S. M. Sonbawne

Mobile Lidar Profiling of Tropical Aerosols and Clouds

Abstract Lidar profiling of atmospheric aerosols and clouds in the lower atmosphere has been in progress at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32′N, 73°52′E, 559 m MSL), India, for more than two decades. To enlarge the scope of these studies, an eye-safe new portable dual polarization micropulse lidar (DPMPL) has been developed and installed at this station. The system utilizes a diode-pumped solid-state (DPSS) neodymium–yttrium–aluminum–garnet (Nd:YAG) laser second harmonic, with either parallel polarization or alternate parallel and perpendicular polarization, as a transmitter and a Schmidt–Cassegrain telescope, with a high-speed detection and data acquisition and processing system, as a receiver. This online system in real-time mode provides backscatter intensity profiles up to about 75 km at every minute in both parallel and perpendicular polarization channels, corresponding to each state of polarization of the transmitted laser radiation. Thus, this versatile lidar system i...

Results of Sun Photometer¿Derived Precipitable Water Content over a Tropical Indian Station

Abstract A compact, hand-held multiband sun photometer (ozone monitor) has been used to measure total precipitable water content (PWC) at the low-latitude tropical station in Pune, India (18°32′N, 73°51′E). Data collected in the daytime (0730–1800 LT) during the period from May 1998 to September 2001 have been used here. The daytime average PWC value at this station is 1.13 cm, and the average for only the clear-sky days is 0.75 cm. PWC values between 0.75 and 1.0 cm have the maximum frequency of occurrence. There is a large day-to-day variability due to varied sky and meteorological conditions. Mainly two types of diurnal variations in PWC are observed. The one occurs in the premonsoon summer months of April and May and shows that forenoon values are smaller than afternoon values. The other type occurs in November and December and shows a minimum around noontime. There is a diurnal asymmetry in PWC in which, on the majority of the days, the mean afternoon value is greater than the forenoon value. This as...

Changes observed in sun photometer derived total column ozone and possible implications on surface-reaching UV radiation over a tropical Indian station

Ozone depletion or unusual decreases in ozone content has now become a global issue because of the corresponding increases in surface reaching UV-B radiation. Sun photometer derived total column ozone along with that from Dobson spectrometer at a tropical urban Indian station, Pune (18°32′N, 73°51′E, 559 m Above Mean Sea Level) for the period 1998–2003 has been investigated. On a monthly mean scale the agreement between ozonometer and Dobson is within ± 3%. Ozone content at Pune during the winter months of January 1999 and 2002 were almost 6–7% lower than that observed during other years. This reduction in ozone content led to enhancement of erythemal dose at the surface during following summer months. Day-to-day variation on some occasions also showed ozone decreases accompanied by significant enhancement in surface reaching UV radiation. Optical remote sensing methods thus help to monitor unusual changes in ozone content.

Characterization of aerosols and pre-cursor gases over Maitri during 24th Indian Antarctica Expedition

Within the framework of the 24th Indian Antarctica Expedition (IAE), observations of total column aerosol optical depth (AOD), ozone (TCO) and precipitable water content (TCW) using a multi-channel solar-radiometer (MICROTOPS-II: Microprocessor-controlled Total Ozone Portable Spectrometer-II), and observations of short-wave global radiative flux using a wide-band pyranometer have been carried out over the Indian Antarctica station Maitri (70.76° S, 11.74° E) and the southern Indian Ocean during December 2004–February 2005. These extensive datasets have been utilized to investigate the aerosol optical, physical and radiative properties, and their interface with simultaneously measured gases. Data over the Oceanic region have been collected from the ship front deck. The daily mean AOD at a characteristic wavelength of 500 nm was found to be 0.042 with an average Angstrom coefficient of 0.24, revealing an abundance of coarse-mode particles. Interestingly, the January fluxes were found to be less by about 20% compared with those in February. The average short-wave direct radiative forcing due to aerosols showed cooling at the surface with an average value of −0.47 Wm−2. The TCO increased from about 252 DU around 38° S to about 312 DU at 70° S, showing a gradual increase in ozone with increasing latitude. The TCO measured by the surface-based ozone monitor matched reasonably well with that observed by the Total Ozone Mapping Spectrometer (TOMS) satellite sensor within 5%. Variability in ozone on a daily scale during the study period was less than 4% over the Antarctica region.

