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T. N. KRISHNAMURTI

Abstract

The equation for steady two-dimensional mountain waves is expressed in the isentropic coordinates. An elliptic equation for the finite amplitude vertical motion field is solved by a numerical marching scheme in atmospheres with varying shear and stability.

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T. N. Krishnamurti
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T. N. Krishnamurti

Abstract

Observational evidence of a major east-west circulation during the northern summer is presented in this paper. The geometry of this circulation is portrayed in the form of the streamlines of the divergent part of the wind for the seasonal mean motion field. It is, furthermore, shown that 1) the intensity of this circulation is comparable to that of the Hadley type circulation, 2) the circulation is thermally direct, 3) there is a generation of kinetic energy by these east-west over-turnings of mass, and 4) this circulation is distinctly different from the so called Walker circulation, the latter being a southern extension of the more vigorous east-west circulation.

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T. N. Krishnamurti

Abstract

Daily upper-wind analysis at 200 mb was carried out between the equator and 45N, around the globe, for December 1955 to February 1956 to map the subtropical jet stream. It turned out that this current formed a pattern of three standing long waves which maintained nearly a steady state. This made possible an analysis of the meteorological variables in a coordinate system fixed with respect to the jet stream axis at 200 mb.

Monthly mean maps and vertical cross sections of wind, temperature, moisture and mass distribution were computed in this coordinate system. The jet-stream core is situated close to the 200-mb surface and has a maximum speed close to 70 m per sec in the mean integrated around the world; this is mainly due to very high speeds over Africa and Asia. Mean velocities close to or above 160 kn are found in the ridges of the meandering jet axis, and lower speeds are observed in the troughs. Computations of mass circulation across the jet-stream axis revealed (for all three months of winter) a thermally direct system of the order of a few knots in the sense of the classical Hadley circulation. The center of the mass-circulation cell was about 15 deg lat equatorward of the jet axis.

Mean structure of the wave is obtained by analysis of fields of temperature, wind and height of constant pressure surfaces.

Some simple calculations are made to evaluate the strength of the computed mass circulation in the heat balance and kinetic-energy balance of the atmospheric general circulation. Rossby's hypothesis of constant vorticity transport is examined.

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T. N. Krishnamurti

Abstract

This paper presents a short summary of the Summer Monsoon Experiment (MONEX). The review is largely based on those papers that have made use of the summer MONEX observations during 1979. 0bservational aspects of this study emphasize the annual march of the monsoon rainfall belt from Indonesia to the foothills of the Himalayas, from the northern winter to the northern summer season and a reverse motion thereafter. The excellent FGGE/MONEX data sets have provided a detailed definition of the divergent wind; these are summarized with reference to the Hadley and the Walker circulations.

The manner in which monsoonal circulations respond to the evolving differential heating fields are presented via the mutual interactions among the rotational and divergent wind components. Specific examples of heat sources from the studies of Luo and Yanai highlight their contrast over different regions of the monsoon including the Tibetan Plateau. A problem of considerable interest in this context is the cooling of the Arabian Sea. A summary of results pertaining to this problem—especially the distribution of the wind stress curl—is highlighted.

The planetary boundary layer is another area of investigation which has drawn much interest, especially over the western Arabian Sea where the Somali jet exhibits interesting properties during summer monsoon. These studies cover modeling, theoretical and observational areas.

The onset and active monsoons were monitored by a large array of ship and research aircraft during MONEX. Studies in this area place an emphasis on observational, theoretical stability analysis and numerical weather prediction. The major results with respect to medium range prediction of the onset of monsoon and the formation and motion of a monsoon depression are summarized in the review.

A component of the MONEX observational program that is examined is the structure and maintenance of desert heat lows. A summary of these results includes the structure of the mixed layer, the day-night differences in the vertical motion profiles and the thermodynamic heat budget.

The final section of this review includes studies on low frequency modes—especially on the time scale of 30 to 50 days. It is becoming apparent that modulations of active and inactive spells of the monsoon are related to wave motions on this time scale. These MONEX data sets provide a strong signal for monitoring these waves. These wave motions on the planetary scale move eastward; on a more regional scale they move northward over the monsoon region. Their behavior is illustrated with respect to the onset, active and break monsoons.

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T. N. Krishnamurti

Abstract

Conventional and commerical aircraft wind observations over the global tropics have been used to prepare daily charts of the field of motion at the 200-mb level for June, July and August 1967. Computations describe the climatology of the field of motion (streamlies and isotachs) and the divergence and vorticity distributions. A number of dynamical computations are made utilizing these data. These include

1) A comparison of the strength of the rotational part of the wind with the total wind.

