Estimating latent heat flux from a vineyard using scintillometry

The combination of a near-infrared and a microwave scintillometer were used to estimate the line-averaged, latent heat flux density (lambda E-s) from a well-irrigated, vineyard valley during summer. The 2 km scintillometer beam passed over the valley floor at a height (z) of 30 m, Eddy covariance towers instrumented at 3 m above the valley surface provided an independent check elf vineyard evaporation. A formula for free convective lambda E-s compared favourably to the eddy covariance values, lambda E-e at low wind speeds and under clear skies. As the convective boundary layer (CBL) developed and passed through the scintillometer beam it caused additional scintillations, in which case lambda E-s >> lambda E-e, During unstable conditions the comparisons were in; close agreement. However lambda E-s tended to overestimate lambda E-e as the wind speed increased. For near-neutral conditions the scintillometer gave unrealistically large values of lambda E-s, The additional scintillations originated from the strong entrainment of advected dry and warm air into the newly formed surface boundary layer. During calm overcast days, cold days and during the early morning periods when the surface fluxes were small, the signal strength of the infrared scintillometer approached the system noise.