Premonsoon aerosol characterization and radiative effects over the Indo‐Gangetic Plains: Implications for regional climate warming
TL;DR: In this paper, a detailed characterization of aerosols over the Indo-Gangetic Plains (IGP) and their radiative effects during the pre-monsoon season (April-May-June) was presented, using ground radiometric and spaceborne observations.
Abstract: The Himalayas have a profound effect on the South Asian climate and the regional hydrological cycle, as it forms a barrier for the strong monsoon winds and serves as an elevated heat source, thus controlling the onset and distribution of precipitation during the Indian summer monsoon. Recent studies have suggested that radiative heating by absorbing aerosols, such as dust and black carbon over the Indo-Gangetic Plains (IGP) and slopes of the Himalayas, may significantly accelerate the seasonal warming of the Hindu Kush-Himalayas-Tibetan Plateau (HKHT) and influence the subsequent evolution of the summer monsoon. This paper presents a detailed characterization of aerosols over the IGP and their radiative effects during the premonsoon season (April-May-June) when dust transport constitutes the bulk of the regional aerosol loading, using ground radiometric and spaceborne observations. During the dust-laden period, there is a strong response of surface shortwave flux to aerosol absorption indicated by the diurnally averaged forcing efficiency of -70 W/sq m per unit optical depth. The simulated aerosol single-scattering albedo, constrained by surface flux and aerosol measurements, is estimated to be 0.89+/- 0.01 (at approx.550 nm) with diurnal mean surface and top-of-atmosphere forcing values ranging from -11 to -79.8 W/sq m and +1.4 to +12 W/sq m, respectively, for the premonsoon period. The model-simulated solar heating rate profile peaks in the lower troposphere with enhanced heating penetrating into the middle troposphere (5-6 km), caused by vertically extended aerosols over the IGP with peak altitude of approx.5 km as indicated by spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization observations. On a long-term climate scale, our analysis, on the basis of microwave satellite measurements of tropospheric temperatures from 1979 to 2007, indicates accelerated annual mean warming rates found over the Himalayan-Hindu Kush region (0.21 C/decade+/-0.08 C/decade) and underscores the potential role of enhanced aerosol solar absorption in the maximum warming localized over the western Himalayas (0.26 C/decade f 0.09 C/decade) that significantly exceed the entire HKHT and global warming rates. We believe the accelerated warming rates reported here are critical to both the South Asian summer monsoon and hydro-glaciological resource variability in the Himalayan-Hindu Kush snowpack and therefore to the densely populated downstream regions.
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TL;DR: In this paper, the authors compare the performance of the Enhanced Deep Blue (DB) and Dark Target (DT) algorithms over land, and a DT over-water algorithm over desert/urban areas.
Abstract: The Moderate Resolution Imaging Spectroradiometer (MODIS) Atmospheres data product suite includes three algorithms applied to retrieve midvisible aerosol optical depth (AOD): the Enhanced Deep Blue (DB) and Dark Target (DT) algorithms over land, and a DT over-water algorithm. All three have been refined in the recent “Collection 6” (C6) MODIS reprocessing. In particular, DB has been expanded to cover vegetated land surfaces as well as brighter desert/urban areas. Additionally, a new “merged” data set which draws from all three algorithms is included in the C6 products. This study is intended to act as a point of reference for new and experienced MODIS data users with which to understand the global and regional characteristics of the C6 DB, DT, and merged data sets, based on MODIS Aqua data. This includes validation against Aerosol Robotic Network (AERONET) observations at 111 sites, focused toward regional and categorical (surface/aerosol type) analysis. Neither algorithm consistently outperforms the other, although in many cases the retrieved AOD and the level of its agreement with AERONET are very similar. In many regions the DB, DT, and merged data sets are all suitable for quantitative applications, bearing in mind that they cannot be considered independent, while in other cases one algorithm does consistently outperform the other. Usage recommendations and caveats are thus somewhat complicated and regionally dependent.
534 citations
Cites background from "Premonsoon aerosol characterization..."
...Observational [Gautam et al., 2010] and model [Henriksson et al....
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...Observational [Gautam et al., 2010] and model [Henriksson et al., 2011] studies concerning the seasonal contribution of different aerosol types to the total AOD in India show that mineral dust makes a large contribution to total AOD during the pre-monsoon season (April–June) while at other times of…...
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TL;DR: In this article, a roadmap is provided for quantifying the aerosol-cloud-precipitation-climate (ACPC) interactions and thereby reducing the uncertainty in anthropogenic climate forcing.
Abstract: Cloud drop condensation nuclei (CCN) and ice nuclei (IN) particles determine to a large extent cloud microstructure and, consequently, cloud albedo and the dynamic response of clouds to aerosol-induced changes to precipitation. This can modify the reflected solar radiation and the thermal radiation emitted to space. Measurements of tropospheric CCN and IN over large areas have not been possible and can be only roughly approximated from satellite-sensor-based estimates of optical properties of aerosols. Our lack of ability to measure both CCN and cloud updrafts precludes disentangling the effects of meteorology from those of aerosols and represents the largest component in our uncertainty in anthropogenic climate forcing. Ways to improve the retrieval accuracy include multiangle and multipolarimetric passive measurements of the optical signal and multispectral lidar polarimetric measurements. Indirect methods include proxies of trace gases, as retrieved by hyperspectral sensors. Perhaps the most promising emerging direction is retrieving the CCN properties by simultaneously retrieving convective cloud drop number concentrations and updraft speeds, which amounts to using clouds as natural CCN chambers. These satellite observations have to be constrained by in situ observations of aerosol-cloud-precipitation-climate (ACPC) interactions, which in turn constrain a hierarchy of model simulations of ACPC. Since the essence of a general circulation model is an accurate quantification of the energy and mass fluxes in all forms between the surface, atmosphere and outer space, a route to progress is proposed here in the form of a series of box flux closure experiments in the various climate regimes. A roadmap is provided for quantifying the ACPC interactions and thereby reducing the uncertainty in anthropogenic climate forcing.
