The oil spill behaviour model of the Canadian atmospheric environment service Part I: Theory and model evaluation

Abstract

This paper describes a model to simulate the behaviour of oil spills in marine environments. The model includes parametrizations of various physical processes representing the movement and weathering of an oil slick. The movement of the slick is affected by wind‐driven, tidal and residual water currents. Turbulent dispersion is an important mechanism influencing the horizontal spreading of the slick for time periods greater than about a day.

The model is used to simulate successfully the movement of spill‐following buoys deployed in the Bay of Fundy, where some of the strongest tidal currents in the world occur. The ability of the model to simulate the horizontal spreading of an oil slick was evaluated with observed data from the Argo Merchant oil spill for a 10‐day period. It was found that the observed shape and extent of the spill could be fairly well described by the parametrization of turbulent dispersion effects.     

Climate and climate change in western canada as simulated by the Canadian regional climate model

Abstract

A þrst climate simulation performed with the novel Canadian Regional Climate Model (CRCM) is presented. The CRCM is based on fully elastic non‐hydrostatic þeld equations, which are solved with an efþcient semi‐implicit semi‐Lagrangian (SISL) marching algorithm, and on the parametrization package of subgrid‐scale physical effects of the second‐generation Canadian Global Climate Model (GCMII). Two 5‐year integrations of the CRCM nested with GCMII simulated data as lateral boundary conditions are made for conditions corresponding to current and doubled CO₂ scenarios. For these simulations the CRCM used a grid size of 45 km on a polar‐stereographic projection, 20 scaled‐height levels and a time step of 15 min; the nesting GCMII has a spectral truncation of T32, 10 hybrid‐pressure levels and a time step of 20 min. These simulations serve to document: (1) the suitability of the SISL numerical scheme for regional climate modelling, (2) the use of GCMII physics at much higher resolution than in the nesting model, (3) the ability of the CRCM to add realistic regional‐scale climate information to global model simulations, and (4) the climate of the CRCM compared to that of GCMII under two greenhouse gases (GHG) scenarios.     

Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian climate centre general circulation model

Abstract

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.

The performance of the scheme and its sensitivity to choices of disposable parameters is illustrated by presenting results from a series of idealized single‐column model tests. These tests demonstrate that the scheme permits establishment of a quasi‐equilibrium between large‐scale forcing and convective response. However, it is also shown that the strength of convective downdrafts is an important factor in determining the nature of the equilibrium state. Relatively strong down‐drafts give rise to an unsteady irregularly fluctuating state characterized by alternate periods of deep and shallow convection.

The effect of using the scheme for GCM climate simulations is illustrated by presenting selected results of a multi‐year simulation carried out with the Canadian Climate Centre GCM using the new parameterization (the CONV simulation). Comparison of these results with those for a climate simulation made with the standard model (the CONTROL simulation, as documented by McFarlane et al., 1992) reveals the importance of other parameterized processes in determining the ultimate effect of introducing the new convective scheme. The radiative response to changes in the cloudiness regime is particularly important in this regard.     

The life cycle of the intense IOP‐14 storm during CASPII. Part I: Analysis and simulations

Abstract

In this study, a 5‐day life‐cycle of the IOP‐14 storm during CASP II is examined using conventional observations and numerical simulations with a mesoscale version of the Canadian Regional Finite‐Element (RFE) model. Observational analysis reveals that the IOP‐14 storm forms from a lee trough, occurring along a strong baroclinic zone with an intense frontogenetic deformation, that interacts with an upper‐level travelling short‐wave trough across the Canadian Rockies. Then the storm experiences a slow, but nearly steady, growth while traversing the North American continent. It deepens explosively as it moves into the Atlantic Ocean. It appears that i) the enhanced large‐scale baroclinicity due to land‐sea temperature contrasts, ii) the tremendous latent heat release due to the transport of high‐θ_e air from the marine boundary layer, Hi) the decrease of surface drag and iv) the favourable westward tilt of the low with an amplifying trough all contribute to the explosive deepening of the storm.

