Part II - Continuous Observations

Surface Layer



Radiation 10 m tower

The BLLAST field campaign focuses on the improved understanding of evening transition of the atmospheric boundary layer. During the transition, the radiation balance, defined as follows (K being the shortwave and L being the longwave part of the spectrum in W/m²),

,

changes drastically, and net radiation is most negative just after the transition. In addition, a series of recent studies has shown that apart from turbulent flux divergence, the divergence of longwave radiation is a substantial contributor to the heat budget (δθ/δt) close to the ground during the evening transition (Ha and Mahrt, 2003; Savijarvi, 2006; Steeneveld et al, 2010),

Some of these studies report even up to 3 Kh-1 of radiative cooling during transitions after clear skies calm summer days. At the same time, it is realised that atmospheric models have limited skill in the transition during low winds. In order to get a complete overview of the boundary layer heat budget, longwave radiation divergence need to be observed. Also, model skill of operational mesoscale models for longwave radiation divergence can be aiming at identifying model biases.

Objectives

Field activities

The field activities consist of two contributions, one by Wageningen Univ. (Netherlands, WUR from now on) and one by the Physikalisch-Meteorologisches Observatorium in Davos, World Radiation Center (PMOD/WRC, Switzerland.
WUR will deploy a tower instrumented by 5 levels of up- and downwelling longwave radiation. Instruments, Hukseflux IR02 pyrgeometers, will be installed at 8, 5, 2, 1, 0.5 m. This instrument has the advantage that it is equipped with heating, avoiding effects of dew deposition at night. Grass below the instruments should be kept relative constant and short (∼3-10 cm). Research has shown that in order to measure flux differences between different levels, the radiometers need to be calibrated relative to each other. Therefore, the instruments will be installed for at least one month at the same height. This will be done before or after the BLLAST experiment at the Wageningen observatory.



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