4.1 Map Composites and Climate Anomalies

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4.0 RESULTS PART I Synoptic Composites for Peak
Emergence Map-Pattern Classification
4.1 Map Composites and Climate Anomalies
JULY-AUGUST CLIMATOLOGY (1948-2003)
PEAK EMERGENCE COMPOSITE (N64)
ANOMOLOGY SIGNIFICANCE (MSLP 500 hPa Heights)

• July-August Climatology
• The regional climatology of British Columbia in
  summer is dominated by the Pacific High, and a
  weak southwesterly flow pattern which generally
  produces light southerly surface winds at YXS due
  to steering by the mountains that flank the
  central-interior plateau.
• Peak Emergence Composite
• The peak emergence events are similar to the mean
  climatology for summer, but are characterized by
  stronger than normal ridging of the Pacific High
  into BC, and an intense heat low centered over
  California that extends northward into the
  southern half of the province.
• The ridge at 500 hPa is centered over the
  composite criteria area, resulting in a relative
  maximum in the instability of the lower
  atmosphere over east-central BC.
• The deviation of the composites from normal
  climatology are shown in the right-hand column
  with statistical significance shaded at the 95
  level.

MSLP (hPa) Tmax (oC)

• Surface Winds
• Winds during peak emergence are generally light,
  but highly variable, and vary both diurnally and
  throughout the 5-day heating cycle.
• Thermal instability mixes the stronger winds
  aloft down to near the surface, resulting in a
  higher frequency of stronger winds with a
  westerly component in the late afternoon, when
  the surface temperature reaches a maximum.
• To further examine this variability, a manual
  map-pattern classification of the 500 hPa
  composite was performed.

500 hPa (dm) LFTX (oC)
4.2 500 hPa Classification of Map Sub-types
RIDGE OVER HEAD (29)
RIDGE TO EAST (31)
RIDGE TO WEST (40)

• Classification Criteria
• An area one reanalysis cell wide, and two cells
  long, centered over YXS was used to classify all
  heating cycle days as exhibiting either a ridge
  to the west (RTW), overhead (ROH) or to the east
  (RTE).

• Frequency Properties of Map-Subtypes
• The synoptic flow varies from northwesterly with
  the RTW, to southwesterly under the RTE scenario,
  and the area of instability in east-central BC is
  larger with the RTE.
• The composite and frequency distribution of the
  map sub-types indicate that the RTW has a greater
  residence time within the 5-day heating cycle,
  and that the RTE is associated with the end of
  the cycle.
• The frequency distribution is therefore
  suggestive of the mean evolution of the synoptic
  conditions during peak emergence events (see Part
  II).
• The variation of the surface winds associated
  with each map sub-type is described below.

4.3 Surface Wind Properties by Map-type
Classification of All Heat Cycle Days
Mean Diurnal Winds Consistency

• Mean afternoon wind speeds are typically less
  than 3 m/s, and on any given day, are only
  greater than 6 m/s approximately 5 of the time.
• The RTW exhibits a more pronounced decrease in
  the mean evening wind speed, and the RTE has a
  higher mean wind speed.
• At the location of YXS, winds generally rotate
  clockwise throughout the daytime hours under the
  RTW and ROH scenario.
• The RTE exhibits greater consistency in the wind
  direction under a southwesterly flow during the
  early part of the day, but with a more sudden
  shift to westerly during the time of maximum
  temperature, and an accompanying increase in wind
  speed.

Frequency Distribution of Daytime Winds (1000
1600 PST)

• Over the course of a typical 5-day heating cycle,
  the day time winds rotate anti-clockwise from a
  northerly to southerly direction.
• Winds with the ROH are generally lighter, and the
  consistency of the wind direction during all
  hours of the day is less than 50.

Variation of Daytime Winds Beneath 500 hPa Ridge
(ROH)

• To better resolve the variation of winds
  surrounding the period of peak emergence, the
  map-pattern classification was repeated using
  only the maps corresponding to day-three of the
  heating cycle, and were classified as exhibiting
  either a RTW, or a RTE.
• Winds have a more pronounced northerly and
  westerly component under the RTW scenario, while
  under the RTE scenario, winds have a greater
  southerly and easterly component.

Classification of Peak Composite