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Impact of Indonesian forest fires during the 1997 El Nino on the aerosol distribution over the Indian Ocean
Advances in Space Research 33 (2004) 1098–1103 www.elsevier.com/locate/asr
Impact of Indonesian forest fires during the 1997 El Nino on the aerosol distribution over the Indian Ocean K. Parameswaran *, Sandhya K. Nair, K. Rajeev Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 1 December 2002; received in revised form 24 March 2003; accepted 25 March 2003
Abstract The El Nino event of 1997–1998 followed by the La Nina in 1998–1999 was the strongest of its kind encountered in the 20th century. Associated with this event Indonesia experienced severe drought leading to large-scale forest fires. Large aerosol plumes from these fires advected over the equatorial Indian Ocean region. Development and decay of this plume and its regional transport are studied using the aerosol optical depth derived from the NOAA14-AVHRR data along with the tropospheric circulation and meteorological conditions derived from NCEP/NCAR reanalysis. The study shows that associated with the large-scale fires in the Indonesian region, a substantially large aerosol plume formed over the equatorial Indian Ocean in the latitude range of 5°N to 10°S, during September–November period of 1997. This plume was confined to the eastern equatorial Indian Ocean in September, and reached up to about 60°E by October. The westward transport of this aerosol plume was driven by the anomalous easterly winds over the tropical Indian Ocean during the El Nino period. A close association is observed between the aerosol optical depth over equatorial Indian Ocean and the fire counts over Southeast Asia derived from Along Track Scanning Radiometer data. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Aerosol optical depth; Regional aerosol distribution; Aerosol transport; Forest fire; El Nino
1. Introduction Burning of biomass releases significant amount of carbonaceous aerosols along with some trace gases into the atmosphere, which can influence the atmospheric chemistry, radiation budget and climate (Crutzen and Andreae, 1990). As the atmospheric aerosols affect the radiation budget on both regional and global scale, directly by scattering and absorbing the incoming solar radiation and indirectly by taking part in cloud process, it is essential to include the influence of biomass burning emissions in climate models (Penner et al., 1992). Fire events at Southeast Asia and its connection with aerosols and trace gases are reported in the literature (Malingreau, 1990; Levine et al., 1999; Laumonier, 1997; Nakajima et al., 1999). During the very strong El Nino event of 1997–1998 (Asnani, 2001), Southeast Asia
*
Corresponding author. Tel.: +91-471-563927; fax: +91-471-415335. E-mail address: [email protected] (K. Parameswaran).
experienced severe drought, which caused large-scale forest fires (Levine et al., 1998). Further, the Northeast monsoonal rainfall over Indonesia was delayed, which let the fires to continue for several weeks. This led to the release of large amount of aerosols into the atmosphere, significantly affecting the regional aerosol distribution (Nakajima et al., 1999). Satellite remote sensing of aerosols remains the most promising tool for studying the regional and global aerosol distributions (Ferrare et al., 1990; Malingreau, 1990; Kaufman et al., 1990, 1997; Stowe et al., 1997), which together with the radiative transfer models can provide information on the impact of individual aerosol plumes on regional radiative effects (Christopher et al., 1996). In this paper, the development and decay of the aerosol plume originated from the forest fires in the Southeast Asia during the September to December period of 1997 over the Indian Ocean is examined based on the aerosol optical depth (AOD) derived from NOAA14-AVHRR data. Fire counts obtained from the Along Track Scanning Radiometer (ATSR) during this
0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0273-1177(03)00736-1
K. Parameswaran et al. / Advances in Space Research 33 (2004) 1098–1103
period are used to study the association between the fire occurrence over Southeast Asia and the regional mean AOD over the equatorial Indian Ocean. The variabilities in the regional aerosol distribution over the Indian Ocean with the corresponding changes in atmospheric circulation and transport are examined based on NCEP/ NCAR reanalysis.
