42 research outputs found
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Investigations of Satellite-Observed, In Situ-Measured, and Model-Simulated Land-Air-Sea Boundary Layer Physics
No full textAir-sea fluxes are essential parameters for quantifying momentum, heat, and humidity exchanges in the marine atmospheric boundary layer (MABL). Traditional techniques, quantifying these parameters in the open ocean, assume stationary and homogeneous conditions. However, these conditions often break down in non-stationary coastal environments with strong horizontal gradients, topography, breaking waves, etc. Also, temporal, in situ instrumentation, alone, cannot characterize the variety of spatial scales necessary for accurate modeling. Furthermore, models often lack the physical parameterizations required for small-scale features or have too coarse-resolution to resolve such features. Therefore, high-resolution, spatial datasets (i.e., satellites) are critical for improving forecast model physics and initial conditions. To combat these limitations, a field experiment named Coastal Land Air-Sea Interaction, conducted in Monterey Bay, CA, aims to combine in situ and satellite data from various instrumentation (on ships, land-based towers, space-borne SAR, etc.) to improve forecasting of the land-air-sea boundary layer within the Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System model (COAMPS). A new wavelet-based technique proposes to autonomously derive state-of-the-art maps of land-air-sea parameters, such as wave speed, wave age, surface roughness length, drag coefficient, and wind stress from SAR at as low as 5-meter resolution. The new technique, called the SAR Land-Air-Sea-Transfer (SLAST) algorithm, successfully quantifies air-sea momentum transfer from the coast-line to the open ocean with validation against traditional buoy and coastal tower anemometer measurements yielding promising results. The SLAST algorithm is valuable to incorporate into COAMPS and other forecasting and mathematical models, such as Large Eddy Simulation (LES) models. SLAST is also applicable in domains across the globe outside Monterey Bay, where in situ measurements are not available or difficult to obtain (such as in hurricane conditions).</p
Identification and Quantification of Wave Breaking Effects in Single-Polarization Along-Track InSAR Imagery of the Coastal Ocean
No full textSpaceborne Along-Track Interferometric Synthetic Aperture Radar (along-track InSAR) has been used successfully to produce estimates of the surface current velocity field on a number of occasions. Along-track InSAR data are comprised of two complex images with a very short time lag, with each pixel containing an amplitude and phase. The phase difference allows a direct measurement of the line-of-sight velocity of the Bragg scattering ripples, which includes contributions of the horizontal surface current as well as the phase velocity of the Bragg ripples and orbital motions of longer waves. To calculate the surface current field, the complicated wave-related contributions to the measured radar velocity need to be estimated and subtracted. Previously, either a single mean velocity correction was used or a spatially varying correction was computed using a relatively simple numerical current-wave interaction model. In areas with large current gradients and spatial depth changes, the resulting complicated surface wave field, such as at our test location at the Columbia River, requires a sophisticated method to estimate the complex corresponding velocity corrections. In this location, the near-shore hindcast model Delft3D with the wave model SWAN, is used to produce 2-D theoretical current and wave fields. To determine the location and magnitude of the wave-related motion contributions, we calculate the Doppler velocity anomaly by subtracting the 1-D component of the theoretical surface current velocities parallel to the radar from the Doppler velocities. Comparing the Doppler velocity anomaly to the SWAN predicted wave height, wave breaking and wave steepness, we confirm the expectation that the Doppler velocity anomaly is closely related to wave breaking and wave steepness. To calculate the required Doppler anomaly correction, we first use the SAR imaging model M4S to simulate ATI-SAR data for wavenumber spectra converted from the SWAN frequency spectra. Unfortunately, the Delft3D implementation of SWAN is unable to output spectral information at the high frequencies that correspond to the high wave numbers to which the radar is sensitive, the gravity-capillary waves. We continue this work by investigating the change in image statistics between areas of high and low Doppler anomaly. We calculate the spatially changing probability density function of the interferogram amplitude and the associated higher order moments: variance, skewness, and kurtosis. We develop an empirical model that relates changes of the image statistics, to the wave motions and the resulting Doppler correction. We apply the empirical model to our data set at the Columbia river as well as to new images and show that we improve the Doppler velocity estimates universally and are able to account for 45%-80% of the required Doppler correction, referenced against the Delft3D model results.</p
Air-Sea Interaction Dynamics Under Hurricane Wind Conditions
