P. Ernest Raj

Identification of aerosol type over the Arabian Sea in the premonsoon season during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB)

A discrimination of the different aerosol types over the Arabian Sea (AS) during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB-06) is made using values of aerosol optical depth (AOD) at 500 nm (AOD500) and A ngstrA¶m exponent (I±) in the spectral band 340-1020 nm (I±340-1020). For this purpose, appropriate thresholds for AOD500 and I±340-1020 are applied. It is shown that a single aerosol type in a given location over the AS can exist only under specific conditions while the presence of mixed aerosols is the usual situation. Analysis indicates that the dominant aerosol types change significantly in the different regions (coastal, middle, and far) of AS. Thus the urban/industrial aerosols are mainly observed in coastal AS, the desert dust particles occur in the middle and northern AS, while clear maritime conditions mainly occur in far AS. Spectral AOD and A ngstrA¶m exponent data were analyzed to obtain information about the adequacy of the simple use of the A ngstrA¶m exponent and spectral variation of a for characterizing the aerosols. Using the least squares method, I± is calculated in the spectral interval 340-1020 nm along with the coefficients a1 and a2 of the second-order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength. The results show that the spectral curvature can effectively be used as a tool for their discrimination, since the fine mode aerosols exhibit negative curvature, while the coarse mode particles exhibit positive curvature. The correlation between the coefficients a1 and a2 with the A ngstrA¶m exponent, and the atmospheric turbidity, is further investigated.

A bistatic lidar for aerosol studies

A bistatic, continuous wave lidar system has been developed at the Indian Institute of Tropical Meteorology, Pune for the purpose of monitoring aerosol characteristics at the place. The system employs a Lexel Model 95–4, 4-Watt Ar+ laser/Spectra-Physics Model 159, 5 Milliwatt He-Ne laser as the transmitter and a 25 cm Newtonian telescope and associated light/electronic measuring equipment as the receiver. A description of the experimental set-up is presented. The procedure for extracting aerosol information from the received scattered laser return signals is outlined.

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.

Study of atmospheric aerosols in a terrain-induced nocturnal boundary layer using bistatic lidar

Abstract The aerosol vertical profiles (20–1000 m) obtained in the atmospheric boundary layer, on four nights, using a bistatic, CW Argon ion lidar system developed at the Indian Institute of Tropical Meteorology, Pune, India are presented. The profiles show a very steep negative gradient of aerosol concentration up to 150 m AGL and thereafter a smooth decrease with altitude. It appears that the aerosol profiles exhibit a stratified layer structure on certain days and show smooth altitudinal variation on other days. An attempt is made to explain the observed features on the basis of terrain effects at the lidar site and stratified turbulence (derived from pilot balloon wind observations) which prevailed on the days of the lidar observations.

Remote sounding of aerosols in the lower atmosphere using a bistatic CW helium-neon lidar

A bistatic, CW (continuous wave) lidar (light detection and ranging) system has been developed at the Indian Institute of Tropical Meteorology, Pune, India for remote sounding of atmospheric aerosols. The system comprises a Spectra-Physics Model 159, 5 mW helium-neon laser as transmitter and a 25 cm Newtonian telescope-photomultiplier assembly as receiver. The details of the experimental set-up and results of preliminary observations of aerosols carried out in the lowest layers (up to 300 m AGL) of the atmosphere are presented. The results were found to be in agreement with those of the model predicted aerosol number density profile obtained by other investigators in the height region of observations. The variations in the height distribution of aerosols are explained in the light of the atmospheric conditions prevailing over the measuring site.

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.

Some results of lidar aerosol measurements and their relationship with meteorological parameters

Abstract Vertical profile measurements of aerosol number density remotely in the lower atmosphere during night-time using a bistatic, continuous wave Argon ion laser radar (lidar) system have been in progress at the Indian Institute of Tropical Meteorology, Pune, India since September 1986. The observational programme includes the measurement of a minimum two and maximum seven vertical profiles of atmospheric aerosols in each month. This paper deals with the results of the analysis of lidar aerosol data archived for a period of one year (October 1986–September 1987) and presents the monthly variations in the height distribution of aerosol number density along with their deviations from the annual mean distribution. Also, presented in this paper are the results of (i) the temporal changes in the aerosol concentration at 30 m AGL and its relationship with surface wind and relative humidity, and (ii) the comparison of the aerosol profiles on some selected days with the near-simultaneously obtained vertical profiles of wind, temperature and relative humidity. The results suggest that variations of aerosol concentration exhibit a certain relationship with those of meteorological parameters, and the atmospheric stability conditions associate with the vertical gradients of concentration at the top of the aerosol layer.

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.

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...

Scattering angle distribution of laser-return signal strength in the lower atmosphere

Abstract The bistatic, continuous wave Argon ion lidar system developed at the Indian Institute of Tropical Meteorology, Pune, India has been used to examine the nature of variation of laser-return signal strength (from a constant altitude in the lower atmosphere) with scattering angle (varied from 90 to 175°). The observed variation correlates well with the calculated variation in Mie scattering cross section assuming a power law type particle size distribution. The values of size index and real part of refractive index at the observation site are found to be around 4.6 and 1.53, respectively. Results suggest the presence of water-soluble, dust-like aerosols with predominance of larger sized particles during the premonsoon (March–May) and SW monsoon (June–September) seasons in the lower atmosphere at the lidar site.