy on this profile. Roughness effect due to the snow on the surface will be discussed as well.

[1] Blein (2016), PhD

[2] Brun et al. (2017), JAS

[3] Oldroyd et al. (2016), BLM

How to cite: Charrondière, C., Brun, C., Obligado, M., Sicart, J.-E., Cohard, J.-M., Guyard, H., Biron, R., and Coulaud, C.: Experimental study of katabatic jets over steep slopes: buoyancy effect and turbulence properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8674, https://doi.org/10.5194/egusphere-egu2020-8674, 2020.

A field experiment took place from July to October 2018 in a narrow valley of the central Pyrenees in order to study local flows and their impacts.
This field experiment is a joint effort by CNRM, Laboratoire d'Aerologie and University of the Balearic Islands. It emerged from a recent numerical study done by the University of the Balearic Islands (Jiménez et al. 2019).
This study suggests that under clear-sky conditions a jet forms in the valley and can be observed several kilometers away from the valley exit.

Several instruments including a Doppler scanning lidar and three meteorological stations were deployed on the main site at the valley exit, where the jet maximum is expected, and on two other sites up valley. Data from the Atmospheric Research Center (part of the Pyrenean Platform for the Observation of the Atmosphere P2OA) in Lannemezan are also used. They include radio-soundings specifically planned for the field experiment. This instrumented platform of Laboratoire d'Aerologie, located about 10 km away from the valley exit, is an important asset for the project.

An overview of the field experiment as well as the valley exit jet main features will be presented. A comparison with outputs from the NWP model AROME will be also shown.

How to cite: Paci, A., Jiménez, M. A., Cuxart, J., Lothon, M., Clary, O., Seity, Y., Martinez-Villagrasa, D., Dabas, A., Rieutord, T., and Román-Cascón, C.: Exit jet in a narrow Pyrenean valley: the Aure Valley 2018 field experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16184, https://doi.org/10.5194/egusphere-egu2020-16184, 2020.

Two experimental campaigns have been carried out in the Cerdanya valley at south side of the Pyrenees (E-W oriented, 35 km long and 9 km wide) during fall 2015 (Cerdanya Cold Pool experiment, CCP’15) and winter 2017 (CCP’17, as a part of the Cerdanya-2017 experiment) to study the cold pool that usually forms there at night. The main site (Das) is placed in the central bottom part of the basin. Conangla et al (2018, IJOC) showed that most cold pool events reported there have a daily cycle, being formed in the evening and destroyed by solar heating of the surface the morning after.

The availability of vertical soundings performed by a tethered balloon and a WindRASS, together with measured surface fluxes of latent and sensible heat and momentum at the surface layer allows to inspect the establishment and evolution of the surface thermal inversion in Das. This area collects also downslope and downvalley flows accumulating cold air in the valley along the night. The organization of the flow at low levels is studied through mesoscale simulations of some selected Intensive Operational Periods (IOPs) and the surface observations at different locations along and across the valley.

The selected IOPs comprise nights with only locally-generated winds and small cloud cover, and with variable surface state including grass, fresh snow and patches of old snow. The evolution of the strength and depth of the surface inversion as seen by the model are compared to the available data. Besides, the organization of the flow at low levels and the contribution of the air from the tributary valleys is analyzed in terms of temperature and wind speed budgets to properly characterize the differences in the strength of the cold pool for the selected studied IOPs.

How to cite: Jiménez, M. A., Cuxart, J., Paci, A., Conangla, L., Martínez-Villagrasa, D., and Martí, B.: Surface thermal inversion evolution in the bottom of a Pyrenean valley studied by observations and mesoscale simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9919, https://doi.org/10.5194/egusphere-egu2020-9919, 2020.

Turbulent flux measurements require high frequency sampling in order to characterize appropriately all the variability scales of the atmosphere. A 3D sonic anemometer coupled with a gas detector allows for applying the eddy-covariance method which has become the standard. However, the high cost of this system often implies to look for alternative methods, specially when multiple stations are required. Turbulent fluxes can also be estimated through the flux-gradient similarity theory, requiring observations of mean quantities of (at least) air temperature and humidity at two levels and wind at one height. This approach is more sensitive to the disturbing influence of heterogeneous and complex surfaces and a comparison between methodologies is required under these conditions.

