Abstract
Well-known cloud-screening algorithms, which are designed to remove cloud-contaminated aerosol optical depths (AOD) from Multifilter Rotating Shadowband Radiometer (MFRSR) and Normal Incidence Multifilter Radiometer (NIMFR) measurements, have exhibited excellent performance at many middle-to-low latitude sites around world. However, they may occasionally fail under challenging observational conditions, such as when the sun is low (near the horizon) and when optically thin clouds with small spatial inhomogeneity occur. Such conditions have been observed quite frequently at the high-latitude Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) sites. A slightly modified cloud-screening version of the standard algorithm is proposed here with a focus on the ARM-supported MFRSR and NIMFR data. The modified version uses approximately the same techniques as the standard algorithm, but it additionally examines the magnitude of the slant-path line of sight transmittance and eliminates points when the observed magnitude is below a specified threshold. Substantial improvement of the multi-year (1999–2012) aerosol product (AOD and its Angstrom exponent) is shown for the NSA sites when the modified version is applied. Moreover, this version reproduces the AOD product at the ARM Southern Great Plains (SGP) site, which was originally generated by the standard cloud-screening algorithms. The proposed minor modification is easy to implement and its application to existing and future cloud-screening algorithms can be particularly beneficial for challenging observational conditions.
Highlights
Atmospheric aerosol particles play an important role in modulating the Earth’s radiation balance by scattering and absorbing incoming solar radiation [1,2]
The expected improvement at the high-latitude North Slope of Alaska (NSA) sites should involve significant reduction of the cloud-contaminated points previously undetected by the original algorithms under the challenging observational conditions at these locations
Multi-year (1999–2012) aerosol optical depth and Angstrom exponent (0.415/0.87 μm) collected at high-latitude Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) sites have been used to illustrate the occasional failure of ARM-supported cloud-screening algorithms, which, have exhibited stable performance at other middle-to-low latitude ARM sites
Summary
Atmospheric aerosol particles play an important role in modulating the Earth’s radiation balance by scattering and absorbing incoming solar radiation [1,2]. Obtaining of aerosol properties with passive remote sensing requires confident identification and exclusion of potential cloud contamination. Such identification and exclusion—so-called cloud-screening—has received increased attention in the recent years. It has been applied when retrieving aerosol properties from ground-based measurements at both permanent and temporary sites supported by the Atmospheric Radiation Measurement (ARM). AOD is representative of the total aerosol burden in the atmosphere, and its spectral dependence, typically described by the Angstrom exponent (AE), indicates particle size [11]. Extinction by large particles shows a relatively small wavelength dependence in the visible spectral region. Small AE values (near zero) suggest the dominance of relatively large particles and larger AE values suggest a reduced contribution of large particles
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