Positive Potential Vorticity Anomalies Generated from Monsoon Convection

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Positive Potential Vorticity Anomalies Generated
from Monsoon Convection
Stephen M. Saleeby and William R.
Cotton Department of Atmospheric Science,
Colorado State University
1. Introduction The CSU-RAMS model has been
utilized to simulate the North American Monsoon
for the 1988, 1993, 1997, and 2000 summer seasons
from June 1 - August 31. These warm season
simulations are examined for linkages between
monsoon convection and precipitation over the
southwest and central U.S. Monsoon bursts result
in enhanced southerly flow from the Gulfs of
California and Mexico, and thus contribute to
increased low-level moisture convective
7. Ten-Day Averaged Potential Vorticity Transport
4. Monthly Averaged 500mb Streamlines
June average
July average
August average
June 3-12, 2000 Avg
Aug 22-31, 2000 Avg
potential over the southwest and Great Plains
however, this does not guarantee development of
precipitation systems over the U.S. Northward
advection of positive potential vorticity
anomalies, generated by monsoon convection over
northwest and central Mexico, aids in the
development of new convection over regions that
would otherwise have minimal background positive
potential vorticity.
The transition of the monsoon ridge is very
distinct between June and July with a large
northward displacement leading to flow up the
Gulf of California into the southwest U.S.
Steering winds also transition from northwesterly
flow near the eastern slopes of the Sierra Madres
to easterly flow that is more suited for
assisting in advection of high PV air and monsoon
moisture into the southern U.S. By August, the
monsoon ridge extends its influence further
north, and more southerly flow persists over
Arizona and New Mexico. The extended ridge in
August is most favorable for advection of PV
anomalies into the west U.S.
Prior to monsoon onset (left) PV transport (PVU x
m/s) into the southwest U.S. is due to baroclinic
systems from the north. During a surge event
(right) there exists a continuous southerly flow
of higher PV air from Mexico into the
Four-Corners states.
The model configuration is shown to the right.
Simulations use single-moment microphysics,
LEAF-2 surface model, two-stream radiation,
Mellor-Yamada TKE for diffusion, Klemp-Wilhelmson
lateral boundary conditions, and Kain-Fritsch
cumulus scheme.
RAMS grid configuration for monsoon seasonal and
surge simulations.
8. Time Progression of PV and Condensate (Aug 29,
00)
2. Monthly Averaged Potential Vorticity
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2
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5. Ten-Day Averaged Potential Vorticity
June average
July average
August average
June 3-12, 2000 Avg
Aug 22-31, 2000 Avg
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5
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The above potential vorticity fields (PVU) are
averaged for the given month over the 1988, 1993,
1997, and 2000 warm seasons. Each of these
seasons, aside from 1988, had U.S. monsoon onset
later than July 1. The monthly averaged PV
increases dramatically over central and western
Mexico from June to July, and over the southwest
U.S. and Great Plains from July to August. This
is coincident with the onset of monsoonal flow
and increased frequency of surge events into late
July and August. The maxima in PV over southwest
Colorado and western Kansas in August is well
correlated with a region of MCS genesis during
this period of the warm season.
In the pre-onset average (left), positive PV
(PVU) is limited to central-eastern Mexico and
the Pacific northwest (from late season
baroclinic systems). During an August surge
period (right), there is a stream of high PV air
extending from Mexico to the Four-corners states
and Great Plains. The intrusion of higher PV air
will aid in development of precipitation systems
to the lee of the Rockies.
A 7.5 km grid spacing simulation was run from Aug
22-31, 2000 to better capture the details of a
monsoon burst period. The resulting time
progression of PV (shaded, PVU) and condensate
(contoured, mm x 10) displays the lifetime of an
individual PV anomaly from its generation over
the mountains of Mexico to its role in generation
of a mesoscale precipitation system to the lee of
the mountains in Colorado.
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3. Monthly Averaged Vertically Integrated
Condensate
June average
July average
August average
9. Summary
The development of widespread monsoon related
convection along the slopes of the SMO in central
Mexico is a source for generation of regionally
strong positive potential vorticity. While
northward surges of moisture from the Gulfs of
Mexico and California provide needed fuel for
convective development over the southwest, a
trigger mechanism is needed in this area
dominated by a large-scale ridge. THE NORTHWARD
ADVECTION OF HIGH POTENTIAL VORTICITY AROUND THE
MONSOON RIDGE INTO THE SOUTHWEST U.S. AIDS IN
DEVELOPMENT OF PRECIPITATION SYSTEMS THAT PROVIDE
CRUCIAL WARM-SEASON RAINS TO THE SOUTHWEST AND
CENTRAL U.S.

In the pre-onset average (left), the presence of
widespread vertically integrated condensate (mm)
is contained in southern Mexico where monsoon
onset is just beginning. During the surge event
in late August (right) there is a clear visual
linkage between the development of convection and
condensate along the SMO and that which is
advected into the western U.S.
The averaged condensate field (mm x 10) increases
substantially over northwest Mexico by July and
over the southwest U.S. by August. This is
visually well correlated with areas of increased
potential vorticity shown above.
Support for this work has been provided by a
grant NA17RJ1228 26 from the National
Oceanic and Atmospheric Administration.