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Eric D. Skyllingstad and James B. Edson

Abstract

Cold air outflow over the Gulf Stream is modeled using a cloud-resolving large-eddy simulation model with three classes of precipitation. Simulations are conducted in a quasi-Lagrangian framework using an idealized sounding and uniform geostrophic winds based on observations taken on 20 February 2007 as part of the World Climate Research Program Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE) project. Two cases are considered, one with an increasing sea surface temperature (SST) representing the crossing of the Gulf Stream front, and a second case with constant SST.

Cloud systems develop in the model with strong convective plumes that spread into regions of stratus clouds at the top of the boundary layer. Simulated boundary layer growth is forced by a combination of evaporative cooling at the cloud top, upward radiative flux, and mechanical entrainment of the overlying warmer and drier air. Constant growth of the boundary layer acts to maintain a near-constant water vapor level in the boundary layer, promoting high latent and sensible heat fluxes. Frictional surface drag is distributed throughout the boundary layer by convection, causing increased shear at the cloud top, qualitatively agreeing with observed sounding profiles. Overall, the frontal case develops stronger precipitation and turbulence in comparison with the constant SST case. A near-uniform stratocumulus layer and stronger radiative cooling are produced in the constant SST case, whereas the frontal case generates open cumuliform clouds with reduced cloud coverage. Cloud evolution in the frontal case is similar to the transition from stratocumulus to shallow cumulus observed in the subtropics, as cumuliform clouds enhance cloud-top entrainment and evaporation of stratus clouds.

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Amanda Plagge, James B. Edson, and Douglas Vandemark

Abstract

Observations of ocean–atmosphere coupling across persistent mesoscale sea surface temperature (SST) gradients are used to examine the controls of atmospheric stability, pressure gradient force, and heat flux that are considered central to oft-observed coupling between wind and SST. Moored air–sea flux measurements near the Gulf Stream are combined with QuikSCAT satellite scatterometer equivalent neutral wind (ENW) data to assess correlations between SST, air–sea fluxes, pressure, and wind perturbations at scales of 10–100 days. The net effect of ocean fronts meandering past the site enabled buoy observation of SST impacts on wind, with coupling coefficients of 0.3–0.5 similar to past studies. Wind stress–SST and ENW–SST correlation coefficients are slightly higher, and roughly 20% of the ENW perturbation is attributed to stratification impacts predicted by Monin–Obukhov (MO) similarity theory. Significantly higher correlation is observed when relating wind or stress perturbations to buoyant heat flux variation. Atmospheric pressure perturbation with SST of order 0.5 hPa °C−1 is observed, as well as high negative correlation between wind and pressure variations. Length and time scales associated with the coupling indicate that peak correlations occur at 50–70 days and 300–500 km, consistent with mesoscale meander scales. Coupling coefficient values vary significantly depending on analysis time scale and exhibit a range near to recently observed interbasin variability. This variability is attributed to the extent of oceanic length scales permitted in the analysis. Together, results affirm the central role of SST-induced turbulent heat flux in controlling pressure field adjustments and thereby the wind perturbations over SST fronts.

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Douglas Vandemark, James B. Edson, and Bertrand Chapron

Abstract

Aircraft altimeter and in situ measurements are used to examine relationships between altimeter backscatter and the magnitude of near-surface wind and friction velocities. Comparison of altimeter radar cross section with wind speed is made through the modified Chelton–Wentz algorithm. Improved agreement is found after correcting 10-m winds for both surface current and atmospheric stability. An altimeter friction velocity algorithm is derived based on the wind speed model and an open-ocean drag coefficient. Close agreement between altimeter- and in situ–derived friction velocities is found. For this dataset, quality of the altimeter inversion to surface friction velocity is comparable to that for adjusted winds and clearly better than the inversion to true 10-m wind speed.

