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Abstract
Case studies are presented which describe a type of lake-effect snowband which forms along the western shore of Lake Michigan when a cold anticyclone to the north sets up an easterly gradient over the lake. Numerical simulations indicate that the snowband coincides with a narrow band of upward motion which results from the convergence of easterly winds over the lake and north to northwesterly winds over land. The northerly winds are part of a land breeze circulation which forms when cold air is heated by the relatively warm lake surface.
Initial data for model simulations are obtained by objective analysis of upper-air data from the eight upper-air stations closest to Lake Michigan at six levels in the lower troposphere. Model results show that a pool of cold air over the lake up to about 850 mb favors rapid growth of the planetary boundary layer over the western half of the lake, and that latent heat release plays an important role in intensifying the land breeze circulation.
Abstract
Case studies are presented which describe a type of lake-effect snowband which forms along the western shore of Lake Michigan when a cold anticyclone to the north sets up an easterly gradient over the lake. Numerical simulations indicate that the snowband coincides with a narrow band of upward motion which results from the convergence of easterly winds over the lake and north to northwesterly winds over land. The northerly winds are part of a land breeze circulation which forms when cold air is heated by the relatively warm lake surface.
Initial data for model simulations are obtained by objective analysis of upper-air data from the eight upper-air stations closest to Lake Michigan at six levels in the lower troposphere. Model results show that a pool of cold air over the lake up to about 850 mb favors rapid growth of the planetary boundary layer over the western half of the lake, and that latent heat release plays an important role in intensifying the land breeze circulation.
Abstract
A three-dimensional, primitive equation, boundary-layer model is used to investigate wintertime mesoscale frontogenesis along the New England coast. Some features included in the model are a terrain-following coordinate system to study the effects of irregular terrain, a stable beating scheme to allow for the release of latent heat and to estimate precipitation, and an upper boundary condition on pressure which permits interaction with the synoptic-scale circulation.
Numerical experiments with the model have verified the hypothesis that coastal fronts form when a cold anticyclone is located just to the north or northeast of New England, and a wave disturbance at 700 mb approaches the northeast from the midwest United States. Polar continental air flowing southwestward around the eastern side of the anticyclone is modified rapidly by sensible and latent heat transfer from the relatively warm Atlantic Ocean. As the 700 mb trough approaches the northeast, surface winds over the ocean veer from northeasterly to a more easterly direction. However, over land the low-level winds do not veer with the geostrophic wind, but rather back from northeasterly to a more northerly direction. This backing results from a combination of factors including surface friction and the effects of heat fluxes from the ocean and latent heat release. The numerical experiments indicate that the effect of heal fluxes from the ocean may be more important than previously suspected. The low-level convergence of maritime air flowing from the cast and cold air over land flowing from the north results in upward motion in a narrow band along the New England coast reaching a magnitude of ∼20 cm s−1 after 15 h of integration.
Abstract
A three-dimensional, primitive equation, boundary-layer model is used to investigate wintertime mesoscale frontogenesis along the New England coast. Some features included in the model are a terrain-following coordinate system to study the effects of irregular terrain, a stable beating scheme to allow for the release of latent heat and to estimate precipitation, and an upper boundary condition on pressure which permits interaction with the synoptic-scale circulation.
Numerical experiments with the model have verified the hypothesis that coastal fronts form when a cold anticyclone is located just to the north or northeast of New England, and a wave disturbance at 700 mb approaches the northeast from the midwest United States. Polar continental air flowing southwestward around the eastern side of the anticyclone is modified rapidly by sensible and latent heat transfer from the relatively warm Atlantic Ocean. As the 700 mb trough approaches the northeast, surface winds over the ocean veer from northeasterly to a more easterly direction. However, over land the low-level winds do not veer with the geostrophic wind, but rather back from northeasterly to a more northerly direction. This backing results from a combination of factors including surface friction and the effects of heat fluxes from the ocean and latent heat release. The numerical experiments indicate that the effect of heal fluxes from the ocean may be more important than previously suspected. The low-level convergence of maritime air flowing from the cast and cold air over land flowing from the north results in upward motion in a narrow band along the New England coast reaching a magnitude of ∼20 cm s−1 after 15 h of integration.
Abstract
The Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5), running on a triply nested grid, was used to simulate the intense lake-effect snowstorm of 4–5 January 1995. On the finest grid (5-km resolution) centered over Lake Ontario, MM5 produced a snowband in the correct location having a size and orientation similar to the band observed by the WSR-88D radar at Binghamton, New York. The simulated precipitation distribution agreed well with the observed snowfall during the first 18 h during the time when the snowband was in its midlake position extending into the Tug Hill plateau. During the last 12 h of the simulation, when both the observed and simulated snowbands lay along the south shore of Lake Ontario, the simulated snowfall at inland locations of Oswego County was less than observed. During this period, the simulated precipitation over Lake Ontario appeared to be excessive, although no radar data or ground truth was available to confirm this.
