Neutral Drag Coefficient Research Articles

Uncertainties of Drag Coefficient Estimates Above Sea Ice from Field Data

Surface turbulent exchanges play a key role on sea ice dynamics, on ocean and sea ice heat budgets and on the polar atmosphere. Uncertainties in parameterizations of surface turbulent fluxes are mostly held by the transfer coefficients and estimates of those transfer coefficients from field data are required for parameterization development. Measurement errors propagate through the computation of transfer coefficients and contribute to its total error together with the uncertainties in the empirical stability functions used to correct for stability effects. Here we propose a methodology to assess their contributions individually to each coefficient estimate as well as the total drag coefficient uncertainty and we apply this methodology on the example of the SHEBA campaign. We conclude that for most common drag coefficient values (between 1.0×10-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1.0\ imes 10^{-3}$$\\end{document} and 2.5×10-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.5\ imes 10^{-3}$$\\end{document}), the relative total uncertainty ranges from 25 and 50%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document}. For stable or unstable conditions with a stability parameter |ζ|>1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|\\zeta |>1$$\\end{document} on average, the total uncertainty in the neutral drag coefficient exceeds the neutral drag coefficient value itself, while for |ζ|<1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|\\zeta |<1$$\\end{document} the total uncertainty is around 25%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document} of the drag coefficient. For closer-to-neutral conditions, this uncertainty is dominated by measurement uncertainties in surface turbulent momentum fluxes which should therefore be the target of efforts in uncertainty reduction. We also propose an objective data-screening procedure for field data, which consists of retaining data for which the relative error on neutral drag coefficient does not exceed a given threshold. This method, in addition to the commonly used flux quality control procedure, allows for a reduction of the drag coefficient dispersion compared to other data-screening methods, which we take as an indication of better dataset quality.

New estimates of pan-Arctic sea ice–atmosphere neutral drag coefficients from ICESat-2 elevation data

Abstract. The effect that sea ice topography has on the momentum transfer between ice and atmosphere is not fully quantified due to the vast extent of the Arctic and limitations of current measurement techniques. Here we present a method to estimate pan-Arctic momentum transfer via a parameterization that links sea ice–atmosphere form drag coefficients with surface feature height and spacing. We measure these sea ice surface feature parameters using the Ice, Cloud and land Elevation Satellite-2 (ICESat-2). Though ICESat-2 is unable to resolve as well as airborne surveys, it has a higher along-track spatial resolution than other contemporary altimeter satellites. As some narrow obstacles are effectively smoothed out by the ICESat-2 ATL07 spatial resolution, we use near-coincident high-resolution Airborne Topographic Mapper (ATM) elevation data from NASA's Operation IceBridge (OIB) mission to scale up the regional ICESat-2 drag estimates. By also incorporating drag due to open water, floe edges and sea ice skin drag, we produced a time series of average total pan-Arctic neutral atmospheric drag coefficient estimates from November 2018 to May 2022. Here we have observed its temporal evolution to be unique and not directly tied to sea ice extent. By also mapping 3-month aggregates for the years 2019, 2020 and 2021 for better regional analysis, we found the thick multiyear ice area directly north of the Canadian Archipelago and Greenland to be consistently above 2.0×10-3, while most of the multiyear ice portion of the Arctic is typically around ∼1.5×10-3.

Effects of sea ice form drag on the polar oceans in the NEMO-LIM3 global ocean–sea ice model

The surface roughness of sea ice is highly variable because of the diversity of discrete obstacles to the flow present on the sea ice surface. These obstacles result in form drag, an effect poorly accounted for in the calculation of surface fluxes over sea ice in climate models. In this study, we implement in the ocean–sea ice model NEMO-LIM3 a variable formulation of the atmospheric and oceanic neutral drag coefficients over sea ice that includes the form drag effect. We examine the impact of this formulation on the ice cover and ocean surface properties over the past decades in the Arctic and Antarctic. Including form drag in the simulations leads to 10–50cm thinner sea ice in the peripheral Arctic seas and ∼10cm thicker sea ice in the Antarctic ice pack on annual means. Form drag significantly impacts summer sea surface temperatures in the marginal ice zone (+0.5 to +1.5°C in the Arctic and -0.5°C in the Antarctic) and sea surface salinities under the ice cover (+0.52 to +0.89g/kg in both polar regions). The seasonality of the ocean surface stress under sea ice is reversed, with a maximum (minimum) ocean surface stress under sea ice in summer (winter) when including form drag. However, this seasonal reversal is likely too intense in our simulations due to the underestimation of the prescribed mean floe lengths in summer. In the Arctic, the long-term declines in sea ice volume and multi-year ice are associated with decreasing neutral drag coefficients and negative trends in ocean surface stress. In the Antarctic, the trends in the neutral drag coefficients control the increase (decrease) in ocean surface stress in the Weddell and Ross (Bellingshausen and Amundsen) Seas. Form drag results in a 22–38% deeper summer mixed layer under polar sea ice and a year-long 10–25m deeper mixed layer in the Central Arctic. All these impacts on the ocean surface properties demonstrate the relevance of form drag for polar ocean modelling.

