Current Airborne Radiometric Measurement Capabilities on NSF-NCAR Aircraft

Measurements of the solar and terrestrial radiation spectra from aircraft have numerous applications for atmospheric process studies. Multiple radiometric sensors are available for use on the NSF-NCAR C-130 and Gulfstream V aircraft, providing data for radiation budget studies, estimates of photolysis rates, and interpretation of hydrometeor, aerosol, and trace gas measurements.

The suite of airborne radiometers covers the electromagnetic spectrum in the visible to infrared range. Energy transfer through horizontal layers is characterized by various zenith- and nadir-viewing radiometers. Two Zeiss spectrometers provide spectrally resolved irradiance measurements in the visible to near-infrared spectral range (0.26 – 2.2 µm). Supplementing these spectrally resolved measurements of irradiance is a set of (broadband) pyranometers and pyrgeometers which extend the observations deeper into the infrared portion of the spectrum (0.2 – 42 µm). Manufactured by Kipp & Zonen, these sensors have been adapted in-house for airborne operations. In addition, zenith- and nadir-viewing radiation pyrometers in the 9.6-11.5 µm band (Wintronics model KT19.85) are available to measure radiometric surface temperature. Finally, measurements of energy flux through a sphere (actinic flux) are accomplished with a set of radiometers designed to be equally responsive to photons from all directions from 0.28-0.68 µm for photochemical rate studies.

Variations in aircraft attitude with respect to horizontal compromise the quality of airborne irradiance measurements by introducing artificial fluctuations into the data, making interpretation of those measurements difficult. To compensate for these aircraft motions, a set of stabilized platforms on which the radiometers are mounted has been designed and tested by the NSF-NCAR Research Aviation Facility. Integrated on the top and bottom of the C130 aircraft, the platforms actively level the Kipp & Zonen sensors based on input from the aircraft Inertial Navigation System (INS). The systems are capable of compensating for aircraft pitch angles of up to ±5° from nominal operating pitch and roll angles of up to ±10°. Platform performance is assessed by evaluating the stability of measurements during attitude changes experienced in research flights and during dedicated flight maneuvers.

The various sensors and calibration methods will be described in this presentation. Data examples and applications from the recent Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR) will also be shown.