Abstract
Abstract. Visibility is an indicator of atmospheric transparency, and it is widely used in many research fields, including air pollution, climate change, ground transportation, and aviation. Although efforts have been made to improve the performance of visibility meters, a significant error exists in measured visibility data. This study conducts a well-designed simulation calibration of visibility meters, which proves that current methods of visibility measurement include a false assumption, leading to the long-term neglect of an important source of visibility error caused by erroneous values of Ångström exponents. This error has two characteristics, namely (1) independence, which means that the magnitude of the error is independent of the performance of the visibility meter. It is impossible to reduce this error by improving the performance of visibility meters. The second characteristic is (2) uncertainty, which means the magnitude of the error does not show a clear change pattern but can be substantially larger than the measurement error of visibility meters. It is impossible to accurately estimate the magnitude of this error or its influence on visibility measurements. Our simulations indicate that, as errors in visibility caused by erroneous values of Ångström exponents are inevitable using current methods of visibility measurement, reliable visibility data cannot be obtained without major adjustments to current measurement methods.
Highlights
Visibility is a fundamental meteorological parameter which is widely used in research related to synoptic meteorology, air quality, climatology, human health, and fields closely related to daily life such as ground transportation, aviation, and navigation (Che et al, 2007; Huang et al, 2009; Hyslop, 2009; Konstantopoulos et al, 2010; Li et al, 2016)
In accordance with the calibration process of visibility meters, we used aerosol optical depth (AOD) data at 0.87 and 0.44 μm to obtain a scheme for calculating the Ångström exponent; we determined the function for calculating AOD at 0.44 μm from the measured AOD0.87 data, and we evaluated the deviation of calculated values (i.e., AOD data calculated to 0.44 μm) from the measured values
The meteorological optical range (MOR) is the visibility measurement benchmark set by World Meteorological Organization (WMO), but its measurement wavelength is neither the same as the wavelength of maximum sensitivity of the human eye nor the measurement wavelength of common visibility meters
Summary
Visibility is a fundamental meteorological parameter which is widely used in research related to synoptic meteorology, air quality, climatology, human health, and fields closely related to daily life such as ground transportation, aviation, and navigation (Che et al, 2007; Huang et al, 2009; Hyslop, 2009; Konstantopoulos et al, 2010; Li et al, 2016). The conclusion reached in this guide states that “Visibility and MOR should be equal if the observer’s contrast threshold is 0.05 (using the criterion of recognition) and the extinction coefficient is the same in the vicinity of the instrument and between the observer and the objects” (WMO, 2018). This conclusion implies that there is no need to consider a deviation from the true values of visibility measured at a wavelength of λ0 (vλT0 ) when converting the visibility measured at λ1 (vλ1 ) to λ0 (vλ0 ), using Eq (3); i.e., the errors in the values of q are negligible. Considering the possible range of q in the atmosphere, it can be inferred that errors in visibility caused by erroneous values of q cannot be ignored; apparent errors exist in the error estimates of current visibility measurements
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