Variability in sun photometer derived summertime total column ozone over the Indian station Maitri in the Antarctic region

Extensive observations of total column ozone (TCO) made over the Antarctic region at the Indian station ‘Maitri’ (70.76oS, 11.74oE) during two summer periods of 2004–2005 and 2006–2007 using a ground-based optical remote sensing instrument (ozonometer) have been examined to study the short-term variations. Mean TCO during 2004–05 summer was 288.7 DU with a variability of 8% and the corresponding values during 2006–2007 summer were 280.4 DU and 11%. Both ozonometer and Dobson spectrophotometer measurements at this Antarctic station during January 2005 showed a short-term decrease in TCO by about 70 DU before regaining original value. Simultaneous changes in precipitable water and surface temperature points to a possible interplay of chemistry and meteorological conditions. Occurrence of an intense solar proton event in January 2005 also suggests the possible role of precipitating charged particles in causing short-lived ozone decrease through production of OH radical.

Lidar remote sensing of the mixed layer height over a tropical urban Indian station

Mixed layer is an important parameter which controls meteorological conditions in the lower atmosphere. Transport and diffusion of pollutants in the lower atmosphere is highly dependant on the structure of the planetary boundary layer, one important feature of which is the height of the well-mixed layer. In the present study, continuous wave, bistatic argon ion lidar-derived scattered signal strength from different heights in the lower troposphere over Pune (18˚ 32′ N, 73˚ 51′ E, 559 m above mean sea level), India during the period April 2007–January 2008 has been recorded remotely and by employing simple statistical tools, the mixed layer height (MLH) and transition layer thickness (TLT) have been estimated. The results show that sufficient mixing of atmospheric constituents such as aerosols exists in the boundary layer in the post-sunset hours during the summer season, enabling estimation of MLH and TLT. On the other hand, during winter months as mixing ceases/weakens by late evening hours, the mixed layer depth is either low or not easily discernible. In view of the importance of mixed layer depth information for various atmospheric applications, the remote sensing tool used and the simple methodology followed here seem promising.

Lidar profiling of aerosols and clouds for regional climate and pollution research

The space-time variability of aerosol inhomogeneities provides unique information on atmospheric behavior needed for climate and environmental research and operational programs. An additional indirect forcing from aerosols results from their involvement in nucleation and growth of cloud droplets, reducing droplet size and thereby potentially influencing cloud albedo. These studies have particular significance over tropics where the convective and dynamical processes associated with high-altitude thunderstorms greatly affect the vertical distributions of aerosols and pre-cursor gases. As the anthropogenic share of the total aerosol loading is quite substantial over many parts of the world, it is essential to monitor the aerosol features systematically over longer time scales. Such observations are very important for understanding the coupling processes that exist between physico-chemical, radiative, dynamical and biological phenomena in the Earths environment, and provide valuable input information for modeling and simulation studies of climate and air quality. The multi-year aerosol number density data acquired during October 1986-September 2000 with a computer-controlled lidar at the Indian Institute of Tropical Meteorology (IITM), Pune, an urban station in India have been utilized to investigate (i) climate variability, (ii) cloud macro-physical parameters and (iii) environmental pollution. The results reveal a long-term trend in aerosol loading, single and multiple layer clouds with low cloud-base during the south-west monsoon months, and high pollution potential during winter late evenings. The trends in aerosol loading and air quality are found to be changing from year to year depending upon meteorological parameters (precipitation in particular). Some of these parameters have also been compared with co-located complementary facilities such as solar radiometers. In order to enlarge the scope of these studies, a dual polarization micro pulse lidar (DPMPL) has been installed at IITM recently to investigate the cloud composition, and aerosol-cloud-climate interactions. The initial results obtained from this state-of-the-art lidar system showed interesting features in the time evolution of nocturnal (stable) boundary layer which have strong bearing on air pollution potential over the experimental station. The complete details of the lidar systems used in the above studies together with discussion of salient results are presented in this paper.