The Impact of Atmosphere\u2013Ocean\u2013Wave Coupling on the Near-Surface Wind Speed in Forecasts of Extratropical Cyclones

Accurate modelling of air–sea surface exchanges is crucial for reliable extreme surface wind-speed forecasts. While atmosphere-only weather forecast models represent ocean and wave effects through sea-state independent parametrizations, coupled multi-model systems capture sea-state dynamics by integrating feedbacks between the atmosphere, ocean and wave model components. Here, we investigate the sensitivity of extreme surface wind speeds to air–sea exchanges at the kilometre scale using coupled and uncoupled configurations of the Met Office’s UK Regional Coupled Environmental Prediction system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 m s^{-1} over the UK. Compared with the atmosphere-only results, coupling to the ocean decreases the domain-average sea-surface temperature by up to 0.5 K. Inclusion of coupling to waves reduce the 98th percentile 10-m wind speed by up to 2 m s^{-1} as young, growing wind waves reduce the wind speed by increasing the sea-surface aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1 m s^{-1}, due to enhanced boundary-layer turbulence which partially offsets air–sea momentum transfer. Using a new drag parametrization based on the Coupled Ocean–Atmosphere Response Experiment 4.0 parametrization, with a cap on the neutral drag coefficient and reduction for wind speeds exceeding 27 m s^{-1}, the atmosphere-only model achieves equivalent impacts on 10-m wind speeds and gusts as from coupling to waves. Overall, the new drag parametrization achieves the same 20% improvement in forecast 10-m wind-speed skill as coupling to waves, with the advantage of saving the computational cost of the ocean and wave models.

Sensitivity of Ice Drift to Form Drag and Ice Strength Parameterization in a Coupled Ice–Ocean Model

ABSTRACTA pan-Arctic sea-ice–ocean prediction system is assessed in terms of its ability to predict sea-ice velocity. This system is based on the Regional Ice Ocean Prediction System running operationally at the Canadian Centre for Meteorological and Environmental Prediction. A form drag parameterization is implemented in the system to allow spatially and temporally varying neutral drag coefficients depending on the sea-ice morphological characteristics. Simulated ice velocity is assessed using data from the International Arctic Buoy Programme, as well as ice motion derived from Environment and Climate Change Canada’s synthetic aperture radar automated ice-tracking system. Results indicate that introducing the form drag parameterization systematically increases the sea-ice velocity and exacerbates a positive bias in summer already present in the previous version in which constant neutral drag coefficients were used. The ice strength parameterization used in the model rheology is found to affect the simulated ice drift significantly. Introducing modifications to the ice strength formulation and the empirical parameters helped alleviate the ice velocity bias.

Stability and Sea State as Limiting Conditions for TKE Dissipation and Dissipative Heating

Abstract This study analyzes high-resolution ship data collected in the Gulf of Mexico during the Lagrangian Submesoscale Experiment (LASER) from January to February 2016 to produce the first reported measurements of dissipative heating in the explicitly nonhurricane atmospheric surface layer. Although typically computed from theory as a function of wind speed cubed, the dissipative heating directly estimated via the turbulent kinetic energy (TKE) dissipation rate is also presented. The dissipative heating magnitude agreed with a previous study that estimated the dissipative heating in the hurricane boundary layer using in situ aircraft data. Our observations that the 10-m neutral drag coefficient parameterized using TKE dissipation rate approaches zero slope as wind increases suggests that TKE dissipation and dissipative heating are constrained to a physical limit. Both surface-layer stability and sea state were observed to be important conditions influencing dissipative heating, with the stability determined via TKE budget terms and the sea state determined via wave steepness and age using direct shipboard measurements. Momentum and enthalpy fluxes used in the TKE budget are determined using the eddy-correlation method. It is found that the TKE dissipation rate and the dissipative heating are largest in a nonneutral atmospheric surface layer with a sea surface comprising steep wind sea and slow swell waves at a given surface wind speed, whereas the ratio of dissipative heating to enthalpy fluxes is largest in near-neutral stability where the turbulent vertical velocities are near zero.

