Abstract

The Mackenzie River basin (MRB) in northwestern Canada is a climatologically important region that exerts significant influences on the weather and climate of North America. The region exhibits the largest cold-season temperature variability in the world on both the intraseasonal and interannual time scales. In addition, some of the strongest recent warming signals have been observed over the basin. To understand the nature of these profound and intriguing observed thermal characteristics of the region, its atmospheric heat budget is assessed by using the NCEP–NCAR reanalysis dataset. The composite heat budgets and large-scale atmospheric conditions that are representative of anomalous winters in the region are examined in unison to study the processes that are responsible for the development of extreme warm/cold winters in the MRB. It is shown that the large winter temperature variability of the region is largely a result of the strong variability of atmospheric circulations over the North Pacific, the selective enhancement/weakening of latent heating of the cross-barrier flow for various onshore flow configurations, and synoptic-scale feedback processes that accentuate the thermal response of the basin to the changes in upwind conditions. The improved understanding of mechanisms that govern the thermal response of the basin to changes in the upstream environment provides a theoretical basis to interpret the climate change and modeling results for the region. In particular, the large recent warming trend observed for the region can be understood as the enhanced response of the basin to the shift in North Pacific circulation regime during the mid-1970s. The strong cold bias that affected the region in some climate model results can be attributed to the underprediction of orographic precipitation and associate latent heating of the cross-barrier flow, and the subsequent weakening of mean subsidence and warming over the basin in the models.

Abstract

A winter oceanic cyclonic cloud system was simulated by using the mesoscale compressible community (MC2) model with different combinations of model resolutions and cloud microphysics packages. Results from these simulations are intercompared to examine the effects of the coarse model grid and simplified model physics on the simulated large-scale storm environment. When aggregated to an area approximately equivalent to the size of a grid box in current GCMs, the results from the models differ significantly in the large-scale cloud and moisture profiles. Although the effects of using different stratiform cloud schemes on the coarse-grid results are appreciable, the effects of different model resolution are shown to be greater on the large-scale frontal cloud field. In particular, the coarse-grid models underestimated the cloudiness and atmospheric moisture content in the warm-frontal region. Such differences in the large-scale model storm environment were consequences of the stronger mean cross-front circulation and mesoscale cloud features in the high-resolution simulation. The stronger cross-front circulation was in turn a result of stronger frontogenetic processes over the region and dynamic influences of the mesoscale cloud bands on the parent storm. Because both the frontal zones and the mesoscale cloud bands are unresolved features in current GCMs, these results suggest that the parameterization of their bulk effects on the large scales should be included in the representation of frontal layered clouds in climate models.

Abstract

Frontal precipitation systems are simulated with a 2D cloud model including ice-phase microphysics. Despite the use of idealized frontogenetic forcing in the simulations, some observed characteristics of frontal zones and their associated cloud and precipitation fields are reproduced in the simulations.

The effects of melting snow on surface frontogenesis is investigated. It is found that the cooling effects of melting snow significantly accelerate surface frontogenesis in winter storm environments, especially when the melting layer is close to the surface. However, the steady-state surface frontal strength in the model is not sensitive to the melting effects.

Finescale thermal and kinematic perturbations inside the frontal zone near the melting level, quite similar to those recently reported in the literature, are evident in the model results. Analysis of the model results suggests that cooling from melting snow may induce these thermal and kinematic perturbations and may enhance baroclinicity, resulting in accelerating frontogenesis. These frontogenetic effects should be strongest when the melting layer is near the surface, thus explaining the often observed coincidence of surface fronts with the surface rain–snow boundary in winter storms.

The Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS) is one of the continental-scale experiments approved specifically by GEWEX to better understand and model water and energy cycling at high latitudes. The project has gone through two phases since its inception in 1994 and conclusion in December 2005. Many scientific results have been achieved through MAGS research to advance our understanding of the Mackenzie River basin climate system. This article is a synthesis of its atmospheric research achievements through an integrative description of the basin's climate system, along with highlights of MAGS research that has advanced our knowledge and understanding of various key aspects of the system. In particular, the significance of MAGS research is discussed in the hancing knowledge of the basin's hydroclimate with focuses on i) the large-scale atmospheric processes that control the transport of water and energy into the basin, and ii) the interactions of the large-scale atmospheric flows with physical features of the basin's environment in affecting the weather and climate of the basin.

Abstract

The 54-yr (1948–2001) NCEP–NCAR reanalysis data as well as other information were used to study the moisture transport and associated circulation features for the severe 2000/01 drought over the western and central Canadian Prairies. Most of the moisture for precipitation over the region is from the Pacific Ocean in winter (November–March) and from the Gulf of Mexico in summer (May–August). An analysis shows that the zonal moisture transport from the Pacific Ocean into both the North American continent and the western and central Canadian Prairies during the winter of the 2000/01 agricultural year was the least over the entire study period, and there was no significantly enhanced moisture influx from the Gulf of Mexico into the region to compensate. Very low winter precipitation was produced over the western and central Canadian Prairies as a consequence. During the ensuing summer period, moisture transport from the Gulf of Mexico was significantly less than normal and no significantly enhanced moisture transport from the Pacific Ocean occurred. These conditions collectively resulted in extremely dry surface conditions for the growing season.

These moisture transport features were mainly associated with prolonged and extraordinarily strong anomalously high pressures over western North America and their related stronger-than-normal air mass sinking over the western and central prairies and adjacent regions. The anomalous high pressures blocked the moisture from flowing into the western and central prairies and were also associated with the splitting of the jet stream that significantly reduced zonal moisture transport by changing the strength and incoming angle of the airflows from the Pacific Ocean during the winter of 2000/01.

