The influence of humidity and temperature in laser-scintillometry and their impact on surface flux estimates

In the intermittently stable nocturnal boundary layer (ISB) surface-fluxes are dominated by short episodes of turbulence in an otherwise laminar flow. These episodes are too short for conventional one-point techniques (eddy-covariance) to probe enough statistically independent eddies to get flux estimates with the desired accuracy. Our group currently investigates if laser scintillometry, which involves the simultaneous probing of all eddies along a path, is a good alternative for measuring fluxes during these short turbulent spells in the ISB.

A dual beam laser-scintillometer measures the variance and correlation of the log-intensities of two horizontal, parallel beams of light at a distance of about 100 meters from the sources. In 1978 Hill has combined the theoretical ideas of Batchelor, Kraichnan, Corrsin, Pao and Leith with the experimental results of Champagne and Williams to construct an empirically optimized analytical model for the dissipative range spectra of a passive scalar, e.g. T and q. Measured values for the log-variance and log-correlation of the scintillometer are used to estimate the two parameters in Hill’s model: the structure parameter for temperature CT2 and the Kolmogorov wavenumber k_d. MOST-theory is then used to relate the spectra of T, q and Tq to surface fluxes.

This presentation focuses on the role of humidity in Hill’s model, which predicts that under advective conditions water vapour will affect the CT2 and k_d observations. Operational methods of scintillometry either ignore humidity or approximate its high wavenumber behaviour with that of temperature. We will discuss what happens with the sensible heat flux and surface friction when this approximation is replaced by Hill’s q-spectrum. We will demonstrate this with two data sets concerning stable conditions, notably: the dry-nocturnal stable data gathered during CASES and the day-time stable advective data collected during the RAPID campaign (both taken place in 1999).