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- Author or Editor: J. D. Schumacher x
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Abstract
Long-term records from four current meters in the Alaskan Stream off Kodiak Island are presented. The net flows decreases with depth and appeared to be in approximate geostrophic equilibrium. Large fluctuations were not common, and the flow was dominated by low-frequency energy. This behavior, which is also supported by temperature and salinity data, suggests a vertically coherent flow with occasional lateral meanders.
The eddy kinetic-energy levels in this region of the Alaskan Stream were quite low, especially in comparison with those in the Kuroshio and Gulf Stream. The flux of momentum across the inshore edge of the Stream appeared to be onshore and to represent a transfer of energy fron3 the mean flow to smaller scales; an eddy viscosity of not more than 106 cm2 s−1 was indicated. The impact on shelf waters of the small, onshore eddy heat flux is unclear.
Abstract
Long-term records from four current meters in the Alaskan Stream off Kodiak Island are presented. The net flows decreases with depth and appeared to be in approximate geostrophic equilibrium. Large fluctuations were not common, and the flow was dominated by low-frequency energy. This behavior, which is also supported by temperature and salinity data, suggests a vertically coherent flow with occasional lateral meanders.
The eddy kinetic-energy levels in this region of the Alaskan Stream were quite low, especially in comparison with those in the Kuroshio and Gulf Stream. The flux of momentum across the inshore edge of the Stream appeared to be onshore and to represent a transfer of energy fron3 the mean flow to smaller scales; an eddy viscosity of not more than 106 cm2 s−1 was indicated. The impact on shelf waters of the small, onshore eddy heat flux is unclear.
Abstract
Data from current moorings at four sites near the shelf break in the Gulf of Alaska are used to present information on the flow, to examine the effects of local winds, and especially to investigate momentum transfer between the offshore and inshore circulation. Net flow at the shelf break in the central and western appears to be similar through the year, but it intensifies appreciably in winter in the northeast Gulf. Only records in the northeast Gulf suggest significant effects on flow by local winds. The eddy fluxes of momentum at the shelf break were extremely small. Although the offshore Alaskan Stream was previously found to transfer momentum toward shore, this flux apparently does not reach the shelf break and influence shelf waters. It appears rather that the gradients of heat and salt observed near the shelf edge result from offshore effects of the coastal flow.
Abstract
Data from current moorings at four sites near the shelf break in the Gulf of Alaska are used to present information on the flow, to examine the effects of local winds, and especially to investigate momentum transfer between the offshore and inshore circulation. Net flow at the shelf break in the central and western appears to be similar through the year, but it intensifies appreciably in winter in the northeast Gulf. Only records in the northeast Gulf suggest significant effects on flow by local winds. The eddy fluxes of momentum at the shelf break were extremely small. Although the offshore Alaskan Stream was previously found to transfer momentum toward shore, this flux apparently does not reach the shelf break and influence shelf waters. It appears rather that the gradients of heat and salt observed near the shelf edge result from offshore effects of the coastal flow.
Abstract
Moored current-meter observations obtained from upstream and downstream locations in Shelikof Strait, between Kodiak and Afognak Islands and the Alaskan mainland in the northwest Gulf of Alaska, show a considerable amount of low-frequency variability superimposed upon the mean southwestward along-channel flow. Moreover, the spectral properties of these low-frequency current fluctuations varied seasonally and according to location within the channel. During fag 1976, the variance spectra of the along-channel fluctuations at the upstream end of Shelikof Strait were sharply peaked at 2.86 days. At the downstream end of the Strait, some 150 km to the southwest, the variance spectrum of the cross-channel fluctuations at 100 m was peaked at 6.15 days. However, the upstream 2.86-day signal was coherent with 2.86-day fluctuations downstream, and the phase relationship between the two locations implied the presence of a downstream-traveling wave with a wavelength of 74 km and a phase speed of 26 km day−1. (Downstream-traveling waves of similar scales have also been observed in satellite images of this region taken during fall 1979.) During winter 1977, on the other hand, the variance spectra of the current fluctuations at the upstream end of the Strait were broadly peaked at 2–3.5 days, and the fluctuations were not coherent with those downstream in this period range. However, the downstream variance spectra were peaked at 4.44 days and this signal was coherent with the upstream fluctuations. The phase relationship between the two locations implied the presence of a downstream-traveling wave with a wavelength of 94 km and a phase speed of 21 km day−1.
