scholarly journals Evidence of Strong Flux Underestimation by Bulk Parametrizations During Drifting and Blowing Snow

2021 ◽  
Author(s):  
Armin Sigmund ◽  
Jérôme Dujardin ◽  
Francesco Comola ◽  
Varun Sharma ◽  
Hendrik Huwald ◽  
...  
Keyword(s):  
Eddy Covariance ◽  
Large Eddy Simulations ◽  
Blowing Snow ◽  
Near Surface ◽  
Surface Mass ◽  
Large Eddy ◽  
Momentum Fluxes ◽  
Moisture Fluxes ◽  
Heat And Moisture ◽  
The Antarctic

AbstractThe influence of drifting and blowing snow on surface mass and energy exchange is difficult to quantify due to limitations in both measurements and models, but is still potentially very important over large areas with seasonal or perennial snow cover. We present a unique set of measurements that make possible the calculation of turbulent moisture, heat, and momentum fluxes during conditions of drifting and blowing snow. From the data, Monin–Obukhov estimation of bulk fluxes is compared to eddy-covariance-derived fluxes. In addition, large-eddy simulations with sublimating particles are used to more completely understand the vertical profiles of the fluxes. For a storm period at the Syowa S17 station in East Antarctica, the bulk parametrization severely underestimates near-surface heat and moisture fluxes. The large-eddy simulations agree with the eddy-covariance fluxes when the measurements are minimally disturbed by the snow particles. We conclude that overall exchange over snow surfaces is much more intense than current models suggest, which has implications for the total mass balance of the Antarctic ice sheet and the cryosphere.

On the way to realistic large eddy simulations – A comparison of virtual measurements with CHEESEHEAD19 field measurements

2021 ◽  
Author(s):  
Luise Wanner ◽  
Sreenath Paleri ◽  
Johannes Speidel ◽  
Ankur Desai ◽  
Matthias Sühring ◽  
...  
Keyword(s):  
Energy Balance ◽  
Eddy Covariance ◽  
Field Measurements ◽  
Transport Processes ◽  
Large Eddy Simulations ◽  
Surface Model ◽  
Closure Problem ◽  
Energy Balance Closure ◽  
Large Eddy ◽  
Eddy Covariance Measurements

<p>Large-eddy simulations are useful tools to study transport processes by mesoscale structures in the atmospheric boundary layer, since in contrast to single-tower eddy covariance measurements, they provide not only temporally but also spatially highly resolved information. Therefore, they are well suited to study the energy balance closure problem, for which the mesoscale transport of latent and sensible heat, triggered by heterogeneous ecosystems, is suspected to be a major cause. However, this requires simulations that are as realistic as possible and thus allow a comparison of real measurements in the field and virtual measurements in the simulation.<br>During the Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors (CHEESEHEAD) experiment in the summer of 2019, a heterogeneous 10x10 square km domain was intensively sampled across scales. This data offers a unique possibility to set up large-eddy simulations with realistic surface heterogeneity. We use PALM to simulate two days and an area of 40 by 40 square kilometers incorporating the CHEESEHEAD site. The large scale atmospheric forcings to inform the boundary conditions are determined from the NCEP HRRR product. As the lower boundary condition, we use a soil and land-surface model coupled with a plant-canopy model, which we adapt to the CHEESEHEAD area based on ground-based and airborne measurements of plant physiological data.<br>In this study, we investigate how well the simulations match with real measurements by comparing simulated profiles and virtual tower measurements with field measurements from radiosonde ascents, lidar measurements of three-dimensional wind and water vapor, eddy-covariance measurements from the 400 meter tower in the center of the study domain, as well as from typical eddy-covariance stations distributed through the study area. This way, we investigate how realistic the simulations actually are and to what extent the knowledge gained from them concerning the energy balance closure problem can be transferred to field measurements.</p>

Estimating the Risk of Extreme Wind Gusts in Tropical Cyclones Using Idealized Large-Eddy Simulations and a Statistical-Dynamical Model

