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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 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

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.

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John D. Marwitz

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.

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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

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.

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Ronald E. Stewart
and
John D. Marwitz

Abstract

The microphysical consequences of seeding stratiform clouds near the Sierra Nevada Mountains are examined. Airborne seeding was conducted with droppable AgI flares released every 250 m and with dry ice pellets released at a rate of 0.1 g m−1 into the clouds having widespread liquid water contents ∼0.1 g m−3. The Wyoming King Air penetrated the AgI curtains for ∼1 h after seeding. The CO2 ice crystal curtain could not be determined beyond ∼10 min because of natural cloud glaciation. Precipitation sized particles grew mainly by diffusion, and particle size spectra at particular levels below cloud top reached and maintained equilibrium shapes as a consequence of particles falling from higher levels.

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Marianne English
and
John D. Marwitz

Abstract

Three convective clouds extending above a stratocumulus layer were identified as being seedable on one day and were then seeded in a random sequence with CO2 pellets, a placebo and droppable AgI flares. The radar and microphysical seeding effects were observed with the Alberta Hail Project S-band radar and with the University of Wyoming Queen Air aircraft. Distinct seeding effects were observed in both seeded clouds by both data systems. The CO2 seeded cloud developed a single curtain of precipitation particles 18 min after seeding which reached the ground 20 min after seeding and ceased precipitating 10 min later. The placebo cloud failed to develop any precipitation-sized particles or radar echo and dissipated after ∼30 min. The AgI seeded cloud developed its first echo 8 min after seeding near the threshold temperature for AgI (−7°C), produced precipitation at the ground 20 min after seeding, and continued to develop a new echo near the −7°C level and precipitate for ∼1 h. A natural echoing storm which occurred nearby was examined by radar and found to develop and evolve in a manner quite unlike the seeded clouds. It is plausible that the AgI continued to generate ice crystals in such a manner as to first initiate and then prolong the lifetime of precipitation while the curtain of CO2 pellets failed to initiate more than a single precipitation curtain.

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Brooks E. Martner
and
John D. Marwitz

Abstract

Measurements of wind from a network of surface anemometers and a 107 m tower have been analyzed for southern Wyoming where a project for large-scale generation of electricity from wind power is underway. Topographically forced channeling of stable air flow across a low region of the Continental Divide is mainly responsible for very high mean wind speeds especially in winter. The seasonal cycle of wind speed exhibits a maximum in winter and minimum in summer. Mean wind speeds are approximately 50% greater in winter months than in summer, and the available wind power density is a factor of ∼4.0 greater in winter than in summer. The diurnal cycle is characterized by minimum speed near sunrise and maximum in afternoon hours. Wind directions are narrowly confined from the west-southwest by topographic channeling of the flow, particularly in winter. Wind speed increases sharply with height at night but the profile becomes much more uniform during daylight hours in response to mixing of the lower atmosphere initiated by surface heating.

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William A. Cooper
and
John D. Marwitz

Abstract

The potential for snowfall augmentation in the San Juan Mountains of southwestern Colorado is considered. We show that the seeding criteria and delivery method used in the Colorado River Basin Pilot Project were not suited to the storm structure and characteristics described in the preceding two papers. New criteria are suggested and compared to the available statistical results. It is suggested that opportunities for precipitation enhancement by seeding occur in the latter part of the storm sequence, are associated with the release of convective instability, and can be identified by the presence of a zone of horizontal convergence upwind of the mountain range.

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