2) A description of the strength and geometry of the Hadley and Walker circulations.

3) An analysis of the ultralong waves (zonal wavenumbers 1 and 2). The waves, which are quasi-stationary, are shown to carry a large proportion of the total variance of the motion field and to have a very pronounced southwest to northeast tilt. The amplitude and motion of transient ultralong waves, wavenumbers 1 and 2, are studied on polar diagrams. It is shown that these waves are intermittent and rapidly moving.

4) An analysis of the amplitude and motion of shorter waves (wavenumbers 7 and 8 of the zonal spectra of the meridional wind), which are intermittent, have been studied on xt diagrams. Their amplitudes and speeds of westward propagation vary respectively between 0 and 6 m sec−1 and 4° to 12° longitude day−1.

5) A computation of the energy exchange between the zonal flow and eddies. Quasi-periodic oscillations in time are shown with a period of roughly 3.5 days. The phenomenon is suggested to be a manifestation of pure barotropic type instability of the tropical upper level flows.

6) Computations of meridional fluxes of momentum and kinetic energy. These are shown to be related to the shapes of the quasi-stationery ultralong waves and possibly also to certain shapes of westward moving shorter waves over the Northern Hemisphere tropics.

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T. N. Krishnamurti

Abstract

A quasi-Lagrangian advective scheme for numerical integration of primitive equations is proposed. The advective scheme is built on a successive approximation procedure where the Fjørtoft-type of quasi-Lagrangian advection form an initial guess. The numerical program is tested by constructing some simple analytic problems containing the non-linear advective terms. It is shown that the scheme is also capable of producing very reasonable numerical forecasts for simple initial conditions for problems of mixed and filtered wave motions.

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T. N. KRISHNAMURTI

Abstract

An arbitrary isobaric surface and the level of non-divergence constitute the two levels of the proposed model. The vorticity equation consistent with the quasi-geostrophic assumptions is applied at two levels. The thermodynamic energy equation for adiabatic motion is applied to the layer between the two levels. A parabolic profile for the vertical motion is assumed. The vertical motion is eliminated between the vorticity and the thermodynamic energy equations to obtain prognostic equations. The prognostic equations are solved by a generalized graphical forecast scheme of successive approximations. Forecasts and forecast errors in a selected meteorological situation are discussed.

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T. N. KRISHNAMURTI

Abstract

In the past most diagnostic studies of frontal cyclone development have been carried out through use of quasi-geostrophic models. In this paper we present the results of vertical motions obtained from a 5-level general balance model. Nongeostrophic effects such as deformation and beta term of the balance equations, divergence, vertical advection, and the twisting term of the complete vorticity equation are retained. Advection of thermal and vorticity fields by the divergent part of the wind are also included in this analysis. Diabatic effect through release of latent heat in regions of saturated dynamic ascent, frictional effects at the lower boundary, and sensible heat transfer from the lake waters to the atmosphere are additional features. The results are presented in a partitioned form. The main results of the calculation reveal that: in the initial difluent stage of the upper trough pronounced sinking motions behind the trough are associated with a strong field of convergence in the northwesterly flow in the upper trough. This sinking motion is partitioned to arise primarily from differential vorticity advection by nondivergent part of the wind, Laplacian of thermal advection by nondivergent part of the wind, and the terrain downslope motion. The upper level development is followed by intense surface cyclogenesis during a period of approximately 36 hr. During the latter stages development is found to be associated with intense rising motion arising from differential vorticity advection by the nondivergent part of the wind, Laplacian of thermal advection by the nondivergent part of the wind, latent heat, and surface friction.

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T. N. Krishnamurti

Abstract

In large-scale tropical flows an estimate of the percentage area covered by active convective cloud elements may be made according to certain theoretical formulations involving parameterization of convective-scale motions. Such an example is Kuo's model cloud which is based on replacing the ambient tropical atmosphere with a moist adiabatic cloud element over a portion of a synoptic-scale grid network. Quantities such as friction layer mass convergence, moisture convergence, and other relevant parameters like net moisture convergence over vertical columns extending over the depth of the troposphere are determined to evaluate the percentage area occupied by convective clouds.

An application to a case study of an easterly wave below a cold low is made. Some radar pictures are presented for comparison purposes.

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