Two consecutive simulations covering a total of 102 h during the storm development are carried out with a grid size of 50 km. The RFE model reproduces very well the formation of the surface low on the lee side of the Rockies, the track and deepening rates, the explosive development and decay of the storm, and various mesoscale phenomena (e.g., a “bent‐back” warm front, a “T‐bone” thermal pattern, a cold frontal “fracture”, an upper‐level “eye” and warm‐core structures), as verified by conventional observations, satellite imagery, flight‐level and dropsonde data from a research aircraft. It is found from potential vorticity (PV) analysis that the storm reaches its peak intensity as the upper‐level dry PV anomaly, the low‐level moist PV anomaly and surface thermal warmth are vertically superposed. PV inversions reveal that these anomalies contribute about 60%, 30% and 10%, respectively, to the 900‐hPa negative height perturbation. It is shown that the warm‐core structure near the cyclone centre is produced by advection of warmer air ahead of the cold front, rather than by adiabatic warming associated with subsidence.     

Is Canadian cloudiness increasing?

Abstract

Cloud amount records for the Canadian mid‐latitudes have been analysed in the context of a “warming world” analogue model that compares records of two 20‐year periods. The cloud amounts increase over practically all these regions while temperatures rise. This historical data set has also been extended temporally to permit analysis of high‐latitude cloudiness trends. These are of particular interest in the “fingerprinting” of CO₂‐induced climatic change. Station records from the Canadian Arctic show distinctive increases in total cloud amount in the last forty years especially in the summer season. This result, unlike the historical analogue analysis, seems to be decoupled from temperature changes.     

Calibration of a three‐component angular distribution model of sky radiance

Abstract

A three‐component (isotropic, circumsolar and horizon‐brightening) model of the angular distribution of sky (short‐wave) radiance has been tested and validated against a data base of measured sky radiance. The data base encompasses cloud cover 0.0 to 1.0 and solar zenith angles 30 to 80°. Empirical constants have been derived for the model enabling the prediction of sky radiances for any sky condition.     

A new adiabatic kernel for the MC2 model

Abstract

Traditional semi‐implicit formulations of nonhydrostatic compressible models may not be stable in the presence of steep terrain when pressure gradient terms are split and lagged in time. If all pressure gradient terms and the divergence are treated implicitly, the resulting wave equation for the pressure contains off‐diagonal cross‐derivative terms leading to a highly nonsymmetric linear system of equations. In this paper we present a more implicit formulation of the Mesoscale Compressible Community (MC2) model employing a Generalized Minimal Residual (GMRES) Krylov iterative solver and a more efficient semi‐Lagrangian advection scheme. Open boundaries now permit exact upwind interpolation and the ability to reproduce simulations to machine precision is illustrated for one‐way nesting at equivalent resolution. Numerical simulations of hydrostatic and nonhydrostatic mountain waves demonstrate the stability and accuracy of the new adiabatic kernel. The computational efficiency of the model is reported for 1D Jacobi and 3D Alternating Direction Implicit (ADI) line relaxation preconditioned implemented with a parallel data transposition strategy.     

Sensitivity of transient eddies to climate change in the CCC general circulation model

Abstract

We use eddy life‐cycle simulations to evaluate the response of atmospheric transient eddies to a global warming caused by CO₂ doubling in the CCC general circulation model. In simulations using Northern Hemisphere winter conditions, transient waves attain larger kinetic energy and encompass a wider range of latitudes in the warmer climate. This behaviour contrasts with a previous investigation that used output from the NCAR and GFDL models. Our analysis indicates two primary factors for the difference between model responses: (1) a smaller change in the mid‐latitude temperature gradient in the CCC model, which allows (2) increased atmospheric water vapour in mid‐latitudes to catalyze a more rapidly evolving life‐cycle.     

A diagnostic study of the southern hemisphere summer circulation of the CCC general circulation model

Abstract

The medium‐scale wave regime, consisting largely of zonal wavenumbers 5–7, frequently dominates the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hövmöller diagrams, space‐time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium‐scale wave regime.

The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with the observations. After comparison with the observed December‐January‐February 1979 distributions, some quantitative differences remain: the heat transport is too weak aloft and too large near the surface, whereas the momentum transport tends to be too weak. The baroclinic and barotropic conversions show a maximum in the medium‐scale waves. The time evolution of the Richardson number of the mean flow suggests that the medium‐scale wave is due to a baroclinic instability.     

