Abstract
A method for determining rapid variations in atmospheric optical depth is proposed. The method is based upon computation of the ratio between close-time spectral measurements of solar direct flux. Use of the ratio avoids the need for absolute calibration of the instruments and minimizes the effects of changes in instrumental conditions (such as temperature or mechanical adjustments) and in air mass. The technique has been applied to some campaigns of measurement for sky conditions ranging from clear skies to scattered-to-broken cloudiness, performed at high frequency (~1Hz) with a system of three array spectrometers, capable of performing very rapid spectral acquisitions, in the 400 to 1700 nm band, thus covering the visible and extending to the near-infrared spectral ranges. Results demonstrate the capacity of this instrumentation and method to detect rapid variation of optical depth, as well as rapid changes in its spectral pattern. The optical depth variability depends on the particular state of the sky and is connected to particle condensation and evaporation processes and to the changes in water vapor content in the transition region between cloud-free and cloudy regions. Thus, the method is suitable for analyzing rapid processes involving particles, either aerosol or cloud droplets, and water vapor, in the cloud boundaries.
Highlights
Techniques based on the measurement of electromagnetic radiation in different spectral bands constitute the main way of monitoring atmosphere conditions and components from satellites, and from the ground
Shortwave radiation coming from the solar flux in the spectral range from the ultraviolet to the near-infrared is affected by the atmospheric gases and particles suspended in the air, in the form of aerosol or clouds, and can be used to derive atmospheric optical and physical properties of these atmospheric components
To overcome some of the above described weaknesses of the array spectrometers, concretely those related with instrument changes and absolute calibration, the ratio between consecutive close-time raw measurements was used
Summary
Techniques based on the measurement of electromagnetic radiation in different spectral bands constitute the main way of monitoring atmosphere conditions and components from satellites, and from the ground. Array-based spectrometers have, over scanning instruments, the major advantage (besides reduced costs) of relatively rapid acquisition of radiation spectra, as the whole spectrum is acquired in one single step. These instruments show great potential to perform measurements in rapidly changing conditions, for instance, when a scattered-to-broken field of clouds occurs in the atmosphere or in the aerosol-droplet continuum in cloud boundaries. Special attention has been devoted to assessing the uncertainties in the measurements and to describing some limitations of the method proposed
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