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- Author or Editor: Nathaniel S. Winstead x
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Abstract
Synthetic aperture radar has shown great promise in detecting surface roughness patterns generated by atmospheric and oceanic features. Those roughness patterns that are the result of sea surface wind stress may be analyzed and related to characteristics of the atmospheric boundary layer. Previously reported examples of detectable atmospheric signatures include gravity waves and Rayleigh–Benard convection in cold-air outbreaks. In this paper, the results from an analysis of an image that contains the signatures of nocturnal-drainage-flow-forced exit jets along the western shore of Chesapeake Bay is presented. A regression analysis is performed that links the length of the surface stress patterns associated with these exit jets to the geometry of their source basins. This analysis differs from previous drainage-flow studies in that a population of drainage flows of varying sizes is studied under identical synoptic conditions. This large sample size provides a unique opportunity to examine the role that topography plays in forcing this kind of flow.
To complement the observational study, a two-dimensional, shallow-fluid model is developed to simulate the drainage-flow exit jets once they leave their source basins. This model allows simulation of the behavior of these flows over the entire range of forcing values observed in the image. This kind of analysis provides physical insight into the dynamics of these hybrid flows and a basis for the development of a similarity theory that relates the physically significant forcing parameters to the characteristic length and speed scales of this phenomenon. The lack of in situ observations unfortunately prevents a direct comparison between model results and observations; however, the model is shown to give characteristic jet length scales that are in reasonable agreement with values obtained from the image analysis.
Abstract
Synthetic aperture radar has shown great promise in detecting surface roughness patterns generated by atmospheric and oceanic features. Those roughness patterns that are the result of sea surface wind stress may be analyzed and related to characteristics of the atmospheric boundary layer. Previously reported examples of detectable atmospheric signatures include gravity waves and Rayleigh–Benard convection in cold-air outbreaks. In this paper, the results from an analysis of an image that contains the signatures of nocturnal-drainage-flow-forced exit jets along the western shore of Chesapeake Bay is presented. A regression analysis is performed that links the length of the surface stress patterns associated with these exit jets to the geometry of their source basins. This analysis differs from previous drainage-flow studies in that a population of drainage flows of varying sizes is studied under identical synoptic conditions. This large sample size provides a unique opportunity to examine the role that topography plays in forcing this kind of flow.
To complement the observational study, a two-dimensional, shallow-fluid model is developed to simulate the drainage-flow exit jets once they leave their source basins. This model allows simulation of the behavior of these flows over the entire range of forcing values observed in the image. This kind of analysis provides physical insight into the dynamics of these hybrid flows and a basis for the development of a similarity theory that relates the physically significant forcing parameters to the characteristic length and speed scales of this phenomenon. The lack of in situ observations unfortunately prevents a direct comparison between model results and observations; however, the model is shown to give characteristic jet length scales that are in reasonable agreement with values obtained from the image analysis.
Abstract
Synthetic aperture radar (SAR) has proven to be a useful tool for observing a wide variety of oceanographic and atmospheric phenomena. This is because capillary waves whose amplitudes are modulated in space and time by oceanic and atmospheric processes are efficient scatterers at SAR wavelengths. In this paper, a SAR image of Lake Michigan taken during the Lake-Induced Convection Experiment is analyzed. The image shows three broad parallel bands identifiable as the components of a shallow, Great Lake–induced thermal circulation:two bands associated with opposing land-breeze circulations, and a middle band containing the signature of boundary layer convection. A cross-frontal cut shows that the width of the two land-breeze fronts varies in a manner consistent with previously reported observations of land and sea breezes superimposed on synoptic flows. The SAR image analysis in conjunction with a mesoscale analysis of a Great Lake–scale convection pattern substantially increases the available knowledge of that pattern. Specifically, the SAR image provides information concerning the precise placement of the surface land-breeze fronts not available from other means. Finally, the SAR analysis shows that the western land-breeze brightness patterns are affected by the shallow terrain along the western shore of Lake Michigan. The latter point therefore suggests that SAR can provide valuable information about the link between variations in surface roughness and/or land use patterns and the horizontal structure of the surface wind stress over coastal regions.
