Airborne surveys of the atmospheric boundary layer above the marginal ice zone on the Newfoundland shelf

Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10^‐3 at 10% coverage to 5 × 10^‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.     

Characteristic Study of the Boundary Layer Parameters over the Arabian Sea and the Bay of Bengal Using the QuikSCAT Dataset

The marine atmospheric boundary layer （MABL） plays a vital role in the transport of momentum and heat from the surface of the ocean into the atmosphere. A detailed study on the MABL characteristics was carried out using high-resolution surface-wind data as measured by the QuikSCAT （Quick scatterometer） satellite. Spatial variations in the surface wind, frictional velocity, roughness parameter and drag coefficient for the different seasons were studied. The surface wind was strong during the southwest monsoon season due to the modulation induced by the Low Level Jetstream. The drag coefficient was larger during this season, due to the strong winds and was lower during the winter months. The spatial variations in the frictional velocity over the seas was small during the post-monsoon season （-0.2 m s^-1）. The maximum spatial variation in the frictional velocity was found over the south Arabian Sea （0.3 to 0.5 m s^-1） during the southwest monsoon period, followed by the pre-monsoon over the Bay of Bengal （0.1 to 0.25 m s^-1）. The mean wind-stress curl during the winter was positive over the equatorial region, with a maximum value of 1.5×10^-7 N m^-3, but on either side of the equatorial belt, a negative wind-stress curl dominated. The area average of the frictional velocity and drag coefficient over the Arabian Sea and Bay of Bengal were also studied. The values of frictional velocity shows a variability that is similar to the intraseasonal oscillation （ISO） and this was confirmed via wavelet analysis. In the case of the drag coefficient, the prominent oscillations were ISO and quasi-biweekly mode （QBM）. The interrelationship between the drag coefficient and the frictional velocity with wind speed in both the Arabian Sea and the Bay of Bengal was also studied.     

A general hydrodynamic theory for mixed-phase microphysics. Part I: drag and fall speed of hydrometeors

A general parameterization for solid and liquid hydrometeors is presented. hydrometeors basically are viewed as porous spheroids with the following variable parameters: diameter, axial ratio, mass, and porosity. Based on this parameterization a functional dependence on the Reynolds number of the drag of hydrometeors is derived, which is based on boundary layer theory. The basic form of this functional dependence is consistent with viscous theory and the inertial drag at low Reynolds numbers is predicted with good accuracy by matching the results from the boundary layer theory with Oseen's theory of creeping motion. Based on this solution a general semi-empirical expression for the Reynolds number and fall speed of particles is found. The results from the present theory are in remarkable agreement with experiments: The errors generally are < 5–10% for a wide variety of hydrometeors in the range of Reynolds numbers 0<N_Re<5×10⁵, including columnar and variously branched planar ice crystals, rimed and unrimed aggregates, lump, conical, and hexagonal graupel, hail, and rain drops. The present parameterisation aims far beyond the limits of the conventional methods since it is suitable for mixed-phase models of the microphysics of precipitation with continuously varying particle mass and shape characteristics and including processes such as depositional growth of ice crystals under varying environmental conditions, collisional growth of particles, and melting.     

The transport of water,heat and salt through the strait of Belle Isle

Abstract

We present an analysis of current‐meter, sea‐level and hydrographic data collected in the Strait of Belle Isle and the northeastern Gulf of St Lawrence. From an array of moorings in the Strait from July to October 1980, we calculate a net transport into the Gulf of 0.13 × 10⁶ m³ s^?1 and show that the mean and eddy fluxes of heat through the Strait represented a net loss of heat to the northeastern Gulf. The estimated rate of loss of heat is less than the long‐term mean computed by Bugden (1981) but becomes comparable if adjusted for interannual changes of transport and water temperature. Moreover, the 1980 data permit the permanent tide‐gauge stations in the Strait at West Ste Modeste and Savage Cove to be levelled relative to one another, thus allowing surface currents to be calculated from sea‐level alone. Hence the long‐term wintertime transport into the Gulf can be calculated after fractional effects on the vertical structure of the flow are considered. During an average winter it appears that advection through the Strait can account for about 35% of the Gulf Intermediate Layer. A multiple regression involving average Intermediate Layer temperatures over 9 years suggests that winter air temperature in the Gulf, representative of atmospheric cooling, and sea‐level difference across the Strait, representative of advection, are equally important variables and together account for 50% of the Layer's temperature variability. Analysis of current‐meter, sea‐level and hydrographic data collected in 1975 supports earlier hypotheses that the strongest inflow of water with ? < 0° C and salinity between 32 and 3 3 should occur in winter. It appears that during the 1975 field program the inflow was about 0.6 × 10⁶ m³ s^?1, which is about twice the long‐term average for January to May.     

