Coupled Boundary Layer Air Sea Transfer Research Articles

Air–Sea Interactions in Light of New Understanding of Air–Land Interactions

Abstract Air–sea interactions are investigated using the data from the Coupled Boundary Layers Air–Sea Transfer experiment under low wind (CBLAST-Low) and the Surface Wave Dynamics Experiment (SWADE) over sea and compared with measurements from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) over land. Based on the concept of the hockey-stick transition (HOST) hypothesis, which emphasizes contributions of large coherent eddies in atmospheric turbulent mixing that are not fully captured by Monin–Obukhov similarity theory, relationships between the atmospheric momentum transfer and the sea surface roughness, and the role of the sea surface temperature (SST) and oceanic waves in the turbulent transfer of atmospheric momentum, heat, and moisture, and variations of drag coefficient Cd(z) over sea and land with wind speed V are studied. In general, the atmospheric turbulence transfers over sea and land are similar except under weak winds and near the sea surface when wave-induced winds and oceanic currents are relevant to wind shear in generating atmospheric turbulence. The transition of the atmospheric momentum transfer between the stable and the near-neutral regimes is different over land and sea owing to the different strength and formation of atmospheric stable stratification. The relationship between the air–sea temperature difference and the turbulent heat transfer over sea is dominated by large air temperature variations compared to the slowly varying SST. Physically, Cd(z) consists of the surface skin drag and the turbulence drag between z and the surface; the increase of the latter with decreasing V leads to the minimum Cd(z), which is observed, but not limited to, over sea.

On the vertical structure and spectral characteristics of the marine Low-Level Jet

The aim of this work is the study of the vertical structure and the spectral characteristics of the marine Low Level Jets (LLJs) which are associated with frontal events. The analyzed data are based on remote sensing (sodar) and in-situ instrumentation measurements, performed during summer 2003, in the frame of the Coupled Boundary Layers Air–Sea Transfer Experiment in Low Winds (CBLAST-Low), at Nantucket Island, MA, U.S.A. The study of the vertical structure of the lower marine Atmospheric Boundary Layer (MABL), during a ten day period, has shown that the first 100 to 200m, is characterized by strongly stable atmospheric conditions which are modified to slightly stable or almost neutral at higher levels. The frequent development of LLJs was also observed and was associated with frontal events, depending on the meteorological conditions. In order to understand the influence of the different physical processes and to study the vertical distribution of the wind intensity variations at the various time scales of interest, the Hilbert–Huang Transform (HHT) algorithm was applied to the time series of the wind data from the sodar, at different levels. Results are presented and discussed for certain LLJ cases, where the observed LLJs were persistent for several hours or days while the analysis of the wind speed data showed high amplitude variations corresponding to contributions not only from inertial but also from diurnal and meso-scale motions.

Directional Wind–Wave Coupling in Fully Coupled Atmosphere–Wave–Ocean Models: Results from CBLAST-Hurricane

Abstract The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in hurricanes push the surface-exchange parameters for temperature, water vapor, and momentum into untested regimes. The Coupled Boundary Layer Air–Sea Transfer (CBLAST)-Hurricane program is aimed at developing improved coupling parameterizations (using the observations collected during the CBLAST-Hurricane field program) for the next-generation hurricane research prediction models. Hurricane-induced surface waves that determine the surface stress are highly asymmetric, which can affect storm structure and intensity significantly. Much of the stress is supported by waves in the wavelength range of 0.1–10 m, which is the unresolved “spectral tail” in present wave models. A directional wind–wave coupling method is developed to include effects of directionality of the wind and waves in hurricanes. The surface stress vector is calculated using the two-dimensional wave spectra from a wave model with an added short-wave spectral tail. The wind and waves are coupled in a vector form rather than through the traditional roughness scalar. This new wind–wave coupling parameterization has been implemented in a fully coupled atmosphere–wave–ocean model with 1.67-km grid resolution in the atmospheric model, which can resolve finescale features in the extreme high-wind region of the hurricane eyewall. It has been tested in a number of storms including Hurricane Frances (2004), which is one of the best-observed storms during the CBLAST-Hurricane 2004 field program. This paper describes the new wind–wave coupling parameterization and examines the characteristics of the coupled model simulations of Hurricane Frances (2004). Observations of surface waves and winds are used to evaluate the coupled model results.

