JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. D13, PAGES 14,551-14,563, SEPTEMBER 20, 1992

Satellite Monitoring of Smoke From the Kuwait Oil Fires

SANJAY S. LIMAYE, STEVEN A. ACKERMAN, AND PATRICK M. FRY

Cooperative Institute for Meteorological Satellite Studies, Space Science and Engineering Center University of Wisconsin-Madison

MAJEED ISA, HABIB ALI, GHULAM ALI, AND ALLAN WRIGHT

Meteorological Service Civil Aviation Affairs, Bahrain International Airport

ART RANGNO

Department of Atmospheric Science, University of Washington, Seattle

The smoke from the oil fires in Kuwait was easily visible in observations from weather satellites in polar and geosynchronous orbits. A portable work station provided these data for planning the National Center for Atmospheric Research and University of Washington research aircraft flights out of Bahrain during the Kuwait Oil-Fire Smoke Experiment. Meteosat visible and infrared satellite observations indicate that the smoke often traveled southeast along the west shore of the Persian Gulf as far as Bahrain, at which point it typically turned west or continued south toward the Arabian coast. The smoke was difi%ult to detect from satellite observations as it moved over water and at large distances from the source during the night from infrared observations. Also notable among the daily satellite images were the frequent, intense dust storms that seemed to form in Syria and northern Iraq and transport dust southeastward over Kuwait, and often to northwestern Saudi Arabia. Clouds were virtually absent during the months of May and june within the first several hundred kilometers along the plume direction. Surface temperatures in Bahrain during April and August 1991 were lower than average by as much as 1ø-3.2øC, and are significant compared to the climatological variability of average minimum and mean temperatures for the summer months.

1. INTRODUCTION

The dispersal of smoke from the oil fires in Kuwait is of interest for several scientific reasons, the main one being regional environmental impact, and others being hazardous air conditions for health and air traffic, possible impact on global climate, and an unfortunate but unprecedented oppor- tunity to study the properties of the smoke itself and its behavior over time. From May 10 to June 14, 1991, a field experiment was undertaken to study the physical, chemical, and radiative properties of the smoke from the oil fires [Hobbs and Radke, 1992]. In situ sampling of the smoke was conducted by the University of Washington and the National Center for Atmospheric Research (NCAR) research aircraft. The intent of this brief contribution is to describe some of the

satellite observations that were used in the planning of the aircraft missions, and the potential environmental impact of the smoke. We summarize the main aspects of the smoke's behavior during summer 1991; the summary is not exhaus- tive, as much analysis remains to be done.

were inadequate to provide the instantaneous smoke plume location. To remedy this situation, a man-computer interac- tive data acquisition system (McIDAS) [Suomi et al., 1983] work station was installed at the Bahrain Meteorological Service and linked via a telephone line to obtain the real-time satellite imagery from the data bank of the McIDAS facility in Madison, Wisconsin [Liraaye et al., 1992]. The delivery of the data was accomplished by automated scheduling of navigation, image subsecting, dialing, and data transmission every half hour using the McIDAS mainframe capabilities. The schedule was prepared and loaded in advance for the duration of the experiment and required little attention at the receiving end in Bahrain. These satellite images allowed continuous monitoring of the smoke dispersal via image animation and provided the smoke plume positions required for planning and conducting the research flight missions. In addition to providing information for the initial planning of the aircraft flight patterns, the air traffic control (ATC) at Bahrain International Airport occasionally relayed the up- dated satellite-determined plume locations to the aircraft

2. FIELD SUPPORT OF SATELLITE OBSERVATIONS

The University of Washington and the NCAR research aircraft used Bahrain International Airport as the base of operations [Hobbs and Radke, 1992]. The Bahrain Meteoro- logical Service provided conventional weather observations; however, the resolution and frequency of these observations

Copyright 1992 by the American Geophysical Union.

Paper number 92JD01100. 0148-0227/92/92JD-01100505.00

TABLE 1. Band-Pass Intervals for AVHRR Instruments on NOAA-10 and NOAA-11

Channel Number Spectral Band Pass,

1 0.58-0.68 2 0.73-1.10 3 3.55-3.93 4 10.3-11.31' 5 11.5-12.5

*NOAA-11 only. On NOAA-10, channel 4 is repeated.

