FIRE and MUD Contents

Mount Pinatubo: A Satellite Perspective of the June 1991 Eruptions

By James S. Lynch¹ and George Stephens¹

¹ National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, Washington, D.C.

ABSTRACT

The June 15, 1991, eruption of Mount Pinatubo was the largest explosive volcanic event detected on this planet with satellite imagery. From June 9 to June 17, many of the Mount Pinatubo eruptions were detected and monitored by NOAA and Japanese operational meteorological satellites (NOAA-10, -11, -12, and GMS). This paper summarizes the satellite information and shows selected examples of the enhanced imagery.

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INTRODUCTION

In June 1991, Mount Pinatubo (15.1N. lat, 120.4E. long) in the Philippines produced a series of violent eruptions culminating in a massive event on June 15. The paroxysmal event was the largest volcanic eruption ever detected on planet Earth with satellite data. On a geological basis this eruption was one of the largest volcanic events of the century--smaller than Novarupta/Katmai (Alaska, USA, 1912), similar to Santa María (Guatemala, 1902) and Cerro Azul/Quizapu (Chile, 1932), and larger than those of El Chichón (Mexico, 1982) and Mount St. Helens (Washington, USA, 1980) (Smithsonian Institution, 1991).

The June eruptions of Mount Pinatubo were monitored by the National Oceanic and Atmospheric Administration's (NOAA's) polar orbiting satellites (NOAA-10, -11, and -12) and the Japanese Geostationary Meteorological Satellite (GMS). Realtime access to GMS imagery provided hourly visible and infrared imagery at approximately 8-km resolution and provided 3-h infrared imagery at about 12-km resolution. Near-realtime and delayed access to imagery from the NOAA spacecraft provided 1.1-km resolution tape recorded coverage over preselected regions and daily coverage at 4.0-km resolution in four or five multispectral bands (two visible and two or three infrared bands; table 1). A priority request for the higher resolution tape-recorded coverage over the Philippines was made by National Environmental Satellite, Data, and Information Service (NESDIS) investigators days before significant eruptions began.

Table 1. Wavelengths measured from the Advanced Very High Resolution Radiometer aboard NOAA's polar orbiting satellites

METHODS FOR ESTIMATION OF MAXIMUM PLUME HEIGHT

Physical scientists around the world have tried a number of approaches to estimate the maximum plume height on the basis of satellite imagery. Four basic techniques are available:

Wind Correlation.--The current methodology used at NESDIS is to correlate ash drift vectors (away from the immediate vicinity of the rising column) with observed atmospheric wind patterns. In general, the ash moves downwind in a direction and at a rate closely matching the prevailing wind. Due to vertical wind shear, the direction and (or) speed of the wind are usually unique at each altitude, so tracking of the plume in successive satellite images allows estimation of plume altitude. This technique is best obtained from nearly continuous geostationary imagery and has proven highly successful since 1990 at NESDIS.

Temperature correlation.--The traditional approach has been to relate plume-top temperatures derived from infrared imagery to environmental temperatures measured by rawinsondes. Simplified parcel theory (that is, assuming 100 percent entrainment of the ambient atmosphere) is not applicable to ascending hot ash plumes. Simplified approaches fail for large eruptions penetrating into the stratosphere because, above the tropopause, ambient temperature increases with height. Additional problems are introduced by increased transmissivity through thin or thinning ash plumes. Woods and Self (1992) described some of the problems associated with using temperatures to estimate column height. In NESDIS operations, temperature correlations are used only as a last resort in the case of weak vertical wind shear.

Stereoscopy.--A technique available when two or more satellites are simultaneously viewing an event is stereoscopy. Geometric and trigonometric calculations can precisely determine column height by using (1) the precise location of the plume and satellites and (2) topographical data. The National Aeronautics and Space Administration (NASA) pioneered this technique in the research environment by monitoring severe thunderstorms with two U.S. geostationary satellites. It is possible that similar approaches can be used on satellites with dual forward-viewing and downward-viewing imagers.

Shadow measurements.--A technique under consideration, but not in operational use, is shadow analysis. Geometric and trigonometric calculations can determine column height precisely by using (1) the precise location of the plume, sun, and satellite; (2) the length of the shadows; and (3) topographical data. Complications arise when shadows fall on a lower cloud layer or on a dark surface such as the ocean. The technique is only useful during daytime hours and with solar zenith angles large enough to produce shadows. NOAA has experimented with this technique for estimating the altitude of thunderstorm canopies.

The heights used in this paper are all derived from wind correlation.

SEQUENCE OF ERUPTIVE EVENTS

Mount Pinatubo began a series of minor ash eruptions on June 9, which became violent on June 12. After a 28-h respite, frequent brief eruptions began on June 14 and culminated with the massive eruptions on June 15. From June 12 to 16, a total of 19 discrete eruptions sent ash and gas into the stratosphere (table 2 and fig. 1).

