A Case Study of Severe Storm Development along a Dryline within a Synoptically Active Environment. Part I: Dryline Motion and an Eta Model Forecast

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations. The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four ''steps'' along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels. The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north-south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

Analysis of a Dryline during IHOP: Implications for Convection Initiation

“…estimated that the secondary dryline formed around this time. The existence of double drylines has been well documented in the literature (e.g., Hane et al 1997;Crawford and Bluestein 1997;Hane et al 2001). It should be noted that the drier air that is suggested by the low refractivity values in the southeast corner of Fig.…”

Dryline Position Errors in Experimental Convection-Allowing NSSL-WRF Model Forecasts and the Operational NAM

“…However, the eastward bias was larger in April and May than in June, it was larger for trough cases than for ridge and zonal cases, and it was larger during 2010 and 2011 than in the other years. One thing the subgroups with larger eastward biases have in common is that they are all likely to be associated with a larger portion of ''active'' drylines, which tend to move much farther east than ''quiescent'' drylines (Hane 2004). Active drylines are typically linked with an eastward-moving synopticscale weather system that helps advect the dryline eastward in addition to vertical mixing processes, while quiescent dryline movement is dominated by processes associated with vertical mixing.…”

“…Past research has found that sensible weather parameters directly linked to boundary layer mixing processes are often associated with large forecast errors (e.g., Coniglio et al 2013 and references therein), but these studies have not explicitly examined forecast drylines. Hane et al (2001) evaluated dryline motion from a limited-area mesoscale model for one case. Ziegler et al (1997) anticipated that mesoscale model output could assist forecasters in preparing refined forecasts of the east-west location of drylines and the potential for dryline convection; however, the study mainly emphasized simulated convection initiation processes associated with drylines.…”

“…Also, only within the last decade or so have operational mesoscale models had adequate grid spacing to fully resolve the sharp moisture gradients associated with drylines. Other difficulties associated with locating and tracking drylines occur because drylines can ''jump'' (Hane et al 2001), sometimes more than one moisture gradient is present (Hane et al 1993;Crawford and Bluestein 1997), and other types of airmass boundaries (e.g., outflow boundaries and cold fronts) have some characteristics similar to those of drylines, making objective dryline identification more difficult. Finally, manually identifying drylines is quite labor intensive.…”

Mesoscale Thermodynamic Influences on Convection Initiation near a Surface Dryline in a Convection-Permitting Ensemble

“…This mesoscale circulation can influence dryline intensity and the characteristic deeper layer 40 of surface-based moisture extending several tens of km downwind from the dryline, which favors 41 CI (e.g., Ziegler et al 1995). Dryline intensity, motion and associated CI may be further 42 influenced by synoptic factors (e.g., Carlson et al 1983; Hane et al 2001) including confluence 43 and implied deformation frontogenesis from larger scales (e.g., Schultz et al 2007). 44…”