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1.
《大气与海洋》2013,51(4):221-233
Abstract

The cell‐to‐cell channel routing schemes used in General Circulation Models (GCMs) and Regional Climate Models (RCMs) are revisited. A simpler parsimonious routing scheme based on Askew's formula (1970) for computing time‐evolving channel lags is implemented and tested against observations and compared with the variable‐velocity scheme of Arora and Boer (1999). The variable‐lag routing scheme agrees very well with the variable‐velocity scheme. The variable‐lag scheme has the advantage of using fewer parameters, which is a major advantage at fine resolution over a large domain, where the uncertainty associated with parameters can be quite large.

The spatial resolutions of RCMs are much finer than those of GCMs and hence there is a need for channel routing at fine spatial resolutions. The task of extending the cell‐to‐cell routing schemes developed for large‐scale routing, as in GCMs, to finer spatial scales, as in RCMs, is addressed. The sensitivity of the variable‐lag scheme to the routing time interval is studied. The choice of the routing time interval is very critical and varies with the spatial resolution as in any hydrological model. A simple method for determining the appropriate range of routing intervals at different spatial resolutions for the variable‐lag scheme is presented.  相似文献   

2.
Abstract

The use of population‐weighted heating degree‐days provides a more realistic estimate of the potential fuel demand for residential heating on a regional, provincial or national basis than the use of unweighted, site‐specific heating degree‐day values. A population‐weighted heating degree‐day programme is already in existence in the United States. The present study has prepared the basic data required to develop a similar programme for Canada using the 1976 Canadian census statistics and the heating degree‐day values for approximately 100 synoptic weather stations across the country.  相似文献   

3.
Abstract

The relationship between the Arctic and subarctic sea‐ice concentration (SIC) anomalies, particularly those associated with the decadal‐scale Greenland and Labrador Seas “Ice and Salinity Anomalies (ISAs) “, and the overlying atmospheric circulation fluctuations is investigated using the singular value decomposition (SVD) and composite map analysis methods. The data analyzed are monthly SIC and sea level pressure (SLP) anomalies, which cover the northern hemisphere poleward of 45°N and extend over the 41‐year period 1954–1994.

The SVD1 (first) mode of the coupled variability, which accounts for 57% of the square covariance, is for the most part an atmosphere‐to‐ice forcing mode characterized by the decadal timescale. The aforementioned ISA anomalies are clearly captured by this mode whose SIC anomalies are dominated by a strong dipole across Greenland. However, as part of the same mode, there is also a weaker SIC dipole in the northern North Pacific which has opposite‐signed anomalies in the Sea of Okhotsk and the Bering Sea. It is also shown that there exists a significant negative correlation between the decadal SIC variability in the Greenland‐Barents Seas region associated with this mode and the North Atlantic Oscillation, whose spectrum also exhibits a quasi‐decadal signal.

The SVD2 mode accounts for 12% of the square covariance and shows no evidence of a dominant forcing field of either SIC or SLP. This SVD mode exhibits very low frequency (interdecadal) variability, and its co‐variability is mainly concentrated in the northern North Pacific. It appears to be a high‐latitude extension of the recently investigated interdecadal North Pacific Oscillation. The spatial structure of the second mode complements the case of the first SVD mode whose co‐variability mainly occurs in the northern North Atlantic.  相似文献   

4.
A semi‐implicit barotropic primitive equations model is integrated over a limited area with time dependent boundary conditions using the standard mesh and a finer mesh. Following a theorem by Charney, a minimum number of variables are specified as boundary conditions for the limited area integrations in order to avoid mathematical over‐specification. The comparison of coarse mesh limited area forecasts with the corresponding forecasts made over a much larger domain demonstrates that the essential features, namely the Rossby type perturbations, are handled almost perfectly. The fine mesh forecasts over the same limited area are also very good. Finally, the effect of specifying inaccurate boundary conditions, in the form of twelve‐hour forecasts, is briefly illustrated.  相似文献   