This paper attempts to explain these low-frequency current fluctuations by investigating the stability of a two-layer flow in a straight channel with a sloping bottom. It is shown that, for representative seasonal values of the mean current shear, density stratification and bottom slope at each end of the Strait, the mean flow is generally baroclinically unstable with respect to downstream-traveling, quasi-geostrophic wave perturbations. (A similar result is also arrived at using the classical Eady model with constant shear, stratification and depth.) More specifically, in an application of the model to the fall 1976 data, it is found that a 2.86-day period wave at the upstream end is unstable and propagates downstream with a wavelength of 92 km and a phase speed of 32 km day−1, in good agreement with the observations. However, at the downstream end, such a wave becomes stable due to the reversal in bottom slope along the Strait. Moreover, at the downstream end, only very long period waves an unstable, with the maximally unstable wave having a period of 5.2 days. Thus the low-frequency spectral peak at 6.15 days observed at the downstream mooring is probably due to locally generated instabilities. From an application of the model to the winter 1977 data, it is found that the large 4.44-day signal observed downstream is likely due to a downstream-travelling wave that is locally unstable but which originated from the upstream end of the Strait where it was marginally unstable.
From these applications and other analyses (e.g., of vertical phase relations and of summer 1978 data), it is concluded that most of the low-frequency current fluctuations in Shelikof Strait are due to baroclinic instability of the mean flow. However, in reaching this conclusion, careful consideration had to be given to the seasonal variations in the mean state and to the changing bottom topography along the Strait.
Abstract
Moored current-meter observations obtained from upstream and downstream locations in Shelikof Strait, between Kodiak and Afognak Islands and the Alaskan mainland in the northwest Gulf of Alaska, show a considerable amount of low-frequency variability superimposed upon the mean southwestward along-channel flow. Moreover, the spectral properties of these low-frequency current fluctuations varied seasonally and according to location within the channel. During fag 1976, the variance spectra of the along-channel fluctuations at the upstream end of Shelikof Strait were sharply peaked at 2.86 days. At the downstream end of the Strait, some 150 km to the southwest, the variance spectrum of the cross-channel fluctuations at 100 m was peaked at 6.15 days. However, the upstream 2.86-day signal was coherent with 2.86-day fluctuations downstream, and the phase relationship between the two locations implied the presence of a downstream-traveling wave with a wavelength of 74 km and a phase speed of 26 km day−1. (Downstream-traveling waves of similar scales have also been observed in satellite images of this region taken during fall 1979.) During winter 1977, on the other hand, the variance spectra of the current fluctuations at the upstream end of the Strait were broadly peaked at 2–3.5 days, and the fluctuations were not coherent with those downstream in this period range. However, the downstream variance spectra were peaked at 4.44 days and this signal was coherent with the upstream fluctuations. The phase relationship between the two locations implied the presence of a downstream-traveling wave with a wavelength of 94 km and a phase speed of 21 km day−1.
This paper attempts to explain these low-frequency current fluctuations by investigating the stability of a two-layer flow in a straight channel with a sloping bottom. It is shown that, for representative seasonal values of the mean current shear, density stratification and bottom slope at each end of the Strait, the mean flow is generally baroclinically unstable with respect to downstream-traveling, quasi-geostrophic wave perturbations. (A similar result is also arrived at using the classical Eady model with constant shear, stratification and depth.) More specifically, in an application of the model to the fall 1976 data, it is found that a 2.86-day period wave at the upstream end is unstable and propagates downstream with a wavelength of 92 km and a phase speed of 32 km day−1, in good agreement with the observations. However, at the downstream end, such a wave becomes stable due to the reversal in bottom slope along the Strait. Moreover, at the downstream end, only very long period waves an unstable, with the maximally unstable wave having a period of 5.2 days. Thus the low-frequency spectral peak at 6.15 days observed at the downstream mooring is probably due to locally generated instabilities. From an application of the model to the winter 1977 data, it is found that the large 4.44-day signal observed downstream is likely due to a downstream-travelling wave that is locally unstable but which originated from the upstream end of the Strait where it was marginally unstable.
