Climate implications of large warming by elevated aerosol over India

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L19809, doi: /2008gl034944, 2008 Climate implications of large warming by elevated aerosol over India S. K. Satheesh, 1,2 K. Krishna Moorthy, 3 S. Suresh Babu, 3 V. Vinoj, 1 and C. B. S. Dutt 4 Received 6 June 2008; revised 16 August 2008; accepted 26 August 2008; published 4 October [1] Wide-ranging multi-platform data from a major field campaign conducted over Indian region was used to estimate the energy absorbed in ten layers of the atmosphere. We found that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers. Three-fold increase in aerosol extinction coefficient was observed at higher atmospheric layers (>2 km) compared to that near the surface and a substantial fraction (as much as 50 to 70%) of aerosol optical depth was found contributed by aerosols above (reflecting) clouds. Consequent absorption and hence strong warming above clouds was found larger by several degrees (K) compared to that near the surface. The aerosol-induced elevated warming was mostly confined below 2 km over northern Indian Ocean while found up to 4 km over central India, thus exhibiting strong meridional gradients (4 K) at atmospheric levels above 2 km. Climate implications of the large elevated warming are discussed. Citation: Satheesh, S. K., K. Krishna Moorthy, S. Suresh Babu, V. Vinoj, and C. B. S. Dutt (2008), Climate implications of large warming by elevated aerosol over India, Geophys. Res. Lett., 35, L19809, doi: / 2008GL distribution of aerosols over the continental India and adjacent oceanic regions are highly limited, excluding a few. [3] During March May, 2006, an extensive, multiinstitutional and multi-platform field experiment, Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was carried out over continental India and adjoining oceans (Arabian Sea, northern Indian Ocean and Bay of Bengal). Major objective of this, one of the largest and most exhaustive field campaigns, ever conducted in the Indian region, was to assess the radiative impact of aerosols over the Indian landmass and the adjoining oceanic regions through intensive and simultaneous multi-platform observations. A network of ground based observatories over the mainland and islands, a dedicated ship cruise over the oceanic regions using a fully equipped research vessel (R/V Sagar Kanya) and altitude profiling over selected regions using an instrumented aircraft formed the three segments of this integrated experiment, which were carried out in tandem (see Moorthy et al. [2008] for details). In this paper, we report major findings from the aircraft campaign carried out during ICARB and climate implications are discussed. 1. Introduction [2] Atmospheric warming due to aerosols still remains a significant source of uncertainty in assessing climate change because most of the current estimates are largely based on model studies [Seinfield and Pandis, 1998; Ramanathan et al., 2007]. The possible role of radiative interactions of aerosols on regional climate (such as monsoon systems), still remains largely uncertain [Lau et al., 2006]. The information on vertical distribution of aerosols with respect to clouds is essential to address complex problems such as radiative impact of aerosols in presence of clouds and hence to estimate altitude profiles of aerosol-induced warming. Optically thick clouds occurring below an aerosol layer effectively serve as a highly reflective background and enhance the absorption by aerosols above clouds [Seinfeld, 2008]. Thus, vertical distribution of aerosols in the atmosphere critically modifies the thermal structure of the atmosphere and hence its stability, with serious implications in cloud formation processes. The knowledge on the vertical 1 Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India. 2 Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 3 Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India. 4 Indian Space Research Organisation, Bangalore, India. Copyright 2008 by the American Geophysical Union /08/2008GL Results [4] The air segment of ICARB was executed using the aircraft of the National Remote Sensing Agency (NRSA) at Hyderabad (India), which dedicated one of its propeller aircraft (Beechcraft 200) to ICARB. In all, 26 aircraft sorties were made (from 23rd March to 03rd May, 2006) and out of which 5 sorties (25th March, 03rd April, 23rd April, 02nd May and 03rd May) correspond to airborne lidar measurements. In addition 12 days of ground-based lidar measurements carried out over continental India (during May 2006) were also used to supplement airborne lidar measurements. A micro pulse lidar (MPL 1000 of Science and Engineering Services Inc., USA) (see Moorthy et al. [2008] for details) was used in the downward-looking mode to obtain vertical profiles of aerosols. We found that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers. Typical vertical profiles of aerosol extinction coefficient (scattering + absorption) derived from the airborne lidar observations (off east coast of India) are shown as time series in Figure 1a as illustrative. The offshore distance is also marked close to time-axis. A cluster of extinction profiles obtained within 50 km of coast are shown in Figure 1b. Multiple aerosol layers, with nearly three-fold increase in aerosol extinction coefficient at higher levels of the atmosphere (3 to 4 km) compared to that of surface, were observed over most of regions explored (from central India to north Indian Ocean) (Figures 2a to f). Surprisingly, a major fraction of aerosol column optical thickness was contributed by aerosols above clouds (Figures 2a 2d, where cloud heights were obtained from L of6