Atmospheric form drag coefficients over Arctic sea ice using remotely sensed ice topography data, spring 2009–2015

Sea ice topography significantly impacts turbulent energy/momentum exchange, e.g., atmospheric (wind) drag, over Arctic sea ice. Unfortunately, observational estimates of this contribution to atmospheric drag variability are spatially and temporally limited. Here we present new estimates of the neutral atmospheric form drag coefficient over Arctic sea ice in early spring, using high‐resolution Airborne Topographic Mapper elevation data from NASA's Operation IceBridge mission. We utilize a new three‐dimensional ice topography data set and combine this with an existing parameterization scheme linking surface feature height and spacing to form drag. To be consistent with previous studies investigating form drag, we compare these results with those produced using a new linear profiling topography data set. The form drag coefficient from surface feature variability shows lower values (&lt;0.5–1 ×10−3) in the Beaufort/Chukchi Seas, compared with higher values (&gt;0.5–1 ×10−3) in the more deformed ice regimes of the Central Arctic (north of Greenland and the Canadian Archipelago), which increase with coastline proximity. The results show moderate interannual variability, including a strong increase in the form drag coefficient from 2013 to 2014/2015 north of the Canadian Archipelago. The form drag coefficient estimates are extrapolated across the Arctic with Advanced Scatterometer satellite radar backscatter data, further highlighting the regional/interannual drag coefficient variability. Finally, we combine the results with existing parameterizations of form drag from floe edges (a function of ice concentration) and skin drag to produce, to our knowledge, the first pan‐Arctic estimates of the total neutral atmospheric drag coefficient (in early spring) from 2009 to 2015.

Coupling hydrodynamic and wave models: first step and sensitivity experiments in the Mediterranean Sea

This work describes the first step towards a fully coupled modelling system composed of an ocean circulation and a wind wave model. Sensitivity experiments are presented for the Mediterranean Sea where the hydrodynamic model NEMO is coupled with the third-generation wave model WaveWatchIII (WW3). Both models are implemented at 1/16° horizontal resolution and are forced by ECMWF 1/4° horizontal resolution atmospheric fields. The models are two-way coupled at hourly intervals exchanging the following fields: sea surface currents and temperature are transferred from NEMO to WW3 by modifying the mean momentum transfer of waves and the wind speed stability parameter, respectively. The neutral drag coefficient computed by WW3 is then passed to NEMO, which computes the surface stress. Five-year (2009–2013) numerical experiments were carried out in both uncoupled and coupled mode. In order to validate the modelling system, numerical results were compared with coastal and drifting buoys and remote sensing data. The results show that the coupling of currents with waves improves the representation of the wave spectrum. However, the wave-induced drag coefficient shows only minor improvements in NEMO circulation fields, such as temperature, salinity, and currents.

Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica

The surface of windy Antarctic snowfields is subject to drifting snow, which leads to the formation of sastrugi. In turn, sastrugi contribute to the drag exerted by the snow surface on the atmosphere and hence influence drifting snow. Although the surface drag over rough sastrugi fields has been estimated for individual locations in Antarctica, its variation over time and with respect to drifting snow has received little attention. Using year-round data from a meteorological mast, seasonal variations in the neutral drag coefficient at a height of 10 m $$(C_{{ DN}10})$$ in coastal Adelie Land are presented and discussed in light of the formation and behaviour of sastrugi based on observed aeolian erosion patterns. The measurements revealed high $$C_{{ DN}10} $$ values $$(\ge $$ 2 $$\times $$ 10 $$^{-3})$$ and limited drifting snow (35% of the time) in summer (December–February) versus lower $$C_{{ DN}10} $$ values $$(\approx $$ 1.5 $$\times $$ $$10^{-3})$$ associated with more frequent drifting snow (70% of the time) in winter (March–November). Without the seasonal distinction, there was no clear dependence of $$C_{{ DN}10} $$ on friction velocity or wind direction, but observations revealed a general increase in $$C_{{ DN}10} $$ with rising air temperature. The main hypothesis defended here is that higher temperatures increase snow cohesion and the development of sastrugi just after snow deposition while inhibiting the sastrugi streamlining process by raising the erosion threshold. This increases the contribution of the sastrugi form drag to the total surface drag in summer when winds are lighter and more variable. The analysis also showed that, in the absence of erosion, single snowfall events can reduce $$C_{{ DN}10} $$ to $$1\,\times \,10^{-3}$$ due to the burying of pre-existing microrelief under newly deposited snow. The results suggest that polar atmospheric models should account for spatial and temporal variations in snow surface roughness through a dynamic representation of the sastrugi form drag.

