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King G.P.,CSIC - Institute of Marine Sciences | Vogelzang J.,KNMI Royal Netherlands Meteorological InstituteDe Bilt Netherlands | Stoffelen A.,KNMI Royal Netherlands Meteorological InstituteDe Bilt Netherlands
Journal of Geophysical Research C: Oceans | Year: 2015

The direction of the energy cascade in the mesoscales of atmospheric turbulence is investigated using near-surface winds over the tropical Pacific measured by satellite scatterometers SeaWinds (QuikSCAT) and ASCAT (MetOp-A). The tropical Pacific was subdivided into nine regions, classified as rainy or dry. Longitudinal third-order along-track structure functions DLLLa and skewness SLa were calculated as a function of separation r for each region and month during the period November 2008 to October 2009. We find that the results support both downscale and upscale interpretations, depending on region and month. The results indicate that normally energy cascades downscale, but cascades upscale over the cold tongue in the cold season and over the west Pacific in summer months. An explanation is offered based on the heating or cooling of the air by the underlying sea surface temperature. It is also found that the signature of intermittent small-scale (<100 km) events could be identified in graphs of SLa, implying that this diagnostic may be useful in the studies of tropical disturbances. © 2014. American Geophysical Union. Source


Vogelzang J.,KNMI Royal Netherlands Meteorological InstituteDe Bilt Netherlands | King G.P.,Nanjing University | Stoffelen A.,KNMI Royal Netherlands Meteorological InstituteDe Bilt Netherlands
Journal of Geophysical Research C: Oceans | Year: 2015

Kinetic energy variance as a function of spatial scale for wind fields is commonly estimated either using second-order structure functions (in the spatial domain) or by spectral analysis (in the frequency domain). Both techniques give an order-of-magnitude estimate. More accurate estimates are given by a statistic called spatial variance. Spatial variances have a clear interpretation and are tolerant for missing data. They can be related to second-order structure functions, both for discrete and continuous data. Spatial variances can also be Fourier transformed to yield a relation with spectra. The flexibility of spatial variances is used to study various sampling strategies, and to compare them with second-order structure functions and spectral variances. It is shown that the spectral sampling strategy is not seriously biased to calm conditions for scatterometer ocean surface vector winds. When the second-order structure function behaves like rp, its ratio with the spatial variance equals (p+1)(p+2). Ocean surface winds in the tropics have p between 2/3 and 1, so one-sixth to one-fifth of the second-order structure function value is a good proxy for the cumulative variance. © 2015. American Geophysical Union. All Rights Reserved. Source

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