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1.
Richard Asselin 《大气与海洋》2013,51(2):44-55
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
2.
Bryan Kerman 《大气与海洋》2013,51(4):417-438
Abstract A new method is presented for the classification of sea ice using multi‐parametric Synthetic Aperture Radar (SAR) imagery. The local textural information, which is in essence a weighted gradient at a point, is computed in two SAR images of similar polarization but differing radar wavelength. The local information from the two images is combined at every pixel using a suggested rule for the addition of an entropy‐like measure. The resulting summation is shown to have the same negative exponential probability distribution found for the information from each separate image, confirming that the combined measure has the properties of information also. It is shown that the resulting joint information categories support a segmentation very similar to one based on consideration of the full complex scattering matrix for three wavelengths. 相似文献
3.
Abstract A numerical method for solving the advection‐diffusion equation, based on the semi‐Lagrangian algorithm of Robert (1981, 1982) is described, analysed and evaluated in comparison with other methods through a series of test problems. It is found that this method is generally better than other semi‐Lagrangian methods, and is a viable alternative to existing methods for LRTAP and other meteorological modelling because of its flexibility in application, its computational stability and its accuracy. 相似文献
4.
《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
5.
《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
6.
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
7.
Lawrence Cheng 《大气与海洋》2013,51(2):279-305
Abstract A simple, moist two‐dimensional numerical model is developed and used to study the mechanisms for the formation of hailstorm feeder clouds. By using high spatial resolution, the model is able to simulate low‐level storm outflows and reproduce the observed separation between hailstorm feeder clouds. The cold outflow emanating from the storm is induced by evaporation of precipitation. The simulations show that feeder clouds develop in response to the gravity waves generated by wind shear that is inherent in the ambient environment and/or created by the interaction between the low‐level inflow and the storm outflow. The subcloud moisture also plays an important role in the formation of a wave‐like line of feeder clouds. It contributes in the cloud development as well as in the enhancement of gravity waves. 相似文献
8.
《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
9.
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
10.
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
11.
Abstract During moist weather under stably stratified and light wind conditions very often “dot” shaped echoes, either distributed randomly or arranged in a stratified layer, have been observed on sodar echograms. They last from a couple of hours to ten hours. Their horizontal widths are up to 200 m while their vertical sizes are up to 40 m. It is argued that dot echoes represent clusters of water vapour translated by the wind in the boundary layer, the back‐scattered acoustic energy being the contribution of correlated fluctuations in temperature and humidity (turbulent mixing) in the inertial subrange. 相似文献
12.
R. Krishna Rao Vupputuri 《大气与海洋》2013,51(3):214-236
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
13.
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
14.
Abstract Airborne measurements of mean wind velocity and turbulence in the atmospheric boundary layer under wintertime conditions of cold offshore advection suggest that at a height of 50 m the mean wind speed increases with offshore distance by roughly 20% over a horizontal scale of order 10 km. Similarly, the vertical gust velocity and turbulent kinetic energy decay on scales of order 3.5 km by factors of 1.5 and 3.2, respectively. The scale of cross‐shore variations in the vertical fluxes of heat and downwind momentum is also 10 km, and the momentum flux is found to be roughly constant to 300 m, whereas the heat flux decreases with height. The stability parameter, z/L (where z = 50 m and L is the local Monin‐Obukhov length), is generally small over land but may reach order one over the warm ocean. The magnitude and horizontal length scales associated with the offshore variations in wind speed and turbulence are reasonably consistent with model results for a simple roughness change, but a more sophisticated model is required to interpret the combined effects of surface roughness and heat flux contrasts between land and sea. Comparisons between aircraft and profile‐adjusted surface measurements of wind speed indicate that Doppler biases of 1–2 m s?1 in the aircraft data caused by surface motions must be accounted for. In addition, the wind direction measurements of the Minimet anemometer buoy deployed in CASP are found to be in error by 25 ± 5°, possibly due to a misalignment of the anemometer vane. The vertical fluxes of heat and momentum show reasonably good agreement with surface estimates based on the Minimet data. 相似文献
15.
Timothy J. Dunkerton 《大气与海洋》2013,51(1):55-68
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
16.
