RELATING EDDY-CORRELATION SENSIBLE HEAT-FLUX TO HORIZONTAL SENSOR SEPARATION IN THE UNSTABLE ATMOSPHERIC SURFACE-LAYER

Measurements of a scalar flux from an extended surface are frequently made with the eddy correlation technique consisting of a vertical velo city sensor and a sensor for the scalar of interest. In many cases the two sensors have to be mounted with a significant horizontal separati on to avoid flow interference. Consequently, the technique will undere stimate the scalar flux. This paper addresses the issue of flux undere stimation due to this separation. A model is developed in the framewor k of Monin-Obukhov similarity for the spatial covariance of vertical v elocity and air temperature in the unstable surface layer. It allows t he underestimation of sensible heat flux to be assessed using informat ion on separation orientation relative to wind direction, atmospheric stability, measurement height, and separation distance. The coefficien t in the model is evaluated with observations made over a potato field and a clover field. The principles established here should also be ap plicable to fluxes of scalars other than sensible heat.