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Changes in the Arctic and Antarctic climate systems are strongly related to processes in the boundary layer and their feedbacks with the free troposphere, ocean and ice. An adequate understanding and quantification of these processes is necessary to improve predictions of future changes in the polar regions and their teleconnection with mid-latitude weather and climate, including meridional transport of heat, moisture and chemical constituents. Processes include atmosphere-ocean-ice (AOI) interactions, physical and chemical snow processes (e.g. snow photochemistry), exchange of chemical constituents including biogeochemical impacts , sources of aerosol, polynya formation processes, sea ice production and loss, and cloud formation, which represent key processes for the atmosphere, ocean and the cryosphere. AOI interactions are also triggered by and have feedbacks with synoptic systems and mesoscale weather phenomena such as cold air outbreaks, katabatic winds and polar lows. Associated processes also include the effect of extreme events such as warm air advection and clouds on the surface energy budget and related boundary layer exchanges. In addition, understanding natural processes including AOI interactions is essential to understand of the background atmosphere to quantify the anthropogenic impacts. Shallow inversions, mostly during winter-time, lead to high air pollutant concentrations. Even though severe air pollution episodes are frequently observed in the Arctic, knowledge on urban emission sources, transport and atmospheric chemical processing of pollution, especially under cold and dark conditions, are poorly understood. Similarly, the polar boundary layer can involve complicated radiative processes such as shallow stable layers with fog present. In addition, polar boundary layers can mediate chemical, aerosol, and isotope exchanges between the atmosphere and the firn important to the interpretation of ice core records.
This session is intended to provide an interdisciplinary forum to bring together researchers working in the area of boundary layer processes and high-latitude weather and climate (including snow physics, air/snow chemistry, and oceanography). Cryosphere and atmospheric chemistry processes (the focus of the IGAC/SOLAS activity “CATCH” and the IGAC/IASC activity “PACES”) are highly relevant to this session. We also encourage preliminary results from field programs such as MOSAiC and other high-latitude research efforts.

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Co-organized by CL2/CR7/OS1
Convener: William Neff | Co-conveners: Jo Browse, Julia Schmale, Michael Tjernström, Gillian Young
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| Attendance Mon, 04 May, 08:30–10:15 (CEST)

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Chat time: Monday, 4 May 2020, 08:30–10:15

Chairperson: Michael Tjernstrom
D3438 |
EGU2020-8728
Amélie Kirchgaessner, John King, and Alan Gadian

We examine the representation of Föhn events across the Antarctic Peninsula Mountains during 2011 as they were observed in measurements by an Automatic Weather Station, and in simulations with the Weather Research and Forecasting Model (WRF) as run for the Antarctic Mesoscale Prediction System (AMPS). On the Larsen Ice Shelf (LIS) in the lee of this mountain range Föhn winds are thought to provide the atmospheric conditions for significant warming over the ice shelf thus leading to the initial firn densification and subsequently providing the melt water for hydrofracturing. This process has led to the dramatic collapse of huge parts of the LIS in 1995 and 2002 respectively.

We find that, while the model generally simulates meteorological parameters very well, and shows good skills in capturing the occurrence, frequency and duration of Föhn events realistically, it underestimates the temperature increase and the humidity decrease during the Föhn significantly, and may thus underestimate the contribution of Föhn to driving surface melt on the LIS. Our results indicate that the misrepresentation of cloud properties and particularly the absence of mixed phase clouds in AMPS, affects the quality of weather simulation under normal conditions to some extent, and to a larger extent the model’s capability to simulate the strength of Föhn conditions - and thus their contribution to driving surface melt on the LIS - adequately.

