Observations of vegetation-breezes and their impact on convection

Aircraft observations over Benin on 17 August 2006 were used to look at the impact of heterogeneities in vegetation cover, primarily between crop and forest/shrub, on the thermodynamic and dynamical properties of the planetary boundary layer (PBL). Isoprene measurements taken on the aircraft were used to link the vegetation patterns to the PBL properties, as it is a biogenic organic compound emitted primarily by woody vegetation species.

Observations from the UK BAe146 atmospheric research aircraft over Benin on 17 August 2006 are used to assess the impact of vegetation heterogeneity on the surface heat fluxes, how these affect the dynamics within the planetary boundary layer (PBL), and their subsequent impact on convection, transport and mixing. Concentrations of isoprene, a biogenic volatile organic compound emitted primarily by woody vegetation species, were measured by the aircraft, and used to link the vegetation patterns to the PBL properties.

The wind measurements at two levels show a clear mesoscale organisation of the meridional winds, with associated areas of enhanced cumulus congestus cover. Areas of low-level convergence correlate both with temperature and with isoprene gradients associated with vegetation boundaries. The results are consistent with cropland areas having lower Bowen ratios, associated with lower evapotranspiration, and lower isoprene fluxes as compared with adjacent forest/shrub cover, thus producing positive temperature anomalies over the cropland. These anomalies lead to temperature gradients at the vegetation boundaries, thus causing the observed mesoscale flows. The areas of convergence were generally found on the southern edge of the warm anomalies, due to the northerly synoptic flow which displaced the convergence zones southward, and strengthened the convergence within the southerly part of the mesoscale flow. The relationships of PBL temperatures with wind velocity, and PBL temperatures with isoprene concentrations, were significant down to scales of 10 and 8km respectively, in line with aircraft observations of soil-moisture induced circulations.

A seasonal climatology of visible satellite data shows enhanced cloud cover in the afternoon over cropland. This is consistent with the presence of land-induced flows, as well as previous studies over the Amazon, and suggests that the aircraft results are not limited to this case-study, but are of climatological significance.