Regional Advection Perturbations in an Irrigated Desert (RAPID) experiment

The RAPID field experiment took place in August-September 1999 at a site 25 km south of Twin Falls, Idaho, USA. The experiment concerned micrometeorological observations over extensive, well-irrigated fields covered with the fast-growing crop alfalfa. During daytime, on a number of days the sensible heat flux was negative and the latent heat flux exceeded net radiation. The energy required for the latent heat flux to be larger than net radiation has to be advected front elsewhere. As the fields were large, we refer to this process as "regional adiection". "Local advection", on the other hand, refers to advection effects, where the wet to dry transition is on a field scale. Evidence is presented that the RAPID data are subject to regional advection conditions. A simple model, based on Penman-Monteith, is derived that describes the regional-advection case rather well. The influence of wind speed under those conditions is illustrated using data and the model. The correlation coefficients between temperature and horizontal wind component appear to be good indicators for advection.