On the turbulent structure of the marine atmospheric boundary layer from CBLAST Nantucket measurements

In this work preliminary results on the characteristics of the turbulent structure of the Marine Atmospheric Boundary Layer (MABL) are presented. Measurements used here were conducted in the framework of the Coupled Boundary Layers Air-Sea Transfer Experiment in Low Wind (CBLAST-Low) project. A number of in situ (fast and slow sensors) and remote sensing (SODAR) instruments were deployed on the coast of Nantucket Island, MA, USA. Measurements of the mean wind, the variances of the three wind components, the atmospheric stability and the momentum fluxes from the acoustic radar (SODAR) revealed the variation of the depth, the turbulent characteristics, and the stability of the MABL in response to the background flow. More specifically, under light south-southwesterly winds, which correspond to the MABL wind directions, the atmosphere was very stable and low values of turbulence were observed. Under moderate to strong southwesterly flow, less stable and neutral atmospheric conditions appeared and the corresponding turbulent quantities were characterized by higher values. The SODAR measurements, with high temporal and spatial resolution, also indicated large magnitude of momentum fluxes at higher levels, presumably associated with the shear forcing near the developed low-level jet. The measurements from the in-situ instrumentation confirmed that the MABL typically has small negative momentum and sensible heat fluxes consistent with stable to neutral stratification while strong diurnal variations were typical for the land surface Atmospheric Boundary Layer (ABL). The developed internal ABL at the experimental site was in general less than 10m during the night and could reach 15m heights during the day, particularly under low-wind conditions.

Air–Sea Enthalpy and Momentum Exchange at Major Hurricane Wind Speeds Observed during CBLAST

Abstract Quantifying air–sea exchanges of enthalpy and momentum is important for understanding and skillfully predicting tropical cyclone intensity, but the magnitude of the corresponding wind speed–dependent bulk exchange coefficients is largely unknown at major hurricane wind speeds greater than 50 m s−1. Since direct turbulent flux measurements in these conditions are extremely difficult, the momentum and enthalpy fluxes were deduced via absolute angular momentum and total energy budgets. An error analysis of the methodology was performed to quantify and mitigate potentially significant uncertainties resulting from unresolved budget terms and observational errors. An analysis of six missions from the 2003 Coupled Boundary Layers Air–Sea Transfer (CBLAST) field program in major hurricanes Fabian and Isabel was conducted using a new variational technique. The analysis indicates a near-surface mean drag coefficient CD of 2.4 × 10−3 with a 46% standard deviation and a mean enthalpy coefficient CK of 1.0 × 10−3 with a 40% standard deviation for wind speeds between 52 and 72 m s−1. These are the first known estimates of CK and the ratio of enthalpy to drag coefficient CK/CD in major hurricanes. The results suggest that there is no significant change in the magnitude of the bulk exchange coefficients estimated at minimal hurricane wind speeds, and that the ratio CK/CD does not significantly increase for wind speeds greater than 50 m s−1.

High-Resolution Satellite Surface Latent Heat Fluxes in North Atlantic Hurricanes

This study presents a new high-resolution satellite-derived ocean surface flux product, XSeaFlux, which is evaluated for its potential use in hurricane studies. The XSeaFlux employs new satellite datasets using improved retrieval methods, and uses a new bulk flux algorithm formulated for high wind conditions. The XSeaFlux latent heat flux (LHF) performs much better than the existing numerical weather prediction reanalysis and satellite-derived flux products in a comparison with measurements from the Coupled Boundary Layer Air–Sea Transfer (CBLAST) field experiment. Also, the XSeaFlux shows well-organized LHF structure and large LHF values in response to hurricane conditions relative to the other flux products. The XSeaFlux dataset is used to interpret details of the ocean surface LHF for selected North Atlantic hurricanes. Analysis of the XSeaFlux dataset suggests that ocean waves, sea spray, and cold wake have substantial impacts on LHF associated with the hurricanes.

Upper-Ocean Response to Hurricane Frances (2004) Observed by Profiling EM-APEX Floats*

Abstract Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air–Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations—the dominance of near-inertial horizontal currents and the phase of these currents—whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2°C, as the surface mixed layer deepened by about 80 m. Based on the time–depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2°C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed–saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32–47 m s−1, CD ~ 1.4 × 10−3.