2. Data and method of analysis All the daytime NOAA14-AVHRR Global Area Coverage (GAC) data in the latitude range of 15°N to 15°S and longitude range of 40°E to 110°E from September to December, 1997 are used for the present study. AOD at the wavelength of 630 nm (50 nm) in the oceanic environment is obtained by inverting the pixel-wise radiance observed at channel 1 of NOAA14AVHRR. Methodology used for deriving the AOD from AVHRR data, the sources of errors and the stringent cloud screening procedures adopted for the AOD retrieval are explained in detail by Rajeev et al. (2000) and Rajeev and Ramanathan (2001). The pixel resolution of AVHRR-GAC data is 4 km at nadir. The degradation of AVHRR sensor is corrected by using the revised post-launch calibration constants (Rao and Chen, 1999). To avoid specular reflection from the ocean surface, which introduces large uncertainties in the derived AOD, data from the anti-solar side of the satellite scan alone is used in the present study. Inversion of satellite measured radiance to derive the AOD is carried out by comparing the observed radiance in channel 1 with that of a theoretically modelled lookup table of AOD and top-of-atmosphere (TOA) radiance. The look-up table is generated using the discrete ordinate radiative transfer method for plane parallel atmosphere (Stamnes et al., 1998), which accounts for multiple scattering due to aerosols and molecules as well as absorption due to aerosols, water vapour and ozone, as explained in Rajeev and Ramanathan (2001). Vertical profiles of pressure, temperature, molecular density and ozone are taken from the model of McClatchey et al. (1972). The measurements of chemical and physical properties of aerosols from the Indonesian fires are sparse (Flint and Richards, 1994). Based on the observations of the properties of smoke aerosols at different tropical regions, mainly the Mexican, Amazonian and African regions (Kreidenweis et al., 2001, and the references there in), we have assumed a smoke aerosol model containing organics (60%), black carbon (5%), sea salt (20%) and sulfate (15%). (Values in the bracket indicate the percentage of each species by mass.) Each of these species is assumed to have a lognormal size distribution with mode radius (rm ) and standard deviation (r) for relative humidity of 70% as (Hess et al., 1998): Sulphate (rm ¼ 0:0285 lm, r ¼ 2:24), Soot (rm ¼
1099
0:0118 lm, r ¼ 2:0), Sea salt (rm1 ¼ 0:38 lm, r ¼ 2:03; rm2 ¼ 3:17 lm, r ¼ 2:03), Organics (rm ¼ 0:0285 lm, r ¼2:24). The aerosols are assumed to be externally mixed with a single scattering albedo of 0.9. The area affected by forest fire can be obtained from the fire counts derived from the satellite observations (Matson et al., 1987; Arino and Rosaz, 1999). The intensity of forest fire is quantified in terms of fire counts observed by ATSR. The monthly mean fire counts, used in this study, is derived from the nighttime data at the 3.7 lm channel of the Along Track Scanning Radiometer (ATSR-2) onboard the European Remote Sensing Satellite (ERS-2). Here the fire count is derived from the number of pixels with the brightness temperature exceeding 312 K, as explained in Arino and Rosaz (1999), and this data was obtained from the ATSR World Fire Atlas of European Space Agency.
3. Results and discussions Fig. 1 shows the 15-days average regional aerosol distribution over the Indian Ocean during September– December 1997 in the latitude–longitude range of 15°N to 15°S and 40°E to 110°E. Significantly high AOD values are observed over equatorial Indian Ocean during this period with varying intensities. The aerosol plume, which was confined to the coastal regions of Indonesia during the first half of September, started extending towards west through the equatorial Indian Ocean reaching 60°E by the first half of October. The strength (in terms of mean AOD) and spatial extent of this aerosol plume increased from the first half of September, reaching their respective maximum values during the second half of October with a very large longitudinal extent covering up to about 60°E. In October, the AOD at some regions of this plume exceeds 1.5. Compared to that in the zonal direction, the spatial gradient in AOD is significantly large along the meridional direction. This is because of the fact that the aerosol plume was mainly confined to the equatorial region and the prevailing horizontal transport was favourable for spreading along the zonal direction. The plume, which started its decay in the first half of November, subsided almost completely by December 1997, with AODs in this geographical region (Nair et al., 2003) resuming its normal value (<0.2). In order to examine the association between the AOD over the equatorial Indian Ocean and the fire occurrence over the Southeast Asian region, the AOD values derived from NOAA14-AVHRR data are compared with fire counts obtained from ATSR. Fig. 2 shows the monthly variation of the average AOD over the eastern equatorial Indian Ocean within the latitude–longitude range of 5°N to 5°S and 85°E to 95°E and the fire counts over Southeast Asia with in the latitude-longitude range of 5°N to
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Fig. 1. Regional maps of the 15-days mean AOD (at wavelength 630 + 50 nm) from September 1997 to December 1997 over the tropical Indian Ocean (N/R indicates regions where AOD could not be retrieved due to presence of clouds).
Fig. 2. Temporal variation of monthly mean AVHRR-derived AOD over the eastern equatorial Indian Ocean region and ATSR-derived fire counts over Southeast Asian region.
5°S and 100°E to 120°E during the period 1997–1999. The average AOD in this region during September 1997 was 0.4 which increased to its peak value of 0.95 in October 1997. The mean AOD decreased to 0.09 in December 1997, which is very close to the normal background AOD value for December, in this region (Nair et al., 2003). The ATSR fire counts also show a prominent peak during September–October period of 1997. The value of monthly total fire count exceeds 6000 in September 1997, during the peak activity and the peak in
AOD occurs in October 1997. Both the mean AOD and the fire counts show a small increase during the March– April period of 1998, though the magnitude of the increase is less than that observed during September– November period of 1997. The monthly mean AOD shows a significant (at 99.99% level) correlation (R ¼ 0:88) with monthly total fire counts, with a small lag of 1 month. These observations clearly show a close association between the AOD at the eastern equatorial Indian Ocean and the fire counts over Sumatra, Indonesia, during the El Nino period of 1997–1998. Spatial distribution of aerosols in the atmosphere is significantly influenced by the prevailing meteorology and atmospheric circulation. The development and decay of the aerosol plume originated from the forest fire would have been influenced by these factors. This aspect has been examined based on the NCEP/NCAR reanalysis data. Fig. 3 shows the prevailing atmospheric circulation at three typical pressure levels, 925 hpa (0.7 km), 500 hpa (5.6 km) and 200 hpa (12 km), during October 1997. The gross features of the circulation pattern are similar for all the months during the period September–December 1997. Though the wind fields are shown only for three typical altitudes, the circulation pattern in the geographic region of the plume generally
K. Parameswaran et al. / Advances in Space Research 33 (2004) 1098–1103
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Fig. 3. Monthly mean winds at the 925, 500 and 200 hpa levels (contours of wind speeds are marked at intervals of 3 ms1 ) along with a plot of composite mean Outgoing Long-wave Radiation (W m2 ) (bottom right panel) during October 1997 obtained from NCEP/NCAR reanalysis.
