On the Variability of In Situ Surface Layer Refractivity Measurements

Abstract

Direct measurements of profiles of atmospheric properties near the ocean surface and within the marine atmospheric surface layer often contain a large degree of variability. The variability observed can be explained by numerous technical and natural reasons such as the temporal variability over the time span a profile is measured (unsteadiness in the mean), spatial variations (inhomogeneity), turbulent fluctuations, and measurement uncertainty. In this study, we explored the observed variability in vertical distributions of refractive index measured with a tethered-balloon-based marine atmospheric profiling system (MAPS). MAPS profiled the atmosphere from approximately 0.5 to 50 m, with instantaneous (order 1 s) measurements performed at each profiled altitude. To explore whether the observed scatter could be largely explained by (inertial-scale) turbulent fluctuations, we simulated refractive index fluctuations with a spectral-based turbulent refractive index fluctuation (TRIF) model. TRIF was optimized based on the MAPS measurements to determine a vertical length scale of the turbulence. The scales computed in the optimization were reasonable based on other estimates in the literature under similar conditions. However, finer-scale trends of the length scale with atmospheric stability did not match expectations, and thus the estimated length scales may be considered more as an order-of-magnitude estimate rather than an exact measurement of this scale. The ability to match the observed variability in the MAPS data using a turbulence model with a reasonable choice of vertical length scale suggests that the MAPS variability is dominated by physical processes such as turbulence rather than being primarily driven by measurement uncertainty.