No full textUnderstanding turbulent fluxes of momentum, mass, and energy across the air-sea boundary are fundamental to our ability to model and parameterize a number of multidimensional geophysical processes, such as wind-wave generation, oceanic circulation, and air-sea gas transfer. The physical nature of the near surface boundary layer remains less known, especially under high winds due to the development of an intermediate substrate layer of large spray droplets known as spume, between the atmosphere and ocean surface. Presence of these spume droplet effects the aerodynamic resistance of the prevailing winds over the surface and thus the behavior of surface drag coefficient. The size-dependent vertical distribution of spume particles in high wind conditions is necessary to understand their effect on air-sea fluxes of heat and momentum. Given spume’s role in mediating air-sea exchange at the base of tropical cyclones or other storm events, the predominant focus of present literature studies on spray dynamics has been within the marine environment. In contrast, spume production in non-seawater bodies have not been extensively studied and potential differences between sea and freshwater are neglected. Thus any significant differences between sea and freshwater remain unquantified. Direct measurements of the physical processes happening at this interface remains scarce till date due to difficulty in making robust measurements in the field. Laboratories on the other hand remains the primary means for directly observing spray processes near the surface, and offers promising aspects for improving our understanding by learning these processes in a controlled environment. There is no standardization on the water type used for these experiments and any potential effects water masses have on the spume generation process is unknown. This adds uncertainty in our ability to make physically realistic spume generation functions that are ultimately applied to the geophysical domain. To address this gap, we have conducted a series of laboratory experiment at the Air-Sea Interaction tank facility (ASIST) of the University of Miami, directly comparing spume concentrations, and surface drag coefficient behavior above fresh and real seawater for 10-m equivalent wind speeds up to 54 m/s. Direct measurements of the near-surface processes were made and directly related to local sources of variance. Droplets in the air above the intensely breaking wind-waves were optically observed and their distribution as functions of wind speed, height, and droplet radius was compared between the two water types. Drag coefficient was calculated using the eddy covariance method on the three-dimensional wind data observed using a sonic anemometer. Our results show significant differences in the spume generation as well as in the surface drag coefficient behavior for the two water types. Substantially higher concentrations of seawater spume were observed as compared to freshwater across all particle sizes and wind speeds. The seawater particles’ vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Statistical analysis of these findings suggest significant differences in the size- and height-dependent distribution response to increased wind forcing between fresh and seawater. Drag coefficient values for seawater were found less than that of freshwater at all wind speeds suggesting modulation of momentum fluxes in the near surface layer due to the presence of spray droplets. These findings were generally unexpected and point to an unanticipated role of physiochemical processes in the spume generation mechanism which may impact spray-mediated flux parameterization over water bodies of different salinities. This body of work represents a multi-faceted approach to understanding physical air-sea interactions in varied regimes and using a wide array of investigatory methods.</p
Investigations of Satellite-Observed, In Situ-Measured, and Model-Simulated Land-Air-Sea Boundary Layer Physics
No full textAir-sea fluxes are essential parameters for quantifying momentum, heat, and humidity exchanges in the marine atmospheric boundary layer (MABL). Traditional techniques, quantifying these parameters in the open ocean, assume stationary and homogeneous conditions. However, these conditions often break down in non-stationary coastal environments with strong horizontal gradients, topography, breaking waves, etc. Also, temporal, in situ instrumentation, alone, cannot characterize the variety of spatial scales necessary for accurate modeling. Furthermore, models often lack the physical parameterizations required for small-scale features or have too coarse-resolution to resolve such features. Therefore, high-resolution, spatial datasets (i.e., satellites) are critical for improving forecast model physics and initial conditions. To combat these limitations, a field experiment named Coastal Land Air-Sea Interaction, conducted in Monterey Bay, CA, aims to combine in situ and satellite data from various instrumentation (on ships, land-based towers, space-borne SAR, etc.) to improve forecasting of the land-air-sea boundary layer within the Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System model (COAMPS). A new wavelet-based technique proposes to autonomously derive state-of-the-art maps of land-air-sea parameters, such as wave speed, wave age, surface roughness length, drag coefficient, and wind stress from SAR at as low as 5-meter resolution. The new technique, called the SAR Land-Air-Sea-Transfer (SLAST) algorithm, successfully quantifies air-sea momentum transfer from the coast-line to the open ocean with validation against traditional buoy and coastal tower anemometer measurements yielding promising results. The SLAST algorithm is valuable to incorporate into COAMPS and other forecasting and mathematical models, such as Large Eddy Simulation (LES) models. SLAST is also applicable in domains across the globe outside Monterey Bay, where in situ measurements are not available or difficult to obtain (such as in hurricane conditions).</p