2021 ◽  
Author(s):  
Daniel P. Stern ◽  
George H. Bryan ◽  
Chia-Ying Lee ◽  
James D. Doyle
Keyword(s):  
Tropical Cyclones ◽  
The United States ◽  
Large Eddy Simulations ◽  
Dynamical Model ◽  
Near Surface ◽  
Southeastern Us ◽  
Wind Gusts ◽  
Extreme Wind ◽  
Large Eddy

AbstractRecent studies have shown that extreme wind gusts are ubiquitous within the eyewall of intense tropical cyclones (TCs). These gusts pose a substantial hazard to human life and property, but both the short-term (i.e., during the passage of a single TC) and long-term (over many years) risk of encountering such a gust at a given location is poorly understood. Here, simulated tower data from large-eddy simulations of idealized TCs in a quiescent (i.e., no mean flow or vertical wind shear) environment are used to estimate these risks for the offshore region of the United States. For both a category 5 and category 3 TC, there is a radial region where nearly all simulated towers experience near-surface (the lowest 200 m) 3-s gusts exceeding 70 m s−1 within a 10-minute period; on average, these towers respectively sample peak 3-s gusts of 110 and 80 m s−1. Analysis of an observational dropsonde database supports the idealized simulations, and indicates that offshore structures (such as wind turbines) in the eyewall of a major hurricane are likely to encounter damaging wind speeds. This result is then incorporated into an estimate of the long-term risk, using analyses of the return period for major hurricanes from both a best-track database and a statistical-dynamical model forced by reanalysis. For much of the nearshore region of the Gulf of Mexico and southeastern US coasts, this analysis yields an estimate of a 30-60% probability of any given point experiencing at least one 70 m s−1 gust within a 30-year period.

scholarly journals An Evaluation of Surface Climatology in State-of-the-Art Reanalyses over the Antarctic Ice Sheet

2019 ◽  
Vol 32 (20) ◽  
pp. 6899-6915 ◽  
Author(s):  
A. Gossart ◽  
S. Helsen ◽  
J. T. M. Lenaerts ◽  
S. Vanden Broucke ◽  
N. P. M. van Lipzig ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Surface Temperature ◽  
Mass Balance ◽  
Ice Sheet ◽  
Surface Mass Balance ◽  
Antarctic Ice Sheet ◽  
Near Surface ◽  
Surface Mass ◽  
Atmospheric Rivers ◽  
The Antarctic

Abstract In this study, we evaluate output of near-surface atmospheric variables over the Antarctic Ice Sheet from four reanalyses: the new European Centre for Medium-Range Weather Forecasts ERA-5 and its predecessor ERA-Interim, the Climate Forecast System Reanalysis (CFSR), and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). The near-surface temperature, wind speed, and relative humidity are compared with datasets of in situ observations, together with an assessment of the simulated surface mass balance (approximated by precipitation minus evaporation). No reanalysis clearly stands out as the best performing for all areas, seasons, and variables, and each of the reanalyses displays different biases. CFSR strongly overestimates the relative humidity during all seasons whereas ERA-5 and MERRA-2 (and, to a lesser extent, ERA-Interim) strongly underestimate relative humidity during winter. ERA-5 captures the seasonal cycle of near-surface temperature best and shows the smallest bias relative to the observations. The other reanalyses show a general temperature underestimation during the winter months in the Antarctic interior and overestimation in the coastal areas. All reanalyses underestimate the mean near-surface winds in the interior (except MERRA-2) and along the coast during the entire year. The winds at the Antarctic Peninsula are overestimated by all reanalyses except MERRA-2. All models are able to capture snowfall patterns related to atmospheric rivers, with varying accuracy. Accumulation is best represented by ERA-5, although it underestimates observed surface mass balance and there is some variability in the accumulation over the different elevation classes, for all reanalyses.