14,551

14,552 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

TABLE 2. Summary of Surface Observations in Bahrain (26ø16N, 50ø39E) Compared to Average for 1960-1990

Sunshine, Monthly Minimum T Monthly Mean T, øC Monthly Maximum T, øC hours

Number of

Smoke Days

Month 1991 Average rms DEP 1991 Average rms DEP 1991 Average rms DEP 1991 DEP Total Surface

January 14.9 14.6 0.3 0.3 17.7 17.0 0.3 0.7 20.7 19.3 0.3 1.4 6.0 -1.6 2 0 February 15.1 15.5 0.7 -0.4 17.9 18.1 0.8 -0.2 23.8 20.6 0.9 3.2 6.5 -1.4 2 1 March 16.8 18.2 1.1 -1.4 19.6 21.1 1.2 -1.5 23.3 24.0 1.3 -0.7 6.1 -1.6 23 7

April 21.3 22.3 1.4 -1.0 25.0 25.8 1.5 -0.8 29.8 28.9 1.7 0.9 7.7 -0.8 21 8 May 23.9 27.1 1.4 -3.2 27.5 30.4 1.4 -2.9 32.9 33.7 1.6 -0.8 8.1 -1.8 24 11 June 27.3 29.5 0.3 -2.2 30.7 32.7 0.3 -2.0 35.7 35.9 0.5 -0.2 11.3 -1.5 22 16

July 28.3 31.1 0.7 -2.8 32.1 34.4 0.6 -2.3 37.2 37.5 0.7 -0.3 10.7 -1.1 30 28 August 28.6 31.3 0.4 -2.7 32.3 34.4 0.4 -2.1 36.8 37.4 0.3 -0.6 10.7 -1.2 28 24

Average is for 1973-1989' rms is computed from monthly mean values. DEP represents departure from 1973-1989 average. Reduction in sunshine is due to smoke, clouds, and dust. Total number of smoke days includes elevated smoke as well as surface. NA, not available.

once they were airborne. The ability of MclDAS to access forecast products from the National Meteorological Center (NMC) and the European Centre for Medium-Range Weather Forecasts (ECMWF), the synoptic conventional surface and upper air weather observations, as well as

satellite observations, enabled us to monitor the meteoro- logical environment with which the oil-fire smoke interacted.

Both geostationary and polar-orbiting weather satellites provided observations for monitoring the smoke dispersal from the fires. Meteosat 4, which is in geostationary orbit

-,

Fig. la

Fig. 1. Appearance of the smoke on four consecutive days, June 12-15, at 0900 UT (local noon in Bahrain). Animation of such images was used in monitoring the positions and changes in the smoke plume.

LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,553

Fig. 1 b

about the Earth at approximately 0 ø longitude nominally provides observations every 30 min and carries a three- channel imaging radiometer (visible, atmospheric window infrared, and midtropospheric water vapor infrared). This study used the visible (0.4-1.05/•m) and infrared (10.5-12.5 /•m) channels to monitor the changes in the smoke, dust, and cloud fields of this geographic region. As reported previously [Limaye et al., 1991], signature of the smoke plume was seldom detectable in the water vapor channel observations, providing indirect evidence that the smoke plume did not affect the midtropospheric water vapor abundance (400-600 mbar). Monitoring the smoke position was accomplished by animating the half-hour Meteosat 4 imagery.

Advanced very high resolution radiometer (AVHRR) ob- servations from two polar orbiting satellites (NOAA-10 and NOAA-11) provided multispectral coverage at higher spatial resolution than Meteosat, although only four times per day. Analysis of the AVHRR data, which have a spatial resolu- tion of 1.1 km at nadir, allows us to further refine the smoke plume location and to study the spectral properties of the smoke. The AVHRR is a five-channel instrument whose

characteristics are summarized in Table 1. These channels

were chosen to minimize the contribution of the atmospheric gases to the measured radiation and are therefore optimum in studying the surface, clouds, and aerosols. The NOAA

polar orbiters are sun-synchronous; thus the images cover- ing a specific geographic region have approximately the same solar illumination on successive days. The NOAA-10 viewed this region at approximately 0300 and 1500 UT, while the NOAA-I 1 had nominal overpasses at 1100 and 2300 UT.

