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Abstract
Low-level (300 m) aircraft observations, taken within an intense extratropical storm during Intensive Observation Period 2 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic, are used to document the near-surface frontal structure in the interior of the storm at 1800 UTC 14 December 1988, when the storm was at its maximum depth (959 mb). The flight data revealed that a well-defined occluded front spiraled into the low, making one-and-one-half turns about the center. The front followed the inner boundary of a spiral cloud and moisture band seen in satellite visible, infrared, and water vapor imagery. The results provide support for the idea that sharp occluded, or occluded-like, fronts can wrap around the core of deep ocean storms and that satellite imagery can be helpful in locating such fronts.
Abstract
Low-level (300 m) aircraft observations, taken within an intense extratropical storm during Intensive Observation Period 2 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic, are used to document the near-surface frontal structure in the interior of the storm at 1800 UTC 14 December 1988, when the storm was at its maximum depth (959 mb). The flight data revealed that a well-defined occluded front spiraled into the low, making one-and-one-half turns about the center. The front followed the inner boundary of a spiral cloud and moisture band seen in satellite visible, infrared, and water vapor imagery. The results provide support for the idea that sharp occluded, or occluded-like, fronts can wrap around the core of deep ocean storms and that satellite imagery can be helpful in locating such fronts.
Abstract
During late springs through early fall the normal regime of low-level westerly and northwesterly flow within the bight of southern California is occasionally interrupted by periods of southerly flow, elevated marine layers, and increased low-level cloudiness. This transition, often termed the Catalina Eddy, is limited to narrow zone of ∼100 km from the coastal mountains and brings cooler temperatures and improved air quality.
This paper describes the initiation and evolution of Catalina Eddy events using both composite and case study approaches. It is found that this phenomenon is produced by the interaction between the synoptic-scale flow and the formidable topography of the region. As a result of synoptic-scale pressure falls along the central California coast and/or mesoscale lee troughing southeast of Point Conception, an alongshore pressure gradient with lower pressure to the north becomes established. The result of such a pressure gradient in a coastal zone with an adjacent topographic barrier is the establishment of southerly flow within a Rossby radius (∼100 km) of the coastal mountains. Since relatively geostrophic northerlies remain offshore, considerable cyclonic vorticity is established in the coastal zone.
During the early stages of an eddy event there may be little or no stratus in the southern California bright even through an eddy circulation is present. As the southerlies and the associated marine layer deepen, coastal stratus develops and thickens. In many, but not all, eddy cases a well-defined stratus eddy forms during the night as the stratus-laden southerly flow surges westsward south of Point Conception and then is advected south by the strong northerly flow at and to the west of the Point.
Catalina Eddy events continue as long as the supporting alongshore pressure gradient remains. As the synoptic pattern evolves and the alongshore pressure gradient weakens and reverses, the normal wind regime in the bight is reestablished.
Although current operational models do not possess the resolution to directly simulate eddy events, they often accurately predict the larger scale flow responsible for eddy formation.
Abstract
During late springs through early fall the normal regime of low-level westerly and northwesterly flow within the bight of southern California is occasionally interrupted by periods of southerly flow, elevated marine layers, and increased low-level cloudiness. This transition, often termed the Catalina Eddy, is limited to narrow zone of ∼100 km from the coastal mountains and brings cooler temperatures and improved air quality.
This paper describes the initiation and evolution of Catalina Eddy events using both composite and case study approaches. It is found that this phenomenon is produced by the interaction between the synoptic-scale flow and the formidable topography of the region. As a result of synoptic-scale pressure falls along the central California coast and/or mesoscale lee troughing southeast of Point Conception, an alongshore pressure gradient with lower pressure to the north becomes established. The result of such a pressure gradient in a coastal zone with an adjacent topographic barrier is the establishment of southerly flow within a Rossby radius (∼100 km) of the coastal mountains. Since relatively geostrophic northerlies remain offshore, considerable cyclonic vorticity is established in the coastal zone.
During the early stages of an eddy event there may be little or no stratus in the southern California bright even through an eddy circulation is present. As the southerlies and the associated marine layer deepen, coastal stratus develops and thickens. In many, but not all, eddy cases a well-defined stratus eddy forms during the night as the stratus-laden southerly flow surges westsward south of Point Conception and then is advected south by the strong northerly flow at and to the west of the Point.
Catalina Eddy events continue as long as the supporting alongshore pressure gradient remains. As the synoptic pattern evolves and the alongshore pressure gradient weakens and reverses, the normal wind regime in the bight is reestablished.
Although current operational models do not possess the resolution to directly simulate eddy events, they often accurately predict the larger scale flow responsible for eddy formation.