Atmospheric turbulent fluxes over snow

On March 26, 1971, eddy fluxes of momentum, sensible heat and water vapour were measured over Lake Mendota, Wisconsin, U.S.A., which was covered by an extensive snowfall. An evaporation rate of about 0.7mm day^–1 (2.2 mW cm^–2) was detected. Wind speeds were light and the atmosphere near the surface was highly stable. In these conditions, the average sensible heat transfer and Reynolds stress were -0.9 mW cm^–2 and 0.10 dyn cm^–2, respectively. Comparison with measured gradients of wind speed, temperature and humidity yield a drag coefficient of about 0.54 × 10^–3, and bulk transfer coefficients for sensible and latent heat of 0.41 × 10^–3 and 0.78 × 10^–3, respectively, applied to 10-m data. When corrected for the effect of atmospheric stability, these three coefficients become (in the same order) 1.2 × 10^–3, 0.9 × 10^–3 and 2.5 × 10^–3. The errors in these estimates are such that the drag coefficient is not significantly different from that corresponding to an aerodynamically smooth surface, while the heat coefficients are similar to those normally applied over liquid water surfaces.     

Coupled ice‐ocean variability in the Greenland Sea

Abstract

A major surface feature of the Greenland Sea during winter is the frequent eastward extension of sea ice south of 75°N and an associated embayment to the north. These features are nominally connected with the East Greenland Current, and both the promontory and the embayment are readily apparent on climatic ice charts. However, there are significant changes in these features on time‐scales as short as a few days. Using a combination of satellite microwave images (SSM/I) of ice cover, meteorological data and in situ velocity, temperature and salinity records, we relate the ice distribution and its changes to the developing structure and circulation of the upper ocean during winter 1988–1989. Our measurements illustrate the preconditioning that leads to convective overturn, which in turn brings warmer water to the surface and results in the rapid disappearance of ice. In particular, the surface was cooled to the freezing point by early December and the salinity then increased through ice formation (about 0.016 m d^‐1) and brine rejection. Once the vertical density gradient was sufficiently eroded, a period of high heat flux (>300 W m^‐2) in late January provided enough buoyancy loss to convectively mix the upper water column to at least 200 m. We estimate vertical velocities at about 3 cm s^‐1 downward during the initial sinking. The deepening of the thermocline raised surface temperatures by over 1°C resulting in nearly 1.5 × 10⁵ km² of ice‐melt within two days. Average rates of ice retreat are about 11 km d^‐1 southwestward, generally consistent with a wind‐driven flow. Comparison of hydrographic surveys from before and after the overturning indicate the fresh water was advected out of the area, possibly to the south and east of our moorings.     

The rose report — A review

Abstract

Estimates of the entrainment rate of salt water into the Fraser River plume have been made using two independent methods. A value of k = 2 × 10 ^‐4 is obtained for he entrainment coefficient relating the vertical to the horizontal velocity from salt conservation arguments at a series of profiles along the plume and from calculations of surface divergence, as measured by drifting drogues. It is also found that entrainment contributes significantly to the deceleration of the river plume after it issues into the Strait of Georgia.     