Abstract
Synthetic aperture radar (SAR) has proven to be a useful tool for observing a wide variety of oceanographic and atmospheric phenomena. This is because capillary waves whose amplitudes are modulated in space and time by oceanic and atmospheric processes are efficient scatterers at SAR wavelengths. In this paper, a SAR image of Lake Michigan taken during the Lake-Induced Convection Experiment is analyzed. The image shows three broad parallel bands identifiable as the components of a shallow, Great Lake–induced thermal circulation:two bands associated with opposing land-breeze circulations, and a middle band containing the signature of boundary layer convection. A cross-frontal cut shows that the width of the two land-breeze fronts varies in a manner consistent with previously reported observations of land and sea breezes superimposed on synoptic flows. The SAR image analysis in conjunction with a mesoscale analysis of a Great Lake–scale convection pattern substantially increases the available knowledge of that pattern. Specifically, the SAR image provides information concerning the precise placement of the surface land-breeze fronts not available from other means. Finally, the SAR analysis shows that the western land-breeze brightness patterns are affected by the shallow terrain along the western shore of Lake Michigan. The latter point therefore suggests that SAR can provide valuable information about the link between variations in surface roughness and/or land use patterns and the horizontal structure of the surface wind stress over coastal regions.
Abstract
For the commonly observed range of air–sea temperature difference and surface wind speed, the static stability of the atmospheric surface layer can have a significant effect on the mean surface stress and its turbulence-scale horizontal variability. While traditional satellite-borne scatterometers have insufficient horizontal resolution to map this turbulence-scale horizontal variability, satellite-borne synthetic aperture radars (SAR) can. This paper explores the potential for applying existing boundary layer similarity theories to these SAR-derived maps of turbulence-scale horizontal variability in air–sea stress.
Two potential approaches for deriving boundary layer turbulence and stability statistics from SAR backscatter imagery are considered. The first approach employs Monin–Obukhov similarity theory, mixed layer similarity theory, and a SAR-based estimate of the atmospheric boundary layer depth to relate the ratio of the mean and standard deviation of the SAR-derived wind speed field to the stability of the atmospheric surface layer and the convective scale velocity of the atmospheric mixed layer. The second approach addresses these same goals by application of mixed layer similarity theory for the inertial subrange of the SAR-derived wind speed spectra. In both approaches, the resulting quantitative estimates of Monin–Obukhov and mixed layer scaling parameters are then used to make a stability correction to the SAR-derived wind speed and to estimate the surface buoyancy flux.
The impact of operational and theoretical constraints on the practical utility of these two approaches is considered in depth. Calibration and resolution issues are found to impose wind speed limits on the approaches’ applicability. The sensitivity of the two approaches’ results to uncertainty in their nondimensional parameters is also discussed.
Abstract
For the commonly observed range of air–sea temperature difference and surface wind speed, the static stability of the atmospheric surface layer can have a significant effect on the mean surface stress and its turbulence-scale horizontal variability. While traditional satellite-borne scatterometers have insufficient horizontal resolution to map this turbulence-scale horizontal variability, satellite-borne synthetic aperture radars (SAR) can. This paper explores the potential for applying existing boundary layer similarity theories to these SAR-derived maps of turbulence-scale horizontal variability in air–sea stress.
Two potential approaches for deriving boundary layer turbulence and stability statistics from SAR backscatter imagery are considered. The first approach employs Monin–Obukhov similarity theory, mixed layer similarity theory, and a SAR-based estimate of the atmospheric boundary layer depth to relate the ratio of the mean and standard deviation of the SAR-derived wind speed field to the stability of the atmospheric surface layer and the convective scale velocity of the atmospheric mixed layer. The second approach addresses these same goals by application of mixed layer similarity theory for the inertial subrange of the SAR-derived wind speed spectra. In both approaches, the resulting quantitative estimates of Monin–Obukhov and mixed layer scaling parameters are then used to make a stability correction to the SAR-derived wind speed and to estimate the surface buoyancy flux.
The impact of operational and theoretical constraints on the practical utility of these two approaches is considered in depth. Calibration and resolution issues are found to impose wind speed limits on the approaches’ applicability. The sensitivity of the two approaches’ results to uncertainty in their nondimensional parameters is also discussed.
Abstract
Previous studies have demonstrated that satellite synthetic aperture radar (SAR) can be used as an accurate scatterometer, yielding wind speed fields with subkilometer resolution. This wind speed generation is only possible, however, if a corresponding accurate wind direction field is available. The potential sources of this wind direction information include satellite scatterometers, numerical weather prediction models, and SAR itself through analysis of the spatial patterns caused by boundary layer wind structures. Each of these wind direction sources has shortcomings that can lead to wind speed errors in the SAR-derived field. Manual and semiautomated methods are presented for identifying and correcting numerical weather prediction model wind direction errors. The utility of this approach is demonstrated for a set of cases in which the first-guess wind direction data did not adequately portray the features seen in the SAR imagery. These situations include poorly resolved mesoscale phenomena and misplaced synoptic-scale fronts and cyclones.