From these applications and other analyses (e.g., of vertical phase relations and of summer 1978 data), it is concluded that most of the low-frequency current fluctuations in Shelikof Strait are due to baroclinic instability of the mean flow. However, in reaching this conclusion, careful consideration had to be given to the seasonal variations in the mean state and to the changing bottom topography along the Strait.
Abstract
A current record during February -August 1980 over the continental slope off Kodiak Island provided the first Eulerian measurements in the high-speed region of the Alaskan Stream. The net flow at 980 m during the 6-month period was 6 cm s−1 at 235°, but there were major low-frequency variations in the current. These appeared to result from the occasional advection of meanders past the mooring, however, rather than from features such as planetary waves. The ratio of fluctuating to mean kinetic energy was much lower than reported values in the Kuroshio and Gulf Stream, probably as a result of important kinematic differences in these flows.
Abstract
A current record during February -August 1980 over the continental slope off Kodiak Island provided the first Eulerian measurements in the high-speed region of the Alaskan Stream. The net flow at 980 m during the 6-month period was 6 cm s−1 at 235°, but there were major low-frequency variations in the current. These appeared to result from the occasional advection of meanders past the mooring, however, rather than from features such as planetary waves. The ratio of fluctuating to mean kinetic energy was much lower than reported values in the Kuroshio and Gulf Stream, probably as a result of important kinematic differences in these flows.
Abstract
Shortly after 0600 UTC (midnight MDT) 9 June 2020, a rapidly intensifying and elongating convective system produced a macroburst and extensive damage in the town of Akron on Colorado’s eastern plains. Instantaneous winds were measured as high as 51.12 m s−1 at 2.3 m AGL from an eddy covariance (EC) tower, and a 50.45 m s−1 wind gust from an adjacent 10-m tower became the highest official thunderstorm wind gust ever measured in Colorado. Synoptic-scale storm motion was southerly, but surface winds were northerly in a postfrontal air mass, creating strong vertical wind shear. Extremely high-resolution temporal and spatial observations allow for a unique look at pressure and temperature tendencies accompanying the macroburst and reveal intriguing wave structures in the outflow. At 10-Hz frequency, the EC tower recorded a 5-hPa pressure surge in 19 s immediately following the strongest winds, and a 15-hPa pressure drop in the following 3 min. Surface temperature also rose 1.5°C in less than 1 min, concurrent with the maximum wind gusts, and then fell sharply by 3.5°C in the following minute. Shifting wind direction observations and an NWS damage survey are suggestive of both radial outflow and a gust front passage, and model proximity soundings reveal a well-mixed surface layer topped by a strong inversion and large low-level vertical wind shear. Despite the greatest risk of severe winds forecast to be northeast of Colorado, convection-allowing model forecasts from 6 to 18 h in advance did show similar structures to what occurred, warranting further simulations to investigate the unique mesoscale and misoscale features associated with the macroburst.
Abstract
Shortly after 0600 UTC (midnight MDT) 9 June 2020, a rapidly intensifying and elongating convective system produced a macroburst and extensive damage in the town of Akron on Colorado’s eastern plains. Instantaneous winds were measured as high as 51.12 m s−1 at 2.3 m AGL from an eddy covariance (EC) tower, and a 50.45 m s−1 wind gust from an adjacent 10-m tower became the highest official thunderstorm wind gust ever measured in Colorado. Synoptic-scale storm motion was southerly, but surface winds were northerly in a postfrontal air mass, creating strong vertical wind shear. Extremely high-resolution temporal and spatial observations allow for a unique look at pressure and temperature tendencies accompanying the macroburst and reveal intriguing wave structures in the outflow. At 10-Hz frequency, the EC tower recorded a 5-hPa pressure surge in 19 s immediately following the strongest winds, and a 15-hPa pressure drop in the following 3 min. Surface temperature also rose 1.5°C in less than 1 min, concurrent with the maximum wind gusts, and then fell sharply by 3.5°C in the following minute. Shifting wind direction observations and an NWS damage survey are suggestive of both radial outflow and a gust front passage, and model proximity soundings reveal a well-mixed surface layer topped by a strong inversion and large low-level vertical wind shear. Despite the greatest risk of severe winds forecast to be northeast of Colorado, convection-allowing model forecasts from 6 to 18 h in advance did show similar structures to what occurred, warranting further simulations to investigate the unique mesoscale and misoscale features associated with the macroburst.