Preliminary results of modeling studies on washout of stable and radioactive analogs of <Superscript>90</Superscript>Sr and <Superscript>137</Superscript>Cs from snow-covered areas

Experimental field and laboratory studies on washout of radionuclides from the snow cover during snow melting were carried out in the winter of 2005/06. In the field studies, a specially equipped runoff site was used. In the laboratory conditions, the experiments were conducted using prepared soil monoliths. In the winter of 2006, 25 g/m² of water-free cesium chloride (CsCl) and 25 g/m³ of strontium chloride (SrCl₂) were put onto the snow cover surface of the runoff site. The snow surface of the soil monolith was coated with a ¹³⁷Cs-bearing solution, then with SrCl₂. Under experimental conditions, practically no surface runoff from the runoff site was recorded. The experiments with the soil monoliths demonstrated that the coefficient of the liquid washout of ¹³⁷Cs normalized to the runoff layer was within 0.9 × 10^?6–1.2 × 10^?4 mm^?1, and that of ⁹⁰Sr normalized to the runoff layer was within 2 × 10^?–1.6 × 10^?4 mm^?1.     

Geometric and aerodynamic roughness of sea ice

The aerodynamic drag of Arctic sea ice is calculated using surface data, measured by an airborne laser altimeter and a digital camera in the marginal ice zone of Fram Strait. The influence of the surface morphology on the momentum transfer under neutral thermal stratification in the atmospheric boundary layer is derived with the aid of model concepts, based on the partitioning of the surface drag into a form drag and a skin drag. The drag partitioning concept pays attention to the probability density functions of the geometric surface parameters. We found for the marginal ice zone that the form drag, caused by floe edges, can amount to 140% of the skin drag, while the effect of pressure ridges never exceeded 40%. Due to the narrow spacing of obstacles, the skin drag is significantly reduced by shadowing effects on the leeward side of floe edges. For practical purposes, the fractional sea-ice coverage can be used to parameterize the drag coefficientC _dn, related to the 10 m-wind. C _dnincreases from 1.2 · 10^-3 over open water to 2.8 · 10^-3 for 55% ice coverage and decreases to 1.5 · 10^-3 for 100% ice coverage.Aircraft turbulence measurements are used to compare the model values of C _dnwith measureents. The correlation between measured and modelled drag coefficients results in r ² = 0.91, where r is the correlation coefficient.     

Baroclinic adjustment in coarse‐resolution numerical ocean models

Abstract

The propagation of baroclinic Kelvin and Rossby waves in a fairly coarse‐resolution numerical reduced‐gravity ocean model is investigated using simple geostrophic adjustment experiments in a box‐like domain. Numerical experiments using three different horizontal resolutions (4° × 5°,2° × 2.5° and l° × 1.25°) with properly scaled eddy viscosity coefficients show that the phase speed of the model Kelvin waves is almost exactly proportional to the grid resolution, but is virtually independent of the model viscosity. These results are consistent with the findings of Hsieh et al. (1983) and Wajsowicz and Gill (1986). It is also shown that the two relevant parameters that govern the propagation and decay of these waves, namely the grid‐resolution parameter Δ = Δx/a (where Δx is the grid size and a is the baroclinic Rossby radius, viz. a = C/f, with C being the phase speed of inviscid internal gravity waves in a continuum) and the viscosity parameterΔ = A_mλ/2πfa³ (where A_m is the eddy viscosity coefficient and λ is the alongshore wavelength) can be replaced with Δ only. This is because in Munk (1950)‐type models, the viscosity parameter Δ scales with Δ³. For Δ³ >1, the Kelvin wave phase speed is c_K ΔC/Δ and the alongshore decay length scale is of the order of the perimeter of the basin, viz., 0(10⁴) km.

In contrast to the case for Kelvin waves, the phase speed of the model Rossby waves is not that much different from its value in a continuum and depends only weakly on the model resolution. This is in good agreement with the theoretical results of Wajsowicz (1986). On the other hand, the model Rossby waves are severely damped, within a distance of the order of a wavelength, by the large eddy viscosity of the model. We therefore extrapolate that for a proper simulanon of Kelvin and Rossby waves in this type of numerical ocean model, we need a grid size smaller than 1° × 1°, and a higher‐order turbulent closure scheme that will reduce the eddy viscosity coefficient.     

Determination of the Drag Coefficient over the Tibetan Plateau

In this paper,a preliminary study is given on the drag (i.e.bulk transfer for momentum) coefficient,on the basis of data from four sets of AWS in Tibet during the first observational year from July 1993 to July 1994 according to China Japan Asian Monsoon Cooperative Research Program.The results show that the drag coefficient over the Tibetan Plateau is 3.3 to 4.4×103.In addition,monthly and diurnal variations of drag coefficient and the relationship among the drag coefficients and the bulk Richardson number,surface roughness length and wind speed at 10 m height are discussed in detail.     