Meteosat Observations

One remarkable aspect of the smoke plumes observed by the satellite imagery was their relatively narrow width (20-50 km) over hundreds of kilometers, indicating the relatively inefficient mixing with the ambient air. Due to the oblique perspective of the smoke, the relatively low spatial resolu- tion, and lack of smoke height perception, the satellites do not readily reveal the complex vertical layering of the smoke plumes from the different fields as they eventually merged, at least horizontally, some distance downwind, as was ob- served by the aircraft flights [Hobbs and Radke, 1992]. In the Persian Gulf region the winds during late spring and summer are northwesterly and climatologically exceptionally steady, at least in the direction from which they blow. This warm, dry wind is referred to as the shamal, the Arabic word for north. The smoke plumes from the Kuwait oil fires were thus often channeled in relatively narrow rivulets toward the southeast. The spread of the smoke over the Arabian pen-

14,554 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

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insula in time lapse satellite imagery thus appears not too different from that of an ordinary smokestack, except on a much larger scale. The lack of spread of the plume orthog- onal to its axis has been somewhat surprising and may result from a stable atmospheric environment, the strength of the low-level Gulf jet, and the lack of vertical shear in wind direction. It is also possible that the smoke plume created a convergence zone during the day when lofting due to solar heating of the smoke, thus contributing to a confined bound- ary. Near the source, the plume had a convective (cumuli- form) aspect that was not apparent 100 km downwind, even when it was entirely over land.

Time sequences of Meteosat imagery (May-July 1991) indicated that the smoke generally moved southeastward and either (1) crossed the southern coast of the Arabian Peninsula, traveled over the northern Arabian Sea, and became more or less invisible to the weather satellite obser-

vations, (2) circulated over the central Arabian desert, where some of the smoke particles settled out as suggested by surface weather reports of smoke at the ground, or (3) drifted directly eastward toward Iran. The northward movement of the smoke was less common during the field experiment conducted in May and June and was observed only a few times during the summer months. This is consistent with the low-level flow patterns expected from climatology. The large

increase in the average temperatures in April (see Table 2) indicates a transitional period between two seasonal flow regimes. In late spring and summer the weather along the coastal region is dominated by northwesterly and northerly winds, essentially no precipitation (occasional precipitation occurs in May), high daytime surface temperatures, and high humidity. During this season, frequent dust storms are expected from climatology, and many were observed as described below.

The interplay between the effects of the smoke on the local atmosphere and the wind conditions over the region occa- sionally led to disproportionately large concentrations of smoke hundreds of kilometers from Kuwait. Thus skies in

Kuwait City, which had burning fields to the north, west, and south a few tens of kilometers away, were often clear of smoke, while other Gulf cities such as Dhahran and Manama (Bahrain) or even the central Arabian city of Riyadh were reporting the presence of smoke. Figure 1 shows the spread of smoke on four consecutive days at local noon from Meteosat visible channel observations, near the end of the experiment (June 12-15, 1991). These four days signified the weakening of the northwesterly flow over Kuwait and the beginning of the more dominant easterly flow in the 700- to 850-mbar layer, as expected from climatology [Ackerman and Cox, 1989]. On June 12 (Figure l a) the smoke traveled

LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,555

Fig. 1 d

down the coast line of Saudi Arabia and the Persian Gulf. By June 13 (Figure lb), this smoke reached the Rub al Khali Desert and Oman, while smoke from the source drifted in a west-northwesterly direction. On June 14 and 15 (Figures 1 c and l d, respectively) the smoke from the source was still moving northwesterly toward Iraq. Such movement of the smoke is clearly evident in time loop sequences of the Meteosat data [Liraaye et al., 1991, 1992], which showed that in late June and July some smoke veils were seen west of the Red Sea, over eastern Libya and further north, over Egypt, and occasionally as far north as the east and west shores of the Caspian Sea. This westward propagation of the smoke is consistent with the climatological upper level flow during the summer months [Newell and Kidson, 1977], and suggests that a fraction of the smaller smoke particles did rise into the upper troposphere. Smoke particles, suspected to have originated in Kuwait, have been detected over Wyoming between 8 and 11 km during March-June 1991 [Deshler and Hoffman, 1992; Sheridan et al., 1992] and also over Japan (particles collected at 7.5 km on April 27, 1991 [Okada et al., 1992]) and Hawaii [Lowenthal et al., this issue]. It thus appears that once the particles rose high into the atmosphere, they were carried globally by the westerly jet.

Plateaus and mountainous regions flank the borders of the

Arabian Peninsula along the Red Sea, the Gulf of Aden, and the Arabian Sea. The absence of terraces on the Arabian side

of the Persian Gulf allows for the transport of smoke into the Rub al Khali Desert, elevation <200 m, where the smoke is expected to be deposited. Satellite observations should indicate the extent and amount of smoke settling through temporal analysis of the blackening of the desert surface.

The height of the smoke plume, when it is visible over the land, can be roughly estimated by comparing the smoke plume shape and drift speed with the constant pressure maps available through either the NMC or ECMWF weather forecasting models, or the streamline analysis of the Bahrain Meteorology Center. Such comparisons indicated that the smoke was typically between 850 and 700 mbar over the Arabian Peninsula, within the first few hundred kilometers of Kuwait. Liraaye et al. [1991] used cloud shadows and measurement of the smoke drift speed to estimate the smoke plume height; however, lack of useful cloud shadows during the summer months precluded direct estimates of the smoke's level during the summer.

Over this geographic region, the smoke was generally within 3-6 km of the surface. Except for a few days during May and June, both cumulus and cirrus clouds were nearly absent in the vicinity of the smoke for distances as far as 1500-2000 km. Scavenging of smoke particles that occurred

14,556 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

METE '"SAT4 01 2•Z MAY 9114:2: 07Z;000 00957 03597 02•00

Fig. 2. Meteosat visible image from 0730 UT on May 22, 1991, over the Persian Gulf region showing extensive airborne dust and smoke.

during February and March in the Gulf region [Liraaye et al., 1991], resulting in numerous reports of "black" rain, was virtually absent during May and June, allowing the smaller smoke particles to survive longer in the atmosphere. As noted previously, no rain is expected during these months from climatology, and, from the satellite observations, little was reported or suspected to have occurred. During the winter the region is influenced by Mediterranean depres- sions, which can bring rain, and thus scavenging of smoke particles such as occurred in January-March was not prev- alent during the summer, within the first few days (time it took to travel a distance of, say, a few hundred kilometers) of the smoke's injection into the atmosphere.

Occasionally midlevel clouds were seen to form at the top of the plume downwind of the fires. Aircraft measurements indicated enhanced relative humidity downwind when the visible smoke top was reaching its maximum elevation. Presumably, some of the moisture originated as a product of the combustion in Kuwait, and the rest from entrainment of the ambient moisture. No deep convective clouds were observed either over Kuwait or over the Arabian Desert

during the summer where the smoke traveled. In addition to observing smoke, the satellite imagery also

indicated frequent occurrences of dust storms that seemed to

form in Syria and northern Iraq and propagate southeast- ward over Kuwait. Early detection of significant amounts of airborne dust was considered important for the safety of the aircraft, and the satellite observations viewed on the porta- ble work station provided a good means to monitor dust outbreaks. A major sandstorm was observed on May 17, with optically thick dust clouds obscuring the surface over a wide area to the southwest of Kuwait, approaching Riyadh, when small cumulus clouds were seen to form above the

smoke plume, which appeared to be at a level higher than the approaching dust. Figure 2 illustrates the situation observed on May 22, 1991, where a dust front could be monitored approaching and finally interacting with the smoke plume. An animated sequence of half-hour satellite imagery clearly indicates the movement of the dust plume and the presence of smoke over southern Iran. In the visible channels the

smoke appears dark due to strong absorption, while the dust appears bright due to scattering. Monitoring the interaction of the dust with the smoke is critical for correct interpreta- tion of smoke characteristics deduced from aircraft observa-

tions. These images also show the smoke "fanning" out from its source region. While this fanning was not common during the field experiment, it is consistent with the stream- line analysis of the Bahrain Meteorological Office. Low-level

LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,557

"OUST 'PLUM'ES

NOR'R"i i ...... cH' ..... l'oe ........ UTC' NORt=t-11 01 1El •UL 91191' 104•'0'0

PERSZRN GULF

tO dULy' ' 1991 00372 01.00

Fig. 3a

Fig. 3. NOAA-11 AVHRR (a) channel I (visible) and (b) channel 4 (infrared) observations for this geographic region on July 10, 1991. (c) Section of the nighttime NOAA-11AVHRR channel 3 data acquired on June 5, 1991, at 2317 UT. The smoke plume is readily apparent for several hundred kilometers downwind of Kuwait as a warm region over the cooler desert surface, opposite to what is observed during the day.

winds were northwesterly, while at 700 mbar the winds were westerly at 45 knots (87 m/s). Thus, the "fan" characteristic of the smoke plume is due to pronounced vertical shear in wind direction.

Observations From NOAA Polar Orbiters

A number of techniques for detecting and monitoring aerosols from the NOAA AVHRR observations have re-

cently been developed. Ackerman [1990] made use of the 3.7- and 11-/am channels to study dust outbreaks, while Rao et al. [1989] used the visible channels to monitor aerosols over oceans. Durkee et al. [1990] uses the shortwave chan- nels in conjunction with the 3.7-/am channel to derive aerosol properties over oceans. Using the differences between 11 and 12/am to monitor volcanic ash clouds was investigated by Prata [1989].

An example of the AVHRR observations for this geo- graphic region is shown in Figure 3 for channels 1 (Figure 3a) and 4 (Figure 3b). The smoke is clearly distinguishable over land using the visible channel (Figure 3a). At the

infrared wavelengths available from the weather satellites (3.7, 11, and 12/am), smoke plumes appear similar in shape but usually with muted texture compared to the signature at the visible wavelengths. In the infrared, both smoke-and dust-laden atmospheres indicate cooler effective tempera- tures than surrounding regions during the day. These cooler effective temperatures may be due to either the infrared or shortwave radiative properties of the aerosol. In the infra- red, the aerosol may reduce the transmittance of the surface- emitted radiance. A cooler surface below the smoke plume may also result from surface cooling due to strong solar absorption by the smoke.

During the night the signature of the smoke plume is typically more readily apparent at the shorter wavelength (channel 3) compared to the longer wavelengths (channels 4 and 5), similar to the early morning observations [Limaye et al., 1991]. This indicates that the smoke is more transparent to the longer wavelengths compared to the shorter infrared wavelength, and thus that the typical particle size is likely to be somewhat smaller than at wavelengths of the longer

14,558 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

PER U c, iRN 13 LF

Fig. 3b

infrared channels. This is confirmed by the aircraft flight sampling of the smoke plume [Johnson et al., 1992; Hobbs and Radke, 1992], which showed that a majority of the smoke particles are approximately 0.1 /am in diameter and form chains several microns in length.

After sunset the desert surface cools rapidly and the brightness temperature contrast between the smoke plume and the adjacent surface (in the infrared images) reverses; for some hours during the night, the brightness temperature of the smoke plume is greater than the adjacent surface, even as far as several hundred kilometers downwind of the fires.

This is depicted in Figure 3c for channel 3 observations on June 5 at 2317 UT (similar results are seen for channel 4); darker regions correspond to warm temperatures. Due to infrared emission by the smoke, the plume is warmer than the adjacent smoke-free regions by several degrees. This is opposite to the daytime situation, as seen in Figure 3b.