Optimizing large-eddy simulations for investigating the energy-balance closure problem at typical flux measurement heights

2020 ◽  
Author(s):  
Luise Wanner ◽  
Frederik De Roo ◽  
Matthias Mauder
Keyword(s):  
Energy Balance ◽  
Eddy Covariance ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Large Eddy Simulations ◽  
Scale Model ◽  
Energy Balance Closure ◽  
Balance Study ◽  
Large Eddy ◽  
Secondary Circulations

<p>The eddy-covariance method generally underestimates sensible and latent heat fluxes, resulting in an energy-balance gap from 10 % to even 30 % across sites worldwide. In contrast to single-tower eddy-covariance measurements, large-eddy simulations (LES) provide information on a 3D array of grid points and can capture atmospheric processes such as secondary circulations on all relevant scales, which makes them a powerful tool to investigate this problem. In order to compare LES results to field measurements at 20 m height from the CHEESEHEAD (Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors) campaign, a LES-setup that provides comparability to the measurements at these low levels is necessary. However, former LES studies have shown that the energy balance is almost closed near the surface, which does not reflect the energy-balance gap in measurements. One possible reason might be the common use of prescribed surface fluxes that cannot adapt to changes in surface temperature and moisture, which would allow for the self-reinforcement of secondary circulations. Therefore, we set up an idealized study, in which we compare the performance of the land-surface and plant-canopy models implemented in PALM to the use of prescribed surface fluxes above homogeneous forest and grassland ecosystems under different atmospheric conditions with respect to realistic energy-balance closure behavior. Furthermore, we evaluate the performance of a dynamic subgrid-scale model, as well as an alternative to the Monin-Obukhov similarity theory (Banerjee et al. 2015, Q. J. R. Met. Soc.).</p>

Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

2009 ◽  
Vol 48 (6) ◽  
pp. 1161-1180 ◽  
Author(s):  
Francis L. Ludwig ◽  
Fotini Katopodes Chow ◽  
Robert L. Street
Keyword(s):  
Dynamic Models ◽  
Turbulence Models ◽  
Coarse Grid ◽  
Large Eddy Simulations ◽  
Near Surface ◽  
Velocity Statistics ◽  
Neutral Boundary Layer ◽  
Regional Prediction ◽  
Large Eddy ◽  
Flow Features

Abstract This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

scholarly journals Antarctic 20th Century Accumulation Changes Based on Regional Climate Model Simulations

2010 ◽  
Vol 2010 ◽  
pp. 1-14 ◽  
Author(s):  
Klaus Dethloff ◽  
Ksenia Glushak ◽  
Annette Rinke ◽  
Dörthe Handorf
Keyword(s):  
Regional Climate Model ◽  
Climate Model ◽  
Regional Climate ◽  
Reanalysis Data ◽  
Horizontal Resolution ◽  
East Antarctica ◽  
Transient Pressure ◽  
Near Surface ◽  
Surface Mass ◽  
The Antarctic

The regional climate model HIRHAM has been applied to Antarctica driven at the lateral and lower boundaries by European Reanalysis data ERA-40 for the period 1958–1998. Simulations over 4 decades, carried out with a horizontal resolution of 50 km, deliver a realistic simulation of the Antarctic atmospheric circulation, synoptic-scale pressure systems, and the spatial distribution of precipitation minus sublimation (P-E) structures. The simulated P-E pattern is in qualitative agreement with glaciological estimates. The estimated (P-E) trends demonstrate surfacemass accumulation increase at the West Antarctic coasts and reductions in parts of East Antarctica. The influence of the Antarctic Oscillation (AAO) on the near-surface climate and the surface mass accumulation over Antarctica have been investigated on the basis of ERA-40 data and HIRHAM simulations. It is shown that the regional accumulation changes are largely driven by changes in the transient activity around the Antarctic coasts due to the varying AAO phases. During positive AAO, more transient pressure systems travelling towards the continent, and Western Antarctica and parts of South-Eastern Antarctica gain more precipitation and mass. Over central Antarctica the prevailing anticyclone causes a strengthening of polar desertification connected with a reduced surface mass balance in the northern part of East Antarctica.

