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- Author or Editor: R. M. Samelson x
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Abstract
The vertical structure of coastal-trapped disturbances in several idealized models of a stably stratified lower atmosphere is examined. The vertical structure and phase speeds of the trapped modes depend on the resting stratification and the height of the orographic step. The presence of a stable layer above the boundary layer inversion increases the gravest-mode phase speed and supports the existence of higher vertical modes. Trapped wave solutions for the step orography are obtained for a lower atmosphere with constant buoyancy frequency. The modes are primarily concentrated below the step but penetrate weakly into the stratified region above the step. The phase speed of the gravest trapped mode is greater than the gravest-mode Kelvin wave speed based on the height of the step. Results from a linear two-layer model suggest that the observed vertical structure of isotherms at the leading edge of a 10–11 June 1994 event may arise during a transition from a directly forced, barotropic, alongshore velocity response to a regime influenced by wave propagation, as the coastal-trapped vertical modes excited by the mesoscale pressure gradients begin to disperse at their respective phase speeds. The results suggest also that the observed vertical structure of alongshore velocity, with largest velocities in the stable layer above the boundary layer, may arise from drag at the sea surface.
Abstract
The vertical structure of coastal-trapped disturbances in several idealized models of a stably stratified lower atmosphere is examined. The vertical structure and phase speeds of the trapped modes depend on the resting stratification and the height of the orographic step. The presence of a stable layer above the boundary layer inversion increases the gravest-mode phase speed and supports the existence of higher vertical modes. Trapped wave solutions for the step orography are obtained for a lower atmosphere with constant buoyancy frequency. The modes are primarily concentrated below the step but penetrate weakly into the stratified region above the step. The phase speed of the gravest trapped mode is greater than the gravest-mode Kelvin wave speed based on the height of the step. Results from a linear two-layer model suggest that the observed vertical structure of isotherms at the leading edge of a 10–11 June 1994 event may arise during a transition from a directly forced, barotropic, alongshore velocity response to a regime influenced by wave propagation, as the coastal-trapped vertical modes excited by the mesoscale pressure gradients begin to disperse at their respective phase speeds. The results suggest also that the observed vertical structure of alongshore velocity, with largest velocities in the stable layer above the boundary layer, may arise from drag at the sea surface.
Abstract
A recent climatology of observed coastal-trapped disturbances in the marine atmospheric boundary layer along the United States west coast motivates the detailed examination, for a specific form of imposed forcing, of a linear shallow-water boundary layer model. The model is forced by a time-dependent pressure field, imposed at a fixed level above the boundary layer, that is an idealized representation of the climatological synoptic evolution: a low pressure center translates westward across the coastal boundary, corresponding to the observed offshore extension of a continental thermal trough. The alongshore structure of the model disturbance is characterized by enhanced northerly flow, a depressed marine layer, and low surface pressure to the north; and southerly flow, a raised marine layer, and high surface pressure to the south. Initially, the marine-layer thickness along the coast responds predominantly to convergence of the ageostrophic cross-shore flow driven by the imposed cross-shore pressure gradient and to convergence (to the south of the low pressure center) and divergence (to the north) of the geostrophic cross-shore flow balanced by the imposed alongshore pressure gradient, lifting in the central and southern parts of the forcing region and failing north of the forcing region. For the parameter values considered here, the amplitude of the coastal-trapped thickness response to the geostrophic cross-shore flow is roughly three times as large as that due to the ageostrophic cross-shore flow, but this ratio is likely to be sensitive to the cross-shore/alongshore aspect ratio of the pressure forcing. The coastal-trapped alongshore velocity disturbance is dominated by the response to the alongshore pressure gradient. There is no alongshore propagation in thickness disturbance during the initial stage of the event, while the alongshore velocity and surface pressure exhibit only weak propagation. In the later stages of the event, when the imposed coastal pressure gradients relax (as the low translates offshore), the cross-shore flow weakens, and the response at the coast is controlled by the convergence and divergence of the alongshore flow. The thickness disturbance, alongshore velocity reversal, and surface pressure perturbation propagate northward along the coast essentially as a Kelvin wave in the later stages of the event. Although both the model and the imposed pressure forcing are highly idealized, the model response is qualitatively and, to some degree, quantitatively consistent with many aspects of existing observations of coastal-trapped wind reversals along the United States west coast.
