Atmospheric Environment 214 (2019) 116831 Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Review article Aerosol-orography-precipitation – A critical assessment T Goutam Choudhurya, Bhishma Tyagia,*, Jyotsna Singhb, Chandan Sarangic, S.N. Tripathid a Department of Earth and Atmospheric Sciences, National Institute of Technology Rourkela, Rourkela, 769008, Odisha, India b Shanti Raj Bhawan, Paramhans Nagar, Kandwa, Varanasi, 221106, India c Pacific Northwest National Laboratory, Richland, United States d Department of Civil Engineering & Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, 208016, India GRAPHICAL ABSTRACT ARTICLE INFO ABSTRACT Keywords: The increasing anthropogenic pollution and its interaction with precipitation received much attention from the Aerosols research community and have been explored extensively for understanding the aerosol-cloud interactions. The Aerosol-orography-precipitation interactions impacts of orography and aerosols on the precipitation processes have unveiled the Aerosol-Orography- Cloud microphysics Precipitation (AOP) interaction as an essential research area. The understanding of AOP interaction is critical for Spillover effect improving the extreme rainfall events prediction over mountainous regions. The phase of clouds (warm or Orographic enhancement factor mixed) along with orography has emerged as a significant factor for influencing the AOP relations. The present work reviews the modelling and observational based studies dealing with the relationship between orography and aerosols on the precipitation. The study reveals the principal role of aerosols in shifting the precipitation pattern for orographic regions. The environmental factors, especially ambient temperature, humidity and flow patterns are also identified to affect the orographic precipitation. The review also discovers that AOP studies exist only to limited areas of the world due to limited observations, and mostly with idealised cases in the modelling framework. 1. Introduction condensate the moisture in the form of solid or liquid precipitation. The precipitating snow accumulates in the high mountains and serves as a Most of the freshwater available to humankind is coming from the necessary freshwater reservoir in many regions of the world. The oro- orographic precipitation (Schär and Frei, 2005). The precipitation over graphic precipitation however, depends on many parameters e.g. the mountains and hills occurs when cloud systems developed with dif- terrain features (mountain height and cross-mountain width) (Colle, ferent mechanisms (e.g., frontal, convective, topographical lift) help to 2004; Roe, 2005; Jiang, 2003 & 2007; Watson and Lane, 2012), * Corresponding author. E-mail address: [email protected] (B. Tyagi). https://doi.org/10.1016/j.atmosenv.2019.116831 Received 18 December 2018; Received in revised form 10 July 2019; Accepted 11 July 2019 Available online 12 July 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved. G. Choudhury, et al. Atmospheric Environment 214 (2019) 116831 atmospheric stability (Schneidereit, 2000; Colle, 2004; Kirshbaum, 2. Model simulations based understanding of aerosol-cloud- 2008; Kunz, 2011), upslope velocity and low-level water content orography interactions (Schneidereit, 2000; Neiman, 2002; Jiang, 2003, 2009; Colle, 2004; Kunz, 2011; Watson, 2012), surface temperature (Kirshbaum, 2008; Aerosols serve as CCN and IN forming liquid droplets and ice Zängl, 2008; Kunz, 2011), the concentration, size spectrum and che- crystals in the atmosphere. Precipitation over orography is a result of mical composition of the aerosols on which the water vapour condenses complex interactions between the microphysical timescales and the (Borys, 2000, 2003; Griffith, 2005; Rosenfeld and Givati, 2006 and advection time scale of the orographic cloud. Thus changing aerosol many more). concentration in such a scenario will modify the cloud droplet number The impact of orography forcing on the amount and distribution of and size distribution, thereby altering the cloud microphysical pro- precipitation depends on the value of Bulk Damköhler number which is cesses, amount, lifetime, distribution and amount of precipitation. The the ratio of the advective time scale (the time it takes for an air parcel to numerical simulations allow the sensitivity studies of various micro- move across the mountain) and microphysical time scales (to convert physical processes, topographical features, the concentrations change in cloud hydrometeors to precipitation) of the cloud system (Jiang, 2003; CCN/IN/Aerosols over different regions of the world using different Miltenberger, 2015). Cloud droplets take time to grow and fall out as initial and boundary conditions