Climatologically Significant Effects of Some Approximations in the Bulk Parameterizations of Turbulent Air–Sea Fluxes

AbstractThis paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulas on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments is examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the period 1982–2014 are computed using SST and surface meteorological state variables from ERA-Interim. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulas. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically on the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%) but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

Observations of surface momentum exchange over the marginal ice zone and recommendations for its parametrisation

Abstract. Comprehensive aircraft observations are used to characterise surface roughness over the Arctic marginal ice zone (MIZ) and consequently make recommendations for the parametrisation of surface momentum exchange in the MIZ. These observations were gathered in the Barents Sea and Fram Strait from two aircraft as part of the Aerosol–Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) project. They represent a doubling of the total number of such aircraft observations currently available over the Arctic MIZ. The eddy covariance method is used to derive estimates of the 10 m neutral drag coefficient (CDN10) from turbulent wind velocity measurements, and a novel method using albedo and surface temperature is employed to derive ice fraction. Peak surface roughness is found at ice fractions in the range 0.6 to 0.8 (with a mean interquartile range in CDN10 of 1.25 to 2.85 × 10−3). CDN10 as a function of ice fraction is found to be well approximated by the negatively skewed distribution provided by a leading parametrisation scheme (Lüpkes et al., 2012) tailored for sea-ice drag over the MIZ in which the two constituent components of drag – skin and form drag – are separately quantified. Current parametrisation schemes used in the weather and climate models are compared with our results and the majority are found to be physically unjustified and unrepresentative. The Lüpkes et al. (2012) scheme is recommended in a computationally simple form, with adjusted parameter settings. A good agreement holds for subsets of the data from different locations, despite differences in sea-ice conditions. Ice conditions in the Barents Sea, characterised by small, unconsolidated ice floes, are found to be associated with higher CDN10 values – especially at the higher ice fractions – than those of Fram Strait, where typically larger, smoother floes are observed. Consequently, the important influence of sea-ice morphology and floe size on surface roughness is recognised, and improvement in the representation of this in parametrisation schemes is suggested for future study.

Wind Stress Dynamics in Chesapeake Bay: Spatiotemporal Variability and Wave Dependence in a Fetch-Limited Environment

AbstractThe spatiotemporal variability of wind stress dynamics in Chesapeake Bay has been investigated using a combination of observations and numerical modeling. Direct measurements of momentum and surface heat fluxes were collected using an ultrasonic anemometer deployed on a fixed tower in the middle reaches of Chesapeake Bay in the spring of 2012 along with collocated wave measurements. These measurements were compared to bulk estimates of wind stress using wave-dependent formulations of the Charnock parameter (alpha). Results indicate that a constant alpha value of 0.018 reasonably represents observed stress values, but estimates can be improved by the inclusion of surface wave information in the parameterization of alpha. Using a wave age formulation of alpha in combination with an optimally interpolated 10-m neutral wind field, a third-generation numerical wave model, Simulating Waves Nearshore (SWAN), was employed to investigate the spatiotemporal variability of wind stress across the estuary. Alpha values were found to be wind speed dependent and displayed spatial distributions that ranged between open-ocean values and strongly fetch-limited values. Model results suggest that variable wind stress dynamics stemming from a combination of variable surface winds and fetch-limited wave growth may result in the 10-m neutral drag coefficient varying by a factor of 2 across the estuary. Up to 20% of these changes can be directly attributed to the effects of variable waves.