Figure 1. The duration, vertical extent, and areal coverage of the 19 eruptions of Mount Pinatubo from June 9 to 17, 1991, as detected from satellite imagery (see table 2). The term "overshooting top" refers to the central ash column in the immediate vicinity of the volcano that punches high into the stratosphere before sinking back to the level of neutral buoyancy, where it then spreads laterally.

Table 2. Preliminary summary of 1991 eruptions of Mount Pinatubo.

² Satellite observations of eruptive episodes were derived from visible and infrared satellite imagery.

³ Duration of event (to nearest 0.5 h) was derived from visible and infrared satellite imagery.

⁴ Plume heights are from PHIVOLCS, USGS, and NOAA/NESDIS (satellite estimates were based on plume motion, correlated with nearby rawinsonde measurements).

⁵ Maximum horizontal extent of "opaque" or "semiopaque" plume determined from infrared satellite imagery.

⁶ Paroxysmal eruption(s) defined here as the period of a nearly continuous, high ash canopy as seen in GMS and polar orbiting satellite imagery. Surface observations (eyewitness reports and instrument records) indicate discrete eruptions from 1027 through 1315 followed by continuous, strong eruption that began at approximately 1342.

From June 9 until the paroxysmal event on June 15, each eruption lasted less than 3 h, and extended over relatively small areas (fig. 2). During daylight hours, visible satellite imagery suggested that most of the ash plumes consisted of steam and "light colored" particulates. Beginning with the eruption of 0553 on June 15, however, the ash particulates became very "dark colored," possibly as a result of the injection of larger amounts of particulate matter (fig. 3). (Note: all times are given as Philippine time, G.m.t. + 8 h.)

Figure 2. NOAA-12 thermal-infrared image taken at 1930 on June 14, 1991, showing three distinct ash plumes. One (the brightest), directly over the volcano, is associated with the 1851 eruption (table 2). Two plumes over the South China Sea west of Luzon are associated with the 1307 and 1410 eruptions. Typhoon Yunya is shown approaching southeastern Luzon. This image was processed from 4-km resolution data. Temperatures derived from the brightest, or coldest, part of the ash plume are around -80°C, and portions of the ash cloud tracked west-southwestward at 30 m/s, corresponding to an altitude of 20 to 25 km.

Figure 3. NOAA-10 false-color multispectral (visible and infrared) image, processed from 1.1-km resolution data, taken at 0730 on June 15, 1991. The darker, circular area near the volcano shows darker particulate matter associated with the 0553 and 0611 eruptions in table 2. The more diffuse plume over the South China Sea west of Luzon is ash from earlier eruptions.

Satellite imagery suggests that the cataclysmic stage of eruptions began about 1027 on June 15 and lasted for over 21 hours. During this period, a stratospheric ash canopy spread across a 2,700,000-km² area (fig. 4). Surface observations (eyewitness reports and instrument records--seismic, infrasonic, and barographs) suggest that five separate eruptive events occurred between 1031 and approximately 1315 and became continuous at approximately 1342.

Figure 4. Rate of expansion of the Mount Pinatubo ash plume following the paroxysmal eruption of June 15-16, 1991. The outer edge of the ash cloud was derived in realtime from 12-km resolution GMS infrared imagery available at 3-h intervals. Gray areas are landmasses. A similar figure has been published by Tokuno (1991).

The paroxysmal eruption(s) appeared to have three distinct phases in the satellite imagery:

Phase 1.--The first phase, by far the most violent phase from a satellite perspective, appeared continuously in infrared imagery from 1031 until 2231 on June 15 (fig. 5). An extensive ash shield achieved heights of 25 to 30 km. In the immediate vicinity of the volcano, the central ash column reached 35 to 40 km; this "overshooting top" was fixed over the volcano for the entire 12-h period. The ash from this phase accounted for over 95 percent of the plume's 2,700,000-km² areal coverage.

Figure 5. NOAA-10 false-color multispectral (infrared) image processed from 1.1-km data (reduced to 2.2-km) taken at 1830 on June 15. The extensive canopy of ash over Luzon and the South China Sea was produced after nearly 5 h of continuous eruption during the paroxysmal event. The total ash plume continued to grow for a total of 18 h. Here, the "overshooting top" can be seen over the volcano. Even though prevailing winds are from the northeast, the force of the eruption was great enough that some ash was forced upwind. The concentric rings around the central core appear to be gravity waves propagating outward. The majority of the plume spread west-southwestward at nearly 35 to 45 m/s and plume-top temperatures were as low as -88°C; the winds correlated with rawinsonde data to heights of 25 to 30 km, and the temperatures were colder than any environmental observations in either the troposphere or stratosphere. The slightly darker portion of the image extending westward from the volcano tracked westward at approximately 50 to 60 m/s and plume-top temperatures were near -60°C; the winds correlated with rawinsonde data to heights of 35 to 40 km.

Phase 2.--The second phase of the eruption was visible as a "ball shaped" plume fixed over the volcano. This phase was seen from 2231 on June 15 until 0331 on June 16. The plume was approximately 26 to 28 km in height.

Phase 3.--The third phase of the eruption, apparent from 0331 through 0731 on June 16, was noted by a "wedge shaped" plume fixed on the volcano (fig. 6). During this phase, ash reached heights of 23 to 25 km.

