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John Marwitz

Abstract

The winter orographic storms over the San Juan Mountains and the Sierra Nevada are compared. The topography of the San Juans is complex while the Sierra barrier is comparatively simple. The barrier jet is well developed upwind of the Sierra Nevada and its development is restricted upwind of the San Juans. The major difference between the storms on the two barriers is that the Sierra Nevada storms are typically maritime while the San Juan storms are continental. The implications for seeding are discussed.

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Yulan Wei and John Marwitz

Abstract

The Front Range blizzard of 6 March 1990 resulted in heavy rain and snow along the foothills of Colorado and in southeast Wyoming. A narrow barrier jet with northerly winds behind a shallow mesoscale front developed concurrently with the heavy precipitation. It was hypothesized by Marwitz and Toth that the mesoscale front was the result of the diabatic process of melting. The CSU RAMS model was used to test the effects of melting, as well as the roles that the ice process and upslope flow played in the storm.

A two-dimensional simulation was initialized with bulk microphysics and with a modified Flagler, Colorado, sounding. The results showed that the simulation was able to produce many features similar to the observations, such as surface cooling, a northerly barrier jet, and a steady. slow-moving shallow mesoscale front. It was found that these features were much less pronounced in an identical simulation but with melting turned off. Furthermore, it was found that diabatically cooled air was accumulated along the foothills and induced a direct circulation. These results supported the hypothesis that melting was a dominant process contributing to the development of the storm. A simulation without ice microphysics showed that ice altered the kinematic and thermodynamic structures. A simulation with halved upslope flow showed that upslope flow was an important factor modifying the precipitation pattern and a critical parameter in determining the kinematic, thermodynamic, and precipitation patterns.

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Naihui Song and John Marwitz

Abstract

A technique for numerical simulation of a stationary, two-dimensional laminar flow process is described. Based on this technique, a model for warm rain microphysics in an orographic cloud was developed. The model includes condensation, coalescence and sedimentation.

The coalescence process depletes the cloud droplets, causing the supersaturation ratio to rise and may cause additional cloud condensation nuclei to activate. The model predicts the initial shape of droplet spectra fairly well for large drops compared with the field observations. There was a discrepancy, however, between the predicted and observed droplet spectra. It was found that the observed coalescence rate was much faster than the calculated rate. The implications are discussed.

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John D. Marwitz

Abstract

The thermodynamic and kinematic structure of two stable orographic storms were described in Part I based on instrumented aircraft data and single Doppler radar data. The precipitation processes in these storms are described in this paper. The storms were deep with cloud top temperatures of about −25°C. Below the melting level the cloud droplet population was continental with a mean droplet diameter <10 μm. Above the melting level the cloud droplet population was maritime with mean droplet diameters of 20 to 30 μm. Near the −5°C level a peak in ice crystal concentration of 30 to 200 L−1 was observed. Since most of the ice crystals were needles, are rime-splintering secondary ice crystal production processes as generally described by Hallett and Mossop was probably occurring.

Calculations of the condensation supply rates were compared with the depletion rates by deposition and accretion. The depletion rates by deposition were less than half the condensation supply rates, and the liquid water contents remained low. Accretion is deduced to be the dominant process, which acts to deplete the condensate to near zero. Deep, stable orographic storms over the Sierra barrier, therefore, develop an efficient glaciation process.

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John D. Marwitz

Abstract

On 12 February 1973 an airflow case study was documented across the San Juan Mountains in south-west Colorado. The main observation system was an NCAR Queen Air aircraft. Several supplementary observations were available from the weather modification project being conducted in the area. The airflow data were synthesized and compared with previous laboratory simulation results over the same area. The orographic cloud contained a number of imbedded convective clouds which had an important effect on the airflow and vertical diffusion processes. A precipitation efficiency was derived using a technique which avoided most of the critical assumptions of previous attempts.

