OCTOBER 2004 HADLEY ET AL. 829

Resuspension of Relic and Dust from Katmai: Still an Aviation Hazard

DAVID HADLEY AND GARY L. HUFFORD Aviation Weather Unit, National Weather Service, Anchorage, Alaska

JAMES J. SIMPSON Digital Image Analysis Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

(Manuscript received 28 October 2003, in ®nal form 29 March 2004)

ABSTRACT Northwest winds were strong enough to continuously resuspend relic volcanic ash from the Katmai cluster and the Valley of Ten Thousand Smokes on 20±21 September 2003. The ash cloud reached over 1600 m and extended over 230 km into the Gulf of Alaska. Several factors in¯uenced the resuspension of the ash: 1) the atmosphere and land surface were very dry prior to the event, further enabling the resuspension and subsequent atmospheric transport of the relic volcanic ash; 2) the production of winds strong enough to entrain and lift the ash over 1600 m into the atmosphere; 3) the complex terrain with numerous mountains interspersed with valleys, channels, and gaps; 4) the superadiabatic lapse rate for the troposphere below 850 mb; and 5) the presence of a strong subsidence inversion around 1400±1600 m. The authors propose that the strong winds are due to accelerations in a superadiabatic atmosphere below 850 mb that is buoyant to both upward and downward perturbations resulting in a hydraulic ¯ow that exposes the lee side of the mountains to sweeping, high-speed turbulent winds near the base of the lee slope. Some unique features of the ash cloud are also examined, including its hazardous nature to aviation. Finally, this paper provides the forecaster with the ability to 1) recognize the conditions needed for relic volcanic ash resuspension and 2) respond immediately to such an event.

1. Introduction and disruption to community infrastructure (Warrick et al. 1981; Blong 1984). The transport of ®ne-grained dust by strong winds Resuspension of ash even after it has fallen to the has been observed and reported over a broad range of ground can be as hazardous as new ash from an erupting spatial scales (see online at www.osei.noaa.gov/Events/ volcano (Sparks et al. 1997). Most studies of resus- Dust). Two major sources of the dust are Asia (Gobi pended ash involve recent eruptions and usually ex- and Mongolian deserts) and Africa (Sahara). Dust events amine only local affects. However, on 20±21 September Ϫ1 occur when friction from surface winds (Ͼ5ms ) 2003, a unique set of conditions produced a very large entrains and lifts the dust particles into the atmosphere resuspension of relic volcanic ash (and probably some and transports the dust across either the North Paci®c dust) from an area around and including the Valley of or the tropical Atlantic Oceans, respectively (Gillette Ten Thousand Smokes in Katmai National Park and 1978). Dust from these large-scale events can affect Preserve, Alaska. The ash cloud extended from the val- radiative forcing and climate (Myhre and Stordal 2001), ley south-southeastward off the coast, over Kodiak Is- biogeochemical cycles (Chadwick et al. 1999), public land, and into the Gulf of Alaska. This paper describes health (Schwartz et al. 1999), and aviation safety (Simp- this event and examines the atmospheric processes that son et al. 2003). generated it. Impact on regional aviation is also cited. Airborne volcanic ash from eruptions is a major threat to aviation safety at all scales (Casadevall 1994; Miller and Casadevall 1999; Hufford et al. 2000; Simpson et 2. Source region of 20±21 September 2003 al. 2000). Like ®ne-grained mineral dust, volcanic ash resuspended ash event affects radiative forcing and climate (Pollack et al. Katmai National Park and Preserve, located on the 1976), public health (Bates and Beggs 1997), vegetation , is one of the most active volcanic (Kobayashi et al. 1988), and can cause property damage regions in the world (Fierstein and Hildreth 2001). There are eight volcanoes in the park that are known as the Corresponding author address: Dr. Gary L. Hufford, NWS Alaska Katmai volcano cluster: Snowy Mountain, Mount Region, Box 23, 222 W. 7th Avenue, Anchorage, AK 99513. Griggs, Mount Katmai, Mount Martin, , E-mail: [email protected] Volcano, , and Alagogshak

᭧ 2004 American Meteorological Society

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FIG. 1. (top) Location of Katmai volcanic cluster on the Alaskan Peninsula. (bottom) Ash fall (red contours) from the 1912 Novarupta eruption. Figure from Fierstein and Hildreth (2000). (Reproduced here courtesy of J. Fierstein.)