scholarly journals Study of near-surface models for large-eddy simulations of a neutrally stratified atmospheric boundary layer

2007 ◽  
Vol 124 (3) ◽  
pp. 405-424 ◽  
Author(s):  
Inanc Senocak ◽  
Andrew S. Ackerman ◽  
Michael P. Kirkpatrick ◽  
David E. Stevens ◽  
Nagi N. Mansour
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Large Eddy Simulations ◽  
Near Surface ◽  
Large Eddy ◽  
Surface Models ◽  
Neutrally Stratified

scholarly journals Cloud and precipitation properties from ground-based remote sensing instruments in East Antarctica

2014 ◽  
Vol 8 (4) ◽  
pp. 4195-4241 ◽  
Author(s):  
I. V. Gorodetskaya ◽  
S. Kneifel ◽  
M. Maahn ◽  
K. Van Tricht ◽  
J. H. Schween ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Bimodal Distribution ◽  
Snow Accumulation ◽  
East Antarctica ◽  
Blowing Snow ◽  
Precipitation Radar ◽  
Radiative Fluxes ◽  
Near Surface ◽  
Surface Mass ◽  
Meteorological Observatory

Abstract. A new comprehensive cloud-precipitation-meteorological observatory has been established at Princess Elisabeth base, located in the escarpment zone of Dronning Maud Land, East Antarctica. The observatory consists of a set of ground-based remote sensing instruments (ceilometer, infrared pyrometer and vertically profiling precipitation radar) combined with automatic weather station measurements of near-surface meteorology, radiative fluxes, and snow accumulation. In this paper, the observatory is presented and the potential for studying the evolution of clouds and precipitating systems is illustrated by case studies. It is shown that the synergetic use of the set of instruments allows for distinguishing ice, mixed-phase and precipitating clouds, including some information on their vertical extent. In addition, wind-driven blowing snow events can be distinguished from deeper precipitating systems. Cloud properties largely affect the surface radiative fluxes, with liquid-containing clouds dominating the radiative impact. A statistical analysis of all measurements (in total 14 months mainly occurring in summer/autumn) indicates that these liquid-containing clouds occur during as much as 20% of the cloudy periods. The cloud occurrence shows a strong bimodal distribution with clear sky conditions 51% of the time and complete overcast conditions 35% of the time. Snowfall occurred 17% of the cloudy periods with a predominance of light precipitation and only rare events with snowfall > 1 mm h−1 water equivalent (w.e.). Three of such intensive snowfall events occurred during 2011 contributing to anomalously large annual snow accumulation. This is the first deployment of a precipitation radar in Antarctica allowing to assess the contribution of the snowfall to the local surface mass balance. It is shown that on the one hand large accumulation events (>10 mm w.e. day−1) during the measurement period of 26 months were always associated with snowfall, but that on the other hand snowfall did not always lead to accumulation. In general, this promising set of robust instrumentation allows for improved insight in cloud and precipitation processes in Antarctica and can be easily deployed at other Antarctic stations.

scholarly journals Diverging future surface mass balance between the Antarctic ice shelves and grounded ice sheet

2021 ◽  
Vol 15 (3) ◽  
pp. 1215-1236
Author(s):  
Christoph Kittel ◽  
Charles Amory ◽  
Cécile Agosta ◽  
Nicolas C. Jourdain ◽  
Stefan Hofer ◽  
...  
Keyword(s):  
Mass Balance ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Surface Mass Balance ◽  
Near Surface ◽  
Surface Mass ◽  
Ice Shelves ◽  
Run Off ◽  
The Antarctic