Abstract
Previous studies have demonstrated that satellite synthetic aperture radar (SAR) can be used as an accurate scatterometer, yielding wind speed fields with subkilometer resolution. This wind speed generation is only possible, however, if a corresponding accurate wind direction field is available. The potential sources of this wind direction information include satellite scatterometers, numerical weather prediction models, and SAR itself through analysis of the spatial patterns caused by boundary layer wind structures. Each of these wind direction sources has shortcomings that can lead to wind speed errors in the SAR-derived field. Manual and semiautomated methods are presented for identifying and correcting numerical weather prediction model wind direction errors. The utility of this approach is demonstrated for a set of cases in which the first-guess wind direction data did not adequately portray the features seen in the SAR imagery. These situations include poorly resolved mesoscale phenomena and misplaced synoptic-scale fronts and cyclones.
Abstract
This paper describes a product that allows one to assess the lower and upper bounds on synthetic aperture radar (SAR)-based marine wind speed. The SAR-based wind speed fields of the current research are generated using scatterometry techniques and, thus, depend on a priori knowledge of the wind direction field. The assessment product described here consists of a pair of wind speed images bounding the wind speed range consistent with the observed SAR data. The minimum wind speed field is generated by setting the wind direction field to be directly opposite to the radar look direction. The maximum wind speed field is generated by setting the wind direction field to be perpendicular to the radar look direction. Although the assessment product could be generated using any marine SAR scene, it is expected to be most useful in coastal regions where the large concentration of maritime operations requires accurate, high-resolution wind speed data and when uncertainty in the a priori knowledge of the wind direction precludes the generation of accurate SAR-based wind speed fields. The assessment product is demonstrated using a case in the northern Gulf of Alaska where synoptic-scale and mesoscale meteorological events coexist. The corresponding range of possible SAR-based wind speed is large enough to have operational significance to mariners and weather forecasters. It is recommended that the product become available to the public through an appropriate government outlet.
Abstract
This paper describes a product that allows one to assess the lower and upper bounds on synthetic aperture radar (SAR)-based marine wind speed. The SAR-based wind speed fields of the current research are generated using scatterometry techniques and, thus, depend on a priori knowledge of the wind direction field. The assessment product described here consists of a pair of wind speed images bounding the wind speed range consistent with the observed SAR data. The minimum wind speed field is generated by setting the wind direction field to be directly opposite to the radar look direction. The maximum wind speed field is generated by setting the wind direction field to be perpendicular to the radar look direction. Although the assessment product could be generated using any marine SAR scene, it is expected to be most useful in coastal regions where the large concentration of maritime operations requires accurate, high-resolution wind speed data and when uncertainty in the a priori knowledge of the wind direction precludes the generation of accurate SAR-based wind speed fields. The assessment product is demonstrated using a case in the northern Gulf of Alaska where synoptic-scale and mesoscale meteorological events coexist. The corresponding range of possible SAR-based wind speed is large enough to have operational significance to mariners and weather forecasters. It is recommended that the product become available to the public through an appropriate government outlet.
Abstract
An important aspect of operational meteorology in and around the Great Lakes region of the United States and Canada in the winter months is the forecasting of lake-effect precipitation. While the synoptic- and mesoscale processes that govern the development of lake-effect precipitation have been well understood for many years, problems observing these bands remain because of the limited boundary layer coverage provided by the Weather Surveillance Radar-1988 Doppler (WSR-88D) network. While traditional visible and infrared satellite imagery helps alleviate these coverage limitations, overcast conditions often negate this advantage.
Here, a new method for observing lake-effect bands by using synthetic aperture radar (SAR) to identify and characterize their surface signatures is presented. SAR is a remote sensing tool that images surface roughness. Over water, this roughness is related to the surface wind stress and, hence, surface wind field. Here, three cases are documented where the SAR aboard the Canadian Radar Satellite-1 imaged the footprints of precipitating bands over the Great Lakes: one case with multiple snowbands west of one main band over Lake Superior, and two cases with shore-parallel bands over each of Lakes Ontario and Michigan. These cases are first documented using traditional observing methods: infrared satellite imagery, WSR-88D, and surface observations. Then, each SAR image is interpreted based upon the traditional observations. The ultimate goal is to demonstrate that SAR is capable of detecting the surface signatures associated with Great Lakes precipitation bands that could be of value to forecasters when data from traditional observation platforms are unavailable.