A planetary boundary-layer study in the Mackenzie Valley,Canada

Detailed wind velocity profiles were obtained by means of a rocket-sonde technique to a height of about 700 m at a site in the Canadian Northwest Territories. Less detailed temperature observations were also made using a balloon sonde. The site was some 100 km east of the easternmost range of the Rocky Mountains. The observations took place in mid-February when the overall atmospheric static stability was considerable. The results showed the presence of an arctic, atmospheric ‘thermocline’ some 500 m above ground, which sloped up or down considerably, with the generators of isothermal surfaces usually parallel to the nearby mountains, in the manner of upwelled or downwelled thermoclines in the ocean near shore. There was often strong baroclinic flow parallel to the mountain range. Noticeable frictional effects were confined to a near-ground layer always less than 100 m and mostly no more than 10 m in height. An Ekman-type boundary layer could only be identified in about one-third of the velocity profiles. The non-dimensionalized depth coefficient of such layers was close to 0.1, the geostrophic drag coefficient about 2.5×10^?4.     

The normal-force coefficient of a thin closed fence

In a full-scale field study, the drag coefficient of a thin closed fence was estimated. A measurement plate was constructed more or less in the center of the barrier in order to estimate the normal-force on the fence. The normal-force coefficient was studied under near-neutral stratification for various Reynolds numbers and different angles of attack.     

Mn(II)-catalysed S(IV) oxidation and its inhibition by acetic acid in acidic aqueous solutions

The kinetics of the S(IV) oxidation by oxygen in the presence of Mn(II) ions and acetic acid has been studied. Experiments were carried out at 25°C, 3.5?≤?pH?≤?5.0, [S(IV)]≈1?×?10^?3 mol/dm³, 1?×?10^?6 mol/dm³?≤?[Mn(II)]?≤?1?×?10^?5 mol/dm³, 1?×?10^?6 mol/dm³?≤?[CH₃COOH]?≤?1?×?10^?4 mol/dm³. Based on the experimental results, rate constants and orders of the reactions were determined. Depending on the reaction conditions, the observed rate constants for the Mn(II)-catalysed S(IV) oxidation ranged between 3.91?×?10^?8 and 8.89?×?10^?7 (mol/dm³) s^?1, and in the presence of acetic acid they ranged between 2.95?×?10^?8 and 7.45?×?10^?7 (mol/dm³) s^?1. The reaction order in S(IV) was zero for both reactions. The effect of Mn(II) ion and acetic acid concentrations as well as an initial pH of the solution on the S(IV) oxidation rate was discussed. It was found that the rate of the S(IV) oxidation depends on the initial pH of the solution but it is independent of the pH change during the reaction. Acetic acid has a weak inhibiting effect on the Mn(II)-catalysed S(IV) oxidation. Under the experimental conditions the S(IV) oxidation rate decreased no more than twice.     

Water interaction with MgCl<Subscript>2</Subscript>×6H<Subscript>2</Subscript>O and NaCl surfaces: measurements of the uptake coefficient

The uptake of water vapor on MgCl₂×6H₂O and NaCl salt dry solid films was studied over the temperature range 240 to 340 K and at 1 Torr pressure of helium using a flow reactor coupled to a modulated molecular beam mass spectrometer. The H₂O to salt uptake data were obtained from the kinetics of H₂O loss on salt coated Pyrex rods. The following Arrhenius expression was obtained for the initial uptake coefficient of H₂O on MgCl₂×6H₂O films: γ ₀ (MgCl₂) = (6.5 ± 1.0) × 10⁻⁶ exp[(470 ± 40)/T] (calculated with specific BET surface area, quoted uncertainties are 1σ statistical). The rate of H₂O adsorption on NaCl was found to be much lower than on MgCl₂×6H₂O, and only an upper limit was determined for the corresponding uptake coefficient: γ (NaCl) ≤ 5.6 × 10⁻⁶ at T = 300 K. The results show that the rate of H₂O adsorption to salt surfaces is drastically dependent on the salt sample composition.     