remained more or less the same (easterly) and a wind reversal is seen only above 150 hpa level. During the non-El Nino years the lower tropospheric wind in the eastern parts of the equatorial Indian Ocean is westerly during September–December. During the El Nino period of 1997, the lower tropospheric wind in this region is predominantly easterly between 22°N and 25°S, as seen in Fig. 3. The easterly wind at the equatorial Indian Ocean is responsible for the transport of aerosols originated from the Indonesian forest fires over to the tropical Indian Ocean. Another important feature observed in Fig. 3 is the large-scale convergence over the western equatorial Indian Ocean between 50°E and 60°E during the September–November period of 1997. Right bottom panel of Fig. 3 shows the outgoing long wave radiation (OLR) flux for October 1997 obtained from NCEP/NCAR reanalysis. This shows abnormally low values of OLR in the geographical region between 5°N to 5°S and 50°E to 70°E. This indicates that large cloudiness and rainfall occurred at the western equatorial Indian Ocean which might have prevented the spreading of this smoke plume further westward of 60°E, particularly during October 1997, as seen in Fig. 1.
The spatial distribution of aerosols (Fig. 1) shows the columnar aerosol loading in the plume and does not provide any information regarding the altitude where the aerosol loading is a maximum. The wind field also does not indicate much in this regard because at all altitudes below 150 hpa level the wind pattern is easterly over the equatorial Indian Ocean. A coarse indication of the altitude variation of aerosols can be obtained by comparing the AVHRR-derived spatial pattern of AOD with aerosol index maps of Total Ozone Mapping Spectrometer (TOMS). TOMS provides information (http://toms.gsdc.nasa.gov/aerosols/aerosols.html) on aerosol loading above 1.5 km and is not quite sensitive to aerosols below this altitude. An examination of daily aerosol index maps from TOMS data during this period shows that the plume is mostly confined to regions east of 90°E and at any rate it did not cross the 80°E longitude. The spatial pattern of aerosol index from TOMS also showed significant day-to-day variability especially in the fringe zone indicating lower values for the aerosol lifetimes. From these it can be inferred that the aerosol plume extending west of 80°E and north of 5°N (as seen from AVHRR) is generally confined to the lower troposphere (mainly below 1.5 km).
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4. Intercomparison with in situ measurements The AVHRR AOD maps in Fig. 1 are further compared with in situ measured AOD at Singapore (1.3°N, 103.8°E) reported by Nakajima et al. (1999). Fig. 4(a) shows the time series of AVHRR derived AOD (630 nm) averaged with in a grid size of 1° l° (open square) around Singapore from September to December 1997 along with the daily mean values of AOD (500 nm) over Singapore (filled circle) observed using sun photometer (Nakajima et al., 1999). The vertical bars denote the standard deviation for AVHRR data. A grid size of 1° 1° is taken to achieve a fine spatial resolution for a fruitful comparison with sun photometer values. This narrow grid size, however, reduces the number of AVHRR pixels where AOD could be derived, due to the presence of clouds. Fig. 4(b) shows a plot of AVHRR AOD values averaged at 2° 2° grid size (filled squares) along with the sun photometer AOD reported by Nakajima et al. (1999) adjusted to 630 nm (open circle) using a multiplication factor of 1.25 which is the mean ratio of AOD at 500 nm to AOD at 630 nm (wavelength exponent of 0.96) observed by Nakajima et al. (1999) during the fire event. Fig. 4 is shown for two purposes: (1) to compare the AVHRR-derived AOD with the in situ values, and (2) to study the details of the time evolution of the mean AOD over this location which is more close to the regions of forest fires. The figure shown in the inset of Fig. 4(a) is the intercomparison of the average AVHRR-derived AOD (at 630 nm) with in a grid size of 1° 1° around Singapore and the corresponding daily mean in situ measured AOD (at 500 nm) at Singapore reported by Nakajima et al. (1999) and that shown in Fig. 4(b) is a similar plot of AVHRR AOD at 2° 2°
grid size with in situ measured AOD adjusted to 630 nm. The slope of the regression fit for both these are shown in the respective figures. The in situ measured values of AOD shown in Fig. 4(a) are slightly larger than the AVHRR-derived AOD because of the difference in wavelengths of these two observations. Except for this, in general, the AVHRR-derived AOD is in good agreement with the daily mean in situ measured AOD. The slope of the intercomparison shown in the inset of Fig. 4(a) is 1.20 and the correlation coefficient is 0.98. This is in fair agreement with the value of 1.25 obtained from Nakajima et al. (1999) considering the error limits. The 2° 2° data is mainly used to examine the AOD variation during the development phase of the plume by expanding the record through plotting the data for September. By increasing the grid size it could be possible to get enough points to depict the variation. The inset on Fig. 4(b) shows that the correlation between AVHRR AOD and in situ AOD at 630 nm is fairly good (even though scatter is slightly more) and the slope of the regression line is 1.06. The AVHRR measurements show that the AOD around Singapore were remaining high (AOD > 1.0) from September up to the second week of November 1997. The almost sudden transition from the hazy conditions to relatively cleaner conditions around 10 November 1997 is clearly seen both in the in situ measurements and the AVHRR derived AOD. It is interesting in this context to note that the mean value of single scattering albedo over Singapore (1.3°N, 103.8°E) measured during the forest fire period (Nakajima et al., 1999) also is about 0.9, the value applicable for the assumed smoke model for our analysis.