Two short-wave troughs interacted with the Lake Ontario snowband. The temporary weakening of the snowband after passage of the first trough was simulated well in the triply nested MM5 simulation. A comparison was made between the operational Eta Model run and an MM5 simulation on a grid of comparable resolution (80 km) in handling the passage of the second more vigorous short wave. Both the Eta and the 80-km MM5 were a few hours too early with the passage of this trough. The nested-grid version of MM5 was correct in simulating the rapid southward movement of the band to Oswego County just after the second trough moved east of the lake. However, because of the timing error with the trough, MM5 was premature by a few hours in the southward shift of the snowband.
Results on the 15-km grid indicated that moisture plumes from Lake Huron and Georgian Bay fed into the Lake Ontario band. In the lowest few hundred meters, these plumes were deflected around the Shelburne Plateau, which lies between Lake Huron and Lake Ontario. Future research will focus on interactions between circulations downwind of Lake Huron and snowbands that form over Lake Ontario.
The results of the 4–5 January 1995 simulation are sufficiently encouraging to suggest that MM5 may be used to make real-time forecasts of lake-effect snowstorms. The lead author is participating in a COMET cooperative project to provide lake-effect snow forecasts, in GEMPAK format, to the National Weather Service Forecast Offices at Buffalo and Binghamton using a 20-km nested grid over Lakes Huron, Erie, and Ontario. Despite relatively coarse resolution, MM5 has produced useful predictions of snowband location and movement during the 1996/97 and 1997/98 lake-effect snow seasons.
Abstract
The Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5), running on a triply nested grid, was used to simulate the intense lake-effect snowstorm of 4–5 January 1995. On the finest grid (5-km resolution) centered over Lake Ontario, MM5 produced a snowband in the correct location having a size and orientation similar to the band observed by the WSR-88D radar at Binghamton, New York. The simulated precipitation distribution agreed well with the observed snowfall during the first 18 h during the time when the snowband was in its midlake position extending into the Tug Hill plateau. During the last 12 h of the simulation, when both the observed and simulated snowbands lay along the south shore of Lake Ontario, the simulated snowfall at inland locations of Oswego County was less than observed. During this period, the simulated precipitation over Lake Ontario appeared to be excessive, although no radar data or ground truth was available to confirm this.
Two short-wave troughs interacted with the Lake Ontario snowband. The temporary weakening of the snowband after passage of the first trough was simulated well in the triply nested MM5 simulation. A comparison was made between the operational Eta Model run and an MM5 simulation on a grid of comparable resolution (80 km) in handling the passage of the second more vigorous short wave. Both the Eta and the 80-km MM5 were a few hours too early with the passage of this trough. The nested-grid version of MM5 was correct in simulating the rapid southward movement of the band to Oswego County just after the second trough moved east of the lake. However, because of the timing error with the trough, MM5 was premature by a few hours in the southward shift of the snowband.
Results on the 15-km grid indicated that moisture plumes from Lake Huron and Georgian Bay fed into the Lake Ontario band. In the lowest few hundred meters, these plumes were deflected around the Shelburne Plateau, which lies between Lake Huron and Lake Ontario. Future research will focus on interactions between circulations downwind of Lake Huron and snowbands that form over Lake Ontario.
The results of the 4–5 January 1995 simulation are sufficiently encouraging to suggest that MM5 may be used to make real-time forecasts of lake-effect snowstorms. The lead author is participating in a COMET cooperative project to provide lake-effect snow forecasts, in GEMPAK format, to the National Weather Service Forecast Offices at Buffalo and Binghamton using a 20-km nested grid over Lakes Huron, Erie, and Ontario. Despite relatively coarse resolution, MM5 has produced useful predictions of snowband location and movement during the 1996/97 and 1997/98 lake-effect snow seasons.
Snowstorms generated over the Great Lakes bring localized heavy precipitation, blizzard conditions, and whiteouts to downwind shores. Hazardous freezing rain often affects the same region in winter. Conventional observations and numerical models generally are resolved too coarsely to allow detection or accurate prediction of these mesoscale severe weather phenomena. The Lake Ontario Winter Storms (LOWS) project was conducted to demonstrate and evaluate the potential for real-time mesoscale monitoring and location-specific prediction of lake-effect storms and freezing rain, using the newest of available technologies. LOWS employed an array of specialized atmospheric remote sensors (a dual-polarization short wavelength radar, microwave radiometer, radio acoustic sounding system, and three wind profilers) with supporting observing systems and mesoscale numerical models. An overview of LOWS and its initial accomplishments is presented.
Snowstorms generated over the Great Lakes bring localized heavy precipitation, blizzard conditions, and whiteouts to downwind shores. Hazardous freezing rain often affects the same region in winter. Conventional observations and numerical models generally are resolved too coarsely to allow detection or accurate prediction of these mesoscale severe weather phenomena. The Lake Ontario Winter Storms (LOWS) project was conducted to demonstrate and evaluate the potential for real-time mesoscale monitoring and location-specific prediction of lake-effect storms and freezing rain, using the newest of available technologies. LOWS employed an array of specialized atmospheric remote sensors (a dual-polarization short wavelength radar, microwave radiometer, radio acoustic sounding system, and three wind profilers) with supporting observing systems and mesoscale numerical models. An overview of LOWS and its initial accomplishments is presented.