Abstract
One of the most important warm season weather phenomena of the west coast of North America is the temporal transition from northerly to southerly flow within a few hundred kilometers offshore from the coastal mountains. Such wind shifts are often accompanied by cooler temperatures, higher pressure, and a change from nearly cloud-free to low overcast conditions. The more vigorous coastal transitions, termed alongshore surges, are characterized by an abrupt change in wind direction, a sudden increase in wind speed to 15 m s−1 or more, a precipitous temperature drop exceeding 10°C, and a sharp rise in sea-level pressure.
This paper presents two detailed case studies of topographically-trapped coastal southerlies: the strong surge event of 15–17 May 1985 and the far weaker case of 3–7 May 1982. It is shown that coastal southerlies and alongshore surges are controlled by the alongshore pressure gradients created by the synoptic scale flow. At low levels and within approximately one Rossby radius of the coastal topography, geostrophic balance with the alongshore pressure gradient is not possible so that air flows downgradient ageostrophically. Under the proper conditions, southerly flow in the coastal zone can propagate northward as a topographically trapped density current.
It is shown that similar phenomena occur near topographic barriers throughout the world.
Abstract
One of the most important warm season weather phenomena of the west coast of North America is the temporal transition from northerly to southerly flow within a few hundred kilometers offshore from the coastal mountains. Such wind shifts are often accompanied by cooler temperatures, higher pressure, and a change from nearly cloud-free to low overcast conditions. The more vigorous coastal transitions, termed alongshore surges, are characterized by an abrupt change in wind direction, a sudden increase in wind speed to 15 m s−1 or more, a precipitous temperature drop exceeding 10°C, and a sharp rise in sea-level pressure.
This paper presents two detailed case studies of topographically-trapped coastal southerlies: the strong surge event of 15–17 May 1985 and the far weaker case of 3–7 May 1982. It is shown that coastal southerlies and alongshore surges are controlled by the alongshore pressure gradients created by the synoptic scale flow. At low levels and within approximately one Rossby radius of the coastal topography, geostrophic balance with the alongshore pressure gradient is not possible so that air flows downgradient ageostrophically. Under the proper conditions, southerly flow in the coastal zone can propagate northward as a topographically trapped density current.
It is shown that similar phenomena occur near topographic barriers throughout the world.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
An explosive cyclogenesis that occurred in the eastern Pacific on 13 November 1981 is described and discussed on the basis of surface and upper air analyses and satellite imagery. During one 12-h period the storm deepened by nearly 40 mb and subsequently attained a central pressure of 950 mb or less. Peak surface winds were in the 45–50 m s−1 range. The Limited Area Fine Mesh (LFM) model 24-h forecast initialized at 0000 GMT on the thirteenth completely failed to predict the storm development.
The development took place within a strong baroclinic zone, as a shallow frontal wave traveled from the relatively stable environment of a long-wave ridge to the unstable environment of a long-wave trough. Winds at jet stream level exceeded 90 m s−1 prior to the explosive deepening. The lifted index in and ahead of the storm diminished and became negative prior to the rapid intensification. Measurements revealed that sensible and latent heat fluxes from the ocean contributed significantly to the reduced stability. Low-level advection of warm, moist air was perhaps an even greater cause of the reduction. Further evidence of the instability was afforded by satellite imagery, which showed outbreaks of deep convection at the leading edge of the warm frontal cloud band and along the upper part of the cold front.
Very dry, ozone-rich air aloft penetrated the cyclone from the rear at the beginning of the rapid-deepening stage. The fully developed cyclone possessed a warm, hurricane-like core in the 1000–500 mb thickness. However, the spiral-shaped cloud pattern was typical of an extratropical occluded cyclone, and surface ship and buoy observations revealed an intertwining of warm and colder air within the core.
A north-south cross section taken through the storm early in the deepening stage showed that the surface front was a shallow feature embedded within a zone of broad and deep baroclinity. A moist-neutral lapse rate prevailed within the frontal cloud band. The band was highly symmetrically unstable and would have remained symmetrically unstable had the lapse rate been considerably more stable.
The rapid intensification is attributed to strong, deep baroclinic forcing in the presence of small effective static stability. The latter term, as used here, denotes the existence of weak or neutral static stability in the precipitating cloud mass combined with below normal dry static stability in the environment.
Abstract
An explosive cyclogenesis that occurred in the eastern Pacific on 13 November 1981 is described and discussed on the basis of surface and upper air analyses and satellite imagery. During one 12-h period the storm deepened by nearly 40 mb and subsequently attained a central pressure of 950 mb or less. Peak surface winds were in the 45–50 m s−1 range. The Limited Area Fine Mesh (LFM) model 24-h forecast initialized at 0000 GMT on the thirteenth completely failed to predict the storm development.
