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  • Author or Editor: John D. Marwitz x
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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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August H. Auer Jr.
and
John D. Marwitz

Abstract

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August H. Auer Jr.
and
John D. Marwitz

Abstract

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August H. Auer Jr.
and
John D. Marwitz

Abstract

Several encounters with hail and graupel (often inadvertent) have occurred while flying an aircraft through the organized updrafts at the base of thunderstorms in the High Plains area. These encounters normally occurred while entering or exiting the organized updrafts and while flying in the vicinity of the strong horizontal reflectivity gradients which commonly border organized updrafts. On several occasions mobile ground crews were beneath the organized updrafts and confirmed the observations from the aircraft. These crews also noted the sequence of precipitation events in this region. The hail which falls in this particular region has the following characteristics: it has a narrow size range, it is large hail (often the largest which falls from the storm), it is commonly not accompanied by rain, and it has a small range of concentrations for a given diameter. The precipitation sequence at the ground over which an organized updraft passes is large hail followed by smaller hail and rain.

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August H. Auer Jr.
and
John D. Marwitz

Abstract

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John D. Marwitz
and
August H. Auer Jr.

Abstract

Air samples and updraft speeds were obtained simultaneously beneath the bases of convective clouds in northeastern Colorado and in South Dakota. Air samples were examined on the ground in a thermal diffusion nuclei chamber in the supersaturation range of 0.1–0.3% and the nuclei spectra determined. Using Twomey's technique, the maximum supersaturation was estimated, thereby estimating the number of cloud nuclei activated.

The updraft speeds were plotted against the concentration of activated cloud nuclei for varying storm intensities. The results indicate the dominate variable, in identifying storm intensity, is updraft strength, with 4–5 m sec−1 being the cutoff point between thunderstorm and hailstorms. The data gave no clear indication that cloud nuclei concentrations played a significant role in identifying storm intensity. Since NV 3k/(2k+4), where V is updraft strength, N number of activated nuclei and the values of k obtained from the analyzed air samples ranged from 0.4 to 2.9, it was determined that in the case of High Plains convection the resulting relation between activated nuclei and updraft strength is NV 0.25 to V 0.89.

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