The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall Over the Central United States
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Journal titleWeather and Forecasting
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AbstractTwenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly lowlevel jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 1408) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature ue advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surfacebased stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximumu e CAPE values of more than 1000 J kg21 in the vicinity of the MCS site and higher values upstream. Maximumu e convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg21) while to the south convection is limited by large values of CIN (greater than 60 J kg21). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.
CitationMoore, J. T., Glass, F.H., Graves, C.E., Rochette, S.M. and Singer, M.J., 2003, The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall Over the Central United States, Wea. Forecasting , 18 (5), 861-878. Available on publisher's site at http://journals.ametsoc.org/doi/abs/10.1175/1520-0434(2003)018%3C0861%3ATEOWET%3E2.0.CO%3B2
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