Synoptic climatological aspects of summer Dry Spells in the Canadian Praries
Date
1973
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Doctoral
Abstract
This is an attempt to measure the frequency of summer Dry Spells over
the Canadian Prairies and to discover their possible causes. Statistical
and synoptic climatological techniques are used as tools in the analysis
and discussion of the occurrence and causes of Dry Spells and, in addition,
make possible the delineation of areas characterized by low or high frequency
of Dry Spells. The analysis reveals a higher frequency of Dry Spells
and a greater variability of precipitation in the summer months in a zone
extending along the Saskatchewan-Alberta boundary.
A very significant factor related to Dry Spells is the position of an
upper level stationary or quasi-stationary ridge (north-south axis) over
the Prairies. The position of the upper level ridge affects the dry conditions
in two ways: firstly, because of the blocking action, the upper
level ridge dislocates the jet stream, tracks of cyclones and associated
moist air masses, so that they lie north of the Prairies; secondly, the
anticyclonic circulation under this ridge of high pressure induces divergence
and subsidence of air accompanied by inversion of temperature at
lower levels.
Other significant factors which contribute to Dry Spells are associated
with surface synoptic conditions. These latter include: an apparent
eastward migration of high and low pressure systems (Aleutian Low, North
Pacific High, Hudson Bay Low and North Atlantic High); and the position of
the migratory anticyclones over the southern and southeastern Prairies. The
dry mPs air mass originating from the Pacific inversion layer also has some
influence on the occurrence of Dry Spells.
The upper level and surface atmospheric flow patterns (i.e., from
surface to 500 millibar levels) are classified into five synoptic Weather
Types and is demonstrated that Weather Type WA (ridge dominant over British
Columbia), Weather Type WB (ridge dominant over central Prairies) and
Weather Type We (ridge dominant over eastern Prairies) correlate with a
high probability of dry conditions over the Prairies. When the preceding
flow patterns are persistent with a quasi-stationary upper level ridge
(north-south axis) over the Prairies, Dry Spells occur. These synoptic
and dynamic atmospheric conditions are summarized in three models explaining
possible relationships pertaining to the probability of dry conditions and
Dry Spells over the Prairies.
Description
Keywords
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Geography
Program
Geography