The dynamics of moist frontogenesis in a semi‐geostrophic model

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.     

Errors near the poles generated by a semi‐Lagrangian integration scheme in a global spectral model

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.     

Short‐term forecasting with a multi‐level spectral primitive equation model part I ‐ model formulation

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.     

Non‐linear coupling between modes in a low‐dimensional model of ENSO

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.     

Surface‐absorbed and top‐of‐atmosphere radiation fluxes for the Mackenzie River basin from satellite observations and a regional climate model and an evaluation of the model

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.     

Short‐term forecasting with a multi‐level spectral primitive equation model part II ‐ Hemispheric prognoses and verification

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.     

Some properties and comparative performance of the semi‐Lagrangian method of Robert in the solution of the advection‐diffusion equation

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.     

The circulation and residence time of the strait of Georgia using a simple mixing‐box approach

Abstract

New observations in the Strait of Georgia, British Columbia, Canada show that temperature and dissolved oxygen have a pronounced seasonal cycle, with a spatially varying phase. Phase lags in oscillating systems arise due to internal time scales which can be interpreted in fluid systems as residence times. Exploiting phase we construct a quantitative and internally consistent circulation scheme for this body of water after dividing it into four regions: the Fraser River plume, the surface waters down to 50 m, the intermediate waters down to 200 m, and the deep water. In this scheme the intermediate water, the largest region by volume, is continually renewed, and its characteristics change in response to continuous changes in the characteristics of source waters. The dependence of the estuarine circulation on variations in fresh inflow is weak. The deep water is volumetrically less important, but seasonal changes in the density of oceanic source waters can produce a variation in the overall circulation by driving an additional inflow which leads to both deep renewal and increased upwelling. In turn, this increased upwelling results in lower surface temperatures than might otherwise be expected. Intermediate water residence times are about 160 days. Deep water is renewed once per year in summer and is affected only by vertical diffusion during the rest of the year. Surface water residence times for the entire Strait are a few months at most, but the Fraser River plume has a freshwater residence time of approximately 1 day. In addition, we find that the residence time of oceanic source waters in the Strait is 1.7 years due to a substantial recirculation in Haro Strait. Other consequences of this scheme are consistent with independent estimates of horizontal transports, air‐sea heat fluxes, subsurface oxygen (O₂) utilization, and primary production. Finally, analysis of the spatial phase variations suggests that the intermediate inflow enters the Strait as a boundary current along the slopes of the Fraser delta.     

Winter‐time near‐surface currents in the strait of Juan de Fuca

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.     

Estimate of the steric contribution to global sea level rise from a comparison of the WOCE one‐time survey with 2006–2008 Argo observations

Abstract

It is well known from observations by altimetric satellites (predominantly TOPEX/Poseidon and Jason‐1) that global sea level is rising. What is less well known is exactly how the observed sea level rise is partitioned between a steric contribution (sea level rising because of changes in ambient temperature and salinity) and a contribution arising from the addition of new water mass to the oceans. Strictly speaking, such a separation is not possible because of the non‐linearity in the equation of state for sea water, but in practice the non‐linearities are sufficiently small to allow this separation as a very good first approximation.

A careful comparison of the World Ocean Circulation Experiment (WOCE) one‐time survey with recent observations by the Argo array indicate a steric component to sea level rise of 2.2 mm y^–1 between the early 1990s and 2006 to 2008. This is a significantly larger rise rate than previously estimated and, along with recent estimates of melt rate from ice sheets, is in much closer agreement with the total rise rate as reported by altimetric satellites, 3.2 ± 0.4 mm y^–1 over this period.     

The performance of an E−l scheme for the atmospheric boundary layer in a mesoscale model with grid spacing as small as 1 km

Summary ¶A new E–l boundary layer scheme is tested within the U.S. Navys COAMPS model. The goal is to give COAMPS the capability to simulate mesoscale cellular convection. The new scheme is aimed to be consistent with both classic results for clear entrainment and recent calibrations, derived from large-eddy simulations, for entrainment into smoke clouds and water clouds. A parameter is included in the scheme that allows sub-grid transport to be reduced so that, when the model has 2km grid spacing or less, more of the transport is forced to occur in resolved convection. At 2km grid spacing, the scheme allows COAMPS to simulate the break up of a stratocumulus cloud deck into mesoscale cellular convection.Received June 7, 2002; accepted August 13, 2002 Published online: February 20, 2003     

Atmospheric modulation of surface heat fluxes at Ocean Weather Station P on a decadal time‐scale

Abstract

A 30‐year record (1951–1980) of surface heat fluxes at Ocean Weather Station P in the northeastern Pacific Ocean (50°N, 145°W) was examined for differences in the interdecadal variation between fail and winter. During the latter part of the 1950s and the early 1960s, the winter surface heat flux from the ocean to the atmosphere diminished significantly whereas the fall heat flux increased slightly This difference in the modulation of the winter heat flux from the fall heat flux during this period appears to be caused by the presence of an atmospheric circulation anomaly resembling that of the Pacific/North America (PNA) low‐frequency variability mode during the winter season.     

The steady‐state structure of the natural stratosphere and ozone distribution in a 2‐D model incorporating radiation and O‐H‐N photochemistry and the effects of stratospheric pollutants

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, HO_X, N₂O,NO_X, 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 NO_x 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 NO_x 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 NO_X perturbation. It is shown that the reduced meridional temperature gradient in the middle and upper stratosphere resulting from the NO_x 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.     

Cross‐shore variations of near‐surface wind velocity and atmospheric turbulence at the land‐sea boundary during CASP

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.     

The climatology of the Canadian Climate Centre general circulation model as obtained from a five‐year simulation

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.     

Sensitivity experiments for polar low forecasting with the CMC mesoscale finite‐element model

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.     

A decision‐analytic study of the joint value of seasonal precipitation and temperature forecasts in a choice‐of‐crop problem

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.     

SPOM: A regional model of the sub‐polar north Atlantic

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.     

Coupled atmosphere–ocean data assimilation experiments with a low-order model and CMIP5 model data

Climate Dynamics - Coupled atmosphere–ocean data assimilation (DA) experiments are performed for estimating the Atlantic meridional overturning circulation (AMOC). Recovery of the AMOC with...