The boundary layer also contributes, through energy advection, to the spatial structure of surface heat flux fields. Patchiness in surface conditions may also result in local thermally-direct circulations within the lower troposphere. Segal et al. [1992] and Avissar [1994, this issue] provide comprehensive reviews of the surface forcing of such conditions and their consequences for the spatial scaling and aggregation of surface flux and land characteristics. An important consequence of these physiographic mesoscale circulations is the complication introduced in scaling and aggregating surface flux and land characteristics. Simple spatial averaging and linear scaling of fluxes are not possible when significant nonlinear dependencies with certain characteristic scales are evident in the two-way coupled land-atmosphere system. Diurnal cycle of heating and cooling also induce so-called land-breeze circulations (so named due to their essentially identical dynamics as sea-breeze).
Kawashima and Ishida [1992] use statistical analysis to relate the observed spatial correlation scale of hydrologic and atmospheric fields to surface conditions. They find that the correlation scale is dependent on the hydroclimate regime through mean temperature and soil wetness. The horizontal scale of air temperature is 50 kilometers on warm days and 100 kilometers on cool days. Localized precipitation due to small scale surface forcing of thermal convection is evident on warmer than average days. Furthermore, the soil thermal inertia is significantly increased when the soil is wetted by liquid water. Thus moist soils respond at a considerably slower rate to radiative forcing and integrate fluctuations in atmospheric forcing. As a result, the spatial scale of spatial correlation in hydrologic and atmospheric processes is larger when the soil is moist. The observational study of Kawashima and Ishida [1992] over central Japan inherently includes spatial interaction introduced through the atmospheric boundary layer coupling to the land surface. The scaling of small scale processes and controls of surface flux partitioning is complicated by the dependence of scaling on the governing energy balance regime.
The scaling (spatial aggregation) of surface flux and land characteristics is simple when the surface and atmospheric boundary layer coupling is one-way (atmospheric forcing of surface conditions). For example, Wood and Lakshmi [1993] show that surface characteristics may be scaled when the aggregation covers 1.5 to 3 correlation lengths or about 1000 to 1500 meters for their study area. Beyond this level of aggregation, Wood and Lakshmi [1993] conclude that a macroscale model based on effective characteristics is sufficient to represent surface flux. The application of simple scaling rules, aggregation and effective macroscale models become considerably more complicated when the land-atmosphere coupling allows two-way interaction [ Jacobs and de Bruin, 1992; Kawashima and Ishida, 1992; Garratt; 1993].
Assessment and testing of land surface parameterizations in atmospheric models are also affected by the degree of land-atmosphere coupling. Similarly, neglecting two-way interactions has serious implications for sensitivity studies. Garratt [1993] provides important guidelines for the assessment of land surface parameterizations; the treatment and representation of atmospheric boundary layer processes significantly alter the sensitivity of land surface parameterizations. The WCRP [1993a] preliminary results from the PILPS (Project for Intercomparison of Land-Surface Parameterization Schemes) suggest that forcing land surface water and energy balance with fixed time-series of atmospheric conditions provide limited measures of the true sensitivity of land surface models.