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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
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
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.
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.
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
Three-dimensional tracks of 21 slow-fall chaff packets have been obtained while the packets were rising in the weak echo regions of eight separate Colorado hailstorms. The chaff packets were released at cloud base in the strong smooth updrafts and tracked with a M-33 track radar. In many cases the chaff was released from an instrumented aircraft. From these data it is shown that the inflow air often has its origin near the surface, the inflow air is typically negatively buoyant below cloud base, there exists a significant non-hydrostatic pressure perturbation in most severe storms, and a vertical velocity maximum typically exists within the weak echo region.
Abstract
Three-dimensional tracks of 21 slow-fall chaff packets have been obtained while the packets were rising in the weak echo regions of eight separate Colorado hailstorms. The chaff packets were released at cloud base in the strong smooth updrafts and tracked with a M-33 track radar. In many cases the chaff was released from an instrumented aircraft. From these data it is shown that the inflow air often has its origin near the surface, the inflow air is typically negatively buoyant below cloud base, there exists a significant non-hydrostatic pressure perturbation in most severe storms, and a vertical velocity maximum typically exists within the weak echo region.
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.
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.
Abstract
Two deep, orographic storms were documented over the Sierra Nevada with an instrumented aircraft and with a single Doppler radar. In both storms the geographic winds were normal to the barrier with speeds of 15 to 30 m s−1. Because of blocking the measured wind component normal to the barrier was substantially less than the geostrophic wind component, especially below barrier crest height. In the very stable case, a barrier jet was present below 1 km AGL and had peak winds of 28 m s−1. In the slightly stable case a barrier jet was present at 1 km AGL and had peak winds of 21 m s−1. Other than the speed and altitude of the barrier jet, the other measured characteristics of the airflow and thermal fields were rather similar between these two storms. The forced ascent of stable air plus the diabatic process of melting caused the isotherms to descend near the barrier. The barrier jet can be expressed by the thermal wind relation and is, therefore, a quasi-geostrophic phenomenon. The derived vertical velocities were estimated to be 0.2 to 0.4 m s−1.
Abstract
Two deep, orographic storms were documented over the Sierra Nevada with an instrumented aircraft and with a single Doppler radar. In both storms the geographic winds were normal to the barrier with speeds of 15 to 30 m s−1. Because of blocking the measured wind component normal to the barrier was substantially less than the geostrophic wind component, especially below barrier crest height. In the very stable case, a barrier jet was present below 1 km AGL and had peak winds of 28 m s−1. In the slightly stable case a barrier jet was present at 1 km AGL and had peak winds of 21 m s−1. Other than the speed and altitude of the barrier jet, the other measured characteristics of the airflow and thermal fields were rather similar between these two storms. The forced ascent of stable air plus the diabatic process of melting caused the isotherms to descend near the barrier. The barrier jet can be expressed by the thermal wind relation and is, therefore, a quasi-geostrophic phenomenon. The derived vertical velocities were estimated to be 0.2 to 0.4 m s−1.
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
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.
This paper reports the results from the latest in a series of scientific exchanges concerning the Soviet anti-hail program. Additional details on the Soviet technique of hail suppression were learned. An impression on the types of storms and the accompanying synoptic conditions was gained. In addition to visual impressions, some case studies are presented.
The North Caucasus region resembles northeastern Colorado from a topographical and meteorological point of view, except that a dry line or dew-point front is not normally present north of the Caucasus Mountains. The abundance of low-level moisture in this region seems to produce storms which resemble cloud streets in that lines of storms are aligned with the wind rather than normal to the wind. Although the large supercell and multicell storms typical of the High Plains of North America do occur in the Caucasus region, they are rare and the Soviet scientists are apparently unable to suppress all the hail on these types of storms.
This paper reports the results from the latest in a series of scientific exchanges concerning the Soviet anti-hail program. Additional details on the Soviet technique of hail suppression were learned. An impression on the types of storms and the accompanying synoptic conditions was gained. In addition to visual impressions, some case studies are presented.
The North Caucasus region resembles northeastern Colorado from a topographical and meteorological point of view, except that a dry line or dew-point front is not normally present north of the Caucasus Mountains. The abundance of low-level moisture in this region seems to produce storms which resemble cloud streets in that lines of storms are aligned with the wind rather than normal to the wind. Although the large supercell and multicell storms typical of the High Plains of North America do occur in the Caucasus region, they are rare and the Soviet scientists are apparently unable to suppress all the hail on these types of storms.
