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The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective

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The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective 1106 WEATHER AND FORECASTING VOLUME 13 The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective CONRAD L. ZIEGLER AND ERIK N. RASMUSSEN NOAA/National Severe Storms Laboratory, Norman, Oklahoma (Manuscript received 21 November 1997, in ®nal form 30 July 1998) ABSTRACT The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Pro®ler Studies project, the Veri®cation of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 ®eld projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets de®ne the ®elds of air¯ow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression. The results of the present study con®rm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modi®cation of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAPE and convective inhibition values according to parcel theory but neglecting vertical boundary layer circulations, are found to have less prognostic value than is conventionally assumed. 1. Introduction induced by the failure to anticipate the development of Improved knowledge of how deep, moist convection large, rain-cooled airmasses and cloudiness. On some is initiated is of fundamental importance to the U.S. occasions during the spring, large regions are forecast National Weather Service (NWS) and other operational to have a moderate or high risk of severe thunderstorms, forecasting groups. Improved nowcasts (i.e., 0±3-h fore- but the skies remain clear. Though statistical techniques casts) and short-term forecasts (6±12 h) of convective may ultimately provide reliable forecasts of precipita- initiation and the mode of early convection could prove tion amounts and coverage, successful mesoscale pre- invaluable to severe local storm forecasters to help focus diction with numerical atmospheric forecast models will their attention on rapidly evolving mesoscale weather require improved understanding and an accurate explicit scenarios. or parameterized representation of convective initiation. It is our impression that dramatic warm season weath- This forecast challenge stems from a fundamental lack er forecast failures are often the result of an inability of knowledge regarding the processes that allow or pre- to anticipate the initiation of convection, or forecasting vent the initiation of deep, moist convection. convection that fails to develop. Incorrect convective In spite of the dif®culties of collecting representative initiation forecasts cause quantitative precipitation fore- in situ measurements on the scales of individual incip- cast errors, and large errors in temperature forecasts are ient clouds, special mesoscale observations collected by a variety of ®xed and mobile instrumented platforms have provided important insights regarding the envi- ronments of developing convection. The capability to Corresponding author address: Dr. Conrad L. Ziegler, Mesoscale detect clear air boundary layer structures near devel- Research and Applications Division, National Severe Storms Labo- ratory, 1313 Halley Circle, Norman, OK 73069. oping moist convection is offered by single or multiple E-mail: [email protected] ground-based Doppler radars (Eymard 1984; Parsons et Unauthenticated | Downloaded 09/29/21 11:18 PM UTC DECEMBER 1998 ZIEGLER AND RASMUSSEN 1107 al. 1991; Wilson et al. 1994) and airborne Doppler ra- in the boundary layer raises the question of whether dars (Wakimoto et al. 1996). Research aircraft have pro- near-surface air feeds developing boundary layer cu- vided in situ observations of air¯ow and thermal prop- mulus clouds (Renno and Williams 1995). erties near boundaries that are believed to play a central Thunderstorms often form near drylines of the south- role in the initiation of convection (e.g., Ziegler and ern U.S. plains (Rhea 1966; Bluestein and Parker 1993), Hane 1993). The subjective interpretation of geosta- but their initiation is dif®cult to forecast. According to tionary satellite data provides fundamentally important an ``ingredients based'' approach to thunderstorm fore- guidance on the initiation of convection along bound- casting (Johns and Doswell 1992; McNulty 1995), the aries (Purdom 1982), while cloud-mesoscale models coincidence of low convective inhibition (CIN) with have played an increasingly important role by providing high convective available potential energy (CAPE) and complete and internally consistent datasets with which deep tropospheric wind shear along a well-de®ned me- to test various hypotheses. This combination of obser- soscale boundary with strong low-level convergence vations and models has led previous investigators to strongly suggest a high likelihood for the initiation of conclude that storm initiation is closely linked to bound- severe convection. During the 1994 and 1995 ®eld ary layer convergence lines. phases of the Veri®cation of the Origins of Rotation in The primary effect of a convergence line is to deepen Tornadoes Experiment (VORTEX) (Rasmussen et al. the moist layer locally and provide a region potentially 1994), deep moist dryline convection did not develop favorable for deep convection. The initiation of moist in several cases despite very favorable ingredients. In convection has been investigated along a variety of each of these cases where deep convection did not de- boundaries in differing geographical regions, including velop, small values of CIN at the dryline suggested that Florida sea breezes (Wakimoto and Atkins 1994; Fank- rising air parcels could easily attain the LCL and LFC hauser et al. 1995); mountain-induced ridge-top and lee and grow into deep convection (Colby 1984). Since se- convergence zones in New Mexico (Raymond and Wil- vere storms developed along several drylines observed kening 1982) and Colorado (Banta 1984), respectively; during the 1991 Central Oklahoma Pro®ler Studies pro- Colorado Front Range convergence zones (Wilson and ject (COPS-91) (e.g., Hane et al. 1993), a comparison Schreiber 1986); and drylines (Hane et al. 1993; Ziegler of the COPS and VORTEX cases affords the opportu- et al. 1997; Hane et al. 1997; Atkins et al. 1998) as well nity to begin exploring the necessary requirements for as the intersection of an east±west baroclinic zone with initiating deep dryline convection. a dryline (Bluestein et al. 1990) on the southern U.S. In accord with a key U.S. Weather Research Program plains. Koch and Ray (1997) use Weather Surveillance (USWRP) objective of re®ning quantitative precipita- Radar-1988 Doppler (WSR-88D) data to document the tion forecasts via improved observations and knowledge initiation of thunderstorms along sea breezes, the Pied- of boundary layer processes (Emanuel et al. 1995), the mont front, and other boundaries in North Carolina. present study focuses on the impact of boundary layer Moist boundary layer air may be elevated to its lifted evolution on the initiation or inhibition of deep moist condensation level (LCL), forming a ``forced'' cumulus, convection along the dryline. Employing selected cases while additional forced lifting may bring enough air to from the 1991, 1994, and 1995 ®eld projects, we ex- the level of free convection (LFC) to form an ``active'' amine the connections between the development of shal- deep, moist convective cloud (Stull 1985). Deep con- low and deep moist convection, the intensity of the hor- vection may form at the intersection of a convergence izontal thermal gradients and the vertical circulations line with horizontal convective rolls where enhanced that accompany cloud initiation, and the realization of updrafts are present (Wilson et al. 1992; Atkins et al. convective instability. Section 2 presents the data 1995) as well as at collision points of thunderstorm sources and analysis techniques, while section 3 pre- out¯ows with other out¯ows or sea breezes (Kingsmill sents the case studies. Discussion of the results in sec- 1995). tion 4 is followed by conclusions in section 5. The sensitivity of boundary layer cumulus convection to wind shear and thermal
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