The data used in this study is part of the ALaiz EXperiment 2017-2018 (ALEX17). This campaign was the last within the New European Altas project. It had a duration of over a year with measurements in complex terrain. The location of the experiment is a valley bounded by two mountain ranges that rise 150 m north and over 600 m south. A central site in the centre of the valley was instrumented with a sodar-RASS, an 80-m tower, a surface energy balance (SEB) station with an eddy-covariance system and a surface-layer station (SLS) with the necessary measurements to estimate the turbulent fluxes. In addition, eight supplementary SLS were deployed along the longitudinal and transverse valley axes to characterize the surface layer variability within the valley.

This communication will present a comparison of the friction velocity and sensible heat flux obtained from both the eddy-covariance system and the flux-gradient method at the central site for a time series of 8 months. Friction velocity is highly comparable between methodologies with a correlation of 0.92 and a standard deviation of 0.05. The performance of the sensible heat flux estimation differs between stable and unstable cases, with a correlation of 0.70 and 0.89, respectively, after applying a quality control procedure. The poorer results obtained under stable conditions points out the need for alternative estimations of the sensible heat flux for these cases.

How to cite: Martí, B., Martínez-Villagrasa, D., and Cuxart, J.: Turbulent flux estimation in the atmospheric surface layer through flux-gradient similarity during the ALEX17 field campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9145, https://doi.org/10.5194/egusphere-egu2020-9145, 2020.

In this work, a micrometeorological assessment of Atmospheric Boundary Layer paremeters is carried out in order to determine the characteristic turbulent scales over complex terrain in the Sierra de Guadarrama, a range in central Spain. Observational data series of temperature and wind velocity measured at high frecuency (10 Hz) are available. These data come from two different stations located in the Bosque de La Herrería and belonging to GuMNet (2020) (Guadarrama Monitoring Network).

Integral scales, both time and spatial, have been determined for different atmospheric conditions, defined by parameters such as wind direction or stability of stratification. Also, energy cascade phenomenon occurence is assessed. In order to carry this out, different time series analysis tools are used, such as autocorrelation functions in time, and normalised power spectra or wavelets. Results obtained are compared with previous works.

In general, results show that under no synoptic forcing there is a clear dependency on diurnal cycle, giving rise to the development of big integral scales at nighttime, while they are small during the day. When synoptic forcing prevails, the scales are also small, both at daytime and nighttime. Moreover, a correlation patterns method has been implemented for scales obtained at two different heights (4 and 8 meters) on the one hand and at two locations on the other. In the first case, integral scales are highly correlated, exceeding the threshold of 0.5. In the second case, temporal scales show high correlation values, but spatial ones do not.  In addition, the slopes of the spectra in the inertial subrange have  been obtained and compared to those over homogeneous terrain (Kaimal et al., 1972), getting similar results for velocity turbulent components but not in case of vertical kinematic momentum and heat fluxes.

References

GuMNet: Guadarrama Monitoring Network (UCM), https://www.ucm.es/gumnet/, 2020.

Kaimal, J. C., Wyngaard, J. C., Izumi, Y., and Coté, O. R.: Spectral characteristics of surface- layer turbulence, Quart. J. R. Met. Soc., 98, 563–589, 1972.

How to cite: García-Pereira, F. and Maqueda, G.: Assessment of turbulent scales over complex terrain in central Spain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4879, https://doi.org/10.5194/egusphere-egu2020-4879, 2020.

A correct spatial representation of the surface energy balance is still a challenge. In a first step, and assuming a correct knowledge of the incoming short-wave radiation, it is the land cover that mostly controls the albedo and the long-wave radiation emitted to the atmosphere, influencing significantly the net radiation available at the surface and the surface temperature. In a second step, the partitioning of this energy into evapotranspiration and sensible heat flux is, in part, controlled by the availability of soil moisture but also by the type, characteristics and physiological state of the vegetation covering the surface, since plants provide a pathway for soil moisture to the atmosphere through transpiration.

Hence, to correctly model the surface energy balance, we face three main challenges: an appropriate representation of the land use, soil moisture and a correct modelling of how plants regulate their stomatal behaviour under different soil-moisture limited conditions.

