Download Positive Potential Vorticity Anomalies Generated from Monsoon Convection

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
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 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.
2. Monthly Averaged Potential Vorticity
4. Monthly Averaged 500mb Streamlines
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.
5. Ten-Day Averaged Potential Vorticity
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.
6. Ten-Day Averaged Condensate and 500mb Winds
3. Monthly Averaged Vertically Integrated Condensate
7. Ten-Day Averaged Potential Vorticity Transport
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 FourCorners states.
8. Time Progression of PV and Condensate (Aug 29, ‘00)
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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.
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.
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.
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.
Support for this work has been provided by a grant NA17RJ1228 #26 from the National
Oceanic and Atmospheric Administration.
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.