O. P. Sharma

Interannual Variations of Summer Monsoons: Sensitivity to Cloud Radiative Forcing

Abstract The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land–sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak So...

On Near-Diffusion-Free Advection over Spherical Geodesic Grids

Abstract A forward trajectory advection scheme has been designed for its use in an icosahedral–hexagonal grid model. The scheme has been evaluated with two-dimensional test cases: solid-body rotation and deformational flow; both depict important characteristics of atmospheric flows. The main motivation of this study is to achieve good accuracy without using higher-order interpolations in a numerical advection scheme, so that it may become viable in fine-resolution GCMs. The computation of the error norm shows its gradient as constant and the scheme is approximately first-order accurate. The other interesting feature of this study is that its downstream search algorithm reduces the complexity from O(n2) to O(n).

Tropical-middle latitude interactions viewed via wave energy flux in the frequency domain

Abstract Over the upper troposphere of the polar latitudes the zonal flows exhibit a large variance on the time scale of the Madden-Julian oscillation, i.e. roughly 30–50 days. The other prominent regions for these intraseasonal oscillations are the Asian and Australian monsoon belts. These two regions are separated by the so-called critical latitude, to the south of which easterlies generally prevail and westerlies are prevalent to the north. A perplexing issue is that of possible tropical-middle latitude interactions across the critical latitude. The notion of the critical latitude emerged from the linear theories for the wave energy flux which assume a constancy in time for the zonal flows. This same problem, viewed in its full non-linear context, can be cast in a frequency domain. Such a formulation does not assume a constancy of the zonal flows in time but does permit the intraseasonal variations of the zonal flows to be present. The computation of the wave energy flux, from the more complete non-linear system in the frequency domain, requires the handling of linear, quadratic and triple product terms via use of Hayashis co-spectral method. These results of the present study, based on 6 years of daily global data sets, show that wave energy flux clearly passes from the latitudes of the monsoon to the polar latitudes. A strong convergence of wave energy flux in the polar latitudes suggests the tropical-middle latitude convergence interactions across the so-called critical latitude—when the problem is viewed in the frequency domain.

A model study for ambient air quality analysis using routine meteorological observations

Abstract In the present study, more realistic and easily adaptable input parameters have been used with a view to investigating the long-range air quality analysis for the dispersion of air pollutants emitted from an area source with a multiple box model. The model formulation has been discussed at length for the ground level sources when convective conditions prevail. The routine meteorological observations have been used for the computation of sensible surface heat flux, friction velocity and mixing depth. A radiation model provides the estimates of the sensible surface heat flux. Based on the similarity theory, an iterative procedure has been adopted for the estimation of friction velocity which provides a coupling of radiation computation and the surface layer of the planetary boundary layer through surface heat flux expression. The important parameters—wind speed and eddy diffusivity profiles—have been derived and have been used to obtain the concentration patterns as hourly averages. The procedure could be easily adopted where observed meteorological parameters may be used for studying the dispersal of pollutants from the ground level sources.

Snowmelt Runoff Simulation Using HEC-HMS in a Himalayan Watershed

The snowmelt induced runoff makes the streamflow perennial that regulates streamflow during spring and summer. The Hydrologic Modeling System (HMS) of the Hydrologic Engineering Centre (HEC) has been adopted for simulating the snowmelt and rainfall runoff. This modeling system, abbreviated as HEC-HMS, uses snow band methodology of US Corps of Engineers as used in Streamflow Synthesis and Reservoir Regulation (SSARR) model. In this paper reliability of HEC-HMS snowmelt model is tested using a temperature index, spatiotemporal analysis of process parameters and variables that characterize Beas sub-basin in the Pirpanjal range of the lower Himalayas above the Manali (1900 m of altitude). The daily and weekly simulations from simple temperature index method have been found satisfactory with R 2 above 0.7 and the results are presented through this paper. The model simulation reveals that ATI Cold/Melt rate functions are important to run and the meteorological model Index (mm) is important for model simulations.

Sensitivity of radiative forcing to global carbonaceous emissions

ABSTRACT Direct radiative forcing at top of the atmosphere for black carbon aerosols from two inventories comes out to be +0.33 W m−2 for Global Emission Inventory Activity (GEIA) and +0.14 W m−2 for BOND (Bond et al., 2004). However, for organic matter aerosols, it is simulated as −0.44 W m−2 for GEIA and −0.11 W m−2 with BOND inventory. Simulated annual global burden and aerosol optical depth of carbonaceous aerosols from GEIA and BOND are also compared. Normalised differences plots show that model simulates generally higher values of carbonaceous aerosols with GEIA, which are far superior in some parts of the globe as compared to those simulated with BOND emission inventory. An evaluation of these quantities with the median of the response of the AeroCom models is considered here as a benchmark – shows that while simulations with GEIA inventory have closer agreement, values of radiative forcing with BOND inventory are comparatively of smaller magnitudes over most parts of the globe. The reasons for this disparity in results for the latter may possibly be attributed to key differences between the two inventories. The main conclusion of this study is that the radiative forcing appears to be highly sensitive to carbonaceous content in aerosol compositions.

