On the mean and tidal currents in Hudson strait

Abstract

The vertical structures of the mean and tidal flows in Hudson Strait are described from moored current‐meter data collected during an 8‐week period in August to October of 1982. The residual flow in the strongly stratified waters off Quebec is directed along the Strait to the southeast, is highly baroclinic and is concentrated near shore (within an offshore length scale of approximately an internal Rossby radius). Maximum mean speeds of 0.3 m s^?1 were observed near‐surface (30 m). In the weakly stratified waters on the northern side of the Strait along Baffin Island the mean flow is northwestward. The maximum speeds are 0.1 m s^?1 near‐surface (30 m) and the current amplitudes decrease to 0.05 m s^?1 at 100 m. The mean southeastward transport is estimated to be 0.93 ±0.23 × 10⁶ m³ s^?1 with a northwestward transport of 0.82 ± 0.24 × 10⁶ m³ s^?1. Over most of the Strait the across‐channel residual currents are directed towards the Quebec shore with velocities ranging from 0.02 to 0.1 ms^?1. Current variability is dominated by the tides, the M₂ being the major tidal constituent. In the vicinity of the mooring the M₂ tide is primarily barotropic, progressive in nature, and has along‐channel current amplitudes varying across the Strait from 0.20 to 0.45 m s^?1. Observed differences in tidal sea‐level elevations across the Strait can be accounted for by the cross‐channel variations characteristic of Kelvin waves.     

Determination of carbon balances for smog chamber terpene oxidation experiments using a 14C tracer technique

In the last decade, numerous studies of monoterpene hydrocarbons in the presence of ozone and oxides of nitrogen have been completed. Although insights about the reactivity of terpenes have been gained, and several products have been identified in the gas and aerosol phases, carbon balances have been generally poor. This paper describes a radiotracer technique which accounts for carbon-containing compounds in a smog chamber at the conclusion of a photooxidation or ozonolysis reaction. Instead of attempting to identify individual products, gas-phase compounds were separated from aerosols, and each phase was analyzed by a liquid scintillation counter. A carbon balance of 79 to 97% was obtained with ¹⁴C--pinene and ozone using this technique. The significance of the results are discussed in terms of the ¹⁴C--pinene concentrations used in this study, which were lower than concentrations used previously by one to four orders of magnitude. In spite of the lower concentrations, the gaseous and aerosol fractions observed in the present investigation are comparable to those observed by others. Possible reaction products are discussed with respect to likely mechanisms for -pinene oxidation.     

Mathematical modelling of the heat exchange between a river and the atmosphere

Some numerical models of various degrees of complexity for studying river atmosphere heat exchange are discussed. Also described are issues related to obtaining the basic meteorological and the required hydrological data. As meteorological and hydrological parameters vary strongly within one year and from year to year, a large amount of information has to be collected and analysed for obtaining a representative picture.The necessary effort depends clearly on the application. It is much larger, when the absolute temperature and the transient response have to be calculated, than when only the average temperature increase due to anthropogenic heat discharges is of interest.Results for the exchange of radiative, latent and sensible heat between atmosphere and water surface including the overall exchange coefficient are given for 21 years, in detail for Basel and summarised for 8 locations along the Rhein. Yearly averages vary little, monthly averages show the expected seasonal variation.     

Gas ventilation of the Saguenay fjord by an energetic tidal front

Abstract

Dissolved noble gas samples were taken during a pilot study in the Saguenay Fjord, Quebec, Canada, in order to determine the contribution of different air‐sea gas exchange mechanisms in an estuary and to assess the contribution of tidal fronts to the aeration of subsurface waters. The noble gases He, Ne, Ar, Kr, and Xe span a large range of molecular diffusivities and solubilities and hence constitute a useful probe of various gas exchange and bubble injection processes. Samples were taken at flood tide upstream and downstream of an energetic tidal front that is generated by a hydraulically controlled flow over a shallow sill at the entrance to the Fjord. The results are interpreted with the help of hydrographic measurements of density and currents along cross‐sill transects describing the physical forcing at the sill. High gas saturations downstream of the sill indicate the aeration of water within the frontal region. An inverse model is used to compare the contribution of bubble injection in the front to diffusion across the air‐sea interface. The large ratio of completely ‘trapped’ bubbles to diffusion suggests that bubbles injected by waves breaking in the front contribute significantly to air‐sea gas exchange with 76% for He, 79% for Ne, 56% for Ar, 47% for Kr, and 35% for Xe.