Figure 6. NOAA-10 false-color multispectral (visible and infrared) composite image processed from 1.1-km data at 0800 on June 16. A wedge-shaped ash cloud, locked over the volcano, is depicted during the third phase of the paroxysmal eruption. Material can be seen rising from the volcano and spreading to form the blue-gray plume and ash cloud.

Large amounts of ash fell across most the island of Luzon and resulted in the closure of Clark Air Base, Cubi Point Naval Air Station, and Manila International Airport. Significant ashfall was reported across the South China Sea and Indochina. Some ash was reported as far away as Thailand.

Following the June 15 eruptions, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the U.S. Geological Survey (USGS) reported nearly continuous minor eruptions (10 to 20 eruptions per day) producing ash to 4 to 5 km in altitude. Most of these events were not detected with infrared satellite imagery, though an occasional plume did appear in the visible imagery. Numerous "secondary explosions" were caused by rain mixing into hot ash and pyroclastic deposits. Occasionally, the eruptions would reach altitudes of 10 to 15 km.

ENCOUNTER WITH THE TYPHOON

Typhoon Yunya approached eastern Luzon on June 14 with sustained winds of 45 m/s (90 knots) and made its closest approach to Mount Pinatubo as a tropical storm around 1400 on June. Yunya's center (or eye) passed within 75 km of Mount Pinatubo during the first phase of the paroxysmal eruption. Many of the rain bands affected the entire island of Luzon, including the volcano, throughout the entire cataclysmic event(s). Upon entering the South China Sea, the typhoon rapidly dissipated. Other than to state that tropical cyclones typically regenerate over the South China Sea, this paper will not discuss or speculate on the causes for Yunya's demise.

Torrential rains mixed with pyroclastic flows and ash deposits and caused numerous lahars. Buildings and homes collapsed under the terrific weight of rain-soaked ash. Most of the fatalities, injuries, and property damage that occurred on June 15 and 16 throughout Luzon were the result of collapsed buildings (C.B. Bautista and others, this volume; Spence and others, this volume) and lahars.

NOAA RESPONSE

NOAA and the Federal Aviation Administration (FAA) established a cooperative effort in 1988 to detect and monitor explosive volcanic eruptions. The primary focus of this effort has been to warn air traffic managers and en-route commercial aircraft of volcanic hazards in the Flight Information Regions assigned to the United States. However, provisions have been made to relay information for explosive eruptions throughout the world.

The NOAA-FAA Volcanic Hazards Alert Program was activated for Mount Pinatubo at the request of the U.S. Department of Defense and USGS. A total of 47 volcanic hazards alerts, consisting of realtime satellite analyses, trajectory forecasts, and other ancillary information, were issued during the June eruptions of Mount Pinatubo.

Immediately following the paroxysmal eruption on June 15, due to the magnitude of the eruption and expected multiyear effects, NOAA activated teams to track aerosol movement, changes in atmospheric chemistry (particularly ozone depletion), and climate response.

The aerosol cloud was tracked around the world by use of the NOAA polar orbiting satellites and both European and NOAA geostationary satellites (METEOSAT and GOES-7). The plume took 3 weeks to completely encircle the planet between 30N. and 20S. lat (fig. 7). Subsequent to this, the aerosol cloud was tracked and monitored by using the operational Aerosol Optical Thickness Product, which is derived from AVHRR data from the NOAA polar orbiting satellites.

Figure 7. GOES-7 visible image of the Pacific Ocean on June 23, 1991, at 0300 UTC (1100 Philippine time). The hazy area in a band from 20° S. lat to 30° N. lat is caused by stratospheric aerosols from the Pinatubo eruption(s).

REFERENCES CITED

Bautista, C.B., this volume, The Mount Pinatubo disaster and the people of central Luzon.

Hoblitt, R.P., Wolfe, E.W., Scott, W.E., Couchman, M.R., Pallister, J.S., and Javier, D., this volume, The preclimactic eruptions of Mount Pinatubo, June 1991.

Sabit, J.P., Pigtain, R.C., and de la Cruz, E.G., this volume, The west-side story: Observations of the 1991 Mount Pinatubo eruptions from the west.

Smithsonian Institution , 1991, Pinatubo: Washington D.C., Smithsonian Institution, Bulletin of the Global Volcanism Network, v. 16 (June 30, 1991), p. 2-5.

Spence, R.J.S., Pomonis, A., Baxter P.J., Coburn, A.W., White M., and Dayrit, M., this volume, Building damage caused by the Mount Pinatubo eruption of June 14-15, 1991.

Tokuno, M., 1991, GMS-4 Observations of Volcanic Eruption Clouds from Mt. Pinatubo, Philippines: Meteorological Satellite Center Technical Note Number 23, Japan Meteorological Agency, p. 1-23.

Woods, A.W., and Self, S., 1992, Thermal disequilibrium at the top of volcanic clouds and its effects of column height, Nature, v. 355, p. 628-630.

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