The Great Plains Dryline

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The Great Plains Dryline

• Just as slope flows occur on a variety of
  mountain-valley scales, so do dry lines
• The North American scale Rocky mountain barrier
  maintains a quasi-permanent dryline on the high
  plains
• The mechanism for the formation of the dryline is
  similar to all drylines.

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• Air on upwind (upslope) side of dry line is
  represented by a deep mixed layer
• Air west of the dryline has broken the inversion
  and so upper level dry and warm air from the EML
  has mixed down
• The inversion breaks on its western extremity
  first because the inversion is closet to the
  surface there
• As the daytime surface mixed layer deepens to the
  inversion throughout the day, the inversion break
  (dryline position) works its way eastward
• West of the dryline, mixing of the deep EML with
  the shallow moist layer produces
• Gusty westerly surface winds
• Very low humidities
• Warm temperatures

• Air east (downslope) of the dryline is relatively
  moist and cool, although there tends to be only a
  small virtual temperature contrast across the
  dryline
• Shallow moist boundary layer
• Source of moisture includes
• Evapotranspiration
• Advection from east (Gulf)
• Moist boundary layer reaches depth of the
  inversion at the point of the dryline
• Daily Evolution
• Late morning and afternoon
• Boundary layer grows below nocturnal inversion
• dryline eats its way eastward
• Evening
• Nocturnal inversion forms
• Moisture trapped under inversion
• Early morning
• Plants become active and transpire raising
  moisture under inversion
• Slope flow commences enhancing inversion

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Example of Dryline Movement Cycle (From Ray)
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Dryline Caught up in synoptic system
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Low-Level Jet

• Strong low-level (900-950mb) southerly jet formed
  over high plains peaking in the early morning
  (0200 LST)
• Thermal slope flow phenomena resulting from
  geostrophic adjustment on meso-alpha scale to
  slope flow
• Two arguments to explain it
• Development of wind maximum at top of nocturnal
  boundary layer
• Inertial oscillation of nocturnal flow above the
  nocturnal boundary but below the earlier daytime
  boundary layer height

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