Nocturnal intermittency in surface CO 2 concentrations in sub-Saharan Africa

Abstract

a b s t r a c tAn exploratory study of CO 2 concentrations and fluxes was conducted during 2013, at a site 12 km North of Harare, Zimbabwe. CO 2 measurements were made over four adjacent fields of differing surface vegetation. The data illustrate the role of atmospheric intermittency as a mechanism for transferring CO 2 between the surface and the atmosphere. At night, limited atmospheric mixing permits CO 2 concentrations to increase to levels well above those conventionally reported (exceeding a spatial average of 450 ppm on some nights), but these high levels are moderated by a periodic intermittency that appears similar to that observed elsewhere and often associated with the presence of strong, synoptic-scale winds aloft (especially low-level jets). The availability of CO 2 data with adequate time resolution facilitates investigation of the general behavior, which is suspected to be a common although rarely observed feature of the lower terrestrial atmosphere. If true, this means that the nocturnal vertical transfer of momentum, heat and mass is not solely through a constrained spectral continuum of turbulence as much as by intermittent bursts, propagating from above and penetrating the surface boundary layer.

On surface fluxes at night – the virtual chamber approach

“…Ratio Energy Balance protocols. The second repeats the analysis, using similar data derived from an earlier study conducted in Zimbabwe (Hicks et al, 2015. The third is based on a study of ammonia emissions from an area previously treated with urea-ammonium nitrate (UAN) as a nitrogenous fertilizer in central Illinois (Nelson et al, 2017(Nelson et al, , 2019.…”

“…Identical BREB 284 instrumentation was set up centrally in four neighboring experimental areas, of which two will 285 be considered here (identified as #1 and #2 in the site depiction in Figure 5). An earlier analysis focused on the nocturnal data obtained, with a time resolution of five minutes so that occurrences of intermittent turbulence were readily apparent (Hicks et al, 2015). In the lack of major upwind surface irregularities, these occurrences were attributed to the gravity wave phenomenon considered in detail by Blackadar (1957) but Mahrt et al (2019) show that there are several alternative causative mechanisms.…”

“…The concept of a 'virtual chamber' analysis to derive flux information from nighttime concentration data (of trace gases like CO 2 , methane (CH 4 ), nitrogen oxide (NO x ), and NH 3 ) arose in examination of data obtained in Zimbabwe, which illustrated the ramp structure at night considered due to dilution of CO 2 accumulation in the stratified layers of air near the surface by nocturnal intermittent turbulence (Hicks et al, 2015). The Zimbabwe dataset is of special interest, because it relates to a field site on the arid Zimbabwe plateau near Harare, at an altitude of more than 1400 m asl.…”

A Case Study of Turbulence in the Nocturnal Boundary Layer During the Indian Summer Monsoon

“…Hicks et al (2015) recently illustrated downwards transfer of momentum associated with a downburst resulting in a periodic increase in wind speed at ground level, attributing this to the synoptic-scale events rather than to surface characteristics. In our study, however, both upwards and downwards transfer of momentum occur simultaneously with strong updrafts, strengthening the proposition that momentum transfer takes place in association with the velocity shear in the atmosphere.…”

On the Relevance of ln(z0/z0T) = kB-1

“…1 are results from two recent studies conducted in widely dissimilar situations. The first study was over a field of grazed pasture in Alabama (Pendergrass et al 2017), the other over a fallow field in the dry season of Zimbabwe Hicks et al 2015). The curves (1) through (4) plotted in Fig.…”