Development of new shipborne aureolemeter to measure the intensities of both direct and circumsolar radiation.

Abstract:

Atmospheric aerosols play important role in the Earth’s radiation budget through the scattering and absorption of solar radiation (direct effect) and modification of cloud properties (indirect effect). A global understanding of the spatial and temporal variations of aerosol optical thickness (AOT) and aerosol optical properties is necessary for assessment of it. Oceans cover about 70% of the Earth’s surface and produce a large quantity of natural aerosols.

The new shipborne aureolemeter was developed to improve the sun-tracking performance for accurate measurements of not only direct but circumsolar radiation, even on a vessel weltering. Sun position is determined by a real-time image processing system with a CCD camera. A round shape is extracted from the captured image. The position of the center of gravity of the round shape is used as the sun position. The accuracy of the sun paosition determination is finer than 0.01˚. The radiometer is tracked the sun under a feedback control with the derived sun position. For a sky radiance distribution measurement, the control target position on the CCD camera image is shifted a pixel corresponding to a measuring scattering angle. In the case of a scattering angle larger than 7˚, the radiometer’s tracking is conducted under feedforward control on the basis of the angle of roll and pitch monitored with a gyroscope. To decide the solid angle of the radiometer, the radiance around the sun was measured in the angle range between the sun and sensor directions from -1.5˚ to +1.5˚ with 0.1˚ resolution. The instrument constants were determined from a Langley plot method. Now, we are conducting a comparison observation between the developed shipborne aureolemeter and an existing sky radiometer.