Neutrally Stratified Turbulent Ekman Boundary Layer: Universal Similarity for a Transitional Rough Surface

The geostrophic Ekman boundary layer for large Rossby number (Ro) has been investigated by exploring the role played by the mesolayer (intermediate layer) lying between the traditional inner and outer layers. It is shown that the velocity and Reynolds shear stress components in the inner layer (including the overlap region) are universal relations, explicitly independent of surface roughness. This universality of predictions has been supported by observations from experiment, field and direct numerical simulation (DNS) data for fully smooth, transitionally rough and fully rough surfaces. The maxima of Reynolds shear stresses have been shown to be located in the mesolayer of the Ekman boundary layer, whose scale corresponds to the inverse square root of the friction Rossby number. The composite wall-wake universal relations for geostrophic velocity profiles have been proposed, and the two wake functions of the outer layer have been estimated by an eddy viscosity closure model. The geostrophic drag and cross-isobaric angle predictions yield universal relations, which are also supported by extensive field, laboratory and DNS data. The proposed predictions for the geostrophic drag and the cross-isobaric angle compare well with data for Rossby number Ro ≥ 10⁵. The data show low Rossby number effects for Ro < 10⁵ and higher-order effects due to the mesolayer compare well with the data for Ro ≥ 10³.     

Theoretical studies on OH and Cl initiated hydrogen atom abstraction of HFE-227pc (CF3OCF2CHF2)

The kinetics of hydrogen atom abstraction reactions of HFE-227pc by OH and Cl was studied by ab initio method. The structural optimization and frequency calculation of the titled compound and the species formed during the abstraction reactions were performed with density functional theory using hybrid meta density functional MPWB1K with 6–31?+?G(d,p) basis set. The energy of the species was further refined by making a single point energy calculation at G3B3 level of theory. The standard enthalpies of formation of reactant and the radical formed after H-atom abstraction was calculated using isodesmic method. The rate constants of abstraction reactions were calculated using Conventional Transition State Theory (CTST) and were found to be 1.5?×?10^?15 and 0.53?×?10^?16 cm³molecule^?1 s^?1 for OH and Cl respectively. The calculated value for the abstraction by OH is close to the experimental value of 2.26?×?10^?15 cm³molecule^?1 s^?1 whereas the same for Cl is found to be about five times lower than that of 2.70?×?10^?16 cm³molecule^?1 s^?1. The theoretical studies yielded the enthalpies of formation and the rate constants that are vital in determining the lifetime of HFE-227pc.     

Aggregation of small ice crystals in an electric field

Abstract

Small ice crystals with average diameter of about 30 μm are produced in a large cold room and allowed to fall in a settling chamber in the presence of a quasi‐uniform electric field. Aggregates (flakes) of ice crystals are collected by permanent replicas. Results show that an electric field above a threshold value of about 4 × 10⁴ V m^‐1 rapidly increases the growth of flakes by the capture of small ice crystals. The influence of the electric field upon the growth of ice aggregates is maximum at a field strength of about 1.5 × 10⁵ V m^‐1. Comparison of the results with Jiusto's mathematical model of the growth rate gives values of the collection efficiency at different field strengths. It is very likely that the electric field increases the adhesion (aggregation) efficiency rather than the collision (cross‐section) efficiency.     

Intercomparison of Two Hydroxyl Radical Measurement Techniques at the Atmosphere Simulation Chamber SAPHIR

At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10⁶ cm^–3) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10⁶ cm^–3, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 – 2) ×10⁶ cm^–3 higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO_x concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.     

Heat transfer coefficients for leaves on orchard apple trees

The coefficient for heat transfer from apple tree leaves was measured from the energy balance of leaves which were prevented from transpiring by applying Vaseline (petroleum jelly). Vaseline had negligible effect on the absorption of short-wave radiation by the leaves. The Nusselt number (Nu) describing heat flux from a leaf in terms of its average temperature was related to Reynolds numbers (Re) in the range 10³ to 10⁴ by Nu = 0.46 Re^0.54 Pr^0.33, where Pr is the Prandtl number. This supports Landsberg and Powell's (1973) wind-tunnel results for transfer from leaves subject to mutual interference.