In this work, by using in situ data we explore the relations between soil moisture and evapotranspiration from several vegetation types at different soil-moisture limited regions: a wetter area in the south of France and a drier one in the south of Spain. For this, we try to distinguish different periods and vegetation states. Since significant differences are observed for the various plant types, we investigate whether using a more realistic and higher-resolution land-use database in the Weather Research and Forecasting (WRF) model improves the simulation of soil moisture and surface fluxes.

How to cite: Román-Cascón, C., Lothon, M., Lohou, F., Brut, A., Hartogensis, O., Merlin, O., Ojha, N., Yagüe, C., Soriguer, R., Díaz-Delgado, R., Andreu, A., and González-Dugo, M. P.: Investigating the relationship between soil moisture and evapotranspiration at different surfaces. Do we improve fluxes just improving the land use in models?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5566, https://doi.org/10.5194/egusphere-egu2020-5566, 2020.

Thermally-driven flows (TDFs) are mesoscale circulations driven by horizontal thermal contrasts in scales ranging from 1 and 100-200 km. The presence of mountains can generate a kind of these TDFs called thermally-driven topographic flows, with a typical daily cycle which is observed when weak synoptic conditions are present. These flows impact the turbulence features in the Atmospheric Boundary Layer (ABL), as well as different scalars (temperature, CO₂, water vapor, pollutants, etc.). Moreover, these circulations, which can be of different scales (from small-scale shallow drainage flows to for example the larger Mountain – Plain flows) can generate gravity waves (GWs) along the transition to the stable boundary layer (SBL) and during the night. In this work, 88 days belonging to an extended period of the BLLAST field campaign^[1] have been analysed. The corresponding nocturnal TDFs have been detected through a systematic and objective algorithm which considers both synoptic and local meteorological conditions. The main objectives of the study are: to characterize the TDFs at CRA (which is placed on a plateau near the Pyrenees in France); to evaluate the performance of the objective algorithm^[2] in obtaining the events of interest; to establish different categories of TDFs and search for driving mechanisms (local, synoptic,..); and finally to explore the connections between TDFs and the generation of Gravity Waves (GWs), often observed in the nocturnal SBL^[3]. Their interaction with turbulence is also analysed using different multiscale techniques, such as wavelets applied to pressure measurements obtained from high accurate microbarometers, and MultiResolution Flux Decomposition –MRFD- applied to sonic anemometer data. The contribution of different scales to turbulent parameters will be deeply evaluated and related to the arrival of TDFs and to the presence of GWs.

[1] Lothon, M., Lohou, F. et al (2014): The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence. Atmos. Chem. Phys., 14, 10931-10960.

 [2] Román-Cascón, C., Yagüe, C., Arrillaga, J.A., Lothon, M., Pardyjak, E,R., Lohou, F., Inclán, R.M., Sastre, M., Maqueda, G., Derrien, S., Meyerfeld, Y., Hang, C., Campargue-Rodríguez, P. & Turki, I. (2019): Comparing mountain breezes and their impacts on CO2 mixing ratios at three contrasting areas. Atmos. Res., 221, 111-126.

[3] Sun, J., Nappo, C.J., Mahrt, L., Belusic, D., Grisogono, B., Stauffer, D.R., Pulido, M., Staquet, C., Jiang, Q., Pouquet, A., Yagüe, C. Galperin, B., Smith, R.B., Finnigan, J.J., Mayor, S.D., Svensson, G., Grachev, A.A. & Neff., W.D.: (2015): Review of wave-turbulence interactions in the stable atmospheric boundary layer, Rev. Geophys., 53, 956–993.

How to cite: Yagüe, C., Román-Cascón, C., Lothon, M., Lohou, F., Arrillaga, J. A., and Maqueda, G.: Observational analysis of thermally-driven topographic flows and their turbulence-waves interaction , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19735, https://doi.org/10.5194/egusphere-egu2020-19735, 2020.

