Kit K. Szeto

Moisture Transport and Other Hydrometeorological Features Associated with the Severe 2000/01 Drought over the Western and Central Canadian Prairies

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 m...

Streamflow hydrology in the boreal region under the influences of climate and human interference

The boreal region has a subarctic climate that is subject to considerable inter-annual variability and is prone to impacts of future warming. Climate influences the seasonal streamflow regime which typically exhibits winter low flow, terminated by spring freshet, followed by summer flow recession. The effects of climatic variation on streamflow cannot be isolated with confidence but the impact of human regulation of rivers can greatly alter the natural flow rhythm, changing the timing of flow to suit human demands. The effect of scenario climate change on streamflow is explored through hydrological simulation. Example of a Canadian basin under warming scenario suggests that winter flow will increase, spring freshet dates will advance but peak flow will decline, as will summer flow due to enhanced evaporation. While this simulation was site specific, the results are qualitatively applicable to other boreal areas. Future studies should consider the role of human activities as their impacts on streamflow will be more profound than those due to climate change.

Mesoscale Circulations Forced by Melting Snow. Part II: Application to Meteorological Features

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 circul...

Mesoscale Circulations Forced by Melting Snow. Part I: Basic Simulations and Dynamics

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 Structure, Water Budget, and Radiational Features of a High-Latitude Warm Front

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 beli...

Midlatitude cyclonic cloud systems and their features affecting large scales and climate

Midlatitude cyclonic cloud systems are common occurrences that significantly impact our climate. In this review, attention is paid to those physical characteristics of these cloud systems with large-scale impacts that must be accounted for in climate simulations. Such attributes include atmospheric forcing, internal structure, surface influences, cloud layering, microphysics, precipitation, water cycling, and radiation. Because of their present limitations associated with, for example, grid sizes and simplified parameterizations, climate models cannot account for all the crucial impacts of these cloud systems. Future advances in the representation of these systems within climate models will need to rely in part on rigorous assessments of model capabilities in a variety of conditions.

A Numerical Investigation of Squall Lines. Part II: The Mechanics of Evolution

Abstract The physical processes involved in the evolution of the model squall line presented in Part I of this study are examined. It is found that both the thermal and dynamic effects are important in the development of the midlevel meso-γ-scale low pressure zone located just to the rear of the convective core. Based on this observation, a positive feedback mechanism is proposed to explain the abrupt transformation of the more or less upright convection line into a quasi-steady meso-β-scale convective system possessing an extensive trailing stratiform region. In order to set the stage for this transformation to take place, the convective updraft is required to possess an initial upshear tilt. Qualitative arguments are given to show that this initial tilt might be the result of a moderate wind shear at low levels and none or reverse shear in the middle to high levels. Results from model sensitivity experiments are presented to support the theory. In addition to the dynamic effects of the low-level wind sh...

On the Extreme Variability and Change of Cold-Season Temperatures in Northwest Canada

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 Pacif...

A Numerical Investigation of Squall Lines. Part I: The Control Experiment

Abstract A two-dimensional, anelastic, cloud-resolving numerical model was used to simulate squall systems. Large domain and fine grid resolutions were utilized so that both the convective and mesoscale components of squall lines could be handled adequately. Detailed cloud microphysics including the ice phase and the Coriolis force have been included in the basic model. Both the life cycle and storm structure of observed squall systems have been simulated successfully. Some details in the observed precipitation and kinematic characteristics of squall lines, such as the locations of front-to-rear jet core, the base of the stratiform cloud, the formation of a transition zone, and the organized mesoscale updraft, have been simulated by the model. The storm-generated meso-γ-scale low pressure center located behind the convective updraft has been shown to be instrumental in the initiation and maintenance of the mesoscale circulation and the associated trailing stratiform region. Diagnostically, the horizontal ...

Precipitation Recycling in the Mackenzie and Three Other Major River Basins

Local evaporation is an important source of moisture for precipitation in many continental regions. The precipitation recycling ratio ( ρ = fraction of total precipitation derived from local evaporation) for four major river basins (the Mackenzie, Mississii, Amazon and Lena) are evaluated by alying the ECMWF ERA-40 reanalysis data to the bulk recycling estimation method of Eltahir and Bras (1994). On an annual basis, recycling is strongest in the Amazon at 30%, and ρ for the other three regions are within 2% of each other with values between 23 and 25%. Apart from the Amazon, the estimated precipitation recycling ratios for all basins exhibit strong seasonal variability, with very low values during the cold season and typically high values during the warm season. The results indicate that close to or more than half of the summer precipitation in the downstream regions of all the test basins is derived from local evaporation. The role of moisture recycling in governing the warm season precipitation variability in the regions is also discussed.