It is difficult to separate the shortwave and longwave effects from the satellite data alone. The infrared extinction

is smaller than the extinction in the visible [Chylek et al., 1981; Ramaswamy and Kiehl, 1985]. At 11/am the extinction efficiency of smoke is very small for particle sizes less than 1 /am in diameter, and remains small until particle size exceeds 10/am [Rarnaswarny and Kiehl, 1985]. The smoke particles were generally submicron in size [Hobbs and

Radke, 1992; Johnson et al., 1992], although millimeter- sized oil droplets were observed close to the fires, and several-micron-long chains of smoke particles were detected by the aircraft. As the smoke ages, the size distribution of smoke particles is thus expected to be different, as a function of the distance from the source, and thus the analysis of the infrared signature of the smoke plume is complicated by competing effects of extinction and surface cooling. Thus the daytime cooler signature of the smoke plume in the infrared window channel observations from Meteosat and AVHRR is

due to extinction of the surface-emitted radiance and emis-

sion by the elevated plume and is not readily distinguishable from the reduced radiance from cooler surfaces expected from the obscuration of the sun.

When smoke and dust are present in the same region either at the same level or in stratified layers, as was encountered during the experiment, the visible and 11-/am infrared observations alone are not able to distinguish be- tween the amount of dust present, as illustrated in Figures 3a and 3b. Note that in this analysis the smoke plume is difficult to discern, indicating small spectral variation in the smoke properties in the infrared window region. However, the signature of the dust shows more difference at the two infrared wavelengths available from AVHRR than does the smoke. Thus, these two channels provide a possible means

LIMAYE ET AL.: SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,559

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$ JUN 911S6 231700 01064 00349 00.50

Fig. 3c

of detecting the presence of dust in the smoke plume. Analysis of the temperature difference between the 11- and 12-/am channels of NOAA-ll, •TB(11-12), is depicted in Figure 4 for the observations shown in Figures 3a and 3b. The dust plumes are easily seen in the •TB(I 1-12) difference image due to the spectral features of dust, especially in the fields north of Kuwait City. The magnitude of •T•(11-12) is a function of the optical thickness of the dust. This type of analysis would be useful for detecting mixtures of smoke and dust over land; visible imagery can be used to detect the presence of smoke, while •T•(11-12) can be used to infer the presence of dust. Presence of dust in the atmosphere is easily seen over water in the visible, unlike the presence of smoke.

Although the spatial resolution of the NOAA satellites is insufficient to monitor each burning well, the short infrared (3.7/•m) NOAA images do indicate the progress made in the number of wells capped throughout the summer and fall of 1991, by the gradual disappearance of hot spots.

3. IMPACT OF SMOKE ON REGIONAL CONDITIONS

The cooling effect of the dark smoke plume on the underlying surface, as expected under the smoke plume position indicated by the satellite observations, is evident in

the surface observations recorded at Manama, Bahrain.

Weather reports of smoke were first reported in the hourly sky conditions at Bahrain on January 26 (two reports), then again on February 10 (one report), 23 (11 reports), and 29 (four reports), for a total of 16 hourly reports. These are consistent with the initial ignition of the oil wells in January and later in February (prior to February 22), when most Of the remaining wells were set on fire. The frequency of the smoke reports rose dramatically in March (253 reports), and continued to increase through August (230 hourly reports of smoke in April, 306 in May). More than half of these occurred during the daylight hours, resulting in reduced insolation at the surface. During April, May, and June the daily maximum, minimum, and average temperatures were lower in Bahrain (Table 2) compared to averages of the last 30 years (1960-1990), and May, June, and July were the coldest months recorded. In all these months the reduction

of the average daily duration of sunshine was between 0.8 and 1.5 hours. At least some of this drop is attributable to the continual presence of smoke over Bahrain blown by the winds and often sinking down to the surface, reducing visibility to as low as 1.0 km (e.g., on May 10, 1991, 1000-1800 UT) or lower. Dust also contributed to the low- ered visibility. The research flights indicated that most of the

14,560 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

0006 ' 0• - i i ':"•i I 'g ?"'•'UL 91191 104"800

Fig. 4. Analysis of the temperature difference between the 11- and 12-/am channels from NOAA-11 (/5TB(11-12)) on July 10, 1991. The dust plumes show up as dark regions, negative differences, while the smoke plume is difficult to distinguish.

