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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.
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.
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.
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.
Abstract
A supercell storm was observed in detail near Grover, Colo., on 18 June 1970. The storm was observed by an S-band radar operated alternately in PPI and RHI modes. An instrumented aircraft was flown near cloud base in the updrafts, and packets of chaff were released and tracked by radar within the weak echo region. Examination of the radar data revealed a remarkable similarity between certain RHI photographs of the Grover storm and the Soviet model of a hailstorm. Comparisons among the environmental conditions observed for other supercell storms revealed certain similar characteristics. The descriptive model of supercell storms by Browning was clarified and refined based on these new observations.
Abstract
A supercell storm was observed in detail near Grover, Colo., on 18 June 1970. The storm was observed by an S-band radar operated alternately in PPI and RHI modes. An instrumented aircraft was flown near cloud base in the updrafts, and packets of chaff were released and tracked by radar within the weak echo region. Examination of the radar data revealed a remarkable similarity between certain RHI photographs of the Grover storm and the Soviet model of a hailstorm. Comparisons among the environmental conditions observed for other supercell storms revealed certain similar characteristics. The descriptive model of supercell storms by Browning was clarified and refined based on these new observations.
Abstract
A case study of a severe hailstorm which occurred in an extremely sheared environment is presented. The storm occurred near Fort Morgan, Colo., on 15 June 1970, and contained a large, persistent bounded weak echo region (WER). The Fort Morgan storm evolved in a manner and displayed several characteristics similar to another storm previously synthesized by Chisholm which also occurred in an extremely sheared environment. It is proposed that the extreme shear probably acted to erode the turbulent air containing precipitation sized particles from around the updraft core, thus allowing the large, bounded WER to persist in each storm. The erosion or detrainment effect is speculated to have accounted for the evolutionary characteristics of these two storms.
Abstract
A case study of a severe hailstorm which occurred in an extremely sheared environment is presented. The storm occurred near Fort Morgan, Colo., on 15 June 1970, and contained a large, persistent bounded weak echo region (WER). The Fort Morgan storm evolved in a manner and displayed several characteristics similar to another storm previously synthesized by Chisholm which also occurred in an extremely sheared environment. It is proposed that the extreme shear probably acted to erode the turbulent air containing precipitation sized particles from around the updraft core, thus allowing the large, bounded WER to persist in each storm. The erosion or detrainment effect is speculated to have accounted for the evolutionary characteristics of these two storms.
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.
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.
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.
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.
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.
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.
Abstract
A north–south-oriented line of convection, associated with a melting-induced mesoscale front, formed south of the Mile High Radar (MHR) during the early hours of the Front Range blizzard of 6–7 March 1990. The kinematic structure of the mesoscale front, that is, precipitation band, was examined by analyzing single-Doppler radar data from MHR with two analysis techniques. The results from the analyses were augmented and compared with observations from three surface stations within the Winter Icing and Storms Project (WISP) area.
Because of the extraction of the latent heat of fusion from the atmosphere as ice particles melted, the temperature dropped rapidly as the rainband passed. This resulted in a large pool of stable, 0°C air behind the mesoscale front. As the melting continued, this pool expanded toward the east where it was not blocked by terrain. It expanded into a convectively unstable air mass with strong southeasterly winds. A band-parallel low-level jet developed east of the band and remained essentially stationary, while the band propagated beneath and to the east of the jet, at which point the band dissipated. The shallow pool of stable air continued to move slowly to the east.
Abstract
A north–south-oriented line of convection, associated with a melting-induced mesoscale front, formed south of the Mile High Radar (MHR) during the early hours of the Front Range blizzard of 6–7 March 1990. The kinematic structure of the mesoscale front, that is, precipitation band, was examined by analyzing single-Doppler radar data from MHR with two analysis techniques. The results from the analyses were augmented and compared with observations from three surface stations within the Winter Icing and Storms Project (WISP) area.
Because of the extraction of the latent heat of fusion from the atmosphere as ice particles melted, the temperature dropped rapidly as the rainband passed. This resulted in a large pool of stable, 0°C air behind the mesoscale front. As the melting continued, this pool expanded toward the east where it was not blocked by terrain. It expanded into a convectively unstable air mass with strong southeasterly winds. A band-parallel low-level jet developed east of the band and remained essentially stationary, while the band propagated beneath and to the east of the jet, at which point the band dissipated. The shallow pool of stable air continued to move slowly to the east.
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.
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.