5.
《大气与海洋》2013,51(3):217-231
Abstract

An intermediate coupled model of the tropical Pacific ocean‐atmosphere system was reduced by projecting the non‐linear model onto a truncated basis set of its own empirical orthogonal functions (EOFs). For moderate coupling strengths, the simulated El Niño/Southern Oscillation (ENSO) variability consists of a dominant quasi‐quadrennial mode with a period of approximately four years and a smaller quasi‐biennial mode at a period of approximately two years. In the absence of a seasonal cycle, the leading two EOFs capture the dynamics of the leading interannual mode, with a further two EOFs being required to capture the secondary oscillation. The presence of seasonal forcing increases the EOF requirement by two, the leading pair of EOFs being dominated by the annual cycle. Normal mode analysis of the reduced models indicates that the quasi‐biennial mode manifests itself, even though it is linearly stable, by non‐linear coupling to the quasi‐quadrennial mode. The nonlinearity does not produce the quasi‐biennial signal unless the spatial degrees of freedom associated with the linear quasi‐biennial mode are present. Other linearly stable modes also couple non‐linearly to the leading interannual mode and to the seasonal cycle, but the quasi‐biennial mode is favoured over other, less‐damped linear modes because of its proximity to a multiple of the quasi‐quadrennial frequency.  相似文献   

6.
《大气与海洋》2013,51(4):443-454
Abstract

Using data from Hydro‐Québec, a spatio‐temporal summary study of cloud‐to‐ground lightning in Quebec (45°‐53 °N; 81 °‐65° W) for the 1996–2005 period was performed on a sample of close to four million lightning strokes. The annual number of lightning strokes and the ratio of negative to positive lightning (76:24) do not differ significantly from one year to the next. Despite the fact that there was an average of 239 lightning days per year, the lightning strokes were concentrated over a period of a few days. Between 1996 and 2005, 50% of the total annual lightning was distributed over 11 days, 75% over 25 days, and 90% over 44 days. Overall, the peak in the average annual cycle occurs on 15 July. Between 1996 and 2002, the number of days with at least one positive lightning stroke remained higher than the number of days with at least one negative lightning stroke. This tendency reversed from 2003 until 2005. Most of the annual lightning occurred during June, July and August. The average minimum number of lightning strokes per hour occurred at approximately 14:00 utc, and the maximum number occurred at 21:00 UTC. The ratio of positive lightning to negative remained constant throughout the day.

Both the density and the number of lightning days were mapped for the 10‐year period. The spatial distribution of lightning indicates a higher density in the southern and western parts of the study area with an average of 0.52 to 1.27 lightning strokes km?2 yr?1. The St. Lawrence Lowlands ecoregion receives the greatest number of lightning strokes annually (from 0.73 to 1.27 km?2 yr?1). The spatial distribution of the number of lightning days per year is approximately the same as that of the density. The same two gradient axes can be observed crossing from north to south and from east to west. The spatial distribution of the percentage of positive lightning strokes varies considerably in the area, ranging from 0 to 65% depending on the location. While the St. Lawrence Lowlands ecoregion has the highest density and highest number of lightning days, it also has the lowest number of positive strokes. Additional research must be done to establish a correlation between our results and environmental variables, such as topography and vegetation, as well as the spatial variations of lightning and instances of forest fire.  相似文献   

7.
《大气与海洋》2013,51(4):235-250
Abstract

A quantification of local energy propagation is employed to distinguish cases of downstream baroclinic development, as described by Orlanski and Sheldon (1995), from among 41 cold‐season cyclones that intensified strongly over the eastern North Pacific Ocean. A group of western North Pacific cyclones is employed to confirm that, in a composite sense, the eastern group is relatively less dependent on baroclinic conversion and more dependent on eddy energy that originates upstream. Based on a proposed set of criteria, about half of the individual eastern cyclones are found to be good examples of downstream baroclinic development. Almost all of this subset appears to have been influenced by a propagation of eddy energy from separate cyclones developing over the western North Pacific a day or two earlier. A primary source of energy feeding this propagation appears to be ascent in the warm sector of the upstream cyclones.  相似文献   

8.
Abstract

We analyse the trajectories of 24 deep‐drogued, satellite‐tracked drifters launched between 50 and 52°N in the northeast Pacific during June and October 1987. Three aspects of the observed motions at the drogue depths of 100 to 120 m are studied: (i) the spatial structure of the mean and variance velocity fields; (ii) the dispersion and eddy diffusion characteristics of the fluctuating motions; and (iii) the properties of selected mesoscale eddies.