Response of an ocean general circulation model to wind and thermodynamic forcings

The stretched-coordinate ocean general circulation model has been designed to study the observed variability due to wind and thermodynamic forcings. The model domain extends from 60‡N to 60‡S and cyclically continuous in the longitudinal direction. The horizontal resolution is 5‡ x 5‡ and 9 discrete vertical levels. First a spin-up experiment has been done with ECMWF-AMIP 1979 January mean fields. The wind stress, ambient atmospheric temperature, evaporation and precipitation have been used in order to derive mechanical and thermodynamical surface forcings. Next, the experiment has been extended for another 30 years (3 cycles each of 10 year period) with varying surface boundary conditions (from January 1979 to December 1988 of ECMWF-AMIP monthly fields for each cycle) along with 120 years extended spin-up control runs results as initial conditions. The results presented here are for the last 10 years simulations. The preliminary results of this experiment show that the model is capable of simulating some of the general features and the pattern of interannual variability of the ocean.

Assessment of the use of satellite derived winds in monsoon forecasting using a general circulation model

Abstract The monsoonal region around India includes vast oceanic area. The observation of winds over the oceans is feasible only through the cloud motions from geostationary satellites. During MONEX-79, a GOES satellite was brought over the Indian ocean and a large number of other observing systems were also employed. A comprehensive well analysed data base is available for the study of the monsoon season of the year 1979. In the present study we have compared the results of monsoon forecasting with a General Circulation Model using FGGE Level III-B data and again using only the satellite derived winds over the Indian ocean. In the latter data set the winds are available only at lower levels (850 mb) and upper levels (200 mb). The observed two level satellite winds are interpolated to required levels using the representative vertical profiles during the onset phase. As a preliminary experiment the winds have been objectively analysed using the Cresman technique. The GCM used is a variant of LMD (France) model - now called MGCM, developed jointly by IIT, Delhi and LMD. This has a variable grid size and has been installed in the CYBER170 computer at New Delhi. The results of the forecast experiment for the onset phase are compared to assess the value of the monsoon forecasting if only the satellite derived winds are used in the model. The desirability of determination of satellite winds at least at one more level is suggested.

Stability of plane Couette flow of a viscoelastic fluid with uniform cross-flow

The linear stability of plane Couette flow of viscoelastic fluids has been the subject of several investigations in recent years. Giesekus (1) has discussed the stability of viscoelastic fluids for circular and plane Couette flows theoretically as well as experimentally. One of the most important facts observed by him is that viscoelasticity gives rise to cellular type of instabilities even in the absence of inertial forces if the second normal stress difference is chosen as positive. In other words inertia only modifies the critical value of the characteristic number associated with the neutral stability. Recently, in a series of subsequent papers Bhatnagar and Giesekus (2, 3) confirmed these ideas for the case of plane channel flow (2) and plane Poiseuille flow (3). For these flows, they also pointed out that two types of disturbances with different cell widths may exist simultaneously provided the parameter representing the ratio of inertial to elastic forces lies in a certain range. While investigating the overstability of plane Couette flow, Giesekus and Bhatnagar (4) found that, in general, the overstable mode is higher than the stationary mode but that both can come close to each other if certain conditions are satisfied.

Variations in sulphate aerosols concentration during winter monsoon season for two consecutive years using a general circulation model

During the field cruises of the Indian Ocean Experiment (INDOEX) extensive measurements on the atmo¬spheric chemical and aerosol composition are undertaken to study the long-range transport of air pollution from south and southeast Asia towards the Indian Ocean during the dry monsoon season in 1998 and 1999. The present paper discusses the temporal and spatial variations in aerosols and aerosol forcing during the winter monsoon season (January-March) for INDOEX first field phase (FFP) in 1998 and INDOEX intensive field phase (IFP) in 1999. An interactive chemistry/aerosol model (LMDZ.3.3) is used to investigate the variation in the spatial distribution of tropospheric sulphate aerosols during 1998 and 1999. The model results depict major enhancement in the sulphate aerosol concentrations, radiative forcing (RF) and optical depth over the Indian subcontinent and adjoining marine areas between INDOEX-FFP and IFP. A significant increase in transport of sulphate aerosols from the continents to the Indian Ocean region has also been simulated during the winter monsoon in 1999. The mean RF over INDOEX-FFP in 1998 is found to be –1.2 Wm -2 while it increased to –1.85 Wm -2 during INDOEX-IFP in 1999. Model results reveal a mean sulphate aerosol optical depth (AOD) of 0.08 and 0.14 over Indian subcontinent during 1998 and 1999, respectively. The model results suggest that elevated AOD downwind of source regions in India can significantly affect the regional air quality and adjoining marine environments.