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Identification and Quantification of Wave Breaking Effects in Single-Polarization Along-Track InSAR Imagery of the Coastal Ocean
No full textSpaceborne Along-Track Interferometric Synthetic Aperture Radar (along-track InSAR) has been used successfully to produce estimates of the surface current velocity field on a number of occasions. Along-track InSAR data are comprised of two complex images with a very short time lag, with each pixel containing an amplitude and phase. The phase difference allows a direct measurement of the line-of-sight velocity of the Bragg scattering ripples, which includes contributions of the horizontal surface current as well as the phase velocity of the Bragg ripples and orbital motions of longer waves. To calculate the surface current field, the complicated wave-related contributions to the measured radar velocity need to be estimated and subtracted. Previously, either a single mean velocity correction was used or a spatially varying correction was computed using a relatively simple numerical current-wave interaction model. In areas with large current gradients and spatial depth changes, the resulting complicated surface wave field, such as at our test location at the Columbia River, requires a sophisticated method to estimate the complex corresponding velocity corrections. In this location, the near-shore hindcast model Delft3D with the wave model SWAN, is used to produce 2-D theoretical current and wave fields. To determine the location and magnitude of the wave-related motion contributions, we calculate the Doppler velocity anomaly by subtracting the 1-D component of the theoretical surface current velocities parallel to the radar from the Doppler velocities. Comparing the Doppler velocity anomaly to the SWAN predicted wave height, wave breaking and wave steepness, we confirm the expectation that the Doppler velocity anomaly is closely related to wave breaking and wave steepness. To calculate the required Doppler anomaly correction, we first use the SAR imaging model M4S to simulate ATI-SAR data for wavenumber spectra converted from the SWAN frequency spectra. Unfortunately, the Delft3D implementation of SWAN is unable to output spectral information at the high frequencies that correspond to the high wave numbers to which the radar is sensitive, the gravity-capillary waves. We continue this work by investigating the change in image statistics between areas of high and low Doppler anomaly. We calculate the spatially changing probability density function of the interferogram amplitude and the associated higher order moments: variance, skewness, and kurtosis. We develop an empirical model that relates changes of the image statistics, to the wave motions and the resulting Doppler correction. We apply the empirical model to our data set at the Columbia river as well as to new images and show that we improve the Doppler velocity estimates universally and are able to account for 45%-80% of the required Doppler correction, referenced against the Delft3D model results.</p
Recommended from our members
Air-Sea Interaction Dynamics Under Hurricane Wind Conditions
No full textUnderstanding turbulent fluxes of momentum, mass, and energy across the air-sea boundary are fundamental to our ability to model and parameterize a number of multidimensional geophysical processes, such as wind-wave generation, oceanic circulation, and air-sea gas transfer. The physical nature of the near surface boundary layer remains less known, especially under high winds due to the development of an intermediate substrate layer of large spray droplets known as spume, between the atmosphere and ocean surface. Presence of these spume droplet effects the aerodynamic resistance of the prevailing winds over the surface and thus the behavior of surface drag coefficient. The size-dependent vertical distribution of spume particles in high wind conditions is necessary to understand their effect on air-sea fluxes of heat and momentum. Given spume’s role in mediating air-sea exchange at the base of tropical cyclones or other storm events, the predominant focus of present literature studies on spray dynamics has been within the marine environment. In contrast, spume production in non-seawater bodies have not been extensively studied and potential differences between sea and freshwater are neglected. Thus any significant differences between sea and freshwater remain unquantified. Direct measurements of the physical processes happening at this interface remains scarce till date due to difficulty in making robust measurements in the field. Laboratories on the other hand remains the primary means for directly observing spray processes near the surface, and offers promising aspects for improving our understanding by learning these processes in a controlled environment. There is no standardization on the water type used for these experiments and any potential effects water masses have on the spume generation process is unknown. This adds uncertainty in our ability to make physically realistic spume generation functions that are ultimately applied to the geophysical domain. To address this gap, we have conducted a series of laboratory experiment at the Air-Sea Interaction tank facility (ASIST) of