• Volume 105, Issue 1

      March 1996,   pages  1-100

    • Atmospheric surface layer parameters during different phases of monsoon over Varanasi from MONTBLEX-90

      D V Viswanadham A N V Satyanarayana

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      The fast and slow response data (8 Hz and 1 Hz respectively), obtained from the MONTBLEX-90 programme, are analysed for computing various surface layer parameters and the fluxes of sensible heat and momentum. In the present paper these fluxes, the Monin-Obukhov length scale (L), the turbulent kinetic energy (TKE), and the intensity of turbulence (σw) over Varanasi have been computed during different phases of monsoon such as dry, weak, moderate and active. Typical days are chosen for studying the above-mentioned parameters.

    • Some observations from the data taken in and around Kharagpur during the onset of the monsoon, 1990

      R Pradhan B Roy U K De D K Rakshit

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      During MONTBLEX 1990, various observational platforms were operated at Kharagpur and the nearby Kalaikunda Air Base. Using the data from all the platforms, one can draw the following conclusions. The temperature and wind data obtained from various sensors have overall compatibility. Sodar wind data indicate the presence of a low level jet at around 300 m above ground. The inversion height may be evaluated from the vertical profile of the sodar back-scatter echo intensity. The sub-synoptic or synoptic scale convergence modulates the inversion height and the presence of cloud-base within the inversion height in turn modulates the sensible heat and momentum fluxes.

    • Fluxes of heat and momentum over sea surface during the passage of a depression in the north Bay of Bengal

      S Sivaramakrishnan M N Patil K G Vernekar

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      Time variation of surface fluxes of heat, moisture and momentum over a sea station (20°N 89°E) in the north Bay of Bengal has been computed by profile method for the period 18th–25th August 1990 using meteorological data of MONTBLEX-90 from ORVSagarkanya. The fluxes showed synoptic and diurnal variations which are marked during depression (20th–21st August) compared to their variation prior to and after this period. Variations of heat and water vapour fluxes were in phase. Night time fluxes are relatively high compared to day time. Average momentum transfer during depression was two to three times large. Variations in Bowen ratio were relatively large during day time. During depression, it varied between 0·2 in day time and about 0·3 at night and in the undisturbed period between −0·1 and 0·2 during day time and 0·2 and 0·25 at night. The study shows that the assumptionCD=CH=CE of the exchange coefficients normally used in estimating the fluxes by the bulk aerodynamic method is not appropriate becauseCH/CD≈2,CE/CD≈1·5 andCH/CE≈1·4.

    • Variability of the oceanic boundary layer characteristics in the northern Bay of Bengal during MONTBLEX-90

      V S N Murty Y V B Sarma D P Rao

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      Variability of the ocean surface boundary layer characteristics on daily time-scale is studied utilizing the 3-hourly hydrographic data collected at a stationary location (20°N, 89°E) in the Bay of Bengal during August (18th–31st) and September (9th–19th), 1990 under MONTBLEX-90 field programme. The daily variations of temperature, salinity, σ0, mixed layer thickness, stability, heat content and rate of change of heat content in the upper 100 m are discussed in relation to prevailing weather (depressions) and hydrographic conditions (influx of fresh water, presence of eddies). The mixed layer thickness is examined through temperatureand σ0-based criteria considering also the surface salinity in the latter. TheT-based mixed layer thickness is always higher than that of σ0-based thickness. The rate of change of heat content is also computed up to the depth of 20°C and 14°C isotherms which takes into account the vertical motion and hence divergence. With the development of a low into a deep depression close to the study area, intense upwelling of subsurface cold waters is noticed from 100 m to the bottom of the surface mixed layer (20m) from 18th to 20th August. The upwelling is weakened by 21st August when the depression moved away from the study location. This variation of upwelling is supported by the variation of surface mixed layer thickness, static stability at 30 m depth, heat content in the upper 100 m and the heat content up to the depth of 20°C isotherm from 18th to 21st August. The rate of change of heat content in the upper 100 m and up to the depths of 20°C and 14°C isotherms leads to net heat storage during August and to net heat depletion during September. This together with the net surface heat gain lead to an import (197Wm−2) and export (233 Wm−2) of heat during August and September respectively through horizontal advective processes. These advective processes are attributed to the presence and movement of a warm core eddy through the study location.

    • Understanding the turbulent structure of the atmospheric boundary layer: A diagnostic approach

      M Chatterjee G K Sen D K Sinha

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      In this paper, we have attempted a diagnostic study of the turbulence characteristics of the ABL by means of two one-dimensional models. The first model uses a first order non-local closure, based on the Transilient Turbulence Theory, for parameterizing turbulent fluxes. while the second model uses second order local closure for parameterizing these. The models have been applied to conduct case studies using the Kytoon data taken at Kharagpur, during 17th–21st June, 1990, as part of the MONTBLEX programme.

      Our findings bring out various interesting features regarding the non-local and local turbulent statistics such as kinematic fluxes, turbulence kinetic energy, vertical velocity variance, the contribution of the eddies of various sizes to the fluxes at different level and the mixing lengths. The one-dimensional anisotropy of the turbulent eddies has been revealed by the findings from the transilient model. The vertical variation of the turbulence kinetic energy, as computed directly by the second order model, is found to be strongly correlated with the vertical velocity variance. In particular, for stably stratified boundary layers, identification of two distinct zones of the turbulence kinetic energy and corresponding vertical velocity maxima is possible, which has been interpreted as positive evidence of patchy turbulence in the boundary layer.

    • Planetary boundary layer over monsoon trough region in a high resolution primitive equation model

      K V J Potty U C Mohanty B Nandi K J Ramesh

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      Numerical experiment with improved boundary layer physics has been performed to study the Planetary Boundary Layer (PBL) characteristics over the monsoon trough region. Details of the evolution and structure of the associated boundary layer processes in the monsoon trough and adjoining oceanic regions are examined by integrating the model up to a period of 48 hours.

      The model used for this study is a high resolution primitive equation, one with 0·5o latitude/longitude horizontal resolution and 16 levels in the vertical (7 levels in the PBL). The boundary layer treatment in the model is based on the Monin-Obukhov similarity theory for the surface layer and Turbulent Kinetic Energy (TKE) formulation based onE-ε approach for the mixed layer. The model is integrated using the FGGE level IIIb analysis of European Centre for Medium Range Weather Forecasts (ECMWF), U.K.

      The study shows that the diurnal variation of TKE over land is a dominant feature comparing with that over the ocean. Along the monsoon trough region, TKE increases from the eastern end to the western side which is mainly associated with the enhancement of sensible heat flux as we move from the eastern wet land to the western desert sector. It may be noted that the low level wind maximum, which is a characteristic feature over the monsoon region, is well simulated by this improved model physics.

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