The parameterization of the drag coefficient (CD) used in the wind stress calculation is taken as a function of wind
speed (W) rather than a constant. :
- Historically, CD was taken as a constant value of 0.0013 to calculate indices from 6-hourly pressure fields and was increased to 0.0026 to calculate indices from a monthly pressure field.
- For the calculations on this page, a non-linear drag coefficient was used (based on Large and Pond (1981) modified for low wind speeds as in
Trenberth et al. (1990)):
- CD = 0.00218 for W < or = 1m/s
- = (0.62+1.56/W) x .001 for 1m/s < W < 3m/s
- = 0.00114 for 3m/s < or = W < 10m/s
- = (0.49 + 0.065W) x .001 for W > or = 10m/s
Non-Linear Drag Coefficient
References:
- Large, W., and S. Pond, 1981: Open Ocean momentum flux measurements in moderate to strong winds. J. Phys. Oceanogr., 11, 324-336.
- Trenberth, K. E., W. G. Large and J. G. Olson, 1990: The mean annual cycle in global ocean wind stress. J. Phys. Oceanogr., 20, 1742-1760.
The effects of the above differences in calculation methods can be seen in the following examples, which compare the
products on this page with PFEL's standard versions of the products at two locations. The magnitude of the effect depends on
location, pressure gradient, wind speed, and other factors such as the original mesh size and possibly proximity to the coast.
The factors affecting the calclation of upwelling index are an area of on-going research at PFEL.
Click on the plot to view a full size image: