scholarly journals Enhanced tropospheric BrO over Antarctic sea ice in mid winter observed by MAX-DOAS on board the research vessel Polarstern

2007 ◽  
Vol 7 (12) ◽  
pp. 3129-3142 ◽  
Author(s):  
T. Wagner ◽  
O. Ibrahim ◽  
R. Sinreich ◽  
U. Frieß ◽  
R. von Glasow ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Vertical Mixing ◽  
Satellite Observations ◽  
Research Vessel ◽  
Optical Absorption Spectroscopy ◽  
Back Trajectory ◽  
First Year ◽  
Polar Regions ◽  
Near Surface

Abstract. We present Multi AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of tropospheric BrO carried out on board the German research vessel Polarstern during the Antarctic winter 2006. Polarstern entered the area of first year sea ice around Antarctica on 24 June 2006 and stayed within this area until 15 August 2006. For the period when the ship cruised inside the first year sea ice belt, enhanced BrO concentrations were almost continuously observed. Outside the first year sea ice belt, typically low BrO concentrations were found. Based on back trajectory calculations we find a positive correlation between the observed BrO differential slant column densities (ΔSCDs) and the duration for which the air masses had been in contact with the sea ice surface prior to the measurement. While we can not completely rule out that in several cases the highest BrO concentrations might be located close to the ground, our observations indicate that the maximum BrO concentrations might typically exist in a (possibly extended) layer around the upper edge of the boundary layer. Besides the effect of a decreasing pH of sea salt aerosol with altitude and therefore an increase of BrO with height, this finding might be also related to vertical mixing of air from the free troposphere with the boundary layer, probably caused by convection over the warm ocean surface at polynyas and cracks in the ice. Strong vertical gradients of BrO and O3 could also explain why we found enhanced BrO levels almost continuously for the observations within the sea ice. Based on our estimated BrO profiles we derive BrO mixing ratios of several ten ppt, which is slightly higher than many existing observations. Our observations indicate that enhanced BrO concentrations around Antarctica exist about one month earlier than observed by satellite instruments. From detailed radiative transfer simulations we find that MAX-DOAS observations are up to about one order of magnitude more sensitive to near-surface BrO than satellite observations. In contrast to satellite observations the MAX-DOAS sensitivity hardly decreases for large solar zenith angles and is almost independent from the ground albedo. Thus this technique is very well suited for observations in polar regions close to the solar terminator. For large periods of our measurements the solar elevation was very low or even below the horizon. For such conditions, most reactive Br-compounds might exist as Br2 molecules and ozone destruction and the removal of reactive bromine compounds might be substantially reduced.

scholarly journals Enhanced tropospheric BrO concentrations over the Antarctic sea ice belt in mid winter observed from MAX-DOAS observations on board the research vessel Polarstern

2007 ◽  
Vol 7 (1) ◽  
pp. 1823-1847
Author(s):  
T. Wagner ◽  
O. Ibrahim ◽  
R. Sinreich ◽  
U. Frieß ◽  
U. Platt
Keyword(s):  
Sea Ice ◽  
Elevation Angle ◽  
Lower Troposphere ◽  
Satellite Observations ◽  
Research Vessel ◽  
Optical Absorption Spectroscopy ◽  
First Year ◽  
Polar Regions ◽  
Near Surface ◽  
The Antarctic

Abstract. We present Multi AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of tropospheric BrO carried out on board the German research vessel Polarstern during the Antarctic winter 2006. Polarstern entered the area of first year sea ice around Antarctica on 24 June 2006 and stayed within this area until 15 August 2006. For the period when the ship cruised inside the first year sea ice belt, enhanced BrO concentrations were almost continuously observed. One interesting exception appeared on 7 July 2006, when the sun elevation angle was < about –2.8° indicating that for low insulation the photolysis of Br2 and/or HOBr is too slow to provide sufficient amounts of Br radicals. Before and after the period inside the first year sea ice belt, typically low BrO concentrations were observed. Our observations indicate that enhanced BrO concentrations around Antarctica exist about one month earlier than observed by satellite instruments. The small BrO concentrations over the open oceans indicate a short atmospheric lifetime of activated bromine without contact to areas of first year sea ice. From detailed radiative transfer simulations we find that MAX-DOAS observations are about one order of magnitude more sensitive to near-surface BrO than satellite observations. In contrast to satellite observations the MAX-DOAS sensitivity hardly decreases for large solar zenith angles and is almost independent from the ground albedo. Thus this technique is very well suited for observations in polar regions close to the solar terminator. Furthermore, combination of both techniques could yield additional information on the vertical distribution of BrO in the lower troposphere.

scholarly journals The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Elem Sci Anth ◽  
2016 ◽  
Vol 4 ◽  
Author(s):  
Peter K. Peterson ◽  
Kerri A. Pratt ◽  
William R. Simpson ◽  
Son V. Nghiem ◽  
Lemuel X. Pérez Pérez ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Ozone Depletion ◽  
Surface Ozone ◽  
Optical Absorption Spectroscopy ◽  
Atmospheric Ozone ◽  
Vertical Profiles ◽  
Large Spatial Scale ◽  
Near Surface ◽  
Air Masses

Abstract Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O3&lt; 10 nmol mol-1) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions.

Evaluation of regional NWP results for sea ice covered Bothnia Bay (Baltic Sea) in winter 2020.

2021 ◽  
Author(s):  
Marta Wenta ◽  
Agnieszka Herman
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Sea Ice ◽  
Surface Temperature ◽  
Atmospheric Boundary Layer ◽  
Baltic Sea ◽  
Polar Regions ◽  
Near Surface ◽  
Accurate Representation ◽  
Complex Processes

&lt;p&gt;The ongoing development of NWP (Numerical Weather Prediction) models and their increasing horizontal resolution have significantly improved forecasting capabilities. However, in the polar regions models struggle with the representation of near-surface atmospheric properties and the vertical structure of the atmospheric boundary layer (ABL) over sea ice. Particularly difficult to resolve are near-surface temperature, wind speed, and humidity, along with diurnal changes of those properties. Many of the complex processes happening at the interface of sea ice and atmosphere, i.e. vertical fluxes, turbulence, atmosphere - surface coupling are poorly parameterized or not represented in the models at all. Limited data coverage and our poor understanding of the complex processes taking place in the polar ABL limit the development of suitable parametrizations. We try to contribute to the ongoing effort to improve the forecast skill in polar regions through the analysis of unmanned aerial vehicles (UAVs) and automatic weather station (AWS) atmospheric measurements from the coastal area of Bothnia Bay (Wenta et. al., 2021), and the application of those datasets for the analysis of regional NWP models' forecasts.&amp;#160;&lt;/p&gt;&lt;p&gt;Data collected during HAOS (Hailuoto Atmospheric Observations over Sea ice) campaign (Wenta et. al., 2021) is used for the evaluation of regional NWP models results from AROME (Applications of Research to Operations at Mesoscale) - Arctic, HIRLAM (High Resolution Limited Area Model) and WRF (Weather Research and Forecasting). The presented analysis focuses on 27 Feb. 2020 - 2 Mar. 2020, the time of the HAOS campaign, shortly after the formation of new, thin sea ice off the westernmost point of Hailuoto island.&amp;#160; Throughout the studied period weather conditions changed from very cold (-14&amp;#8451;), dry and cloud-free to warmer (~ -5&amp;#8451;), more humid and opaquely cloudy. We evaluate models&amp;#8217; ability to correctly resolve near-surface temperature, humidity, and wind speed, along with vertical changes of temperature and humidity over the sea ice. It is found that generally, models struggle with an accurate representation of surface-based temperature inversions, vertical variations of humidity, and temporal wind speed changes. Furthermore, a WRF Single Columng Model (SCM) is launched to study whether specific WRF planetary boundary layer parameterizations (MYJ, YSU, MYNN, QNSE), vertical resolution, and more accurate representation of surface conditions increase the WRF model&amp;#8217;s ability to resolve the ABL above sea ice in the Bay of Bothnia. Experiments with WRF SCM are also used to determine the possible reasons behind model&amp;#8217;s biases. Preliminary results show that accurate representation of sea ice conditions, including thickness, surface temperature, albedo, and snow coverage is crucial for increasing the quality of NWP models forecasts. We emphasize the importance of further development of parametrizations focusing on the processes at the sea ice-atmosphere interface.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Reference:&lt;/p&gt;&lt;p&gt;Wenta, M., Brus, D., Doulgeris, K., Vakkari, V., and Herman, A.: Winter atmospheric boundary layer observations over sea ice in the coastal zone of the Bay of Bothnia (Baltic Sea), Earth Syst. Sci. Data, 13, 33&amp;#8211;42, https://doi.org/10.5194/essd-13-33-2021, 2021.&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;br&gt;&lt;br&gt;&lt;br&gt;&lt;br&gt;&lt;br&gt;&lt;br&gt;&lt;/p&gt;

