Estimating Cn2 Over Snow And Sea Ice From Meteorological Quantities

Abstract

Because turbulent fluctuations in the atmospheric refractive index (n) at wavelength x are related to turbulent fluctuations in the temperature (t) and humidity (q) by n = A(x,P,T,9)t + B(x,P,T,Q)q, it is possible to estimate the refractive index structure parameter C in the atmospheric surface layer from meteorological quantities. I describe two such estimation procedures, one based on the velocity, temperature, and humidity scales u*' t*, and q*, and a second based on the routine meteorological quantities U,, T -Th, and %-Q,. Subscript h here denotes the wind speed (U,), temperature (TO, and humidity (Qh) at rVference height h; subscript s indicates the sUrface value. I tabulate the coefficients A and B as functions of λ, the atmospheric pressure (I)), and the ambient temperature (T) and humidity (Q) in four wavelength regions--visible (including near-infrared), an infrared window, near-millimeter, and radio. A sensitivity analysis of the two estimation procedures demonstrates that the accuracy of the Cn2 estimate is a strong function of the Bowen ratio (Bo), the ratio of sensible to latent heat flux at the surface. At two Bo values within the interval [-10,10], one dependent on λ and the other on enyironmental conditions, the uncertainty in the Cn2 estimate becomes infinite. I focus on C values over snow and sea ice, and my examples are for these surfaces, but the estimation procedures presented can be applied to any geophysical surface that is horizontally homogeneous.