Turbulence and mean meteorological parameters
In the BLLAST experiment, 9 masts will be equipped with a total of 26 instruments measuring turbulence. Measured properties are: wind, temperature, CO2 concentration and water vapour concentration. The first aim of all the stations is to get a clearer picture of the fluxes in the diverse landscape that is the BLLAST experimental site and integrated in the airborne measurements. Beyond this purpose, most of the surface stations are implemented to also deal with other objectives.
- The southern Corn, moor and forest EC-stations will sample three contiguous large areas with relatively homogeneous vegetation . The moor site has some heterogeneity in the vegetation, but this is uniform throughout the field. These three sites are devoted to study the impact of surface heterogeneities on the late afternoon transition. The surface energy balance time-shift between the different vegetation cover and the secondary circulation between the fields during this phase of the day will be investigated. The EC-data here are complemented with tethered balloons and unmanned aircraft data. In addition, the forest site tower will be instrumented at two levels, just above the canopy and 5 meters above, to study the flow near the canopy.
- The Micro-site main focus is the small surface heterogeneities study. Over a 100m square flat surface, bare soil, small bushes, grass, and small puddles constitute a very heterogeneous surface whose soil characteristics (temperature, humidity) will be extensively mapped. Added to the Ec- station, the temperature vertical profile up to 1.5m will be measured.
- The 60m mast is equipped with quick (10Hz) sensors at three levels. With this mast, a vertical profile can be made in and above the surface layer. This mast is a permanent platform at the CRA and provides year-round flux-data.
- The Skinflow-mast is a 10m high mast equipped with a total of 6 sonic anemometers. The aim of this set-up is to investigate near-surface divergence of the sensible heat flux. The lowest sonics are at a height of less than one meter. Complimentary to the sonic anemometers is a set of fast thermocouples, close to the ground. A twin tower will be settled near the skinflow-mast to measure the radiative divergence.
- At the edge site, a set-up of three masts, all equipped with sonic anemometers and fast water vapour and CO2 sensors, is used to validate a flux-footprint model. Two stations will be set up in a grass and wheat field respectively and one station will be located at the edge. With a model that includes the land-uses in the surrounding area, representative fluxes can be reconstructed.
The following table presents the vegetation cover, the site latitude, longitude and height above the sea level and the properties measured at all EC-station.
| Site name | Land use | Latitude (N) Longitude (E) | Height ASL [m] | Properties measured |
|---|---|---|---|---|
| Corn (Site 2) | Corn | 43°05’24 » 0°21’30 » | 645±5 | Temperature, wind, H2O, CO2, pressure, radiation, rain, soil temperature, soil humidity, soil heat flux |
| Moor (Site 2) | Mixed moor-like vegetation | 43°05’24 » 0°21’42 » | 641±3 | Temperature, wind, H2O, CO2, pressure, radiation, rain, soil temperature, soil humidity, soil heat flux |
| Forest (Site 2) | Douglas Spruce | 620 | Temperature, wind, H2O, CO2, pressure | |
| 60-m tower (Site 1) | Mixed | 43°07’27.15″ 0°21’45.33″ | 602 | Temperature, wind, H2O, CO2, pressure, radiation |
| Micro (Site 1) | Grass and shrubs | 43°07’26 » 0°21’51.3″ | 601 | Temperature, wind, H2O, CO2, pressure, radiation |
| Skinflow (Site 1) | Grass | 43°07’39.2″ 0°21’57.4″ | 591±5 | Temperature, wind |
| Edge (Site 1) | Wheat, rhye and peas | 43°07’56.0″ 0°21’37.3″ | 582 | Temperature,wind,H2O, CO2, pressure, wet-bulb-temperature, soil temperature, soil humidity, soil heat flux |
| Edge (Site 1) | Grass | 43°07’52.5″ 0°21’33.9″ | 581 | Temperature,wind,H2O, CO2, pressure, wet-bulb-temperature, soil temperature, soil humidity, soil heat flux |
| Edge (Site 1) | Mixed | 43°07’53.4″ 0°21’35.2″ | 581 | Temperature, wind, H2O, CO2, pressure |
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
- To measure the radiation balance at the surface.
- To measure radiation divergence, and examine its relative contribution in the heat budget.
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.
Microbarometers-Sonic
During the BLLAST field campaign (14 June to 8 July), three microbarometers PAROSCIENTIFIC (Model 6000-16B) will be deployed in supersite 1 (See Fig. 7). The configuration will be forming a triangular array of 150m aprox. and at 1m a.g.l.. In order to avoid contamination from wind speed, static pressure ports (GILL 230-61002) are connected to the microbarometers, so that almost all the dynamic perturbation produced by the wind is filtered. These high precision digital instruments can detect very small pressure perturbations, of the order of thousandths of hPa. During BLLAST, a sampling rate of 2Hz is chosen as a compromise between having a good temporal resolution and registering small enough pressure perturbations (with this sampling rate the resolution is around 0.002 hPa).
The objective is to study the small scale static pressure fluctuations produced in the atmospheric boundary layer. These fluctuations can be due to turbulent motions (high frequency) and also to the propagation of waves of different types (gravity waves, Kelvin-Helmholtz instabilities, etc), produced by different mechanisms such as orographic forcing, fronts, convective forcing, geostrophic adjustment, shear instability, etc. The typical amplitudes of the pressure fluctuations related to waves is in the range of 0.01-0.1 hPa with a periodicity ranging from 1 to 40 minutes, although some episodes of intense mesoscale gravity waves with amplitudes up to 2-3 hPa have been reported.
The array configuration of the three microbarometers can be used during specific periods of the field campaigns, to detect and characterize wave events by means of lag analysis, cross-correlation or methods based on wavelets decompositions (wavelength, phase speed and direction of propagation of these waves can be evaluated).
Additionally, an Ultrasonic Wind Sensor, uSonic-3 Scientific (previously USA-1) (METEK) will be deployed above the Microbarometer A, at z=2.4m agl measuring the 3 velocity components (u, v and w) and sonic temperature (T) at 20 Hz sampling rate. 
Fig. 7: Deployment of the microbarometers array in BLLAST (site 1)
Scintillometers
Scintillometers allow us to measure heat fluxes along a path which separates the emitter from the receiving telescope. The fluxes result from eddies which are crossing the path along a given time window. Thus, it is an integrated measurement of surface fluxes.
Three scintillometers will be settled during BLLAST:
- On the « edge site » at site 1, a mini-scintillometer from MAQ (University of Wageningen), installed from one field to the other, with a 40 m path across the edge.
- Another 3 km path scintillometer from MAQ was installed between the top of the church of Campistrous and the roof of site 1 main building.
- A large aperture scintillometer will be installed by Météo-France GAME/GMEI, between the cheminée of the KNOF industry company and the roof of site 1 main building, 4 km apart one from the other.