Abstract
A recent climatology of observed coastal-trapped disturbances in the marine atmospheric boundary layer along the United States west coast motivates the detailed examination, for a specific form of imposed forcing, of a linear shallow-water boundary layer model. The model is forced by a time-dependent pressure field, imposed at a fixed level above the boundary layer, that is an idealized representation of the climatological synoptic evolution: a low pressure center translates westward across the coastal boundary, corresponding to the observed offshore extension of a continental thermal trough. The alongshore structure of the model disturbance is characterized by enhanced northerly flow, a depressed marine layer, and low surface pressure to the north; and southerly flow, a raised marine layer, and high surface pressure to the south. Initially, the marine-layer thickness along the coast responds predominantly to convergence of the ageostrophic cross-shore flow driven by the imposed cross-shore pressure gradient and to convergence (to the south of the low pressure center) and divergence (to the north) of the geostrophic cross-shore flow balanced by the imposed alongshore pressure gradient, lifting in the central and southern parts of the forcing region and failing north of the forcing region. For the parameter values considered here, the amplitude of the coastal-trapped thickness response to the geostrophic cross-shore flow is roughly three times as large as that due to the ageostrophic cross-shore flow, but this ratio is likely to be sensitive to the cross-shore/alongshore aspect ratio of the pressure forcing. The coastal-trapped alongshore velocity disturbance is dominated by the response to the alongshore pressure gradient. There is no alongshore propagation in thickness disturbance during the initial stage of the event, while the alongshore velocity and surface pressure exhibit only weak propagation. In the later stages of the event, when the imposed coastal pressure gradients relax (as the low translates offshore), the cross-shore flow weakens, and the response at the coast is controlled by the convergence and divergence of the alongshore flow. The thickness disturbance, alongshore velocity reversal, and surface pressure perturbation propagate northward along the coast essentially as a Kelvin wave in the later stages of the event. Although both the model and the imposed pressure forcing are highly idealized, the model response is qualitatively and, to some degree, quantitatively consistent with many aspects of existing observations of coastal-trapped wind reversals along the United States west coast.
Abstract
A mesoscale atmospheric model, nested in operational global numerical weather prediction fields, is used to estimate low-level winds and surface wind stress through Nares Strait, between Ellesmere Island and Greenland, during 2 yr from August 2003 to July 2005. During most of the year, the model low-level winds are dominated by intense, southward along-strait flow, with monthly-mean southward 10-m winds reaching 10 m s−1 in winter. Summertime flow is weak and distributions of hourly along-strait winds during the 2-yr period are strongly bimodal. The strong southward low-level winds are associated with ageostrophic, orographically channeled flow down the pressure gradient from the Lincoln Sea to Baffin Bay and are highly correlated with the pressure difference along Nares Strait. The 2-yr means and leading EOFs of monthly-mean 10-m winds and wind stress place the strongest winds and stress in the southern parts of Smith Sound and of Kennedy Channel, at the openings to Baffin Bay and Kane Basin, at known sites of polynya formation, including the North Water polynya in Smith Sound, suggesting that the locally intensified winds may cause these persistent polynyas. An intense wind event observed in Nares Strait by a field camp, with surface winds exceeding 30 m s−1, generally follows the typical pattern of these low-level flows. Based on the model correlation of winds and pressure difference, a 51-yr time series of estimated winds in Nares Strait is reconstructed from historical surface pressure measurements at Thule, Greenland, and Alert, Canada. The pressure difference and reconstructed wind time series are correlated with the Arctic Oscillation at annual and longer periods, but not on monthly periods.
Abstract
A mesoscale atmospheric model, nested in operational global numerical weather prediction fields, is used to estimate low-level winds and surface wind stress through Nares Strait, between Ellesmere Island and Greenland, during 2 yr from August 2003 to July 2005. During most of the year, the model low-level winds are dominated by intense, southward along-strait flow, with monthly-mean southward 10-m winds reaching 10 m s−1 in winter. Summertime flow is weak and distributions of hourly along-strait winds during the 2-yr period are strongly bimodal. The strong southward low-level winds are associated with ageostrophic, orographically channeled flow down the pressure gradient from the Lincoln Sea to Baffin Bay and are highly correlated with the pressure difference along Nares Strait. The 2-yr means and leading EOFs of monthly-mean 10-m winds and wind stress place the strongest winds and stress in the southern parts of Smith Sound and of Kennedy Channel, at the openings to Baffin Bay and Kane Basin, at known sites of polynya formation, including the North Water polynya in Smith Sound, suggesting that the locally intensified winds may cause these persistent polynyas. An intense wind event observed in Nares Strait by a field camp, with surface winds exceeding 30 m s−1, generally follows the typical pattern of these low-level flows. Based on the model correlation of winds and pressure difference, a 51-yr time series of estimated winds in Nares Strait is reconstructed from historical surface pressure measurements at Thule, Greenland, and Alert, Canada. The pressure difference and reconstructed wind time series are correlated with the Arctic Oscillation at annual and longer periods, but not on monthly periods.
