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- Author or Editor: Ronald E. Stewart x
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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.
Abstract
The 54-yr (1948–2001) NCEP–NCAR reanalysis data as well as other information were used to study the moisture transport and associated circulation features for the severe 2000/01 drought over the western and central Canadian Prairies. Most of the moisture for precipitation over the region is from the Pacific Ocean in winter (November–March) and from the Gulf of Mexico in summer (May–August). An analysis shows that the zonal moisture transport from the Pacific Ocean into both the North American continent and the western and central Canadian Prairies during the winter of the 2000/01 agricultural year was the least over the entire study period, and there was no significantly enhanced moisture influx from the Gulf of Mexico into the region to compensate. Very low winter precipitation was produced over the western and central Canadian Prairies as a consequence. During the ensuing summer period, moisture transport from the Gulf of Mexico was significantly less than normal and no significantly enhanced moisture transport from the Pacific Ocean occurred. These conditions collectively resulted in extremely dry surface conditions for the growing season.
These moisture transport features were mainly associated with prolonged and extraordinarily strong anomalously high pressures over western North America and their related stronger-than-normal air mass sinking over the western and central prairies and adjacent regions. The anomalous high pressures blocked the moisture from flowing into the western and central prairies and were also associated with the splitting of the jet stream that significantly reduced zonal moisture transport by changing the strength and incoming angle of the airflows from the Pacific Ocean during the winter of 2000/01.
Consequences of the stronger-than-normal subsidence were hot and dry surface air during the summer and less precipitation. Collectively, these dynamic factors are favorable for both the formation and the maintenance of droughts.
Abstract
The 54-yr (1948–2001) NCEP–NCAR reanalysis data as well as other information were used to study the moisture transport and associated circulation features for the severe 2000/01 drought over the western and central Canadian Prairies. Most of the moisture for precipitation over the region is from the Pacific Ocean in winter (November–March) and from the Gulf of Mexico in summer (May–August). An analysis shows that the zonal moisture transport from the Pacific Ocean into both the North American continent and the western and central Canadian Prairies during the winter of the 2000/01 agricultural year was the least over the entire study period, and there was no significantly enhanced moisture influx from the Gulf of Mexico into the region to compensate. Very low winter precipitation was produced over the western and central Canadian Prairies as a consequence. During the ensuing summer period, moisture transport from the Gulf of Mexico was significantly less than normal and no significantly enhanced moisture transport from the Pacific Ocean occurred. These conditions collectively resulted in extremely dry surface conditions for the growing season.
These moisture transport features were mainly associated with prolonged and extraordinarily strong anomalously high pressures over western North America and their related stronger-than-normal air mass sinking over the western and central prairies and adjacent regions. The anomalous high pressures blocked the moisture from flowing into the western and central prairies and were also associated with the splitting of the jet stream that significantly reduced zonal moisture transport by changing the strength and incoming angle of the airflows from the Pacific Ocean during the winter of 2000/01.
Consequences of the stronger-than-normal subsidence were hot and dry surface air during the summer and less precipitation. Collectively, these dynamic factors are favorable for both the formation and the maintenance of droughts.
Abstract
This article examines the types of winter precipitation that occur near 0°C, specifically rain, freezing rain, freezing drizzle, ice pellets, snow pellets, and wet snow. It follows from a call by M. Ralph et al. for more attention to be paid to this precipitation since it represents one of the most serious wintertime quantitative precipitation forecasting (QPF) issues. The formation of the many precipitation types involves ice-phase and/or liquid-phase processes, and thresholds in the degree of melting and/or freezing often dictate the types occurring at the surface. Some types can occur simultaneously so that, for example, ensuing collisions between supercooled raindrops and ice pellets that form ice pellet aggregates can lead to substantial reductions in the occurrence of freezing rain at the surface, and ice crystal multiplication processes can lead to locally produced ice crystals in the subfreezing layer below inversions. Highly variable fall velocities within the background temperature and wind fields of precipitation-type transition regions lead to varying particle trajectories and significant alterations in the distribution of precipitation amount and type at the surface. Physically based predictions that account for at least some of the phase changes and particle interactions are now in operation. Outstanding issues to be addressed include the impacts of accretion on precipitation-type formation, quantification of melting and freezing rates of the highly variable precipitation, the consequences of collisions between the various types, and the onset of ice nucleation and its effects. The precipitation physics perspective of this article furthermore needs to be integrated into a comprehensive understanding involving the surrounding and interacting environment.
Abstract
This article examines the types of winter precipitation that occur near 0°C, specifically rain, freezing rain, freezing drizzle, ice pellets, snow pellets, and wet snow. It follows from a call by M. Ralph et al. for more attention to be paid to this precipitation since it represents one of the most serious wintertime quantitative precipitation forecasting (QPF) issues. The formation of the many precipitation types involves ice-phase and/or liquid-phase processes, and thresholds in the degree of melting and/or freezing often dictate the types occurring at the surface. Some types can occur simultaneously so that, for example, ensuing collisions between supercooled raindrops and ice pellets that form ice pellet aggregates can lead to substantial reductions in the occurrence of freezing rain at the surface, and ice crystal multiplication processes can lead to locally produced ice crystals in the subfreezing layer below inversions. Highly variable fall velocities within the background temperature and wind fields of precipitation-type transition regions lead to varying particle trajectories and significant alterations in the distribution of precipitation amount and type at the surface. Physically based predictions that account for at least some of the phase changes and particle interactions are now in operation. Outstanding issues to be addressed include the impacts of accretion on precipitation-type formation, quantification of melting and freezing rates of the highly variable precipitation, the consequences of collisions between the various types, and the onset of ice nucleation and its effects. The precipitation physics perspective of this article furthermore needs to be integrated into a comprehensive understanding involving the surrounding and interacting environment.