The development took place within a strong baroclinic zone, as a shallow frontal wave traveled from the relatively stable environment of a long-wave ridge to the unstable environment of a long-wave trough. Winds at jet stream level exceeded 90 m s−1 prior to the explosive deepening. The lifted index in and ahead of the storm diminished and became negative prior to the rapid intensification. Measurements revealed that sensible and latent heat fluxes from the ocean contributed significantly to the reduced stability. Low-level advection of warm, moist air was perhaps an even greater cause of the reduction. Further evidence of the instability was afforded by satellite imagery, which showed outbreaks of deep convection at the leading edge of the warm frontal cloud band and along the upper part of the cold front.
Very dry, ozone-rich air aloft penetrated the cyclone from the rear at the beginning of the rapid-deepening stage. The fully developed cyclone possessed a warm, hurricane-like core in the 1000–500 mb thickness. However, the spiral-shaped cloud pattern was typical of an extratropical occluded cyclone, and surface ship and buoy observations revealed an intertwining of warm and colder air within the core.
A north-south cross section taken through the storm early in the deepening stage showed that the surface front was a shallow feature embedded within a zone of broad and deep baroclinity. A moist-neutral lapse rate prevailed within the frontal cloud band. The band was highly symmetrically unstable and would have remained symmetrically unstable had the lapse rate been considerably more stable.
The rapid intensification is attributed to strong, deep baroclinic forcing in the presence of small effective static stability. The latter term, as used here, denotes the existence of weak or neutral static stability in the precipitating cloud mass combined with below normal dry static stability in the environment.
Abstract
This paper describes a severe, leeside windstorm that struck parts of western Washington State on 23–25 December 1983. It is shown that strong winds, some exceeding 50 m s−1, were mainly limited to swaths downwind of gaps in the Cascade mountains. An examination of the December 1983 event and previous major windstorms indicates that they are generally associated with strong sea level pressure gradients, a stable layer near crest level east of the Cascades which subsides west of the mountains, and light to moderate easterly flow at crest level. The strongest surface winds during the December 1983 event were associated with the period in which the sea level pressure gradient across the Cascades was increasing rapidly. Furthermore, the variation of the leeside winds appears to be modulated by the flow normal to the mountains. It is shown that the horizontal pressure gradients can explain the strong winds and that lee wave amplification, although possible, is probably not of major importance.
Abstract
This paper describes a severe, leeside windstorm that struck parts of western Washington State on 23–25 December 1983. It is shown that strong winds, some exceeding 50 m s−1, were mainly limited to swaths downwind of gaps in the Cascade mountains. An examination of the December 1983 event and previous major windstorms indicates that they are generally associated with strong sea level pressure gradients, a stable layer near crest level east of the Cascades which subsides west of the mountains, and light to moderate easterly flow at crest level. The strongest surface winds during the December 1983 event were associated with the period in which the sea level pressure gradient across the Cascades was increasing rapidly. Furthermore, the variation of the leeside winds appears to be modulated by the flow normal to the mountains. It is shown that the horizontal pressure gradients can explain the strong winds and that lee wave amplification, although possible, is probably not of major importance.
Abstract
This study examines the changes in Cascade Mountain spring snowpack since 1930. Three new time series facilitate this analysis: a water-balance estimate of Cascade snowpack from 1930 to 2007 that extends the observational record 20 years earlier than standard snowpack measurements; a radiosonde-based time series of lower-tropospheric temperature during onshore flow, to which Cascade snowpack is well correlated; and a new index of the North Pacific sea level pressure pattern that encapsulates modes of variability to which Cascade spring snowpack is particularly sensitive.
Cascade spring snowpack declined 23% during 1930–2007. This loss is nearly statistically significant at the 5% level. The snowpack increased 19% during the recent period of most rapid global warming (1976–2007), though this change is not statistically significant because of large annual variability. From 1950 to 1997, a large and statistically significant decline of 48% occurred. However, 80% of this decline is connected to changes in the circulation patterns over the North Pacific Ocean that vary naturally on annual to interdecadal time scales. The residual time series of Cascade snowpack after Pacific variability is removed displays a relatively steady loss rate of 2.0% decade−1, yielding a loss of 16% from 1930 to 2007. This loss is very nearly statistically significant and includes the possible impacts of anthropogenic global warming.