Abstract. The future surface mass balance (SMB) will influence the ice dynamics and the contribution of the Antarctic ice sheet (AIS) to the sea level rise. Most of recent Antarctic SMB projections were based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5). However, new CMIP6 results have revealed a +1.3 ∘C higher mean Antarctic near-surface temperature than in CMIP5 at the end of the 21st century, enabling estimations of future SMB in warmer climates. Here, we investigate the AIS sensitivity to different warmings with an ensemble of four simulations performed with the polar regional climate model Modèle Atmosphérique Régional (MAR) forced by two CMIP5 and two CMIP6 models over 1981–2100. Statistical extrapolation enables us to expand our results to the whole CMIP5 and CMIP6 ensembles. Our results highlight a contrasting effect on the future grounded ice sheet and the ice shelves. The SMB over grounded ice is projected to increase as a response to stronger snowfall, only partly offset by enhanced meltwater run-off. This leads to a cumulated sea-level-rise mitigation (i.e. an increase in surface mass) of the grounded Antarctic surface by 5.1 ± 1.9 cm sea level equivalent (SLE) in CMIP5-RCP8.5 (Relative Concentration Pathway 8.5) and 6.3 ± 2.0 cm SLE in CMIP6-ssp585 (Shared Socioeconomic Pathways 585). Additionally, the CMIP6 low-emission ssp126 and intermediate-emission ssp245 scenarios project a stabilized surface mass gain, resulting in a lower mitigation to sea level rise than in ssp585. Over the ice shelves, the strong run-off increase associated with higher temperature is projected to decrease the SMB (more strongly in CMIP6-ssp585 compared to CMIP5-RCP8.5). Ice shelves are however predicted to have a close-to-present-equilibrium stable SMB under CMIP6 ssp126 and ssp245 scenarios. Future uncertainties are mainly due to the sensitivity to anthropogenic forcing and the timing of the projected warming. While ice shelves should remain at a close-to-equilibrium stable SMB under the Paris Agreement, MAR projects strong SMB decrease for an Antarctic near-surface warming above +2.5 ∘C compared to 1981–2010 mean temperature, limiting the warming range before potential irreversible damages on the ice shelves. Finally, our results reveal the existence of a potential threshold (+7.5 ∘C) that leads to a lower grounded-SMB increase. This however has to be confirmed in following studies using more extreme or longer future scenarios.

scholarly journals Near-Surface Intensification of Tornado Vortices

2007 ◽  
Vol 64 (7) ◽  
pp. 2176-2194 ◽  
Author(s):  
D. C. Lewellen ◽  
W. S. Lewellen
Keyword(s):  
Analytical Model ◽  
Broad Class ◽  
Surface Flow ◽  
Large Eddy Simulations ◽  
Fine Tuning ◽  
Near Surface ◽  
Corner Flow ◽  
Large Eddy ◽  
End Wall ◽  
Corner Flows

Abstract An idealized analytical model and numerical large-eddy simulations are used to explore fluid-dynamic mechanisms by which tornadoes may be intensified near the surface relative to conditions aloft. The analytical model generalizes a simple model of Barcilon and Fiedler and Rotunno for a steady supercritical end-wall vortex to more general vortex corner flows, angular momentum distributions, and time dependence. The model illustrates the role played by the corner flow swirl ratio in determining corner flow structure and intensification; predicts an intensification of near-surface swirl velocities relative to conditions aloft of Iυ ∼ 2 for supercritical end-wall vortices in agreement with earlier analytical, numerical, and laboratory results; and suggests how larger intensification factors might be achieved in some more general corner flows. Examples of the latter are presented using large-eddy simulations. By tuning the lateral inflow boundary conditions near the surface, quasi-steady vortices exhibiting nested inner and outer corner flows and Iυ ∼ 4 are produced. More significantly, these features can be produced without fine tuning, along with an additional doubling (or more) of the intensification, in a broad class of unsteady evolutions producing a dynamic corner flow collapse. These scenarios, triggered purely by changes in the far-field near-surface flow, provide an attractive mechanism for naturally achieving an intense near-surface vortex from a much larger-scale less-intense swirling flow. It is argued that, applied on different scales, this may sometimes play a role in tornadogenesis and/or tornado variability. This phenomenon of corner flow collapse is considered further in a companion paper.

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