Swell and the drag coefficient

Simultaneous measurements of waves and turbulent fluxes were collected from a moored surface buoy in the Philippine Sea. Waves were partitioned into their wind sea and swell components, and the ratio of swell to wind sea energy was used to assign a swell index. The 10-m neutral drag coefficient was calculated using the eddy correlation method. Four hundred hours of data were processed in 30 minute runs for wind speeds 8.5 to 16.5 m s−1 when the peak wave direction was within 90° of the wind direction and included observations during mixed seas, swell dominant, and wind sea dominant conditions. The data were analyzed to explore the influence of swell on the drag coefficient. It was found that when compared to periods of equal wind speed, the drag coefficient was reduced up to 37 % when swell energy was twice that of the wind sea energy. It is believed that this reduction was due to a decrease in the turbulent flux around the swell frequency, suggesting that the swell diminishes the surface aerodynamic roughness.

The spatial‐temporal variability of air‐sea momentum fluxes observed at a tidal inlet

AbstractCoastal waters are an aerodynamically unique environment that has been little explored from an air‐sea interaction point of view. Consequently, most studies must assume that open ocean‐derived parameterizations of the air‐sea momentum flux are representative of the nearshore wind forcing. Observations made at the New River Inlet in North Carolina, during the Riverine and Estuarine Transport experiment (RIVET), were used to evaluate the suitability of wind speed‐dependent, wind stress parameterizations in coastal waters. As part of the field campaign, a small, agile research vessel was deployed to make high‐resolution wind velocity measurements in and around the tidal inlet. The eddy covariance method was employed to recover direct estimates of the 10 m neutral atmospheric drag coefficient from the three‐dimensional winds. Observations of wind stress angle, near‐surface currents, and heat flux were used to analyze the cross‐shore variability of wind stress steering off the mean wind azimuth. In general, for onshore winds above 5 m/s, the drag coefficient was observed to be two and a half times the predicted open ocean value. Significant wind stress steering is observed within 2 km of the inlet mouth, which is observed to be correlated with the horizontal current shear. Other mechanisms such as the reduction in wave celerity or depth‐limited breaking could also play a role. It was determined that outside the influence of these typical coastal processes, the open ocean parameterizations generally represent the wind stress field. The nearshore stress variability has significant implications for observations and simulations of coastal transport, circulation, mixing, and general surf‐zone dynamics.

Variability of Arctic sea‐ice topography and its impact on the atmospheric surface drag

AbstractOver the polar oceans, near‐surface atmospheric transport of momentum is strongly influenced by sea‐ice surface topography. The latter is analyzed on the basis of laser altimeter data obtained during airborne campaigns between 1995 and 2011 over more than 10,000 km of flight distance in different regions of the Arctic Ocean. Spectra of height and spacing between topographic features averaged over 10 km flight sections show that typical values are 0.45 m for the mean height and about 20 m for the mean spacing. Nevertheless, the variability is high and the spatial variability is stronger than the temporal one. The total topography spectrum is divided into a range with small obstacles (between 0.2 m and 0.8 m height) and large obstacles (≥0.8 m). Results show that large pressure ridges represent the dominant topographic feature only along the coast of Greenland. In the Central Arctic, the concentration of large ridges decreased over the years, accompanied by an increase of small obstacles concentration and this might be related to decreasing multiyear ice. The application of a topography‐dependent parameterization of neutral atmospheric drag coefficients reflects the large variability in the sea‐ice topography and reveals characteristic differences between the regions. Based on the analysis of the two spectral ranges, we find that the consideration of only large pressure ridges is not enough to characterize the roughness degree of an ice field, and the values of drag coefficients are in most regions strongly influenced by small obstacles.

Impact of Variable Atmospheric and Oceanic Form Drag on Simulations of Arctic Sea Ice*

Abstract Over Arctic sea ice, pressure ridges and floe and melt pond edges all introduce discrete obstructions to the flow of air or water past the ice and are a source of form drag. In current climate models form drag is only accounted for by tuning the air–ice and ice–ocean drag coefficients, that is, by effectively altering the roughness length in a surface drag parameterization. The existing approach of the skin drag parameter tuning is poorly constrained by observations and fails to describe correctly the physics associated with the air–ice and ocean–ice drag. Here, the authors combine recent theoretical developments to deduce the total neutral form drag coefficients from properties of the ice cover such as ice concentration, vertical extent and area of the ridges, freeboard and floe draft, and the size of floes and melt ponds. The drag coefficients are incorporated into the Los Alamos Sea Ice Model (CICE) and show the influence of the new drag parameterization on the motion and state of the ice cover, with the most noticeable being a depletion of sea ice over the west boundary of the Arctic Ocean and over the Beaufort Sea. The new parameterization allows the drag coefficients to be coupled to the sea ice state and therefore to evolve spatially and temporally. It is found that the range of values predicted for the drag coefficients agree with the range of values measured in several regions of the Arctic. Finally, the implications of the new form drag formulation for the spinup or spindown of the Arctic Ocean are discussed.