The mean Lagrangian velocity field is consistent with the mean flow pattern derived from the historical dynamic height topography. Fluctuating motions within the region are dominated by mesoscale eddies and meanders. Several instances of persistent O(100 days) westward flowing countercurrents were also observed. Based on the Lagrangian integral time‐ and length scales, drifter motions become decorrelated within a period of 10 days and a separation of 100 km. The mean zonal and meridional integral time‐scales of 4.5 and 3.6 days, respectively, are nearly identical with those obtained by Krauss and Böning (1987) from deep‐drogued drifter tracks in the North Atlantic. Because of the relatively small (<100 cm2 s?2) kinetic energy values in the northeast Pacific, the corresponding mean Lagrangian length scales of 29.4 and 29.9 km are roughly half those for the Atlantic.

The observed drifter dispersion is generally consistent with Taylor's (1921) theory for single‐particle dispersion in homogeneous isotropic turbulence. Estimates obtained using 476 pseudo‐drifter tracks generated from the original records indicate that the dispersion increases linearly with time, t, within the first 3 to 5 days of launch and subsequently increases as t1/2 (the random‐walk regime) within 10 days of launch. The respective peak zonal and meridional eddy diffusion coefficients of 4.1 × and 3.8 × 107 cm2 s?1 are reached within 30 days of deployment. Similar estimates for the peak eddy diffusivities are obtained using dispersion curves for sets of 4 drifters launched at the same location during the June and October deployments. The dispersion of these clusters followed an exponential rather than a t1/2 dependence over the first 70 days after release.

Eddies are predominantly clockwise rotary and are characterized by radii of 26 ± 16 km, periods of rotation of 16.0 ± 5.2 days, and azimuthal current speeds of 12.7 ± 8.6 cm s?1. One eddy was tracked for over 10 months. Oceanographic data collected during the October deployment period showed the eddies have vertical extents of 500 to 700 m and are linked to isotherm depressions of over 100 m in the main pycnocline. All eddies in the bifurcation zone propagate to the west at roughly 1.5 ± 0.4 cm s?1 counter to the prevailing mean flow and winds. These speeds are consistent with the westward phase speeds of first mode baroclinic planetary (Rossby) waves.  相似文献   

9.
Abstract

A static decision‐analytic method is used to investigate the economic value of bivariate ‐ precipitation and temperature ‐ seasonal forecasts of the form currently issued by the U.S. National Weather Service. This method is applied to a corn versus spring wheat choice‐of‐crop decision‐making problem by considering a transect of four counties across the northwestern margin of the North American corn belt. Numerical results indicate that seasonal forecasts of current quality can be of appreciable value (≥$1/ha) for some locations when the optimal action chosen on the basis of climatological information is only marginally preferred to another action. Increases in forecast value follow from hypothetical increases in the quality of both the precipitation and temperature components of the forecasts in the spring wheat region, whereas forecast value increases primarily as a function of the quality of the precipitation forecasts alone in the corn belt region. The results are very sensitive to absolute and relative crop prices.  相似文献   

