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Iowa State Univ.: "Large-scale signals for mesoscale phenomena affecting Iowa"

Final Report


The project proposed to study 2-3 mesoscale winter weather systems that produced narrow snowbands in the Iowa region. We examined roughly 5 years of data, and found 2 cases where mesoscale cyclones produced snow bands. Because of the limited number of these events, we extended the research to examine a classic Alberta Clipper, to compare and contrast the larger-scale features during a Clipper event with that of these meso-low events.

Large-scale diagnostic budget studies were performed for both the February 15 1996 winter mesoscale vortex case described in Gallus and Bresch (Monthly Weather Review, 1997) and a somewhat weaker, less organized case occurring on December 4 1997. In addition, budgets were computed also for the Alberta Clipper event of December 30 1998. NCEP forecast data were used to explore the large-scale forcings associated with the system. The diagnostic analyses focused on three aspects: (1) the three-dimensional structure of the synoptic- scale environment, (2) the vorticity budget, and (3) the heat budget. The major goal of this work was to search for the genesis mechanisms of these meso-vortices from a synoptic perspective.

Our findings are as follows:

(1) The two meso-lows were generated within synoptic-scale systems, which consisted of upper short-wave troughs coupled with lower-tropospheric lows ahead of the troughs (a typical developing baroclinic wave). However, the most unusual feature associated with these synoptic systems was the meridional juxtaposition of the trough and ridge lines extending westward from the lower-tropospheric low, especially in the 2/15/96 event. These trough and ridge lines established the meridional stratification of the synoptic- scale background vorticity.

(2) Vorticity budget analyses revealed that the eastward propagation of the synoptic-scale troughs advects the meridionally stratified vorticity and facilitates the genesis of the meso-lows.

(3) Strong low-level cold air advection induces low-level upward motion to the south of the front in each case. Coupling between the vortex stretching induced by this upward motion and synoptic-scale vorticity advection results in the genesis of the meso-lows.

(4) The larger-scale environments for both the meso-low events and the Alberta Clipper case were somewhat similar. All of the events are shallow in nature, and the intensity of the disturbances is strongly influenced by low-level static stability. It appears the February 15, 1996 event was unique in that lower-level static stability was unusually small, and the larger scale environment acted to restrict the surface signal of the forcing to a very small area. The other meso-low event was much weaker with more stable low-level conditions. In addition, the larger-scale environment in the Alberta Clipper case is more favorable for surface cyclone organization on a larger scale.

In addition to these accomplishments related directly to the original objectives, the following related tasks were also assisted by the interaction stemming from this project:

(1) Hands-on forecasting exercises were developed at Iowa State University (funded under other sources) to improve science education. These exercises involve forecasting of warm season convective severe weather, and disruptive winter weather events such as the meso-lows studied in
the Partners Project. Several datasets representing severe convective events were transferred to DMX-NWS. In addition, the web-based forecasting exercise is now available for use by the DMX office, and any interested offices, where it can serve as professional training material, particularly for new forecasters. The exercise is described in a paper (Gallus et al.) to be published in the Bulletin of the Atmospheric Sciences in summer 2000.

(2) Craig Cogil and Karl Jungbluth of the NWS provided Doppler radar instruction for an undergraduate instrumentation class at ISU. This included a lecture session at ISU and two labs demonstrating the WSR-88D radar at the NWS office. As mentioned in (1), ISU archived data
has been utilized for local research and training on severe storms at the NWS.

(3) Karl Jungbluth presented a combined lecture/lab for the Instrumentation class at Iowa State on April 24.


As mentioned above, NWS personnel were able to assist in teaching of a university course. In addition, William Gallus presented early results from the research at a seminar at the DMX office in November 1999. An undergraduate student funded under the Partners Project also volunteered time to assist the DMX office in an informal internship mode. In addition, another undergraduate student who played a prominent role in the development of the web-based severe weather forecasting activity has also begun to assist the DMX office with computational needs. The student has implemented NCEP's most recent workstation version of the Eta model on an Iowa State computer where we plan to run the model with 10-15 km resolution over Iowa in real-time and post output to the Internet for use by the DMX office. The student is also assisting in getting that model running locally on a machine at the DMX office; however, the computational resources available there are not as large, which may limit the use of the model.


Gallus, W. A., Jr., T.-C. Chen, and S. Krentz, 1999: Large-scale signals for mesoscale phenomena affecting Iowa. Seminar, presented at National Weather Service, Des Moines, IA, November 22.

T.-C. Chen and W. A. Gallus, Jr., 2000: Large-scale signals for mesoscale phenomena affecting Iowa. Manuscript under preparation; to be submitted to Monthly Weather Review.

Gallus, W. A., Jr., T.-C. Chen, and S. Krentz, 2000: A comparison of large-scale patterns for an Alberta Clipper case and an intense winter mesoscale vortex. Seminar planned for NWS-DMX Cold Season Training Symposium in Fall 2000.


4.1 The primary benefit to Iowa State University was the enhancement of an already good and active research relationship with the DMX office. National Weather Service personnel were able to assist in the teaching of radar applications in an upper-level undergraduate/graduate course on instrumentation. The collaboration allowed the university researchers to learn about AWIPS, which will be helpful in the teaching of synoptic meteorology. Feedback at a seminar given at the DMX office helped the university researchers maintain some focus on application of the research in the operational setting. In addition, the DMX office has offered to provide ISU with some new observing system data.

4.2 Analysis results of the first case have been shared with the NWS, helping us to understand the numerical modeling complexities and predictability of such small scale events. NWS forecasters have learned that simply adding data density at the surface may not solve our mesoscale forecast problems. ISU is being recognized by the NWS as a source of expertise in numerical modeling and mesoscale analysis. One student who worked on this project was a Student Volunteer at the Des Moines NWS Office, so he already carried some understanding of the forecast process with him to the project.