smoke was typically between 2 and 6 km above mean sea level [Hobbs and Radke, 1992]. On days when the smoke reached down to the surface, one could smell the smoke, and the absorption of the sunlight was so substantial that, in late afternoon, sunspots could be seen through the smoke with the naked eye. Smoke was observed to descend to the surface even during the day, a notion contrary to the lofting mechanism that should lift the smoke due to solar absorp- tion. Lofting could still have taken place in the upper layers of the plume. Probable explanations include vertical mixing and large-scale descending motions associated with the anticyclonic circulation prevalent to the region.

As the data in the Table 2 indicate, the monthly averaged minimum, maximum, and mean temperatures are smaller than the climatological values (as 30-year average minimum and maximum values were not readily available for all months, 16-year averages and root-mean-square (rms) devi- ations have been given in Table 2 for consistency in monthly mean, minimum, and maximum temperatures). For June, July, and August, the year to year variability of the monthly mean temperature, as indicated by the standard deviation, is between 0.3 ø and 0.7øC, much smaller than the drop ob- served in the minimum and mean temperatures. Observa-

tions recorded at Dhahran, only a short distance away, show a similar cooling. The prevalence of smoke over Bahrain at least circumstantially links the recorded drop to the reduc- tion of the sunshine during the daylight hours. Surprisingly, the monthly average maximum temperatures recorded dur- ing the summer months of 1991 are within climatological variability, whereas the monthly average minimum and mean temperatures are lower by more than two or three standard deviations. This supports the notion that the smoke is only weakly absorbing in the infrared, thereby allowing continued cooling during the evening and nighttime. If the smoke absorbed strongly in the infrared, this would suppress surface radiative cooling during the night. It is apparent, however, that a more thorough analysis of the influence of smoke on the temperatures recorded at the surface is war- ranted that takes into account the influence of the synoptic conditions and the smoke optical thickness, which can be estimated from the satellite.

Why the monthly mean maximum temperatures are not lower by as much as the mean temperature may have to do with above-normal temperatures on days when smoke was not present or, in view of the frequent presence of the smoke near the surface during the day, when solar heating of the

LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,561

DHAHRAN 1991

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TEMPERATURE (c) Fig. $. Temperature and moisture atmospheric soundings from Dhahran on June 12 and 15, 1990 (Julian days 163

and 166, respectively). Several elevated temperature inversions are evident. Aircraft flights showed that smoke was trapped in these layers, and the inversions may have been frequently created by the presence of the plumes themselves.

plume may have elevated the surface temperature. Also, the time of day when the smoke is close to the surface is important.

Figure 5 depicts two temperature and dew point soundings acquired on June 12 and 15, 1991, at 1200 UT from Dhahran, Saudi Arabia, just a short distance across the causeway from Bahrain. Satellite observations indicated the presence of smoke on June 12 and none on June 15 (see Figure 1). Elevated temperature inversions between 400 and 600 mbar are typically observed in this region and denote the top of the dust layer [Ackerman and Cox, 1989]. The sounding on June 12 shows an elevated temperature inversion near 850 mbar, which is associated with the top of the smoke plume. No low-level inversion exists on June 15, when the smoke was absent. Also the lower atmosphere is cooler, and therefore the atmosphere is more stable, on the day with smoke. Again, more detailed study of the rawinsonde data and satellite observations needs to be undertaken to fully under- stand the impact of the smoke on the region's weather.