Estimates of the 10-m Neutral Sea Surface Drag Coefficient from Aircraft Eddy-Covariance Measurements

Abstract The 10-m neutral drag coefficient (CDN10) over the sea is calculated using a large observational dataset consisting of 5800 estimates of the mean flow and the fluxes from aircraft eddy-covariance measurements. The dataset includes observations from 11 different experiments with four different research aircraft. One of the goals is to investigate how sensitive CDN10 is to the analysis method. As such, CDN10 derived from six unique processing schemes that involve different methods for averaging the surface stress and the wind speed are compared. Especially in weak winds, the resulting CDN10 values depend on the choice of processing. Four distinct regimes of CDN10 are identified: weak winds where calculating CDN10 is not well posed, moderate winds (4 to 10 m s−1) where CDN10 is a constant, strong winds (10 to 20 m s−1) where CDN10 increases linearly with increasing wind speed, and very strong winds (20 to 24 m s−1) where CDN10 steadily decreases with increasing wind speed. However, as this last regime is based on data from a single experiment, additional data are needed to confirm this apparent decrease in CDN10 for winds exceeding 20 m s−1.

Effect of sea ice morphology during Arctic summer on atmospheric drag coefficients used in climate models

Realistic modeling of polar sea ice dynamics and atmospheric processes over sea ice needs a detailed representation of the near‐surface atmospheric fluxes of momentum. In this study, parametrizations of neutral drag coefficients mostly used in different general circulation models are compared with a recently developed parametrization including the impact of sea ice morphology. The new parametrization, using the sea ice and melt pond fraction as governing parameters, accounts for the effect of form drag caused by edges at leads, melt ponds, and floes. Based on remote sensing data of ice and melt pond fraction, it is shown that during Arctic summer the traditionally used drag coefficients differ from the new ones by a factor 0.5–1.2. The geographic distribution of drag coefficients obtained from both parametrizations is very different. Differences are due to a nonlinear and non‐monotonic dependence of drag coefficients on sea ice concentration in the new parametrization.

Wave–induced Characteristics of Atmospheric Turbulence Flux Measurements

In this paper, we present the air–sea flux of momentum obtained with the eddy correlation method applied to data measured from a moored discus buoy deployed approximately 600 m off a research Air Sea Interaction Tower at Martha's Vineyard Coastal Observatory, Massachusetts during spring 2010. We discuss corrections to account for the wave–induced motion of buoy. Our analysis confirm that the neutral drag coefficient depends upon wave age and wind speed. The data scatter between wind and momentum stress in the low wind speed are significantly larger for those with higher wind speeds. Furthermore, the momentum transfer is investigated using a wave model with sea state dependent drag coefficient. This wave age dependent drag reveals fairly good agreement compared to the observed drag coefficient.

A simple extension of “An alternative approach to sea surface aerodynamic roughness” by Zhiqiu Gao, Qing Wang, and Shouping Wang

Based on the data previously collected during the Humidity Exchange over the Sea Main Experiment (HEXMAX), the methods used to parameterize aerodynamic roughness (z0), friction velocity (u*), and the neutral drag coefficient (CDN) under moderate wind speed conditions originally proposed by Gao et al. (2006) were extended by using the nondimensional significant wave height (gHs/u*2 or gHs/U10N2) instead of wave age (cp/u* or cp/U10N), where g is the acceleration of gravity, Hs is the significant wave height, U10Nis the horizontal wind speed at 10‐m height under the neutral atmospheric condition, andcp is the phase velocity of the peak wave spectrum. The results show (1) u* = 0.024U10N(gHs/U10N2)−1/4, (2) z0 = 10 × exp[−4.797(gHs/u*2)1/6] or z0 = 10 × exp[−16.613(gHs/U10N2)1/4], and (3) CDN = 0.007(gHs/u*2)−1/3 or CDN = 5.76 × 10−4(gHs/U10N2)−1/2. The present parameterization schemes were experimentally tested.