Fine-Scale Observations of a Pre-Convective Convergence Line in the

1. Fine-Scale Observations of a Pre-Convective Convergence Line in the Central Great Plains on 19 June 2002 JP3J.1 JP3J.1 Benjamin Daniel Sipprell, sipprell@uwyo.edu, and Bart Geerts , University of Wyoming, Laramie, USA The Problem Questions: 1. How do mesoscale atmospheric processes and surface fluxes alter the convective boundary layer (CBL) to generate a dryline boundary? 2. How is dryline convergence maintained, at very small scales (Ziegler and Rasmussen 1998)? 3. How does deep convection initiate along a dryline at those scales? Sustained Convergence Density Current Dynamics Vertical Structure and Evolution DOW3 along with mobile mesonet data clearly show strong confluence near the dryline and along the UWKA flight track. A first stepped traverse shows that the dry air is cooler (θv lower), consistent with the westward tilt of the dryline & the negative solenoidal circulation. Early soundings through the CBL demonstrate the presence of a strong capping inversion at 850 mb and increasing values of CAPE. Objectives: 1. To describe the kinematic and thermodynamic properties of a pre- convective dryline at very high resolutions and in vertical cross sections. 2. To demonstrate via a case study that fine-scale convergence is driven by the buoyancy gradient, sustained by density current dynamics. Dual-Doppler analysis confirms the dryline tilt to the W and the negative solenoidal circulation. The first stepped traverse observes θv 0.5K cooler on the dry side than moist side, anda peak of 0.5K at the dryline. First series of stepped traverses shows θe and ‘r’ differences 3 K and 1.5 g kg-1, resp. across the dryline, later to increase to 10 K and 6 g kg-1 indicating dryline strengthening. During the 3rd stepped traverse the dryline becomes quasi-stationary and better-defined, according to DOW3 data. Discussion By 21 UTC cross dryline confluence increases with values of 10 ms-1over distances of hundreds of meters. Murphey et al. (2005) find that horizontal shearing along the dryline due to confluence yields high vertical vorticity along a contorted dryline on this day. There is a weak θv (virtual pot. temp.) gradient across the dryline. This gradient is consistent with the vertical tilt of the echo plume, and the vertical velocity couplet, indicating a thermally direct solenoidal circulation. The circulation and tilt reverse when the θv (virtual pot. temp.) gradient reverses. At the fine-line convergence zone (the dryline), anomalously high θv occurred, deepening in time till CI. Early (unusual eastward tilt) A remarkable transformation occurs between the 2nd and 3rd dryline stepped traverses: the dryline shifts from a westward to an eastward tilt with a consistent θv gradient reversal. Denser air flips from the W side to the E side of the dryline, possibly because of larger surface sensible heat flux to the W. The vertical velocity dipole consistently shifts, the solenoidal circulation becomes positive, and the eastward propagating fine-line becomes stationary. All this is consistent with density current theory. Late (classic westward tilt) Conclusions LearJet dropsondes and UWKA stepped traverses observe a deep core of positive buoyancy near the dryline. By the last series of transects the CBL depth above the dryline exceeds 3200 m AGL. Advection of high θe air into the CBL ‘dome’ results in the erosion of CIN. • Updrafts greater than 5 m/s are observed, collocated with anomalously high values of θv • This leads to local deepening of the CBL along the fine-line, leading to CI. • Dual-Doppler wind field demonstrates the existence of solenoidal circulations consistent with horizontal density differences, and changes in dryline propagation speed. DOW3 data, local mesonets and mobile mesonets show an increasingly intense southerly jet and thus increasing confluent flow into a evolving stationary dryline. REFERENCES: Murphey, Hanne V. and Wakimoto, Roger M., 2005: Dryline on 19 June 2002 during IHOP. Part I: Airborne Doppler and LEANDRE II Analysis of the Thin Line Structure and Convection Initiation. Mon. Wea. Rev.: in press. Ziegler, Conrad L. and Rasmussen, Erik N., 1998: The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective. Wea. and Forecasting: Vol. 13, No. 4, pp. 1106–1131.