Water samples were analyzed for helium isotopes and tritium in order to explore the possibility of constraining ventilation time scales. The relationship between tritium and salinity revealed two end‐member waters: a freshwater component from the Saguenay River of 23.6 ± 0.5 TU, likely a residual of bomb‐produced tritium, and a seawater end‐member of approximately 1.5 TU originating in the subpolar Atlantic. An unexpected contribution of radiogenic 4He was detected in the deep waters of the St. Lawrence Estuary, likely a consequence of out‐gassing from old, uranium and thorium rich granitic terrain.     

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×10⁵ to 2.9×10⁶ molecules cm^-3, corresponding to midday maxima of 3 to 12×10⁶ molecules cm^-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.     

Atmospheric and tidal forcing of the exchange between Prince William Sound and the Gulf of Alaska

Current meter data from a series of oceanographic moorings spanning a total of five years was analyzed to quantify the tidal and subtidal exchange of water between Prince William Sound and the adjacent continental shelf in the northern Gulf of Alaska. Velocity profiles were used to quantify the exchange in terms of a transport through each of the two largest passages: Montague Strait and Hinchinbrook Entrance. Buoy wind and atmospheric pressure observations, as well as bottom pressure records, are then used to elucidate the role of atmospheric forcing on the exchange.An EOF analysis shows that the barotropic component accounts for 62% or more of the variance in the velocity profiles even after tides are removed by low-pass filtering, and thus the analysis is concerned primarily with depth-integrated transport. The estimated depth-integrated transport can reach ±0.6 Sv in Montague Strait, and ±1.5 Sv in Hinchinbrook Entrance. The largest fluctuations occur in response to the semidiurnal tides. Transport variations on subtidal time scales, which can reach −0.2 Sv in Montague Strait, and +0.6 Sv in Hinchinbrook Entrance, are shown by a frequency domain analysis to be dominated by easterly wind stress events which occur at periods of 2–5 days in both summer and winter. Atmospheric pressure has much less impact on transport, but there is some evidence that it might play a small role on time scales of a few weeks.Bottom pressure records suggest that easterly wind events set up a sea level height gradient in Hinchinbrook Entrance such that it tilts up to the east, which under geostrophy drives a barotropic flow into Prince William Sound. The same winds also raise the sea level in Hinchinbrook Entrance relative to Montague Strait, encouraging an outflow there in agreement with the ADCP observations. There is no evidence that the wind drives a vertically sheared bi-directional flow in either entrance, as has been observed in some estuaries. It is hypothesized that the lack of such a flow is possible because Prince William Sound has two major connections to the shelf, which alters the mass conservation requirement for each passage when compared to a system with just one entrance.     

Impact of Hudson Bay on the energy balance in the Hudson Bay Lowlands and the potential for climatic modification

Abstract

Three sites were instrumented to measure all components of the energy balance. The sites were located in the Churchill, Manitoba region and comprised a Sea Site on a sand spit 1 km seaward from the mainland, a Nearcoast Site 2 km inland from the coast and an Inland Site 65 km inland. Measurements were made continuously over a 90‐day period from 19 May to 16 August 1984. This period encompassed the bulk of the growing season.

The measurements were stratified into onshore and offshore wind directions and were compared for 10‐day periods. The comparisons show very significant differences attributable to the cold summer conditions promoted by the sea ice in Hudson Bay. The ground heat flux and latent heat flux were much greater during offshore winds but the sensible heat flux was greatest for onshore winds. Air temperatures averaged 7°C warmer for offshore than for onshore winds. The reasons for these differences are detailed and the climatic modifications that would probably result from earlier sea‐ice melt are discussed. Some implications of climatic modification are also noted.     

