Search Results
You are looking at 1 - 10 of 19 items for :
- Author or Editor: Ronald E. Stewart x
- Article x
- Refine by Access: All Content x
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
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.
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.
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.
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.
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.
Abstract
The NCEP–NCAR reanalysis data were used to calculate the atmospheric moisture fluxes into and out of the Saskatchewan River basin for the period 1948–2001. Although bias exists in the estimated moisture flux divergence, the data are still very useful for characterizing the general features of the basin's water vapor fluxes. The direction of the meridional moisture fluxes over the Saskatchewan River basin changes with seasons, but that of the zonal moisture fluxes does not. Moisture flows into the basin from the west (the Pacific Ocean) during all seasons. Moisture influxes from the south in early summer are usually related to the long-distance meridional transport of water vapor from the Gulf of California and the Gulf of Mexico. Moisture flows into the basin from the north in all seasons except for late spring and early summer. The moisture outflow to the east mainly arises from the extensive zonal transport across the basin in all seasons, although this is most pronounced in late summer and autumn. In addition to the two primary moisture sources, the Pacific Ocean and the Gulf of Mexico, the Arctic Ocean is also a moisture source for the Saskatchewan River basin during most seasons. Hudson Bay is another moisture source although this occurs infrequently. Moisture fluxes for the Saskatchewan River basin show some similarities with and differences from those experienced by the Mackenzie River basin. Differences in topography and surface properties between these two basins are key factors generating the differences in water vapor transport. Differences also exist in moisture sources for the two basins. However, there are connections between them through seasonal moisture exchange across the shared boundary.
Abstract
The NCEP–NCAR reanalysis data were used to calculate the atmospheric moisture fluxes into and out of the Saskatchewan River basin for the period 1948–2001. Although bias exists in the estimated moisture flux divergence, the data are still very useful for characterizing the general features of the basin's water vapor fluxes. The direction of the meridional moisture fluxes over the Saskatchewan River basin changes with seasons, but that of the zonal moisture fluxes does not. Moisture flows into the basin from the west (the Pacific Ocean) during all seasons. Moisture influxes from the south in early summer are usually related to the long-distance meridional transport of water vapor from the Gulf of California and the Gulf of Mexico. Moisture flows into the basin from the north in all seasons except for late spring and early summer. The moisture outflow to the east mainly arises from the extensive zonal transport across the basin in all seasons, although this is most pronounced in late summer and autumn. In addition to the two primary moisture sources, the Pacific Ocean and the Gulf of Mexico, the Arctic Ocean is also a moisture source for the Saskatchewan River basin during most seasons. Hudson Bay is another moisture source although this occurs infrequently. Moisture fluxes for the Saskatchewan River basin show some similarities with and differences from those experienced by the Mackenzie River basin. Differences in topography and surface properties between these two basins are key factors generating the differences in water vapor transport. Differences also exist in moisture sources for the two basins. However, there are connections between them through seasonal moisture exchange across the shared boundary.
Abstract
Cloud physics data measured by aircraft during two successive winter field seasons (1978–79 and 1979–80) of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective clouds provide the greatest likelihood of significant supercooled water. The Sierra barrier appears to optimize these conditions 40 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although these conditions were greatly reduced at temperatures less than −10°C. The dominance of liquid water content over ice crystal concentration was maximized 7–10 h after the 700 mb trough passage. Area-wide and banded clouds, which make up the remaining precipitation events, showed only small amounts of supercooled water and general abundance of ice crystals. The largest liquid water contents were observed at the greatest temperatures, usually 0° to −5°C. Such climatological information suggests that a weather modification program to enhance snowfall should concentrate primarily on the convective clouds.
Abstract
Cloud physics data measured by aircraft during two successive winter field seasons (1978–79 and 1979–80) of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective clouds provide the greatest likelihood of significant supercooled water. The Sierra barrier appears to optimize these conditions 40 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although these conditions were greatly reduced at temperatures less than −10°C. The dominance of liquid water content over ice crystal concentration was maximized 7–10 h after the 700 mb trough passage. Area-wide and banded clouds, which make up the remaining precipitation events, showed only small amounts of supercooled water and general abundance of ice crystals. The largest liquid water contents were observed at the greatest temperatures, usually 0° to −5°C. Such climatological information suggests that a weather modification program to enhance snowfall should concentrate primarily on the convective clouds.
Abstract
The phase of precipitation formed within the atmosphere is highly dependent on the vertical temperature profile through which it falls. In particular, several precipitation types can form in an environment with a melting layer aloft and a refreezing layer below. These precipitation types include freezing rain, ice pellets, wet snow, and slush. To examine the formation of such precipitation, a bulk microphysics scheme was used to compare the characteristics of the hydrometeors produced by the model and observed by a research aircraft flight during the 1998 ice storm near Montreal, Canada. The model reproduced several of the observed key precipitation characteristics. Sensitivity tests on the precipitation types formed during the ice storm were also performed. These tests utilized temperature profiles produced by the North American Regional Reanalysis. The results show that small variations (±0.5°C) in the temperature profiles as well as in the precipitation rate can have major impacts on the types of precipitation formed at the surface. These results impose strong requirements on the accuracy needed by prediction models.
Abstract
The phase of precipitation formed within the atmosphere is highly dependent on the vertical temperature profile through which it falls. In particular, several precipitation types can form in an environment with a melting layer aloft and a refreezing layer below. These precipitation types include freezing rain, ice pellets, wet snow, and slush. To examine the formation of such precipitation, a bulk microphysics scheme was used to compare the characteristics of the hydrometeors produced by the model and observed by a research aircraft flight during the 1998 ice storm near Montreal, Canada. The model reproduced several of the observed key precipitation characteristics. Sensitivity tests on the precipitation types formed during the ice storm were also performed. These tests utilized temperature profiles produced by the North American Regional Reanalysis. The results show that small variations (±0.5°C) in the temperature profiles as well as in the precipitation rate can have major impacts on the types of precipitation formed at the surface. These results impose strong requirements on the accuracy needed by prediction models.
Abstract
The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the conclusion that the snowflakes originated as 3–5-mm dendritic crystals in an area of weak convective instability at 5 km and were transported downwind in a strongly sheared airflow. The dendrites aggregated, fell into an existing snowzone (supported in some regions by vertical motion with velocities ranging from 0.2–0.6 m s−1), and continued to descend along a deep, downward sloping layer with temperatures near 0°C. Rapid aggregation occurred in the near 0°C region in particular and without appreciable particle breakup. An exponential fit to the particle size distribution in the region of very large snowflakes had a slope parameter on the order of 100 m−1.
Abstract
The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the conclusion that the snowflakes originated as 3–5-mm dendritic crystals in an area of weak convective instability at 5 km and were transported downwind in a strongly sheared airflow. The dendrites aggregated, fell into an existing snowzone (supported in some regions by vertical motion with velocities ranging from 0.2–0.6 m s−1), and continued to descend along a deep, downward sloping layer with temperatures near 0°C. Rapid aggregation occurred in the near 0°C region in particular and without appreciable particle breakup. An exponential fit to the particle size distribution in the region of very large snowflakes had a slope parameter on the order of 100 m−1.