Abstract
An important aspect of operational meteorology in and around the Great Lakes region of the United States and Canada in the winter months is the forecasting of lake-effect precipitation. While the synoptic- and mesoscale processes that govern the development of lake-effect precipitation have been well understood for many years, problems observing these bands remain because of the limited boundary layer coverage provided by the Weather Surveillance Radar-1988 Doppler (WSR-88D) network. While traditional visible and infrared satellite imagery helps alleviate these coverage limitations, overcast conditions often negate this advantage.
Here, a new method for observing lake-effect bands by using synthetic aperture radar (SAR) to identify and characterize their surface signatures is presented. SAR is a remote sensing tool that images surface roughness. Over water, this roughness is related to the surface wind stress and, hence, surface wind field. Here, three cases are documented where the SAR aboard the Canadian Radar Satellite-1 imaged the footprints of precipitating bands over the Great Lakes: one case with multiple snowbands west of one main band over Lake Superior, and two cases with shore-parallel bands over each of Lakes Ontario and Michigan. These cases are first documented using traditional observing methods: infrared satellite imagery, WSR-88D, and surface observations. Then, each SAR image is interpreted based upon the traditional observations. The ultimate goal is to demonstrate that SAR is capable of detecting the surface signatures associated with Great Lakes precipitation bands that could be of value to forecasters when data from traditional observation platforms are unavailable.
Abstract
This paper investigates the temporal and spatial climatology of coastal barrier jets in the Gulf of Alaska. The jets are divided into two categories based upon the origin of the air involved: “classic” barrier jets fed primarily by onshore flow and “hybrid” jets fed primarily by gap flow from the continental interior. The analyses are compiled from five years (1998–2003) of synthetic aperture radar images from the Radarsat-1 satellite totaling 3000 images. Thermodynamic and kinematic data from the NCEP reanalysis is used in the interpretation of the results.
The majority of coastal barrier jets occur during the cool season, with the coastline near Mount Fairweather and the Valdez–Cordova mountains experiencing the greatest number of barrier jets. Hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of Cross Sound, Yakutat Bay, and Icy Bay. Some interannual variability in the total number of jets is observed.
Coastal barrier jet formation is associated with onshore wind directions and maximum terrain heights exceeding 2 km within 100 km of the coast, features that support low-level flow blocking. Hybrid jet formation requires the additional condition of an abnormally large offshore-directed pressure gradient force.
Half of the barrier and hybrid jets exhibit surface wind speeds in excess of 20 m s−1 (strong gale), although their widths are typically less than 100 km. The maximum speed of both types of jet tends to be 2–3 times that of the ambient synoptic flow. A small percentage of the jets detach from the coastline, with the typical detachment distance being 10 km.
Abstract
This paper investigates the temporal and spatial climatology of coastal barrier jets in the Gulf of Alaska. The jets are divided into two categories based upon the origin of the air involved: “classic” barrier jets fed primarily by onshore flow and “hybrid” jets fed primarily by gap flow from the continental interior. The analyses are compiled from five years (1998–2003) of synthetic aperture radar images from the Radarsat-1 satellite totaling 3000 images. Thermodynamic and kinematic data from the NCEP reanalysis is used in the interpretation of the results.
The majority of coastal barrier jets occur during the cool season, with the coastline near Mount Fairweather and the Valdez–Cordova mountains experiencing the greatest number of barrier jets. Hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of Cross Sound, Yakutat Bay, and Icy Bay. Some interannual variability in the total number of jets is observed.
Coastal barrier jet formation is associated with onshore wind directions and maximum terrain heights exceeding 2 km within 100 km of the coast, features that support low-level flow blocking. Hybrid jet formation requires the additional condition of an abnormally large offshore-directed pressure gradient force.
Half of the barrier and hybrid jets exhibit surface wind speeds in excess of 20 m s−1 (strong gale), although their widths are typically less than 100 km. The maximum speed of both types of jet tends to be 2–3 times that of the ambient synoptic flow. A small percentage of the jets detach from the coastline, with the typical detachment distance being 10 km.
Abstract
This paper investigates the large-scale flow and thermodynamic structures associated with barrier jets along the Alaskan coast using the National Centers for Environmental Prediction (NCEP) reanalysis, as well as the average wind, moisture, and thermodynamic soundings at Yakutat, Alaska (YAK), and Whitehorse, Yukon Territory, Canada (YXY). Large-scale and sounding composites are constructed for all barrier jets objectively identified around YAK using synthetic aperture radar (SAR) imagery during the cool and warm seasons of 1998–2003. During the cool season the jet events are separated into those with sharp upstream wind gradients (shock jets), highly variable (“gustlike”) surface winds (variable jets), and the other jet events (other jets).