Measurements of bromine monoxide over four halogen activation seasons in the Canadian high Arctic

2020 ◽  
Author(s):  
Kristof Bognar ◽  
Xiaoyi Zhao ◽  
Kimberly Strong ◽  
Rachel Y.-W. Chang ◽  
Udo Frieß ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Vertical Mixing ◽  
Temperature Inversion ◽  
The Arctic ◽  
Optical Absorption Spectroscopy ◽  
Blowing Snow ◽  
Atmospheric Conditions ◽  
Coarse Mode ◽  
Arctic Haze

&lt;p&gt;&lt;span&gt;Bromine explosions and corresponding ozone depletion events (ODEs) are common in the Arctic spring. The snowpack on sea ice and sea salt aerosols (SSA) are both thought to release bromine, but the relative contribution of each source is not yet known. Furthermore, the role of atmospheric conditions is not fully understood. Long-term measurements of bromine monoxide (BrO) provide useful insight into the underlying processes of bromine activation. Here we present a four-year dataset (2016-2019) of springtime BrO partial columns retrieved from Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Eureka, Canada (80.1&amp;#176; N, 86.4&amp;#176; W). Due to the altitude of the measurement site (610 m), the measurements often represent BrO above the shallow boundary layer, and the strength of the temperature inversion has limited impact on the BrO partial columns. When the boundary layer is deep, however, the effects of the enhanced vertical mixing manifest as an increase in the minimum BrO values (and reduced ODE frequency) for wind speeds of ~8 m/s or greater. We find that BrO events show two modes differentiated by local wind direction and air mass history. Longer time spent in first-year sea ice areas corresponds to increased BrO for one of these modes only. We argue that snow on multi-year ice (salted and acidified by Arctic haze) might also contribute to bromine release. The MAX-DOAS measurements show that high aerosol optical depth is required to maintain lofted BrO. In situ measurements indicate that accumulation mode aerosols (mostly Arctic haze) have no direct correlation with BrO. The presence of coarse mode aerosols, however, is a necessary and sufficient condition for observing enhanced BrO at Eureka. The measurements of coarse mode aerosols are consistent with SSA generated from blowing snow. The good correlation between BrO and coarse mode aerosols (R&lt;sup&gt;2&lt;/sup&gt; up to 0.57) supports the view that SSA is a direct source of bromine to the polar troposphere.&lt;/span&gt;&lt;/p&gt;

scholarly journals Observations and the source investigations of boundary layer BrO in Ny-Ålesund Arctic

2017 ◽  
Author(s):  
Yuhan Luo ◽  
Fuqi Si ◽  
Haijin Zhou ◽  
Ke Dou ◽  
Yi Liu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Loss Rate ◽  
Elemental Mercury ◽  
Optical Absorption Spectroscopy ◽  
Polar Regions ◽  
Gaseous Elemental Mercury ◽  
Bay Area ◽  
Boundary Layer Ozone ◽  
Kings Bay

Abstract. Bromine monoxide is a reactive halogen species which has crucial impact on the chemistry of the tropospheric polar boundary layer. During polar spring, BrO enhancement can be detected in both northern and southern Polar Regions, while the boundary layer ozone depletion events occur. A considerable challenge for understanding enhanced BrO and the associated ODEs is the difficulty of real-time observations. In this study, a typical process of enhanced bromine and depleted ozone in late April, 2015 at Ny-Ålesund boundary layer was observed using ground-based Multi Axis-Differential Optical Absorption Spectroscopy (MAX-DOAS) technique. The results showed that there were as high as 8 × 1014 molecular cm−2 BrO slant columns above the Kings Bay area in 26 April. Considering meteorology, sea ice distribution and air mass history, the floating sea ice in the Kings Bay area was considered as the major source of this bromine enhancement event. During this period, the boundary layer ozone and gaseous elemental mercury (GEM) was synchronously reduced by 85 % and 90 % separately. The kinetic calculation showed that the ozone loss rate is 10.3 ppbv h−1, which is extremely high compared to other area. The GEM loss rate is about 0.25 ng m−3 h−1. The oxidized GEM may directly deposit to snow/ice and thereby influence the polar ecosystem.

