PNNL

Abstract

A novel ground-based radiometer, referred to as the Shortwave Array Spectroradiometer-Hemispheric (SAS-He), is introduced. This radiometer uses the shadow band technique to report total irradiance and its direct and diffuse components frequently (every 30 sec) with continuous spectral coverage from 350-1700 nm and moderate spectral (~2.5 nm ultraviolet/ visible, and ~6 nm shortwave-infrared) resolution. The SAS-He’s performance is evaluated using integrated datasets collected over coastal regions during three field campaigns supported by the U.S. Department of Energy’s (DOE’s) Atmospheric Radiation Measurement (ARM) Program, namely (1) Two-Column Aerosol Project (TCAP), (2) Tracking Aerosol Convection Interactions Experiment (TRACER), and (3) Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE). We compare (i) aerosol optical depth (AOD) and total optical depth (TOD) derived from the direct irradiance, (ii) the diffuse irradiance, and diffuse-to-direct ratio (DDR) calculated from two components of the total irradiance. As part of the evaluation, both AOD and TOD derived from the SAS-He direct irradiance are compared to those provided by collocated Cimel sunphotometer (CSPHOT) at five (380, 440, 500, 675, 870 nm) and two (1020, 1640 nm) wavelengths, respectively. Additionally, the SAS-He diffuse irradiance and DDR are contrasted with their counterparts offered by collocated Multi-Filter Rotating Shadowband Radiometer (MFRSR) at six (415, 500, 615, 675, 870, 1625 nm) wavelengths. Overall, reasonable agreement is demonstrated between the compared products despite the challenging observational conditions associated with varying aerosol loadings and diverse types of aerosols and clouds. For example, the AOD- and TOD-related values of root-mean-square error are within the expected measurement uncertainty of AOD (0.01-0.02).