Date: May 10th 2007

Dear Ocean Vector Winds Community,

Several members of the science user community have found a problem with the wind stress values derived from the Large and Pond algorithms in the PO DAAC Wind Stress Products The problem has been verified independently by PO DAAC, and we are in the process of alerting the user community about its nature Please note that the wind stress values derived from the Liu et al algorithms do not suffer from the error described as follows:

The Large and Pond wind stress values were derived using the drag coefficient documented in "Oceanic Vertical Mixing: A Review and a Model with Nonlocal Boundary Layer Parameterization" (Large et al, 1994, REV of GEOPHYSICS, p 389, eqn A1a) The drag coefficient is defined as follows:

10^3 Cd = 2 70 U10^-1 + 0 142 + 0 0764 U10, where Cd is the Drag Coefficient and U10 is the wind velocity at 10m above sea level

The equation for wind stress from Deacon and Webb (1962, The Sea, Vol 1, pp 43-87) is:

tau = rho_a * Cd * U10 * abs(U10) where rho_a is the density of air (1 2kg/m^3)

Therefore, wind stress using the Large and Pond drag coefficient should be defined as:

tau= rho_a * [0 0027 (U10) + 0 000142 (U10^2) + 0 0000764 (U10^3) where the stress is oriented along the QuikSCAT/SeaWinds wind direction

Unfortunately, as shown in the document entitled, "SeaWinds on QuikSCAT Level 3 - Derived Multialgorithm Surface Wind Stress (2003, JPL Document D25767, http://poseidon jpl nasa gov:2031/DATASET_DOCS/qscat_L3stress html), the rho_a term was not included in the PO DAAC derivation of wind stress using the Large and Pond drag coefficient Therefore, the users of the Large and Pond stresses have been using a stress value which is approximately 22% lower (constant scaling factor) than the intended stress

It is important to reiterate that the Liu et al wind stresses, distributed in the same products as the Large and Pond stresses, did not suffer from the lack of the air density, and were correctly coded

The specific products affected are:

SeaWinds on QuikSCAT L2B-Derived Multialgorithm Surface Wind Stress (#182)
SeaWinds on QuikSCAT L3-Derived Multialgorithm Surface Wind Stress (#183)
SeaWinds on ADEOS-II L2B-Derived Multialgorithm Surface Wind Stress (#195)
SeaWinds on ADEOS-II L3-Derived Multialgorithm Surface Wind Stress (#196)

These products are not official products of the QuikSCAT project or of its science team They were developed by PO DAAC in 2003 in response to requests from several science users and have been available since 2003

The products have been removed from distribution and will be replaced in several weeks (June 1) with corrected wind stress products The corrected wind stress product will use the Large and Pond equation with the correct air density The online user guide documentation will be updated and corrected In addition, we are examining the rain flags in these products

We apologize to the ocean vector winds community for the problems these product errors may have caused and wish to thank those scientists both external and internal to JPL for their help in identifying and resolving this issue PO DAAC will not only replace this product and documentation but is examining its internal review processes to improve quality control of products prior to release We also encourage our user community to report any problems they may find to PO DAAC so that corrective action may be pursued in the speediest fashion

If you have questions or comments please contact us at podaac@podaac jpl nasa gov

Patricia Liggett PO DAAC Project Manager JPL

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