Three-Wavelength Method of Measuring Path-Averaged Turbulent Heat Fluxes

Abstract

Conceptually, electro-optical measurements of the path-averaged refractive index structure parameter Cn2 at three wavelengths should yield measurements of the vertical fluxes of sensible (Hs) and latent (HL) heat. With three independent Cn2 measurements we can compute?at least formally?the meteorologically relevant temperature, humidity, and temperature?humidity structure parameters Ct2, Cq2, and Ctq, respectively. The heat fluxes Hs and HL derive from these and a simultaneous electro-optical measurement of the path-averaged turbulent kinetic energy dissipation rate ? through inertial?dissipation calculations. A sensitivity analysis shows that, with the best current technology (wavelengths of 0.94 µm, 10.6 µm, and 3.33 mm), the three-wavelength method would yield measurements of Ct2 accurate to ±20% when the Rowen ratio, Bo=Hs/HL, is in the range 0.1<|Bo|≤10. The measurement of Cq2 is potentially accurate to ±10%?but only when 0.01≤|Bo|<0.5; outside this range, the accuracy is much worse. And the accuracy of the Ctq measurement is poor. The predicted uncertainty is no better than ±40%. This three-wavelength combination, however, can yield the sign of Ctq?an important piece of information?when 0.015<|Bo|<0.5. If instead of the 10.6-µm wavelength, we substituted a wavelength of 18.8 µm, where laser measurements are more difficult, the Bowen ratio ranges over which we could measure both Cq2 and the sign of Ctq expand. For Cq2, the useful Bowen ratio range is now 0.01≤|Bo|<1; and for the sign of Ctq, it is roughly 0.02<|Bo|<2.