(Fig. 1). These volcanoes form a 25-km-long line of from these volcanoes have deposited volcanic ash in the contiguous stratovolcanoes on the drainage divide of Katmai region at least 15 times in the past 10 000 yr. the Alaska Peninsula. Typical elevations of these vol- The 1912 Novarupta eruption, the largest of the twen- canoes range from 1830 to 2320 m. Major eruptions tieth century, produced at least 17 km3 of ash fall de-

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FIG. 2. Valley of Ten Thousand Smokes ash ¯ow sheet (yellow), which erupted from Novarupta in Jun 1912 overlaid on local topography. (Figure provided courtesy of U.S. Geological Survey.)

posits and 11 km3 of ash ¯ow (pumiceous pyroclastic of the ¯at ¯oored valley (Fig. 1), which still remains ¯ow) in about 60 h (Hildreth 1983). The Novarupta vent largely vegetation free (Hildreth 1987). is located on the north foot of Trident Volcano. This Ground-based observers (tourists/park rangers) have 1912 eruption is virtually unique among major historical reported seeing from time to time the resuspension of eruptions in that it generated a large volume ash ¯ow some volcanic ash from the ground in Katmai National that all came to rest on land. The ash ¯ow moved like Park and Preserve. However, resuspension on the scale a sheet northwestward from Novarupta and formed the of the 2003 event described herein is rare. The only Valley of Ten Thousand Smokes, which covers an area potential evidence of a similar past event in Alaska is of about 120 km2 (Fig. 2). The thickness of the ash in suggested by Riehle et al. (1987) for the Aniakchak the valley varies up to about 250 m in depth. Griggs, Volocano. A pilot reported volcanic ash over Port Hei- Katmai, Trident, and Mageik partially surround the head den, Alaska, near the volcano. There was no coincident

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FIG. 3. Resuspended ash cloud extending from Katmai over Kodiak Island into the Gulf of Alaska as seen in MODIS 250-m visible data at 2123:05 UTC 21 Sep 2003. volcanic eruption at the time so the conclusion in the surface winds over the Katmai area during the entire report was that it was resuspended volcanic ash. On 20± resuspended ash cloud event. Examination of the 850- 21 September 2003, weather satellite imagery suggested mb wind and temperature analysis from the NCEP Eta strong winds entrained and lifted ash from the surface Model for 1200 UTC 21 September, shows the extent of the Valley of Ten Thousand Smokes high into the of cold advection and 30 kt (15.4 m sϪ1) winds (Fig. atmosphere (Ͼ1600 m). This event appears to be much 5). For the ¯ying community, large-scale winds of larger than any previously reported event: the length of 20 kt (10.3 m sϪ1) or greater are the criteria for clas- the cloud extended in excess of 230 km from the source sifying wind as strong in mountainous areas (Carney et and the downstream cloud exhibited both a south and al. 2000). a eastward component (Fig. 3). Upper-air observations from National Weather Ser- vice (NWS) Alaskan stations, King Salmon (ϳ100 km 3. Meteorological conditions upstream of the Katmai area) and Kodiak (ϳ150 km downstream), provide vertical pro®les of the atmosphere a. Background from 0000 UTC 21 September to 0000 UTC 22 Sep- An analysis of the large-scale patterns in the National tember (Fig. 6). Both King Salmon and Kodiak show a Centers for Environmental Protection (NCEP) sea level very dry surface layer; a rare condition in northwesterly pressure charts during the 20±21 September ash cloud ¯ow for this time of year. The temperature pro®le below event shows 1) a weakening quasi-stationary cyclonic 850 mb for King Salmon has a superadiabatic lapse rate. center located just south of Prince William Sound in the A very shallow moisture layer is con®ned just below a Gulf of Alaska [992 mb at 0000 UTC 21 September capping inversion (1380±1500 m) throughout the King (not shown) to 1010 mb at 0600 UTC 22 September]; Salmon soundings, and winds are generally greater than and 2) two anticyclonic centers (1024±1028 mb), one 20 kt (10.3 m sϪ1) and consistently from the northwest. in the Bering Sea and the other on the Alaskan North A shallow moisture layer is not present at Kodiak until Slope (Fig. 4). This pressure ®eld produced northwest 0000 UTC 22 September when it forms below a strong

