Abstract
Abstract Subgrid-scale (SGS) velocity variations result in gridscale sea surface flux enhancements that must be parameterized in weather and climate models. Traditional parameterizations are deterministic in that they assign a unique value of the SGS velocity flux enhancement to any given configuration of the resolved state. In this study, we assess the statistics of SGS velocity flux enhancement over a range of averaging scales (as a proxy for varying model resolution) through systematic coarse-graining of a convection-permitting atmospheric model simulation over the Indian Ocean and west Pacific warm pool. Conditioning the statistics of the SGS velocity flux enhancement on 1) the fluxes associated with the resolved winds and 2) the precipitation rate, we find that the lack of a separation between “resolved” and “unresolved” scales results in a distribution of flux enhancements for each configuration of the resolved state. That is, the SGS velocity flux enhancement should be represented stochastically rather than deterministically. The spatial and temporal statistics of the SGS velocity flux enhancement are investigated by using basic descriptive statistics and through a fit to an anisotropic space–time covariance structure. Potential spatial inhomogeneities of the statistics of the SGS velocity flux enhancement are investigated through regional analysis, although because of the relatively short duration of the simulation (9 days) distinguishing true inhomogeneity from sampling variability is difficult. Perspectives for the implementation of such a stochastic parameterization in weather and climate models are discussed.
Highlights
Near-surface winds exert an important control on exchanges of mass, energy, and momentum between the atmosphere and the underlying surface
Based on theoretical foundations, these parameterizations are generally largely empirical. They are averaged in time, the expressions relate fluxes at a single point in space to the atmospheric state at that location
We have considered only pointwise statistics of the error «N,n and the residuals zN,n and cN,n
Summary
Near-surface winds exert an important control on exchanges of mass, energy, and momentum between the atmosphere and the underlying surface. Consistent with the behavior of the relative error, the median of «N,n increases with coarsening scale: coarser grids result in larger differences between resolved and true fluxes for all values of FN(R,n).
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