K. K. Dani

Lidar measurements of aerosol column content in an urban nocturnal boundary layer

Abstract Lidar aerosol measurements made at Pune (lat. 18°32′N, long. 73°51′E, 559 m AMSL), India, a tropical urban station, during the nine-year period from October 1986 to September 1995 have been used to study the temporal variations in the aerosol-loading in the nocturnal boundary layer. There is a long-term increasing trend in the aerosol column content in the 50–1100 m layer that has been attributed to be due to the increasing anthropogenic activity around the lidar site. The seasonal variations in aerosol content show a maximum in the pre-monsoon month of May and a minimum in the SW monsoon month of July. The percentage contribution of the 50–200 m layer to the total loading in the 50–1100 m layer is about 41% which points to the predominant surface source of particulate matter. On a seasonal scale, there is a decrease of about 36% in the aerosol content from premonsoon (March-May) to monsoon (June-September) season and this decrease on year-to-year basis is directly related to the amount of rainfall received at the ground at Pune during the monsoon season. This effect has not changed appreciably over the years whatever be the increase in the aerosol loading. The temporal evolution of aerosol content in the nighttime showed a rapid decrease soon after sunset and a slower rate of decrease in the midnight hours. The study also showed that surface relative humidity and winds influence the temporal variations in the aerosol column content.

Relationship between lidar‐based observations of aerosol content and monsoon precipitation over a tropical station, Pune, India

This paper reports the results of the aerosol lidar experiments that have been performed at the Indian Institute of Tropical Meteorology (IITM), Pune (18.54°N, 73.85°E, 559 m amsl), a tropical station in India. The lidar-observed cloud macro-physical parameters (cloud-base and cloud-ceiling heights, vertical thickness, etc.) and polarisation characteristics and their association with surface-generated aerosols at the experimental site are presented and discussed. The correspondence among the lidar-derived aerosol distributions, meteorological parameters and south-west (SW) monsoon (June–September) activity over Pune during 12 successive SW monsoon seasons (1987–98) including two pairs of contrasting seasons of 1987–8 and 1993–4 is also examined. The results indicate an association between variations in aerosol loading in the boundary layer during the pre-monsoon season (March–May) and precipitation intensity during the ensuing monsoon season. Moreover, the decrease in aerosol content from pre-monsoon to monsoon season is found to follow the SW monsoon season total precipitation. Thus the results suggest that (i) the IITM lidar can also be a useful remote sensor for aerosol characterisation studies from polarisation measurements, and some important physical properties of clouds in the lower atmosphere over the station, and (ii) there exists a correspondence between boundary-layer aerosol content and SW monsoon precipitation over Pune, which is explained in terms of the type of aerosols and the environmental and meteorological processes, particularly during pre-monsoon and monsoon months prevailing over the experimental station. Copyright

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...

Study of total column atmospheric aerosol optical depth, ozone and precipitable water content over Bay of Bengal during BOBMEX-99

The spatial and temporal variations in aerosols and precursor gases over oceanic regions have special importance in the estimation of radiative forcing parameters and thereby in the refinement of general circulation models. Extensive observations of the columnar aerosol optical depth (AOD), total column ozone (TCO) and precipitable water content (PWC) have been carried out using the on-line, multi-band solar radiometers onboard ORV Sagar Kanya (Cruise # SK 147B) over Bay of Bengal during 11th–28th August 1999. Aerosol optical and physical properties (optical depth and angstrom parameter) have been estimated at six wavelengths covering from UV to NIR (380–1020 nm) while TCO and PWC have been determined using the UV band around 300 nm and NIR band around 940 nm, respectively. Added, concurrent meteorological and satellite observations during this field phase of BOBMEX-99 have been utilized to investigate spectral-temporal variations of AOD, TCO and PWC in marine environment.The results indicate lower AODs (around 0.4 at characteristic wavelength of 500 nm) and size distributions with abundance of coarse-mode particles as compared to those aerosols of typical land origin. An interesting result that is found in the present study is the significant reduction in AOD at all wavelengths from initial to later part of observation period due to cloud-scavenging and rain-washout effects as well as signature of coastal aerosol loading. The clear-sky daytime diurnal variation of TCO shows gradual increase during post-sunrise hours, broad maximum during afternoon hours and gradual decrease during pre-sunset hours, which is considered to be due to photochemical reactions. The diurnal variation curve of PWC showed maximum (~ 4 cm) during morning hours and gradual decrease (~ 3.5 cm) towards evening hours, which are found to be greater as compared to typical values over land. Another interesting feature observed is that although the PWC values are very high, there was no proportionate or appreciable enhancement in AOD—a feature that can be utilized to infer composition of aerosols over the study region.