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FIG. 4. NCEP North Paci®c surface analyses for dates and time indicated. Highlighted wedge shows area of interest. capping inversion at about 1500 m. Winds at Kodiak terrain (Fig. 2); and 2) there are numerous mountains are from the west in the lower troposphere at the be- interspersed with valleys, channels, and gaps through ginning of the resuspension event, becoming northwest the mountain barrier. In addition, the sounding at King by 1200 UTC 21 September. Salmon shows that the lower troposphere is very stable, The wind speeds observed in the soundings from except below 850 mb where the atmosphere is super King Salmon appear too weak to produce the dynamic adiabatic. In this case, the synoptic-scale pressure gra- vertical forcing necessary to lift the relic volcanic ash dient of stable ¯ow is both nearly perpendicular to the well into the atmosphere (Ͼ1600 m). However, there mountain barrier with its channels and gaps and is in are several important factors that likely had a strong phase with local pressure-driven channeling. With the local in¯uence on the winds that lifted the relic ash into presence of the low-level superadiabatic lapse rate and the atmosphere: 1) The Katmai Cluster and the Valley a strong subsidence inversion to de¯ect the parcel down- of Ten Thousand Smokes are located in very complex ward, we have factors that can contribute to acceleration

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FIG. 5. NCEP Eta Model 850-mb wind and temperature forecast chart for Alaska area 0000 UTC 22 Sep 2003. of downslope winds that would entrain and lift the relic mal through the summer until mid-August at which time volcanic ash from the ground and form the ash cloud the departure through September was 51.6 mm (2.03 observed on 21±22 September 2003 (Fig. 3). The air in.) below normal [58.4 mm (2.30 in.)]. These obser- descending the lee slopes of the mountainous terrain in vations, along with park ranger observations, indicate a superadiabatic atmosphere will become immediately that surface conditions in the region, including Valley colder than the surrounding environment and accelerate of Ten Thousand Smokes on 20±21 September 2003 toward the earth. Impending turbulent mixing will then were going through a rare dry spell for the area. Simpson occur up to the inversion level. et al. (2002a) conducted a climatological study of Alas- ka and found that the Katmai area has maximum pre- cipitation in October with September the second wettest b. Con®rmation and validation of conditions month. This differs from King Salmon where the wettest To entrain the relic volcanic ash and probably some month is August. dust from the surface of the Valley of Ten Thousand Three pilot reports (PIREPS) over Kodiak reported Smokes, the surface must have been dry for several days the presence of the ash cloud that reduced visibilities prior to the event. Sandi Fowler (National Park Service, (Table 1). In addition, two of the pilots reported that Katmai 2003, personal communication) states that at the ash was being blown up from the ground. One pilot Brooks Camp, the level of the lake lowered throughout reported the top of the ash cloud at 5500 ft, and another the summer of 2003 and boats had dif®culty using the at 6000 ft. This information con®rms entrainment from docks because of the unusually low water level. There the ground and that the top of the ash cloud was con- are no temperature or precipitation observations taken strained by the presence of the temperature inversion. inside the park and preserve. An examination of Na- We postulate that winds over the mountains and tional Climatic Data Center (NCDC) precipitation rec- through the channels and gaps that surround the Valley ords for King Salmon showed precipitation above nor- of Ten Thousand Smokes accelerated because of the