for a better understanding of the AOP precipitation, and the elevation and slope of the terrain limit the lo- interactions. Depending on the height of the topographical region, cations of precipitation occurrence. For example, if the time scale of orographic clouds either can be a warm phase or mixed phase. In microphysical processes is less than the time scale of advection across general, for warm clouds when everything else remains constant, in- the orography, then under ideal conditions, the cloud may precipitate crease in aerosols results in more small cloud droplets (Twomey, 1977) over the windward side. Otherwise the cloud may cross the barrier and and an increase in cloud lifetime (Albrecht, 1989). This increase may may increase the leeward precipitation (spillover effect). Since aerosols, result in the suppression of precipitation due to the decrease in coa- by acting either as cloud condensation nuclei (CCN) or ice-nuclei (IN), lescence efficiencies because of the reduced drop size (Rosenfeld, affect the time scales of cloud microphysical processes, they play a vital 2005). However, for mixed-phase clouds, the understanding of aerosol role in modifying the orography forcing. impact is still in progress because of complex microphysics involvement In the present scenario of understanding the climate change and its in ice nucleation. We can summarise the warm and mixed phase cloud association with various processes, the aerosol-cloud-climate interac- studies related to orography and aerosol as follows. tions are vital to understand the extreme rainfall/drought events globally (Intergovernmental Panel on Climate Change (IPCC), 2013; 2.1. Warm phase clouds Stocker, 2014 Koren et al., 2014; Fan et al., 2015; Sarangi et al., 2015; Fan et al., 2016). With evident increase in aerosols, especially over The growth processes for warm phase clouds are condensation and developing countries (e.g. Gogikar and Tyagi, 2016; Gogikar et al., collision – coalescence (C–C). Condensation, the change of water va- 2018; Singh et al., 2019; Gogikar et al., 2019; Sahu et al., 2019), the pour into liquid water, usually occurs in the atmosphere when warm air investigation of extreme precipitation patterns association with aero- rises, cools and lose its capacity to hold water vapour. The excessive sols are more crucial. As explored by many researchers, the aerosol- water vapour results in cloud droplet formation via condensation. cloud interactions are having direct impacts on the associated rainfall Initial growth of droplet by condensation is high, but it reduces with over any region of interest and follow feedback mechanism (e.g. Baker time, and is not sufficient to produce the large raindrops alone. The and Charlson, 1990; Xue and Feingold, 2006; Morrison et al., 2012; dominant growth process in warm phase clouds is C–C, which has three Bollasina et al., 2013; Altaratz et al., 2014; Saleeby et al., 2015; Fan categories: et al., 2016). The mechanism is further complicated when we are trying to understand aerosol-cloud interactions modulated by topographical (i) Autoconversion – The initial phase with formation of small cloud features. However, understanding how topographical features interact droplets which results in drizzle drops after coalescence with aerosols, clouds and associated rainfall is becoming more and (ii) Self-collection – The large drizzle drop formation from the smaller more crucial with the increasing extreme rainfall events (cloud-burst) ones, i.e. small raindrop (drizzle drop) collects other small rain- in the vicinity of orographic structures (Chaudhuri et al., 2014; Fan drop (drizzle drop) to form larger ones and et al., 2015; Dimri et al., 2017). Various researchers are exploring the (iii) Accretion – The cloud drops collection by the large drizzle drops aerosol-orography-precipitation (AOP) interactions over different re- formed in the previous step. gions of the world with both observational and modelling analysis (e.g. Rosenfeld and Givati, 2006; Lynn et al., 2007; Fan et al., 2014; Fan Out of the three processes, the efficiency of accretion is higher than et al., 2017). self-collection (Ochs III et al., 1984), with the well-established re- The aerosol-cloud interactions are well explored (e.g., Khain, 2009; lationship between the warm phase cloud and precipitation efficiency Tao et al., 2012; Fan et al., 2016; Kant et al., 2017). The AOP under- (e.g., Testik and Barros, 2007; Villermaux and Bossa, 2009; Wilson and standing is still under progress. The present paper is an attempt to re- Barros, 2014). However, the understanding related to warm phase view the findings of the AOP interaction, with a focus on the indirect clouds and orography is under progress with fewer