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Paul A. Frederickson, Kenneth L. Davidson, and James B. Edson

Abstract

Comparisons are made between surface wind stress measurements obtained by the inertial-dissipation and direct covariance methods on a stable offshore tower and by the inertial-dissipation and bulk methods on a ship. The shipboard inertial-dissipation friction velocity measurements agreed very well with both the tower inertial-dissipation and direct covariance values, to within ±2% in the mean and with a 10% or lower rms scatter. The inertial-dissipation determinations also exhibited less scatter than the tower direct covariance measurements. A detailed error analysis indicates that shipboard inertial-dissipation wind stress values can have an accuracy of better than 15% in near-neutral conditions, as compared to an accuracy of roughly 30% for the bulk method. The accuracy of shipboard inertial-dissipation values was shown to be equal to that of direct covariance measurements from a tower. Errors in inertial-dissipation wind stress values are most likely due primarily to deviations from the assumed balance between turbulent kinetic energy production and dissipation and to errors in determining the wind speed variance spectra. Errors in direct covariance measurements are most likely due primarily to finite time averaging and to flow distortion effects, unless great care is taken to minimize or correct for flow distortion. The high accuracy of inertial-dissipation wind stress values found in this study, combined with the well-known difficulties in shipboard direct covariance measurements due to platform motion and flow distortion, demonstrate that the inertial-dissipation method is the best option at present for determining the wind stress from a ship.

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Peter P. Sullivan, James B. Edson, Tihomir Hristov, and James C. McWilliams

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.

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Jeffrey E. Hare, Tetsu Hara, James B. Edson, and James M. Wilczak

Abstract

Previous field investigations of the wave-induced pressure field have focused on determination of the momentum input from wind to the surface waves. This is useful for the estimation of wave growth rate and, in particular, the wave growth parameter β. Due to the difficult nature of experimental study of airflow very close to the wave surface, it has been necessary to extrapolate elevated measurements of the wave-induced pressure field to the surface. This practice may be incorrect without adequate knowledge of the complex vertical structure of the pressure field. In addition, the wave-induced pressure and velocity fields are coupled to the near-surface turbulence. Hence, understanding the nature of the wave-induced flow fields is critical for modeling of the near-surface wind and wave fields.

Utilizing a simple similarity hypothesis, detailed vertical structure of the wave-induced pressure and velocity components is examined. Results of this analysis are presented using data obtained in the spring and fall of 1994 during the Risø Air–Sea Experiment program. These results demonstrate that, when compared to theory, simple extrapolation of measurements of the wave-induced pressure field from a fixed height above the surface may contribute to the uncertainty of measured growth rates. In addition, it is demonstrated that an analogous similarity relationship for the wave-induced vertical velocity field yields results that are consistent with previous laboratory studies.

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Simon P. de Szoeke, James B. Edson, June R. Marion, Christopher W. Fairall, and Ludovic Bariteau

Abstract

Dynamics of the Madden–Julian Oscillation (DYNAMO) and Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) observations and reanalysis-based surface flux products are used to test theories of atmosphere–ocean interaction that explain the Madden–Julian oscillation (MJO). Negative intraseasonal outgoing longwave radiation, indicating deep convective clouds, is in phase with increased surface wind stress, decreased solar heating, and increased surface turbulent heat flux—mostly evaporation—from the ocean to the atmosphere. Net heat flux cools the upper ocean in the convective phase. Sea surface temperature (SST) warms during the suppressed phase, reaching a maximum before the onset of MJO convection. The timing of convection, surface flux, and SST is consistent from the central Indian Ocean (70°E) to the western Pacific Ocean (160°E).

Mean surface evaporation observed in TOGA COARE and DYNAMO (110 W m−2) accounts for about half of the moisture supply for the mean precipitation (210 W m−2 for DYNAMO). Precipitation maxima are an order of magnitude larger than evaporation anomalies, requiring moisture convergence in the mean, and on intraseasonal and daily time scales. Column-integrated moisture increases 2 cm before the convectively active phase over the Research Vessel (R/V) Roger Revelle in DYNAMO, in accordance with MJO moisture recharge theory. Local surface evaporation does not significantly recharge the column water budget before convection. As suggested in moisture mode theories, evaporation increases the moist static energy of the column during convection. Rather than simply discharging moisture from the column, the strongest daily precipitation anomalies in the convectively active phase accompany the increasing column moisture.