How to cite: Kirchgaessner, A., King, J., and Gadian, A.: The representation of Föhn events to the east of the Antarctic Peninsula in simulations by the Antarctic Mesoscale Prediction System (AMPS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8728, https://doi.org/10.5194/egusphere-egu2020-8728, 2020

D3439 |
EGU2020-3043
Étienne Vignon, Ghislain Picard, Claudio Durán-Alarcón, Simon P. Alexander, Hubert Gallée, and Alexis Berne

The offshore extent of Antarctic katabatic winds exert a strong control on sea ice production and the formation of polynyas. In this study, we combine ground-based remotely-sensed and meteorological measurements at Dumont d’Urville (DDU) station, satellite images and simulations with the WRF model to analyze a major katabatic wind event in Adélie Land. Once developed over the slope of the ice sheet, the katabatic flow experiences an abrupt transition near the coastal edge. The transition consists in a sharp increase in the boundary layer depth, a sudden decrease in wind speed and a decrease in Froude number from 3.5 to 0.3. This so-called ‘katabatic jump’ visually manifests as a turbulent ‘wall’ of blowing snow in which updrafts exceed 5 m s −1 . The wall reaches heights of 1000 m and its horizontal extent along the coast is more than 400 km. By destabilizing the boundary-layer downstream, the jump favors the trapping of a gravity wave train  with an horizontal wavelength of 10.5 km. The trapped gravity waves exert a drag that significantly slows down the low-level outflow. Moreover, atmospheric rotors form below the first wave crests. The wind speed record measured at DDU in 2017 (58.5 m s −1 ) is due to the vertical advection of momentum by a rotor. A statistical analysis of observations at DDU reveals that katabatic jumps and low-level trapped gravity waves occur frequently over coastal Adélie Land. It emphasizes the important role of such phenomena in the coastal Antarctic dynamics.

How to cite: Vignon, É., Picard, G., Durán-Alarcón, C., Alexander, S. P., Gallée, H., and Berne, A.: Gravity wave excitation during the coastal transition of an extreme katabatic flow in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3043, https://doi.org/10.5194/egusphere-egu2020-3043, 2020

D3440 |
EGU2020-3634
Anna Jones, Neil Brough, and Paul Griffiths

We present first observations of OH and (HO2 + RO2) carried out in Antarctica outside the summer season. Measurements were made over 23 days in spring at the coastal Antarctic station Halley. Increases in concentrations were evident during the measurement period due to rapidly increasing solar irradiance, and clear diurnal cycles were present throughout. There were also notable differences in air mass composition depending on wind direction. Air masses that had traversed the sea-ice-zone had both higher concentrations of OH and a larger OH:(HO2 + RO2) ratio. We use steady-state kinetic arguments and a 0-D box model to probe the chemical drivers. We find that differences in bromine chemistry, previously measured at Halley, are sufficient to account for the observed differences in OH concentration as well as the ratio. There is some evidence also that chlorine chemistry is influencing concentrations of RO2.

Sea ice in the polar regions is undergoing considerable change. Our results suggest that changes in the characteristics and extent of the sea-ice-zone that lead to changes in abundance of atmospheric halogens, will also result in a change in OH. For example, a shift towards more new sea ice formation, with its higher salinity over multi-year ice, would be expected to increase the abundance of halogens; conversely, overall reduction in sea ice extent would ultimately reduce abundance of halogens. OH radicals play a key role in oxidation reactions that remove pollutants from the atmosphere. Especially given anticipated expansion of industrial activities in the Arctic, this is a further factor to take into account when considering the wider impacts of sea ice loss.

How to cite: Jones, A., Brough, N., and Griffiths, P.: Influence of sea-ice-derived halogens on atmospheric HOx as observed in springtime coastal Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3634, https://doi.org/10.5194/egusphere-egu2020-3634, 2020

