Transcript Drylines

Drylines
By: Allie Vegh
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Definition: A dryline is a zone of strong
horizontal moisture gradient separating
warm, moist air from hot, dry air in the
boundary layer.
The warm, moist air originates from the Gulf
of Mexico.
The hot, dry air originates in the elevated
terrain of southwestern United States and
northern Mexico.
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The hot, dry air caps the warm, moist
air to create an inversion referred to as
the dry front.
The intersection of the dry front with
the topography is known as the dryline.
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Drylines tend to form east of the Rocky Mountains in
the western Great Plains.
Drylines are present on approximately 40% of spring
and early summer days.
They are typically about 500 to 1000 km in length
and 1 to 20 km wide.
Drylines appear on surface charts as sharp gradients
in the dewpoint temperatures up to ~ 18 K in a
distance of 1 to 10 km.
The depth of the low-level moisture out in front of
the dryline will determine its speed. The less
moisture in the lower atmosphere the faster the
dryline will move.
Types of Dryline Environments
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The two types are:
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Synoptically Active
Quiescent
Synoptically Active
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The dryline typically extends from a surface
low pressure center.
Its motion is generated by turbulent vertical
mixing in the boundary layer due to diurnal
heating and horizontal and vertical motions
from a mid- or upper-level short-wave trough.
This allows for drier air aloft to mix with the
moist boundary layer air allowing the dryline
to propagate eastward.
Quiescent
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These drylines form in an environment that
does not contain synoptic-scale forcing.
The dryline’s motion is mainly due to
turbulent vertical mixing as a result of diurnal
heating.
During the late morning and early afternoon
hours the dryline propagates in an eastward
direction, and during the late evening and
overnight hours the dryline retrogrades back
to the west.
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Most drylines tend to be quiescent in form with a
diurnal pattern.
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Eastward movement during the day and westward
retrogression during the night.
The overall net motion of the dryline is typically to the east.
The slope of the dry front strengthens during the day
and weakens at night.
Surface winds in the moist air however maximize
around midnight and are weaker during the day.
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This is due to the nocturnal low level jet which can form as a
result of the dryline earlier in the day.
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Drylines tend to be areas of locally enhanced
convergence at the surface.
Due to the enhanced convergence drylines
tend to be a favorable location for convective
initiation and isolated severe storms.
Approximately 70% of the time convective
storms form within 200 nm of the dryline.
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These storms account for a large percentage of
the precipitation the Great Plains receive during
the spring and summer months.
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Dryline bulges:
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Area of strong surface winds that
accelerate a portion of the dryline farther
east than the rest of the dryline.
Bulges tend to be areas of increased
convergence and have a greater potential
for convective activity.
References
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Journal of the Atmospheric Sciences
Monthly Weather Review
Weather & Forecasting
Weatherwise