Positive Potential Vorticity Anomalies Generated from Monsoon Convection

Stephen M. Saleeby and William R. CottonDepartment of Atmospheric Science, Colorado State University

1. IntroductionThe 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

2. Monthly Averaged Potential Vorticity

Support for this work has been provided by a grant NA17RJ1228 #26 from the National Oceanic and Atmospheric Administration.

RAMS grid configuration for monsoon seasonal and surge simulations.

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.

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.

3. Monthly Averaged Vertically Integrated Condensate

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.

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.

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.

5. Ten-Day Averaged Potential Vorticity

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.

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 Four-Corners states.

8. Time Progression of PV and Condensate (Aug 29, ‘00)

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. SummaryThe 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.

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.

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76. Ten-Day Averaged Condensate and 500mb Winds