We investigate sea-breeze (SB) frontal passages troughout a 10-year period. Spanning the whole period, numerical simulations from the Weather Research and Forecasting (WRF) model are compared with a comprehensive observational database from the Cabauw Experimental Site (Ruisdael Project). On the one hand, a fine horizontal resolution of 2 km is employed in the numerical simulations, and the observational vertical levels within the first 200 m above the surface are replicated. On the other hand, an algorithm based on objective and strict filters is applied to both observations and simulations to select the SB events. This methodology allows to investigate the atmospheric scales influencing the SB formation and their interaction with local turbulence in a robust and objective way.

By carrying out a filter-by-filter comparison, we find that the simulated large-scale conditions show a good rate of coincidence with the observations (69%). Small biases in the large scale wind direction, however, induce important deviations in the surface-wind evolution. Regarding the mesoscale forcings, the land-sea temperature gradient is overestimated in average up to 4 K, producing stronger SB fronts in WRF. The analysis of the SB frontal characteristics and impacts is carried out by classifying the events into three boundary-layer regimes (convective, transition and stable) based on the value of the sensible-heat flux at the moment of the SB onset. The stronger SB in the model leads to enhanced turbulence particularly in the convective and transition regimes: the friction velocity, for instance, is overstated by around 50% at the SB onset. In addition, the arrival of the SB front enhances the stable stratification and gives rise to faster afternoon and evening transitions compared with situations solely driven by local atmospheric turbulence.

The obtained results can be considered a benchmark of the aspects to be improved in order to produce finer SB forecasts and more adequate representations of the associated physical processes, particularly during the afternoon and evening transition of the ABL.

How to cite: Arrillaga, J. A., Jiménez, P., Vilà-Guerau de Arellano, J., Jiménez, M. A., Román-Cascón, C., Sastre, M., and Yagüe, C.: Analyzing the atmospheric scales involved in sea-breeze formation and frontal characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3041, https://doi.org/10.5194/egusphere-egu2020-3041, 2020.

Numerical weather prediction models rely heavily on boundary-layer theories, which poorly capture the interactions between the Earth’s heterogeneous surface and the internal boundary layers aloft. Further, in relation to these theories, there remains outstanding questions that still require new understanding, such as the closure of the surface energy balance, advection quantification, and surface-flux interaction. We hypothesize that under certain conditions of unstable and neutral stratification, surface thermal heterogeneities can significantly influence the flow structure and alter momentum and scalar transport. To be able to access this hypothesis, we designed the Idealized horizontal Planar Array experiment for Quantifying Surface heterogeneity (IPAQS). IPAQS took place during the summers of 2018 and 2019 at the Great Salt Lake Desert playa in western Utah at the U.S. Army Dugway Proving Ground’s Surface Layer Turbulence and Environmental Test (SLTEST) facility. The site is characterized by a long uninterrupted fetch with uniform surface roughness and large thermal and moisture heterogeneities covering a wide range of scales. Observations were made with an array of 2-m high, temporally-synchronized, fast-response sonic anemometers, and finewire thermocouples, which were deployed on a coarse grid covering an area of 800 m x 800 m with 200-m spacing. Results provide valuable insight into the spatial and temporal evolution of the flow. Fine-scale turbulence was measured using Nano-Scale Thermal Anemometry Probes (NSTAP). Meanwhile, larger-scale turbulence was captured with Doppler wind LiDARs. Presented is an overview of the experiment and initial results.

How to cite: Morrison, T., Calaf, M., Pardyjak, E., Hultmark, M., Higgins, C., Iungo, G., Drake, S., Hoch, S., Zajic, D., Perelet, A., Bingham, A., Brunner, C., DeBell, T., Gunawardena, N., Huang, Y.-C., Mogollon, G., Najafi, B., Pandya, Y., Puccioni, M., and Kumar Singh Sr, D.: An atmospheric surface layer study: The Idealized horizontal Planar Array experiment for Quantifying Surface Heterogeneity (IPAQS) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12081, https://doi.org/10.5194/egusphere-egu2020-12081, 2020.

