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- Author or Editor: Ronald E. Stewart x
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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.
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.
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.
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.
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.
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.
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.
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.
