Satellite detection of hazardous volcanic clouds and ... - Savaa - NILU

Nat Hazardsdominating at latitudes north of the equator in July. Similar plots of the mean meridionalwind component for January and July are shown in Fig. 5a, b. Meridional winds in Januarytend to be more northerly and can reach 6 m s -1 in the high latitudes of the winterhemisphere. Equatorial winds are weak at both times of the year. Some general commentscan be made about the likely spread of volcanic clouds that reach 10 km altitude based onthese mean wind circulation patterns. For eruptions that occur near equatorial latitudes inJuly, the dispersion will most likely be westwards with little transport polewards. In factthis was observed for the eruption of Pinatubo for clouds that reached much higher altitudes,since the wind structure at 10 km is similar up to 30 km or so (see Fig. 6). Eruptionsthat occur at higher latitudes will spread volcanic debris in directions and at speeds thatdepend on the hemisphere and the season. In July, Chilean eruption clouds will travelrapidly eastwards and southwards and it is very unlikely the clouds will spread to thenorthern hemisphere and more likely they will be confined to circumnavigate the southernhemisphere at latitudes south of 30° S. This was observed for the August, 1991 eruption ofMt Hudson, which spread volcanic clouds eastwards towards Australia causing some minoraviation incidents (Barton et al. 1992). Eruptions occurring in Japan and northwards alongthe Kamchatkan peninsula will spread clouds eastwards and at greater speeds in Januarythan in July. The meridional winds will take these clouds southwards in summer (July) andnorthwards in winter (January). The north-eastward spreading of volcanic clouds emanatingfrom Japanese or Kamchatkan volcanoes in winter is likely to create greater risk foraviation using the busy north Pacific air routes that connect Japan to the USA.The main air corridor connecting air traffic between USA and continental Europe liesbetween latitudes 45–65° N and crosses the north Atlantic—the so-called North AtlanticTracks (NATs). Volcanic eruptions from Iceland could potentially disrupt air traffic usingthe NATs. Greatest impact would occur when meridional winds are northerly and zonalwinds weak. This would bring clouds southwards directly intersecting the NATs. A wintertimeeruption fits this scenario better than a summertime event. At lower altitudes (\5 km)the meridional winds in January tend to be southerly, keeping lower altitude eruptionclouds away from Europe and the main air routes. A more typical trajectory for volcanicclouds originating from Iceland is transport eastwards with northward confinement andcircumnavigation of the pole. While this trajectory has been documented (Carn et al.2008), it is a much less serious threat to aviation, due to the low density of air traffic atthese high latitudes. Another possible trajectory for Icelandic eruption clouds consists ofrapid eastward and then southward transport which brings the clouds over eastern Europeand into the busy continental air routes. This trajectory has also been documented from aneruption of Grimsvotn in 2004 (Witham et al. 2007), which caused some grounding ofEuropean aviation but no serious incidents were reported.Over the busy (especially for air freight) trans-Pacific air routes between Japan and Asiaand continental USA, the main threat is from Japanese volcanoes, volcanoes in Kamchatkaand volcanoes along the Aleutian chain and into south-western Alaska. The whole region isvolcanically active and there have been several aviation related incidents with ash cloudsgenerated by volcanoes in this region. At most times of the year the zonal winds traveleastwards, taking volcanic debris from Japan/Kamchatka towards continental USA andinto higher density air traffic. Two well-documented cases of aviation incidents haveoccurred in this region, both during the winter season. In the case of the KLM 747commercial jet on route to Anchorage, Alaska from Amsterdam, Netherlands the encounteroccurred in December 1989 and close to the Mt Redoubt volcano (60.485° N, 152.742° W)(Casadevall 1994). The aircraft was travelling south-eastwards towards the ash plume,which was moving north-westwards, with the mean winter winds (see Figs. 4a and 5a).123