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Sébastien P. Bigorre, Robert A. Weller, James B. Edson, and Jonathan D. Ware

Abstract

A surface mooring was deployed in the Gulf Stream for 15 months to investigate the role of air–sea interaction in mode water formation and other processes. The accuracies of the near-surface meteorological and oceanographic measurements are investigated. In addition, the impacts of these measurement errors on the estimation and study of the air–sea fluxes in the Gulf Stream are discussed. Pre- and postdeployment calibrations together with in situ comparison between shipboard and moored sensors supported the identification of biases due to sensor drifts, sensor electronics, and calibration errors. A postdeployment field study was used to further investigate the performance of the wind sensors. The use of redundant sensor sets not only supported the filling of data gaps but also allowed an examination of the contribution of random errors. Air–sea fluxes were also analyzed and computed from both Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parameterization and using direct covariance measurements. The basic conclusion is that the surface buoy deployed in the Gulf Stream to support air–sea interaction research was successful, providing an improved 15-month record of surface meteorology, upper-ocean variability, and air–sea fluxes with known accuracies. At the same time, the coincident deployment of mean meteorological and turbulent flux sensors proved to be a successful strategy to certify the validity of the bulk formula fluxes over the midrange of wind speeds and to support further work to address the present shortcomings of the bulk formula methods at the low and high wind speeds.

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L. Mahrt, Edgar L Andreas, James B. Edson, Dean Vickers, Jielun Sun, and Edward G. Patton

Abstract

Summertime eddy correlation measurements from an offshore tower are analyzed to investigate the dependence of the friction velocity for stable conditions on the mean wind speed V, air–sea difference of virtual potential temperature δθ υ, and nonstationary submeso motions. The quantity δθ υ sometimes exceeds 3°C, usually because of the advection of warm air from land over cooler water at this site. Thin stable boundary layers result. Unexpectedly, does not depend systematically on the stratification δθ υ even for weak winds. For weak winds, increases systematically with increasing submeso variations of the wind. The relationship for a given V is greater in nonstationary conditions. Additionally, this study examines as a function of wind direction. The relationship appears to be affected by swell direction for weak winds and advection from land for short fetches.

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Martin Flügge, Mostafa Bakhoday Paskyabi, Joachim Reuder, James B. Edson, and Albert J. Plueddemann

Abstract

Direct covariance flux (DCF) measurements taken from floating platforms are contaminated by wave-induced platform motions that need to be removed before computation of the turbulent fluxes. Several correction algorithms have been developed and successfully applied in earlier studies from research vessels and, most recently, by the use of moored buoys. The validation of those correction algorithms has so far been limited to short-duration comparisons against other floating platforms. Although these comparisons show in general a good agreement, there is still a lack of a rigorous validation of the method, required to understand the strengths and weaknesses of the existing motion-correction algorithms. This paper attempts to provide such a validation by a comparison of flux estimates from two DCF systems, one mounted on a moored buoy and one on the Air–Sea Interaction Tower (ASIT) at the Martha’s Vineyard Coastal Observatory, Massachusetts. The ASIT was specifically designed to minimize flow distortion over a wide range of wind directions from the open ocean for flux measurements. The flow measurements from the buoy system are corrected for wave-induced platform motions before computation of the turbulent heat and momentum fluxes. Flux estimates and cospectra of the corrected buoy data are found to be in very good agreement with those obtained from the ASIT. The comparison is also used to optimize the filter constants used in the motion-correction algorithm. The quantitative agreement between the buoy data and the ASIT demonstrates that the DCF method is applicable for turbulence measurements from small moving platforms, such as buoys.

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