Flow and Transport over Forested Hills in Neutral and Stable Conditions

NCAR Biogeosciences

ACD/MMM Seminar

Flow and Transport over Forested Hills in Neutral

and Stable Conditions

John Finnigan

CSIRO Atmospheric Research, Canberra, Australia

Wednesday, 7 July 2004

Foothills Lab, Building 2, Room 1022

3:30 p.m.

The linear analytic treatment of flow over rough hills by Hunt et al. (1988) (HLR) has now been extended to include a plant canopy as a lower boundary (Finnigan and Belcher, 2004). The model applies to low hills (, where H is hill height and L horizontal length scale) and deep canopies, where practically all the momentum is absorbed as drag on the foliage rather than as friction on the ground. The theory predicts that the HLR results are substantially modified near the ground and that separation can occur within the canopy on the lee side of the hill, even on very gentle hills. This prediction has now been confirmed by wind tunnel tests.

Using this windfield model to drive a model of carbon dioxide transport on a forested hill with realistic treatment of photosynthesis, Katul, Finnigan and Leuning (2004) have shown that the strong symmetry breaking effect of the within-canopy separation leads to large advective terms and points to problems in measuring scalar exchange in hilly terrain from single towers such as those normally used in the Fluxnet.

Most recently, the windfield model has been extended to include the influence of stable stratification. The model results go some way to explaining field and wind tunnel measurements that show that the presence of a canopy promotes a strong, stable gravity current, essentially decoupled from the flow above the vegetation, when the surface layer is moderately to strongly stably stratified. Furthermore the linear theory shows that the strength of the gravity current depends on the potential temperature deficit in the canopy and the slope length, L rather than the slope angle, H/L. This might provide a reason for the failure to close the carbon dioxide mass balance in stable nighttime conditions at many Fluxnet sites with gentle topography.