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Diagnostic study of a midlatitude frontal squall line




Lin, Xin, author

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Heat, moisture and momentum budgets of a squall line that occurred in the central United States on 26-27 June 1985 are investigated throughout its developing, mature and dissipating stages. The slow propagating behavior of the squall line made the 26-27 June data set unique since it covers a large fraction of the squall line life cycle, much longer than previous diagnostic studies. Budget studies have been performed at six different times at time intervals of 90 minutes using OK PRE-STORM (Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central) rawinsonde data. Secondary circulation forcing and frontogenetical/frontolytical effects are also computed to evaluate the importance of diabatic effects using diagnosed heating/ cooling distributions. The squall line was followed by a low-level cold front and was characterized by a narrow stratiform region and an intermediate asymmetric feature during the early stages. The flow pattern normal to the line was generally similar to previous squall line studies except that a low-level rear inflow associated with the cold front was superimposed upon expected squall line FTR/ RTF (front to rear/rear to front) flows . The midlevel RTF flow was quite weak well behind the squall line during the developing and mature stages and significantly strengthened during the dissipating stage, suggesting that the stratiform region plays an important role in RTF flow development. A convergence band resulting from system RTF and FTR flows extended rearward from low levels near the leading edge up to the rear of the system. During the developing and mature stages. peak convergence was located at low levels around the leading edge. At the dissipating stage. midlevel convergence behind the convective region intensified as stratiform region developed, while low level convergence near the leading edge gradually weakened. During the developing stage, a narrow vertical band of upward motion corresponding to a narrow convective line was present around the leading edge over Oklahoma. Moderately strong compensating downward motion occurred behind and ahead of the system. A wide band of upward motion was noticed over Kansas where stratiform and convective regions co-existed. The squall line became symmetric during its later stages. A low-level mesoscale downdraft developed as the stratiform region developed. Weak convergence and downward motion took place near tropospause above the squall line system. Both Q1 and Q2 showed tilted structure when the stratiform region developed, as did the w field. Distinct profiles are found for the convective and stratiform regions respectively, similar to previous diagnostic results. The system-averaged heating peak Q1 was located at middle levels between 500 and 550 hPa throughout the evolution. much lower than the 10-11 June case. The moisture sink Q2 clearly showed the evolutional contribution from the convective and stratiform regions at different stages. A single drying peak, which resulted from the convective region. was evident at low levels around 700 hPa through most of the developing and early mature stages. Midlevel drying feature during the early stages may probably be induced by two possible mechanisms: (1) midlevel drying generated by the trailing stratiform region. and (2) vertical eddy transport of water vapor within the convective region. During the late mature and dissipating stages, a double-peak structure became very pronounced. suggesting that mixture of convective and stratiform drying may be the causal mechanism at these stages. At later stage, a single drying peak resulting from the stratiform region was present at middle levels around 475 hPa. During the developing and mature stages, the squall line had little effect on the geopotential height field. At the late mature and dissipating stages, a pronounced mesoscale low was present within the stratiform region. It was also followed by a mesohigh. The couplet of the mesolow and mesohigh may explain most of the dramatic strengthening of the rear inflow at the back edge of the squall line during the dissipating stage. In the convective region. the line-normal momentum flux was generally negative, along with negative wind shear present above 600 hPa, the momentum flux transport was upgradient throughout most of its evolution, consistent with other studies. In the stratiform region during the dissipating stage, the momentum flux became positive and the flux transport appeared to be downgradient. The contribution of diabatic heating/cooling effect to secondary circulation forcing and frontogenetical/frontolytical effects is investigated using diagnosed Q1 data. Different from "dry convection" cases, the contribution of the diabatic effect to secondary circulation forcing above the low-level cold front within the system was one order of magnitude larger than the geostrophic stretching deformation and geostrophic shearing deformation terms, suggesting the important role the diabatic effect played in mesoscale circulation at middle and upper levels. At low levels around the cold front , the three terms contributed nearly equally to the forcing. In the frontogenesis equation, although the differential diabatic effect was one order of magnitude larger than the confluence term above the front , it was strongly opposed by the tilting term. Therefore, although the low-level cold front contributed to triggering the intense convection within the squall line, any feedback to frontogenesis appeared to be small. At low levels during the system developing stage, frontogenetical effect was present around and ahead of the leading edge and frontolytical effect behind. But at the dissipating stage, weak frontogenetical effect only occurred in a small region within the frontal zone, with frontolysis took place ahead and behind. This feature may help to explain why the movement of both the low level cold front and the squall line increased during the early stages and decreased rapidly and almost ceased at the dissipating stage.


Fall 1992.
Also issued as author's thesis (M.S.) -- Colorado State University, 1992.

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