• G S Bhat

      Articles written in Journal of Earth System Science

    • Surface meteorological instrumentation for BOBMEX

      G S Bhat S Ameenulla

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      Although India has a long experience in ship-borne experiments and oceanographic instrumentation, the atmospheric component has not received much attention in the past. In this paper, the basis of the atmospheric instrumentation system assembled for use on board ORV Sagar Kanya for the BOBMEX-Pilot experiment is described along with some representative results. Wherever possible, Woods Hole's IMET recommendations for meteorological sensors for applications in the marine environment have been followed to keep our measurements in par with international standards. The sensors were tested during the BOBMEX-Pilot experiment and all sensors worked well. Velocity, humidity and temperature data have been successfully collected using fast sensors. It is shown that the component due to the ship's pitching motion can be removed from the measured vertical velocity by making use of an accelerometer. This makes it possible to calculate the surface fluxes by direct methods.

    • Atmospheric boundary layer characteristics during the BOBMEX-Pilot experiment

      G S Bhat S Ameenulla M Venkataramana K Sengupta

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      The atmospheric boundary layer characteristics observed during the BOBMEX-Pilot experiment are reported. Surface meteorological data were acquired continuously through an automatic weather monitoring system and manually every three hours. High resolution radiosondes were launched to obtain the vertical thermal structure of the atmosphere. The study area was convectively active, the SSTs were high, surface air was warm and moist, and the surface air moist static energy was among the highest observed over the tropical oceans. The mean sea air temperature difference was about 1.25‡C and the sea skin temperature was cooler than bucket SST by 0.5‡C. The atmospheric mixed layer was shallow, fluctuated in response to synoptic conditions from 100 m to 900 m with a mean around 500 m.

    • Inertial-Dissipation flux measurements over south Bay of Bengal during BOBMEX-Pilot experiment

      M Venkataramana K Sengupta G S Bhat S Ameenulla J V S Raju

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      This paper describes measurement of air-sea parameters and estimation of sensible and latent heat fluxes by the “Inertial-Dissipation” technique over south Bay of Bengal. The data were collected on ORV Sagar Kanya during BOBMEX-Pilot cruise during the period 23rd October 1998 to 12th November 1998 over south Bay of Bengal. The fluxes are estimated using the data collected through fast response sensors namely Gill anemometer, Sonic anemometer and IR Hygrometer. In this paper the analyses carried out for two days, one relatively cloud free day on November 3rd and the other cloudy with rain on November 1st, are presented. Sea surface and air temperatures are higher on November 3rd than on November 1st. Sensible heat flux for both the days does not show any significant variation over the period of estimation, whereas latent heat flux is more for November 3rd than November 1st. An attempt is made to explain the variation of latent heat flux with a parameter called thermal stability on the vapor transfer from the water surface, which depends on wind speed and air to sea surface temperature difference.

    • Some salient features of the atmosphere observed over the north Bay of Bengal during BOBMEX

      G S Bhat

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      This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and 89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August 1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection. It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water vapor content, and wind direction show the major changes between the active and weak phases of convection.

    • On the measurement of the surface energy budget over a land surface during the summer monsoon

      G S Bhat S C Arunchandra

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      The measurement of surface energy balance over a land surface in an open area in Bangalore is reported. Measurements of all variables needed to calculate the surface energy balance on time scales longer than a week are made. Components of radiative fluxes are measured while sensible and latent heat fluxes are based on the bulk method using measurements made at two levels on a micrometeorological tower of 10 m height. The bulk flux formulation is verified by comparing its fluxes with direct fluxes using sonic anemometer data sampled at 10 Hz.Soil temperature is measured at 4 depths. Data have been continuously collected for over 6 months covering pre-monsoon and monsoon periods during the year 2006. The study first addresses the issue of getting the fluxes accurately.It is shown that water vapour measurements are the most crucial. A bias of 0.25% in relative humidity,which is well above the normal accuracy assumed by the manufacturers but achievable in the field using a combination of laboratory calibration and field intercomparisons, results in about 20 W m−2 change in the latent heat flux on the seasonal time scale. When seen on the seasonal time scale,the net longwave radiation is the largest energy loss term at the experimental site. The seasonal variation in the energy sink term is small compared to that in the energy source term.

    • On the marine atmospheric boundary layer characteristics over Bay of Bengal and Arabian Sea during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)

      Denny P Alappattu D Bala Subrahamanyam P K Kunhikrishnan K M Somayaji G S Bhat R Venkatesan C B S Dutt A Bagavath Singh V K Soni A S Tripathi

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      Detailed measurements were carried out in the Marine Atmospheric Boundary Layer (MABL) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) which covered both Arabian Sea and Bay of Bengal during March to May 2006. In this paper, we present the meteorological observations made during this campaign. The latitudinal variation of the surface layer turbulent fluxes is also described in detail.

    • Equatorial modes observed in atmospheric variables

      L Kiranmayi G S Bhat

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      Wavenumber –frequency spectral analysis of different atmospheric variables has been carried out using 25 years of data.The area considered is the tropical belt 25°S $–$25°N. A combined FFT- wavelet analysis method has been used for this purpose.Variables considered are outgoing long- wave radiation (OLR),850 hPa divergence,zonal and meridional winds at 850,500 and 200 hPa levels,sea level pressure and 850 hPa geopotential height.It is shown that the spectra of different variables have some common properties,but each variable also has few features different from the rest. While Kelvin mode is prominent in OLR and zonal winds,it is not clearly observed in pressure and geopotential height fields;the latter two have a dominant wavenumber zero mode not seen in other variables except in meridional wind at 200 hPa and 850 hPa divergences.Different dominant modes in the tropics show significant variations on sub-seasonal time scales.

    • Comparison of CloudSat and TRMM radar reflectivities

      K D Sindhu G S Bhat

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      Comparison of reflectivity data of radars onboard CloudSat and TRMM is performed using coincident overpasses. The contoured frequency by altitude diagrams (CFADs) are constructed for two cases: (a) only include collocated vertical profiles that are most likely to be raining and (b) include all collocated profiles along with cloudy pixels falling within a distance of about 50 km from the centre point of coincidence. Our analysis shows that for both cases, CloudSat underestimates the radar reflectivity by about 10 dBZ compared to that of TRMM radar below 15 km altitude. The difference is well outside the uncertainty value of ∼2 dBZ of each radar. Further, CloudSat reflectivity shows a decreasing trend while that of TRMM radar an increasing trend below 4 km height. Basically W-band radar that CloudSat flies suffers strong attenuation in precipitating clouds and its reflectivity value rarely exceeds 20 dBZ though its technical specification indicates the upper measurement limit to be 40 dBZ. TRMM radar, on the other hand, cannot measure values below 17 dBZ. In fact combining data from these two radars seems to give a better overall spatial structure of convective clouds.

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