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Ronald E. Stewart

The Canadian Atlantic Storms Program (CASP) field project was conducted from 15 January to 15 March 1986 over Atlantic Canada in conjunction with the American Genesis of Atlantic Lows Experiment (GALE). The goals of CASP were to begin the process of understanding and eventually better predicting the mesoscale structure of East Coast storms as well as the storms themselves. Conceptual models of the storms have been formulated, the nature of cyclogenesis and the structure of frontal surfaces have been investigated, and precipitation regions and precipitation type transitions have been studied. Numerical weather simulations have been used to better understand critical parameters affecting storm behavior and improvements in instrumentation have been made. Future research activities are needed to better understand the interaction of the storms with surface features such as coastlines and sea ice.

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Ronald E. Stewart

Winter storms often produce snow, freezing rain, and ice pellets. The characteristics and formation of these forms of precipitation as well as their location within storms are reviewed. Phenomena such as accretion and fog can be related to this precipitation as well.

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John M. Hanesiak
and
Ronald E. Stewart

Abstract

On 1–2 February 1992 a major storm produced a prolonged period (6 h) of ice pellets over St. John's, Newfoundland. At least two key features contributed to the prolonged duration. First, a subsaturated region within an inversion led to a reduction in the melting rate of particles that eventually meant that they could completely refreeze in the lower subfreezing region. This subsaturated region formed within descending air aloft identified by Doppler radar observations. Second, a cold core of air between the surface and the inversion was critically important for the refreezing of partially melted particles. Results from an airmass transformation model were used to show that the ice pellet duration was extended as a result of air traveling over sea ice as opposed to over the ocean. In addition, this study showed that Doppler radar velocity information may be capable of estimating the base height of the above freezing temperature regime during freezing rain/drizzle. Furthermore, the Doppler velocity information may also be used as a warning for possible freezing rain/drizzle conditions. A conceptual model of this storm has been developed to integrate all of the observations and it was also compared to other storms producing ice pellets. Only one other storm possessed a period of sole ice pellets and it was also the only other storm that exhibited a pronounced subsaturated region within the inversion.

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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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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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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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Julie M. Thériault
and
Ronald E. Stewart

Abstract

Several types of precipitation, such as freezing rain, ice pellets, and wet snow, are commonly observed during winter storms. The objective of this study is to better understand the formation of these winter precipitation types. To address this issue, detailed melting and refreezing of precipitation was added onto an existing bulk microphysics scheme. These modifications allow the formation of mixed-phase particles and these particles in turn lead to, or affect, the formation of many of the other types of precipitation. The precipitation type characteristics, such as the mass content, liquid fraction, and threshold diameters formed during a storm over St John’s, Newfoundland, Canada, are studied and compared with observations. Many of these features were reproduced by the model. Sensitivity experiments with the model were carried out to examine the dependence of precipitation characteristics in this event on thresholds of particle evolution in the new parameterization.

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Gordon A. Mcbean
and
Ronald E. Stewart

Abstract

A frontal system passed over the Storm Transfer and Response Experiment project area on 17 November 1980. As revealed by ship-born rawinsonde, surface, and radar information, this northeast Pacific storm exhibited three components: it linked surface and 500-mb troughs with marked surface windshift and associated confluence and horizontal wind shear, but exhibiting no surface temperature change; a midtropospheric cold katafront; and an upper moisture front that was moving ahead more rapidly than the other components. The upper moisture front was marked by a moisture gradient across the trailing edge of a large cloud region that moved with the winds at 500 mb or higher. Due to the horizontal thermal homogeneity of the marine boundary layer, the confluence associated with the surface trough did not cause low-level cold frontogenesis. High θw values of the upper portion of the frontal system indicate that air was advected from much farther south. Important characteristics of frontal system evolution over the eastern sections of oceans where the atmosphere may not have had time to reach equilibrium with the underlying ocean surface are noted.

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Ronald E. Stewart
and
Patrick King

Abstract

Rain–snow boundaries in two southern Ontario storms are examined. Radar and satellite information were used to illustrate the nature and extent of the associated precipitation and cloud regions. The deepest radar echoes and clouds occurred close to the boundary. Surface temperature and pressure were related to the boundary; some of the changes in these parameters were shown to be attributable to melting snowflakes. These radar, satellite, and surface observations are consistent with a mesoscale circulation driven by melting snow.

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Ronald E. Stewart
and
Patrick King

Abstract

The mesoscale storm structure and the evolution of precipitation type are examined during freezing precipitation episodes over southern Ontario. Precipitation bands linked to both warm and cold fronts were mainly responsible for the precipitation during these episodes. One feature detected by radar and related to freezing rain and/or ice pellets in most cases was the initial radar overhang. Observations of mixed precipitation types, including snow and ice pellets with freezing rain, are partially a consequence of size-dependent differences in melting and refreezing within an upper level invention and a lower level subfreezing region, respectively. Inadequate time for refreezing in the lower subfreezing region may, however, lead to particles at the ground being composed of a mixture of water and ice. Prediction techniques for this type of severe weather need to account for its mesoscale nature and for the actual types of precipitation involved.

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