Modelling the sea ice-ocean seasonal cycle in Hudson Bay, Foxe Basin and Hudson Strait, Canada

The seasonal cycle of water masses and sea ice in the Hudson Bay marine system is examined using a three-dimensional coastal ice-ocean model, with 10 km horizontal resolution and realistic tidal, atmospheric, hydrologic and oceanic forcing. The model includes a level 2.5 turbulent kinetic energy equation, multi-category elastic-viscous-plastic sea-ice rheology, and two layer sea ice with a single snow layer. Results from a two-year long model simulation between August 1996 and July 1998 are analyzed and compared with various observations. The results demonstrate a consistent seasonal cycle in atmosphere-ocean exchanges and the formation and circulation of water masses and sea ice. The model reproduces the summer and winter surface mixed layers, the general cyclonic circulation including the strong coastal current in eastern Hudson Bay, and the inflow of oceanic waters into Hudson Bay. The maximum sea-ice growth rates are found in western Foxe Basin, and in a relatively large and persistent polynya in northwestern Hudson Bay. Sea-ice advection and ridging are more important than local thermodynamic growth in the regions of maximum sea-ice cover concentration and thickness that are found in eastern Foxe Basin and southern Hudson Bay. The estimate of freshwater transport to the Labrador Sea confirms a broad maximum during wintertime that is associated with the previous summers freshwater moving through Hudson Strait from southern Hudson Bay. Tidally driven mixing is shown to have a strong effect on the modeled ice-ocean circulation.     

Understanding precipitation recycling over the Tibetan Plateau using tracer analysis with WRF

The precipitation recycling (PR) ratio is an important indicator that quantifies the land-atmosphere interaction strength in the Earth system’s water cycle. To better understand how the heterogeneous land surface in the Tibetan Plateau (TP) contributes to precipitation, we used the water-vapor tracer (WVT) method coupled with the Weather Research and Forecasting (WRF) regional climate model. The goals were to quantify the PR ratio, in terms of annual mean, seasonal variability and diurnal cycle, and to address the relationships of the PR ratio with lake treatments and precipitation amount. Simulations showed that the PR ratio increases from 0.1 in winter to 0.4 in summer when averaged over the TP with the maxima centered at the headwaters of three major rivers (Yangtze, Yellow and Mekong). For the central TP, the highest PR ratio rose to over 0.8 in August, indicating that most of the precipitation was recycled via local evapotranspiration in summer. The larger daily mean and standard deviation of the PR ratio in summer suggested a stronger effect of land-atmosphere interactions on precipitation in summer than in winter. Despite the relatively small spatial extent of inland lakes, the treatment of lakes in WRF significantly impacted the calculation of the PR ratio over the TP, and correcting lake temperature substantially improved both precipitation and PR ratio simulations. There was no clear relationship between PR ratio and precipitation amount; however, a significant positive correlation between PR and convective precipitation was revealed. This study is beneficial for the understanding of land-atmosphere interaction over high mountain regions.

     

双线偏振多普勒雷达对一次降雹超级单体发展减弱阶段的演变分析

2015年4月28日,在高空冷涡的天气背景下,一个伴有较长生命史中气旋的超级单体在上海南汇双线偏振多普勒雷达观测范围内经历了发展减弱阶段,并产生了冰雹、雷雨大风、短时强降水等灾害性天气。利用常规天气观测、南汇双线偏振多普勒雷达、双雷达反演风场等资料分析发现,超级单体在发展成熟阶段呈现出回波悬垂、低层入流缺口、中气旋以及三体散射等经典特征,以及表示雷暴处于发展加强阶段的差分反射率因子柱。差分反射率因子柱通常意味着雷暴中上升气流的加强,同时说明大量的水滴可以被强上升气流托举到0℃层以上,形成过冷水滴,从而有利于冰雹的形成。超级单体经过的区域0~6 km垂直风切变达到了22~26 m·s-1,强垂直风切变环境有利于水平涡度发展,南汇雷达观测范围内中气旋维持了100 min左右,有利于雷暴的发展维持。此外,基于模糊逻辑法对此次降雹强对流天气过程开展了相态识别,结果表明,模糊逻辑法对此次强对流天气过程中的冰雹、降水等不同性质的降水能够进行有效的识别,有助于对雷暴降水相态本质的认识。     

A study of ozone mini-hole formation using a tracer advection model driven by barotropic dynamics