Spectral characteristics of urban aerosols and their association with relative humidity

Abstract Multi-spectral extinction measurements made with co-located high-spectral resolution radiometer (spectroradiometer) and sunphotometer at the Indian Institute of Tropical Meteorology (IITM), Pune (18°32′N, 73°5′E, 559 m AMSL) on 188 cloud-free days between March 1993 and May 1995 are presented. The observed aerosol optical size spectrum was approximatd by a composite power-law distribution function. The seasonal mean size distributions, inferred from both the radiometers, exhibit a power-law type distribution with different exponents changing at an intermediate size. The mean Junge size exponents ( ν 1 and ν 2 ) and switching radius ( r 0 ) obtained with spectroradiometer are found to be in good agreement with those obtained with sunphotometer. The relationships among aerosol optical depths and derived size distributions, and meteorological parameters (height-integrated) are discussed.

Atmospheric aerosol–cloud-stability relationship as observed with optical and radio remote sensing techniques

Abstract Atmospheric aerosol characteristics are being monitored, on a routine basis, with lidars and spectroradiometer since 1985 and 1992, respectively, at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m AMSL), a tropical Indian urban station. The lidar-derived nocturnal atmospheric structures, transport of surface-generated aerosols (pollutants) and their participation in the formation of clouds and their time evolution in the planetary boundary layer (PBL) are studied in relation to the local diabatic conditions and height-integrated relative humidity (RH) inferred from radiometersonde and pilot balloon (pibal) observations. This paper mainly addresses (i) how the integrated lidar, spectroradiometer and coincident aerometric data can be used to delineate the atmospheric structure and transport mechanisms, and (ii) the extent to which the lidar observations during different atmospheric conditions can be useful to explain the phenomenon of cloud scavenging of aerosols over the experimental station.

Time evolution of monsoon low-level jet observed over an Indian tropical station during the peak monsoon period from high-resolution Doppler wind lidar measurements

Doppler wind lidar measurements of horizontal winds at an Indian tropical station, Mahbubnagar (16.73°N, 77.98°E, 445 m above mean sea level), were used to investigate the time evolution of the monsoon low-level jet (MLLJ) during the southwest monsoon season. Vertical profiles of zonal wind in the altitude range of 100 to 3000 m above surface (at every 50 m height interval and 5 min time averaged) obtained during the period 25 July to 23 August 2011 are considered for the analysis. The zonal winds in the altitude up to 3000 m above ground are predominantly westerly throughout the period and on almost all the days there is a westerly wind speed maximum around 500 m above ground during nighttime. Soon after local sunrise, the core of this wind speed maximum (jet) gets lifted up and by afternoon, the westerly wind maximum is shifted to a higher altitude of 2000 m–2500 m without much change in its magnitude. Analysis of the high-resolution lidar data strongly indicates that the same nocturnal LLJ seems to be moving up and evolving into a daytime westerly MLLJ reported in several previous studies. Wind speed and direction derived from the wind lidar agree reasonably well with simultaneously observed GPS upper air sounding wind measurements. Further analysis shows that the time-height evolution of the jet core is closely associated with daytime convection and boundary layer growth. The presence of clouds over the region seems to inhibit this type of time evolution.

Tropical urban aerosol distributions during pre-sunrise and post-sunset as observed with lidar and solar radiometer at Pune, India

Abstract The pre-sunrise and post-sunset differences in the tropical urban aerosol distributions have been studied by conducting coordinated experiments using a continuous wave, bistatic Argon ion lidar and a spectroradiometer at the Indian Institute of Tropical Meteorology, Pune, India during 1997–2000. The results of the study indicate higher aerosol concentration in the air layers close to the ground, and lower concentration aloft on all the days of observations. Further, the concentrations are found to be greater in the early-morning (pre-sunrise) hours and lower in the late-evening (post-sunset) hours during the winter season and vice versa during the pre-monsoon season. These deviations are considered to be due to the convective activity and associated turbulent mixing during the pre-monsoon and close-to-ground and elevated haze layer formations during the winter months over the experimental station.

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...

Aerosol optical depth and ozone variations during the total solar eclipse of 24 October 1995

Abstract The total solar eclipse of 24 October 1995 has provided an important opportunity to investigate a variety of eclipse-related atmospheric phenomena. The path of totality of the eclipse passed through a few stations in India. Field campaign experiments have been carried out with an objective to investigate the effects of solar eclipse phenomenon on atmospheric aerosol characterization and columnar ozone over Robertsgunj (24°42′N, 83°04′E, 312 m AMSL), Uttar Pradesh, India during 23–25 October 1995 using a multi-channel radiometer. The solar irradiance measured in the ultra-violet (UV) to near-infrared (NIR) wavelength region has been utilized to study the spectral–temporal variations of height-integrated aerosol optical depth and corresponding aerosol size distribution, and total column ozone during the pre- and post-eclipse period (control days) and on the total solar eclipse day. The study has revealed that aerosol optical characteristics and ozone variations observed on the eclipse day, especially during the period of totality, differ significantly from those noticed on the control days. These deviations are explained on the basis of changes in meteorological parameters induced by the eclipse phenomenon.