Numerical analysis of flux footprints for different landscapes

SummaryA model for the canopy – planetary boundary layer flow and scalar transport based on E-ɛ closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns – forest with clear-cuts as well as hypothetical heterogeneous relief – a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown – the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends on the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Solling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites.

[1]  Günter Gross,et al.  Numerical simulation of canopy flows , 1993 .

[2]  Monique Y. Leclerc,et al.  Footprint prediction of scalar fluxes using a Markovian analysis , 1990 .

[3]  A. Ibrom,et al.  Eddy-correlation measurements of fluxes of CO2 and H2O above a spruce stand , 1996 .

[4]  John D. Wilson,et al.  A second-order closure model for flow through vegetation , 1988 .

[5]  David R. Miller,et al.  A first-order closure scheme to describe counter-gradient momentum transport in plant canopies , 1985 .

[6]  W. Oechel,et al.  FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities , 2001 .

[7]  Ü. Rannik,et al.  Flux footprints over complex terrain covered by heterogeneous forest , 2004 .

[8]  Hans Peter Schmid,et al.  Experimental design for flux measurements: matching scales of observations and fluxes , 1997 .

[9]  Monique Y. Leclerc,et al.  Footprint prediction of scalar fluxes from analytical solutions of the diffusion equation , 1990 .

[10]  R. Kormann,et al.  An Analytical Footprint Model For Non-Neutral Stratification , 2001 .

[11]  T. A. Black,et al.  E-ε modelling of turbulent air flow downwind of a model forest edge , 1996 .

[12]  Ü. Rannik,et al.  Footprints and Fetches for Fluxes over Forest Canopies with Varying Structure and Density , 2003 .

[13]  G. Campbell,et al.  An Introduction to Environmental Biophysics , 1977 .

[14]  George L. Mellor,et al.  A Simulation of the Wangara Atmospheric Boundary Layer Data , 1975 .

[15]  Stephen E. Belcher,et al.  TURBULENT FLOW OVER HILLS AND WAVES , 1998 .

[16]  B. Launder,et al.  The numerical computation of turbulent flows , 1990 .

[17]  M. Raschendorfer,et al.  The energy budget of a spruce forest: field measurements and comparison with the forest–land–atmosphere model (FLAME) , 1998 .

[18]  A. Chorin.  Numerical solution of the Navier-Stokes equations , 1968 .

[19]  J. Lloyd,et al.  Using a One-and-a-Half Order Closure Model of the Atmospheric Boundary Layer for Surface Flux Footprint Estimation , 2004 .

[20]  T. W. Horst,et al.  Footprint estimation for scalar flux measurements in the atmospheric surface layer , 1992 .

[21]  R. Pielke.  Mesoscale Meteorological Modeling , 1984 .

[22]  J. Finnigan,et al.  A Second-Order Closure for Neutrally Stratified Vegetative Canopy Flows , 1999 .

[23]  I. P. Castro,et al.  A LIMITED-LENGTH-SCALE k-ε MODEL FOR THE NEUTRAL AND STABLY-STRATIFIED ATMOSPHERIC BOUNDARY LAYER , 1997 .

[24]  M. Heimann,et al.  A simple three-dimensional canopy - planetary boundary layer simulation model for scalar concentrations and fluxes , 2002 .