The atmospheric boundary layer (ABL) is the layer of air closest to the ground which is directly influenced on a daily basis by the heating and cooling of the earth's surface. The exact depth of the ABL varies according to the time of day. During the daytime it is usually between 1 and 3 km; during the night it is much shallower. The ABL is important because is links the fluxes of heart and water vapour observed at the surface to the general circulation of the atmosphere. To model climate correctly it is necessary that the ABL is well understood and represented in the model.
Because the air in the ABL is turbulent, small scale variations (about 1km or less) in evaporation and heat flux at the surface are smoothed, with the temperature, humidity and depth of the ABL being uniform over the entire area. Larger scale variations (on the scale of 10km or more) may lead to differences in ABL properties between the different surface types. Such differences may cause local atmospheric circulations to develop which may be important for the local climate of an area.

A satellite photograph
of the area around
Ji-Paraná, Rondônia.
The lighter areas have
been cleared.
The ABRACOS sites
were chosen to
represent a large forest
and a large cleared area
suitable for ABL studies.



During ABRACOS three ABL measurement campaigns were carried out. These campaigns were called the Rondônia Layer Experiment (RBLE) 1, 2 and 3 and were held as Ji-Paraná where the scale of the forested and deforested areas is large enough for each surface type to develop its own ABL. The campaigns were held during the dry season when the difference in evaporation between the two surfaces types, forest and pasture, is at its greatest. Measurements were made with both free-flying radiosondes which measure temperature, humidity and wind up to about 12km and with a tethered balloon which makes more detailed measurements in the lowest 1km of the atmosphere.
Measurements were made at both the forest and clearing sites. Profiles of potential temperature measured during RBLE2 show that the daytime ABL was deeper over the clearing than the forest. The data have been used to test several models of ABL development. It appears that the ABL over the pasture grows more rapidly than predicted by the models, possibly because of the increased turbulence generated by the strips of forest typical of this area. The data have also been used to initialise one dimensional climate models used in experiments to investigate the sensitivy of climate to land surface parameters, and to initialise a mesoscale model which can predict local effects on climate caused by the pattern of deforestation in this area.

RBLE1

RBLE2

RBLE3

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References

Available data

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