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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.
《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
3.
《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
4.
Abstract The effects of condensational heating on the semi‐geostrophic dynamics of frontogenesis are studied using a two‐dimensional deformation model. The model includes water vapour and allows the formation of stratiform clouds. Analysis and numerical results show that heating due to stratiform clouds has the effect of reducing stability to slantwise convection, as found in previous studies (Thorpe and Emanuel, 1985). In addition, heating‐induced potential vorticity and temperature anomalies play a very important role in the frontal circulation. The ageostrophic flow induced by these anomalies tends to reinforce the effect of heating and increases the strength of frontal cloud. The model is also able to produce the low‐level jet maximum ahead of a cold front at an elevated level, in agreement with observations, owing to the explicit condensation scheme used in the model. 相似文献
5.
《大气与海洋》2013,51(4):445-463
Abstract A regional model of the sub‐polar North Atlantic has been developed for use in process and variability studies of this important high‐latitude area. Open boundary conditions handle connections with the rest of the Atlantic Ocean at 38°N, while buffer zones are used in the northern boundary regions. Extensive testing and experimentation has led to a model which can reproduce major elements of the hydrography and circulation in the region, although limitations exist. A key model feature is the inclusion of a finite volume partial cell topographic representation that significantly improves the structure of the underlying bottom topography. Improvements include a tighter and sharper gyre structure, increased transports, sub‐polar mode water formation sites linked to the topographic slope along the outside of the gyre and a more reasonable representation of Labrador Sea water properties and dispersal pathways. The choice of inflow conditions for the open southern boundary affects the deep western boundary current, as well as the representation of Mediterranean Water, which has a significant effect on Labrador Sea water in the eastern basin. 相似文献
6.
The spectral global baroclinic primitive equation model described in Part I of this paper has been extensively tested. The model has been run daily from operational analyses for over a year. From this large sample of forecasts, verification statistics have been collected and compared with similar statistics collected from three competitive grid‐point models. The spectral model is also compared with the grid‐point models in a synoptic case study. A second case study demonstrates the effect of horizontal resolution and physical effects on spectral model forecasts. The results of these experiments demonstrate that the spectral model is highly competitive with other models, in terms of both accuracy and computational efficiency. On 18 February 1976 the spectral model became the operational Canadian large‐scale forecast model. 相似文献
7.
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
8.
Abstract High‐resolution versions of the Canadian operational regional finite‐element model (RFE) have been developed to assess their potential in simulating mesoscale, difficult‐to‐forecast and potentially dangerous weather systems commonly referred to as polar lows. The operational (1989) 100‐km version and a 50‐km version of the model have been run for two different polar low cases: one over Hudson Bay and one over Davis Strait. More integrations have also been performed on the Hudson Bay event both at 50 and 25 km to assess the model sensitivity to ice cover. As expected, the reduction in spatial truncation errors provided by the increase in resolution results in a better simulation of the systems. Moreover, when run at higher resolutions the model shows a significant sensitivity to ice cover. The results of the ice‐cover experiments also put into perspective the interaction between the heat and moisture fluxes at the surface, the low‐level wind structure, and the relation of these to the development of the polar low. This study suggests that the improved forecast accuracy obtained from increased resolution is limited by the correctness of the analysis of the ice cover, which acts as a stationary forcing for the entire forecast period. 相似文献
9.
A previous study (Suckling and Hay, 1976a) described a method for calculating hourly values of the direct and diffuse solar radiation for cloudless sky conditions. This paper presents an extension which incorporates the effects of clouds through the use of hourly values of cloud amount and type for up to four layers and hourly bright sunshine totals. The latter data provide a more accurate measure of the length of time the direct radiation of the sun is not attenuated by cloud. On an average, the cloud layer‐sunshine (CLS) model estimated daily total solar radiation at five Canadian locations to within ±15 per cent of the measured values. This was an improvement over an earlier model (Davies et al., 1975) based on cloud data alone, but the relative advantage, as well as the overall errors themselves, were diminished as the averaging period was increased to five and ten days. The CLS model has the additional advantage of calculating the separate direct and diffuse components of the total solar radiation. 相似文献
10.
T.J. Simons 《大气与海洋》2013,51(4):169-179
Numerical models were used to compute water circulations throughout the 1970 shipping season for Lake Erie and for the 1972 International Field Year on Lake Ontario. Simultaneous computations of surface elevations were compared with observed water levels to adjust the model results after the fact. As a by‐product of these simultations, effective stress coefficients over water can be estimated. The results support earlier evidence that the effective wind stress over water is larger than indicated by atmospheric boundary layer measurements. 相似文献
11.