scholarly journals MAX-DOAS observations of the total atmospheric water vapour column and comparison with independent observations

2013 ◽  
Vol 6 (1) ◽  
pp. 131-149 ◽  
Author(s):  
T. Wagner ◽  
M. O. Andreae ◽  
S. Beirle ◽  
S. Dörner ◽  
K. Mies ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Water Vapour ◽  
Satellite Observations ◽  
Shielding Effect ◽  
Optical Absorption Spectroscopy ◽  
Atmospheric Water ◽  
Near Surface ◽  
Atmospheric Water Vapour ◽  
Independent Observations ◽  
Good Agreement

Abstract. We developed an algorithm for the retrieval of the atmospheric water vapour column from Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations in the yellow and red spectral range. The retrieval is based on the so-called geometric approximation and does not depend on explicit a priori information for individual observations, extensive radiative transfer simulations, or the construction of large look-up tables. Disturbances of the radiative transfer due to aerosols and clouds are simply corrected using the simultaneously measured absorptions of the oxygen dimer, O4. We applied our algorithm to MAX-DOAS observations made at the Max Planck Institute for Chemistry in Mainz, Germany, from March to August 2011, and compared the results to independent observations. Good agreement with Aerosol Robotic Network (AERONET) and European Centre for Medium-Range Weather Forecasting (ECMWF) H2O vertical column densities (VCDs) is found, while the agreement with satellite observations is less good, most probably caused by the shielding effect of clouds for the satellite observations. Good agreement is also found with near-surface in situ observations, and it was possible to derive average daily H2O scale heights (between 1.5 km and 3 km). MAX-DOAS measurements use cheap and simple instrumentation and can be run automatically. One important advantage of our algorithm is that the H2O VCD can be retrieved even under cloudy conditions (except clouds with very high optical thickness).

scholarly journals Observations and source investigations of the boundary layer bromine monoxide (BrO) in the Ny-Ålesund Arctic

2018 ◽  
Vol 18 (13) ◽  
pp. 9789-9801 ◽  
Author(s):  
Yuhan Luo ◽  
Fuqi Si ◽  
Haijin Zhou ◽  
Ke Dou ◽  
Yi Liu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Elemental Mercury ◽  
Arctic Sea Ice ◽  
Optical Absorption Spectroscopy ◽  
Mercury Deposition ◽  
Gaseous Elemental Mercury ◽  
Bay Area ◽  
Short Period ◽  
Kings Bay

Abstract. During polar spring, the presence of reactive bromine in the polar boundary layer is considered to be the main cause of ozone depletion and mercury deposition. However, many uncertainties still remain regarding understanding the mechanisms of the chemical process and source of the bromine. As Arctic sea ice has recently been dramatically reduced, it is critical to investigate the mechanisms using more accurate measurements with higher temporal and spatial resolution. In this study, a typical process of enhanced bromine and depleted ozone in the Ny-Ålesund boundary layer in late April 2015 was observed by applying ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) technique. The results showed that there were bromine monoxide (BrO) slant columns as high as 5.6 × 1014 molec cm−2 above the Kings Bay area on 26 April. Meanwhile, the boundary layer ozone and gaseous elemental mercury (GEM) were synchronously reduced by 85 and 90 %, respectively. Based on the meteorology, sea ice distribution and air mass history, the sea ice in the Kings Bay area, which emerged for only a very short period of time when the enhanced BrO was observed, was considered to be the major source of this bromine enhancement event. The oxidized GEM may be directly deposited onto snow/ice and thereby influence the polar ecosystem.

scholarly journals The mechanical properties of saline ice under uniaxial compression

1994 ◽  
Vol 19 ◽  
pp. 39-48 ◽  
Author(s):  
Gary A. Kuehn ◽  
Erland M. Schulson
Keyword(s):  
Mechanical Properties ◽  
Sea Ice ◽  
Strain Rate ◽  
Uniaxial Compression ◽  
Growth Direction ◽  
First Year ◽  
Polar Regions ◽  
Deformation And Fracture ◽  
Specimen Orientation ◽  
Ductile To Brittle Transition

