The Kinematics of Orographic Airflow During Sierra Storms

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  • 1 Department of atmospheric Science, University of Wyoming, Laramie 82701
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Abstract

Two case studies of the kinematics of the airflow over the Sierra barrier are presented. The observations consisted of rawinsondes and single Doppler RHI and velocity azimuth display (VAD) analysis of PPI scans. The RHI scans were made orthogonal to the nearly two-dimensional Sierra barrier. The cloud in the first case study contained a strong stable layer at 0°C while the second storm was highly unstable.

The radar bright band and soundings near the radar indicated that an ∼250 m thick 0°C isothermal layer was present in response to the diabatic process of melting. When the bright band was impinging upon the barrier, the associated 0°C isothermal layer was inferred to expand until it finally reached the ground. At that point it was ∼1 km in depth. Direct thermodynamic soundings am presented for a similar situation which agrees with the modified soundings inferred for this study. The resulting effects on the airflow and precipitation are discussed.

The unstable case contained a line of deep convection oriented parallel to the crest The deep convection acted to transport a substantial amount of low-level unstable air upward and the convection also blocked the airflow such that a wake was present downwind of the line of convection.

Abstract

Two case studies of the kinematics of the airflow over the Sierra barrier are presented. The observations consisted of rawinsondes and single Doppler RHI and velocity azimuth display (VAD) analysis of PPI scans. The RHI scans were made orthogonal to the nearly two-dimensional Sierra barrier. The cloud in the first case study contained a strong stable layer at 0°C while the second storm was highly unstable.

The radar bright band and soundings near the radar indicated that an ∼250 m thick 0°C isothermal layer was present in response to the diabatic process of melting. When the bright band was impinging upon the barrier, the associated 0°C isothermal layer was inferred to expand until it finally reached the ground. At that point it was ∼1 km in depth. Direct thermodynamic soundings am presented for a similar situation which agrees with the modified soundings inferred for this study. The resulting effects on the airflow and precipitation are discussed.

The unstable case contained a line of deep convection oriented parallel to the crest The deep convection acted to transport a substantial amount of low-level unstable air upward and the convection also blocked the airflow such that a wake was present downwind of the line of convection.

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