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ABSTRACT
This study identifies the common large-scale environments associated with the development of derechoproducing convective systems (DCSs) from a large number of events. Patterns are identified using statistical clustering of the 500-mb geopotential heights as guidance. The majority of the events (72%) fall into three main patterns that include a well-defined upstream trough (40%), a ridge (20%), and a zonal, low-amplitude flow (12%), which is identified as an additional warm-season pattern. Consequently, the environmental large-scale patterns idealized in past studies only depict a portion of the full spectrum of the possibilities associated with the development of DCSs.
In addition, statistics of derecho proximity-sounding parameters are presented relative to the derecho life cycle as well as relative to the forcing for upward motion. It is found that the environments ahead of maturing derechos tend to moisten at low levels while remaining relatively dry aloft. In addition, derechos tend to decay as they move into environments with less instability and smaller deep-layer shear. Low-level shear (instability) is found to be significantly higher (lower) for the more strongly forced events, while the low-level storm-relative inflow tends to be much deeper for the more weakly forced events. Furthermore, discrepancies are found in both low-level and deep-tropospheric shear parameters between observations and the shear profiles considered favorable for strong, long-lived convective systems in idealized simulations. This study highlights the need to examine DCS simulations within more realistic environments to help reconcile these disparities in observations and idealized models and to provide improved information to forecasters.
1. Introduction
A derecho-producing convective system (DCS) is a type of mesoscale convective system (MCS; Zipser 1982) that produces large swaths of severe "straight-line" winds at the earth's surface. Straight-line winds attributed to convection are typically grouped into those generated by "mesohighs" (Johnson and Hamilton 1988), "gust fronts" (Goff 1976; Wakimoto 1982), or "downbursts" (Fujita and Wakimoto 1981). Much of the damage owing to nontornadic convective winds has been attributed to derechos (Wakimoto 2001). Although derechos appear to occur most frequently during the summer months across the midwestern United States, they have been observed during all months of the year and in most locales east of the Rocky Mountains (Johns and Hirt 1987; Bentley and Mote 1998).
Johns and Hirt (1987; JH87 hereafter) define...