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IOVWST Meeting
Online, 2023

Stress or Wind: Definitions and Biases

Presenting Author: Professor Mark Bourassa

Scatterometers have long been considered to respond to surface turbulent stress. They have been calibrated to wind-like quantities (equivalent neutral winds) due to the paucity of stress observations. Equivalent neutral winds are winds that have been modified to be more physically consistent with stress. However, for historical reasons, equivalent neutral winds were made consistent with friction velocity, which is the square root of the kinematic stress (the stress divided by atmospheric density). If scatterometers do respond to stress, this approach introduces an error (a gain) in wind speed related to the square root of atmospheric density. For scatterometer-derived stress, the error would be proportional to density raised to the 1 to 1.5 power, depending on the wind speed. This dependency on air density is examined, with the goal of showing that scatterometer backscatter is more closely a function of surface stress than wind speed or equivalent neutral wind. The influence of air density is a gain, a proportional increase or decrease, making the impact far greater than a random error. These considerations could be important in areas of strong temperature gradients (e.g., the Gulf Stream, equatorial cold tongues, and oceanic fronts), cold air outbreaks, and for large-scale circulation studies. For large gyre circulations, the atmospheric density can change by „b10% at the equator-ward and pole-ward extremes of the gyre, relative to the average over the gyre. Other relevant applications are scalar fluxes, such as sensible heat, latent heat or moistures, and some gas fluxes (e.g., CO2). For scalar fluxes, the gain is proportional to the ˇĄproportional error in densityˇ¦ raised to the 0.5 to 0.75 power. These errors are likely to be physically important when considering global budgets. Collocated data from buoys and the SeaWinds on QuikSCAT scatterometer are used to demonstrate that scatterometer responses are more stress-like than wind-like.

2023 International OVWST Meeting
Wednesday Oct. 4, Nov. 1, and Nov. 15 Online
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