摘要:

Current understanding of the possible nature of climatic change at the regional scale is limited by the spatial resolution of General Circulation Models (GCM). The use of GCM outputs without correction linked to the spatial variability of the variables can bring significant errors in their utilization at the regional scale. The potential of the Canadian GCM for regional applications in Quebec has been analysed by comparison to the climatic normals of temperature and precipitation, measured over the Quebec climatological network, on an annual and seasonal basis. This analysis has been undertaken with the support of a geographical information system (GIS) (PAMAP). In summary, a difference between the climatic normal and the GCM output has been estimated at 20% for temperature and 30% for precipitation. We present an analysis of a corrected regionalized scenario for the province of Quebec of the possible climatic change simulated by the Canadian GCM under the hypothesis of a doubling of atmospheric CO2. Results show an increase of the annual average temperature of 4° C for summer and 6°C for winter, associated with an average increase of 80 mm (10%) in annual precipitation, reaching 25% in some regions.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649538

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified adjustment time scale.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649539

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In this study, a 24‐h high‐resolution numerical prediction of a prefrontal squall line associated with the 14 July 1987 Montreal flood is employed to investigate the origin and role of mesoscale gravity waves in the development of the squall system. The 24‐h integration using an improved mesoscale version of the Canadian regional finite‐element model is first validated against available observations; then non‐observable features are diagnosed to reveal the relationship between deep convection and gravity wave events. It is shown that the model reproduces well many aspects of the squall line, such as the propagation and organization of the convective system, as well as its associated precipitation. It is found that gravity waves are first excited near Lake Erie, following the initiation of early convective activity. Then, these waves propagate eastward and northeastward at speeds of 20 and 35 m s‐1, respectively. As the waves propagate downstream, deep convection radiates rapidly behind the wave trough axis, forming a long line of squall convection. Because the squall line moves with the gravity waves in a “phase‐locked” manner, deep convection has a significant influence on the structure and amplitude of the gravity waves. The sensitivity of the wave‐squall prediction to various parameters in convective parameterization is also examined.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649540

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In this study, the internal circulation structures of the 14 July 1987 intense mesoscale convective system (MCS) are investigated using an improved high‐resolution version of the Canadian regional finite‐element model. It is found that although the MCS is characterized by a leading convective line followed by a trailing stratiform rainband, the associated circulation structures differ substantially from those in the classical midlatitude squall system. These include the rapid propagation and separation of the leading convection from the trailing rainband, the development of a surface‐based instead of an elevated rear‐to‐front descending flow and a shallow front‐to‐rear ascending flow associated with the stratiform precipitation, the generation of low‐ and mid‐level rather than mid‐ to upper‐level stratiform cloudiness and the development of a strong anticyclonic vorticity band at the back edge of the stratiform region. It is shown that the trailing stratiform rainband is dynamically forced by frontogenetical processes, and aided by the release of conditional symmetric instability and local orographical lifting. The intense leading and trailing circulations result from latent heat released by the convective and explicit cloud schemes, respectively. Sensitivity experiments reveal that the proper coupling of these two cloud schemes is instrumental in obtaining a realistic prediction of the above‐mentioned various mesoscale components. Vorticity budget calculations show that tilting of horizontal vorticity contributes the most to the amplification of the anticyclonic vorticity band, particularly during the squall's incipient stage. The sensitivity of the simulated squall system to other model physical parameters is also examined.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649541

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Historically, tornadoes often have been modelled in open domains in which the boundary conditions on the top of the modelled vortex are imposed, specified by flow‐straightening, or extrapolated from upstream. In current three‐dimensional modelling efforts, reasons of computational efficiency still impose a need to model the tornado in isolation from the storm using what could be called a “one‐way nested model”. The effect of such isolation on a tornado simulation is investigated here with a redundant one‐way nested model embedded within a high‐resolution, axisymmetric storm model. The tornado in the nested model is compared with the tornado in the complete model. (The complete model by itself can also be thought of as containing a “fully interactive, two‐way nested model"). The use of extrapolated or straightened outflow boundary conditions on the nested model is shown to have a profound effect on the tornado solution. Maximum windspeeds in the nested model can differ by more than a factor of two from those in the corresponding region of the complete model. Only subcritical vortices are considered here.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649542

