Snow, Shrubs, Grasses, and Footprint Theory: Measuring Moisture and Energy Fluxes in Patchy Landscapes

When measuring sensible and latent heat flux from a tower within a heterogeneous landscape, one must consider which part of the landscape influences the flux sampled by the instruments. This variable landscape fraction, known as a footprint, is dependent upon wind direction, wind speed and atmospheric stability (thermal and mechanical). From 1 December 2002 - 31 March 2003, the FLuxes Over Snow Surfaces II (FLOSS II) field campaign measured sensible and latent heat fluxes at various heights on a 34 m tower in North Park, Colorado. North Park is an intermountain basin covered with a mixture of shrubs and graminoids (grasses and sedges) that interact with winter snow and wind to produce heterogeneous snow covers and, depending on the depth, protruding vegetation. During this period, snow depth measurements were made along transects extending 400-600 m upwind of the tower roughly every ten days. These snow depth data, in combination with blowing-snow model (SnowTran-3D) simulations, provided daily snow-depth distributions on a 1-meter grid over the area surrounding the flux tower. In addition, shrub height and vertical biomass profiles were measured and combined with a vegetation map having a 1-meter sampling scale. Merging the snow-depth distributions with the vegetation-height map allowed us to quantify the amount of vegetation protruding above the snow. This, in turn, allowed us to analyze the influence of exposed vegetation on observed energy and moisture fluxes. In this poster we describe our model for identifying the landscape fraction gauged by the flux-tower instruments as a function of commonly observed atmospheric conditions.