S. S. Singh

Numerical simulation of a super cyclonic storm, Orissa 1999: impact of initial conditions

Numerical simulations are performed using the Penn State University/ National Center for Atmospheric Research Mesoscale Model (MM5) to study the impact of initial conditions on the super cyclone which hit the coast of Orissa in 1999. Because analysis of the cyclones circulation was inadequate in the initial fields owing to the coarse resolution of the operational analysis systems and sparse oceanic data coverage, synthetic vortex data were generated using empirical relations and used in the analysis. Four-dimensional data assimilation is performed in order to assimilate the synthetic vortex in the initial stage to the model. Considerable improvement in the track is obtained by using the synthetic vortex. With better specification of the initial vortex structure, the model successfully simulated the typical tropical cyclone characteristics, such as asymmetries in the wind field: the strongest winds occurred in the east and close to the cyclones centre, strong wind gradients were found between the centre and the maximum wind region, and there was a slow decrease in wind speed up to the middle troposphere. Despite failing to produce the intense pressure drop observed for this cyclone, the model shows much better cyclone development with enhanced initial condition than the analysis. Copyright

Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.

Impact of modified physics in limited area model forecasts

A number of physical factors have been introduced to improve limited area model forecasts. The factors include land surface fluxes, shallow convection and radiation. The model including these additional physical factors (modified physics) is run for five cases of monsoon depression which made landfall over the Indian coast, and the results are compared with those of the control run. The forecasts are verified by computing the root mean square and mean errors. The differences in these skill scores between the two model runs are tested for their statistical significance. It is found that the modified physics has a statistically significant effect on the model skill with the maximum impact on the mean sea level pressure and the temperature.Detailed analyses of mean sea level pressure, wind, rainfall and temperature further confirm that the modified physics has maximum impact on mean sea level pressure and temperature and marginal impact on wind and rainfall. Furthermore, analyses of some model parameters related to physics at a grid point for one case of depression were done. The results show that the inclusion of the land surface physics, shallow convection and radiative processes have produced a better precipitation forecast over the grid point.

Upper and lower tropospheric energetics of standing and transient eddies in wave number domain during summer monsoon of 1991

Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi.The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere.The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2.It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.

Momentum transport of wave zero during March: A possible predictor for the Indian summer monsoon

Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.

Intra-seasonal variations of kinetic energy of lower tropospheric zonal waves during northern summer monsoon

Space spectral analysis of zonal (u) and meridional (v) components of wind and time spectral analysis of kinetic energy of zonal waves at 850 hPa during monsoon 1991 (1st June 1991 to 31st August 1991) for the global belt between equator and 40°N are investigated. Space spectral analysis shows that long waves (wavenumbers 1 and 2) dominate the energetics of Region 1 (equator to 20°N) while over Region 2 (20°N to 40°N) the kinetic energy of short waves (wavenumbers 3 to 10) is more than kinetic energy of long waves. It has been found that kinetic energy of long waves is dominated by zonal component while both (zonal and meridional) the components of wind have almost equal contribution in the kinetic energy of short waves.Temporal variations of kinetic energy of wavenumber 2 over Region 1 and Region 2 are almost identical. The correlation matrix of different time series shows that (i) wavenumber 2 over Regions 1 and 2 might have the same energy source and (ii) there is a possibility of an exchange of kinetic energy between wavenumber 1 over Region 1 and short waves over Region 2. Wave to wave interactions indicate that short waves over Region 2 are the common source of kinetic energy to wavenumber 2 over Regions 1 and 2 and wavenumber 1 over Region 1. Time spectral analysis of kinetic energy of zonal waves indicates that wavenumber 1 is dominated by 30–45 day and bi-weekly oscillations while short waves are dominated by weekly and bi-weekly oscillations.The correlation matrix, wave to wave interaction and time spectral analysis together suggest that short period oscillations of kinetic energy of wavenumber 1 might be one of the factors causing dominant weekly (5–9 day) and bi-weekly (10–18 day) oscillations in the kinetic energy of short waves.

Cumulus convection and lateral boundary conditions in a limited area model

Three versions of Kuos cumulus parameterization have been tested in a limited area model to investigate their comparative performances. Results show that the version of Anthes produces better forecasts than those produced by other versions. To identify a suitable scheme of lateral boundary conditions for the limited area model, impact of two time-invariant and two time-dependent boundary conditions have been examined. The forecasts suggest that the time-dependent tendency modification scheme, based on large-scale tendencies obtained from observed data, is a better boundary scheme for the model. Furthermore, the forecast produced with the revised version of the model incorporating improved versions of Kuos scheme and lateral boundary conditions shows an overall improvement.

Short range prediction with a multi-level primitive equation model

A five-level primitive equation model in a (x, y, p, t) coordinate system has been developed. A fairly sophisticated scheme of physical processes has been incorporated in the model. The model physics include air-sea interaction, cumulus parametrization, large scale condensation, dry convective adjustment, horizontal and vertical diffusion and simulated radiation. The initial balance between mass and motion fields has been obtained through a dynamic initialization scheme. The model has been integrated upto 48 hr using input data of a case of monsoon depression. The results of initialization and forecast have been presented and discussed. Wind, temperature and vertical velocity fields have been found to retain the observed map features; after the initialization, however, the surface pressure has been considerably modified. The model produced a reasonably good forecast up to 24 hr as far as the flow fields, rainfall region, structure of the depression and the movement of cyclonic circulation were concerned and beyond that damped rapidly. The rainfall rates were underestimated. Some of the shortcomings of the model are also discussed.

Application of semi-implicit scheme of integration to barotropic prediction

Application of a semi-implicit version of primitive equation barotropic model to predict monsoon depressions is proposed. Forecasts upto 48 hr have been quite satisfactory. The results are compared with those obtained from an explicit version of primitive equation barotropic model developed earlier. The flow patterns from these two versions are similar with few exceptions in isolated pockets, however, the forecast movement obtained from the semi-implicit version is comparable or slightly superior to those obtained from the explicit version. The computational time in semi-implicit and explicit schemes is approximately in the ratio of 1∶2.