Abstract
Estimates of three components of an uncertainty budget for a coastal ocean model in a wind-forced regime are made based on numerical simulations. The budget components behave differently in the shelf regime, inshore of the 200-m isobath, and the slope-interior regime, between the 200-m isobath and a fixed longitude (126°W) that is roughly 150 km offshore. The first of the three budget components is an estimate of the uncertainty in the ocean state given only a known history of wind stress forcing, with errors in the wind forcing estimated from differences between operational analyses. It is found that, over the continental shelf, the response to wind forcing is sufficiently strong and deterministic that significant skill in estimating shelf circulation can be achieved with knowledge only of the wind forcing, and no ocean data, for wind fields with these estimated errors. The second involves initial condition error and its influence on uncertainty, including both error growth with time from well-known initial conditions and error decay with time from poorly known initial conditions but with well-known wind forcing. The third component is that of boundary condition error and its influence on the interior solutions, including the dependence of that influence on the specific location along the boundary of the boundary condition error. Boundary condition errors with amplitude comparable to the root-mean-square variability at the boundary lead eventually to errors equal to the root-mean-square variability in the slope-interior regime, and somewhat smaller errors in the shelf regime. Covariance estimates based on differences of the wind-forced solutions from the ensemble mean are not dramatically different from those based on the full fields, and do not show strong state dependence.
Abstract
Estimates of three components of an uncertainty budget for a coastal ocean model in a wind-forced regime are made based on numerical simulations. The budget components behave differently in the shelf regime, inshore of the 200-m isobath, and the slope-interior regime, between the 200-m isobath and a fixed longitude (126°W) that is roughly 150 km offshore. The first of the three budget components is an estimate of the uncertainty in the ocean state given only a known history of wind stress forcing, with errors in the wind forcing estimated from differences between operational analyses. It is found that, over the continental shelf, the response to wind forcing is sufficiently strong and deterministic that significant skill in estimating shelf circulation can be achieved with knowledge only of the wind forcing, and no ocean data, for wind fields with these estimated errors. The second involves initial condition error and its influence on uncertainty, including both error growth with time from well-known initial conditions and error decay with time from poorly known initial conditions but with well-known wind forcing. The third component is that of boundary condition error and its influence on the interior solutions, including the dependence of that influence on the specific location along the boundary of the boundary condition error. Boundary condition errors with amplitude comparable to the root-mean-square variability at the boundary lead eventually to errors equal to the root-mean-square variability in the slope-interior regime, and somewhat smaller errors in the shelf regime. Covariance estimates based on differences of the wind-forced solutions from the ensemble mean are not dramatically different from those based on the full fields, and do not show strong state dependence.
Abstract
The predictability of coastal ocean circulation over the central Oregon shelf, a region of strong wind-driven currents and variable topography, is studied using ensembles of 50-day primitive equation ocean model simulations with realistic topography, simplified lateral boundary conditions, and forcing from both idealized and observed wind time series representative of the summer upwelling season. The main focus is on the balance, relevant to practical predictability, between deterministic response to known or well-predicted forcing, uncertainty in initial conditions, and sensitivity to instabilities and topographic interactions. Large ensemble and single-simulation variances are found downstream of topographic features, associated with transitions between along-isobath and cross-isobath flow, which are in turn related both to the time-integrated amplitude of upwelling-favorable wind forcing and to the formation of small-scale eddies. Simulated predictability experiments are conducted and model forecasts are verified by standard statistics including anomaly correlation coefficient, and root-mean-square error. A new variant of relative entropy, the forecast relative entropy, is introduced to quantify the predictive information content in the forecast ensemble, relative to the initial ensemble. The results suggest that, even under conditions of relatively weak wind forcing, the deterministic response is stronger than instability growth over the 3–7-day forecast intervals considered here. Consequently, important elements of the coastal circulation should be accessible to predictive, dynamical forecasts on the nominal 7-day predictability time scale of the atmospheric forcing, provided that sufficiently accurate initializations are available. These results on predictability are consistent with inferences drawn from recent modeling studies of coastal ocean circulation along the central Oregon shelf, and should have general validity for other, similar regions.
