Monday, 10 February 2003
A new automated method of MFRSR-based optical depth analysis
Modern robotic spectral solar instruments designed for aerosol optical depth (AOD) retrievals, such as the Multi-Filter Rotating Shadowband Radiometer (MFRSR), operate in an unattended mode. Their raw data sets sample a wide range of atmospheric conditions, most of which are undesirable for spectral channel calibration via the Langley method, i. e., extrapolation to the extraterrestrial, or zero air mass, signal. Recently, a method that utilizes component solar measurements (direct and diffuse) to identify totally clear-sky and non-hazy periods has been successfully used to screen MFRSR data for calibration Langley plots. This method of clear-sky detection is completely automated, thus making it attractive for operational use. Periods identified as clear represent times of unobscured measurements and clear views of the sun, thus mimicking the process by which manual sun photometer measurements are made, i. e., only when clear views of the sun are possible. Several of these calibration Langley plots over a multi-week period provide a stable sample of zero air mass intercepts (Il0) from which a reliable mean, or representative Il0 is computed. Within this period, the calibration Il0 may be combined with any MFRSR measurements to construct two-point Langley plots from which time series of AOD are computed. Ancillary components of the clear-sky analysis results may also be used to screen out cloud contamination in the operational measurements that can't be used for AOD retrievals. Although this method is completely automated, it has been shown to work only in a test-of-concept mode. It has not been applied operationally, nor has it been applied to more than one location. In this work, this new method of MFRSR calibration and aerosol optical depth analysis is applied and tested operationally for several locations, and at various times of year. Results are compared to independent aerosol optical depth measurements from other MFRSR networks, and from the U.S. component of AERONET.