Fig. 4. Time series of the AVHRR-derived AOD (630 nm) averaged with in a grid size of 1° 1° (open square) during September to December 1997 along with in situ measured AOD (closed circle) using sun photometer at Singapore (Nakajima et al., 1999), inset showing a regression plot of the two (a) and Time series of the AVHRR-derived AOD (630 nm) averaged with a grid size of 2° 2° (solid square) at Singapore along with in situ AOD adjusted to 630 nm (open circle), inset showing a regression plot of the two (b).
K. Parameswaran et al. / Advances in Space Research 33 (2004) 1098–1103
5. Conclusions Impact of the El Nino related Indonesian forest fires, which occurred during the September to December period of 1997, on the aerosol distribution over the tropical Indian ocean is studied based on the satellite-derived (NOAA14-AVHRR) AOD over the Indian Ocean, the wind data obtained from the NCEP/NCAR reanalysis and the ATSR-derived fire counts over the Southeast Asian region. The El Nino related weather and atmospheric conditions have significantly influenced the regional aerosol distribution over the Indian Ocean. The major El Nino event of 1997 caused large-scale forest fires in Indonesia producing massive amounts of carbonaceous aerosols. A substantially large aerosol plume formed over the equatorial Indian Ocean, during September to November period of 1997. This plume started in September reached up to about 60°E by October and decayed almost completely by December. Westward transport of the aerosol plume was driven by the anomalous easterly winds over the tropical Indian Ocean during the El Nino period and was arrested at 60°E by wet removal. The development and decay of the plume is highly correlated with the fire counts in the Indonesian region and lower tropospheric circulation.
Acknowledgements The fire counts were provided by ATSR World Fire Atlas, European Space Agency, ESA/ESRIN, via Galileo Galilei, CP 64, 00044 Frascati, Italy through website http://sharkl.esrin.esa.it/FIRE/AF/ATSR/ and NCEP/ NCAR reanalyses by the NOAA-CIRES Climate Diagnostics Center, Boulder through Website http://www/ cdc.noaa.gov.
References Arino, O., Rosaz, J.M., 1997 and 1998 World ATSR fire atlas using ERS-2 ATSR-2 data, in: Proceedings of Joint Fire Science Conf., Boise, Idaho, Published by the University of Idaho and the International Association of Wildland Fire. pp. 177–182, 1999. Asnani, G.C., El Nino of 1997–1998 and Indian Monsoon. Mausam 52, 57–66, 2001. Christopher, S.A., Kliche, D.V., Chou, J., Welch, R.M. First estimates of the radiative forcing of aerosols generated from biomass burning using satellite data. J. Geophys. Res 101, 21265–21273, 1996. Crutzen, P.J., Andreae, M.O. Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669–1678, 1990. Flint, E.P., Richards, J.F. Trends in carbon content of vegetation in south and Southeast Asia associated with changes in land use, in: Dale, V.H. (Ed.), Effects of Land-use Change on Atmospheric CO2 Concentrations: South and Southeast Asia as a Case Study. Springer-Verlag, pp. 201–299, 1994.