Abstract
The melting of snow extracts latent heat of fusion from the environment. The basic response of the atmosphere to this cooling-by-melting mechanism is investigated by using a nonlinear two-dimensional numerical model. It is found that the resultant melting-induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves. The characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions. Model results show that the melting associated with realistic precipitation rates (up to 10 mm h−1) can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second. Since the gravity waves and the cold outflow current propagate away from the source, they can have significant dynamic effects on the environment remote from the precipitation region. Moreover, the melting-induced, near O°C isothermal layer in the atmosphere alters the local static stability. It is inferred that thew melting-induced effects may significantly influence the momentum and moisture transports in mesoscale precipitation systems.
Abstract
The melting of snow extracts latent heat of fusion from the environment. The basic response of the atmosphere to this cooling-by-melting mechanism is investigated by using a nonlinear two-dimensional numerical model. It is found that the resultant melting-induced circulations consist of a forced downdraft which spreads out laterally like a gravity current and transients which are gravity waves. The characteristics of these mesoscale thermally driven circulations are studied under idealistic atmospheric conditions. Model results show that the melting associated with realistic precipitation rates (up to 10 mm h−1) can induce horizontal wind perturbations of several meters per second and vertical motions of tens of centimeters per second. Since the gravity waves and the cold outflow current propagate away from the source, they can have significant dynamic effects on the environment remote from the precipitation region. Moreover, the melting-induced, near O°C isothermal layer in the atmosphere alters the local static stability. It is inferred that thew melting-induced effects may significantly influence the momentum and moisture transports in mesoscale precipitation systems.
Abstract
Various authors have proposed that the cooling associated with melting precipitation contributes significantly to the dynamics of mesoscale precipitation systems. In this study, we use the numerical model described in Part I of this paper to investigate the effects of the cooling-by-melting mechanism in three specific situations: rain/snow boundaries, the production of deep 0°C isothermal layers, and the trailing stratiform region associated with mesoscale convective systems.
It is found that melting in the vicinity of a rain/snow boundary produces a thermally indirect mesoscale vertical circulation that may be responsible for enhanced precipitation near a rain/snow boundary. Melting in the presence of warm air advection above the melting layer and cold advection at and below it are necessary for producing deep 0°C layers within realistic times. The dynamic effects of cooling associated with melting and evaporation in the stratiform region of a mature squall line system produce a mesoscale circulation qualitatively similar to that recently reported in the literature.
Abstract
Various authors have proposed that the cooling associated with melting precipitation contributes significantly to the dynamics of mesoscale precipitation systems. In this study, we use the numerical model described in Part I of this paper to investigate the effects of the cooling-by-melting mechanism in three specific situations: rain/snow boundaries, the production of deep 0°C isothermal layers, and the trailing stratiform region associated with mesoscale convective systems.
It is found that melting in the vicinity of a rain/snow boundary produces a thermally indirect mesoscale vertical circulation that may be responsible for enhanced precipitation near a rain/snow boundary. Melting in the presence of warm air advection above the melting layer and cold advection at and below it are necessary for producing deep 0°C layers within realistic times. The dynamic effects of cooling associated with melting and evaporation in the stratiform region of a mature squall line system produce a mesoscale circulation qualitatively similar to that recently reported in the literature.
Abstract
The microphysical characteristics of a precipitation type transition region within a midlatitude winter storm are discussed in relation to the background thermodynamic and kinematic fields. A deep region in which the temperature was close to 0°C (the transition region) was observed along the Atlantic coastline of Nova Scotia. This transition region was approximately 30 km wide and about 2 km deep. At 80 kPa, a large horizontal temperature gradient marked the boundary between the transition region and the colder air. The observed thermal structure is linked to diabatic processes, and in particular, to the freezing of small droplets, the refreezing of semi-melted particles and the melting of precipitation. Large, partially melted aggregates were located just downwind of the deep transition region. Particle trajectories near the transition region are very sensitive to the background temperature and wind fields and may lead to regions of reduced and enhanced concentrations at the surface and aloft. A conceptual model of the flow fields suggests that this case resembles warm and cold conveyor belts similar to those found in synoptic systems, but on a smaller scale. The transition region in this case is located at the boundary between the warm and cold conveyor belts.
Abstract
The microphysical characteristics of a precipitation type transition region within a midlatitude winter storm are discussed in relation to the background thermodynamic and kinematic fields. A deep region in which the temperature was close to 0°C (the transition region) was observed along the Atlantic coastline of Nova Scotia. This transition region was approximately 30 km wide and about 2 km deep. At 80 kPa, a large horizontal temperature gradient marked the boundary between the transition region and the colder air. The observed thermal structure is linked to diabatic processes, and in particular, to the freezing of small droplets, the refreezing of semi-melted particles and the melting of precipitation. Large, partially melted aggregates were located just downwind of the deep transition region. Particle trajectories near the transition region are very sensitive to the background temperature and wind fields and may lead to regions of reduced and enhanced concentrations at the surface and aloft. A conceptual model of the flow fields suggests that this case resembles warm and cold conveyor belts similar to those found in synoptic systems, but on a smaller scale. The transition region in this case is located at the boundary between the warm and cold conveyor belts.
Abstract
Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze ) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.
Abstract
Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze ) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.