the University of Miami, directly comparing spume concentrations, and surface drag coefficient behavior above fresh and real seawater for 10-m equivalent wind speeds up to 54 m/s. Direct measurements of the near-surface processes were made and directly related to local sources of variance. Droplets in the air above the intensely breaking wind-waves were optically observed and their distribution as functions of wind speed, height, and droplet radius was compared between the two water types. Drag coefficient was calculated using the eddy covariance method on the three-dimensional wind data observed using a sonic anemometer. Our results show significant differences in the spume generation as well as in the surface drag coefficient behavior for the two water types. Substantially higher concentrations of seawater spume were observed as compared to freshwater across all particle sizes and wind speeds. The seawater particles’ vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Statistical analysis of these findings suggest significant differences in the size- and height-dependent distribution response to increased wind forcing between fresh and seawater. Drag coefficient values for seawater were found less than that of freshwater at all wind speeds suggesting modulation of momentum fluxes in the near surface layer due to the presence of spray droplets. These findings were generally unexpected and point to an unanticipated role of physiochemical processes in the spume generation mechanism which may impact spray-mediated flux parameterization over water bodies of different salinities. This body of work represents a multi-faceted approach to understanding physical air-sea interactions in varied regimes and using a wide array of investigatory methods.</p
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Spectral properties of radar return from the ocean surface according to a Bragg-based composite surface model
No full textDoppler spectra of the radar return from the ocean surface include information on the line-of-sight velocity of the scatterers and can thus be evaluated for direct measurements of surface currents and the orbital motions of long waves. However, experimental results obtained from along-track interferometric SAR (ATI) and Doppler scatterometry indicate that measured Doppler spectra can significantly deviate from first-order model predictions. The author presents an improved composite surface model for the computation of Doppler spectra which is based on Bragg scattering theory and includes contributions associated with the complete two-dimensional ocean wave spectrum. In addition to geometric effects, the asymmetric distribution of short ripple waves along longer waves due to hydrodynamic modulation is taken into account. The author compares model results with scatterometer data, and discusses the consequences of model predictions for wave and current measurements by scatterometer, SAR, and ATI
COVID-19 Booster Vaccination Status and Long COVID in the United States: A Nationally Representative Cross-Sectional Study
No full textEarly studies have found that the initial COVID-19 vaccination series was protective against severe symptoms and long COVID. However, few studies have explored the association of booster doses on severe disease outcomes and long COVID. This cross-sectional analysis used data from the 2022 US National Health Interview Survey data to investigate how vaccination status correlates with COVID-19 infection severity and long COVID among previously infected individuals. Participants were categorized into three groups: those who had received at least one booster, those with only the initial complete vaccination series, and those with either an incomplete series or no vaccinations. Out of 9521 survey respondents who reported a past positive COVID-19 test, 51.2% experienced moderate/severe infections, and 17.6% experienced long COVID. Multivariable regression models revealed that receiving at least one booster shot was associated with lower odds of experiencing moderate/severe symptoms (aOR = 0.78, p p = 0.003). Completing only the primary vaccine series did not significantly decrease the likelihood of severe illness or long COVID. These findings support the continued promotion of booster vaccinations to mitigate long COVID risks in vulnerable populations
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Doppler spectra of the radar backscatter from the sea surface obtained from a three-scale composite surface model
No full textThe Doppler shift of the backscattered radar signal from the sea surface can be used for determining the line-of-sight velocity of the scatterers and thus for measuring surface currents and ocean wave spectra. The bandwidth of the instantaneous Doppler spectrum, which is associated with the distribution of the line-of-sight velocity within the radar resolution cell, is a measure for the scene coherence time which enters into the SAR imaging mechanism for ocean scenes. Experimental results indicate that the measured Doppler shift can significantly exceed the values expected from simple first-order considerations. Furthermore, the Doppler bandwidth has been found to vary along long ocean waves, and a difference between the Doppler bandwidths for upwind and downwind looking radars has been observed. The author presents a new model for the computation of Doppler spectra which includes the hydrodynamic modulation of short waves by longer waves. The proposed model can reproduce the experimental results at least qualitatively at the present stage.<
