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John D. Marwitz
and
Jim Toth

Abstract

Heavy snowfall occurred in central Oklahoma on 14 December 1997. The snowfall continued throughout the following day, with over 30 cm of snow falling from Oklahoma to Michigan. The snow in Oklahoma fell from a warm-frontal snowband that was oriented northeast-southwest.

An objective analysis of the synoptic-scale data indicated that frontogenesis was present in the warm-frontal region and a direct circulation pattern around the warm front. The ageostrophic winds above the warm front forced the conditionally unstable air to ascend and release its instability. The forcing mechanisms for the snowband were, therefore, frontogenetic forcing and convective buoyancy.

A series of plan position indicator volume scans were obtained with the NSSL Doppler radar at Norman, Oklahoma. Single-Doppler analysis techniques were used to calculate the mesoscale kinematic properties of the snowband. The data from a special rawinsonde released within the band was combined with the radar-derived kinematic structure to reveal the mesoscale thermodynamic structure of the band.

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Ronald E. Stewart
and
John D. Marwitz

Abstract

The effects of particle fallspeeds on the downwind spread of initially vertical columns or curtains are examined in environments with wind shear. Sets of equations describing the column width as a function of time and distance below column top are derived by assuming, first, that the particles fall at a constant rate and, second, that particle fallspeed changes with time. These predictions are compared with measurements of a seeding curtain within a non-turbulent stratus cloud with high wind shear (0.017 s−1). The comparison implies that differential fallspeed effects in a non-turbulent sheared environment can account for much of the spread of the curtains.

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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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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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Theodore S. Karacostas
and
John D. Marwitz

Abstract

The objective of this study is to describe the characteristics of the airflow and turbulence structure over mountainous terrain. Turbulent characteristics of the airflow were measured using well-instrumented aircraft. The shear, buoyancy, transport of energy and eddy dissipation rate terms were obtained from direct measurements. The turbulent kinetic energy budgets were determined with respect to height and horizontal distance upwind and downwind of the mountain. The change of turbulence intensity was also demonstrated by comparing power spectra as a function of height, as well as a function of distance upwind and downwind of the mountain. The results show that all measurable terms were significant. The shear production and the eddy dissipation rate were the dominant terms. The buoyancy and vertical transport terms were smaller but still important. The imbalance term was estimated to be relatively small.

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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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John D. Marwitz
and
Ronald E. Stewart

Abstract

Airborne seeding experiments were conducted over the Sierra Nevada Mountains in essentially ice-free convective clouds on two days in March 1979 as part of the Sierra Cooperative Pilot Project. On 18 March towering cumuli which extended above a stratiform layer of clouds were seeded, while on 21 March individual towering cumuli were seeded as they developed and moved over the windward side of the mountains. Each cloud was seeded with a vertical curtain oriented perpendicular to the winds during a single pass through the cloud top. The seeding mode was either a low (∼0.1 g m−1) or high (∼1 g m−1) CO2 rate or AgI flares (one 20-gram flare per 250 m).

The seeded curtains were penetrated a number of times by the University of Wyoming King Air. The high CO2 rate apparently overseeded the cloud in that the liquid water was depleted and the cloud dissipated in ∼35 min. Even though much of the liquid water was depleted in the other seeded clouds, they persisted and precipitated for over an hour because additional liquid water was condensed through the additional release of convective instability from orographic lifting. The clouds seeded with a low CO2 rate and with AgI flares yielded similar microphysical characteristics and both methods appeared to have converted the non-precipitating clouds to continuously precipitating clouds.

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Gary P. Ellrod
and
John D. Marwitz

Abstract

The subcloud inflow and outflow structures of two multicell thunderstorms were synthesized from data by a variety of observation systems. The systems included digitized radar, instrumented aircraft, radar chaff, rawinsondes and surface data. The thunderstorms occurred on two consecutive days in northeast Colorado. The data were particularly examined for evidence of horizontal accelerations of the inflow air and hence pressure perturbations in the inflow region and were also examined for indications of interactions between the inflow and outflow air.

From the horizontal accelerations it was inferred that widespread but weak mesolows (≤1 mb) existed in the inflow region of each storm. Most of the acceleration appeared to have occurred in the region between the aircraft and environmental rawinsonde observations. The thermal structures of the inflow and outflow regions of both storms are presented. The location of the strongest updrafts near cloud base was generally above and to the rear of the ground-level wind-shift line. This position suggested that the updrafts were enhanced by interaction with the cold air outflow. Possible explanations for the decay of the storms are offered.

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John D. Marwitz
and
Edwin X. Berry

Abstract

A severe hailstorm having many of the characteristics of Browning's right-moving severe local storms occurred in Alberta on 28 July 1969. This storm was systematically scanned by the Alberta Hail Studies high-resolution 10-cm radar and by the 3-cm radar in the Desert Research Institute's B-26 research aircraft. The former obtained reflectivity factor data throughout the volume of the storm while the latter obtained ground-reference PPI radar contours at flight levels varying from cloud base (7000 ft MSL) to 16,000 ft, and updraft measurements on the southern side of the storm in the Weak Echo Region (WER). Updrafts were smooth and reached a speed of 3500 ft min−1 (18 m sec−1). The width of the WER narrowed from ∼4 mi near cloud base to 2 mi at 16,000 ft. The radar echo was found to tilt approximately 40° from the vertical toward the right of the mean environmental winds. The echo intensity reached 30 dBZ at 25,000 ft directly above the WER.

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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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