Summary An ozone mini-hole is a region of strongly depleted column total ozone amounts, associated with the growth of synoptic-scale wave disturbances. Their formation is illustrated here using a sequence of idealised model experiments. Simplified barotropic dynamics are used to drive an ozone tracer advection model on an f-plane representing a hemisphere. Firstly, the Contour Dynamics method is used to integrate the barotropic vorticity equation. Vorticity contours are initialised to simulate typical planetary wave structures and the developing wind field advects components of the ozone model. The vertical profiles of ozone mixing ratio are represented by simple linear functions, separated by a tropopause height field and capped by an upper model boundary. Integrating these profiles thus yields a total column ozone field which is closely dependent on tropopause height. In addition to horizontal advection, a vertical motion parametrisation is included, based on a quasi-geostrophic theory for tropopause displacement. The model is also used to simulate the formation of an actual mini-hole which occurred over northern Europe. Here, observed fields of vorticity, ozone and tropopause height are employed and the system integrated using a pseudo-spectral method. The mini-hole is successfully simulated, despite the simple model dynamics. The results demonstrate the correlation between column total ozone and the tropopause height and confirm the crucial role played by vertical air motions and by the meridional gradients of mid-stratospheric ozone mixing ratios for the formation of ozone mini-holes.With 12 Figures     

ATTILA: atmospheric tracer transport in a Lagrangian model

The model ATTILA has been developed to treat the global-scale transport of passive trace species in the atmosphere within the framework of a general circulation model (GCM). ATTILA runs online within the GCM ECHAM4 and advects the centroids of 80.000 to 190.000 constant mass air parcels. Each trace constituent is thereby represented by a mass mixing ratio in each parcel. ATTILA contains state-of-the-art parameterizations of convection, turbulent boundary layer mixing and inter-parcel transport, and provides an algorithm to map the tracer concentrations from the trajectories to the ECHAM model grid. The transport characteristics of ATTILA are evaluated against observations and the standard semi-Lagrangian transport scheme of ECHAM by two experiments. (1) We simulate the distribution of the short-lived tracer radon (²²²Rn) in order to examine fast vertical transport over continents, and long-range transport from the continents to remote areas. (2) We simulate the distribution of radiocarbon (¹⁴C) from nuclear weapon tests in order to examine upper tropospheric and stratospheric transport characteristics. Contrary to the semi-Lagrangian scheme, ATTILA shows a greatly reduced meridional transport in the upper troposphere and lower stratosphere, and a reduced downward flux from the stratosphere to the troposphere, especially in mid-latitudes. Since ATTILA is a numerically non-diffusive scheme, it is able to maintain steep gradients, which compare better to the observations than the rather smooth gradients produced by the semi-Lagrangian scheme.     

Concentration fluctuation measurements in tracer plumes using high and low frequency response detectors

A set of tracer experiments designed to compare two concentration fluctuation detectors and measure fluctuation statistics at high frequencies is described. A detector which has been used in several previous fluctuation experiments (the TIP photoionisation detector manufactured by Photovac of Canada) is compared with another with a much higher frequency response (the flame ionisation detector — FID — made by Cambustion of the UK). Good agreement is found and results show that the signal optimization system used in previous work with the TIP provides an accurate enhancement of the instrument output, thus improving confidence in the results of previous papers. They also confirm that the TIP detector is able to resolve most of the concentration variance in most situations of interest, but not at very short range. Measurements of the high frequency end of the fluctuation spectrum using the FID show inertial-convective subrange behaviour at frequencies not resolved by the TIP, supporting earlier work. Fluctuation spectra measured very close to the source are also shown to have a characteristic +2/3 power law behaviour (when nSc (n) is plotted against n) at lower frequencies, in agreement with theoretical predictions.     

Prediction of solar irradiance distribution in a wheat canopy using a laser technique

A new measurement technique, the “laser technique”, is proposed for providing timely information concerning the solar irradiance distribution in a vegetative canopy. The method is conceptually similar to the point quadrat method and involves measuring the height where a collimated light beam of very small cross section, aimed at the canopy, first hits foliage. To demonstrate its use, the technique is used to determine power and energy budgets of radiation intercepted by the various foliage components of a wheat (Triticum turgidum L.) canopy. It was found for the wheat canopy in the dough maturity stage that the middle and upper canopy layers intercept the bulk of the solar irradiance during the middle hours of the day, and that the soil and leaves intercept no sunlight for low sun angles.In comparison with other available measuring methods the laser technique is shown to have two major advantages. First, it is amenable to automation, and, using such an automated system, to rapid acquisition of large data sets. Second, it is applicable to canopies of various heights to about 30 m. Several potential sources of error in application of the technique, methods of error minimization, and improvements to the technique are discussed.     

Probability distribution function of a forced passive tracer in the lower stratosphere

The probability distribution function （PDF） of a passive tracer, forced by a ＂mean gradient＂, is studied. First, we take two theoretical approaches, the Lagrangian and the conditional closure formalisms, to study the PDFs of such an externally forced passive tracer. Then, we carry out numerical simulations for an idealized random flow on a sphere and for European Center for Medium-Range Weather Forecasts （ECMWF） stratospheric winds to test whether the mean-gradient model can be applied to studying stratospheric tracer mixing in midlatitude surf zones, in which a weak and poleward zonal-mean gradient is maintained by tracer leakage through polar and tropical mixing barriers, and whether the PDFs of tracer fluctuations in midlatitudes are consistent with the theoretical predictions. The numerical simulations show that when diffusive dissipation is balanced by the mean-gradient forcing, the PDF in the random flow and the Southern-Hemisphere PDFs in ECMWF winds show time-invariant exponential tails, consistent with theoretical predictions. In the Northern Hemisphere, the PDFs exhibit non-Gaussian tails. However, the PDF tails are not consistent with theoretical expectations. The long-term behavior of the PDF tails of the forced tracer is compared to that of a decaying tracer. It is found that the PDF tails of the decaying tracer are time-dependent, and evolve toward flatter than exponential.     

Long-wave radiative analysis of cloudy scattering atmospheres using a net exchange formulation

The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer equation. It was proposed in 1967 by Green [Q. J. R. Meteorol. Soc. 93 (1967) 371] for atmospheric sciences and by Hottel [H.C. Hottel, A.F. Sarofim. Radiative Transfer McGraw Hill, New York, 1967] for engineering sciences. Until now, the NEF has been used only in a very few cases for atmospheric studies. Recently we have developed a long-wave radiative code based on this formulation for a GCM of the Mars planet. Here, we will present results for the Earth atmosphere, obtained with a Monte Carlo Method based on the NEF. In this method, fluxes are not addressed any more. The basic variables are the net exchange rates (NER) between each pair of atmospheric layer (i, j), i.e. the radiative power emitted by i and absorbed by j minus the radiative power emitted by j and absorbed by i. The graphical representation of the NER matrix highlights the radiative exchanges that dominate the radiative budget of the atmosphere and allows one to have a very good insight of the radiative exchanges. Results will be presented for clear sky atmospheres with Mid-Latitude Summer and Sub-Arctic Winter temperature profiles, and for the same atmospheres with three different types of clouds. The effect of scattering on long-wave radiative exchanges will also be analysed.     

Effective crop evapotranspiration measurement using time-domain reflectometry technique in a sub-humid region

Theoretical and Applied Climatology - The primary objective of this study was to evaluate the performance of the time-domain reflectometry (TDR) technique for daily evapotranspiration estimation of...     

On the initial streakness of a dispersing tracer in two- and three-dimensional turbulence

If a spot of tracer is released into a turbulent flow, the peak concentration at some subsequent time will initially be much greater than that implied by a solution for the ensemble average concentration at fixed points. For two-dimensional turbulence three areas may be defined: (1) an area A_d related to the ensemble average concentration field; (2) an area A_p defined in terms of the relative dispersion of particles seeded into the patch after a short initial diffusion time; and (3) the area A_t occupied by tracer. It is argued that A_d grows linearly with time, whereas A_p and A_t grow exponentially; A_p faster than A_t. Thus, the concentration field is significantly streaky, even within the particle domain, until A_t becomes comparable with A_d. The time taken for this to occur is estimated; after this time, fluctuations about the ensemble average concentration field should not be greater than those given by a simple mixing length argument. In three-dimensional turbulence the volume V_t of the tracer domain grows much more rapidly than the volume V_p of the particle domain if the merging of streaks is ignored. However, V_t cannot be greater than V_p so streaks must merge and V_p can be used to provide a rough estimate of peak concentration, or concentration variance.