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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

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

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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Yihui Ding
and
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
and
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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Gregory J. Tripoli
and
T. N. Krishnamurti

Abstract

An assimilation of satellite low-cloud vector data and conventional meteorological data is presented in this paper. The domain of the study is the GATE A-scale area. The period is the summer months of 1972. Objective analysis of the data for 93 individual days was carried out for this entire domain. One of the important climatological findings of this study is the presence of a velocity maximum in the southeast trades along the Brazilian coast. Mean speeds for three months exceed 10 m/s in this region; daily values occasionally are as large as 25 m/s. Besides showing the monthly mean motion field, we have examined in detail one-level barotropic energy exchanges and fluxes in the GATE A-scale domain. The number of conventional plus non-conventional wind observations are about 400 per day. This is more than has been used in most previous studies. Some of these results of the energetics, especially with regards to the period when Hurricane Agnes formed, are thus of considerable interest.

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Arindam Chakraborty
and
T. N. Krishnamurti

Abstract

This study addresses seasonal forecasts of rains over India using the following components: high-resolution rain gauge–based rainfall data covering the years 1987–2001, rain-rate initialization, four global atmosphere–ocean coupled models, a regional downscaling of the multimodel forecasts, and a multimodel superensemble that includes a training and a forecast phase at the high resolution over the internal India domain. The results of monthly and seasonal forecasts of rains for the member models and for the superensemble are presented here. The main findings, assessed via the use of RMS error, anomaly correlation, equitable threat score, and ranked probability skill score, are (i) high forecast skills for the downscaled superensemble-based seasonal forecasts compared to the forecasts from the direct use of large-scale model forecasts were possible; (ii) very high scores for rainfall forecasts have been noted separately for dry and wet years, for different regions over India and especially for heavier rains in excess of 15 mm day−1; and (iii) the superensemble forecast skills exceed that of the benchmark observed climatology. The availability of reliable measures of high-resolution rain gauge–based rainfall was central for this study. Overall, the proposed algorithms, added together, show very promising results for the prediction of monsoon rains on the seasonal time scale.

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