Abstract In a series of 5‐day forecasts with a 3‐layer/2.8° hemispheric model, horizontal diffusion schemes of the second and fourth degree are compared with a numericalfilter technique. The results, which are discussed mainly in terms of spectral energetics in zonal wavenumber space, indicate that fourth‐degree diffusion is more scale selective than second‐degree and equivalent to filtering. The seventh‐order filter applied only intermittently to the prognostic variables is superior to fourth‐degree diffusion from the viewpoint of computational economy. Excessive dissipation of the long waves may inhibit the production of eddy kinetic energy from eddy available potential energy. 相似文献
12.
Abstract When a semi‐Lagrangian integration scheme was introduced into a global spectral model by Ritchie (1988), large errors developed in the neighbourhood of the poles. It took approximately 6 months of effort to diagnose the problem and find an appropriate correction. The method that was used to diagnose the source of error was quite tedious, but it was successful. Furthermore, it seems that this method could be used to diagnose other sources of error that occasionally show up in numerical integrations. For this reason, it was felt that this method should be described in a separate article. This is the main objective of the presentation that follows. An integration is carried out with the original version of the model and some results are presented in order to illustrate these errors. In order to identify their exact cause, the model is stripped down in two steps. At each step, some checks are made to ensure that the errors are stillpresent in the degraded version of the model. In the end, the remaining equations are sufficiently simple to ensure that the cause of the errors becomes obvious. The diagnosis immediately suggests some alternative computational designs. A modification that completely eliminates these errors is then proposed and tested. An integration with the modified spectral model is carried out and results are presented to show that the errors have disappeared. 相似文献
13.
Abstract The climatology of the Canadian Climate Centre atmospheric general circulation model is presented and compared with the observed climatology of the atmosphere. The model climatology is obtained from a simulation over five annual cycles and the results are presented in terms of averages for the four seasons. The climatology of the model is discussed in terms of zonally and time averaged values of the primary atmospheric variables as well as in terms of the spatial distributions of the important surface parameters and of the rotational and divergent components of the tropospheric flow. Some measure of model variability is also presented. The model is generally quite successful in reproducing the mean observed climatology of the atmosphere. 相似文献
14.
《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
15.
Billie L. Taylor 《大气与海洋》2013,51(3):261-268
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
16.
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
17.
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
18.
R.K.R. Vupputuri 《大气与海洋》2013,51(4):728-738
Abstract A 2‐D time‐dependent Eulerian model that takes into account the major interaction between chemistry and dynamical transports in the stratosphere is used to simulate, in a self‐consistent manner, the anomalous spring variations of ozone, temperature and other relevant minor constituents in the southern polar region. The normal transport coefficients and the standard chemistry have been altered to represent the anomalous dynamical and chemical conditions in the extremely cold lower stratosphere of Southern Hemisphere (south of 65° S) spring. The results show that it is necessary to invoke both dynamical and chemical mechanisms in order to explain the observed rapid spring decline of total ozone in the southern polar region. 相似文献
19.
《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
20.
《大气与海洋》2013,51(4):455-471
Abstract Integrated atmospheric moisture has been derived from a network of Global Positioning System (GPS) receivers established in southern Alberta. GPS receivers and post‐processing techniques provide the ability to estimate integrated precipitable water vapour (PWV) at temporal and spatial scales not usually available using conventional observational techniques and without costly expendables. GPS‐derived PWV was evaluated during the Alberta GPS Atmospheric Moisture Evaluation (A‐GAME) using nearby radiosonde observations from the Airdrie, Olds‐Didsbury and Sundre airports during field campaigns in the summers of 2003 and 2004. For the 2004 A‐GAME period, the regional (15 km) Global Environmental Multiscale model (GEM)‐modelled PWV was compared to the GPS derived PWV using a distance weighting approach. GEM model performance was assessed with regards to prognosis time (from 0 to 9 hours), grid cell elevation, location and the presence of storms in the study region. The results show that there is good agreement between radiosonde‐derived PWV and PWV derived from nearby GPS sites with correlations (r2) ranging from 0.76 to 0.84; the GPS‐derived PWV showed a small dry bias averaging 0.6 mm. When compared to GPS‐derived PWV, GEM model performance was found to be favourable out to the hour‐3 prognosis with an overall correlation (r2) of 0.63. Performance decreased with increasing prognosis time and as a result of the presence of storm activity in the study region but did not decrease with increasing grid cell elevation. 相似文献