Download Climatology of Dry Lines

What is a dryline
MET 171B: Drylines
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Mike Voss
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An airstream boundary separating moist air
originating over the Gulf of Mexico from dry
air originating over the plateaus of
southwestern U.S. and Mexico.
The dryline typically forms along the high
plains in the spring months and is a frequent
site of convection initiation.
Dry Lines
Climatology of Dry Lines
Defined: Narrrow zone of strong moisture gradient (dew
points) at the surface
…. Really a dry line is a dew point front…
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Where???
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Why??
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This typically represents a boundary between air masses
(moist maritime vs. dry continental)
Typically located over the Plains from Texas
northward to the Dakotas in spring and summer
Rarely seen east of the Mississippi
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Moist air masses that originate over the Gulf
mingle with air masses that were modified and or
develop over the interior West
Topography Matters
Typical Dryline
Dryline Symbology
1. Descending air in lee of Rockies is warm and dry
2. Descending air also leads to lee side trough
What process would initiate southerly flow from Gulf
and westerly flow from SW over a place like
KS/OK??
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Our friend lee… and a surface high over the SE US
Temperatures in
degrees Celsius
©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology in Midlatitudes, Volume II
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Synoptic conditions associated with strong and weak
periods in dryline are outlined in the handout journal
paper
Strong defined by: high Delta Td, high Delta U
Weak defined by: opposite
See paper for details
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Air on upwind (upslope) side of
dry line is represented by a deep
mixed layer
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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 :
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Gusty westerly surface winds
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Very low humidities
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Warm temperatures
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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
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Daily Evolution:
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Typical Cross Section View of
Dryline (see images from lecture!)
Shallow moist boundary layer
Source of moisture includes
z Evapotranspiration
z Advection from south east
(Gulf)
Moist boundary layer reaches
depth of the inversion at the point
of the dryline
Late morning and afternoon:
z Boundary layer grows below
nocturnal inversion
z dryline eats its way eastward
Evening:
z Nocturnal inversion forms
z Moisture trapped under
inversion
Early morning
z Plants become active and
transpire raising moisture
under inversion
z Slope flow commences
enhancing inversion
1. Elevated Mixed Layer
advected east over moist layer
4. Heating of the day allows
inversion to mix out from
west to east. Dry line
moves.
Topography
West Side
East Side
Mixed Layer, breaks through
shallow inversion early in day
Moist (cool) air in boundary
layer, grows during nocturnal
inversion, mixes with heating
of day
Interior West
Plateau
3. The Intersection with the
Topography to the West is
the Dry Line
2. Warm Mixed Layer Overrides
the Moist Air to create an
Inversion
Heating, no inversion
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Quiescent (non-synoptic forced)
Types of Dryline Environments
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The two types are:
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Quiescent (absence of synoptic forcing)
Synoptically Active
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Synoptically Active
The dryline typically
extends from a surface
low pressure center.
Often positioned ahead
of the cold front
Vertical motions from a
mid- or upper-level
short-wave trough
exacerbate quiescent
diurnal pattern
This allows for drier air
aloft to mix with the
moist boundary layer air
allowing the dryline to
propagate eastward.
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The dryline’s motion is mainly due to vertical mixing as
a result of diurnal heating along topography
Moves eastward during the late morning and early
afternoon hours (advances mix the cool moist air with
the well mixed component as heating progresses) +
advection
Moves westward during the late evening and overnight
hours the dryline retrogrades
Dryline Bulge
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Non uniform tropospheric flow can advance portions of
the dryline. The combination of low-level convergence +
shear can locally enhance convective uplift.
Features are typically pronounced with synoptic forced
dryline
Drylines = Convergence
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Drylines tend to be areas of locally enhanced
convergence at the surface.
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Approximately 70% of the time convective storms
form within 200 km 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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