D3441 |
EGU2020-9258
Silvia Becagli and Rita Traversi and the BioAPRoS Team

The Biogenic Aerosol and Primary Production in the Ross Sea – BioAPRoS project, funded by funded by the Ministry for the Education, University and Scientific Research (MIUR) through the National Antarctic Research Programme (PNRA) aims to improve the understanding of the ocean-atmosphere interactions with particular attention to the interconnections between oceanic primary production and atmospheric gaseous and particulate compounds. These processes have a strong climatic relevance due to the aerosol interaction with solar radiation, its possible interaction with cloud formation and properties, in a region where other aerosol sources are very limited. To achieve the objectives of the project, measurements and sampling in the atmosphere (dimethylsulfide, in the gas phase, and methanesulfonic acid, sugars, amino acids and methoxyphenols in the aerosols) and in sea water (nutrients, chlorophyll, phytoplankton composition and physiological state, DMSP as a precursor of atmospheric DMS) were carried out simultaneously for the first time at the Italian "Mario Zucchelli" Station (MZS; 74.7°S, 164.1°E).

We report here the data obtained in two Antarctic field campaigns carried out in summers of 2018-19 and 2019-20. The DMS atmospheric concentration was measured directly in situ by Gas Chromatography.  It showed concentrations up to 921 pptv (the highest value obtained in both campaigns); the timing of maximum concentration was strongly related to the timing of sea ice melting in the surrounding oceanic areas. Within the project, the low-cost ACHAB (Antartic low-Cost Hydro Arduino Bio-optic profiler) probe has been developed for the acquisition of physical and bio-optical data along the water column, during the 2019-20 campaign. Furthermore, the Phyto-VFP (Phytoplankton Variable Fluorescence Production) bio-optical model was refined to be applied to the Southern Ocean for the estimation of primary production in Terranova Bay and Ross Sea at micro and mesoscale resolutions, respectively. Phyto-VFP was specifically set-up using as input chl a satellite data (merged products based on MODIS-A, MERIS, SeaWIFS, VIIRS-N for low resolution images and Sentinel-2 for high resolution ones) as well as the photosynthetic parameters obtained from a series of laboratory experiments conducted on polar species, enabling to take into account the effect of a nutrient limitation on their photosynthetic performance.

The evolution of concentration of the atmospheric compounds arising from phytoplankton activity was investigated with respect to oceanic parameters (chlorophyll and primary productivity, in turn related to the phytoplankton taxonomic composition and physiological state), to the variations of solar and photosynthetically active radiation, and to the dynamics of sea ice in the Ross Sea.

Understanding and quantifying the correlation between atmospheric compounds and oceanic primary productivity (affecting the oceanic and atmospheric CO2 budget) has a relevant importance in studies on global change because this interaction is influenced by, and in its turn influences, climatic variations.

How to cite: Becagli, S. and Traversi, R. and the BioAPRoS Team: Preliminary results on the correlation between biogenic aerosol and primary production in the Ross Sea – (PNRA-BioAPRoS Project), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9258, https://doi.org/10.5194/egusphere-egu2020-9258, 2020

D3442 |
EGU2020-20267
Roya Ghahreman, Wanmin Gong, Ann-Lise Norman, Stephen R. Beagley, Ayodeji Akingunola, and Paul A. Makar

Atmospheric dimethyl sulfide, DMS, is the main biogenic source of sulfate particles in the Arctic atmosphere. Sulfate particles have a net cooling effect, which can partially offset Arctic warming from absorbing aerosols, such as black carbon. As efficient cloud condensation nuclei (CCN), sulfate particles are also able to influence the cloud’s microphysical properties. 

DMS production and emission to the atmosphere increase during the Arctic summer, due to a greater ice-free sea surface area and higher biological activity. In the model simulation of a field campaign conducted over the Canadian high Arctic during the summer of 2014 (NETCARE; Abbatt et al. 2019), the inclusion of DMS in the model, GEM-MACH, resulted in a significant increase, up to 100%, in the modelled atmospheric SO2 in some regions of the Canadian Arctic. Analysis of the modelled size-segregated aerosol sulfate indicated that DMS has the most significant impact on particles in the size range of 50 – 200 nm in this case. Simulations have shown that localized regions of high seawater DMS can have a significant impact on atmospheric concentrations.

