Footprint Analysis to Assess the Conditioning of Temperature and Humidity Measurements in a Weather Station Vicinity

Equations for estimating the influence of upwind fetch on air temperature and humidity measurements are applied to assess fetch impacts on agricultural weather stations. The equations are derived from flux and scalar footprint models and may serve as a first approximation of the amount of impact (F = 0 to 1) that upwind fetch distance has on measured air temperature or humidity at height z above the surface. This information is useful for judging adequacy of green fetch upwind of a weather station and for judging the amount of impact that a dry surface in the vicinity of a weather station has on measurements. The equations were applied to various fetch lengths of clipped grass and dry, bare soil over a range of wind speed. Results indicate that F increases more-or-less logarithmetically with fetch distance for both surface conditions and show F to increase with decreasing wind speed for a specified fetch distance. Results indicate that the 100:1 fetch distance:measurement height rule-of-thumb applies to unstable boundary layer conditions (positive Bowen ratio), but may underestimate the fetch requirement for neutral and stable conditions. Values for F for short fetch lengths, for example 5 and 10 m, show the fallacy of locating a weather station over a small area of grass or other vegetation, but surrounded by dry, poorly vegetated further upwind. Essentially none of the sensor signal (T and e measurement) is conditioned by the grass when wind speed is greater than 2 m s^–1.