Abstract
Volcanic ash can travel great distances and sometimes persist for several weeks, necessitating the monitoring of large areas for potential aviation hazards. Ash can affect aircraft in several ways, from engine failure to manageable impacts if the ash concentration is low. The location and properties of even low levels of ash are, therefore, required in order to make informed safety decisions. Infrared (IR) satellite-borne sensors are vital to this monitoring because of their high spatial coverage day and night. Interpretation of IR ash observations generally relies on exploiting the difference between an observation and the observation anticipated for the same scene under ash-free conditions, the so-called ash signal. We apply forward-model observations of andesitic ash with a fixed particle size distribution to investigate the sensitivity of ash signals to the ambient atmosphere. The variability in the ash signals attributable to atmospheric effects is found to be at least as high as the variability attributable to differences in ash concentration and altitude, implying that different concentrations could be retrieved if atmospheric effects were ignored. Ash with different optical properties is likely to correspond to a slightly different ash signal and may exhibit a different sensitivity to atmospheric effects to that presented here.
Highlights
The serious threat posed to aircraft by volcanic ash even at large distances from the eruption site is widely recognized.[1]
The mean plots show the mean signals produced by ash under different atmospheric conditions and show that a different relationship exists between concentration, altitude, and the ash signal when the ash is present in a different type of atmosphere (Figs. 4 and 5)
The signal produced by the same ash concentration, at the same altitude, with the same composition can vary greatly when the ash is present in an atmosphere with different properties, even if the two atmospheres being compared share similar characteristics
Summary
The serious threat posed to aircraft by volcanic ash even at large distances from the eruption site is widely recognized.[1]. Sensors with a wide range of detection capabilities were used to monitor the ash during the Eyjafjallajökull crisis, exploiting already existing techniques as well as developing new ones. Some established methods for ash detection and monitoring use visible and ultraviolet (UV) imagery (e.g., see Ref. 4), while others consider only infrared (IR) (e.g., see Ref. 5), and further approaches combine the information from these different parts of the spectrum It is likely that in the daytime, with the addition of information from visible and UV measurements, ash can be more reliably detected and its properties can be more reliably retrieved in all atmospheric conditions
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