Pe Raj

Investigations of aerosol optical depth variations using spectroradiometer at an urban station, Pune, India

Abstract The high spectral resolution radiometer at the Indian Institute of Tropical Meteorology (IITM), Pune [18°, 32′N, 73° 51′ E, 559 m Above Mean Sea Level (AMSL)], India has been operated on 79 clear-sky days during April 1993–April 1995 in order to study the temporal-spectral variations in aerosol optical depth (AOD) at the station. From the records of solar irradiance at different zenith angles of the sun, AODs were estimated at a spectral interval of 5 nm in the wavelength region 400–700 nm. The temporal variations in AOD in different spectral regions showed a relationship with those observed in the concurrent surface-level meteorological parameters at the experimental site. The seasonal variations in AOD indicated maximum during summer months and minimum during winter months. In addition, the afternoon (AN) AODs are found to be higher as compared to the forenoon (FN) depths in summer months of the year, whereas the FN AODs dominate in winter months at almost all wavelengths over the observing station.

Real-time monitoring of atmospheric aerosols using a computer-controlled lidar

Regular monitoring of aerosol parameters during nighttime in the tropical urban lower atmosphere using a tunable, continuous wave, bistatic argon ion laser radar (lidar) system was initiated at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°N51′E), India in 1986. So far, the measurements have been made from manual operation of the lidar which consumes a lot of time and inhibits interpretation of results immediately after completion of the experiment. In order to improve and expedite the data acquisition-processing and to avoid extra manual work to a considerable extent, the lidar has recently been made fully automatic. This paper discusses the on-line control and digital data system that provides real-time acquisition, analysis and display of lidar data. The improvements in the data quality and application of the system to the real-time observation of various features connected with atmospheric aerosol variations in both space and time are presented. The previously used lidar data acquisition and handling techniques and those of the present auto-mode operation are compared. Such real-time aerosol observations on a long-term basis are expected to lead to a better understanding of the optical properties of aerosols and their interaction with clouds and climate and will hopefully add to the global monitoring efforts and attempts at incorporating aerosols into climate models.

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

Retrieval of Aerosol Size Index From High-Resolution Spectroradiometer Observations

ABSTRACT Aerosol characteristics over an urban station were studied by utilizing aerosol optical depths at 61 wavelengths in the region 400–700 nm (5-nm interval) obtained with a high spectral resolution radiometer during the period from April 1993 to April 1995 at the Indian Institute of Tropical Meteorology (HTM), Pune (18°32′ N, 73°51′ E, 559-m AMSL), India. The retrieved columnar aerosol size index values are compared with those derived from colocated bistatic argon ion lidar angular scattering measurements. The spectroradiometer-derived size index (daytime) values are found to vary between 2.91 and 4.04, whereas the lidar-derived (nighttime) value is found to vary from 2 to 4.5 during the observational period. The temporal and short-term variations in aerosol size index are presented. The seasonal variation in aerosol size index over the experimental station indicates larger values during winter months and smaller values during pre-monsoon months, which suggests relatively larger-sized particles duri...

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...

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.

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.

Remote sensing of spectral signatures of tropospheric aerosols

AbstractWith the launch of the German Aerospace Agencys (DLR) Modular Opto-electronic Scanner (MOS) sensor on board the Indian Remote Sensing satellite (IRS-P3) launched by the Indian Space Research Organization (ISRO) in March 1996, 13 channel multi-spectral data in the range of 408 to 1010 nm at high radiometric resolution, precision, and with narrow spectral bands have been available for a variety of land, atmospheric and oceanic studies. We found that these data are best for validation of radiative transfer model and the corresponding code developed by one of the authors at Space Applications Centre, and called ATMRAD (abbreviated for ATMospheric RADiation). Once this model/code is validated, it can be used for retrieving information on tropospheric aerosols over ocean or land. This paper deals with two clear objectives, viz.,1Validation of ATMRAD model/code using MOS data and synchronously measured atmospheric data, and if found performing well, then to2derive relationship between MOS radiances and Aerosol Optical Thickness (AOT). The data validation procedure essentially involves•near-synchronous measurements of columnar aerosol optical thickness and altitude profiles of aerosol concentration using ground-based multi-filter solar radiometers and Argon-ion Lidar, respectively and•computation of the top-of-the-atmosphere (TOA) radiances from a low reflecting target (near clear water reservoir in the present study) using the ATMRAD model. The results show that the model performance is satisfactory and a relationship between the spectral parameters of MOS radiances and aerosol optical thickness can be established. In this communication, we present the details of the experiments conducted, database, validation of the ATMRAD model and development of the relationship between AOT and MOS radiance.

Tropical atmospheric stability and wind structures as inferred from the Indian MST radar observations

SummaryThe MST (Mesosphere-Stratosphere-Tropospher) Radar Facility at Gadanki (13.47° N, 79.18° E), near Tirupati, Andhra Pradesh, India has been operated over seven diurnal cycles—three in November 1994, one in September 1995 and three in January–February 1996 with an objective to study the wind and stability characteristics in the troposphere and lower-stratosphere. The radar-measured height profiles of both zonal (EW) and meridional (NS) wind components and near-simultaneous radiosonde measurements from Madras (13.04° N, 80.7° E) and Bangalore (12.85° N, 77.58° E), the two stations close to either side of the radar site, have been compared and they are found to be in gross agreement within the limitations of the sensing techniques.The results of the study also indicated multiple stable and turbulent structures/stratification throughout the height region from about 4 to 30 km. It is noticed that the stable layers are well marked around the altitudes 4 km, 12 km and the tropopause while the turbulent layers exist a few kilometers below the tropopause. These stable and turbulent layer structures showed good correspondence with the radar-measured wind gradients and also with the radiosonde-derived temperature and wind distributions over Madras. The maximum positive gradient in the signal-tonoise ratio (SNR) which corresponds to ‘radar tropopause’ is found to coincide with the greater potential temperature gradient and smaller wind gradient. The time evolution of atmospheric stability structure, derived from the SNR, spectral width and vertical wind revealed a ‘diffused tropopause’ or ‘tropopause weakening’ which is found to be associated with broader spectral width and larger gradients of winds. This feature is considered to be due either to the instability associated with large vertical gradients in horizontal winds (dynamical instability) or to the instability generated by the convection (convective instability).

Advances in atmospheric, hydrographic and vegetation remote sensing with lidar

Lidar has evolved as one of the powerful and versatile techinqne for atmospheric monitoring and geophysical and hydrographic studies from different observational platforom. This paper discusses the recent advances that have taken place in the lidar technology for rnulti-dimensional atmospheric, hydragraphic and vegetation monitoring together with the results of some specific atmospheric studies carried our for the past ten years using the lider systems built at the Indian Institute of Tropical Meteorotogy (IITm) Pune.