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Kit K. Szeto

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.

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Kit Kong Szeto and Han-Ru Cho

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 pressure gradient forces associated with this low center drove the front-to-rear flow as well as the front portion of the rear-to-front flow. The front-to-rear flow destabilized the upper troposphere to the rear of the squall line, thus providing a suitable environment for the development of the mesoscale updraft and stratiform precipitation. The storm-relative rear-to-front flow possessed a double jet core structure that was found to be forced by different zones of horizontal pressure gradient force in the interior of the storm.

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Kit Kong Szeto and Han-Ru Cho

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 shear and the strength of the storm-induced cold pool, model results show that other storm-induced features and environmental factors such as the middle-level wind shear also play an important role in determining the evolution of squall systems.

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Kit Kong Szeto and Han-Ru Cho

Abstract

The effects of various microphysical processes and the Coriolis force on the dynamics of squall systems were investigated with a two-dimensional, anelastic numerical model. The incorporation of ice-phase microphysics into the model has been found to be important in the successful simulation of realistic storm structure and evolution of squall lines. The significance of the ice-phase microphysics is largely accounted for by the small terminal velocities of ice particles and cooling by melting.

The response of the atmosphere to the cooling by melting is a complicated one and has been shown to play an important role in shaping the kinematic and precipitation characteristics of the observed and modeled squall systems. The interaction between the front-to-rear (FTR) flow and cooling by melting would both intensify (by enhancing the mesoscale updraft and the FTR flow above the melting layer) and limit (by partially driving the rear-to-front flow at the back of the stratiform region) the stratiform precipitation development.

The Coriolis force has also been found to have significant effects on the simulated squall systems. The rotational component of the storm flow field constrains the strength of the divergent wind field, which in turn limits the horizontal scale of the mesoscale circulation and the associated stratiform region. The model squall lines seemed to be most sensitive to the variations of f in the range between f = 0.7 × 10−4 s−1 and f = 1 × 10−4 s−1.

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Jinliang Liu, Ronald E. Stewart, and Kit K. Szeto

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.

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Kit Szeto, Peter Gysbers, Julian Brimelow, and Ronald Stewart
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Kit K. Szeto, Ronald E. Stewart, and Charles A. Lin

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.

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Kit Szeto, Xuebin Zhang, Robert Edward White, and Julian Brimelow
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Kit K. Szeto, Charles A. Lin, and Ronald E. Stewart

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.

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Julian Brimelow, Kit Szeto, Barrie Bonsal, John Hanesiak, Bohdan Kochtubajda, Fraser Evans, and Ronald Stewart

Abstract

In the spring and early summer of 2011, the Assiniboine River basin in Canada experienced an extreme flood that was unprecedented in terms of duration and severity. The flood had significant socioeconomic impacts and caused over $1 billion (Canadian dollars) in damage. Contrary to what one might expect for such an extreme flood, individual precipitation events before and during the 2011 flood were not extreme; instead, it was the cumulative impact and timing of precipitation events going back to the summer of 2010 that played a key role in the 2011 flood. The summer and fall of 2010 were exceptionally wet, resulting in above-normal soil moisture levels at the time of freeze-up. This was followed by record high snow water equivalent values in March and April 2011. Cold temperatures in March delayed the spring melt, resulting in the above-average spring freshet occurring close to the onset of heavy rains in May and June. The large-scale atmospheric flow during May and June 2011 favored increased cyclone activity in the region, which produced an anomalously large number of heavy rainfall events over the basin. All of these factors combined generated extreme flooding. Japanese 55-year Reanalysis Project (JRA-55) data are used to quantify the relative importance of snowmelt and spring precipitation in contributing to the unprecedented flood and to demonstrate how the 2011 flood was unique compared to previous floods. This study can be used to validate and improve flood forecasting techniques over this important basin; the findings also raise important questions regarding floods in a changing climate over basins that experience pluvial and nival flooding.

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