Land-surface , boundary layer and cloud-field coupling over the Amazon in ERA-40

Models are a powerful tool for understanding the coupling of physical processes. We illustrate this using ERA-40 data for the Madeira river, a south-western basin of the Amazon, which has a large seasonal cycle with a dry season in the austral winter. Daily averaged data, derived from basin-averaged hourly data, give a powerful description of this coupling of physical processes at the land-surface. The links that are visible on a daily timescale can also be seen on the seasonal timescale. Several important surface processes are strongly influenced by soil moisture: relative humidity which gives the mixed sub-cloud layer depth, low cloud cover and the surface net long-wave flux. The link between soil moisture and equivalent potential temperature can therefore be clearly seen once the temperature dependence is filtered. Surface evaporation is controlled as much by the feedback of the cloud field on the surface radiation budget as by soil moisture. Above the surface the cloud field and precipitation is coupled to the large-scale dynamics, specifically the mid-tropospheric omega field. The shortwave cloud forcing of the atmosphere and the surface is given by the cloud field albedo at the top of the atmosphere to better than 1%. We have developed a powerful methodology for comparing the feedback processes in different models with each other and with data.

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