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Ferrare, R.A., Fraser, R.S., Kaufman, Y.J. Satellite measurements of large-scale air pollution: measurements of forest fire smoke. J. Geophys. Res. 95, 9911–9925, 1990. Hess, M., Kopke, P., Schult, I. Optical properties of aerosols and clouds: the software package Opac. Bull. Am. Meteorol. Soc. 79, 831–844, 1998. Kaufman, Y.J., Setzer, A., Justice, C., Tucker, C.J., Pereira, M.C., Fung, I. Remote sensing of biomass burning in the tropics, in: Goldammer, J.G. (Ed.), Fires in the Tropical Biota. SpringerVerlag, pp. 371–399, 1990. Kaufman, Y.J., Tanre, D., Remer, L.A., Vermote, E.F., Chu, A., Holben, B.N. Operational remote sensing of tropospheric aerosol over land from EOS moderate resolution imaging spectroradiometer. J. Geophys. Res. 102, 17051–17067, 1997. Kreidenweis, S.M., Remer, L.A., Bruintjes, R., Dubovik, O. Smoke aerosols from biomass burning in Mexico: hygroscopic smoke optical model. J. Geophys. Res. 106, 4831–4844, 2001. Laumonier, Y. The Vegetation and Physiography of Sumatra. Kluwer Academic Publishers, Dordrecht, 1997. Levine, J.S., Edwards, T.D., McReynolds, T.E., Dull, C.W., Gaseous and particulate emissions released to the atmosphere during the Fires in Kalimantan and Sumatra, Indonesia in 1997, WMO Workshop on Regional Transboundary Smoke and Haze in Southwest Asia, Singapore, 2–5 June 1998. Levine, J.S., Bobbe, T., Ray, N., Witt, R.G., Singh, A.,Wildland fires and the environment: a global synthesis. Environment Information and Assessment Technical Report, p. 46, 1999. Malingreau, J.P. The contribution of remote sensing to the global monitoring of fires in tropical and subtropical ecosystems, in: Goldammer, J.G. (Ed.), Fires in the Tropical Biota. SpringerVerlag, pp. 337–370, 1990. Matson, M., Stephens, G., Robinson, J. Fire detection using data from the NOAA-N satellites. Int. J. Remote Sensing 8, 961–970, 1987. McClatchey, R.A., Fenn, R.W., Selby, J.E.A., Volz, F.E., Garing, J.S., Optical properties of the atmosphere, Environmental Research Papers, No. 411, Air Force Cambridge Research Laboratories, Bedford, MA, 1972. Nair, S.K., Rajeev, K., Parameswaran, K. Winter time regional aerosol distribution and the influence of continental transport over the Indian Ocean. J. Atmos. Sol. Terr. Phys. 65, 149–165, 2003. Nakajima, T., Higurashi, A., Takeuchi, N., Herman, J.R. Satellite and ground-based study of aerosol properties of 1997 Indonesian forest fire aerosols. Geophys. Res. Lett. 26, 2421–2424, 1999. Penner, J.E., Dickinson, R.E., OÕNeill, C.A. Effects of aerosol from biomass burning on the global radiation budget. Science 256, 1432– 1434, 1992. Rajeev, K., Ramanathan, V., Meywerk, J. Regional aerosol distribution and its long-range transport over the Indian Ocean. J. Geophys. Res. 105, 2029–2043, 2000. Rajeev, K., Ramanathan, V. Direct observations of clear-sky aerosol radiative forcing from space during the Indian Ocean Experiment. J. Geophys. Res. 106, 17,221–17,236, 2001. Rao, C.R.N., Chen, J. Revised post launch calibration of the visible and near infrared channels of the advanced very high resolution radiometer (AVHRR) on the NOAA-14 spacecraft. Int. J. Remote Sensing 20, 3485–3491, 1999. Stamnes, K., Tsay, S.C., Wiscombe, W., Jayaweera, K. A numerically stable algorithm for discrete-ordinate method radiative transfer in multiple scattering and emitting layered media. Appl. Opt. 27, 2502–2509, 1998. Stowe, L.L., Ignatov, A.M., Singh, R.R. Development, validation, and potential enhancements to the second-generation operational aerosol product at the National Environmental Satellite, Data, and Information Service of the National Oceanic and Atmospheric Administration. J. Geophys. Res. 102, 16923–16934, 1997.
Impact of Indonesian forest fires during the 1997 El Nino on the aerosol distribution over the Indian Ocean K. Parameswaran *, Sandhya K. Nair, K. Rajeev Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 1 December 2002; received in revised form 24 March 2003; accepted 25 March 2003
Abstract The El Nino event of 1997–1998 followed by the La Nina in 1998–1999 was the strongest of its kind encountered in the 20th century. Associated with this event Indonesia experienced severe drought leading to large-scale forest fires. Large aerosol plumes from these fires advected over the equatorial Indian Ocean region. Development and decay of this plume and its regional transport are studied using the aerosol optical depth derived from the NOAA14-AVHRR data along with the tropospheric circulation and meteorological conditions derived from NCEP/NCAR reanalysis. The study shows that associated with the large-scale fires in the Indonesian region, a substantially large aerosol plume formed over the equatorial Indian Ocean in the latitude range of 5°N to 10°S, during September–November period of 1997. This plume was confined to the eastern equatorial Indian Ocean in September, and reached up to about 60°E by October. The westward transport of this aerosol plume was driven by the anomalous easterly winds over the tropical Indian Ocean during the El Nino period. A close association is observed between the aerosol optical depth over equatorial Indian Ocean and the fire counts over Southeast Asia derived from Along Track Scanning Radiometer data. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Aerosol optical depth; Regional aerosol distribution; Aerosol transport; Forest fire; El Nino
1. Introduction Burning of biomass releases significant amount of carbonaceous aerosols along with some trace gases into the atmosphere, which can influence the atmospheric chemistry, radiation budget and climate (Crutzen and Andreae, 1990). As the atmospheric aerosols affect the radiation budget on both regional and global scale, directly by scattering and absorbing the incoming solar radiation and indirectly by taking part in cloud process, it is essential to include the influence of biomass burning emissions in climate models (Penner et al., 1992). Fire events at Southeast Asia and its connection with aerosols and trace gases are reported in the literature (Malingreau, 1990; Levine et al., 1999; Laumonier, 1997; Nakajima et al., 1999). During the very strong El Nino event of 1997–1998 (Asnani, 2001), Southeast Asia
*
Corresponding author. Tel.: +91-471-563927; fax: +91-471-415335. E-mail address: [email protected] (K. Parameswaran).
