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T. N. Krishnamurti
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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

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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

Abstract

This paper outlines a theory for a diagnostic balance model. A unique manner of partitioning baroclinic vertical motions into various forcing mechanisms is proposed as a natural extension of the quasi-geostrophic problem. Forcing functions include advection of vorticity and temperature by the nondivergent and the divergent part of the wind. Role of various terms of the complete vorticity and the so-called balance equations are included in the analysis. Other features of the diagnostic model are air flow over terrain, frictional contributions at the lower boundary, sensible heat transfer from water surfaces, and stable and unstable formulations of latent heat release. Typical magnitudes and physical interpretations of several nongeostrophic mechanisms are illustrated. Two applications of the above mentioned diagnostic model appear in this issue, a study of a frontal cyclone development by Krishnamurti and a study of a low latitude disturbance by Baumhefner.

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T. N. KRISHNAMURTI

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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

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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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Yihui Ding
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T. N. Krishnamurti

Abstract

The heat budget of the Siberian high is investigated by using a compositing method. Ninteen cases of strong Siberian highs that moved over China from the northwest were selected from datasets covering five winters (December through February of 1980–1984). The apparent heat source (Q 1)and the apparent moisture sink (Q 2) were estimated via budget calculations. The diabatic heating terms were also calculated via direct use of physical parameterization schemes. The vertical transports of sensible and latent heat from subgrid-scale of motions were estimated as residuals from the thermodynamic energy equation.

Over the region of the region of the Siberian high, strong radiative cooling and large-scale descending motion (with large-scale mass convergence over the upper and middle tropospheric and divergence over the lower troposphere) contributes to a rapid buildup of the Siberian high. Heating in the upper troposphere due to subgrid-scale sensible heat transfer is also an important factor in the maintenance of mass convergence in the upper troposphere through enhancement of the warm upper-tropospheric anticyclone lying over the Siberian high.

The transformation of the Siberian high commences shortly after it moves away from its source region. As the high arrives at lower latitudes, the sensible heating due to condensation in the lower troposphere enhance the transformation process of the cold high.

A comparison of the heat budgets of the Siberian high at low latitudes is carried out over the land and the warm oceanic area to the southeast.

In section 7 of this paper we present some planetary-scale signatures associated with the motion of the Siberian high. This motion is shown to be accompanied by an eastward shift of the tropical planetary-scale divergent circulation. This aspect is very similar to a shift of divergent circulation centers that one finds between non-El Niñ and El Niñ years.

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T. N. Krishnamurti
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H. S. Bedi

Abstract

The eastern and the western tropical oceans usually show a considerable zonal asymmetry in the extent and depth of deep cumulus convection. Earlier versions of a simple cumulus parameterization based on GATE observations have revealed some limitations in differentiating this type of zonal asymmetry. The aim of the proposed scheme is to provide global statistical corrections to a Kuo-type cumulus parameterization scheme and thus to optimize the moistening, heating and rainfall rates over different regions. The base data for this study are the recently analyzed global FGGE IIIb datasets. Three months of daily datasets during the global experiment were utilized in order to evaluate the coefficients of a multiple regression analysis. These multiple regression coefficients vary in space and provide different measures of a moistening parameter b and a mesoscale convergence parameter η. A clear distinction in the strength of convection is found, based on the regression parameters, between the western and the eastern oceans. This generalization of a modified Kuo-type scheme is derived for a spectral resolution of 42 waves. The impact of the aforementioned scheme is investigated in several medium range prediction experiments. Forecast comparison with a simpler version of the Kuo scheme is also carried out. Our interest in these experiments is an evaluation of precipitation forecasts, for which the proposed global cumulus parameterization is compared with other experiments that were based on GATE coefficients and with the observed measures of precipitation. The results of the global forecasts show a very marked improvement in the short range (1 to 2 day) prediction from the use of the globally varying parameterization coefficients. On the other hand, the precipitation amounts predicted from an application of the local GATE coefficients underestimate the rainfall rates over most regions.

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Masao Kanamitsu
and
T. N. Krishnamurti

Abstract

We contrast the 200 mb flow regimes during a drought year (1972) with those during a normal rainfall year (1967) over the global tropics for the northern summer months. It is shown that the deficient rainfall over central India and western Africa during 1972 may be related to the following: 1) warm sea surface temperatures over the equatorial Pacific; 2) excessive number of typhoon days over the western Pacific; 3) strong east-northeasterlies over the equatorial eastern Indian ocean (related to upper level outflows from typhoons); 4) weaker tropical easterly jet; 5) weaker meridional pressure gradient over India; 6) weaker Tibetan high; 7) a southeastward shift of the major circulation patterns as well as of several dynamical parameters; 8) weaker vertical wind shear and a weaker measure of the combined barotropic-baroclinic instability over West Africa; and 9) weaker westward steering for rain-producing disturbances over India and a consequent stronger influence of the mountains.

A sequential interrelationship of the above phenomenological aspects of the drought problem are discussed in this paper.

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D. E. Pedgley
and
T. N. Krishnamurti

Abstract

In this paper we present the results of a detailed synoptic compositing technique to determine the structure of a West African monsoon cyclone. The cyclone was in an early stage of development and was confined to the lower troposphere. Coastal weather along West Africa indicated the passage of this disturbance. This was noted from surface observations from ships of opportunity. A calculation of the meridional gradient of potential vorticity was carried out in the region of this disturbance. This calculation showed that the necessary condition for the existence of the combined barotropic-baroclinic instability was satisfied by these data. In order to go one step beyond the necessary conditions of this instability mechanism two very short range numerical prediction experiments are illustrated whose energetics confirm these results. Finally, we also present dynamic vertical motion distributions for this disturbance.

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