Investigations of a Winter Mountain Storm in Utah. Part I: Synoptic Analyses, Mesoscale Kinematics, and Water Release Rates

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  • 1 Atmospheric Sciences Center, Desert Research Institute, Reno, Nevada
  • | 2 Centre de Recherches Atmospheriques, Universite Paul Sabatier-Observatoire Midi-Pyrenees, Lannemezan, France
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Abstract

A winter storm passing across the north–south-orientated Tushar Mountains in southwest Utah is investigated in this multipart paper. This Part I describes the evolving synoptic pattern, mesoscale kinematics, and calculated water release rates (condensation or deposition) in clouds over the western upstope part of the mountains. Horizontal mesoscale kinematic variables come from direct application of Volume Velocity Processing to single C-band Doppler radar data. Water release rates are computed from updrafts derived from the radar data and from the vertical gradient of saturation mixing ratio obtained from soundings.

In Stage I of the storm altostratus was present on the leading side of a long-wave trough. Weak updrafts occurred only at the higher altitudes within the clouds where there was convergence and large-scale synoptically forced lift. Downdrafts as great as −0.6 m s−1 occurred in the lower parts of the cloud where there was divergence. The downdrafts were induced in part by sublimation cooling of solid (ice) precipitation falling from the altostatus. Only virga was observed and the radar echoes did not reach the surface.

Stage II was initially dominated by passage of a short-wave aloft. Drier air associated with the short-wave led to complete evaporation of the altostratus of Stage I. The lower parts of this cloud (≤4.5 km MSL) eventually redeveloped into altocumulus.

Later in Stage II the wind veered more perpendicular to the mountains. Simultaneously, convergence developed in the lower 900–1200 m of the atmosphere, and mesoscale updrafts of 0.1–0.2 in m s−1 were calculated. Maxima in the water release rate were associated with the updrafts.

During Stage III a passing cold front influenced the kinematics and cloud and precipitation. From prior to frontal passage to a few hours afterward the wind beneath the frontal surface veered from southwesterly to northerly. There was strong convergence at low altitudes just upwind of the Tushar Mountains. It was accompanied by strong, deep mesoscale updrafts extending from near the ground up through the frontal surface and by water release maxima.

The storm changed character after the wind at low altitudes had veered to northerly and had become parallel to the Tushar Mountains. Convergence maxima continued to be present beneath the frontal surface but weaker. They preceded by ∼0.5 h maxima in the convergence above the frontal surface. Associated with these paired convergence features were updraft maxima located above the frontal surface. Water release rates were generally lower than earlier in Stage III. The decrease was greatest at low altitudes beneath the frontal surface where the wind had veered to northerly, where there was little uplift by the Tushar Mountains, and where updrafts were weak. Above the frontal surface the decrease in water release rate was not as great inasmuch as lift by the frontal surface was still occurring.

The storm dissipated in Stage IV. The axis of the longwave trough passed through the area, winds at higher altitudes beneath the frontal surface veered more northerly, and there was substantial drying at all altitudes above and below the frontal surface. The winds beneath the frontal surface were divergent, indicative of subsidence, and mesoscale downdrafts were present.

Abstract

A winter storm passing across the north–south-orientated Tushar Mountains in southwest Utah is investigated in this multipart paper. This Part I describes the evolving synoptic pattern, mesoscale kinematics, and calculated water release rates (condensation or deposition) in clouds over the western upstope part of the mountains. Horizontal mesoscale kinematic variables come from direct application of Volume Velocity Processing to single C-band Doppler radar data. Water release rates are computed from updrafts derived from the radar data and from the vertical gradient of saturation mixing ratio obtained from soundings.

In Stage I of the storm altostratus was present on the leading side of a long-wave trough. Weak updrafts occurred only at the higher altitudes within the clouds where there was convergence and large-scale synoptically forced lift. Downdrafts as great as −0.6 m s−1 occurred in the lower parts of the cloud where there was divergence. The downdrafts were induced in part by sublimation cooling of solid (ice) precipitation falling from the altostatus. Only virga was observed and the radar echoes did not reach the surface.

Stage II was initially dominated by passage of a short-wave aloft. Drier air associated with the short-wave led to complete evaporation of the altostratus of Stage I. The lower parts of this cloud (≤4.5 km MSL) eventually redeveloped into altocumulus.

Later in Stage II the wind veered more perpendicular to the mountains. Simultaneously, convergence developed in the lower 900–1200 m of the atmosphere, and mesoscale updrafts of 0.1–0.2 in m s−1 were calculated. Maxima in the water release rate were associated with the updrafts.

During Stage III a passing cold front influenced the kinematics and cloud and precipitation. From prior to frontal passage to a few hours afterward the wind beneath the frontal surface veered from southwesterly to northerly. There was strong convergence at low altitudes just upwind of the Tushar Mountains. It was accompanied by strong, deep mesoscale updrafts extending from near the ground up through the frontal surface and by water release maxima.

The storm changed character after the wind at low altitudes had veered to northerly and had become parallel to the Tushar Mountains. Convergence maxima continued to be present beneath the frontal surface but weaker. They preceded by ∼0.5 h maxima in the convergence above the frontal surface. Associated with these paired convergence features were updraft maxima located above the frontal surface. Water release rates were generally lower than earlier in Stage III. The decrease was greatest at low altitudes beneath the frontal surface where the wind had veered to northerly, where there was little uplift by the Tushar Mountains, and where updrafts were weak. Above the frontal surface the decrease in water release rate was not as great inasmuch as lift by the frontal surface was still occurring.

The storm dissipated in Stage IV. The axis of the longwave trough passed through the area, winds at higher altitudes beneath the frontal surface veered more northerly, and there was substantial drying at all altitudes above and below the frontal surface. The winds beneath the frontal surface were divergent, indicative of subsidence, and mesoscale downdrafts were present.

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