experienced severe drought, which caused large-scale forest fires (Levine et al., 1998). Further, the Northeast monsoonal rainfall over Indonesia was delayed, which let the fires to continue for several weeks. This led to the release of large amount of aerosols into the atmosphere, significantly affecting the regional aerosol distribution (Nakajima et al., 1999). Satellite remote sensing of aerosols remains the most promising tool for studying the regional and global aerosol distributions (Ferrare et al., 1990; Malingreau, 1990; Kaufman et al., 1990, 1997; Stowe et al., 1997), which together with the radiative transfer models can provide information on the impact of individual aerosol plumes on regional radiative effects (Christopher et al., 1996). In this paper, the development and decay of the aerosol plume originated from the forest fires in the Southeast Asia during the September to December period of 1997 over the Indian Ocean is examined based on the aerosol optical depth (AOD) derived from NOAA14-AVHRR data. Fire counts obtained from the Along Track Scanning Radiometer (ATSR) during this
0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0273-1177(03)00736-1
K. Parameswaran et al. / Advances in Space Research 33 (2004) 1098–1103
period are used to study the association between the fire occurrence over Southeast Asia and the regional mean AOD over the equatorial Indian Ocean. The variabilities in the regional aerosol distribution over the Indian Ocean with the corresponding changes in atmospheric circulation and transport are examined based on NCEP/ NCAR reanalysis.
2. Data and method of analysis All the daytime NOAA14-AVHRR Global Area Coverage (GAC) data in the latitude range of 15°N to 15°S and longitude range of 40°E to 110°E from September to December, 1997 are used for the present study. AOD at the wavelength of 630 nm (50 nm) in the oceanic environment is obtained by inverting the pixel-wise radiance observed at channel 1 of NOAA14AVHRR. Methodology used for deriving the AOD from AVHRR data, the sources of errors and the stringent cloud screening procedures adopted for the AOD retrieval are explained in detail by Rajeev et al. (2000) and Rajeev and Ramanathan (2001). The pixel resolution of AVHRR-GAC data is 4 km at nadir. The degradation of AVHRR sensor is corrected by using the revised post-launch calibration constants (Rao and Chen, 1999). To avoid specular reflection from the ocean surface, which introduces large uncertainties in the derived AOD, data from the anti-solar side of the satellite scan alone is used in the present study. Inversion of satellite measured radiance to derive the AOD is carried out by comparing the observed radiance in channel 1 with that of a theoretically modelled lookup table of AOD and top-of-atmosphere (TOA) radiance. The look-up table is generated using the discrete ordinate radiative transfer method for plane parallel atmosphere (Stamnes et al., 1998), which accounts for multiple scattering due to aerosols and molecules as well as absorption due to aerosols, water vapour and ozone, as explained in Rajeev and Ramanathan (2001). Vertical profiles of pressure, temperature, molecular density and ozone are taken from the model of McClatchey et al. (1972). The measurements of chemical and physical properties of aerosols from the Indonesian fires are sparse (Flint and Richards, 1994). Based on the observations of the properties of smoke aerosols at different tropical regions, mainly the Mexican, Amazonian and African regions (Kreidenweis et al., 2001, and the references there in), we have assumed a smoke aerosol model containing organics (60%), black carbon (5%), sea salt (20%) and sulfate (15%). (Values in the bracket indicate the percentage of each species by mass.) Each of these species is assumed to have a lognormal size distribution with mode radius (rm ) and standard deviation (r) for relative humidity of 70% as (Hess et al., 1998): Sulphate (rm ¼ 0:0285 lm, r ¼ 2:24), Soot (rm ¼
1099
0:0118 lm, r ¼ 2:0), Sea salt (rm1 ¼ 0:38 lm, r ¼ 2:03; rm2 ¼ 3:17 lm, r ¼ 2:03), Organics (rm ¼ 0:0285 lm, r ¼2:24). The aerosols are assumed to be externally mixed with a single scattering albedo of 0.9. The area affected by forest fire can be obtained from the fire counts derived from the satellite observations (Matson et al., 1987; Arino and Rosaz, 1999). The intensity of forest fire is quantified in terms of fire counts observed by ATSR. The monthly mean fire counts, used in this study, is derived from the nighttime data at the 3.7 lm channel of the Along Track Scanning Radiometer (ATSR-2) onboard the European Remote Sensing Satellite (ERS-2). Here the fire count is derived from the number of pixels with the brightness temperature exceeding 312 K, as explained in Arino and Rosaz (1999), and this data was obtained from the ATSR World Fire Atlas of European Space Agency.
