FAAM CORE Radiometers, performing total, red and infrared upwelling/downwelling radiation measurements
 
The FAAM broadband radiometer instruments include 2 clear dome pyranometers, 2 red dome pyranometers and two pyrgeometers. One each of these instruments are mounted on the upper and lower surfaces of the aircraft to measure downwelling and upwelling radiation. The clear and red measurements are made using modified versions of Eppley pyranometers and the infrared measurements are made using purpose built Foot Pyrgeometers.

Pyranometers

The pyranometers fitted to the aircraft are essentially the model PSP instrument made by the Eppley Laboratory in the USA which are widely used for ground based solar radiation measurements. They are however made from cast aluminium instead of bronze to reduce the aircraft installation weight and give good thermal response to temperature changes (Foot, 1982). The thermopile detector is made from a coiled constantan wire which is partially plated with copper to produce a set of junctions half of which are thermally connected to the blackened detector surface and half to the instrument body which acts as a heat sink. The sensitivity of this thermopile is of the order of 10 µV W-1m2. The detector is covered with a double glass dome arrangement which serves two purposes, firstly it protects the surface of the thermopile and secondly it defines the operating wavelength range. Two types of Schott glass are used for the outer dome, WG295 (clear dome) and RG715 (red dome). The numerical part of the identifier is the cut-on wavelength (nm) and both glasses transmit out to about 3000 nm. The inner dome (WG295 in both cases) improves the stability of the instrument by reducing the longwave radiation exchange between the outer dome and the thermopile. A removable dessicant holder contains silica gel to keep the inside of the instrument dry thus avoiding condensation on the inside of the dome. The sensitivity of the thermopile is temperature dependent and the manufacturer incorporates a thermistor-resistor compensation circuit. According to the Eppley data this reduces the variation to better than ±1 % for the sink temperature range -40°C to +30°C. In addition to the compensation circuit the MRF instruments are fitted with an additional thermistor to monitor the sink temperature and this information is available as a separate signal.

Pyranometer calibration

The pyranometers are calibrated annualy by comparison with two Kipp and Zonen pyranometers with a spectral range of 300 to 2800 nm which are in turn calibrated by the Met Office Calibration Labs against a pyrheliometer. It is assumed that the signal voltage is proportional to the incoming radiation flux so the calibration uses the flux measured by the kipp and Zonen instruments to define the constant of proportionality. To calibrate the red pyranometers the red domes are replaces with clear WG295 domes.

Data

Pyranometer data is part of FAAM's core data set which can be found on CEDA.
 
Following ascents and descents the pyranometers can be out of thermal equilibrium (th einner dome temperature may take longer to stablise than the outer dome) which can cause errors and negative values in the data. Therefore it is recommended that the data is used in straight and level runs only. For more information please refer to the attachedMRF technical note, which also suggests the pyranometers are allowed 5 minutes following a profile to stablise.
 
 
To help ensure good BBR data during a campaign there should be (1) regular cleaning of the domes, (2) a box pattern calibration and (3) regular pirouettes on the run way.

Pyrgeometers

The Foot pyrgeometer was designed after standard Eppley pyrgeometers were found to function below their optimal performance on an aircraft which regularly changed altitude moving into different temperature regimes. It consists of a circular detector divided into 8 slices alternately blackened or coated with gold. The detector is house below a glass dome from an Eppley pyrgeometer. The inner surface of the dome is coated with a vacuum-deposited interference filter providing transmission over the range of approximately 3.5 to 50 micrometres. The Instrument uses a spiral themopile with adjacent junctions below blackened and reflective slices of the detector. In this way the thermopile measures the temperature difference between blackeded and reflective portions of the detector and the IR flux is derived from this measurement.

Pyrgeometer calibration

The facilities to reliably calibrate the pyrgeometers do not exist at FAAM and therefore the data from these instruments should only be used for qualitative analysis only. In particular the instruments take some time to equilibrate when the temperature changes (of the order minutes), the variation of their sensitivity with temperature is unquantified and they tend to suffer from occasional step changes in their calibration caused by parts of the gold coating delaminating.

Further Readings

McConnelll, C. L. (2009). Aircraft Measurements of Saharan Mineral Dust. Phd Thesis.
 
Saunders, R., Foot, J., Kilsby, C., and Seymour, J. (1991). Report of broad-band radiative fluxes working group. Technical report, UK Met Office. MRF Technical note no. 5.
 
Saunders, R., Brogniez, G., Buriez, J., Meerkötter, R., and Wendling, P. (1992). A Comparison of Measured and Modeled Broadband Fluxes from Aircraft Data during the ICE’89 Field Experiment. Journal of Atmospheric and Oceanic Technology, 9(4), 391– 406.

Further Details

Contact Alan Woolley at FAAM

Additional information