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John D. Marwitz

Abstract

Two case studies are presented of multi-cell storms in Alberta which displayed separate modes of propagation. Discrete propagation occurred on the right flank of both storms as in multi-cell storms previously documented by Browning and Ludlam in England and Chisholm in Alberta. The storms which were synthesized by Browning and Ludlam and by Chisholm deviated to the right due only to discrete propagation. The individual cells of the first storm (Alhambra storm) propagated continuously to the right in addition to the discrete propagation, which caused the Alhambra storm to deviate ∼55° to the right of the mean environmental winds. On the other hand, the individual cells in the second storm (Rimbey storm) were observed to propagate continuously to the left of the mean environmental winds. The continuous propagation of the cells to the left was offset by the discrete propagation to the right. Schematic models of the Wokingham, Alhambra and Rimbey storms are presented.

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John D. Marwitz

Abstract

The Colorado River Basin Pilot Project was conducted over the San Juan Mountains in southwestern Colorado and ran for five winter seasons, terminating in 1974–75. The objective of the project was to demonstrate the feasibility of increasing the amount of snowpack and, therefore, the amount of available runoff. The Bureau of Reclamation, through its contractors, conducted the project. A number of statistical evaluations of the program have been made. This series of papers represents the principal physical evaluation of the seeding potential of San Juan storms.

The synthesis of several well-documented San Juan storms indicates that most storms evolve through four distinct stages which are related to thermodynamic stability. The stages in sequence are stable, neutral, unstable and dissipation. During the stable stage, much of the flow below mountain top level is blocked and diverted toward the west. During the neutral stage, the storm is deep; it typically extends throughout much of the troposphere. During the unstable stage, a zone of horizontal convergence appears to form near the surface at the base of the mountain on the upwind side and a convective cloud line is often present over this convergence zone. Subsidence at mountain top height causes dissipation. Rare but well-organized storms containing a baroclinic zone that extends throughout the troposphere also pass over the San Juans. Blocked flow does not appear to occur in the well-organized storms.

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John Marwitz and James Toth

Abstract

Over 1 m of snow fell in the foothills of southeast Wyoming and northeast Colorado during the storm of 6–7 March 1990. The heavy snowfall combined with strong winds to product blizzard conditions resulting in major highways being closed for several days. The heaviest snow fell in the vicinity of a narrow northerly barrier jet that developed in place along the Front Range of the Colorado Rockies. Strong warm-air advection from the southeast was observed during the development of the barrier jet; there was no cold-air advection from the north. Rapid intensification of the barrier jet took place only after precipitation started and was concurrent with the development of heavy precipitation. A mesoscale front marked the transition from southeasterly to northerly flow. This front remained very close to the rain-snow line and progressed toward the east at 1–2 m s−1 for about 15 h. Melting precipitation appears to be the dominant process contributing to the development of the barrier jet and mesoscale front.

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Hunter Coleman and John Marwitz

Abstract

A unique wintertime storm occurred on 12 February 1992 during the Stormscale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST) field project. This storm consisted of a narrow east–west-oriented snow swath (max 20-cm snow depth) with an extensive area of freezing rain to the south. The event was well observed by various networks and systems. Included in these systems were about twice the normal sounding sites, which released rawinsondes every 3 h, single- and dual-Doppler capabilities, and NCAR King Air and Wyoming King Air aircraft. Four aspects of the storm were investigated: the temporal evolution of the low-level jet, the vertical stability of various layers, the development of isothermal layers, and the dynamical effects resulting from melting.

The freezing rain related to this storm was a result of an overrunning situation, and the snow swath was produced from conditional symmetric instability with respect to the ice process in the overrunning cloud layer. The warm frontal layer was dynamically unstable in terms of the Richardson number and contained some shear-induced gravity waves. A convective boundary layer was present near the surface. A low-level jet was present at the top of the convective boundary layer and along the snow–freezing rain interface. Isothermal layers developed just below the bright band as a result of diabatic cooling because of melting. The diabatic process of melting appears to have enhanced the speed of the low-level jet and triggered and focused the release of the thermodynamic instability so that an enhanced precipitation rate occurred over the snow–freezing rain interface.

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