Part II - Continuous Observations
|Turbulence and mean meteorological parameters |||Radiation 10 m tower |||Microbarometers-Sonic |||Scintillometers|
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.
- To measure the radiation balance at the surface.
- To measure radiation divergence, and examine its relative contribution in the heat budget.
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
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.