The dates of maximum snowpack and 90% melt out have shifted 5 days earlier since 1930. Both shifts are statistically insignificant. A new estimate of the sensitivity of Cascade spring snowpack to temperature of −11% per °C, when combined with climate model projections of 850-hPa temperatures offshore of the Pacific Northwest, yields a projected 9% loss of Cascade spring snowpack due to anthropogenic global warming between 1985 and 2025.
Abstract
This study examines the changes in Cascade Mountain spring snowpack since 1930. Three new time series facilitate this analysis: a water-balance estimate of Cascade snowpack from 1930 to 2007 that extends the observational record 20 years earlier than standard snowpack measurements; a radiosonde-based time series of lower-tropospheric temperature during onshore flow, to which Cascade snowpack is well correlated; and a new index of the North Pacific sea level pressure pattern that encapsulates modes of variability to which Cascade spring snowpack is particularly sensitive.
Cascade spring snowpack declined 23% during 1930–2007. This loss is nearly statistically significant at the 5% level. The snowpack increased 19% during the recent period of most rapid global warming (1976–2007), though this change is not statistically significant because of large annual variability. From 1950 to 1997, a large and statistically significant decline of 48% occurred. However, 80% of this decline is connected to changes in the circulation patterns over the North Pacific Ocean that vary naturally on annual to interdecadal time scales. The residual time series of Cascade snowpack after Pacific variability is removed displays a relatively steady loss rate of 2.0% decade−1, yielding a loss of 16% from 1930 to 2007. This loss is very nearly statistically significant and includes the possible impacts of anthropogenic global warming.
The dates of maximum snowpack and 90% melt out have shifted 5 days earlier since 1930. Both shifts are statistically insignificant. A new estimate of the sensitivity of Cascade spring snowpack to temperature of −11% per °C, when combined with climate model projections of 850-hPa temperatures offshore of the Pacific Northwest, yields a projected 9% loss of Cascade spring snowpack due to anthropogenic global warming between 1985 and 2025.
Abstract
A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 km) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical processes important for its development. Observations show that the low formed near the ice edge in a region of moderate low-level baroclinicity and cold-air advection when an upper-level trough, or lobe of anomalously large potential vorticity (PV), broke off from a migratory, upper-level cold low over Siberia and advanced into the region.
A full physics model experiment, initialized 24 h prior to the appearance of the polar low, produced a small, intense cyclone having characteristics similar to the observed low. The simulated low more closely resembled an extratropical cyclone than a typical circularly symmetric hurricane, possessing a thermal structure with frontlike features and an asymmetric precipitation shield. Although the simulated low developed southeast of, and earlier than, the observed low, the basic similarity of the observed and modeled systems was revealed by a comparison of the sequence of weather elements at a point in the path of the simulated low with the sequence of observations from nearby St. Paul Island, Alaska.
A series of experiments was performed to test the sensitivity of the simulated polar low development to various physical processes. Four experiments of 48-h duration each were initialized 24 h before the low appeared. In the first experiment, in which surface fluxes were turned off, the low failed to develop. In the second experiment, in which the fluxes were switched on after a 24-h delay, only a weak low formed. In the third experiment, in which the ice edge was shifted a degree of latitude to the north, thus increasing the overwater fetch of the cold air, the low’s evolution was slightly altered but the final outcome was little changed. A fourth high-horizontal resolution experiment (6.67-km spacing) displayed more plentiful and sharper mesoscale features but on the storm scale yielded results that were similar to those of the full-physics run. A full-physics experiment initialized 24 h later, at the time the low first appeared, and run for 24 h, produced a system of similar intensity to that in the 48-h full-physics run but somewhat better positioned. Corresponding sensitivity experiments showed that with both surface fluxes and latent heating omitted, the low weakened and nearly died away. Experiments retaining only surface fluxes in one case and only latent heating in the other, produced similar cyclones of moderate depth.
The results suggest that the development of some, if not most, polar lows can be regarded as fundamentally similar to that of midlatitude ocean cyclones. In both cases a mobile upper-level PV anomaly interacts with a low-level thermal or PV anomaly produced by thermal advection and/or diabatic heating. The polar low lies at the end of the spectrum of extratropical cyclogenesis in which concurrent surface fluxes of sensible and latent heat and the immediately ensuing condensation heating in organized convection dominate the development of the low-level anomaly.
Abstract
A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 km) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical processes important for its development. Observations show that the low formed near the ice edge in a region of moderate low-level baroclinicity and cold-air advection when an upper-level trough, or lobe of anomalously large potential vorticity (PV), broke off from a migratory, upper-level cold low over Siberia and advanced into the region.
