Comparison of thermal variance dissipation rates from moored and profiling instruments at the equator

Abstract

As a quantitative test of moored mixing measurements using xpods, a comparison experiment was conducted at 08, 1408Win October-November 2008. The following three measurement elements were involved: (i) NOAA's Tropical Atmosphere Ocean (TAO) mooring with five xpods, (ii) a similar mooring 9 km away with seven xpods, and (iii) Chameleon turbulence profiles at an intermediate location. Dissipation rates of temperature variance and turbulent kinetic energy are compared. In all but 3 of 17 direct comparisons 15-day mean values of χ T agreed within 95% bootstrap confidence limits computed with the conservative assumption that individual 1-min xpod averages and individual Chameleon profiles are independent. However, significant mean differences occur on 2-day averages. Averaging in time reduces the range (95%) in the observed differences at two locations from a factor of 17 at 1-day averaging time to less than a factor of 2 at 15 days, presumably reflecting the natural variability in both the turbulence and the smallscale fluid dynamics that lead to instability and turbulence. The motion of xpod on a mooring beneath a surface buoy is complex and requires a complete motion package to define in detail. However, perfect knowledge of the motion of the sensor tip is not necessary to obtain a reasonable measure of χ T. A sampling test indicated that the most important motion sensor is a pressure sensor sampled rapidly enough to resolve the surface wave-induced motion. © 2012 American Meteorological Society.