WRF-Chem simulations of a typical pre-monsoon dust storm in northern India: influences on aerosol optical properties and radiation budget

Open Access Atmospheric Chemistry and Physics Atmos. Chem. Phys., 14, 2431–2446, 2014 www.atmos-chem-phys.net/14/2431/2014/ doi:10.5194/acp-14-2431-2014 © Author(s) 2014. CC Attribution 3.0 License. WRF-Chem simulations of a typical pre-monsoon dust storm in northern India: influences on aerosol optical properties and radiation budget R. Kumar1,2 , M. C. Barth2 , G. G. Pfister2 , M. Naja3 , and G. P. Brasseur1,4 1 Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado, USA 2 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA 3 Aryabhatta Research Institute of Observational Sciences, Nainital, India 4 Climate Service Center, Helmholtz Zentrum Geesthacht, Hamburg, Germany Correspondence to: R. Kumar () Received: 12 June 2013 – Published in Atmos. Chem. Phys. Discuss.: 23 August 2013 Revised: 16 January 2014 – Accepted: 4 February 2014 – Published: 10 March 2014 Abstract. The impact of a typical pre-monsoon season (April–June) dust storm event on the regional aerosol optical properties and radiation budget in northern India is analyzed. The dust storm event lasted from 17 to 22 April 2010 and the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) estimated total dust emissions of 7.5 Tg over the model domain. Both in situ (AERONET – Aerosol Robotic Network) and satellite observations show significant increase (> 50 %) in local to regional scale aerosol optical depth (AOD) and decrease (> 70 %) in the Ångström exponent (α) during this period. Amongst the AERONET sites in this region, Kanpur was influenced the most, where the AOD reached up to 2.1 and the α decreased to −0.09 during the dust storm period. The WRFChem model reproduced the spatial and temporal distributions of dust plumes and aerosol optical properties but generally underestimated the AOD. The average MODIS and WRF-Chem AOD (550 nm) values in a subregion (70–80◦ E, 25–30◦ N) affected the most by the dust storm are estimated as 0.80 ± 0.30 and 0.68 ± 0.28, respectively. Model results show that dust particles cool the surface and the top of the atmosphere, but warm the atmosphere itself. The radiative perturbation due to dust aerosols averaged over the subregion is estimated as −2.9 ± 3.1 W m−2 at the top of the atmosphere, 5.1 ± 3.3 W m−2 in the atmosphere and −8.0 ± 3.3 W m−2 at the surface. The simulated instantaneous cooling under the dust plume was much higher and reached −227 and −70 W m−2 at the surface and the top of the atmosphere, re- spectively. The impact of these radiative perturbations on the surface energy budget is estimated to be small on a regional scale but significant locally. 1 Introduction Dust storms frequently occur throughout the desert regions of the world, especially during springtime, injecting large amounts of mineral dust aerosols into the atmosphere. Dust aerosols have a wide range of potential consequences for ambient air quality, global climate, atmospheric chemistry, and biogeochemical processes. Higher levels of particulate matter during dust storms can lead to serious health problems (e.g., Dey et al., 2004; El-Askary et al., 2006). Dust aerosols perturb the Earth’s radiation budget directly by interacting with both short- and long-wave radiation (e.g., Seinfeld et al., 2004; Ge et al., 2010; Zhao et al., 2011) and indirectly by modifying cloud microphysics and cloud optical properties (Haywood and Boucher, 2000; Satheesh and Moorthy, 2005). They provide large surfaces for heterogeneous chemistry and deposition of different trace gases (e.g., Dentener et al., 1996; Wang et al., 2012), and influence oceanic and terrestrial biogeochemistry by transporting nutrients like iron (Jickells et al., 2005; Kalenderski et al., 2013). Dust aerosols are ice nuclei and potentially cloud condensation nuclei and can therefore affect cloud properties and precipitation (Miller et al., 2004; Zhao et al., 2011; Teller et al., 2012). Dust aerosols Published by Copernicus Publications on behalf of the European Geosciences Union. 2432 R. Kumar et al.: WRF-Chem simulations of a typical pre-monsoon dust storm can also impact the dynamics of the atmosphere through radiative effects (e.g., Tompkins et al., 2005; Stanelle et al., 2010; Chaboureau et al., 2011). In general, dust aerosols are emitted from the arid and desert areas around the globe with about 90 % of the total emissions occurring in the Northern Hemisphere (mostly in northern Africa) (Li et al., 2008). Estimates of global total dust emissions by global models are highly uncertain and range from 500 to 6000 Tg yr−1 , as reported by the Aerosol Comparisons between Observations and Models (AeroCom) project (Textor et al., 2006; Prospero et al., 2010; Huneeus et al., 2011). Dust aerosols are removed from the atmosphere by dry and wet deposition, with dry deposition removing larger particles near the source regions and wet deposition dominating during long-range transport over the oceanic regions. Previous observations and simulations show that dust aerosols are not only confined to the source region but can be transported as far as 1000 km or more (Tegen and Fung, 1994; Ginoux et al., 2001; Prospero et al., 2002; Prospero and Lamb, 2003; Mahowald et al., 2005; Uno et al., 2006; Li et al., 2008), and thus can potentially affect the aerosol optical properties and radiation budget of downwind regions. These impacts of dust aerosols on local to regional scale have been studied in many parts of the world, including India (e.g., Dey et al., 2004; Prasad et al., 2007; Zhao et al., 2010, 2011, Han et al., 2011; Kalenderski et al., 2013). However, the studies over India have only provided information on the local scale, as they focused on integrating in situ observations at a few sites using one dimensional radiative transfer modeling (e.g., Dey et al., 2004; Chinnam et al., 2006; Hegde et al., 2007; Prasad and Singh, 2007; Prasad et al., 2007; Pandithurai et al., 2008; Sharma et al., 2012). Knowledge of the regional scale distribution of dust aerosols and their impact on the regional radiation budget over the Indian region is still very limited. Dust storms are often observed in the northern part of India where their primary source is the Thar Desert (also known as the Great Indian Desert) located in northwestern India (Washington et al., 2003; Gautam et al., 2009). The dust storms over this region show a distinct seasonal cycle with higher frequency and intensity during the pre-monsoon season (April–June) (Sikka, 1997; Dey et al., 2004; Prasad and Singh, 2007). The generation of dust storms over northern India in the pre-monsoon season is supported by hotter and drier weather conditions (Kumar et al., 2012a). The prevailing westerly/southwesterly winds in the lower atmosphere facilitate the transport of dust aerosols from the Thar Desert to the Indo-Gangetic Plain (IGP) region (Sikka, 1997). The IGP region is one of the most densely populat (...truncated)