A full physics model experiment, initialized 24 h prior to the appearance of the polar low, produced a small, intense cyclone having characteristics similar to the observed low. The simulated low more closely resembled an extratropical cyclone than a typical circularly symmetric hurricane, possessing a thermal structure with frontlike features and an asymmetric precipitation shield. Although the simulated low developed southeast of, and earlier than, the observed low, the basic similarity of the observed and modeled systems was revealed by a comparison of the sequence of weather elements at a point in the path of the simulated low with the sequence of observations from nearby St. Paul Island, Alaska.
A series of experiments was performed to test the sensitivity of the simulated polar low development to various physical processes. Four experiments of 48-h duration each were initialized 24 h before the low appeared. In the first experiment, in which surface fluxes were turned off, the low failed to develop. In the second experiment, in which the fluxes were switched on after a 24-h delay, only a weak low formed. In the third experiment, in which the ice edge was shifted a degree of latitude to the north, thus increasing the overwater fetch of the cold air, the low’s evolution was slightly altered but the final outcome was little changed. A fourth high-horizontal resolution experiment (6.67-km spacing) displayed more plentiful and sharper mesoscale features but on the storm scale yielded results that were similar to those of the full-physics run. A full-physics experiment initialized 24 h later, at the time the low first appeared, and run for 24 h, produced a system of similar intensity to that in the 48-h full-physics run but somewhat better positioned. Corresponding sensitivity experiments showed that with both surface fluxes and latent heating omitted, the low weakened and nearly died away. Experiments retaining only surface fluxes in one case and only latent heating in the other, produced similar cyclones of moderate depth.
The results suggest that the development of some, if not most, polar lows can be regarded as fundamentally similar to that of midlatitude ocean cyclones. In both cases a mobile upper-level PV anomaly interacts with a low-level thermal or PV anomaly produced by thermal advection and/or diabatic heating. The polar low lies at the end of the spectrum of extratropical cyclogenesis in which concurrent surface fluxes of sensible and latent heat and the immediately ensuing condensation heating in organized convection dominate the development of the low-level anomaly.
Abstract
Many coastal locations around the world experience rapid transitions from warm, dry continental air to cool, moist marine air. These onshore “pushes” or surges of marine air can be accompanied by strong winds, large temperature drops and a substantial increase in low clouds. A detailed case study of a typical Pacific Northwest event as well as a composite of several events are presented. It is shown that all major surges are initiated by synoptic-scale changes and that the mesoscale topography of the region “amplifies” the synoptic signal. Annual, monthly and diurnal climatologies are discussed. This paper also discusses the origin of associated West Coast phenomena such as heat troughs and narrow coastal pressure ridges.
Abstract
Many coastal locations around the world experience rapid transitions from warm, dry continental air to cool, moist marine air. These onshore “pushes” or surges of marine air can be accompanied by strong winds, large temperature drops and a substantial increase in low clouds. A detailed case study of a typical Pacific Northwest event as well as a composite of several events are presented. It is shown that all major surges are initiated by synoptic-scale changes and that the mesoscale topography of the region “amplifies” the synoptic signal. Annual, monthly and diurnal climatologies are discussed. This paper also discusses the origin of associated West Coast phenomena such as heat troughs and narrow coastal pressure ridges.
Abstract
This paper describes a localized windstorm that struck some areas of northwest Washington State on 28 December 1990 with winds exceeding 45 m s−1, resulting in extensive property damage, treefalls, and power outages. Arctic air, originating within the interior of British Columbia, descended into a mesoscale gap in the Coast/Cascade Mountains and then accelerated ageostrophically to the west. This gap acceleration is explained quantitatively by a three-way balance among the pressure gradient force, friction, and inertia. The flow maintained its integrity as a narrow current of high-speed air as it exited the gap and subsequently accelerated over water. Troughing in the lee of the Cascade Mountains enhanced the horizontal pressure gradient over northwest Washington; this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration. Farther south the flow decelerated as the current spread out horizontally.
Abstract
This paper describes a localized windstorm that struck some areas of northwest Washington State on 28 December 1990 with winds exceeding 45 m s−1, resulting in extensive property damage, treefalls, and power outages. Arctic air, originating within the interior of British Columbia, descended into a mesoscale gap in the Coast/Cascade Mountains and then accelerated ageostrophically to the west. This gap acceleration is explained quantitatively by a three-way balance among the pressure gradient force, friction, and inertia. The flow maintained its integrity as a narrow current of high-speed air as it exited the gap and subsequently accelerated over water. Troughing in the lee of the Cascade Mountains enhanced the horizontal pressure gradient over northwest Washington; this pressure gradient approximately balanced frictional drag resulting in only minimal acceleration. Farther south the flow decelerated as the current spread out horizontally.
