Search Results

You are looking at 1 - 3 of 3 items for :

  • Author or Editor: K. K. Szeto x
  • Journal of Climate x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
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.

Full access
K. K. Szeto
and
H. Guan

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.

Full access
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.

Full access