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FIG. 6. Simpli®ed skew T±log p plots at 0000 UTC 21 Sep, 1200 UTC 21 Sep, and 0000 UTC 22 Sep 2003 for King Salmon and Kodiak, Alaska, respectively. See appendix in Simpson et al. (2000) for detailed discussion of skew T± log p plot. factors discussed earlier. The Alaska Marine Ferry Tus- tained for the coastal waters off the Katmai area at 0342 tumena (call sign WNGW) at 57.9ЊN and 154.3ЊW (just UTC 22 September (Fig. 7). See Monaldo (2000) for offshore of the Katmai cluster in Shelikof Strait) re- details of SAR-derived winds. The SAR image shows ported a surface wind of 39 kt (21.8 m sϪ1), about double strong surface winds both north and south of Kodiak the speed of the wind observed upstream of the Katmai Island blowing from the Alaska Peninsula. A ship ob- area. servation at 0000 UTC 22 September from the Tustu- A synthetic aperture radar (SAR) image of derived mena at 58.8ЊN and 152.1ЊW con®rms that the sur- surface winds from the RADARSAT satellite was ob- face winds at the northern tip of Kodiak were 37 kt

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TABLE 1. Airborne volcanic ash sightings by general aviation pilots of Katmai event. Date Time (2003) (UTC) Location Aircraft Flight level Remarks 21 Sep 1825 Terror Bay DH2 6500 ft Observed airborne ash at 4000 ft, 5 miles visibility, ash being blown up from ground

21 Sep 1825 Uganik Bay DH2 6500 ft Observed airborne ash at 2000 ft, 3 miles visibility.

21 Sep 1911 Kodiak C207 6000 ft Observed airborne volcanic ash below Island 6000 ft, 3±5 miles of visibility, ash be- ing blown up from ground

21 Sep 2018 Afognak PA18 7000 ft Airborne volcanic ash, visibility 1 mile Island and variable, for most part, tops of ash 5500 ft

FIG. 7. Derived synthetic aperture radar surface wind image for Alaska Peninsula±Kodiak coastal area for 0342 UTC 22 Sep 2003.

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Ϫ1 (19.0 m s ) in good agreement with the SAR image. and 12-␮m(T 5) brightness temperature (BT) difference A unique feature in the SAR image is the zone of little (⌬T ϭ T 4 Ϫ T 5). Meteorological clouds are associated or no wind that extends from the Katmai coast across with positive ⌬Ts (Yamanouchi et al. 1987) while vol- Kodiak Island and into the Gulf of Alaska. This zone canic plumes/clouds should have negative ⌬Ts (Prata is a ``shadow'' of the resuspended volcanic ash cloud 1989). Class separation becomes increasingly dif®cult at that time. It appears that the ash cloud was suf®ciently as ⌬T → 0 (Simpson et al. 2002b). Other factors [e.g., dense to attenuate the SAR signal from the RADARSAT atmospheric water vapor, ice coating of the airborne ash, satellite so that no surface winds below it could be mea- particle size, chemical composition, particle shape (de- sured. A search of the literature shows no studies on parture from spherical), errors in satellite sensor radio- the attenuation of the SAR signal by airborne volcanic metric calibration, surface emissivity and temperature, ash over water. The National Weather Service, Alaska ground water and juvenile water in the magna, optically Region, looks at SAR-derived coastal winds daily, and active coatings] may compromise accurate detection us- did not observe this pattern of attenuation either before ing the split window method (see Simpson et al. 2000, or after this relic ash event. Unfortunately, observations 2001; Pieri et al. 2002 and the reference contained there- from the King Salmon Weather Surveillance Radar- in). Likewise, because of its chemical composition (sil- 1988 Doppler (WSR-88D) weather radar were not use- icon, iron, aluminum, and calcium) and its particle size ful for this event. The 1.5Њ elevation beam of the Dopp- distribution (mean aerodynamic diameter 2±4 ␮m), both ler radar cannot ``see'' below about 1700 m over the airborne Asian desert dust (e.g., Gobi Desert) and Sa-

