Characteristics of the Surface Roughness Layer

Textbook micrometeorology defines several different regions of the lower atmosphere, among which the surface boundary layer (SBL) is most commonly addressed. Within this layer, fluxes of the defining quantities, heat and momentum, are assumed to be constant with height. There is strong awareness that the SBL does not extend all the way to the surface. The SBL is assumed to be limited at its lower level by the height at which there are no longer significant effects of individual surface roughness elements – vegetation, buildings and the like. The layer of the atmosphere between the SBL (so determined) and the actual effective surface is surface roughness layer (SRL). Both the SBL and the SRL are within the planetary boundary layer (PBL) which can be considered as the envelope of the atmosphere that contains the diurnal cycle of heat content. Often, there is mention of the “mixed layer” which is often defined by the vertical extent to which convection extends in daytime, usually but not always associated with the tops of any convective clouds that might be present. In stably stratified conditions, the definition of the mixed layer becomes elusive.

These various layers are convenient descriptions, adopted by those who desire simple views of the changing atmosphere. In practice it could be more appropriate to define the top of the SBL as being that height at which formulations based on surface considerations have the same consequences as those based on larger-scale (e.g. synoptic or PBL) approaches. The corresponding criterion defining the bottom of the SBL (and the top of the SRL) could be the spatial average height (Zr) at which local roughness elements no longer influence profiles. This height is commonly related to the familiar quantities z₀ and d, with a historic rule of thumb often summarized as Zr ≈ d + 10z₀. In this context, note that d and z₀ are quantities determined by extrapolation of gradients and fluxes within the SBL to the surface, hence ignoring the presence and influence of the SRL.

Most experimental observations that relate to these considerations have been made in daytime. However, studies of CO₂ profiles conducted though a growing maize canopy in 2023 offer an opportunity to investigate the nighttime situation using CO₂ as a tracer. The measurements reveal that the nocturnal strongly stratified SRL is often characterized by the linear profile such as reported by Verma and Rosenberg (1976) and examined by Galmiche and Hunt (2002). The use of CO₂ measurements satisfies some of the main experimental requirements that obscure examination of the nighttime SRL. In particular, the nocturnal profiles studied result largely from soil effluxes of CO₂ that are not greatly affected by atmospheric quantities. This is not the case for the profiles of heat and wind, since these are strongly influenced by atmospheric changes. However, the case of SRL temperature profiles has recently been examined (Boekee et al., 2024) with results that align with the present CO₂ observations.

It is proposed that CO2 can serve as a useful tracer of opportunity, to investigate many aspects of air-surface exchange as if affects crops and plantations.