Thirty Years of Atmospheric Extinction from Telescopes of the North Atlantic Canary Archipelago

Abstract

This study examines 30 years of atmospheric extinction, τ, obtained from both stellar and solar telescope measurements, at ~2.4 km MSL, from the North Atlantic Canary Archipelago—an island chain located at approximately 28°N, around 100 km from the west coast of Africa. Data from three AERONET monitors, located at varying heights on one of the main islands, were also used, although these are only available over a shorter (<10 yr) period. The Canary Archipelago is regularly affected by dust intrusions into the local atmosphere as they intersect one of the primary export pathways of mineral dust from the Sahara. The τ of “baseline” and “dust influenced” conditions were statistically distinguished by fitting normal-gamma mixture distributions to the observations using Markov chain Monte Carlo methods, and then the seasonal and long-term characteristics of these data were examined. The telescope data show that baseline conditions are usually stable at τ < 0.1 (except during periods influenced by volcanic aerosols) and indicate the existence of a low-amplitude () seasonal variation. During dust-influenced conditions, τ regularly reaches values of a factor of 2–6 times higher than normal. The majority of dust intrusions take place during the months of July and August, when they may occur 44 ± 15% of the time, predominantly at high altitudes (with ~94.3 ± 1.6% of intrusions occurring ≥ 2.4 km), whereas during the months of November–May, dust intrusions occur far less frequently (~19 ± 7%) and are more common at lower altitudes—with intrusions at <2.4 km comprising ~ 79.5 ± 3.2% of all outbreaks. Year-to-year variations in the frequency of dust-influenced conditions (of ~9%) were found but no long-term trend over the observed 30-yr period.

Atmospheric Extinction Coefficients in the Ic Band for Several Major International Observatories: Results from the BiSON Telescopes, 1984–2016

“…If the summer and winter periods are analysed separately, then the winter does indeed show a mean of approximately 0.05 to 0.06, and the summer a much higher mean of 0.14 to 0.15 with correspondingly higher standard deviation. A similar atmospheric analysis on data from telescopes on the Canary Archipelago, which included our BiSON data from this instrument, was made by Laken et al (2016) and the modal values are consistent given the uncertainties. Laken et al (2016) provides a thorough investigation into the occurrence of dust events, and how they change over both short seasonal periods and longer time scales.…”

“…A similar atmospheric analysis on data from telescopes on the Canary Archipelago, which included our BiSON data from this instrument, was made by Laken et al (2016) and the modal values are consistent given the uncertainties. Laken et al (2016) provides a thorough investigation into the occurrence of dust events, and how they change over both short seasonal periods and longer time scales. Several authors have investigated this in more detail {see, e.g., Guerrero et al (1998) site based at Izaña.…”

Recent changes and drivers of the atmospheric evaporative demand in the Canary Islands

“…Nevertheless, it is also worth noting that global radiation measurements and sunshine duration records contain a signal of the direct effects of aerosols (Sanroma et al, 2010;Sanchez-Romero et al, 2014;Wild, 2015) in the Canary Islands. Nevertheless, the Canary Islands is a region mostly free of anthropogenic aerosols given the large frequency and intensity of trade winds (Mazorra et al, 2007), and it is not expected that the frequency of Saharan dust events, which could affect incoming solar radiation, has noticeably changed over the last decades (Flentje et al, 2015;Laken et al, 2015). Consequently, in the Canary Islands we can consider high accuracy in determining the radiative component using sunshine duration series.…”

“…The PM method can be used globally, and has been widely verified based on lysimeter data from diverse climatic regions (Itenfisu et al, 2000;López-Urrea et al, 2006). Allen et al (1998) …”

“…Although these variables are generally poorly measured and highly inhomogeneous over both space and time, numerous regional studies analysed the evolution of the AED by means of the robust Penman-Monteith (PM) equation using long time series of these variables. The available regional studies show quite contradictory results, where some studies showed AED negative trends, including those in China (Xu et al, 2006;Ma et al, 2012;Zhang et al, 2007;Liu et al, 2015) and northwestern India (Jhajharia et al, 2014). In contrast, other regional studies have found positive trends in AED, including those in central India (Darshana et al, 2012), Iran (Kousari and Ahani, 2012;Tabari et al, 2012), Florida (Abtew et al, 2011), continental Spain (Espadafor et al, 2011;Vicente-Serrano et al, 2014a;Azorin-Molina et al, 2015), France (Chaouche et al, 2010) and Moldova (Piticar et al, 2015).…”