Those cool season barrier jets without shock or variable characteristics are associated with an anomalously deep upper-level trough approaching the Gulf of Alaska and an anomalous ridge over western Canada and interior Alaska. The associated surface cyclone and surface ridging result in strong low-level southerlies over southeast Alaska and the advection of 850-mb warm anomalies northward from the subtropics to Alaska. In contrast, the shock events have significant cold anomalies at 850 mb over the interior, while both the shock and variable jets have less upper-level ridging over the interior. The warm season other-jet composite is similar to that for the cool season, except that an 850-mb cool anomaly develops near the coast and the approaching upper-level trough is not significantly deeper than climatology.
The sounding composite at YAK of the other-jet type during the cool season is more stable, moist, and slightly cooler at lower levels than the nonjet events. The largest low-level cool, dry, and high stability anomalies are for the shock events at YAK and YXY, which suggests that this cold and dry air source over the interior is an important ingredient for the development of sharp frontlike boundaries to the barrier jet. In contrast, the variable jets have weaker low-level stability, which favors the subsequent mixing of higher momentum to the surface in localized areas. The warm season jets also have cooler lower levels than those for the nonjet events, but the lower levels are nearly well mixed with little stratification, especially over the interior.
Abstract
This paper investigates the large-scale flow and thermodynamic structures associated with barrier jets along the Alaskan coast using the National Centers for Environmental Prediction (NCEP) reanalysis, as well as the average wind, moisture, and thermodynamic soundings at Yakutat, Alaska (YAK), and Whitehorse, Yukon Territory, Canada (YXY). Large-scale and sounding composites are constructed for all barrier jets objectively identified around YAK using synthetic aperture radar (SAR) imagery during the cool and warm seasons of 1998–2003. During the cool season the jet events are separated into those with sharp upstream wind gradients (shock jets), highly variable (“gustlike”) surface winds (variable jets), and the other jet events (other jets).
Those cool season barrier jets without shock or variable characteristics are associated with an anomalously deep upper-level trough approaching the Gulf of Alaska and an anomalous ridge over western Canada and interior Alaska. The associated surface cyclone and surface ridging result in strong low-level southerlies over southeast Alaska and the advection of 850-mb warm anomalies northward from the subtropics to Alaska. In contrast, the shock events have significant cold anomalies at 850 mb over the interior, while both the shock and variable jets have less upper-level ridging over the interior. The warm season other-jet composite is similar to that for the cool season, except that an 850-mb cool anomaly develops near the coast and the approaching upper-level trough is not significantly deeper than climatology.
The sounding composite at YAK of the other-jet type during the cool season is more stable, moist, and slightly cooler at lower levels than the nonjet events. The largest low-level cool, dry, and high stability anomalies are for the shock events at YAK and YXY, which suggests that this cold and dry air source over the interior is an important ingredient for the development of sharp frontlike boundaries to the barrier jet. In contrast, the variable jets have weaker low-level stability, which favors the subsequent mixing of higher momentum to the surface in localized areas. The warm season jets also have cooler lower levels than those for the nonjet events, but the lower levels are nearly well mixed with little stratification, especially over the interior.
Abstract
The synthetic aperture radar ocean surface signature of atmospheric internal gravity waves in the vicinity of a synoptic-scale warm front is examined via a classic Kelvin–Helmholtz velocity profile with a rigid lower boundary and a sloping interface. The horizontal distance that the waves extend from the surface warm front is consistent with a bifurcation along the warm frontal inversion from unstable to neutral solutions. Similarity theories are derived for the wave span and the location of maximum growth rate relative to the surface front position. The theoretical maximum wave growth rate is demonstrated to occur near this bifurcation point and, hence, to explain the observed pattern of wave amplitude. Finally, a wave crest-tracing procedure is developed to explain the observed acute orientation of waves with respect to the surface warm front.
Abstract
The synthetic aperture radar ocean surface signature of atmospheric internal gravity waves in the vicinity of a synoptic-scale warm front is examined via a classic Kelvin–Helmholtz velocity profile with a rigid lower boundary and a sloping interface. The horizontal distance that the waves extend from the surface warm front is consistent with a bifurcation along the warm frontal inversion from unstable to neutral solutions. Similarity theories are derived for the wave span and the location of maximum growth rate relative to the surface front position. The theoretical maximum wave growth rate is demonstrated to occur near this bifurcation point and, hence, to explain the observed pattern of wave amplitude. Finally, a wave crest-tracing procedure is developed to explain the observed acute orientation of waves with respect to the surface warm front.