Abstract
Extensive hydrographic surveys were conducted in Shelikof Strait in March and October 1985. The data are used to describe circulation and property distributions and the changes that occurred. The upper layer flows to the southwest throughout the year, but greatest speeds occur in the fall when surface waters are least saline because of a maximum in freshwater discharge. The deep water has its source to the south, and the properties seem to result from vertical mixing of this southern water. Thus Shelikof Strait has an estuarine-like circulation with a northward, deep inflow.
Property distribution showed that isolines were usually deepest on the right side of the channel looking to the southwest; greatest baroclinic speeds were often there also. Differential Ekman pumping may contribute to the development of this structure and its changes. Volume transport estimates varied considerably. In October the southwest flow bifurcated, with part continuing along the Alaska Peninsula and the rest exiting the main channel to the south; in March all upper-layer flow followed the main channel. Shelikof Strait appears to be a system influenced by both density-driven and wind-driven effects.
Abstract
Extensive hydrographic surveys were conducted in Shelikof Strait in March and October 1985. The data are used to describe circulation and property distributions and the changes that occurred. The upper layer flows to the southwest throughout the year, but greatest speeds occur in the fall when surface waters are least saline because of a maximum in freshwater discharge. The deep water has its source to the south, and the properties seem to result from vertical mixing of this southern water. Thus Shelikof Strait has an estuarine-like circulation with a northward, deep inflow.
Property distribution showed that isolines were usually deepest on the right side of the channel looking to the southwest; greatest baroclinic speeds were often there also. Differential Ekman pumping may contribute to the development of this structure and its changes. Volume transport estimates varied considerably. In October the southwest flow bifurcated, with part continuing along the Alaska Peninsula and the rest exiting the main channel to the south; in March all upper-layer flow followed the main channel. Shelikof Strait appears to be a system influenced by both density-driven and wind-driven effects.
Abstract
Shortly after 0600 UTC (midnight local time) 9 June 2020, a convective line produced severe winds across parts of northeast Colorado that caused extensive damage, especially in the town of Akron. High-resolution observations showed gusts exceeding 50 m s−1, accompanied by extremely large pressure fluctuations, including a 5-hPa pressure surge in 19 s immediately following the strongest winds and a 15-hPa pressure drop in the following 3 min. Numerical simulations of this event (using the WRF Model) and with horizontally homogeneous initial conditions (using Cloud Model 1) reveal that the severe winds in this event were associated with gravity wave dynamics. In a very stable postfrontal environment, elevated convection initiated and led to a long-lived gravity wave. Strong low-level vertical wind shear supported the amplification and eventual breaking of this wave, resulting in at least two sequential strong downbursts. This wave-breaking mechanism is different from the usual downburst mechanism associated with negative buoyancy resulting from latent cooling. The model output reproduces key features of the high-resolution observations, including similar convective structures, large temperature and pressure fluctuations, and intense near-surface wind speeds. The findings of this study reveal a series of previously unexplored mesoscale and storm-scale processes that can result in destructive winds.
Significance Statement
Downbursts of intense wind can produce significant damage, as was the case on 9 June 2020 in Akron, Colorado. Past research on downbursts has shown that they occur when raindrops, graupel, and hail in thunderstorms evaporate and melt, cooling the air and causing it to sink rapidly. In this research, we used numerical models of the atmosphere, along with high-resolution observations, to show that the Akron downburst was different. Unlike typical lines of thunderstorms, those responsible for the Akron macroburst produced a wave in the atmosphere, which broke, resulting in rapidly sinking air and severe surface winds.