Spectral characteristics of turbulence in the hurricane boundary layer over the ocean between the outer rain bands

Spectra and cospectra of wind velocity, potential temperature and humidity have been analysed using data collected in the atmospheric boundary layer in Hurricanes Fabian (2003) and Isabel (2003) during the Coupled Boundary Layer Air– Sea Transfer (CBLAST)—Hurricane experiment. The spectra and cospectra are normalized following the surface layer scaling methods according to similarity theory. It is found that the CBLAST data gathered in the mixed layer between the outer rain bands below 400 m can be grouped into well-defined curves for spectra of wind velocity, potential temperature and humidity, and for cospectra of momentum and humidity flux. However, the cospectra of sensible heat flux do not exhibit well-defined universal shape. The CBLAST universal shape spectra and cospectra generally resemble the shapes of those from previous studies, but shift to higher frequencies that correspond to smaller wavelengths of turbulent eddies that contain most of the energy. This work highlights the structural difference between the hurricane boundary layer and the standard atmospheric boundary layer over land and ocean. Copyright © 2010 Royal Meteorological Society

Turbulence Structure of the Hurricane Boundary Layer between the Outer Rainbands

Abstract As part of the Coupled Boundary Layers Air–Sea Transfer (CBLAST)-Hurricane program, flights were conducted to directly measure turbulent fluxes and turbulence properties in the high-wind boundary layer of hurricanes between the outer rainbands. For the first time, vertical profiles of normalized momentum fluxes, sensible heat and humidity fluxes, and variances of three-dimensional wind velocities and specific humidity are presented for the hurricane boundary layer with surface wind speeds ranging from 20 to 30 m s−1. The turbulent kinetic energy budget is estimated, indicating that the shear production and dissipation are the major source and sink terms, respectively. The imbalance in the turbulent kinetic energy budget indicates that the unmeasured terms, such as horizontal advection, may be important in hurricane boundary layer structure and dynamics. Finally, the thermodynamic boundary layer height, estimated based on the virtual potential temperature profiles, is roughly half of the boundary layer height estimated from the momentum flux profiles. The latter height where momentum and humidity fluxes tend to vanish is close to that of the inflow layer and also of the maximum in the tangential velocity profiles.

High SST variability south of Martha's Vineyard: Observation and modeling study

High, small-scale SST variability (6 °C over 5–10 km) observed South of Martha's Vineyard during the low-wind component of the Coupled Boundary Layers Air–Sea Transfer (CBLAST-Low) oceanographic field program in August 2003 is investigated using the Navy Coastal Ocean Model (NCOM), with atmospheric forcing provided by the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®). 1 1 COAMPS® is a registered trademark of the Naval Research Laboratory. The ocean model includes tidal boundary forcing by the eight major tidal constituents, which is superimposed on the non-tidal lateral boundary conditions obtained from the 1/8° global NCOM real-time hindcast. The simulation is conducted with a high-resolution, 200-m grid, with bathymetry from the NGDC 3-arc-second Coastal Relief Model. The COAMPS fields, tidal forcing and NCOM results are evaluated with the CBLAST-Low observations and previous results. Both the simulation and observation analyses reveal that SST variability south of Martha's Vineyard is significant on August 18 and 25 and is strongly related to the cooling events on August 17 to 18 and August 24 to 25. The northeast winds passing through Muskeget Channel generate sharp horizontal SST gradients on August 18 by accelerating the westward transport of cold water from the cold, tidally-mixed Nantucket Shoals and by wind-induced upwelling and surface-cooling-induced vertical mixing. The mechanism of SST change on August 25 is differentiated from the change on August 18 by the northwest winds being unfavorable to the westward transport of cold water. The SST cooling on August 25 is mainly caused by local vertical mixing induced by heat lost.

First direct measurements of enthalpy flux in the hurricane boundary layer: The CBLAST results

Hurricanes extract energy from the warm ocean through enthalpy fluxes. As part of the Coupled Boundary Layer Air‐Sea Transfer (CBLAST) experiment, flights were conducted to measure turbulent fluxes in the high‐wind boundary layer of hurricanes. Here we present the first field observations of sensible heat and enthalpy flux for 10m wind speeds to 30 ms−1. The analyses indicate no statistically significant dependence of these bulk exchange coefficients on wind speed. As a measure of hurricane development potential, we compute the mean ratio of the exchange coefficient for enthalpy to that for momentum and find it to be significantly below the lowest threshold estimated by previous investigators. This suggests that the enthalpy flux required for hurricane development may come from sources other than turbulent fluxes, such as lateral fluxes from the vortex warm core, or sea spray. Alternatively, it demands a re‐evaluation of the theoretical models used to derive the threshold.

