An Investigation of the Elevated Aerosol Layer Using a Polarization Lidar Over a Tropical Rural Site in India
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An Investigation of the Elevated Aerosol Layer Using a Polarization Lidar Over a Tropical Rural Site in India R. Vishnu1
· Y. Bhavani Kumar2 · Anish Kumar M. Nair2,3
Received: 7 August 2019 / Accepted: 8 September 2020 © Springer Nature B.V. 2020
Abstract Polarization lidar observations were made to study the transport of an elevated aerosol layer over Gadanki, India (13.45° N, 79.17° E) during the pre-monsoon period of the year 2009. Observations show significant aerosol layering within and above the boundary layer. Coordinated observations with radiosondes were carried out from 2 to 10 April 2009. Temporal and spatial variations of the parameters are studied for the boundary layer (≈ 2.5 km) and up to 5 km. The backscattering coefficient and the depolarization ratio are observed to increase and decrease with an increase in humidity, respectively. Clouds are not formed, indicating less efficiency of the aerosol in acting as condensation nuclei. The transport of the elevated aerosol layer is investigated using a back-trajectory analysis, revealing that the transported layer originating from the central Indian region has a depolarization ratio of at least 0.05. From model analysis and satellite fire-count data, it is inferred that the source of the aerosol layer is wildfire events over the central Indian region. The elevated smoke-aerosol layer (not mixing with the boundary layer) has implications for the altering of the temperature profile of the atmosphere and the suppression of cloud formation. Keywords Boundary layer · Elevated layer · Lidar · Depolarization ratio
1 Introduction The vertical distribution of aerosols affects atmospheric stability through radiative heating, altering the cloud microphysics and radiation budget, and subsequently changing the weather and climate of the Earth’s atmospheric system (Ramanathan 2001; Yorks et al. 2009; Li et al. 2016; Vishnu 2018). Hygroscopic particle behaviour is an important factor that alters the scattering properties of aerosols (Satheesh and Krishna Moorthy 2005; Nishizawa et al. 2010). The water-vapour content in the atmosphere influences the aerosol distribution and
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R. Vishnu [email protected]
1
Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, Kerala 695022, India
2
National Atmospheric Research Laboratory, Gadanki, Andhra Pradesh 517112, India
3
Meteorological Facility, Thumba Equatorial Rocket Launching Station, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India
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thus affects the radiative forcing and total radiation budget (Hegg et al. 1993; Rajeev et al. 2010). Sakai and Shibata (1997) report that a strong negative correlation exists between the polarization of the aerosol and associated water-vapour content in the boundary layer (Murayama et al. 1996; Sakai and Shibata 1997). Atmospheric boundary-layer aerosols may be locally generated or transported aloft as elevated aerosol layers, which then have a high importance in terms of the radiative forcing (Rajeev et al. 2000; Rajeev and Raman
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