Regular, nocturnal fog is a defining and seasonally varying feature in the Namib desert. Historical observations were limited to the binary measure of fog occurrence and the concurrent fog water input is quantified only since 2014 via the FogNet using Juvik fog collectors. This installation opened new avenues of research such as the efficiency of the transport mechanism, sampling and spatial variation thereof. An eddy covariance setup of a cloud droplet probe and collocated sonic(s) was installed in turns at the two FogNet stations Vogelfederberg (23.10°S, 15.03°E, 515 m above sea level) and Gobabeb (23.56°S, 15.04°E, 406 m above sea level) for 2 years in the frame of the Namib Fog Life Cycle Analysis Field Measurements (NaFoLiCA-F) project. With this setup, we gathered duration, droplet size distribution, droplet concentration, liquid water content, turbulent liquid water flux and the fog water input via the Juvik fog collector with a total of over 150 fog events. We found that fog appears suddenly and front-like as seen by an increase of droplet numbers by several magnitudes and dissolves more gradually towards the morning. All droplet classes of the resolved range of 2 to 50 µm are present, but at the Vogelfederberg with around 2 to 3 times larger fog water input, the mean and median of the distribution are lower due to comparably fewer large droplets. Liquid water fluxes at both sites resulted in a net gain for the surface but the spatial discrepancy between fog water input recorded by fog collectors and the liquid water content indicates that drizzle, i.e. droplets outside the resolved range, may contribute to the larger total water deposition at Vogelfederberg.

How to cite: Spirig, R., Feigenwinter, C., and Vogt, R.: Turbulent transport and deposition of fog droplets in the Central Namib, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16201, https://doi.org/10.5194/egusphere-egu2020-16201, 2020.

We analyzed the flow characteristics in strep-up street canyons using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations (H_u/H_d = 0.33, 0.6 and L/S = 1, 2, 3, and 4; H_u, H_d, L, and S respectively indicate the upwind, downwind building heights, the building length and street-canyon width) as those in the wind-tunnel experiments. The CFD model reproduced the in-canyon vortex, recirculation zones above the downwind buildings, and stagnation point position reasonably well. Furthermore, we analyze the flow characteristics in the step-up street canyons based on the numerical results. The in-canyon flows simulated in the shallow (H_u/H_d = 0.33) and deep (H_u/H_d = 0.6) street canyons underwent two stages (development and mature stages) as the building-length ratio increased. In the development stages, one clockwise-rotating vortex was formed in the step-up street canyons and its center was slightly tilted toward the wall of the upwind building. However, in the mature stages, two clockwise-rotating vortices were formed in the upper and lower layers. A clockwise vortex and a counterclockwise vortex were stabilized as the building width ratio increased.

How to cite: Park, S.-J., Kim, J.-J., Pardyjak, E., and Hong, J.-Y.: CFD Simulations of Flows in Step-up Street Canyons , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12121, https://doi.org/10.5194/egusphere-egu2020-12121, 2020.

  In this study, we analyzed the observation environments of the automated synoptic observing systems (ASOSs) using a computational fluid dynamics (CFD) model, focusing on the observational environments of air temperatures, wind speeds, and wind directions. The computational domain sizes are 2000 m × 2000 m × 750 m, and the grid sizes are 10 m × 10 m × 5 m in the x-, y-, and z- directions, respectively. We conducted the simulations for eight inflow directions (northerly, northeasterly, easterly, southeasterly, southerly, southwesterly, westerly, northwesterly) using the ASOS-observation wind speeds and air temperatures averaged in August from 2010 to 2019. We analyzed the effects of the surrounding buildings and terrains on the meteorological observations of the ASOSs, by comparing the wind speeds, wind directions, and air temperatures simulated at the ASOSs with those of inflows. The results showed that the meteorological observation environments were quite dependent on whether there existed the obstacles and surface heating on their surfaces at the observation altitude of the ASOSs.

How to cite: Kang, J.-E. and Kim, J.-J.: Assessment of Observational Environments of the Automated Synoptic Observing Systems in Korea Using a CFD Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12244, https://doi.org/10.5194/egusphere-egu2020-12244, 2020.

Large-eddy simulations are performed to investigate the effects of background wind on the secondary circulations (SCs) in the convective boundary layer. Heterogeneities are produced by a prescribed two-dimensional surface sensible heat flux pattern of chessboard-type and have a size which is a bit larger than the boundary layer height.

When the wind blows along the diagonal of the chessboard-like pattern, the roll-like SCs are observed even when the background wind speed is as large as 10m/s, with whose axes are oriented along the diagonal of the pattern. Another case with wind direction along neither the diagonal nor the side of the chessboard-like pattern and weak wind speed shows the roll-like SCs still exist but lack symmetry. The SCs become much weaker and change their axes orientation when the wind speed increases.