Abstract
The predictability of coastal ocean circulation over the central Oregon shelf, a region of strong wind-driven currents and variable topography, is studied using ensembles of 50-day primitive equation ocean model simulations with realistic topography, simplified lateral boundary conditions, and forcing from both idealized and observed wind time series representative of the summer upwelling season. The main focus is on the balance, relevant to practical predictability, between deterministic response to known or well-predicted forcing, uncertainty in initial conditions, and sensitivity to instabilities and topographic interactions. Large ensemble and single-simulation variances are found downstream of topographic features, associated with transitions between along-isobath and cross-isobath flow, which are in turn related both to the time-integrated amplitude of upwelling-favorable wind forcing and to the formation of small-scale eddies. Simulated predictability experiments are conducted and model forecasts are verified by standard statistics including anomaly correlation coefficient, and root-mean-square error. A new variant of relative entropy, the forecast relative entropy, is introduced to quantify the predictive information content in the forecast ensemble, relative to the initial ensemble. The results suggest that, even under conditions of relatively weak wind forcing, the deterministic response is stronger than instability growth over the 3–7-day forecast intervals considered here. Consequently, important elements of the coastal circulation should be accessible to predictive, dynamical forecasts on the nominal 7-day predictability time scale of the atmospheric forcing, provided that sufficiently accurate initializations are available. These results on predictability are consistent with inferences drawn from recent modeling studies of coastal ocean circulation along the central Oregon shelf, and should have general validity for other, similar regions.
Abstract
Measurements of surface wind stress by the SeaWinds scatterometer on NASA's Quick Scatterometer (QuikSCAT) satellite are analyzed and compared with several different atmospheric model products, from an operational model and two high-resolution nested regional models, during two summer periods, June through September 2000 and 2001, in the coastal region west of Oregon and northern California. The mean summer wind stress had a southward component over the entire region in both years. Orographic intensifications of both the mean and fluctuating wind stress occurred near Cape Blanco, Cape Mendocino, and Point Arena. Substantial differences between the model products are found for the mean, variable, and diurnal wind stress fields. Temporal correlations with the QuikSCAT observations are highest for the operational model, and are not improved by either nested model. The highest-resolution nested model most accurately reproduced the mean observed stress fields, but slightly degrades the temporal correlations due to incoherent high-frequency (0.5–2 cpd) fluctuations. The QuikSCAT data reveal surprisingly strong diurnal fluctuations that extend offshore 150 km or more with magnitudes that are a significant fraction of the mean wind stress. Wind stress curl fields from QuikSCAT and the models show local cyclonic and anticyclonic maxima associated with the orographic wind intensification around the capes. The present results are consistent with the hypothesis of a wind-driven mechanism for coastal jet separation and cold water plume and anticyclonic eddy formation in the California Current System south of Cape Blanco.
Abstract
Measurements of surface wind stress by the SeaWinds scatterometer on NASA's Quick Scatterometer (QuikSCAT) satellite are analyzed and compared with several different atmospheric model products, from an operational model and two high-resolution nested regional models, during two summer periods, June through September 2000 and 2001, in the coastal region west of Oregon and northern California. The mean summer wind stress had a southward component over the entire region in both years. Orographic intensifications of both the mean and fluctuating wind stress occurred near Cape Blanco, Cape Mendocino, and Point Arena. Substantial differences between the model products are found for the mean, variable, and diurnal wind stress fields. Temporal correlations with the QuikSCAT observations are highest for the operational model, and are not improved by either nested model. The highest-resolution nested model most accurately reproduced the mean observed stress fields, but slightly degrades the temporal correlations due to incoherent high-frequency (0.5–2 cpd) fluctuations. The QuikSCAT data reveal surprisingly strong diurnal fluctuations that extend offshore 150 km or more with magnitudes that are a significant fraction of the mean wind stress. Wind stress curl fields from QuikSCAT and the models show local cyclonic and anticyclonic maxima associated with the orographic wind intensification around the capes. The present results are consistent with the hypothesis of a wind-driven mechanism for coastal jet separation and cold water plume and anticyclonic eddy formation in the California Current System south of Cape Blanco.
Abstract
Fog and low-level stratus during April–September 2009 are examined in a set of coupled ocean–atmosphere numerical simulations of the northern California Current System (CCS). The model configurations differ only in the choice of planetary boundary layer (PBL) parameterization scheme and, in one case, surface flux scheme. The results suggest that fog formation in this region primarily occurs through condensation at the surface induced locally by surface cooling, when moist offshore air is advected over cold upwelled waters and the shallow coastal marine PBL is further stabilized by warm, dry, continental air that extends offshore above the PBL inversion. These results are consistent with some but not all prior hypotheses for fog formation in the CCS region. Fog formation by downward growth of a preexisting stratus layer is also found in the simulations, but dominates only in those simulations with PBL schemes that produce an extensive and evidently unphysical stratus layer at 200 m height, which serves as the source for the downward growth. The stronger fog response in later summer months arises from seasonal warming of offshore SST, which increases the moisture content and temperature of the upstream air mass, while cool coastal SSTs are maintained by upwelling. On synoptic time scales, a similar influence of fog response on upstream conditions is found but controlled instead by changes in wind direction. These results suggest that the critical factors determining the evolution of the coastal fog regime in a warming climate are likely the temperature of upwelling source waters and the offshore flow of continental air.
