An approach to deriving roughness length and zero-plane displacement height from satellite data, prototyped with BOREAS data

Abstract

Global climate models typically use relatively crude estimates of roughness length, often based on constant values for land cover category. However, large variations may exist within any of the vegetation categories. Global large-scale estimates of roughness length, such as required by climate models, cannot rely on field experiments for practical reasons and must, therefore, be derived from satellite observations. This paper develops a satellite retrieval method for the roughness parameters, momentum roughness length, z o, and displacement height, d. The formulas, based on the observational data of Raupach [Raupach, M.R., 1992. Bound.-Layer Meteor. 60, 375–395; Raupach, M.R., 1994. Bound.-Layer Meteor. 71, 211–216] and Lindroth [Lindroth, A., 1993. Bound.-Layer Meteor. 66, 265–272], are applicable to most types of vegetation. Raupach relates the momentum roughness length and zero-plane displacement height to the frontal area index, λ, and to the height of the vegetation, h. The frontal area index is related to the shape of an average tree or plant crown and to the average density of the canopy elements. The crown shape factor includes the aspect (height-to-width) ratio of the crown and the geometric shape of the crown. Lindroth relates the momentum roughness length to the leaf area index (LAI) and to the height of the trees. The functional form of z o/ h is peaked in both frontal area index, with the maximum occurring at λ=0.152, and average plant LAI ( L p), with the maximum occurring at L p=0.878. As λ and L p increase, d/ h also increases and approaches an upper limit asymptotically at high values of λ and L p. As the vegetation height is difficult to obtain from present satellite data, retrievals are restricted to fractional values (i.e. z o/ h and d/ h) for global application. This retrieval then requires data for the crown aspect ratio, the canopy density (fractional overstory canopy cover) and the LAI. Data from BOREAS ( BOReal Ecosystem Atmosphere Study) are used to illustrate this satellite retrieval method.