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John D. Marwitz
and
Jim Toth

Abstract

Heavy snowfall occurred in central Oklahoma on 14 December 1997. The snowfall continued throughout the following day, with over 30 cm of snow falling from Oklahoma to Michigan. The snow in Oklahoma fell from a warm-frontal snowband that was oriented northeast-southwest.

An objective analysis of the synoptic-scale data indicated that frontogenesis was present in the warm-frontal region and a direct circulation pattern around the warm front. The ageostrophic winds above the warm front forced the conditionally unstable air to ascend and release its instability. The forcing mechanisms for the snowband were, therefore, frontogenetic forcing and convective buoyancy.

A series of plan position indicator volume scans were obtained with the NSSL Doppler radar at Norman, Oklahoma. Single-Doppler analysis techniques were used to calculate the mesoscale kinematic properties of the snowband. The data from a special rawinsonde released within the band was combined with the radar-derived kinematic structure to reveal the mesoscale thermodynamic structure of the band.

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John D. Marwitz
and
Paul J. Dawson

Abstract

A number of low-level flights were conducted with an instrumented aircraft to investigate wind characteristics in the planetary boundary layer over the low regions of the continental divide in southern Wyoming. The airflow upwind of the continental divide is stably-stratified and as it converses through Wyoming's “wind corridor&rdquo, a strong horizontal thermal gradient or baroclinic zone develops. The coldest air moves through the northern part of the wind corridor and is ovelain by a very stable layer. Trapped lee waves are prevalent in the planetary boundary layer of the wind corridor. Through the corridor the forces normal to the flow an in approximate balance and the flow is an anomalous anticyclonic flow around a low. Downwind of the wind corridor the airflow forms a convergence zone on its southern boundary with air from north-central Colorado. The convergence zone appears to remain distinct for ∼ 100 km downwind of the Medicine Bow Mountains.

The flow through smaller gaps was also investigated. Flows with hydraulic jump characteristics were observed.

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Kenneth L. Grandia
and
John D. Marwitz

Abstract

Data were obtained for three High Plains thunderstorms in which penetrations were made of the weak echo region by an instrumented aircraft. The data from one of the cases are presented in detail. Two of the storms were steady state, as revealed by chaff analysis and subsequent subcloud passes. The third storm dissipated during the penetrations. The three storms were each characterized by negatively buoyant air at cloud base. Chaff released into the updrafts of the storms did not decelerate below the level of free convection (LFC). A vertical pressure perturbation gradient, therefore, existed below the LFC and within the weak echo region which acted to accelerate the air parcels in the presence of negative buoyancy. The analysis of the equivalent potential temperature fields for the two steady storm cases revealed considerable entrainment of environment air into the weak echo region. The mixing of the entrained parcels probably caused the observed increase of turbulence with height.

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