10.
《大气与海洋》2013,51(3):361-376
Abstract

The goal of this study is to evaluate the impact of incorporating the marine surface winds retrieved from the ERS‐2 scatterometer in the Canadian three‐dimensional variational analysis system, (3D‐var). The aspects of the 3D‐var most relevant to the assimilation of surface ‐wind observations and a general method for resolving the directional ambiguity of the retrieved scatterometer ‐winds are first described. A comparison ‐with 6‐h forecasted winds is then made to demonstrate that these data are of high quality, but exhibit a speed bias that can be removed by increasing their amplitudes by about 5%. The analysis increment from a single scatterometer wind observation is calculated to illustrate the response of the 3D‐var to surface wind observations. As a consequence of the forecast error covariance model, the assimilation of surface wind observations produces meteorologically consistent increments for both the rotational and divergent wind components and the mass field. The results from a series of cross‐validation experiments using ship‐based wind data demonstrate a positive impact of assimilating scatterometer winds and the effectiveness of a simple method for estimating and removing the speed bias. The impact of assimilating scatterometer data within a short assimilation cycle is also evaluated. Overall, the results show that including scatterometer data in the analysis decreases the 6‐h forecast error of surface wind by 13%. Over the northern extra‐tropics the improvement is only 4% and for the southern extra‐tropics it is 16%. Results from a series of two‐day forecasts produced using the analyses from the assimilation cycles with and without retrieved scatterometer winds included are also presented. Using radiosonde observations at 850 hPa, 500 hPa, 250 hPa and 100 hPafor verification, the impact on the forecasts is nearly neutral in the northern hemisphere and the tropics. Conversely, a significant positive impact is found on both wind and mass fields in the southern hemisphere over the entire two‐day forecast.  相似文献   

11.
Abstract

During November 1976 to February 1977 near‐surface wind, current and temperature measurements were made at three sites along the Strait of Juan de Fuca. Strong tidal currents and major intrusions of warmer, fresher offshore coastal water were superimposed upon the estuarine circulation of near‐surface seaward flow. The r.m.s. amplitudes of the diurnal and semidiurnal tidal currents were ～30 cms‐1 and 30–47 cm s‐1, respectively. The vector‐mean flow at 4 m‐depth was seaward and decreased in speed from 28 cm s‐1 at 74 km from the entrance to 9 cm s‐1 at 11 km from the entrance. On five occasions intrusions of 1–3 C warmer northeast Pacific coastal water occurred for durations of 1–10 days. The 25 cm s‐1 up‐strait speed of the intrusive lens agreed to within 20% of the gravity current speed computed from Benjamin's (1968) hydraulic model. The near‐surface currents associated with the intrusions and the southerly coastal winds were significantly correlated, indicating that the intrusions were initiated when shoreward Ekman currents advected Pacific coastal water into the Strait. The reversals were not significantly coherent with the along‐strait sea surface slope measured along the north side of the Strait nor were they strongly related to local wind forcing.  相似文献   

12.
《大气与海洋》2013,51(4):251-265
Abstract

In this paper, 441 Conductivity Temperature Depth (CTD) casts from the North Water (NOW) Polynya study were used to calculate geostrophic currents between the 10 and 200 dbar surface during April, May and June 1998. Results for April and May indicated a surface intensified southward flow of 10 to 15 cm s–1 with a small return flow along the Greenland coast in agreement with inferred currents described by Melling et al. (2001) and surface ice drifts found by Wilson et al. (2001). Southward transports at this time were 0.4–0.55 Sv in April and May. In June, however, surface currents diminished markedly: southward transports declined to 0.1–0.35 Sv, coincident with a decrease in directly measured winds over the polynya and in the surface barometric pressure difference between Grise Fjord and the Carey Islands that was used as a surrogate for the local north wind speed. There was no evident decrease in air pressure difference between Resolute and Grise Fjord, indicative of the strength of the north wind over the eastern Arctic in general. The results are consistent with present thinking that the NOW Polynya is primarily a latent heat polynya, forced by dominant north winds. The idea, broached here, is that the polynya creates its own microclimate which sustains the polynya's ice‐free condition after its initial formation. The mechanism is identified by an anomalous low pressure region associated with surface buoyancy flux in the polynya and is pursued through the application of a simple geostrophic adjustment model that suggests two self‐sustaining mechanisms. Firstly, the frontal intrusion of the cold ambient terrestrial air mass drives a significant surface wind that transports frazil ice to the edge of the polynya before it can congeal. Secondly, rotation at these high latitudes restricts the penetration of the front into the polynya, essentially insulating the centre from freezing temperatures.  相似文献   

