Department of Atmospheric and Oceanic Sciences, UW-Madison
Conundrums in eddy covariance flux towers: How many towers are enough? How tall is a tower tall enough?
Room 811 AOSS, March 29, 2017, 2:30 PM
Eddy covariance flux towers have been used extensively across the world to observe heat and gas exchanges between the land surface and the atmosphere. This technique is key to monitor surface-atmosphere interactions, the carbon cycle, and the ecosystem functioning since last century. Despite of decades of efforts, flux towers still confront two major challenges: the footprint bias and the energy balance non-closure. The footprint of eddy covariance towers varies temporally and is usually within the areas of 0.1-10 km², which is far smaller than the canopy scale and the resolution of most earth system models (102-103 km²), causing a scale mismatch in model-data comparison. Further, almost all flux towers suffer from an imbalanced energy budget, a problem that the sum of measured sensible and latent heat flux is about 20% less than the available net radiation. Previous studies show that these two problems can be mitigated (1) under more homogeneous landscape, (2) with multiple towers, (3) by building a taller tower, and (4) by applying some scaling methods. Here we ask: (1) How homogeneous is a landscape homogeneous enough? (2) How many towers are towers enough? (3) How tall is a tower tall enough? (4) How upscaling is upscaling enough? We use large eddy simulations as well as an upscaling method, environmental response function, to answer above questions.