Effects of Roll Vortices on Turbulent Fluxes in the Hurricane Boundary Layer

Boundary-layer secondary circulations or ‘roll vortices’ can have a significant influence on the turbulent exchange of momentum, sensible heat and moisture throughout the hurricane boundary layer. In this study, analyses of data from a WP-3D aircraft of the National Oceanic and Atmospheric Administration (NOAA) are presented. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-hurricane experiment sponsored through the Office of Naval Research and NOAA’s annual hurricane research program, flights were conducted to investigate energy exchange across the air–sea interface. We present the first in-situ aircraft-based observations of rolls in the hurricane boundary layer and investigate their influence on energy and momentum exchange. The rolls detected in Hurricane Isidore (year 2002) have a characteristic wavelength of about 900 m, in good agreement with analyses of data from a synthetic aperture radar image captured by the Canadian Space Agency’s RADARSAT satellite in the same storm. Our analyses of the airborne data suggest that roll vortices may be a significant factor modulating the air–sea momentum exchange.

Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves

Abstract Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases by a factor of 4 compared to a flat surface. The mean wind profile, turbulence variances, and vertical momentum flux are then dependent on the state of the wave field. The LES results are compared with measurements from the Coupled Boundary Layers Air–Sea Transfer (CBLAST) field campaign. A quadrant analysis of the momentum flux from CBLAST verifies a wave age dependence predicted by the LES solutions. The measured bulk drag coefficient CD then depends on wind speed and wave state. In situations with light wind following swell, CD is approximately 50% lower than values obtained from standard bulk parameterizations that have no sea state dependence. In extreme cases with light wind and persistent swell, CD < 0.

On the Temporal Distribution of Bursts and Events in the Stable Marine Atmospheric Surface Layer

A methodology for tracing the bursting phenomenon occurring in the turbulent boundary-layer is applied to stable marine atmospheric boundary-layer (MABL) data, collected within the framework of the Coupled Boundary Layer Air-Sea Transfer, Low wind component experiment at Nantucket Island, 2003, Massachusetts, USA. For the definition of the events contributing to the bursting phenomenon, the quadrant analysis of the momentum flux with varying threshold value is utilized. The present work aims at the better understanding of the time scales of the microstructures in the MABL, by providing the number of the events and the groups and their mean duration and the time difference of all quadrants of the quadrant analysis, for different thresholds and under different meteorological conditions. The interrelation of the different time scales as well as the correlation between the quadrants are investigated and discussed. Also, it is demonstrated that the procedure of grouping events leads to bursts with a mean duration that is independent of the threshold value, for the most important quadrants of the momentum transfer (2 and 4). It is found that the combination of near-neutral conditions with low wind speed permits the development of larger duration events within the stable MABL. Finally, it is shown that the groups of events maintain adequate time and flux coherence only for quadrants 2 and 4.

Turbulent Fluxes in the Hurricane Boundary Layer. Part II: Latent Heat Flux

Abstract As part of the recent ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Departmental Research Initiative, an aircraft was instrumented to carry out direct turbulent flux measurements in the high wind boundary layer of a hurricane. During the 2003 field season flux measurements were made during Hurricanes Fabian and Isabel. Here the first direct measurements of latent heat fluxes measured in the hurricane boundary layer are reported. The previous wind speed range for humidity fluxes and Dalton numbers has been extended by over 50%. Up to 30 m s−1, the highest 10-m winds measured, the Dalton number is not significantly different from the Humidity Exchange over the Sea (HEXOS) result, with no evidence of an increase with wind speed.

Turbulent Fluxes in the Hurricane Boundary Layer. Part I: Momentum Flux

Abstract An important outcome from the ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s−1. Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.

Quadrant analysis of the scalar and momentum fluxes in the stable marine atmospheric surface layer

The quadrant technique, a conditional sampling approach that allocates Reynolds stresses into four different types of events (ejections or bursts, sweeps, inward interactions and outward interactions), is applied to stable marine atmospheric boundary-layer data, collected in the framework of the Coupled Boundary Layer Air—Sea Transfer, Low wind component experiment at Nantucket Island, Massachusetts, USA. The general properties of both scalar and momentum transport are analyzed under the scope of quadrant analysis experimentally and theoretically. It is shown that the third-order Gram–Charlier series is necessary and even sufficient in most of the cases, in describing the experimental time and flux contributions of each quadrant to the total transfer, for both scalar and momentum transport, while the ability of the Gaussian distribution is limited to outlining the general pattern of these quantities. Moreover, a threshold value is applied to the conditional analysis, separating the most important events from the less significant ones and the sensitivity of the flux and especially the time fraction of each quadrant on the choice of this value is presented and discussed. Also, a set of numerically extracted equations, completing the analytical relations, is derived, enabling the prediction of the time and flux fractions of each quadrant, for a wide range of correlation coefficient and threshold values. Finally, the sensitivity of the analysis to the atmospheric stability and the Reynolds averaging scales showed that correlated and uncorrelated motions tend to balance for increasingly stable conditions and/or for large time scales.

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.