Spatial variations of surface moisture ¯ux from aircraft data L. Mahrt a, * , Dean Vickers a , Jielun Sun b a College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA b National Center for Atmospheric Research, Boulder, CO 90307, USA Received 10 August 2000; received in revised form 15 November 2000; accepted 2 February 2001 Abstract This study surveys the existing conceptual views of the in¯uence of surface heterogeneity on spatial variability of ¯uxes into the atmosphere, and constructs an approach for using low-level aircraft to measure spatial variations of the surface moisture ¯ux. This approach is applied to Canadian Twin Otter aircraft data collected during the Southern Great Plains Experiment. The response of turbulent moisture ¯uxes to surface heterogeneity is reduced by horizontal mixing by the turbulent eddies. During the evolution of the daytime convective boundary layer, the eddy size increases and the spatial variation of the moisture ¯ux into the atmosphere is con®ned to larger horizontal scales. The in¯uence of the surface heterogeneity on moisture ¯uxes into the atmosphere is framed here in terms of a horizontal blending scale, which can be used as guidance for partitioning the aircraft track. Problems with the esti- mation of the spatial variability of surface moisture ¯uxes from aircraft eddy correlation data are examined. Obtaining an adequate sample size of the moisture ¯uxes over heterogeneous surfaces is considerably more dicult than over homogeneous surfaces. Sampling requirements determine the maximum spatial resolution of the measured surface ¯uxes. The time-dependence of the spatial variation of the surface moisture ¯ux is even more dicult to estimate since simple compositing of dierent aircraft passes would eliminate the time-dependence. A new scheme for estimating the time±space dependence of surface ¯uxes is developed. As an application example, this method is used to show that the evaporative fraction varies only slowly from morning to afternoon for the surface types examined here. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Surface moisture ¯ux; Blending height; Evaporative fraction; Surface heterogeneity; Aircraft ¯ux measurements 1. Introduction Heterogeneity of surface moisture ¯uxes occurs on a wide range of scales from millimeters to the scale of continents and oceans. Surface heterogeneity on the scale of the surface roughness elements grass blades, bushes, trees, etc.) is not normally referred to as het- erogeneity from an atmospheric point of view because the eect of the individual roughness elements on the time-averaged atmospheric ¯ow is con®ned to the roughness sublayer Fig. 1). Above the roughness sub- layer, in the surface layer, the eect of heterogeneity on the scale of individual roughness elements is eliminated through horizontal mixing by turbulence. Monin± Obukhov similarity theory, normally used to predict surfce ¯uxes into the atmosphere, requires that the mean atmospheric variables be measured within the surface layer [21]. The surface layer is assumed to be thin compared to the atmospheric boundary-layer depth so that the values of the ¯uxes within the surface layer are close to the surface values. Fluxes over heterogeneous surfaces are sometimes formulated as the footprint problem [11,27]. When the surface heterogeneity is con®ned to a single horizontal discontinuity, with surface homogeneity on either side, the ¯ow adjustment is normally viewed in terms of de- velopment of an internal boundary layer [10], de®ned as the layer of air in¯uenced by the new surface down- stream from the discontinuity. This situation not con- sidered here. For more complex surface heterogeneity, the concept of the blending height [4,21,25,31] is often applied where the in¯uence of surface heterogeneity becomes small above the blending height. The blending height is level in the atmospheric boundary layer, above which the in¯uence of a given horizontal scale of surface heterogeneity is small, but not necessarily zero. The blending height is a scaling depth that describes the gradual decrease of the in¯uence of surface hetero- geneity with height. There is no level where the in¯uence of surface heterogeneity suddenly ceases. The blending Advances in Water Resources 24 2001) 1133±1141 www.elsevier.com/locate/advwatres * Corresponding author. Tel.: +1-541-737-5691; fax: +1-541-737- 2540. E-mail address: mahrt@oce.orst.edu L. Mahrt). 0309-1708/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0309-170801)00045-8