2 Figure 1. Vertical distribution of aerosols inferred from air-borne lidar. (a) Typical vertical profiles of extinction coefficient derived from the air-borne lidar (east coast of India) are shown as time series. Offshore distances estimated from GPS are also shown near time-axis. (b) A cluster of extinction coefficient profiles obtained within 50 km of coast. MODIS onboard TERRA and AQUA satellites, approximate range of which over southern & central India is shown as solid vertical bar in Figure 1b). It may be interesting to note that ship-borne measurements have shown an order of magnitude spatial variation in aerosol optical thickness (in the range of 0.07 to 1.2) over the oceans [Moorthy et al., 2008]. While most of aerosol was confined below 1 km over northern Indian Ocean, a substantial amount was found up to 4 km (exti nction coefficient 0.25 at 4 km) in case of Central India (Figure 2e). Strong meridional gradients in the height of aerosol layer (defined in this paper as height up to which presence of aerosol was felt significantly in terms of extinction coefficient) as well as aerosol extinction coefficient at higher atmospheric levels (2 km) came as a surprise. The GPS sonde ascents from the R/V Sagar Kanya research vessel located close to the locations of aerial measurements have shown that aerosols layers are located well above the marine boundary layer (MABL). [5] Vertical profiles of aerosol microphysical properties and aerosol black carbon (BC) concentrations were also made using the same aircraft (about 4 to 5 hours prior to the airborne lidar measurements). The percentage contribution of aerosol BC to layer extinction (estimated from measurements of BC mass concentration onboard aircraft and airborne lidar measurements of the layer extinction), revealed large latitudinal structures with values ranging from 5% to as high as >20%, as compared to the column average value of 10% reported earlier over Indian Ocean [Satheesh et al., 1999]. Corresponding single scattering albedo (ratio of scattering to extinction) values (Figure 3a), which are in the range of 0.85 to 0.95 at the surface and drop down to <0.80 at an altitude of 2 km, indicating substantially higher absorption in atmospheric layers aloft (2 km). The measured surface single scattering albedo values over the coastal and oceanic regions (based on data from research vessel) were in the range of 0.84 to [6] The layer extinction coefficient and contribution of BC to layer extinction are used to estimate downward and upward flux at several layer boundaries (<4 km) for with aerosol and without aerosol conditions. We have used a discrete ordinate radiative transfer model described by Ricchiazzi et al. [1998] for this purpose. The net fluxes (downward minus upward) due to aerosols at the layer boundaries were differenced to obtain the energy absorbed in that layer, from which the warming rate under clear-sky conditions (24-hr average) are estimated (Figure 3b). The aerosol-induced warming rates observed in our study are comparable (at 2 km) or larger (at higher and lower levels) than those reported by Ramanathan et al. [2007] for Indian Ocean. We find that at local noon, the warming due to aerosol absorption at higher levels exceeds that at the surface layers by several degrees (as much as 3 to6k). Airborne measurements of over central India have shown presence of substantial amount (>1000 ng m 3 ) of aerosol black carbon above 2 km [Moorthy et al., 2004]. While considering the large aerosol-induced extinction at higher levels over central India, this translates to a warming of 1.5 K/day (24-hr average) or 3 to 5 K at local noon (the range of which is shown as green horizontal bar in Figure 3b). An illustration of the elevated aerosol layer over India and features of its radiative impacts are shown in Figure Climate Implications and Discussion [7] The large warming of the atmospheric layers aloft (by 2 to 6 degrees) observed at several locations of India starting all the way from southern to central India during summer and pre-monsoon season is striking and raises several issues. What are the climate implications of large warming by the elevated aerosol over most of the Indian region? [8] The range of cloud top heights (indicated in Figures 1b and 2a 2d) suggests that a substantial fraction of aerosol column optical thickness (30 to 70% at different locations) is contributed by aerosols located above clouds. When (reflecting) clouds are embedded within an elevated absorbing aerosol layer (as observed during our experiment), the 2of6

