Climate
Patterns of the GVRD: Regional
and Local Scale Precipitation Regimes
The
topography has a major influence on the precipitation regime within the GVRD.
The range of annual precipitation between different locations can vary from
below 1,000 mm to more than 3,000 mm. The southern portions of the lower Fraser
Valley such as Delta and Tsawwassen typically receive approximately 1,000 mm
or less of rain annually. The high ridges of the Northshore Mountains, such
as Hollyburn Ridge and Grouse Mountain, normally receive more than 2,500 mm
of precipitation annually. Overall, there seems to be a trend between elevation
and enhancement of precipitation that can be attributed to a strong orographic
effect (Hetherington, 1979; GVRD, 2002).
Orographic effect is the general term for the phenomenon of precipitation enhancement
attributed by uplifting and cooling of warm air masses over a high terrain.
It has been studied in detail for the better part of the 20th century. Barry
(1981) gives a good review of the studies of the orographic effect that have
been done since as early as 1945. Barry states that the amount of orographic
precipitation depends on three factors operating on different scales: (1) air
mass characteristics and the synoptic-scale pressure pattern; (2) local vertical
uplifting motion due to the topography; and (3) microphysical processes in the
cloud and the evaporation of falling drops. Furthermore, it is not only affected
by the terrain but also by the proximity to moisture sources and the aspect
relative to the approaching wind (Whiteman, 2000). Figure
4 shows a simple illustration of orographic effect.