Further investigation of DMS impact on the Arctic summer cloud microphysics was carried out by using a fully coupled version of GEM-MACH. Overall, the model simulations show that the inclusion of DMS in model leads to an increase in cloud droplet number concentrations (CDNC) and a decrease in droplet mean mass diameters (MMD), and has no significant effects on liquid water content (LWC). The impact of DMS on Canadian weather forecasts will be evaluated using operational forecast tools.

How to cite: Ghahreman, R., Gong, W., Norman, A.-L., Beagley, S. R., Akingunola, A., and Makar, P. A.: Role of dimethyl sulfide on the formation and growth of aerosols, and its impact on liquid clouds in the Arctic summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20267, https://doi.org/10.5194/egusphere-egu2020-20267, 2020

D3443 |
EGU2020-9643
Gillian Young, Jutta Vüllers, Peggy Achtert, Paul Field, Jonathan Day, Ewan O'Connor, Ian Brooks, Michael Tjernström, John Prytherch, and Ryan Neely III

State-of-the-art numerical models such as the UK Met Office Unified Model and European Centre for Medium-Range Weather Forecasting Integrated Forecasting System are crucial tools for forecasting future Arctic warming. However, their ability to reproduce clouds and boundary layer meteorology in the high Arctic has not been thoroughly evaluated following significant model developments over the last 10 years. Model evaluation is key to understanding where remaining process weaknesses lie, thus informing further parametrization developments to improve the simulated surface energy budget.

Here, we evaluate model performance with comparison to observations made during the Arctic Ocean 2018 expedition, where a suite of remote-sensing instrumentation was active aboard the Swedish icebreaker Oden measuring summertime Arctic cloud and boundary layer properties. We find that both models do not reproduce cloud fractions well at altitude (up to 8 km) and overestimate the occurrence of low (<1 km) clouds during the sea ice melt period of the expedition. Low cloud agreement with observations improves when the sea ice begins to refreeze; however, the underestimation of cloud aloft remains consistent regardless of sea ice conditions. In this presentation, we will indicate which model processes need to be improved to capture these summertime Arctic clouds more effectively.

How to cite: Young, G., Vüllers, J., Achtert, P., Field, P., Day, J., O'Connor, E., Brooks, I., Tjernström, M., Prytherch, J., and Neely III, R.: Evaluating Arctic meteorology modelled with the Unified Model and Integrated Forecasting System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9643, https://doi.org/10.5194/egusphere-egu2020-9643, 2020

D3444 |
EGU2020-15116
Sara Khosravi, Annette Rinke, Wolfgang Dorn, Christof Lüpkes, Vladimir Gryanik, Dmitry Chechin, Ralf Jaiser, and Dörthe Handorf

Climate models have deficits in reproducing Arctic circulation and sea ice development. The air-sea ice-ocean interaction parametrizations could be a potential reason of this shortcoming. In most climate models air-sea ice-ocean interaction are parametrized based on mid-latitude conditions which is not appropriate for polar region. The POLEX project, funded by Helmholtz Association and Russian Science Foundation, is studying the impact of improved representation of Arctic air-sea ice-ocean interaction on changes in Arctic atmospheric circulation and Arctic-midlatitude linkages. We have used a new suite of parametrizations, which are easily applicable for climate simulations and have been developed based on SHEBA expedition data by Gryanik and Lüpkes (2018). We implemented the new parametrizations in the global atmospheric model (ECHAM6) in the framework of POLEX to estimate its effect on regional Arctic and large-scale circulation changes. Several steps have been defined for implementing the new parameterization to be able to distinguish and understand better the impact of its parameters. Roughness length and stability functions for stable stratification have been modified. Here the initial results of ECHAM6 sensitivity runs for different steps of the parameterization will be presented. We will present first results from process-oriented evaluation over the Arctic sea ice, e.g. how is the impact on the simulation of the two states of the Arctic boundary layer in winter. Furthermore, we will show that the large-scale circulation reacts to the new parametrization in different months and years differently.
Reference:
Gryanik, V.M. and C. Lüpkes (2018) An efficient non-iterative bulk parametrization of surface fluxes for stable atmospheric conditions over polar sea-ice, Boundary-Layer Meteorol., 166, 301-325