Consequences of the stronger-than-normal subsidence were hot and dry surface air during the summer and less precipitation. Collectively, these dynamic factors are favorable for both the formation and the maintenance of droughts.

Abstract

The melting of snow extracts latent heat of fusion from the environment. The basic response of the atmosphere to this cooling-by-melting mechanism is investigated by using a nonlinear two-dimensional numerical model. It is found that the resultant melting-induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves. The characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions. Model results show that the melting associated with realistic precipitation rates (up to 10 mm h⁻¹) can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second. Since the gravity waves and the cold outflow current propagate away from the source, they can have significant dynamic effects on the environment remote from the precipitation region. Moreover, the melting-induced, near O°C isothermal layer in the atmosphere alters the local static stability. It is inferred that thew melting-induced effects may significantly influence the momentum and moisture transports in mesoscale precipitation systems.

Abstract

Various authors have proposed that the cooling associated with melting precipitation contributes significantly to the dynamics of mesoscale precipitation systems. In this study, we use the numerical model described in Part I of this paper to investigate the effects of the cooling-by-melting mechanism in three specific situations: rain/snow boundaries, the production of deep 0°C isothermal layers, and the trailing stratiform region associated with mesoscale convective systems.

It is found that melting in the vicinity of a rain/snow boundary produces a thermally indirect mesoscale vertical circulation that may be responsible for enhanced precipitation near a rain/snow boundary. Melting in the presence of warm air advection above the melting layer and cold advection at and below it are necessary for producing deep 0°C layers within realistic times. The dynamic effects of cooling associated with melting and evaporation in the stratiform region of a mature squall line system produce a mesoscale circulation qualitatively similar to that recently reported in the literature.

Abstract

A severe ice storm affected the east coast of Canada during the Canadian Atlantic Storms Project II. A hierarchy of cloud-resolving model simulations of this storm was performed with the objective of enhancing understanding of the cloud and mesoscale processes that affected the development of freezing rain events. The observed features of the system were reasonably well replicated in the high-resolution simulation. Diagnosis of the model results suggests that the change of surface characteristics from ocean to land when the surface warm front approaches Newfoundland disturbs the (quasi-) thermal wind balance near the frontal region. The cross-frontal circulation intensifies in response to the thermal wind imbalance, which in turn leads to the development of an extensive above-freezing inversion layer in the model storm. Depending on the depth of the subfreezing layer below the inversion, the melted snow may refreeze within the subfreezing layer to form ice pellets or they may refreeze at the surface to form freezing rain. Such evolution of surface precipitation types in the model storm was reasonably well simulated in the model. Model results also show that the horizontally differential cooling by melting near the nose of the above-freezing inversion layer enhances the local baroclinicity, which in turn induces perturbations on the cross-front flow. Depending on stability of the ambient flow, such local flow perturbations may trigger symmetric or convective overturning above the region and consequently enhance the local precipitation production via a positive feedback mechanism.

Abstract

This study represents the first attempt at developing a comprehensive climatology of atmospheric and surface water and energy budgets for the Mackenzie River basin (MRB). Different observed, remotely sensed, (re)analyzed, and modeled datasets were used to obtain independent estimates of the budgets. In particular, assimilated datasets, including the National Centers for Environmental Prediction Global Reanalysis 2 (NCEP-R2), the global 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), the NCEP North American Regional Reanalysis (NARR), and the Canadian Meteorological Centre (CMC) operational regional analysis as well as results from the Canadian Regional Climate Model (CRCM) simulations, are used in the study. Apart from the development of state-of-the-art budget estimates for the MRB, the relative merits of current models, data assimilation systems, and global blended datasets in representing aspects of the water and energy cycle of this northern and data-sparse region were also assessed. In addition, the levels of uncertainty in assessing the budgets as well as their sources are discussed. The regional water budget for the MRB is closed within 10% of the observed runoff by using the moisture flux convergence from ERA-40, NARR, CMC, or CRCM. While these are noted improvements over previous water closure assessments for the region, magnitudes of the residuals in balancing the budgets are often comparable to the budget terms themselves in all the model and analysis datasets, and the spreads of budget estimates from the different datasets are also typically large, suggesting that substantial improvements to the models and observations are needed before the assessments of water and energy budgets for this northern region can be vastly improved.

Abstract

On 30 September 1994 an Arctic low pressure system passed over the southern Beaufort Sea area of northern Canada and research aircraft observations were made within and around the warm front of the storm. This study is unique in that the warm front contained subzero centigrade temperatures across the entire frontal region. The overall structure of the warm front and surrounding region was similar to midlatitude storms; however, the precipitation rates, liquid water content magnitudes, horizontal and vertical winds, vertical wind shear, turbulence, and thermal advection were very weak. In addition, a low-level jet and cloud bands were aligned parallel to the warm front, near-neutral stability occurred within and around the front, and conditional symmetric instability was likely occurring. A steep frontal region resulted from strong Coriolis influences that in turn limited the amount of cloud and precipitation ahead of the system. The precipitation efficiency of the storm was high (60%) but is believed to be highly dependent on the stage of development. The mesoscale frontogenetic forcing was primarily controlled by the tilting of isentropic surfaces with confluence/convergence being the secondary influence. Sublimation contributions may have been large in the earlier stages of storm development. Satellite and aircraft radiometers underestimated cloud top heights by as much as 4 km and this was mostly due to the near transparency of the lofted ice layer in the upper portion of the storm. Maximum surface solar radiation deficits ranged between 91 W m⁻² and 187 W m⁻² at two surface observing sites. This common type of cloud system must have a major impact on the water and energy cycles of northern Canada in the autumn and therefore must be well accounted for within climate models.