Katmai Mountains. haran dust produce a negative T 4 Ϫ T 5 signal, compa- rable, if not stronger, to that often produced by airborne volcanic ash (Simpson et al. 2003). 4. Discussion A time series of AVHRR T 4 Ϫ T 5 scenes over the a. Winds and the superadiabatic lapse rate Katmai area for 20±21 September 2003 (Fig. 8) shows a strong negative T 4 Ϫ T 5 signal associated with the Peltier and Clarke (1979) describe a severe down- resuspension of relic volcanic ash from the 1912 No- slope windstorm at Boulder, Colorado, in January 1972 varupta eruption. This uniquely strong resuspension that involved resonant ampli®cation of the lee wave event combines attributes associated with both airborne through the depth of the troposphere. Modeled surface volcanic ash and airborne Asian dust. The chemical winds closely ®t the actual winds that exceeded 3 times structure of the relic volcanic ash, coupled with its small the mean ¯ow speed. However, the Katmai case involves particle size, contributes to its straightforward detection the presence of a nonstandard atmosphere with a su- by the split window method. Moreover, for resuspension peradiabatic lapse rate below 850 mb, and a strong sub- of the relic volcanic ash to occur, the atmosphere has sidence inversion between 740 and 850 mb, which likely to be dry and the winds must be strong, conditions also rules out the possibility of wave ampli®cation due to favorable for the atmospheric transport of desert dust. resonance through a deep layer. Under such conditions, A very dry atmosphere likewise mitigates several of the Carney et al. (2000) reported downslope winds in excess problems associated with the accurate detection of air- of 100 kt (51.4 m sϪ1). Because the atmosphere is su- borne volcanic ash by the split window method. peradiabatic below 850 mb, the air is buoyant to both upward and downward perturbations. Gap winds have been known to exceed the mean wind c. Impacts on aviation speed by a factor of 2 or more because of the Bernoulli The National Weather Service Alaska Aviation effect (Carney et al. 2000). In the Katmai case, this Weather Unit issued a SIGMET warning to the aviation means that wind speeds could have reached 60 kt (30.8 community on the presence of the ash/dust cloud at 1915 msϪ1) to 90 kt (46.3 m sϪ1). Indeed, if the gap winds UTC 21 September 2003. The Kodiak Federal Aviation that ¯owed between the summits of Katmai, Trident, Administration (FAA) Flight Service Station received and Mageik Volcanoes began a descent through a su- reports of some very light ash fallout in the Kodiak peradiabatic atmosphere, then it could have been pos- area. The FAA also reported that a regional airline can- sible for those wind speeds to accelerate in excess of celled a couple of ¯ights into Brooks Camp until after 100 kt (51.4 m sϪ1) before impeding on the relic, dry the ash event ceased. The airlines based their decision ash deposits. on past experiences with ash where one of their aircraft suffered minor engine damage (scoring of the cylin- b. Split window detection of relic volcanic dust ders). Fortunately for this event, the Kodiak±Katmai region had mostly clear skies and the ash/dust cloud Operational detection of airborne volcanic ash by Vol- remained below 1700 m in the atmosphere. Pilots that canic Ash Advisory Centers (VAACs) often relies on did ¯y could easily see the ash/dust cloud and avoid it. satellite remote sensing using the ``split window meth- If signi®cant meteorological clouds had been present, od.'' This method evaluates the Advanced Very High then this could have compromised the ash/dust detection

Resolution Radiometer (AVHRR) or equivalent 11-(T 4) technique (Simpson et al. 2000, 2002b) and thereby

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FIG. 8. Time series of AVHRR T4 Ϫ T5 images. Katmai source region and Ko- diak Island are shown (green). (a) The airborne relic volcanic ash appears over the source region (Katmai) as black and in (b)±(m) is advected south-southeast over Kodiak Island and into air routes.

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5. Conclusions REFERENCES

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