On November 6, 1991, the last well was capped; however, the capping of the wells does not end the potential effect the fires can have on the region's climate. The surface charac- teristics play an important role in determining climate of this region. The settling of smoke particles immediately down- wind of the oil fields and over the interior of the Arabian

Peninsula has changed the surface albedo, which may feed back on the region's local climate (e.g., result in greater

daytime surface temperature due to increased solar absorp- tion). The effect on the surface is clearly seen in the vicinity of the oil fields in the NOAA AVHRR image shown in Figure 6 (see arrow). Several dark areas near the oil fields in Kuwait are seen which are elongated along the prevailing wind direction. The darkening is due to the settling of the larger smoke particles or chains that rained out of the plume as the smoke drifted [Hobbs and Radke, 1992]. Changes further downwind over the interior of the peninsula are certainly more subtle. Detailed analysis of prefire and postfire AVHRR and Meteosat images can be used to determine the effect of smoke deposition on the surface reflectance, sur- face temperature, and cloud cover. Deposition of the smoke also changes the surface chemical composition and may have an impact on the ability of the wind to raise the dust off the surface and into the atmosphere. On the other hand, due to the large number of dust events in this region which may originate further north and west, the deposited smoke parti- cles may be quickly covered by dust. Satellite observations can assess changes in dust storm frequency between prefire and postfire conditions.

4. SUMMARY

The smoke from the oil fires in Kuwait was readily visible in conventional observations by weather satellites in polar and geostationary orbits. The temporal and spectral cover-

14,562 LIMAYE ET AL.' SATELLITE MONITORING OF SMOKE FROM KUWAIT

...... 0004 ............ NORR-i2 04 12' NOV 91316 043600 00368 00368 02.00

Fig. 6. NOAA-11AVHRR visible (channel 1) image of Kuwait's environs acquired on November 12, 1991. A large area of lowered albedo is evident as a result of the settlement of the smoke particles. The elongated shape of the darkened area is a signature of the strong directional persistence of the winds during the period when the fires burned.

age provided by these observations complements the higher resolution observations gathered by System Probatoire d'Observation de la Terre (SPOT) and Landsat satellites. The summer 1991 observations from satellites and aircraft

showed that bulk of the smoke remained in the lower

troposphere, well below 5-6 km within 1000-1500 km of the source region. Settling of the larger particles has left several darkened, elongated regions downwind of the oil fields, parallel to the prevalent wind direction during the period that the oil wells were on fire. Daily time lapse sequences of visible channel images did show occasional presence of smoke further to the west and north as well as to the east, suggesting that some smaller smoke particles did rise some- what higher. Some 500 km downwind of Kuwait, the mean monthly mean and minimum surface temperatures recorded in Bahrain during May-August 1991 were 2.0-3.2øC lower than the climatological average values (preceding 16 years), well above the standard deviation for the monthly mean and minimum temperatures. During May and June, smoke was reported at the surface in Bahrain on 11 and 16 days respectively, and this seems to be the dominant reason for the temperature drop. It is apparent that a more thorough

analysis of the influence of the smoke on the regional climatic conditions is needed for a better understanding of the degree of impact.

The success and usefulness of a portable McIDAS work station in locating and tracking the smoke demonstrate the value of current satellite data in planning and conducting of field experiments. Satellite observations provide valuable information on the transportation and deposition of the smoke. Also evident is the potential role of the satellite observations for determining the impact of the smoke on regional weather and climate; however, a more detailed correlative study is required before the impact can be fully assessed. Such studies are planned for the future.

Acknowledgments. We gratefully acknowledge the cooperation and support provided by EUMETSAT and, in particular, by Horst Faas, who disseminated the Meteosat visible images early in the day expressly for the purpose of the field experiment. We also wish to acknowledge the staff of the Meteorological Service, Civil Aviation Affairs, Bahrain International Airport. This work was supported by National Science Foundation through a contract with the National Center for Atmospheric Research (NCAR), which is operated by the

LIMAYE ET AL..' SATELLITE MONITORING OF SMOKE FROM KUWAIT 14,563

University Corporation for Atmospheric Research (UCAR), and by the Space Science and Engineering Center of the University of Wisconsin-Madison. Assistance of G. Peltzer has been valuable.

Finally, we thank two anonymous referees for their useful com- ments.

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G. Ali, H. Ali, M. Isa, and A. Wright, Meteorological Service Civil Aviation Affairs, Bahrain International Airport, Bahrain.

A. Rangno, Department of Atmospheric Science, University of Washington, Seattle, WA 98195.

(Received February 7, 1992; revised May 3, 1992;

accepted May 6, 1992.)