How to cite: Khosravi, S., Rinke, A., Dorn, W., Lüpkes, C., Gryanik, V., Chechin, D., Jaiser, R., and Handorf, D.: The role of air-sea ice-ocean interaction processes for Arctic-midlatitude linkages, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15116, https://doi.org/10.5194/egusphere-egu2020-15116, 2020

D3445 |
EGU2020-10670
Alden Adolph, Wesley Brown, Karina Zikan, and Robert Fausto

As Arctic temperatures have increased, the Greenland Ice Sheet has exhibited a negative mass balance, with a substantial and increasing fraction of mass loss due to surface melt. Understanding surface energy exchange processes in Greenland is critical for our ability to predict changes in mass balance. In-situ and remotely sensed surface temperatures are useful for monitoring trends, melt events, and surface energy balance processes, but these observations are complicated by the fact that surface temperatures and near surface air temperatures can significantly differ due to the presence of inversions that exist across the Arctic. Our previous work shows that even in the summer, very near surface inversions are present between the 2m air and surface temperatures a majority of the time at Summit, Greenland. In this study, we expand upon these results and combine a variety of data sources to quantify differences between surface snow/ice temperatures and 2m air temperatures across the Greenland Ice Sheet and investigate controls on the magnitude of these near surface temperature inversions. In-situ temperatures, wind speed, specific humidity, and albedo data are provided from automatic weather stations operated by the Programme for Monitoring of the Greenland Ice Sheet (PROMICE). We use the Clouds and the Earth's Radiant Energy System (CERES) cloud area fraction data to analyze effects of cloud presence on near surface temperature gradients. The in-situ temperatures are compared to Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) and Moderate Resolution Imaging Spectrometer (MODIS) ice surface temperature data to extend findings across the ice sheet. Using PROMICE in-situ data from 2015, we find that these 2m temperature inversions are present 77% of the time, with a median strength of 1.7°C. The data confirm that the presence of clouds weakens inversions. Initial results indicate a RMSE of 3.9°C between MERRA-2 and PROMICE 2m air temperature, and a RMSE of 5.6°C between the two datasets for surface temperature. Improved understanding of controls on near surface inversions is important for use of remotely sensed snow surface temperatures and for modeling of surface mass and energy exchange processes.

How to cite: Adolph, A., Brown, W., Zikan, K., and Fausto, R.: Analyzing Near Surface Temperature Inversions Across the Greenland Ice Sheet Using In-situ, Remote Sensing, and Reanalysis Data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10670, https://doi.org/10.5194/egusphere-egu2020-10670, 2020

D3446 |
EGU2020-15227
Maurice Van Tiggelen, Paul Smeets, Carleen Reijmer, Brice Noël, Jakob Steiner, Emile Nieuwstraten, Walter Immerzeel, and Michiel van den Broeke

Over ice sheets and glaciers, the turbulent heat fluxes are, next to the radiative fluxes, the second largest source of energy driving the ablation. In general, most (climate) models use a bulk turbulence parametrization for the estimation of these energy fluxes. Recent work suggest that the turbulent heat fluxes might be greatly underestimated by such models. Unfortunately, only a few direct and long-term observations of turbulent fluxes are available over ice sheets to evaluate their inclusion in models. 

In this study, we developed a vertical propeller eddy-covariance method to continuously monitor the sensible heat fluxes over the Greenland ice sheet (GrIS). We quantify its contribution to surface ablation using three years of data from the K-transect, located in the western ablation area of the GrIS. The direct flux measurements are also compared to those from several bulk turbulence models, and to a high-resolution regional climate model (RACMO2), in order to quantify modelling uncertainty.