Abstract
Shortly after 0600 UTC (midnight local time) 9 June 2020, a convective line produced severe winds across parts of northeast Colorado that caused extensive damage, especially in the town of Akron. High-resolution observations showed gusts exceeding 50 m s−1, accompanied by extremely large pressure fluctuations, including a 5-hPa pressure surge in 19 s immediately following the strongest winds and a 15-hPa pressure drop in the following 3 min. Numerical simulations of this event (using the WRF Model) and with horizontally homogeneous initial conditions (using Cloud Model 1) reveal that the severe winds in this event were associated with gravity wave dynamics. In a very stable postfrontal environment, elevated convection initiated and led to a long-lived gravity wave. Strong low-level vertical wind shear supported the amplification and eventual breaking of this wave, resulting in at least two sequential strong downbursts. This wave-breaking mechanism is different from the usual downburst mechanism associated with negative buoyancy resulting from latent cooling. The model output reproduces key features of the high-resolution observations, including similar convective structures, large temperature and pressure fluctuations, and intense near-surface wind speeds. The findings of this study reveal a series of previously unexplored mesoscale and storm-scale processes that can result in destructive winds.
Significance Statement
Downbursts of intense wind can produce significant damage, as was the case on 9 June 2020 in Akron, Colorado. Past research on downbursts has shown that they occur when raindrops, graupel, and hail in thunderstorms evaporate and melt, cooling the air and causing it to sink rapidly. In this research, we used numerical models of the atmosphere, along with high-resolution observations, to show that the Akron downburst was different. Unlike typical lines of thunderstorms, those responsible for the Akron macroburst produced a wave in the atmosphere, which broke, resulting in rapidly sinking air and severe surface winds.
Abstract
Conductivity and temperature versus depth (CTD) and expendable bathythermograph (XBT) data taken during the ice-free seasons of 1975–77 define a structural front paralleling the 50 m isobath. This front forms a narrow transition separating a well-mixed coastal domain from a two-layered central shelf domain. In early spring, prior to frontogenesis, we believe that temperature and salinity are continuous across the 50 m isobath. Thus, the front does not result from the confluence of water masses; rather the front permits the evolution of different water masses following frontogenesis. The changing balance between buoyant energy input and tidal stirring determines the frontal location and the frontal width correlates with bottom slope. The front is similar to those reported around the British Isles, but we find that in the Bering Sea the salinity distribution is important, that the ice cover influences the seasonal evolution of the hydrographic structure, and that the geostrophic (baroclinic) speed differences across the front are small (<2 cm s−1). We hypothesize that frontogenesis depends critically on positive feedback between stratification and mixing.
Abstract
Conductivity and temperature versus depth (CTD) and expendable bathythermograph (XBT) data taken during the ice-free seasons of 1975–77 define a structural front paralleling the 50 m isobath. This front forms a narrow transition separating a well-mixed coastal domain from a two-layered central shelf domain. In early spring, prior to frontogenesis, we believe that temperature and salinity are continuous across the 50 m isobath. Thus, the front does not result from the confluence of water masses; rather the front permits the evolution of different water masses following frontogenesis. The changing balance between buoyant energy input and tidal stirring determines the frontal location and the frontal width correlates with bottom slope. The front is similar to those reported around the British Isles, but we find that in the Bering Sea the salinity distribution is important, that the ice cover influences the seasonal evolution of the hydrographic structure, and that the geostrophic (baroclinic) speed differences across the front are small (<2 cm s−1). We hypothesize that frontogenesis depends critically on positive feedback between stratification and mixing.
Abstract
Many of our generation’s most pressing environmental science problems are wicked problems, which means they cannot be cleanly isolated and solved with a single “correct” answer. The NSF AI Institute for Research on Trustworthy AI in Weather, Climate, and Coastal Oceanography (AI2ES) seeks to address such problems by developing synergistic approaches with a team of scientists from three disciplines: environmental science (including atmospheric, ocean, and other physical sciences), artificial intelligence (AI), and social science including risk communication. As part of our work, we developed a novel approach to summer school, held from 27 to 30 June 2022. The goal of this summer school was to teach a new generation of environmental scientists how to cross disciplines and develop approaches that integrate all three disciplinary perspectives and approaches in order to solve environmental science problems. In addition to a lecture series that focused on the synthesis of AI, environmental science, and risk communication, this year’s summer school included a unique “trust-a-thon” component where participants gained hands-on experience applying both risk communication and explainable AI techniques to pretrained machine learning models. We had 677 participants from 63 countries register and attend online. Lecture topics included trust and trustworthiness (day 1), explainability and interpretability (day 2), data and workflows (day 3), and uncertainty quantification (day 4). For the trust-a-thon, we developed challenge problems for three different application domains: 1) severe storms, 2) tropical cyclones, and 3) space weather. Each domain had associated user persona to guide user-centered development.