Abstract Inertial oscillations in current records collected from May to September, 1977, at ten mooring sites 20–300 km apart in the semi‐enclosed sea off northwest British Columbia are analysed. Near‐surface oscillations were wind‐driven, clockwise rotary and circularly polarized; near‐bottom oscillations at depths of 155–330 m were clockwise rotary, less than 10% of near‐surface amplitudes, highly elliptical and poorly correlated with surface winds. In the open southwest sector of the region, near‐surface spectra possessed well‐defined peaks centred roughly 3.5% above the local inertial frequency (f), whereas spectra for the semi‐enclosed northern sector had broad peaks centred at f. The peak spectral frequency at the southeast corner of the mooring array was 6.5% below f and is linked to a Doppler shift by mean flow advection of comparatively high wavenumber inertial oscillations. A particularly vigorous wind‐generated surface “event” in mid‐June was coherent to 99% confidence over a distance of 300 km and persisted for more than 8 days at most locations and 11 days at a mooring at the edge of the continental shelf. (Typical durations for single wave groups were ～2 1/2 days.) This event, together with a similar less energetic event in August, was due to quasi‐resonant forcing by frontal winds associated with sequences of regularly spaced, eastward travelling extratropical cyclones. Estimated inertial wavelengths ranged from 300–700 km over the main portion of the sea to 85–95 km in the southeast corner. 相似文献
17.
Abstract A new approach to fetch‐limited wave studies is taken in this paper. Using data from five towers arranged along a line from the eastern shore of Lake St Clair, the differential growth between towers is explored as a function of local wave age. It is argued that this method avoids the usual fetch‐limited pitfall of inhomogeneity over long fetches and, in particular, the changes in wind speed downfetch of an abrupt roughness change. It is found that the growth rate decreases uniformly downfetch as the waves approach full development. This differential method leads to a smooth transition from rapidly growing short fetch waves to the asymptotic invariant state of full development. When the variation in wind speed after an abrupt (land to water) roughness change is taken into account, the idea of a universal fetch‐limited growth curve is called into question. 相似文献
18.
David A. Clausi 《大气与海洋》2013,51(3):183-194
Abstract Image texture interpretation is an important aspect of the computer‐assisted discrimination of Synthetic Aperture Radar (SAR) sea‐ice imagery. Co‐occurrence probabilities are the most common approach used to solve this problem. However, other texture feature extraction methods exist that have not been fully studied for their ability to interpret SAR sea‐ice imagery. Gabor filters and Markov random fields (MRF) are two such methods considered here. Classification and significance level testing shows that co‐occurrence probabilities classify the data with the highest accuracy, with Gaborfilters a close second. MRF results significantly lag Gabor and co‐occurrence results. However, the MRF features are uncorrelated with respect to co‐occurrence and Gabor features. The fused co‐occurrence/MRF feature set achieves higher performance. In addition, it is demonstrated that uniform quantization is a preferred quantization method compared to histogram equalization. 相似文献
19.
An estimate of the energy content of near‐surface internal waves in the Strait of Georgia is obtained from a combination of aerial photographs and in‐situ measurements. The role of these waves in the tidal energy budget and in the mixing processes in the Strait is discussed. 相似文献
20.
《大气与海洋》2013,51(3):129-139
Abstract Both the earth‐reflected shortwave and outgoing longwave radiation (OLR) fluxes at the top of the atmosphere (TOA) as well as surface‐absorbed solar fluxes from Canadian Regional Climate Model (CRCM) simulations of the Mackenzie River Basin for the period March 2000 to September 2003 are compared with the radiation fluxes deduced from satellite observations. The differences between the model and satellite solar fluxes at the TOA and at the surface, which are used in this paper to evaluate the CRCM performance, have opposite biases under clear skies and overcast conditions, suggesting that the surface albedo is underestimated while cloud albedo is overestimated. The slightly larger differences between the model and satellite fluxes at the surface compared to those at the TOA indicate the existence of a small positive atmospheric absorption bias in the model. The persistent overestimation of TOA reflected solar fluxes and underestimation of the surface‐absorbed solar fluxes by the CRCM under all sky conditions are consistent with the overestimation of cloud fraction by the CRCM. This results in a larger shortwave cloud radiative forcing (CRF) both at the TOA and at the surface in the CRCM simulation. The OLR from the CRCM agrees well with the satellite observations except for persistent negative biases during the winter months under all sky conditions. Under clear skies, the OLR is slightly underestimated by the CRCM during the winter months and overestimated in the other months. Under overcast conditions the OLR is underestimated by the CRCM, suggesting an underestimation of cloud‐top temperature by the CRCM. There is an improvement in differences between model and satellite fluxes compared to previously reported results largely because of changes to the treatment of the surface in the model. 相似文献