Understanding the mechanical properties of saline ice is important for engineering design as well as for operations in polar regions. In order to gain understanding of the basic mechanisms of deformation and fracture, laboratory-grown columnar saline ice, representative of first-year sea ice, was tested in uniaxial compression under a variety of conditions of Strain rate (10−7to 10−1s−1), temper-aiure (−40°, −20°, −10° and −5°C) and orientation (loading vertically or horizontally: i.e. parallel or perpendicular to the growth direction). The range of strain rate spanned the ductile-to-brittle transition for every combination of temperature and specimen orientation. The results of over 250 tests are reported. Mechanical properties, failure mode and ice structure are analyzed with respect to the testing conditons. The results show that strength is dependent upon the ice structure, orientation, strain rate and temperature. During loading in the ductile regime the structure is altered (e.g. by recrystallization), whereas in the brittle regime the majority of the structural change is through cracking. The results are compared to results from the literature on both natural sea ice and laboratory-grown saline ice. Where possible, they are interpreted in terms of micromechanica] models.

scholarly journals NO<sub>x</sub> emissions, isoprene oxidation pathways, vertical mixing, and implications for surface ozone in the Southeast United States

2016 ◽  
Author(s):  
Katherine R. Travis ◽  
Daniel J. Jacob ◽  
Jenny A. Fisher ◽  
Patrick S. Kim ◽  
Eloise A. Marais ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Surface Ozone ◽  
Vertical Mixing ◽  
Satellite Observations ◽  
Oxidation Products ◽  
Large Contribution ◽  
Nox Emissions ◽  
Tropospheric No2 ◽  
Isoprene Oxidation ◽  
Southeast Us

Abstract. Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx ≡ NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model (CTM) at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high in the Southeast and nationally by 50 %. This is demonstrated by SEAC4RS observations of NOx and its oxidation products, by surface network observations of nitrate wet deposition fluxes, and by OMI satellite observations of tropospheric NO2 columns. Upper tropospheric NO2 from lightning makes a large contribution to the satellite observations that must be accounted for when using these data to estimate surface NOx emissions. Aircraft observations of upper tropospheric NO2 are higher than simulated by GEOS-Chem or expected from NO-NO2-O3 photochemical stationary state. NOx levels in the Southeast US are sufficiently low that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft and from ozonesondes, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 8 ± 13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to 0.2 km altitude, whereas GEOS-Chem has no such gradient because of efficient boundary layer mixing. We conclude that model biases in simulating surface ozone over the Southeast US may be due to a combination of excessive NOx emissions and excessive boundary layer vertical mixing.

scholarly journals Parameterizing radiative transfer to convert MAX-DOAS dSCDs into near-surface box averaged mixing ratios and vertical profiles

2012 ◽  
Vol 5 (5) ◽  
pp. 7641-7673 ◽  
Author(s):  
R. Sinreich ◽  
A. Merten ◽  
L. Molina ◽  
R. Volkamer
Keyword(s):  
Boundary Layer ◽  
Radiative Transfer ◽  
Mexico City ◽  
Vertical Gradient ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Transfer Model ◽  
Model Calculations ◽  
Near Surface ◽  
Mixing Ratios

Abstract. We present a novel parameterization method to convert Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) differential Slant Column Densities (dSCDs) into near-surface box averaged volume mixing ratios. The approach is applicable inside the planetary boundary layer under conditions with significant aerosol load, does not require a-priori assumptions about the trace gas vertical distribution and builds on the increased sensitivity of MAX-DOAS near the instrument altitude. It parameterizes radiative transfer model calculations and significantly reduces the computational effort. The biggest benefit of this method is that the retrieval of an aerosol profile, which usually is necessary for deriving a trace gas concentration from MAX-DOAS dSCDs, is not needed. The method is applied to NO2 MAX-DOAS dSCDs recorded during the Mexico City Metropolitan Area 2006 (MCMA-2006) measurement campaign. The retrieved volume mixing ratios of two elevation angles (1° and 3°) are compared to volume mixing ratios measured by two long-path (LP)-DOAS instruments located at the same site. Measurements are found to agree well during times when vertical mixing is expected to be strong. However, inhomogeneities in the air mass above Mexico City can be detected by exploiting the different horizontal and vertical dimensions probed by MAX-DOAS measurements at different elevation angles, and by LP-DOAS. In particular, a vertical gradient in NO2 close to the ground can be observed in the afternoon, and is attributed to reduced mixing coupled with near surface emission. The existence of a vertical gradient in the lower 250 m during parts of the day shows the general challenge of sampling the boundary layer in a representative way and emphasizes the need of vertically resolved measurements.

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