3. Results and discussions Fig. 1 shows the 15-days average regional aerosol distribution over the Indian Ocean during September– December 1997 in the latitude–longitude range of 15°N to 15°S and 40°E to 110°E. Significantly high AOD values are observed over equatorial Indian Ocean during this period with varying intensities. The aerosol plume, which was confined to the coastal regions of Indonesia during the first half of September, started extending towards west through the equatorial Indian Ocean reaching 60°E by the first half of October. The strength (in terms of mean AOD) and spatial extent of this aerosol plume increased from the first half of September, reaching their respective maximum values during the second half of October with a very large longitudinal extent covering up to about 60°E. In October, the AOD at some regions of this plume exceeds 1.5. Compared to that in the zonal direction, the spatial gradient in AOD is significantly large along the meridional direction. This is because of the fact that the aerosol plume was mainly confined to the equatorial region and the prevailing horizontal transport was favourable for spreading along the zonal direction. The plume, which started its decay in the first half of November, subsided almost completely by December 1997, with AODs in this geographical region (Nair et al., 2003) resuming its normal value (<0.2). In order to examine the association between the AOD over the equatorial Indian Ocean and the fire occurrence over the Southeast Asian region, the AOD values derived from NOAA14-AVHRR data are compared with fire counts obtained from ATSR. Fig. 2 shows the monthly variation of the average AOD over the eastern equatorial Indian Ocean within the latitude–longitude range of 5°N to 5°S and 85°E to 95°E and the fire counts over Southeast Asia with in the latitude-longitude range of 5°N to
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Fig. 1. Regional maps of the 15-days mean AOD (at wavelength 630 + 50 nm) from September 1997 to December 1997 over the tropical Indian Ocean (N/R indicates regions where AOD could not be retrieved due to presence of clouds).
Fig. 2. Temporal variation of monthly mean AVHRR-derived AOD over the eastern equatorial Indian Ocean region and ATSR-derived fire counts over Southeast Asian region.
5°S and 100°E to 120°E during the period 1997–1999. The average AOD in this region during September 1997 was 0.4 which increased to its peak value of 0.95 in October 1997. The mean AOD decreased to 0.09 in December 1997, which is very close to the normal background AOD value for December, in this region (Nair et al., 2003). The ATSR fire counts also show a prominent peak during September–October period of 1997. The value of monthly total fire count exceeds 6000 in September 1997, during the peak activity and the peak in
AOD occurs in October 1997. Both the mean AOD and the fire counts show a small increase during the March– April period of 1998, though the magnitude of the increase is less than that observed during September– November period of 1997. The monthly mean AOD shows a significant (at 99.99% level) correlation (R ¼ 0:88) with monthly total fire counts, with a small lag of 1 month. These observations clearly show a close association between the AOD at the eastern equatorial Indian Ocean and the fire counts over Sumatra, Indonesia, during the El Nino period of 1997–1998. Spatial distribution of aerosols in the atmosphere is significantly influenced by the prevailing meteorology and atmospheric circulation. The development and decay of the aerosol plume originated from the forest fire would have been influenced by these factors. This aspect has been examined based on the NCEP/NCAR reanalysis data. Fig. 3 shows the prevailing atmospheric circulation at three typical pressure levels, 925 hpa (0.7 km), 500 hpa (5.6 km) and 200 hpa (12 km), during October 1997. The gross features of the circulation pattern are similar for all the months during the period September–December 1997. Though the wind fields are shown only for three typical altitudes, the circulation pattern in the geographic region of the plume generally
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Fig. 3. Monthly mean winds at the 925, 500 and 200 hpa levels (contours of wind speeds are marked at intervals of 3 ms1 ) along with a plot of composite mean Outgoing Long-wave Radiation (W m2 ) (bottom right panel) during October 1997 obtained from NCEP/NCAR reanalysis.
remained more or less the same (easterly) and a wind reversal is seen only above 150 hpa level. During the non-El Nino years the lower tropospheric wind in the eastern parts of the equatorial Indian Ocean is westerly during September–December. During the El Nino period of 1997, the lower tropospheric wind in this region is predominantly easterly between 22°N and 25°S, as seen in Fig. 3. The easterly wind at the equatorial Indian Ocean is responsible for the transport of aerosols originated from the Indonesian forest fires over to the tropical Indian Ocean. Another important feature observed in Fig. 3 is the large-scale convergence over the western equatorial Indian Ocean between 50°E and 60°E during the September–November period of 1997. Right bottom panel of Fig. 3 shows the outgoing long wave radiation (OLR) flux for October 1997 obtained from NCEP/NCAR reanalysis. This shows abnormally low values of OLR in the geographical region between 5°N to 5°S and 50°E to 70°E. This indicates that large cloudiness and rainfall occurred at the western equatorial Indian Ocean which might have prevented the spreading of this smoke plume further westward of 60°E, particularly during October 1997, as seen in Fig. 1.
The spatial distribution of aerosols (Fig. 1) shows the columnar aerosol loading in the plume and does not provide any information regarding the altitude where the aerosol loading is a maximum. The wind field also does not indicate much in this regard because at all altitudes below 150 hpa level the wind pattern is easterly over the equatorial Indian Ocean. A coarse indication of the altitude variation of aerosols can be obtained by comparing the AVHRR-derived spatial pattern of AOD with aerosol index maps of Total Ozone Mapping Spectrometer (TOMS). TOMS provides information (http://toms.gsdc.nasa.gov/aerosols/aerosols.html) on aerosol loading above 1.5 km and is not quite sensitive to aerosols below this altitude. An examination of daily aerosol index maps from TOMS data during this period shows that the plume is mostly confined to regions east of 90°E and at any rate it did not cross the 80°E longitude. The spatial pattern of aerosol index from TOMS also showed significant day-to-day variability especially in the fringe zone indicating lower values for the aerosol lifetimes. From these it can be inferred that the aerosol plume extending west of 80°E and north of 5°N (as seen from AVHRR) is generally confined to the lower troposphere (mainly below 1.5 km).
