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Kamal Puri

Abstract

The impact of low-frequency baroclinic gravity modes on the tropical and extratropical circulation has been studied in a global spectral model. The procedure used was to filter out these gravity modes at every time step in a 120 day integration. Comparing the results of this integration with a control integration in which these modes were retained shows that they have a major influence on the tropical divergent circulation, which is severely damped in their absence. The precipitation rate in the filtered integration is considerably reduced and this leads to lower temperatures in the troposphere. The changes in the tropical circulation are accompanied by a considerable reduction in the amplitude of 40–50 day oscillation as indicated by the time variation of the mean sea level pressure and the upper and lower tropospheric winds at five tropical stations indicating the important role of the gravity modes in these oscillations. Finally the control and filtered integrations were repeated with a positive sea surface temperature anomaly of 4°C in the Indonesian-North Australian region. Then integrations indicate that the low frequency gravity modes resulting from convective activity in the tropics also play a strong role in tropical-midlatitude interactions.

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Kamal Puri

Abstract

The local response to tropical diabatic heating is studied by using diabatic normal mode initialization procedures. It is shown that diabatic initialization is able to produce a well-balanced state in regions of heating provided the appropriate vertical modes are initialized. However, the persistence of balance during model integrations is strongly dependent on the compatibility between the specified heating during initialization and the heating during the model integration. Finally, a procedure is developed in which cloud-top temperature data is used to identify regions of deep convection where heating rates are specified during initialization. Although the procedure provides a means of combining the analysis of the divergent wind field and initialization, a number of problems remain, namely the specification of the intensity and vertical profiles of the heating rates.

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Kamal Puri

Abstract

Experiments with insertion of height data in a free surface spectral model were conducted to explain the effectiveness of local geostrophic correction of the wind field in accelerating the assimilation of the height data by the model. It is shown that the local geostrophic correction projects most of the inserted data on to the slow manifold of the model.

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Kamal Puri

Abstract

By expanding model forecasts with and without convection in terms of three-dimensional normal mode functions it is shown that convective adjustment mainly influences gravity modes for vertical mode 4 of the nine-level ANMRC spectral model. It is also shown that the dominant influence of convective adjustment occurs in the lowest frequency gravity modes for all zonal wavenumbers and in particular wavenumbers 0 to 8. By applying various gravity mode filters it is shown that the Hadley circulation in this model is also dominated by these low-frequency gravity modes.

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Kamal Puri

Abstract

A series of twelve 4-day forecasts for the Southern Hemisphere was made, including both summer and winter cases using a hemispheric spectral model. The model had a resolution of wavenumber 31 in the horizontal and nine levels in the vertical. The model forecasts were verified in terms of various objective measures and a subjective evaluation of the synoptic charts. From this limited study it appears that the winter forecasts are significantly better than the summer forecasts. Given the present data network, analysis procedures and the parameterization of physical processes in the model used, the useful forecast period is about two days for summer and three days for winter.

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Kamal Puri

Abstract

The ability of the Bureau of Meteorology Research Centre global spectral model to simulate features of the Australian summer monsoon during January and February 1987 has been studied. Two timescales were considered: namely, the short range, defined by periods of less than 5 days, and the long range (30–40 days). It is shown that the mean circulation and precipitation simulated by the model are sensitive to the sea surface temperature. However, this sensitivity is realized only if appropriate parameterizations of boundary layer fluxes are used. The short-term predictions of monsoon onset are sensitive to the initial moisture analysis and the parameterization of cumulus convection.

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Kamal Puri

Abstract

It is shown that the tropical divergent circulation in the nine-level Geophysical Fluid Dynamics Laboratory (GFDL) spectra model is dominated by the lowest frequency gravity modes for vertical mode 4. The importance of diabatic heating during normal mode initialization is studied and results indicate that a considerable amount of divergent circulation can be generated during initialization if the true heating rates are known; in the absence of a knowledge of the heating rates it is preferable to leave the lowest-frequency gravity modes uninitialized. Model integrations with and without various physical parameterizations show that processes connected with convection exert the greatest influence on the evolution of the velocity potential field.

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Roger Daley and Kamal Puri

Abstract

Four-dimensional data assimilation is the analysis technique that has been devised to cope with the large quantities of asynoptic data received from the new remote observing systems. In this analysis method, an atmospheric simulation model is integrated in time with observed data being inserted into the model whenever it becomes available.

In the present study, the four-dimensional data assimilation process is analyzed in terms of the normal modes of the assimilating model and the slow manifold concept of Leith (1979). The problem of “data rejection” and the spurious excitation of transient gravity waves can be shown to have a simple geometrical interpretation in the slow manifold methodology. Using these ideas it is possible to define an ideal assimilation technique. Various realizable assimilation techniques which approach this ideal are proposed and tested.

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Kamal Puri and William Bourke

Abstract

Free surface and non-divergent spectral models have been integrated using varying resolutions with both analytic and meteorological initial fields. The results have been interpreted in terms of convergence of solutions. Both types of integrations show that convergent solutions are obtained over a period of a few days provided that sufficient resolution is used. Energy, enstrophy, and error distributions with planetary wavenumber also indicate crucial differences between the highest and lowest resolution integrations.

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Kamal Puri and P. Lönnberg

Abstract

The sensitivity of analysis of a well-observed tropical cyclone to high-resolution structure functions and modification of the analysis scheme to accept data in the vicinity of the cyclone is studied. It is shown that these changes in the analysis system lead to a much improved location of the cyclone. Additionally, the high-resolution structure functions also lead to an analysed structure in the vertical that is closer to the observed data and so is more realistic. However, the strong wind shears in the vertical are still not satisfactorily analysed.

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