Small-Scale Spatial Variation of the Nocturnal Wind Field

This study examines the spatial variability of the nocturnal wind field using eight networks of surface observations ranging in horizontal width from 500 m to 65 km. The wind field is partitioned into small-scale variability (submeso motions) and the spatially-averaged wind vector. The vector-averaged wind is analogous to the wind resolved by a numerical model, posed here in terms of the wind that is vector averaged over an observational network. The small-scale variability represents the unresolved subgrid (sub-network) variation estimated in terms of the spatial variation of the wind vector within the observational domain. The bulk formula for the spatially-averaged heat flux is modified to account for the subgrid variation of the wind field. Investigation of the spatial variability of the wind field is also motivated by the need to estimate the representativeness of observations of the wind vector at an individual measurement site with respect to the wind field over the surrounding landscape. The small-scale variability of the observed wind field is contrasted between the networks as a function of the spatially-averaged wind vector, stratification, size of the network, and the topography. A strong dependence on topography emerges in spite of different instrumentation, deployment strategy, and processing for each network. Even weak topography can be important. A better design for future observational networks is briefly discussed.