Abstract
Many of our generation’s most pressing environmental science problems are wicked problems, which means they cannot be cleanly isolated and solved with a single “correct” answer. The NSF AI Institute for Research on Trustworthy AI in Weather, Climate, and Coastal Oceanography (AI2ES) seeks to address such problems by developing synergistic approaches with a team of scientists from three disciplines: environmental science (including atmospheric, ocean, and other physical sciences), artificial intelligence (AI), and social science including risk communication. As part of our work, we developed a novel approach to summer school, held from 27 to 30 June 2022. The goal of this summer school was to teach a new generation of environmental scientists how to cross disciplines and develop approaches that integrate all three disciplinary perspectives and approaches in order to solve environmental science problems. In addition to a lecture series that focused on the synthesis of AI, environmental science, and risk communication, this year’s summer school included a unique “trust-a-thon” component where participants gained hands-on experience applying both risk communication and explainable AI techniques to pretrained machine learning models. We had 677 participants from 63 countries register and attend online. Lecture topics included trust and trustworthiness (day 1), explainability and interpretability (day 2), data and workflows (day 3), and uncertainty quantification (day 4). For the trust-a-thon, we developed challenge problems for three different application domains: 1) severe storms, 2) tropical cyclones, and 3) space weather. Each domain had associated user persona to guide user-centered development.
Abstract
The National Hurricane Center Hurricane Probability Program, which estimated the probability of a tropical cyclone passing within a specific distance of a selected set of coastal stations, was replaced by the more general Tropical Cyclone Surface Wind Speed Probabilities in 2006. A Monte Carlo (MC) method is used to estimate the probabilities of 34-, 50-, and 64-kt (1 kt = 0.51 m s−1) winds at multiple time periods through 120 h. Versions of the MC model are available for the Atlantic, the combined eastern and central North Pacific, and the western North Pacific. This paper presents a verification of the operational runs of the MC model for the period 2008–11 and describes model improvements since 2007. The most significant change occurred in 2010 with the inclusion of a method to take into account the uncertainty of the track forecasts on a case-by-case basis, which is estimated from the spread of a dynamical model ensemble and other parameters. The previous version represented the track uncertainty from the error distributions from the previous 5 yr of forecasts from the operational centers, with no case-to-case variability. Results show the MC model provides robust estimates of the wind speed probabilities using a number of standard verification metrics, and that the inclusion of the case-by-case measure of track uncertainty improved the probability estimates. Beginning in 2008, an older operational wind speed probability table product was modified to include information from the MC model. This development and a verification of the new version of the table are described.
Abstract
The National Hurricane Center Hurricane Probability Program, which estimated the probability of a tropical cyclone passing within a specific distance of a selected set of coastal stations, was replaced by the more general Tropical Cyclone Surface Wind Speed Probabilities in 2006. A Monte Carlo (MC) method is used to estimate the probabilities of 34-, 50-, and 64-kt (1 kt = 0.51 m s−1) winds at multiple time periods through 120 h. Versions of the MC model are available for the Atlantic, the combined eastern and central North Pacific, and the western North Pacific. This paper presents a verification of the operational runs of the MC model for the period 2008–11 and describes model improvements since 2007. The most significant change occurred in 2010 with the inclusion of a method to take into account the uncertainty of the track forecasts on a case-by-case basis, which is estimated from the spread of a dynamical model ensemble and other parameters. The previous version represented the track uncertainty from the error distributions from the previous 5 yr of forecasts from the operational centers, with no case-to-case variability. Results show the MC model provides robust estimates of the wind speed probabilities using a number of standard verification metrics, and that the inclusion of the case-by-case measure of track uncertainty improved the probability estimates. Beginning in 2008, an older operational wind speed probability table product was modified to include information from the MC model. This development and a verification of the new version of the table are described.
