TT2a: EOP Surface Flux Measurements

TT-leaders: Colin Lloyd, Chris Taylor

Detailed information: TT2a document : EOP Surface Flux Measurements (latest update: 23.01.2006, pdf, 0.2 MB)

Scientific justification and objectives

An important component of the AMMA project is the monitoring of surface conditions continuously over the course of the Extended Observation Period (EOP). Over West Africa, the seasonal evolution of the atmosphere is accompanied by pronounced changes in surface properties, driven by rainfall. These properties (notably soil moisture and leaf area) control fluxes of heat and moisture into the atmosphere and thus affect characteristics of the coupled system. We have at this time little knowledge of the evolution of these properties and associated fluxes during the annual cycle, and from one year to the next. Furthermore, their poor representation in large scale models affects the overall simulation of the monsoon system. Where we do have observations, these are for a limited period over a relatively small area (e.g. HAPEX-Sahel).

Flux observations are not representative of the areal average fluxes even at the mesoscale, due to strong heterogeneity in factors such as vegetation cover, soil characteristics, and rainfall. At the larger scale, these detailed observations by themselves provide little insight into processes occurring elsewhere on the vegetation gradient between the natural closed canopy forest in southern West Africa and the desert in the north of the region. The primary aim of the EOP surface flux monitoring in AMMA is therefore to make continuous observations from a selection of typical land covers under a broad range of climatic conditions over a minimum of 2 years. The ensemble of instruments (a network of flux stations augmented by other micrometeorological instrumentation e.g. radiation, soil physics) will provide a coherent set of observations with which to understand how the land surface typically evolves at representative points on the gradient. Crucially, the flux stations will provide ground truth to link with earth observation data, and to constrain surface models. This will feed into the land data assimilation scheme, the measurements playing a critical role in attempts to quantify the daily to interannual variability of the land surface at scales from the meso- to the monsoon scale. In addition, the systems will provide flux observations to support field/catchment scale studies.

The provision of surface fluxes, surface meteorological measurements and soil physics measurements during the EOP/SOP is required in various of the Work Packages

The location and operation of the flux systems provides, not only the time-series of regional point measurements of sensible and latent heat and carbon dioxide fluxes for the above work packages, but also the relevant long-term data to vegetation, soil moisture, energy balance and aerosol experiments at each of the supersites. There are therefore very strong links between TT2a and TT2b, TT3, TT4 and TT5.