McCue Scott J.

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McCue
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Scott J.
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  • Preprint
    Air-sea gas transfer velocity estimates from the Jason-1 and TOPEX altimeters : prospects for a long-term global time series
    ( 2006-03-02) Glover, David M. ; Frew, Nelson M. ; McCue, Scott J.
    Estimation of global and regional air–sea fluxes of climatically important gases is a key goal of current climate research programs. Gas transfer velocities needed to compute these fluxes can be estimated by combining altimeter-derived mean square slope with an empirical relation between transfer velocity and mean square slope derived from field measurements of gas fluxes and small-scale wave spectra [Frew, N.M., Bock, E.J., Schimpf, U., Hara, T., Hauβecker, H., Edson, J.B., McGillis, W.R., Nelson, R.K., McKenna, S.P., Uz, B.M., Jähne, B., 2004. Air–sea gas transfer: Its dependence on wind stress, small-scale roughness and surface films, J. Geophys. Res., 109, C08S17, doi: 10.1029/2003JC002131.]. We previously reported initial results from a dual-frequency (Ku- and C-band) altimeter algorithm [Glover, D.M., Frew, N.M., McCue, S.J., Bock, E.J., 2002. A Multi-year Time Series of Global Gas Transfer Velocity from the TOPEX Dual Frequency, Normalized Radar Backscatter Algorithm, In: Gas Transfer at Water Surfaces, editors: Donelan, M., Drennan, W., Saltzman, E., and Wanninkhof, R., Geophysical Monograph 127, American Geophysical Union, Washington, DC, 325–331.] for estimating the air–sea gas transfer velocity (k) from the mean square slope of short wind waves (40–100 rad/m) and derived a 6-year time series of global transfer velocities based on TOPEX observations. Since the launch of the follow-on altimeter Jason-1 in December 2001 and commencement of the TOPEX/Jason-1 Tandem Mission, we have extended this time series to 12 years, with improvements to the model parameters used in our algorithm and using the latest corrected data releases. The prospect of deriving multi-year and interdecadal time series of gas transfer velocity from TOPEX, Jason-1 and follow-on altimeter missions depends on precise intercalibration of the normalized backscatter. During the Tandem Mission collinear phase, both satellites followed identical orbits with a mere 73-s time separation. The resulting collocated, near-coincident normalized radar backscatter (σ°) data from both altimeters present a unique opportunity to intercalibrate the two instruments, compare derived fields of transfer velocity and estimate the precision of the algorithm. Initial results suggest that the monthly gas transfer velocity fields generated from the two altimeters are very similar. Comparison of along-track Ku-band and C-band σ° during the collinear phase indicates that observed discrepancies are due primarily to small offsets between TOPEX and Jason-1 σ°. The Jason-1 k values have an apparent bias of + 4% relative to TOPEX, while the precision estimated from the two observation sets is 5–7% and scales with k. The resultant long-term, global, mean k is 16 cm/h.
  • Article
    A new approach to estimation of global air-sea gas transfer velocity fields using dual-frequency altimeter backscatter
    (American Geophysical Union, 2007-11-03) Frew, Nelson M. ; Glover, David M. ; Bock, Erik J. ; McCue, Scott J.
    A new approach to estimating air-sea gas transfer velocities based on normalized backscatter from the dual-frequency TOPEX and Jason-1 altimeters is described. The differential scattering of Ku-band (13.6 GHz) and C-band (5.3 GHz) microwave pulses is used to isolate the contribution of small-scale waves to mean square slope and gas transfer. Mean square slope is derived for the nominal wave number range 40–100 rad m−1 by differencing mean square slope estimates computed from the normalized backscatter in each band, using a simple geometric optics model. Model parameters for calculating the differenced mean square slope over this wave number range are optimized using in situ optical slope measurements. An empirical relation between gas transfer velocity and mean square slope, also based on field measurements, is then used to derive gas transfer velocities. Initial results demonstrate that the calculated transfer velocities exhibit magnitudes and a dynamic range which are generally consistent with existing field measurements. The new algorithm is used to construct monthly global maps of gas transfer velocity and to illustrate seasonal transfer velocity variations over a 1-year period. The measurement precision estimated from >106 duplicate observations of the sea surface by TOPEX and Jason-1 altimeters orbiting in tandem is better than 10%. The estimated overall uncertainty of the method is ±30%. The long-term global, area-weighted, Schmidt number corrected, mean gas transfer velocity is 13.7 ± 4.1 cm h−1. The new approach, based on surface roughness, represents a potential alternative to commonly used parameterizations based on wind speed.