Meanwhile, the results are different when the Coriolis force is considered. When the background wind is weak, the asymmetry of the SCs become more significant with the development of boundary layer when the Coriolis force is considered, while the SCs tend to be symmetrical without the Coriolis force. When the background wind strengthens, the SCs are more difficult to maintain in the case of Coriolis force.

Further analysis through rotational and divergent decomposition suggests which part contributes more to the maintenance of the SCs.

How to cite: Xiang, Z., Sun, J., and Zou, J.: Characteristics of the secondary circulations in the convective boundary layer over two-dimensional heterogeneous surfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12391, https://doi.org/10.5194/egusphere-egu2020-12391, 2020.

WRF model provides a potentially powerful framework for coupled simulations of flow covering a wide range of
spatial and temporal scales via a successive grid nesting capability. Nesting can be repeated down to turbulence
solving large eddy simulation (LES) scales, providing a means for significant improvements of simulation of
turbulent atmospheric boundary layers. We will present the recent progress on our WRF-LES simulations of
the Perdigao Experiment performed over mountainous terrain. We performed multi-scale simulations using
WRF’s different Planetary Boundary Layer (PBL) parameterizations as well as Large Eddy Simulation (LES)
and compared the results with the detailed field measurements. WRF-LES model improved the mean flow field
as well as second-order flow statistics. Mean fluctuations and turbulent kinetic energy fields from WRF-LES
solution are investigated in several cross-sections around the hill which shows good agreement with measurements.

How to cite: Kirkil, G.: High-resolution atmospheric boundary layer simulations using WRF-LES, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21760, https://doi.org/10.5194/egusphere-egu2020-21760, 2020.

The lowest portion of the planetary boundary layer (PBL), where the turbulent fluxes are assumed to be constant, is known as the atmospheric surface layer (ASL). Within the surface layer, the surface exchange processes play an important role in land-atmosphere interaction. Thus, a precise formulation of the surface fluxes is crucial to ensure an adequate atmospheric evolution by numerical models. The Monin–Obukhov Similarity Theory (MOST) is a widely used framework to estimate the surface turbulent fluxes within the surface layer. MOST uses similarity functions of momentum (φ_m) and heat (φ_h) for non-dimensional wind and temperature profiles. Over the years, various formulations for these similarity functions have been proposed by the researchers ranging from linear to non-linear forms. These formulations have limitations in the weak wind, stable, and unstable atmospheric conditions. In the surface layer scheme currently available in the Community Atmosphere Model version 5 (CAM5.0), the stable and unstable regimes are divided into four parts, and the corresponding similarity functions are the functions proposed by Kader and Yaglom (1990) for strong unstable stratification, by Businger et al. (1971) for weak unstable stratification, functions by Dyer (1974) for weak stable stratification, and for moderate to strongly stable stratification, the functions from Holtslag et al. (1990) have been utilized. The criteria used for this classification are somewhat ad-hoc, and there is an abrupt transition between different regimes encountered.            

       In the present study, an effort has been made to implement the similarity functions proposed by Grachev et al. (2007) for stable conditions and Fairall et al. (1996) for unstable conditions in the surface layer scheme of Community Land Model (CLM) for CAM5.0. In the modified version, the similarity functions for unstable conditions are a combination of commonly used Paulson type expressions for near-neutral stratification and an expression proposed by Carl et al. (1973) that takes in to account highly convective conditions. Similarly, the formulation proposed by Grachev et al., for stable conditions, can cover a wider range of stable stratifications. The simulations with CAM5 model using the existing and modified version of surface layer scheme have been performed with 1° resolution for ten years, and the impact of modified functions on the simulation of various important near-surface variables over the tropical region is analyzed. It is found that the scheme with modified functions improving the simulation of surface variables as compared with the existing scheme over the tropical region. In addition, the limitations arbitrarily imposed on particular variables in the existing surface layer scheme can be eliminated or suppressed by using these modified functions.  

References:

Fairall CW, Bradley EF, Rogers DP, Edson JB, Young GS (1996) Bulk parameterization of air-sea fluxes for tropical ocean global atmosphere coupled-ocean atmosphere response experiment. J Geophys Res 101(C2):3747–3764

Grachev, A.A., Andreas, E.L., Fairall, C.W., Guest, P.S. and Persson, P.O.G. (2007a) SHEBA: flux–profile relationships in stable atmospheric boundary layer. Boundary-Layer Meteorology,124, 315–333.

Keywords:

Boundary layer, Turbulence, Climate Model, Surface Fluxes

How to cite: Namdev, P., Sharan, M., and Mishra, S. K.: Sensitivity studies with different formulations for similarity functions used in surface layer scheme, over the tropical region, in a climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16539, https://doi.org/10.5194/egusphere-egu2020-16539, 2020.

In the frame of the COMTESSA (Camera Observation and Modelling of 4D Tracer Dispersion in the Atmosphere) project, tracer dispersion release experiments were performed during three field campaigns in Norway in July 2017, 2018, and 2019.  The main goal of the project is to improve our understanding of turbulence and plume dispersion on local scale in the planetary boundary layer by bringing together full four-dimensional (space and time) observations of a (nearly) passive tracer (sulfur dioxide, SO₂), with advanced data analysis and turbulence and dispersion modelling. By means of tomographic reconstruction of the 3D tracer concentration distribution, not only the mean but also higher moments of the probability density function of the tracer concentration field can be revealed. In 2017 first field tests were made, releasing SO₂ in continuous plumes and puffs from a 10 m tower, while in the following years SO₂ was released from a 60 m tower, located in the centre of a fenced-in 900 m x 400 m wide flat gravel field. The masts were equipped with eddy covariance measurement systems to continuously record turbulent fluxes of heat and momentum during the field campaigns. Up to six ultraviolet (UV) and in 2019 also three infrared (IR) SO₂ cameras, were placed in a ring around the SO₂ release tower at varying distances up to ~1.2 km to simultaneously image the movement and spread of the 2d integrated SO₂ tracer column densities.

Here we present an overview of the field experiments and lessons learned, with focus on results from the 2019 summer campaign. It was a challenge to find a location where hazardous gas could be released and a main obstacle for the imaging-based experiment were the unfavourable weather conditions. Despite these challenges, progress was made throughout the years. During consecutive summers the release equipment was improved and optimized and in 2019 puff releases were made by filling balloons with SO₂ and exploding them. The cameras were continuously developed, the setup of the cameras at the site was adjusted to allow observations for longer timescales.  During July 11-28, 2019 ~130 puffs were released from balloons holding between 250 g and 325 g SO₂. Those are used to give an overview of the image/data processing and type of results that can be obtained from our observations, e.g. relative dispersion and meandering, Eulerian and Lagrangian integral time scales and their relation, tomographic reconstruction. The focus lies on the plume spread, i.e. relative dispersion processes we recorded under different stability conditions in July 2019.

How to cite: Stebel, K., Cassiani, M., Ardeshiri, H., Bernardo, C., Dinger, A. S., Kylling, A., Park, S.-Y., Pisso, I., Schmidbauer, N., and Stohl, A.: Camera Observation and Modelling of 4D Tracer Dispersion in the Atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18036, https://doi.org/10.5194/egusphere-egu2020-18036, 2020.

Turbulence coherent structures play an important role in the transport of momentum, sensible heat, water vapour and carbon dioxide fluxes over atmospheric surface layer (ASL) . Using eddy covariance system measurements on a 50m tower in Zengcheng, Guangdong province, we develop a novel method based on quadrant analysis to detect turbulent coherent structures. We presume that turbulent flux events’ durations smaller than threshold t are isotropic turbulence. Therefore, the durations of small-time-scale (duration< t) turbulent flux events of each quadrant are expect to be equal, which can be regarded as the criterion of threshold . A deviation of the similarity between four quadrant small-time-scale turbulent flux events’ durations is set to determine the  value. Contour map of momentum flux joint probability density function on quadrant domain proves our hypothesis. Coherent structures can be identified from large-time-scale (duration>t) turbulent flux events.