13.
A mean meridional circulation model of the stratosphere, incorporating radiative heating and photochemistry of the oxygen‐hydrogen‐nitrogen atmosphere, is used to simulate the meridional distributions of O3, HOX, N2O,NOX, temperature and the three components of mean motion for the summer and winter seasons under steady‐state conditions. The results are generally in good agreement with the available observations in the normal stratosphere. The model has been applied to assess the effects of water vapour and nitrogen oxide perturbations resulting from aircraft emissions in the stratosphere. It is found that a fleet of 500 Boeing‐type sst's, flying at 20 km and 45°N in the summer hemisphere and inserting NOx at a rate of 1.8 megatons per year, has the effect of reducing the global total ozone by 14.7%. Similar calculations for 342 Concorde/TU‐114's, cruising at 17 km and injecting NOx at a rate of 0.35 megatons per year, show a global‐average total‐ozone reduction of 1.85%. Although water vapour is considered important, because of its ability to convert NO2 into HNO3, the direct effect on global‐average total‐ozone reduction resulting from the 100% increase in the stratospheric water content is less than 1%. The changes in the chemical structure (HO^NO^), temperature, and mean motions associated with the ozone reduction are also investigated in the case of the 1.8‐megaton‐per‐year NOX perturbation. It is shown that the reduced meridional temperature gradient in the middle and upper stratosphere resulting from the NOx perturbation leads to the weakening of the tropical easterly jet in the summer hemisphere and mid‐latitude westerlies in the winter season.

The sensitivity of the model solutions to an alternate choice of input parameters (diffusion coefficients and solar photodissociation data) is tested and the main deficiency of the model is pointed out.  相似文献   

14.
Abstract

On‐site and off‐site forecasts for the CFB Summerside aerodrome, written coincidentally over a six‐week period, are verified using the Ranked Probability Score and compared by means of a two‐tailed test of paired differences. The results show that, at a 10% level of significance, forecasts made on‐site are significantly better than forecasts made off‐site for at least four hours into the forecast period. When compared with persistence, both forecast offices were superior. At no time did the persistence forecasts score significantly better than those issued by a meteorologist. When compared with climatology, on‐site forecasts were significantly better for the first nine hours, while off‐site forecasts were significantly better for the first ten hours.  相似文献   

15.
《大气与海洋》2013,51(4):227-250
Abstract

The mid‐latitude ocean's response to time‐dependent zonal wind‐stress forcing is studied using a reduced‐gravity, 1.5‐layer, shallow‐water model in two rectangular ocean basins of different sizes. The small basin is 1000 km × 2000 km and the larger one is 3000 km × 2010 km; the aspect ratio of the larger basin is quite similar to that of the North Atlantic between 20°N and 60°N. The parameter dependence of the model solutions and their spatio‐temporal variability subject to time‐independent wind stress forcing serve as the reference against which the results for time‐dependent forcing are compared.

For the time‐dependent forcing case, three zonal‐wind profiles that mimic the seasonal cycle are considered in this study: (1) a fixed‐profile wind‐stress forcing with periodically varying intensity; (2) a wind‐stress profile with fixed intensity, but north–south migration of the mid‐latitude westerly wind maximum; and (3) a north–south migrating profile with periodically varying intensity. Results of the small‐basin simulations show the intrinsic variability found for time‐independent forcing to persist when the intensity of the wind forcing varies periodically. It thus appears that the physics behind the upper ocean's variability is mainly controlled by internal dynamics, although the solutions’ spatial patterns are now more complex, due to the interaction between the external and internal modes of variability. The north–south migration of wind forcing, however, does inhibit the inertial recirculation; its suppression increases with the amplitude of north–south migration in the wind‐stress forcing.

