Introduction: Renaissance of Scintillometry

In the 1970s impressive progress was made in research on turbulence and the use of scintillometers. Muschinski and Lenschow (2002) summarised the outcome of the workshop on ‘Future Directions for Research on Meterand Submeter-Scale Atmospheric Turbulence’ held at Boulder, Colorado, U.S.A. where the development in this field was discussed. One of their comments noted that, after a blooming period in the 1970s and consolidation in the 1980s, funding and interest in scintillometry faded rapidly in the early 1990s. This special BLM issue is a result of a renewed interest, a renaissance, in scintillometry. Of work done in the last 40 years, some of the most important contributions include the work of Tatarskii (1961), the textbook by Monin and Yaglom (1975), the collection of scintillation papers brought together by Andreas (1990) and reviews by e.g., Hill (1997). Here are some main lines. A scintillometer is an instrument that consists of a transmitter and a receiver. The receiver measures intensity fluctuations in the radiation emitted by the transmitter caused by refractive scattering of turbulent eddies in the scintillometer path. For laser sources, or small aperture scintillometers (SAS), the observed intensity fluctuations are a measure of the structure parameter of the refractive index, C2 n, and the inner scale of turbulence, 0. For large aperture scintillometers (LAS), they are a measure of C2 n only. At optical wavelengths the contribution of temperature fluctuations dominates, i.e., the structure parameter of temperature C2 T can be deduced from the C2 n measurement. For radio wavelengths (>1 mm) on the other hand, water vapour fluctuations contribute most to the scintillometer signal, i.e. the structure parameter of moisture C2 q can be deduced from the C 2 n measurement. Surface fluxes of sensible heat, latent heat and momentum can be determined from the obtained C2 T , C 2 n and 0 respectively by applying Monin–Obukhov similarity theory (MOST). A number of length scales play a role in scintillometry: • The inner and outer length scales of turbulence. The first, 0, is proportional to the Kolmogorov length (ν3/ )1/4, where ν is the molecular kinematic viscosity and the dissipation rate of turbulent kinetic energy (TKE). The outer length scale is proportional to the height above the surface, z, in the surface layer; • the wavelength of the light source of the scintillometer, λ; • the aperture, D, of the scintillometer. Note that the transmitter and receiver have the same aperture in the scintillometers used in the studies presented here;