sashemfr (b1)

Abstract

The shortwave array spectroradiometer—hemispheric (SASHE) uses a hemispheric spectralon diffuser and rotating shadowband similar to the MFRSR and RSS instruments, so it also provides spectrally resolved, direct-normal, diffuse-horizontal, and total-horizontal irradiances, and can be calibrated in situ via Langley regression. The SASHE incorporates two Avantes fiber-coupled spectroradiometers (Avaspec ULS 2048 CCD and Avaspec NIR256-1.7) for visible and near-infrared detection in the wavelength range 350-1700 nm. The spectral resolution is 2.4 nm for the ULS 2048 CCD and 6 nm for the NIR256-1.7. Like the SWS and SASZE, an external collector mounted outdoors is connected via fiber-optic cable to a pair of spectroradiometers housed indoors, so the entire spectral range is measured simultaneously for both spectroradiometers. For the typical shadowband measurement, the spectroradiometers are operated with one measurement per second, but the entire shadowband sequence takes about 20 seconds, so this is the measurement interval for the irradiances. The SASHE operating sequence is configurable and supports alternate programmed sequences as a function of solar zenith angle. For example, it currently employs different sequences for high sun angles, low sun angles, and twilight conditions when the sun is below the horizon but the sky is not yet fully dark. It also includes a mode where visible spectra are collected rapidly while the band is moved over a range of near-sun angles to characterize the forward-scattered radiation. The SASHE data are preliminarily calibrated through comparison with calibrated lamps. The accuracy of irradiance produced by the SASHE is expected to be better than ±5%, which includes calibration and instrument stability errors. Under suitably uniform atmospheric conditions, higher calibration accuracy and precisions (±1%) can be obtained when the responsivity is tied to the solar extraterrestrial irradiance via Langley regression plots. This approach is recommended when high-precision measurements of optical depth are required.