to appear in proceedings of 19th Conference on Severe Local Storms,
American Meteorological Society - Minneapolis, Sept., 1998
Department of Atmospheric Sciences and
National Center for Supercomputing Applications
University of Illinois at Urbana-Champaign
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1. INTRODUCTION
Thunderstorms are often organized into squall lines, frequently along existing or developing mesoscale features such as fronts and drylines. Several patterns of cell development and organization are common, and such mesoscale convective systems (MCSs) are sometimes accompanied by significant precipitation and/or severe weather in the southern plains (Bluestein and Jain, 1985, hereafter BJ85; Houze et al., 1990, hereafter H90). Many of these convective systems eventually exhibit the leading-line/trailing stratiform precipitation structure discussed by H90, who further classified such lines into symmetric (about the line center) and asymmetric types. Severe weather in the form of hail and tornadoes was more common with asymmetric systems, in which new cell formation was preferred on one (typically south) end of the line, and decaying cells accompanied by a larger stratiform region were found on the other. Such asymmetric MCSs formed in environments of greater low-level along-line shear and lower bulk Richardson number than their symmetric counterparts.
BJ85 discussed the initiation and evolution of convective lines (under 50 km
wide) responsible for severe weather in Oklahoma. Among the MCSs meeting their
criteria, broken and back-building lines were responsible for just over half of
all severe events. Bluestein, Marx and Jain (1987) found that 70% of
back-building and 47% of broken lines were found to produce severe weather.
Broken lines form as individual cells develop and the line begins to fill
in, perhaps forming a solid line on radar. Back-building lines, which
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fall
into H90's asymmetric model, may start with an individual storm and build
southward over great distances. Back-building squall lines have been noted and studied for some time. Newton (1963) suggested a front moving at an angle into a region of greater instability might yield back-building convection, though BJ85 showed that this need not be the case. Newton and Fankhauser (1964) outlined the structure of such lines and discussed the larger anvil area and stratified precipitation downwind, the growth of new cells upwind and the vertical wind profiles accompanying such systems. BJ85 found that back-building lines formed along drylines, cold fronts, and near intersections of boundaries, and suggested back-building behavior might be linked to along-line variation of the capping inversion combined with surface heating. Several questions remain concerning these squall lines. BJ85's composite soundings suggest that back-building lines formed in conditions of slightly less instability, larger convective inhibition, and greater vertical shear compared to broken lines, though these differences were not statistically significant in their study. In addition, they note similarities between the environments of back-building lines and supercells, and suggest that back-building behavior might be internally forced, rather than a consequence of the mesoscale forcing. Indeed, the relationship between the back-building lines and the forcing is not completely known. We seek to better understand the development of back-building lines with (possibly) distinct southern end cells, and to understand the processes which distinguish back-building from broken-line development. We will evaluate the premise that along-line variation in convective inhibition and instability is sufficient, under mesoscale forcing, to produce back-building behavior. This is |
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distinct from recent modeling work investigating the role of along-line
differences of environmental shear and rear-to-front flow (Bélair and
Zhang, 1997) in squall line structure and from studies explaining symmetric and
asymmetric-shaped (and sometimes supercellular) squall lines under uniform
thermodynamic conditions (Rotunno et al., 1988; Weisman, 1993; Skamarock et
al., 1994).
2. METHODOLOGY
We have previously modeled broken line behavior by simulating a cold front in two dimensions (slab-symmetry, with along-front velocity predicted) and moving the fields to three dimensions. Random perturbations were introduced and three-dimensional broken line convection developed (Jewett and Wilhelmson, 1996). We plan to study back-building convection by incorporating along-line variation in instability when moving to 3-D. This meridional gradient will result in increasing temperature, moisture, instability and (for the shear profiles of interest) a greater capping inversion southward. The increased inversion strength may result in a final southernmost cell with no further extension of the line; such cells in nature are typically the strongest in the line. We will compare back-building cells forming in this environment with those with the mesoscale forcing removed to test the hypothesis that the forcing is an important part of the back-building process and that back-building is not solely an intrinsic property of thunderstorms forming in the local stability and shear along the mesoscale boundary. We will compare our results to simulations producing distinct right-flank cells by Weisman and Klemp (1984) and Dudhia and Moncrieff (1989). Preliminary simulations are underway and results will be presented at the Conference.
3. ACKNOWLEDGMENTS
Simulations were carried out on the Pittsburgh Supercomputing Center C90 and the NCSA CRAY Origin2000. Additional computing and other support was provided by NCSA. This work was supported by the National Science Foundation under grant ATM 96-33228. |
4. REFERENCES
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