Parametrization schemes of incident radiation in the North Water polynya

Abstract

An evaluation of the Canadian Land Surface Scheme (CLASS) 3.1 snow cover simulations at four sites included in the Snow Model Intercomparison Project (SnowMIP) revealed that CLASS was able to provide realistic representations of snow cover accumulation, melt and physical properties over a range of snow cover climates. The modified snow aging parametrization in CLASS 3.1 provided improved simulations of snowpack density which resulted in a marked reduction in the root‐mean‐square (rms) error for daily snow depth, and slight improvements in snow surface temperature. CLASS 3.1 still exhibited a tendency to overestimate snow cover duration which is attributed to the way shallow snow ablation is treated. CLASS provided generally realistic simulations of daily and seasonal variation in snow albedo although cold snow albedo was underpredicted by 0.10 to 0.15 at a site with a deep (> 2 m) cold snowpack. CLASS also exhibited a tendency to overpredict late spring snow albedo which was reduced by the addition of a snow layer subroutine that kept track of snow albedo by precipitation event. CLASS had a noticeable cold bias averaging 3°–4°C at two mountain sites included in the comparison. The bias was closely linked to atmospheric stability and could exceed 10°C under conditions of strong radiative cooling and low wind speeds. The CLASS energy deficit under these conditions was determined to be ～20–40 W m^?2 and was mostly accounted for by introducing a windless exchange coefficient into the calculation of sensible heat fluxes following the approach used in a number of other physical snowpack models. CLASS provided realistic simulations of daily snowmelt runoff with the exception of the Weissfluhjoch site which was characterized by a deep cold snowpack. A preliminary assessment of snow water equivalent (SWE) rms error for the 23 models participating in SnowMIP showed that CLASS was one of the better single layer snow models included in the comparison. CLASS performance was comparable to the multi‐layer CROCUS snowpack model in the evaluations carried out in this study.     

A study of planetary wave errors in a spectral numerical weather prediction model

Abstract

A series of fifteen 96‐h forecasts made with a spectral numerical weather prediction model is studied with reference to errors in the planetary wavelengths. The major contributor to the short (less than 48‐h) range forecast error is identified as an external mode. The medium range forecast error (96 h) is internal in character and reflects a deficiency in the simulation of the quasi‐stationary components.     

Skill assessment of seasonal hindcasts from the Canadian historical forecast project

Abstract

The performance of seasonal hindcasts produced with four global atmospheric models in the second phase of the Canadian Historical Forecasting Project is evaluated. Deterministic and probabilistic forecast skill assessments are carried out using common verification measures. Several methods of combining multi‐model output to produce deterministic and probabilistic forecasts of near‐surface air temperature, 500 hPa geopotential height, and 700 hPa temperature for zero‐month and one‐month leads are considered. A variance‐based weighting modestly improves the skill of deterministic and probabilistic hindcasts in some cases. A parametric Gaussian probability estimator is superior to a non‐parametric count‐method estimator for producing multi‐model probability forecasts. Statistical adjustment is beneficial for deterministic and probabilistic hindcasts of near‐surface temperature over the ocean but not always over land. Skill improves with the number of different models used for a given total ensemble size. The four‐model ensemble is shown to be a reasonable multi‐model configuration.     

Graupel growth and trajectories in a shallow cb cloud determined by a forced 1‐D model

Abstract

A kinematic model is applied to graupel growth. The vertical velocity and ther‐modynamic field data are taken from the forced 1‐D time‐dependent model of Cb cloud developed by Curie and Jane (1988). The graupel embryo pocket was released at the height of the — 10°C isotherm. The influence of the forced lifting on further graupel growth and its trajectory is analysed by sensitivity experiments based on the amplitude of the forced lifting, and initial graupel radius, density and cloud droplet concentration for the forced lifting initiation time derived from the model and the forced lifting duration time that agreed with observations. In particular, the sensitivity analysis was carried out for the forced lifting initiation and duration times.

It is shown that for large values of the forced lifting amplitudes, the residence time of the graupel within cloud and the final graupel radius may be significantly larger compared with those in the non‐forced case. The residence time in a cloud can also be significantly larger for the smallest amplitude, whereas the final radius is rather insensitive owing to oscillations around the melting level. For some cases the forced lifting causes recycling inside the updraft, contrary to the results of previous non‐forced numerical models. The recycling mechanism is sensitive to the forced lifting duration time and the time interval between the graupel pocket injection in cloud and the initiation of the forced lifting. Initially the observed recycling mechanism is a consequence of the periodic forced lifting mechanism, but then combines with recycling of the Pflaum type (1980).     