The differences between observations and models highlight the need for upgrading the bulk turbulence parameterizations and especially the model parameters, such as the surface roughness lengths. We also find that during short but extreme warm events, the turbulent heat fluxes become the largest source for surface ablation. Typical for such intense events on the K-transect are fast changes in wind direction, which cause changes in the surface roughness parameters due to the anisotropic feature of the ice hummocks. These parameters are critical for modelling the turbulent fluxes in bulk parameterizations, but are often variable and unknown. We conclude with drone topography measurements to better constrain the surface roughness locally, and discuss methods to improve the modelling of turbulent surface fluxes on the whole GrIS.

How to cite: Van Tiggelen, M., Smeets, P., Reijmer, C., Noël, B., Steiner, J., Nieuwstraten, E., Immerzeel, W., and van den Broeke, M.: Contribution of turbulent heat fluxes to surface ablation on the Greenland ice sheet., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15227, https://doi.org/10.5194/egusphere-egu2020-15227, 2020

D3447 |
EGU2020-2005
William Neff, Christopher Cox, and Mathew Shupe

The ICECAPS field program (Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit) has operated at Summit Station (over 3000 m ASL) since the spring of 2010 with a broad range of instruments to study the role of clouds and precipitation over the Greenland Ice Sheet (GIS).  In addition, a high-resolution minisodar has been operated nearby since 2008 (initially as part of an ice-atmosphere chemical exchange study).  The sodar provides detailed views of the thermodynamic structure of the boundary layer from 2 to 160 m above the surface. Several other collaborating programs support additional boundary-layer measurements such as broadband radiation and turbulent flux measurements. The sodar has proven useful in the interpretation of chemical interactions with the snow surface and underlying firn as well as comparisons of boundary layer depth estimators (Van Dam et al, 2013, 2015).  In addition it has documented the response of the boundary layer to changing cloud forcing (Shupe et al. 2013).  In addition, it has been used to study the wintertime boundary layer with super-cooled fog layers present (Cox et al, 2019).  Additional observations have added to an already rich data set, such as those of stable water vapor isotopes (e.g. Berkelhammer et al. 2016). 

As in the 2012 melt episode that encompassed nearly the entire ice sheet, atmospheric rivers (ARs) bring moisture from the south along the coasts of Greenland and have been increasing (Mattingly et al., 2018; Neff 2018).  We will present a climatology from 2000 to 2012 of ARs some of which are associated with increased transport of moisture from the subtropics at times in concert with hurricanes and tropical storms that follow the same path.  This climatology reveals a distinct low-high pressure pattern spanning from NE Canada to the central Atlantic: the boundary between these systems provides the pathway for moisture to flow from the subtropics.  In this presentation will describe the characteristic cloud/clear skies sequence and accompanying boundary layer structure at Summit Station during these events.  A typical sequence is one of ARs trapped along the west coast and then spreading moisture over the GIS in subsequent days.

To understand the origin of the moisture arriving at Summit Station we also carried out back trajectory analyses that show connections to both ARs and extratropical remnants of hurricanes that follow the same path to Greenland.  Of particular interest will be the boundary layer behavior during the dramatic melt episodes of June and then July 2019 that had their origins in heat waves off of Africa and over Europe. 

 

How to cite: Neff, W., Cox, C., and Shupe, M.: The Boundary Layer and its Response to External Forcing at Summit Station Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2005, https://doi.org/10.5194/egusphere-egu2020-2005, 2020

D3448 |
EGU2020-18102
Daniela Littmann, Wolfgang Dorn, Hélène Bresson, Marion Maturilli, and Markus Rex

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) is the largest one-year-long research expedition within the central Arctic and has started in September 2019 to gather comprehensive climate data from an almost unreachable region. The gathered observational data in combination with concurrent high-resolution modeling provide new insights that play a key role for the improvement of our understanding of the interaction processes between the atmosphere, ocean, and sea ice and eventually global climate change. The present study focuses on the influence of the surface conditions on the atmospheric boundary layer by applying the large eddy simulation model configuration of the icosahedral non-hydrostatic model (ICON-LES). ICON-LES is used here with a grid spacing between 50 m and 800 m and set up to a domain with radii of 10 km to 100 km around the MOSAiC drift track. The model is driven by output data from weather forecast simulations for selected stormy and rather calm days. Results of simulations with various spatial horizontal resolutions and with different surface conditions such as ice fraction, ice thickness, snow cover will be compared and evaluated against observational data from MOSAiC.

