Wind-Speed Controls on Scalar Gradients, Fluxes, and Cospectra in the Stable Boundary Layer

Abstract

Vertical profiles of temperature, water vapour, and carbon dioxide ($\mathrm {CO}_{2}\,$) densities are analyzed, together with the respective turbulent fluxes of these scalars during 9 nights when there was a consistent wind decay along the period, with a corresponding stable boundary layer regime transition from weak to very stable. The purpose is to understand whether the vertical gradients and fluxes of these scalars respond differently to the mean wind decay. A multi-resolution spectral decomposition is used to address how the patterns found depend on their temporal scale. Case studies show that only water vapour gradients and latent heat fluxes switch sign along the nights. The signs of sensible heat and $\mathrm {CO}_{2}\,$ fluxes on low-frequency scales oppose those on turbulent time scales, while the same does not always occur for latent heat. The covariance between fluctuations of temperature and those of both water vapour and $\mathrm {CO}_{2}\,$ are also analyzed, along with their spectral decompositions. Their role in the corresponding scalar flux budgets is evaluated by comparing two relevant terms in the budget equations. It is found that the term related to the vertical gradient exceeds the one related to the temperature–scalar covariance for wind speeds above a threshold, which is larger for latent heat flux than for both sensible heat and $\mathrm {CO}_{2}\,$ fluxes. The implications of this result for flux parametrizations are addressed.