3 Figure 2. The altitude of aerosols relative to clouds. (a d) Vertical distribution of aerosols at four representative locations (central India (marked as CI), southern central India (SCI), southern coast (SC) and north Indian Ocean (NIO)) inferred using an airborne micro pulse lidar. Cloud top heights obtained from MODIS data are indicated using a cartoon symbol of cloud. Fractions of extinction coefficient contributed by aerosols above clouds (t AC ) are also marked. (e) Aerosol layer height and extinction coefficient at 3 km at four representative locations described in Figures 2a 2d and 2f. (f) Four representative locations marked in map of India. 3of6

4 Figure 3. Vertical distribution of single scattering albedo and aerosol-induced warming rates. (a) Range of single scattering albedo at various altitudes inferred from aircraft measurements. The range of values observed during the campaign is shown as a function of altitude. Red in color bar indicates larger absorption while blue indicates lower absorption. (b) Aerosol-induced warming rate under clearsky conditions (24-hr average). aerosols interact not only with the radiation from the Sun, but also with that reflected from the cloud layer below, leading to an enhanced absorption. Adding to the complexity, recent investigations have shown that when black carbon exists in mixed state with other aerosols, its radiative impact is substantially larger [Jacobson, 2001; Chandra et al., 2004]. Recent reports from India (including SEM images) indicated the presence of aerosols in mixed forms [Satheesh et al., 2006]. In such a situation, the aerosolinduced warming could be much more than those shown in Figure 3b (which correspond to clear-sky conditions). If warming by elevated absorbing aerosols over India is considerably larger, the issues such as cloud burn off proposed by Ackerman et al. [2000] become increasingly relevant. [9] Large elevated warming close to Himalayan region is a disturbing fact. Multi-year measurements of aerosols made at Manora Peak in central Himalayas (29 N, 79 E, 2 km above mean sea level) have shown that visible optical thicknesses are very low (0.1) in winter (November to February) and increases rather steeply to reach high values (0.5) in summer and pre-monsoon (April June) [Dumka et al., 2008]. This observation is consistent with the Figure 2e which indicates increasing trend in aerosol extinction coefficient above 2 km from north Indian Ocean to Central India. Synthesizing ground-based spectral optical depth measurements with satellite (METEOSAT) data, Moorthy et al. [2007] have also shown that dust absorption efficiency over deserts of northwest India is much higher compared to those of the Saharan regions. The cloud top pressure data from MODIS indicates that cloud top is below 2 km over most of northern India during April May period. Therefore, enhanced short wave absorption above clouds, resulting from dust contaminated with black carbon (hence with large absorbing efficiency) close to Himalayas may have a role in the drastic decrease observed in Himalayan glacier area in just a few decades reported by Kulkarni et al. [2007]. [10] Kulkarni et al. [2007] have investigated Himalayan glacial retreat using data from satellite sensors (with a spatial resolution of 5.8 meters) onboard the Indian Remote Sensing (IRS) satellites. These studies have shown a reduction of 21% in glacier area from 1962 to Using data from a network of sun photometers over several locations in India, Satheesh et al. [2002] have shown a three-fold increase in aerosol optical depth from 1985 to When we examine these two observations in conjunction with the alarming warming rates at higher atmospheric levels (2 km) and its strong meridional dependence (increasing towards central and north India) reported in this paper, it emerges that the large elevated warming by absorbing aerosols above (reflecting) clouds contribute to Himalayan glacial retreat, the response time of which is unknown. However, readers may note that to assess the effect of the estimated warming of the elevated layers on Himalayan glaciers data over a long period needs to be examined. The present study is only indicative of this possibility. Also multi-year observations are needed to obtain a statistics of the aerosol vertical distribution and elevated warming. [11] Lastly, the role of aerosols on Indian monsoon is not yet understood well. Recent studies [e.g., Ramanathan et al., 2005] have shown that large reduction of solar radiation reaching the Earth s surface by anthropogenic aerosols might slow down the hydrological cycle, weaken the monsoon circulation and decrease Indian summer monsoon rainfall. Recently, Lau et al. [2006] have shown that absorption of solar radiation and consequent warming by aerosols over Tibetan Plateau (elevated land) acts like an elevated heat pump (EHP), which draws in warm and moist air over the Indian sub continent leading to advancement and subsequent intensification of Indian summer monsoon. Observations carried out in our study from the equatorial Indian Ocean to central Himalayas show that during pre-monsoon season, most of Indian region is characterized by elevated aerosol layers with high absorption properties, a major fraction of which located above the (reflecting) clouds, which substantially warm atmospheric 4of6