A tidal model of the northeast pacific

Abstract

The development of a tidal model for the west coast of Canada is described. The model is intermediate in resolution between coarse‐gridded global models and fine‐gridded local models; it provides a good representation of the main shelf regions and also includes a substantial area of the neighbouring ocean. The physical processes relevant to tides in both deep and shallow water are included. Calculations have been carried out for the M₂ and K₁ constituents and the model results were compared with extensive tide‐gauge observations and empirically based charts. For M₂, the agreement between model results and observations is generally excellent, but for K₁, which contains more small‐scale variability, the model results are not quite so good. The variability in K₁ is associated with tidally generated continental shelf waves. Examination of the computed currents and energy fluxes suggests that shelf‐wave components are present in the model solution but, for the Vancouver Island shelf, their propagation is not reproduced accurately. This may be due to deficiencies in the model and/or to the influences of stratification and mean currents, which are neglected here. The model predicts that shelf‐wave components should also occur in diurnal tides on the Alaskan shelf.

The significance of the tide‐generating potential and advection are also examined and further work proposed.     

Modelling Sea Surface Temperature (SST) in the Hudson Bay Complex Using Bulk Heat Flux Parameterization: Sensitivity to Atmospheric Forcing,and Model Resolution

ABSTRACT

Sea surface temperature (SST) from four Nucleus for European Modelling of the Ocean (NEMO) model simulations is analyzed to study the bulk flux parameterization to compute SST over the Hudson Bay Complex (HBC) for the summer months (August and September) from 2002 to 2009. The NEMO simulation was forced with two atmospheric forcing sets with different resolutions: the Coordinated Ocean-ice Reference Experiment, version 2 (COREv2), as the lower resolution and the Canadian Meteorological Centre’s Global Deterministic Prediction System Reforecasts (CGRF) as the higher resolution. The CGRF forcing is also implemented in the third and fourth runs using different runoff data and different NEMO resolutions (1/12° versus 1/4°). Results show that all four modelled SSTs followed observed SST patterns, with regional differences in SST bias between simulations with different atmospheric forcing. The SST differences are small between simulations forced with the same atmospheric forcing but with different model resolution or runoff. This implies that the model resolution and runoff have a small effect on the simulated SST in the HBC. Moreover, to better capture the effect of near-surface temperature (T_air) on simulated SST, we conducted three analyses using the Haney flux linearization formula. Results from these assessments did not indicate any direct influence on the model-simulated SSTs by T_air. Looking at the heat flux as a signature for SST showed that both averaged spatial distribution and time series of net heat flux produced by the three CGRF forcing simulations were higher than the net heat flux generated by the CORE 2 simulation. This was generally true for all four components of the total heat flux (sensible, latent, shortwave, and longwave) individually as well. Total heat flux in summer is governed by the shortwave heat flux, with values up to 120?W?m^?2 in August, and the longwave heat flux is the main contributor to the total heat flux differences. These heat flux differences lead to corresponding colder model SSTs for the CGRF runs and warmer SSTs for the CORE 2 simulations.     

Airborne radar measurements of ocean wave spectra and wind speed during the grand banks ERS‐1 SAR wave experiment

Abstract

Measurements of ocean directional wave spectra, significant wave height, and wind speed over the Grand Banks of Newfoundland were made using the combined capabilities of the radar ocean wave spectrometer (ROWS) and scanning radar altimeter (SRA). The instruments were flown aboard the NASA P‐3A aircraft in support of the Grand Banks ERS‐1 Synthetic Aperture Radar (SAR) Wave Experiment. The NASA sensors use proven techniques, which differ greatly from SAR, for estimating the directional long‐wave spectrum; thus they provide a unique set of measurements for use in evaluating SAR performance. ROWS and SRA data are combined with spectra from the SAR aboard the Canadian Centre for Remote Sensing (CCRS) CV‐580 aircraft, the first‐generation Canadian Spectral Ocean Wave Model (CSOWM) hindcast, and other available in situ measurements to assess the ERS‐1 SAR's ability to correctly resolve wave field components along a 200‐ to 300‐km flight line for four separate satellite passes. Given the complex seas present on the Grand Banks, the complementary nature of viewing the sea spectrum from the perspectives of multiple sensors and a wave prediction model is apparent. The data intercomparisons show the ERS‐1 SAR to be meeting the expected goals for measuring swell, but the data also show evidence of this remote sensor's inability to detect the shorter waves travelling in the azimuth or along‐track direction. Example SAR spectra simulations are made using a non‐linear forward transform with ROWS measurements as input. Additionally, surface wind and wave height estimates made using the ROWS altimeter channel are presented. These data demonstrate the utility of operating the system in its new combined altimeter and spectrometer configuration.     

