SCALE ISSUES IN BOUNDARY-LAYER METEOROLOGY - SURFACE-ENERGY BALANCES IN HETEROGENEOUS TERRAIN

This paper, part review and pare new work, falls into three main secti ons. The first is a review of scale issues in both hydrology and meteo rology, focusing on their origins in the water and energy conservation equations, integrated over control volumes of different scales. Sever al guidelines for scale translations are identified. The second sectio n reviews the upscaling problem in boundary-layer meteorology, setting out two 'flux-matching' criteria for upscaling models of land-air flu xes: the conservation requirement that surface fluxes average linearly and the practical requirement that model form be preserved between sc ales. By considering the effects of boundary conditions, it is shown t hat the combination or Penman-Monteith equation is a model for element al energy fluxes which leads to physically consistent flux-matching ru les for upscaling surface descriptors (resistances). These rules are t ested, along with some other possibilities, and found to perform well. The third section tests the hypothesis that regionally averaged energ y balances over land surfaces are insensitive to the scale of heteroge neity, X. Heterogeneity is classified as microscale when X less than o r equal to UmT, mesoscale when UmT*less than or equal to X less than or equal to UmTe, and macroscale when UmTe less than or equal to X [wh ere U-m is the mean wind speed in the convective boundary layer (CBL) and T and T-e the convective and entrainment time scales, respectivel y]. A CBL slab model is used to show that regionally averaged energy f luxes are remarkably insensitive to X in both the microscale and macro scale ranges. Other reviewed evidence suggests that the mesoscale rang e behaves similarly in dry conditions. Questions remain about the cons equences of clouds and precipitation for regionally averaged surface e nergy fluxes.