Transcript Dryline

The Dryline
•The dryline can be defined as the near surface convergence zone between moist air
flowing off the Gulf of Mexico and dry air flowing off of semi-arid high plateau regions of
Mexico and the southwestern United States:
Temperatures in
degrees Celsius
Dry Line
• A surface boundary between warm, moist
air and hot, dry air.
• Found in the western Great Plains in the
United States
• Also found in China, India, and over
Central West Africa
Dry Line
• Surface boundary between warm, moist
air and hot, dry air.
New Mexico /
Tx Panhandle
Hot, Dry Air
Abilene, TX
Warm, Moist Air
Houston, TX
Dry Line
• After sunrise, the sun will warm the surface
which will warm the air near the ground.
• This air will mix with the air above the ground.
• Since the air above the moist layer is dry (and
is much larger than the moist layer), the
mixed air will dry out.
• The dry line boundary will progress toward
the deeper moisture.
• The dryline tends to “jump” in discrete steps
Dry Line
Top of moist layer
before mixing
Hot, Dry Air
Boundary after mixing
New Mexico /
Tx Panhandle
Abilene, TX
Warm, Moist Air
Houston, TX
Initial Position
of the Dry Line
Position of the
Dry Line after
mixing
Dryline Evolution and Movement
Here is the situation in the morning hours before surface heating creates thermals in the
boundary layer:
New Mexico /
Tx Panhandle
Abilene, TX
Houston, TX
Dryline Evolution and Movement
With time:
Dryline Evolution and Movement
With time, the dryline moves east as thermal turbulence mixes out the western part of
the shallow moist layer.
Dry Line
• After sunset, a nocturnal inversion forms and
the winds in the moist air respond to surface
pressure features.
• The dry line may progress back toward the
west – in sharp contrast to fronts (i.e. the
dryline advances eastward in the day and
retreats back toward the west at night)
• Dry line can act as a focal point for surface
convergence and severe weather formation.
• When radar cells exist within 200 miles either
side of the dryline, the first echoes usually
develop within 10 miles of it
Dry Line
Southern
Plains
Dry Line
©1993 Oxford
University Press
-- From:
Bluestein,
SynopticDynamic
Meteorology in
Midlatitudes,
Volume II
Temperatures in
degrees Celsius
Dry Line -- Southern Plains
USA
Behind the Dry Line
In Front of the Dry Line
Westerly winds (often strong)
S or SE winds
Clear Skies
Hazy or Cloudy Skies
Warm Temperatures
Warm Temperatures
Low Moisture
High Moisture
Dryline Example
ABQ
DFW
NCAR
Sounding
West
of the
Dryline
Sounding
East
of the
Dryline
The Dryline
•The dryline is observed generally from April through June about 40% of the time
•It’s observed from the southern Great Plains up into the Dakotas – from the Rockies
eastward to about 96 degrees W longitude.
•It is easiest to identify with a moisture variable such as mixing ratio:
From Shaefer (86)
Storm Initiation Along the Dryline
Dryline Bulges
•Somewhat common
•80-100 km in scale
•Preferred locations for
convective initiation
Shaefer (86)