317 citations
TL;DR: In this paper, the authors examined the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/southern slopes of the Himalayas during the 2009 pre-monsoon season.
Abstract: . We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/southern slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April–June) aerosol optical depths of over 0.6 – highest over Southern Asia. The influence of dust loading is greater over the Western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). Transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA550 nm
232 citations
Cites background or methods or result from "Premonsoon aerosol characterization..."
...Influx and accumulation of aerosols over northern and central India extends to elevated altitudes (above 3–4 km) (Gautam et al., 2010; Kuhlmann and Quass, 2010; Kar et al., 2010) and can be transported above the boundary layer clouds leading to enhanced absorption of solar radiation and regional warming (Satheesh et al....
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...Here, the methodology followed for model inputs and calculations in this paper is similar to our previous aerosol radiative forcing estimation over Kanpur (Gautam et al., 2010)....
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...This tuning procedure is used to account for aerosol absorption by constraining the model with observed forcing efficiency as a means to achieve reasonable estimates of aerosol forcing for the region; this approach is similar to that previously applied over Kanpur (Gautam et al., 2010)....
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...01 at Kanpur, estimated from a similar methodology (Gautam et al., 2010), suggesting greater absorption in the IGP....
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...Several satellite-based studies indicate the IGP as a major pollution hot spot due to dust transport, anthropogenic aerosols and biomass burning during pre-monsoon season (Jethva et al., 2005; Habib et al., 2006; Gautam et al., 2009b, 2010)....
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01 Dec 2005
TL;DR: The recent trend of declining winter and spring snow cover over Eurasia is causing a land-ocean thermal gradient that is particularly favorable to stronger southwest (summer) monsoon winds, raising the possibility that the current warming trend of the Eurasian landmass is making the Arabian Sea more productive.
Abstract: The recent trend of declining winter and spring snow cover over Eurasia is causing a land-ocean thermal gradient that is particularly favorable to stronger southwest (summer) monsoon winds. Since 1997, sea surface winds have been strengthening over the western Arabian Sea. This escalation in the intensity of summer monsoon winds, accompanied by enhanced upwelling and an increase of more than 350% in average summertime phytoplankton biomass along the coast and over 300% offshore, raises the possibility that the current warming trend of the Eurasian landmass is making the Arabian Sea more productive.
194 citations
TL;DR: In this article, the authors explore the southern slopes of Mt Everest, analyzing the time series of temperature and precipitation reconstructed from seven stations located between 2660 and 5600 m asl during 1994-2013, complemented with the data from all existing ground weather stations located on both sides of the mountain range (Koshi Basin) over the same period.
Abstract: Studies on recent climate trends from the Himalayan range are limited, and even completely absent at high elevation (> 5000 m asl) This study specifically explores the southern slopes of Mt Everest, analyzing the time series of temperature and precipitation reconstructed from seven stations located between 2660 and 5600 m asl during 1994–2013, complemented with the data from all existing ground weather stations located on both sides of the mountain range (Koshi Basin) over the same period Overall we find that the main and most significant increase in temperature is concentrated outside of the monsoon period Above 5000 m asl the increasing trend in the time series of minimum temperature (+0072 °C yr−1) is much stronger than of maximum temperature (+0009 °C yr−1), while the mean temperature increased by +0044 °C yr−1 Moreover, we note a substantial liquid precipitation weakening (−93 mm yr−1) during the monsoon season The annual rate of decrease in precipitation at higher elevations is similar to the one at lower elevations on the southern side of the Koshi Basin, but the drier conditions of this remote environment make the fractional loss much more consistent (−47% during the monsoon period) Our results challenge the assumptions on whether temperature or precipitation is the main driver of recent glacier mass changes in the region The main implications are the following: (1) the negative mass balances of glaciers observed in this region can be more ascribed to a decrease in accumulation (snowfall) than to an increase in surface melting; (2) the melting has only been favoured during winter and spring months and close to the glaciers terminus; (3) a decrease in the probability of snowfall (−10%) has made a significant impact only at glacier ablation zone, but the magnitude of this decrease is distinctly lower than the observed decrease in precipitation; (4) the decrease in accumulation could have caused the observed decrease in glacier flow velocity and the current stagnation of glacier termini, which in turn could have produced more melting under the debris glacier cover, leading to the formation of numerous supraglacial and proglacial lakes that have characterized the region in the last decades
166 citations
Cites background from "Premonsoon aerosol characterization..."
...According to Ramanathan et al. (2007), Gautam et al. (2010) argued that the observed warming during the pre-monsoon period (April–June) can be ascribed not only to the global greenhouse warming, but also to the solar radiation absorption caused by the large amount of aerosol (mineral dust mixed…...
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References
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01 Jan 2007
TL;DR: The first volume of the IPCC's Fourth Assessment Report as mentioned in this paper was published in 2007 and covers several topics including the extensive range of observations now available for the atmosphere and surface, changes in sea level, assesses the paleoclimatic perspective, climate change causes both natural and anthropogenic, and climate models for projections of global climate.
Abstract: This report is the first volume of the IPCC's Fourth Assessment Report. It covers several topics including the extensive range of observations now available for the atmosphere and surface, changes in sea level, assesses the paleoclimatic perspective, climate change causes both natural and anthropogenic, and climate models for projections of global climate.
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TL;DR: The operation and philosophy of the monitoring system, the precision and accuracy of the measuring radiometers, a brief description of the processing system, and access to the database are discussed.
6,535 citations