We apply this method to the momentum, sensible heat, water vapour and carbon dioxide fluxes and obtain individual turbulent coherent structures time-series of different fluxes. It is found that numbers and durations of turbulent coherent structures are similar. Secondly, threshold  is not sensitive to the change of ASL satiability. Compared with k method (NARASIMHA 2007), our  method stands for more physical background as it can be seen as the time-scale of isotropic turbulence, which makes our detecting method more efficient.

How to cite: Wang, Y., Wang, B., and Fang, R.: An approach to detect turbulence coherent structures based on quadrant analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21055, https://doi.org/10.5194/egusphere-egu2020-21055, 2020.

Dust devils are atmospheric air vortices with a vertical axis, which are formed by strong sun radiation and the resulting vertical temperature gradient. The structure of a typical dust devil is dominated by a radial inflow near the surface and a vertical upward flow within the vortex. Since experimental investigations are limited to local in-situ measurements of the atmosphere, many mechanisms related to their formation and their characteristic properties are insufficiently understood. We will present an idea how dust devils can be generated in a laboratory experiment and how processes, which contribute to their formation, can be investigated.

We have chosen the so-called Rayleigh-Bénard set-up as an appropriate model experiment for our investigations. The "Barrel of Ilmenau" is a test facility, which consists mainly of an air-filled, cylindrical tank with a total diameter of 7 m and a total height of 8 m. At the bottom of the tank is a heating plate, whose temperature can be set precisely between 20°C and 80°C. A second plate, which can be positioned at any height between 0.2m and 6.3m above the heating plate, is used for cooling and can be set to temperatures as low as 10°C...30 °C. This results in a total temperature difference of up to 70K which is significantly beyond the temperature difference of the atmospheric boundary layer. The side wall of the tank is adiabatic. Due to the isothermal plates and a compensation heating system, a temperature that is constant over time is reached and controlled boundary conditions can be assumed.

For the characterization of Dust Devil-like vortices, an optical measurement method is used to obtain the trajectories of single particles. Since there are no commercial systems that are suitable for such a large measurement volume, we created our own system. The measurements in the RB cell are carried out with an aspect ratio Γ=3 and a Ra number of Ra=2x10¹⁰ and Ra=8x10¹⁰.  With the help of the measurements we want to show that Dust Devil-like vortices are created. The structure should be characterized and the results will be compared to DNS.

How to cite: Lösch, A. and du Puits, R.: Experimental investigation of Dust Devil like vortices with 3D particle tracking velocimetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22451, https://doi.org/10.5194/egusphere-egu2020-22451, 2020.

Diurnal and seasonal variations of sensible heat (H) and latent heat (LE) fluxes observed at an agricultural site on the campus of Obafemi Awolowo University, Ile-Ife, southwest Nigeria have been reported in this paper. The deductions are made based on half-hourly flux data acquired from an open-path eddy covariance (OPEC) system measured continuously over a two-year observation period (2017-2018) at the study site. The study area is within tropical wet and dry climate of West Africa, thereby experiencing an alternating wet (that is, April – October) and dry (that is, December – February) seasons (monsoonal). Our results showed that peak hourly values of H and LE occurred at about 13:00 LT and 14:00 LT respectively, a lag of approximately one hour between them at the location. The diurnal range for H and LE during wet season was 75.3 W m^-2 and 177.0 W m^-2 respectively, while for dry season it was 182.0 W m^-2 and 89.9 W m^-2 respectively. The daily mean value of H for wet season was 19.7 ± 27.2 W m^-2 and it was 52.1 ± 63.5 W m^-2 for LE. For dry season, daily mean values for H and LE were 44.0 ± 66.4 W m^-2 and 26.6 ± 33.7 W m^-2 respectively. A transition of seasons from wet (Bowen ratio, Bo < 1) to dry (Bo > 1) was observed in November and reversal in March.

Keywords: Diurnal and Seasonal Variations, Sensible and Latent Heat Fluxes, Tropical Wet and Dry Climate

How to cite: Ajao, A., Obisesan, O., Ayoola, M., and Jegede, O.: Diurnal and Seasonal Variations of Sensible and Latent Heat Fluxes at an Agricultural Site in Ile-Ife, Southwest Nigeria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-25, https://doi.org/10.5194/egusphere-egu2020-25, 2020.