Kamal Puri

Spectral Representation of Three-Dimensional Global Data by Expansion in Normal Mode Functions

Abstract To represent atmospheric data spectrally in three indices (zonal wavenumber, and meridional and vertical modal indices), we propose to use three-dimensional normal mode functions (NMFs) to express the wind and mass fields simultaneously. The NMFs are constructed from the eigensolutions of a global primitive equation model and they are orthogonal functions. The vertical parts are obtained from the solutions of the vertical structure equation with the equivalent height as the eigenvalue. The vertical modal index is associated with a different value of the equivalent height. The horizontal parts of NMFs are Hough harmonics with zonal wavenumber and meridional modal index as two-dimensional scalings. The expansion of global data in terms of NMFs permits the partition of energy into two distinct kinds of motions-gravity-inertia modes and rotational modes of Rossby/Haurwitz type. Both kinds of motion are also partitioned into different vertical modes. Results of the spectral distribution of atmosph...

A Global Spectral Model for Simulation of the General Circulation

Abstract A global general circulation for mean January conditions has been conducted with a nine-level, wavenumber 15 (rhomboidal) spectral model. A semi-implicit algorithm has been used in the time integration, thereby enhancing computational economy. The simulation reproduces many qualitative aspects of the observed January climatology confirming this type of model as an attractive alternative to models using finite-difference formulations.

The Use of Satellite Data in the Specification of Convective Heating for Diabatic Initialization and Moisture Adjustment in Numerical Weather Prediction Models

Abstract Although diabatic processes play an important role in the tropical circulation, current analysis schemes and numerical weather prediction models are unable to adequately include diabatic heating information. In this paper, procedures for using outgoing longwave radiation data as proxy data for diabatic heating rates and to initialize the moisture field are proposed. The feasibility of using this data in the ECMWF data assimilation system is demonstrated, and it is further shown that using appropriate diabatic heating together with moisture initialization has the potential to substantially reduce the spinup time in numerical weather prediction models.

Sensitivity of ECMWF analyses-forecasts of tropical cyclones to cumulus parameterization

Abstract The sensitivity of the ECMWF analysis-forecast system for four tropical cyclones during the period spanned by the Australian Monsoon Experiment to cumulus parameterization and model resolution is presented. Two parameterization schemes were compared: namely, the Kuo cumulus parameterization and the Betts-Miller adjustment scheme. Both analyses and forecasts show considerable sensitivity with the Betts-Miller scheme generating more intense cyclonic systems as indicated by maps of sea-level pressure, low-level winds, vorticities, and cross sections in the neighborhood of the cyclones. The Betts-Miller scheme also results in better consistency of the fields in the vertical between the divergent circulation and vertical velocity. Of the two cases considered, one (for tropical cyclone Jason) showed a marked sensitivity to increased model resolution with the higher resolution forecast showing considerable improvement.

Four-Dimensional, Data Assimilation and the Slow Manifold

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.

The Simulation of Stationary and Transient Geopotential-Height Eddies in January and July with a Spectral General Circulation Model

Abstract We examine the characteristics of stationary and transient eddies in the geopotential-height field as simulated by a spectral general circulation model. The model possesses a realistic distribution of continents and oceans and realistic, but smoothed, topography. Two simulations with perpetual January and July forcing by climatological sea surface temperatures, sea ice, and insulation were extended to 1200 days, of which the final 600 days were used for the results in this study. We find that the stationary waves are well simulated in both seasons in the Northern Hemisphere, where strong forcing by orography and land-sea thermal contrasts exists. However, in the Southern Hemisphere, where no continents are present in midlatitudes, the stationary waves have smaller amplitude than that observed in both seasons. In both hemispheres, the transient eddies are well simulated in the winter season but are too weak in the summer season. The model fails to generate a sufficiently intense summertime midlati...

The Relationship Between Convective Adjustment Hadley Circulation and Normal Modes of the ANMRC Spectral Model

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.

Some experiments on the use of tropical diabatic heating information for initial state specification

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.

Implications of Horizontal Resolution in Spectral Model Integrations

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.

Sensitivity of Low-Latitude Velocity Potential Field in a Numerical Weather Prediction Model to Initial Conditions, Initialization and Physical Processes

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.