Diagnostic simulations of the summer circulation in the Canadian arctic archipelago

Abstract

Gridded fields of potential temperature and salinity, interpolated to the time of minimal ice coverage, are constructed for the Canadian Arctic Archipelago based on archived data. In order to overcome the large variations in the horizontal coverage of the observations, the gridding is performed in an iterative procedure where the horizontal correlation scales depend on the data coverage as well as on the flow field. The mean flow corresponding to the temperature and salinity fields are calculated with a diagnostic numerical ocean model. The simulations show that the relative flow through the different straits depends on the elevation difference from the Arctic Ocean to Baffin Bay, and on the density distribution and baroclinic pressure gradients. A 5‐cm increase in the Arctic‐Baffin elevation difference can double the transport. Mean values of the summer flow are a total transport of 0.9 Sv, with 34% flowing through Barrow Strait, 20% through Jones Sound, and 46% through Nares Strait.     

Application of the Canadian regional climate model to the Laurentian great lakes region: Implementation of a lake model

Abstract

This study reports on the implementation of an interactive mixed‐layer/thermodynamic‐ice lake model coupled with the Canadian Regional Climate Model (CRCM). For this application the CRCM, which uses a grid mesh of 45 km on a polar stereographic projection, 10 vertical levels, and a timestep of 15 min, is nested with the second generation Canadian General Circulation Model (GCM) simulated output. A numerical simulation of the climate of eastern North America, including the Laurentian Great Lakes, is then performed in order to evaluate the coupled model. The lakes are represented by a “mixed layer” model to simulate the evolution of the surface water temperature, and a thermodynamic ice model to simulate evolution of the ice cover. The mixed‐layer depth is allowed to vary spatially. Lake‐ice leads are parametrized as a function of ice thickness based on observations. Results from a 5‐year integration show that the coupled CRCM/lake model is capable of simulating the seasonal evolution of surface temperature and ice cover in the Great Lakes. When compared with lake climatology, the simulated mean surface water temperature agrees within 0.12°C on average. The seasonal evolution of the lake‐ice cover is realistic but the model tends to underestimate the monthly mean ice concentration on average. The simulated winter lake‐induced precipitation is also shown, and snow accumulation patterns on downwind shores of the lakes are found to be realistic when compared with observations.     

Frequency analysis of short‐duration rainfalls

Abstract

An investigation was carried out to determine the appropriateness of a probability distribution and fitting technique commonly used in Canada for rainfall frequency analysis.

The extreme value type 1 (EV1) distribution was assessed for three long‐term Canadian stations: Victoria, St Thomas and Québec. The EV1 distribution appears to provide a reasonable fit for durations varying from 5 min to 6 h, but is not clearly superior to another two‐parameter distribution, the lognormal.

The fitting technique, known as modified moments or regression, was assessed by comparing it with three other fitting techniques: moments, maximum likelihood and adjusted maximum likelihood. This comparison was carried out using Monte Carlo simulation techniques over the parameter space deemed to be representative of short duration rainfall data for Canada. In terms of estimating rainfall amounts of specified return period, the modified moments technique was the poorest with regard to both bias and efficiency. In general, the maximum likelihood estimates were the most efficient and were relatively unbiased.     

Application of a coupled ice‐ocean model to the Labrador Sea

Abstract

The steady, coupled ice‐ocean circulation model of Willmott and Mysak (1989) for a meridional channel is applied to the Labrador Sea for the winter season. The model consists of a thermodynamic reduced‐gravity ocean combined with a variable thickness ice cover that is in thermal equilibrium. Upon specifying the forcing fields of surface air temperature, wind stress and water temperature along the open southern boundary, the winter climatological ice‐edge position, ice thickness, ocean circulation and temperature fields are determined in the channel domain. The sensitivity of the results to the various model parameters is examined. In particular, the optimum heat exchange coefficients for the interfaces of air‐water, ice‐water and air‐ice are found.

The model ice‐edge position compares favourably with the 50% winter climatological ice concentration isoline obtained from an analysis of 32 years (1953–84) of sea‐ice concentration data. The simulations of the ocean temperature and ice thickness are also quite realistic according to the observed records available. The model is also applied to two specific winters (1981 and 1983) during which anomalous sea‐ice and weather conditions prevailed in the Labrador Sea.