Model solutions in the larger rectangular basin and at smaller viscosity exhibit more realistic recirculation gyres, with a small meridional‐to‐zonal aspect ratio, and an elongated eastward jet; the low‐frequency variability of these solutions is dominated by periodicities of 14 and 6–7 years. Simulations performed in this setting with a wind‐stress profile that involves seasonal variations of realistic amplitude in both the intensity and the position of the atmospheric jet show the seven‐year periodicity in the oceanic circulation to be robust. The intrinsic variability is reinforced by the periodic variations in the jet's intensity and weakened by periodic variations in the meridional position; the two effects cancel, roughly speaking, thus preserving the overall characteristics of the seven‐year mode.  相似文献   

16.
Abstract

It is shown that oscillating mean flow solutions exist in the one‐dimensional Holton‐Lindzen (1972) model in the presence of a single Kelvin wave, mean flow diffusion, and an easterly zonal force per unit mass that is constant in height and time except at those points in the time‐height cross‐section where the latitudinally‐integrated mean flow is less than some prescribed easterly value. The latter forcing is intended to crudely represent the absorption of quasi‐stationary planetary Rossby waves at the tropical zero‐wind line. Our results suggest an alternative, and somewhat simpler, possible interpretation of the quasi‐biennial mean zonal wind oscillation in the equatorial lower stratosphere.  相似文献   

17.
《大气与海洋》2013,51(4):325-338
Abstract

A portable ground‐based instrument has been constructed for the automated measurement of vertical column abundances of a number of gases pertinent to stratospheric ozone chemistry. The instrumentation is described in this paper and results are presented from the first set of field measurements, made during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) 1998 field campaign at Vanscoy, Saskatchewan, Canada. Zenith‐sky spectra in the near ultraviolet and visible wavelength regions were recorded for a period of seven days, prior to and following the launch of the MANTRA balloon on 24 August 1998. The spectra were then analysed using the differential optical absorption spectroscopy (DOAS) technique in conjunction with a radiative transfer model to determine vertical column amounts of ozone and NO2. Ozone measurements compared favourably with concurrent observations by ozonesondes, a Brewer spectrophotometer, and satellite instruments. Vertical NO2 columns were in broad agreement with those determined by the Global Ozone Monitoring Experiment (GOME) satellite instrument.  相似文献   

18.
A global baroclinic primitive equation model using the spectral technique has been constructed for short‐ and medium‐range numerical weather prediction. The spectral technique, which is a special case of the Galerkin method, employs spherical harmonic basis functions in the evaluation of all horizontal derivatives. The use of a transform technique allows all the horizontal operations to be performed efficiently and allows physical processes to be evaluated in real space. The model employs a semi‐implicit algorithm for time integration and finite differencing in the vertical. Physical processes include orography, moist convection, large scale precipitation and boundary layer processes.  相似文献   

19.
Abstract

Data assimilation in numerical weather forecasting corrects current forecast values by subtracting a portion of interpolated forecast‐minus‐observation differences at the points of a three‐dimensional grid. Deviations used in updating a forecast data field are forecast errors obtained or derived from observations available at update time. When observations are missing at mandatory levels, construction of full vertical soundings by interpolation introduces extraneous errors. The present paper is concerned with determination of the error in vertical extrapolations of surface winds, and of aircraft and satellite cloud‐tracked winds. In addition it examines the effect on accuracy of using location‐specific statistics compared to averaged statistics as the basis for the interpolation weighting scheme and compares errors of one‐ and two‐variable interpolations.

Interpolation accuracy tests demonstrate the influence of the interpolation scheme on the quality of interpolated information used in forecast updating. The results show that the level of accuracy exceeds the benchmark provided by monthly mean forecast error values only with bivariate interpolation of wind components from off‐level data sources.  相似文献   

20.
Abstract

The conclusion that there is no vorticity and no vorticity transport in a boundary current, assuming no slip at the boundary, is shown to be robust to the level (L1/L2)2, where L1 is a characteristic width and L2 a characteristic length of the boundary current. Vorticity transport into the interior from the boundary is shown to be related to the stress gradient at the boundary, which is in turn equal to the pressure gradient along the boundary. On western boundaries the flow is down the pressure gradient, a circumstance that usually leads to thin boundary layers. It is shown that in the inner, fractional, boundary layer both the stress and the energy dissipation depend on the thickness of the inner layer and become small when it is thin.  相似文献   

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