Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: Dean Vickers x
  • Journal of Physical Oceanography x
  • Refine by Access: All Content x
Clear All Modify Search
Dean Vickers
,
Larry Mahrt
, and
Edgar L Andreas

Abstract

The 10-m neutral drag coefficient (C DN10) over the sea is calculated using a large observational dataset consisting of 5800 estimates of the mean flow and the fluxes from aircraft eddy-covariance measurements. The dataset includes observations from 11 different experiments with four different research aircraft. One of the goals is to investigate how sensitive C DN10 is to the analysis method. As such, C DN10 derived from six unique processing schemes that involve different methods for averaging the surface stress and the wind speed are compared. Especially in weak winds, the resulting C DN10 values depend on the choice of processing.

Four distinct regimes of C DN10 are identified: weak winds where calculating C DN10 is not well posed, moderate winds (4 to 10 m s−1) where C DN10 is a constant, strong winds (10 to 20 m s−1) where C DN10 increases linearly with increasing wind speed, and very strong winds (20 to 24 m s−1) where C DN10 steadily decreases with increasing wind speed. However, as this last regime is based on data from a single experiment, additional data are needed to confirm this apparent decrease in C DN10 for winds exceeding 20 m s−1.

Full access
L. Mahrt
,
Dean Vickers
,
Edgar L Andreas
, and
Djamal Khelif

Abstract

The variation of the sea surface sensible heat flux is investigated using data from the Gulf of Tehuantepec Experiment (GOTEX) and from eight additional aircraft datasets representing a variety of surface conditions. This analysis focuses on near-neutral conditions because these conditions are common over the sea and are normally neglected, partly because of uncertain reliability of measurements of the small air–sea temperature difference. For all of the datasets, upward heat flux is observed for slightly stable conditions. The frequency of this “countergradient” heat flux increases with increasing wind speed and is possibly related to sea spray or microscale variations of surface temperature on the wave scale. Upward area-averaged sensible heat flux for slightly stable conditions can also be generated by mesoscale heterogeneity of the sea surface temperature (SST). Significant measurement errors cannot be ruled out.

The countergradient heat flux for weakly stable conditions is least systematic for weaker winds, even though it occurs with weak winds in all of the datasets. In an effort to reduce offset errors and different SST processing and calibration procedures among field programs, the authors adjusted the SST in each field program to minimize the countergradient flux for weak winds. With or without this adjustment for the combined dataset, the extent of the upward heat flux for weakly stable conditions increases with increasing wind speed.

Full access
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.

Full access
James B. Edson
,
Venkata Jampana
,
Robert A. Weller
,
Sebastien P. Bigorre
,
Albert J. Plueddemann
,
Christopher W. Fairall
,
Scott D. Miller
,
Larry Mahrt
,
Dean Vickers
, and
Hans Hersbach
Full access
James B. Edson
,
Venkata Jampana
,
Robert A. Weller
,
Sebastien P. Bigorre
,
Albert J. Plueddemann
,
Christopher W. Fairall
,
Scott D. Miller
,
Larry Mahrt
,
Dean Vickers
, and
Hans Hersbach

Abstract

This study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 s and b = −0.005. When combined with a parameterization for smooth flow, this formulation gives better agreement with the stress estimates from all of the field programs at all winds speeds with significant improvement for wind speeds over 13 m s−1. Wave age– and wave slope–dependent parameterizations of the surface roughness are also investigated, but the COARE 3.5 wind speed–dependent formulation matches the observations well without any wave information. The available data provide a simple reason for why wind speed–, wave age–, and wave slope–dependent formulations give similar results—the inverse wave age varies nearly linearly with wind speed in long-fetch conditions for wind speeds up to 25 m s−1.

Full access