Abstract
Fog and low-level stratus during April–September 2009 are examined in a set of coupled ocean–atmosphere numerical simulations of the northern California Current System (CCS). The model configurations differ only in the choice of planetary boundary layer (PBL) parameterization scheme and, in one case, surface flux scheme. The results suggest that fog formation in this region primarily occurs through condensation at the surface induced locally by surface cooling, when moist offshore air is advected over cold upwelled waters and the shallow coastal marine PBL is further stabilized by warm, dry, continental air that extends offshore above the PBL inversion. These results are consistent with some but not all prior hypotheses for fog formation in the CCS region. Fog formation by downward growth of a preexisting stratus layer is also found in the simulations, but dominates only in those simulations with PBL schemes that produce an extensive and evidently unphysical stratus layer at 200 m height, which serves as the source for the downward growth. The stronger fog response in later summer months arises from seasonal warming of offshore SST, which increases the moisture content and temperature of the upstream air mass, while cool coastal SSTs are maintained by upwelling. On synoptic time scales, a similar influence of fog response on upstream conditions is found but controlled instead by changes in wind direction. These results suggest that the critical factors determining the evolution of the coastal fog regime in a warming climate are likely the temperature of upwelling source waters and the offshore flow of continental air.
Abstract
The influence of mesoscale sea surface temperature (SST) variations on wind stress and boundary layer winds is examined from coupled ocean–atmosphere numerical simulations and satellite observations of the northern California Current System. Model coupling coefficients relating the divergence and curl of wind stress and wind to downwind and crosswind SST gradients are generally smaller than observed values and vary by a factor of 2 depending on planetary boundary layer (PBL) scheme, with values larger for smoothed fields on the 0.25° observational grid than for unsmoothed fields on the 12-km model grid. Divergence coefficients are larger than curl coefficients on the 0.25° grid but not on the model grid, consistent with stronger scale dependence for the divergence response than for curl in a spatial cross-spectral analysis. Coupling coefficients for 10-m equivalent neutral stability winds are 30%–50% larger than those for 10-m wind, implying a correlated effect of surface-layer stability variations. Crosswind surface air temperature and SST gradients are more strongly coupled than downwind gradients, while the opposite is true for downwind and crosswind heat flux and SST gradients. Midlevel boundary layer wind coupling coefficients show a reversed response relative to the surface that is predicted by an analytical model; a predicted second reversal with height is not seen in the simulations. The relative values of coupling coefficients are consistent with previous results for the same PBL schemes in the Agulhas Return Current region, but their magnitudes are smaller, likely because of the effect of mean wind on perturbation heat fluxes.
Abstract
The influence of mesoscale sea surface temperature (SST) variations on wind stress and boundary layer winds is examined from coupled ocean–atmosphere numerical simulations and satellite observations of the northern California Current System. Model coupling coefficients relating the divergence and curl of wind stress and wind to downwind and crosswind SST gradients are generally smaller than observed values and vary by a factor of 2 depending on planetary boundary layer (PBL) scheme, with values larger for smoothed fields on the 0.25° observational grid than for unsmoothed fields on the 12-km model grid. Divergence coefficients are larger than curl coefficients on the 0.25° grid but not on the model grid, consistent with stronger scale dependence for the divergence response than for curl in a spatial cross-spectral analysis. Coupling coefficients for 10-m equivalent neutral stability winds are 30%–50% larger than those for 10-m wind, implying a correlated effect of surface-layer stability variations. Crosswind surface air temperature and SST gradients are more strongly coupled than downwind gradients, while the opposite is true for downwind and crosswind heat flux and SST gradients. Midlevel boundary layer wind coupling coefficients show a reversed response relative to the surface that is predicted by an analytical model; a predicted second reversal with height is not seen in the simulations. The relative values of coupling coefficients are consistent with previous results for the same PBL schemes in the Agulhas Return Current region, but their magnitudes are smaller, likely because of the effect of mean wind on perturbation heat fluxes.
