Lidar investigation of tropical nocturnal boundary layer aerosols and cloud macrophysics
Introduction
Aerosols and clouds are vital components of the earth–atmosphere radiation balance as well as the hydrological cycle of the earth system (Ramanathan et al., 2001). Aerosols and clouds act to regulate the radiative equilibrium by absorption and scattering of both the incoming and outgoing radiation (Ardanuy et al., 1989, Charlson et al., 1992, Satheesh and Ramanathan, 2000, Latham et al., 2008, Panicker et al., 2008, Manoj et al., 2011, Srivastava et al., 2012, Paasonen et al., 2013). Aerosols may amend the cloud properties and precipitation formation by modifying the concentration and size of cloud droplets (Albrecht, 1989, Rosenfeld, 2000, Koren et al., 2004, Koren et al., 2005, Rosenfeld et al., 2008, Manoj et al., 2012, Sporre et al., 2012, Topping et al., 2013). Since a significant fraction of the total aerosol loading in the atmosphere comes from the boundary layer (Parameswaran et al., 2004, Tiwari et al., 2013, Pachauri et al., 2013), it is worthwhile to explore the influence of the meteorological parameters first on aerosol characteristics and dispersion in the atmosphere (e.g., Reddy et al., 2007) and further on their interaction with low-level clouds. Some studies in the literature (for example, Stull, 1988) have already focused on the influence of boundary layer aerosols on clouds. Under cloud-free conditions, the onset of turbulence aids incorporation of fresh aerosols in the mixed layer. Based on field experiments, Nilsson et al. (2001) have suggested that the adiabatic cooling in the rising convective plumes and the turbulent fluctuations in temperature and humidity by the entrainment flux might further enhance aerosol formation.
The aerosol particles need updrafts to stay airborne, which is provided by convection and turbulence in the boundary layer (Frisch et al., 1999, Parameswaran et al., 2004). Under favorable condition, some of them act as cloud condensation nuclei (CCN) and an increase in the CCN number may have a significant impact on the microphysics of clouds and thus climate (Albrecht, 1989, Topping et al., 2013). Leaitch et al. (1996) reported that the cloud droplet number concentration increases with increasing out-of-cloud aerosol number concentration for stratocumulus clouds, and suggested that turbulence measurements are essential to quantify the aerosol influence on cloud physics. Subsequently, the clouds thus forming at the top of the boundary layer have been widely recognized for their role in radiative processes, and thus have a strong effect on regional and global climate. However, the formation of continental boundary layer clouds is complicated not only by changes of the thermo-dynamical and hydrological conditions of the land surface, but also by geographical conditions such as terrain and variations in the sources and abundance of CCN. Therefore, understanding these clouds is critical to both weather and climate. Due to the large spatio-temporal variabilities and heterogeneous physical and chemical characteristics of aerosols, the interplay between aerosols and clouds under different atmospheric conditions is complex but really a challenging topic for the atmospheric scientists. Lidars play an important role in these studies because of their ability to make very precise continuous measurements of different aerosol and cloud parameters (e.g., McCormick et al., 1993, Pahlow et al., 2006, Devara et al., 2008). With the advent of technology, sophisticated lidar remote sensing systems are now used to study many different aspects of the atmosphere and its components. For instance, lidars are used to study the properties of high-altitude cirrus clouds over equatorial regions (Omar and Gardner, 2001), high-latitude polar stratospheric clouds (Santacesaria et al., 2001), stratospheric ozone (Leblanc and McDermid, 2000), along with stratospheric aerosols (Zuev et al., 2001) and tropospheric aerosols (Barnaba and Gobbi, 2001). In addition, lidars find their application in the studies of air pollution and trace gas concentration, boundary layer–troposphere–stratosphere–mesosphere structures and their composition and dynamics (e.g., Kulkarni et al., 2008). Lidars use radiation in the ultraviolet, visible or infrared region of the electromagnetic spectrum. Different types of physical processes in the atmosphere are related to different types of light scattering. Choosing various types of physical processes (such as Mie, Rayleigh, Resonance and Raman scattering and Doppler shift) allow atmospheric composition, temperature and wind to be measured. In this paper, we report some results obtained from the analysis of high space–time resolution data archived using a dual polarization micro-pulse lidar (DPMPL) and concurrent meteorological data, showing the role of nocturnal boundary layer (NBL) processes in the cloud formation in addition to illustrating how low-level clouds modulate the thermal structure of the NBL. Thus, the present study reinforces feedback mechanisms that exist between NBL and clouds.
The description of observational site and synoptic background is given in Section 2, and details of experimental setup and data analysis are provided in Section 3. Results from our analysis are documented in Section 4, and summarized in Section 5.
Section snippets
Experimental site and synoptic background
The study has been carried out at Pune (18°32′ N, 73°51′ E, 559 m AMSL), a semi-arid tropical urban site in the west-central India, which is situated around 200 km inward to the eastern side of the Arabian sea coast and also on the leeward side (rain-shadow region) of the Western Ghats. The experimental site is surrounded by hillocks of about 150 m high to its three sides (Devara and Raj, 1991), and it often favors strong shear of winds and thereby well mixing of boundary layer aerosols (Manoj and
Experiment and data
The current investigation is undertaken with an advanced dual polarization micro-pulse lidar (Foretech Model DPMPL 0.3C), which has been installed at the Indian Institute of Tropical Meteorology (IITM), Pune, India. It is a uni-axial mono-static lidar, operating at 532 nm wavelength, acquiring vertical profiles of backscatter intensity up to stratospheric altitudes at fine spatial resolution of 30 cm and temporal resolution of about a minute in real-time mode. Based on the technical
Results and discussion
Generally, the ability to understand the NBL and its interaction with cloud formation aloft is limited by the paucity of systematic high-vertical-resolution measurements from the surface to well above the top of the NBL (Mahrt et al., 1979). However, by employing the lidar technique, we present here some specific results related to the structure and shape of nocturnal low-level clouds observed on some typical days representing the monsoon seasons. A possible explanation is given to the observed
Summary and conclusions
High-resolution lidar backscatter data and concurrent meteorological parameters have been used to examine the role of aerosols and boundary layer processes in modifying cloud structural properties over a tropical station. Attempts were also made to study the interaction of clouds with the underlying boundary layer structure and evolution. The aerosols are dispersed into the nocturnal atmosphere by different processes such as wind, its vertical shear induced turbulence, and buoyancy production
Acknowledgments
The authors are grateful to the Director, IITM for the encouragement and infrastructure facilities to undertake the present study. The Mesoscale and Micro-scale Division of National Center for Atmospheric Research, United States, is acknowledged for access to the WRF‐ARW model. The authors acknowledge with thanks the ECMWF, UK and NCEP/NCAR, USA for the vertical profiles of meteorological fields used in the study. The authors are highly grateful to the “Aerosol and Cloud Physics Laboratory for
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