• P C S Devara

      Articles written in Journal of Earth System Science

    • Wind gradients at meteor heights over mid-latitude stations

      P C S Devara G Chandrasekhar M I Ahmed

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      The gradients of amplitude and phase of diurnal and semidiurnal components of zonal and meridional winds in the height intervals of 80–90 km and 90–100 km are studied using the meteor wind radar data collected at Atlanta (34°N, 84°W) during the period August 1974–March 1978. The results are compared with those at Adelaide (35°S, 139°E). It is found that the gradients vary in an opposite manner between the two height intervals.

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

      K K Dani R S Maheskumar P C S Devara

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      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.

    • Remote sensing of spectral signatures of tropospheric aerosols

      M B Potdar S A Sharma V Y Parikh P C S Devara P E Raj Y K Tiwari R S Maheskumar K K Dani S K Saha S M Sonbawne Y Jaya Rao G Pandithurai

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      With the launch of the German Aerospace Agency's (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.,

      Validation of ATMRAD model/code using MOS data and synchronously measured atmospheric data, and if found performing well, then to

      derive 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.

    • Lidar observation of aerosol stratification in the lower troposphere over Pune during pre-monsoon season of 2006

      P Ernest Raj S K Saha S M Sonbawne S M Deshpande P C S Devara Y Jaya Rao K K Dani G Pandithurai

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      Lidar observations of aerosol vertical distributions in the lower troposphere along with observations of horizontal and vertical winds from collocated UHF radar (Wind Profiler) over a tropical Indian station, Pune during the pre-monsoon season (March–May) of 2006 as part of an ISRO-GBP national campaign (ICARB) have been examined. Lidar vertical profiles showed high aerosol concentrations in the surface layers and a subsequent gradual decrease with height. Results showed the presence of an elevated stratified aerosol layer around 2000–3500m height which persisted throughout the months of March and April. Observed strong vertical gradients in both horizontal and vertical winds in the lower troposphere seem to be a possible cause for the formation of elevated aerosol layers. Further, high daytime temperatures accompanied by dry conditions at the surface help to enhance the aerosol loading in the lower layers over this location.

    • A study of lightning activity over land and oceanic regions of India

      Asha Nath G K Manohar K K Dani P C S Devara

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      Monthly variations of lightning activity over typical land and oceanic regions of India were examined using satellite data (OTD)for a 5-year period (1995-1999).It is noted that the nature of variation between surface air maximum temperature (𝑇_max) ,thunderstorm days (Th_n) ,and lightning flash count over ER and WR showed remarkable correspondence and sensitivity with each other on monthly time scale.As we move out of winter season and enter the monsoon season,via pre-monsoon season,the WR undergoes cooling relative to the ER in the range 0.1-1.2°C. As a result,WR experiences reduction of thunder days and lowering in flash count. This decrease in 𝑇_max Th_{n} and flash count over WR may also be associated with relatively small values of 𝑇𝜃 w and CAPE in comparison with similar values over ER during the monsoon sea-son.Our observation of associated reduction in Th_n and lightning count per 1°C cooling in surface air maximum temperature suggests reduction of ∼3.5 thunderstorms per station and 73 flashes. Comparison of lightning flashes between pairs of coastal,oceanic,arid-zone,hilly,and island stations reveals distinct relationship between climate regime and intensity of lightning activity. We may conclude the results of this study by saying that the overhead lightning activity is a clear reflection of the status of the underlying ground-earth properties. A close and continuous monitoring of lightning activity may be considered as a need of present day scientific studies.

    • A study of lightning activity over land and oceanic regions of India

      Asha Nath G K Manohar K K Dani P C S Devara

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