CENTRAL ILLINOIS AND INDIANA
The Central Illinois Chapter and the Indiana Chapter of the American Meteorological Society held a joint meeting at the Beef House restaurant in Covington, Indiana on Thursday, June 27, 2003. Approximately 40 were in attendance from both states, traveling from as far away as Lincoln in Illinois and Mishawaka and Muncie in Indiana.
This meeting is an annual tradition as many members enjoy the pilgrimage to the Beef House, often listed as the "Best Steakhouse in Indiana." This year the food and service lived up to their reputation.
Central Illinois President Ed Kieser opened the meeting at 6:30 PM with some pre-dinner welcoming remarks. After dinner Ed Kieser and Dr. David Arnold, President of the Indiana Chapter, each spoke about upcoming events. Both mentioned that the next meetings for each chapter will be held in September, with the Indiana Chapter meeting at WISH-TV in Indianapolis, IN and the Central Illinois Chapter meeting at Weldon Springs State Park in Clinton, IL. Details on dates and times will be announced later in the summer. Elections for officers will be held for each chapter at the September meetings. Announcements were also made on preparations for the Midwest Extreme and Hazardous Weather Region Conference to be held in Champaign on October 17-18, 2003.
The speaker for the evening was Dr. Jeff Trapp, recently appointed as Associate Professor of Earth and Atmospheric Sciences at Purdue University. The title of his presentation was New Insight into Tornadoes and Damaging Winds with Squall Lines and Bow Echoes.
Dr. Trapp started his presentation by showing a radar reflectivity image of a "quasi-linear convective system" (QLCS) from near Jackson, Mississippi on November 11, 1995. This squall line produced an F3 tornado. Most of the time we think of tornadoes as coming from parent storms that are individual cells, particularly super cells. Yet, many tornadoes do occur with squall lines. The speaker indicated that we should care about studying tornadoes associated with squall lines and bow echoes because (1) tornadoes can occur anywhere along an extensive line, (2) radar precursors are not as clearly understood for tornadoes associated with lines, so there tends to be less lead time for warnings, and (3) the climatological characteristics of squall line tornadoes are unknown, such as whether these tornadoes are weak and short lived, whether they occur with the same frequency as "cell" tornadoes, and whether they occur at the same general time of day.
To determine the climatological characteristics of QLCS tornadoes, the following steps were followed: (1) gather tornado reports from Storm Data online, (2) use composite radar reflectivity images to classify each parent storm type as cell or line, (3) repeat steps 1 and 2 for all tornadoes during 1998-2000, and (4) analyze the results. For 1998-2000 there were 3827 reported tornadoes in the continental U.S. Of those, 80% were classified as having a parent storm as a "cell," 18% as from a "line," and 2% from "other" types like hurricane rainbands.
In terms of tornado intensity, there were fewer F0 tornadoes reported than expected for lines. The speaker speculated that F0 "line" tornadoes might be underreported, perhaps due to low visibility or that many might occur at night. The distribution of tornadoes by local time showed the typical peak in the late afternoon and evening for "cell" tornadoes. For "line" tornadoes the frequency was also greater in the afternoon and evening, but the overall distribution was more even, with indeed many more "line" tornadoes occurring at night than "cell" tornadoes.
In Illinois and Indiana, a higher percentage of tornadoes were reported from lines than the national average. From 1998 through 2000 there were 274 tornadoes reported in Illinois and Indiana, with 59% from cells and 41% from lines, compared to 80% to 18% ratio nationally.
More needs to be learned about QLCS tornadoes as it was found that (1) a high percentage of QLCS tornadoes occur in certain geographical areas, such as in Illinois and Indiana, (2) such tornadoes can be strong or violent, although they are rare events, for both line and cell classifications, and (3) QLCS tornadoes frequently occur during nighttime hours. At night there is less warning information due to the spotter value being diminished and there is poorer warning dissemination as many affected citizens are sleeping.
Next, Dr. Trapp spoke on damaging non-tornadic winds with squall lines and bow echoes. He presented Fujita's (1985) conceptualization of high-wind generation in (type "B") bow echoes and asked if it tells the whole story. He used the Klemp-Wilhelmson Model to simulate a damaging QLCS. The model was initialized using a high value for CAPE and moderate low-level shear.
The simulated squall line evolved into a bow echo with "straight-line" surface winds found at the bow-echo apex and additionally in association with numerous low-level mesovortices. For certain environmental shears, the mesovortex-induced winds are more damaging in terms of intensity, duration, and areal extent that are the descending rear-inflow winds at the apex.
Finally, research is continuing in part with the Bow Echo and MCV Experiment (BAMEX) this summer. Dr. Trapp's objectives from BAMEX include conducting detailed aerial and ground surveys of wind damage following bow echo events and relating the severity and scale of damage to (radar observed) location/structural characteristics. They recently collected good data from a case on 31 May 2003, and look forward to analyzing data from BAMEX.
After questions the meeting adjourned at about 9:15 PM.---Ed Kieser.
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