How to cite: Littmann, D., Dorn, W., Bresson, H., Maturilli, M., and Rex, M.: Large Eddy Simulations of the Arctic atmospheric boundary layer around the MOSAiC drift track, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18102, https://doi.org/10.5194/egusphere-egu2020-18102, 2020

D3449 |
EGU2020-19187
Vladimir M. Gryanik, Andrey Grachev, Christof Lüpkes, and Dmitry Sidorenko

The calculation of the near-surface turbulent fluxes of energy and momentum in climate and weather prediction models requires transfer coefficients. Currently used parametrizations of these coefficients are based on stability functions derived from measurements over land and not over sea ice. However, recently, a non-iterative parametrization has been proposed by Gryanik and Lüpkes (2018), which can be applied to climate and weather prediction models as well but uses stability functions of Grachev et al. (2007). These functions had been obtained from measurements during the Surface Heat Budget over the Arctic Ocean campaign (SHEBA) and thus from measurements over sea ice. A drawback of the scheme of Gryanik and Lüpkes (2018) is that there is still some complexity due to the complexity of the SHEBA based functions.

Thus new stability functions are proposed for the stable boundary layer, which are also based on the SHEBA measurements but avoid the complexity. It is shown that the new functions are superior to the former ones with respect to the representation of the measured relationship between the Obukhov length and the bulk Richardson number. Moreover, the resulting transfer coefficients agree slightly better with the SHEBA observations in the very stable range. Nevertheless, the functions fulfill the same criteria of applicability as the earlier functions and contain furthermore as an extension a dependence on the neutral Prandtl number. Applying the new functions, an efficient non-iterative parametrization of the near-surface turbulent fluxes of momentum and heat is developed where transfer coefficients result as a function of the bulk Richardson number (Rib) and roughness parameters. The new transfer coefficients, which are recommended for weather and climate models, agree well with the SHEBA data in a large range of stability (0< Rib<0.5) and with those based on the Dyer-Businger functions in the range Rib <0.08.

References

Grachev A.A., Andreas E.L, Fairall C.W., Guest P.S., Persson POG (2007) Boundary-Layer Meteorol., 124, 315–333.

Gryanik, V.M. and Lüpkes C. (2018) An Efficient Non-iterative Bulk Parametrization of Surface Fluxes for Stable Atmospheric Conditions Over Polar Sea-Ice,Boundary-Layer Meteorol 166:301-325

How to cite: Gryanik, V. M., Grachev, A., Lüpkes, C., and Sidorenko, D.: New modified and extended stability functions for the stable boundary layer based on SHEBA data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19187, https://doi.org/10.5194/egusphere-egu2020-19187, 2020

D3450 |
EGU2020-22372
Micha Gryschka, Xu Zhou, and Matthias Sühring

Leads are Chanel-like openings in the sea-ice through which heat of several 100 Watt/m2 is transferred from the ocean into the atmosphere. Even though leads account only for a view percent to the total ice coverage in polar regions, they modify the polar boundary layer significantly. Therefore, leads need to be considered in numerical weather and climate models. Since, generally leads are not explicitly resolved in these models it is important to understand the overall effect of leads of different sizes onto the boundary layer for different meteorological conditions.