5 Figure 4. Illustration of the elevated aerosol over India and features of its radiative impacts. DT NS is the north-south gradient (between north Indian Ocean and central India) in aerosol-induced warming above 3 km and DH NS is the same for the height of the aerosol layer. DT is the aerosol-induced warming above clouds which is in the range of 2 4 K over central India while close to zero over north Indian Ocean. Cloud top heights obtained from MODIS data are indicated using a cartoon symbol of cloud. Fractions of extinction coefficient contributed by aerosols above clouds (t AC ) are also marked DF S is the reduction of surface radiation due to aerosols. TP EHP represents altitude of the elevated heat pump hypothesis reported by Lau et al. [2006]. layers aloft (Figure 4). This indicates the possibility of EHP effect over most of continental Indian and adjacent oceans. The consequence of these contrasting processes needs to be understood before arriving at conclusions on the aerosol impact on regional climate system. 4. Conclusions [12] Major conclusions are listed below. [13] (a) During pre-monsoon season, most of Indian region is characterized by elevated aerosol layers. [14] (b) Three-fold increase in aerosol extinction coefficient was observed at higher atmospheric layers (>2 km) compared to that near the surface. [15] (c) Substantial fraction (as much as 50 to 70%) of aerosol optical depth was found contributed by aerosols above (reflecting) clouds, consequently enhancing absorption and hence strong warming above clouds by several degrees (K) compared to that near the surface. [16] (d) The aerosol-induced elevated warming was mostly confined below 2 km over northern Indian Ocean while found up to 4 km over central India, thus leading to strong meridional gradients (4 K) at atmospheric levels above 2 km. [17] (e) Strong warming above clouds and while examined in conjunction with large aerosol optical thickness (0.5) found over central Himalayas during pre-monsoon season, indicates potential role of elevated aerosol warming in Himalayan glacier retreat, the response time of which is unknown. [18] (f) The impact of large elevated warming over India and its meridional dependence, coupled with strong aerosolinduced cooling of Earth s surface on Asian summer monsoon needs to be understood. [19] Acknowledgments. This research work was carried out under Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB) field campaign of ISRO-GBP, and the authors thank V. Jayaraman and R. Sridharan for their support. The authors thank Department of Science and Technology, New Delhi, for supporting Micro Pulse Lidar (MPL) used in this work. Authors are grateful to K. Radhakrishnan, aircraft team (headed by K. Kalyanaraman and V. Raghu Venkataraman), Rasik Ravindra, and M. Sudhakar for their excellent cooperation and support through out the campaign. Part of this research was carried out when one of the authors (SKS) was supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. References Ackerman, A. S., et al. (2000), Reduction of tropical cloudiness by soot, Science, 288(5468), Chandra, S., S. K. Satheesh, and J. Srinivasan (2004), Can the state of mixing of black carbon aerosols explain the mystery of excess atmospheric absorption?, Geophys. Res. Lett., 31, L19109, doi: /2004gl Dumka, U. C., et al. (2008), Short period modulations in aerosol optical depths over central Himalayas: Role of mesoscale processes, J. Appl. Meteorol. Climatol., 47, Jacobson, M. Z. (2001), Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols, Nature, 409(6821), Kulkarni, A. V., et al. (2007), Glacial retreat in Himalaya using Indian remote sensing satellite data, Curr. Sci., 92, Lau, K. M., M. K. Kim, and K. M. Kim (2006), Aerosol induced anomalies in the Asian summer monsoon The role of the Tibetan Plateau, Clim. Dyn., 26, , doi: /s z. Moorthy, K. K., S. S. Babu, S. V. Sunilkumar, P. K. Gupta, and B. S. Gera (2004), Altitude profiles of aerosol BC, derived from aircraft measurements over an inland urban location in India, Geophys. Res. Lett., 31, L22103, doi: /2004gl Moorthy, K. K., S. S. Babu, S. K. Satheesh, J. Srinivasan, and C. B. S. Dutt (2007), Dust absorption over the Great Indian Desert inferred using ground-based and satellite remote sensing, J. Geophys. Res., 112, D09206, doi: /2006jd Moorthy, K. K., et al. (2008), Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB): An overview, J. Earth Syst. Sci., 117, Ramanathan, V., et al. (2005), Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle, Proc. Natl. Acad. Sci. U. S. A., 102, , doi: /pnas of6

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