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4. Intercomparison with in situ measurements The AVHRR AOD maps in Fig. 1 are further compared with in situ measured AOD at Singapore (1.3°N, 103.8°E) reported by Nakajima et al. (1999). Fig. 4(a) shows the time series of AVHRR derived AOD (630 nm) averaged with in a grid size of 1° l° (open square) around Singapore from September to December 1997 along with the daily mean values of AOD (500 nm) over Singapore (filled circle) observed using sun photometer (Nakajima et al., 1999). The vertical bars denote the standard deviation for AVHRR data. A grid size of 1° 1° is taken to achieve a fine spatial resolution for a fruitful comparison with sun photometer values. This narrow grid size, however, reduces the number of AVHRR pixels where AOD could be derived, due to the presence of clouds. Fig. 4(b) shows a plot of AVHRR AOD values averaged at 2° 2° grid size (filled squares) along with the sun photometer AOD reported by Nakajima et al. (1999) adjusted to 630 nm (open circle) using a multiplication factor of 1.25 which is the mean ratio of AOD at 500 nm to AOD at 630 nm (wavelength exponent of 0.96) observed by Nakajima et al. (1999) during the fire event. Fig. 4 is shown for two purposes: (1) to compare the AVHRR-derived AOD with the in situ values, and (2) to study the details of the time evolution of the mean AOD over this location which is more close to the regions of forest fires. The figure shown in the inset of Fig. 4(a) is the intercomparison of the average AVHRR-derived AOD (at 630 nm) with in a grid size of 1° 1° around Singapore and the corresponding daily mean in situ measured AOD (at 500 nm) at Singapore reported by Nakajima et al. (1999) and that shown in Fig. 4(b) is a similar plot of AVHRR AOD at 2° 2°
grid size with in situ measured AOD adjusted to 630 nm. The slope of the regression fit for both these are shown in the respective figures. The in situ measured values of AOD shown in Fig. 4(a) are slightly larger than the AVHRR-derived AOD because of the difference in wavelengths of these two observations. Except for this, in general, the AVHRR-derived AOD is in good agreement with the daily mean in situ measured AOD. The slope of the intercomparison shown in the inset of Fig. 4(a) is 1.20 and the correlation coefficient is 0.98. This is in fair agreement with the value of 1.25 obtained from Nakajima et al. (1999) considering the error limits. The 2° 2° data is mainly used to examine the AOD variation during the development phase of the plume by expanding the record through plotting the data for September. By increasing the grid size it could be possible to get enough points to depict the variation. The inset on Fig. 4(b) shows that the correlation between AVHRR AOD and in situ AOD at 630 nm is fairly good (even though scatter is slightly more) and the slope of the regression line is 1.06. The AVHRR measurements show that the AOD around Singapore were remaining high (AOD > 1.0) from September up to the second week of November 1997. The almost sudden transition from the hazy conditions to relatively cleaner conditions around 10 November 1997 is clearly seen both in the in situ measurements and the AVHRR derived AOD. It is interesting in this context to note that the mean value of single scattering albedo over Singapore (1.3°N, 103.8°E) measured during the forest fire period (Nakajima et al., 1999) also is about 0.9, the value applicable for the assumed smoke model for our analysis.
Fig. 4. Time series of the AVHRR-derived AOD (630 nm) averaged with in a grid size of 1° 1° (open square) during September to December 1997 along with in situ measured AOD (closed circle) using sun photometer at Singapore (Nakajima et al., 1999), inset showing a regression plot of the two (a) and Time series of the AVHRR-derived AOD (630 nm) averaged with a grid size of 2° 2° (solid square) at Singapore along with in situ AOD adjusted to 630 nm (open circle), inset showing a regression plot of the two (b).
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5. Conclusions Impact of the El Nino related Indonesian forest fires, which occurred during the September to December period of 1997, on the aerosol distribution over the tropical Indian ocean is studied based on the satellite-derived (NOAA14-AVHRR) AOD over the Indian Ocean, the wind data obtained from the NCEP/NCAR reanalysis and the ATSR-derived fire counts over the Southeast Asian region. The El Nino related weather and atmospheric conditions have significantly influenced the regional aerosol distribution over the Indian Ocean. The major El Nino event of 1997 caused large-scale forest fires in Indonesia producing massive amounts of carbonaceous aerosols. A substantially large aerosol plume formed over the equatorial Indian Ocean, during September to November period of 1997. This plume started in September reached up to about 60°E by October and decayed almost completely by December. Westward transport of the aerosol plume was driven by the anomalous easterly winds over the tropical Indian Ocean during the El Nino period and was arrested at 60°E by wet removal. The development and decay of the plume is highly correlated with the fire counts in the Indonesian region and lower tropospheric circulation.
Acknowledgements The fire counts were provided by ATSR World Fire Atlas, European Space Agency, ESA/ESRIN, via Galileo Galilei, CP 64, 00044 Frascati, Italy through website http://sharkl.esrin.esa.it/FIRE/AF/ATSR/ and NCEP/ NCAR reanalyses by the NOAA-CIRES Climate Diagnostics Center, Boulder through Website http://www/ cdc.noaa.gov.
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