With numerous Large-Eddy Simulations we investigated the dependency of the lead averaged surface heat flux on the lead width in a range between 50 m and 25 000 m for different meteorological conditions. Generally, we found under same temperature differences between ice and water and same meteorological conditions an increase of the the lead averaged heat flux with increasing lead width by more then 200% for some situations. We like to give some brief explanations of the possible causes for this behavior as well as to oppose these results to other former studies in this field, which might disagree to them in some points.

How to cite: Gryschka, M., Zhou, X., and Sühring, M.: Dependency of the turbulent heat exchange over polar leads on the lead width – an LES study , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22372, https://doi.org/10.5194/egusphere-egu2020-22372, 2020

D3451 |
EGU2020-7849
Eleftherios Ioannidis, Kathy Law, Jean-Christophe Raut, Tatsuo Onishi, William R. Simpson, Rachel M. Kirpes, and Karri A. Pratt

The Arctic is influenced by long-range transport of aerosols, for example, sulphate, black carbon, and dust from mid-latitude emissions, especially in winter and spring, leading to the formation of Arctic Haze with enhanced aerosol concentrations. However, more recently, local sources of aerosols, such as wood-burning or resource extraction, are highlighted as already being important, but many uncertainties about sources and aerosol processes still remain. For example, the formation of secondary aerosols, such as sulphate, in winter despite very low temperatures and the absence of sunlight.

In this study, which contributes to the international PACES-ALPACA initiative, the Weather Research Forecasting (WRF) and WRF-Chem models are used to investigate wintertime pollution over Alaska with a focus on regions influenced by local pollution, such as Fairbanks and by Arctic Haze, such as Utqiagvik (formerly known as Barrow). Fairbanks is the most polluted city in the United States during wintertime due to high emissions and the occurrence of strong surface temperature inversions.

As a first step, background aerosols originating from remote sources were evaluated in large- scale quasi-hemispheric WRF-Chem runs using ECLIPSE anthropogenic emissions. The model performs quite well over Alaska at background sites (e.g. Denali Park) compared to observations from the US Environmental Protection Agency (EPA). Discrepancies in modelled aerosols due to formation mechanisms and aerosol acidity are being investigated.

Secondly, in order to better simulate Arctic aerosols and local pollution episodes, different schemes in WRF were tested over Alaska with a particular focus on improving simulations of the Arctic boundary layer structure and, in particular, wintertime temperature inversions which trap pollution at the ground. In order to simulate these extreme/cold meteorological conditions, different schemes linked to boundary layer physics, surface layer dynamics and the land surface have been tested and evaluated against Integrated Global Radiosonde Archive (IGRA2) and Integrated Surface Database (ISD). The model captures the cold meteorological conditions over Alaska, for example, capturing strong temperature inversions over Utqiagvik and Fairbanks in winter 2012.

Thirdly, WRF-Chem is used to simulate background and local Arctic air pollution, using the improved WRF setup for meteorology over Alaska for winter 2013-2014. The model is being run with Hemispheric Transport of Air Pollution version 2 (HTAP v2) and other high-resolution emission inventories and evaluated against available aerosol data (PM2.5, black carbon, sulphate) over Alaska including data on aerosol chemical properties. The model is used to examine aerosol composition in locally produced and remote aerosols and to identify the origins contributing to aerosol distributions. The sensitivity of modelled aerosols to, for example, meteorological factors, such as humidity, is examined.

How to cite: Ioannidis, E., Law, K., Raut, J.-C., Onishi, T., Simpson, W. R., Kirpes, R. M., and Pratt, K. A.: Wintertime Arctic Air Pollution over Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7849, https://doi.org/10.5194/egusphere-egu2020-7849, 2020

D3452 |
EGU2020-7987
Xuhua Cheng

 Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over

the southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter,

located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55oE-78oE, 

and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer

accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing,

thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere

by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode

(SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while

has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the

reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere.

How to cite: Cheng, X.: The impact of SST front on the surface wind in the southern Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7987, https://doi.org/10.5194/egusphere-egu2020-7987, 2020