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new infrared (IR) radiation scheme for extending the CCC/GCM (Canadian Climate Center General Circulation Model) into the middle atmosphere is proposed. It combines the previous CCC/GCM radiation scheme including the effects of H2O, CO2, O3, CH4, N2O, CFC11 and CFC12, clouds and aerosols, with a new computationally efficient matrix parameterization for the cooling rate in the middle atmosphere for both LTE (local thermodynamic equilibrium) and non‐LTE layers. The matrix parameterization includes the effects of both the 15 μm CO2and the 9.6 μm O3bands and provides a proper lower boundary condition for the non‐LTE recurrence formula. The new scheme shows satisfactory agreement with line‐by‐line calculations. The absolute error does not exceed 0.8 ? day‐1for vastly different atmospheric conditions. Introducing the new radiation module into the CCC/GCM results in deviations of the simulated temperature from the CIRA‐1986 model of not more than 10 ? throughout most of the altitude‐latitude domain.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649543

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Monthly‐mean fluxes computed from the Comprehensive Ocean‐Atmosphere Data Set (COADS) are used to describe the variability in air‐sea heat exchange over the eastern Newfoundland Shelf and examine its role in ocean temperature variability. Attention is focussed on the climatological annual variation and interannual (monthly) anomalies of fluxes and ocean temperature during 1951–1988 in two areas: Station 27 on the inner shelf and the central Grand Bank. A vertical diffusion model is used to quantify the contribution of the fluxes to ocean temperature variability, and examine the temporal and vertical structure of the vertical eddy diffusivity. For the climatological annual temperature variation, the model indicates that local air‐sea fluxes can account for 89% and 77% of the variance for Sta. 27 and the central Grand Bank, respectively, although there are also influences from ocean advection. Strong seasonal and vertical variations in the diffusivity are suggested by the model, with values generally in the range (0.5 — 20) × 10‐4m s‐1. For interannual ocean temperature variability, the model and flux estimates indicate that local air‐sea fluxes can account for only a small fraction of the observed variability, although there is a strong suggestion of coupled atmosphere‐ocean temperature variability, particularly at low frequencies. Ocean advection, sea ice influences and flux inaccuracies are discussed as possible factors in the interannual variability results. Collectively, the study results indicate that ocean temperature variability on the eastern Newfoundland Shelf is part of an atmosphere‐ice‐ocean interconnection involving both regional and large‐scale processes.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649544

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Observations in both the ice and slush layers suggest that sea water intrudes into the snow layer following a snow storm. Ice temperature values recorded at 1 cm below the snow‐ice interface show that the upward flux of sea water is of short duration. This is followed by a period of intense brine drainage characterized by the migration of a salty brine layer, with salinities up to 42 psu. These results suggest that a snow storm induces a complete (upward) flushing of the brine channel network and major modifications of snow and ice characteristics.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649545

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

It is shown that the parameters in a quasi‐three‐dimensional numerical tidal model can be estimated accurately by assimilation of data from current meters and tide gauges. The tidal model considered is a semi‐linearized one in which advective nonlinearities are neglected but nonlinear bottom friction is included. The parameters estimated are the eddy viscosity, bottom friction coefficient, water depth and wind drag coefficient, the first three of which are allowed to be position‐dependent. The adjoint method is used to construct the gradient of a cost function defined as a certain norm of the difference between computed and observed current and surface elevations. On the basis of a number of tests, it is shown that very effective estimation of the nodal values of the parameters can be achieved using the current data either alone or in combination with elevation data. When random errors are introduced into the data, the estimated parameters are quite sensitive to the magnitude of the errors, and in particular the eddy viscosity is unstably sensitive. The sensitivity of the viscosity can be stabilized by incorporating an appropriate penalty term in the cost function or alternatively by reducing the number of estimated viscosity values via a finite element approximation. Once stabilized, the sensitivity of the estimates to data errors is significantly reduced by assimilating a longer data record.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649546

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A depth‐independent numerical model of the Juan de Fuca/Strait of Georgia system reproduces the broad structure of the observed depth‐averaged residual circulation in the Central Strait of Georgia but underestimates its magnitude (Marinone and Fyfe, 1992). Here we present some new calculations based on a re‐parameterization of the unresolved eddies in terms of “statistical dynamical tendencies” instead of the previous eddy‐viscosity treatment. With the new parameterization, the simulated time‐mean flow is closer to the observed circulation both in structure and magnitude. While not specifically designed to do so, the new parameterization also leads to a modest improvement in the low‐pass filtered component of the flow. Based on these results, the depth‐averaged residual currents in the region are conjectured to involve a four‐way balance between the hitherto ignored effect of “statistical dynamical tendencies” and conventional tidal, atmospheric and buoyancy forcing.

ISSN:0705-5900    

DOI:10.1080/07055900.1995.9649547

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor