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N. Ramanathan
and
K. Srinivasan

Abstract

In many of the short-range numerical weather prediction models, an accurate specification of grid size is essential in mountainous and valley regions to adequately represent terrain forcing without resorting to subgrid-scale parameterization. Since the geomorphology of the earth is quite varied, an improper specification of grid size will neither be adequate nor realistic. In this study, the spectral method is used to determine the required optimum horizontal grid size for numerical simulations enclosing Kashmir Valley and its surroundings.

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N. Ramanathan
and
K. Srinivasan

Abstract

The airflow over the Kashmir Valley for a summer day was studied using a numerical mesoscale model. Srinagar observations were used as initial data. The surface orography, soil moisture variations, cloud cover, and vegetation effects were included in the computations. The combined effect of these factors on the development of atmospheric circulations in the valley was obtained quantitatively, and the three-dimensional model simulated results are compared with available observations. The following principal results were obtained. (a) The simulated surface temperature pattern shows a close correlation with the terrain elevations and prevailing atmospheric stabilities, (b) the intensities of katabatic and anabatic winds developed at the slopes are governed by terrain asymmetries and aspect ratio of the slopes, (c) the boundary layer depths developed at different locations in the valley are found to be nonuniform, and (d) the convergence zone formed during nighttime shows an irregular distribution.

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T. N. Krishnamurti
and
Y. Ramanathan

Abstract

This paper examines some observational aspects of the evolution of energy exchanges and differential heating during the GARP Monsoon Experiment MONEX. The main findings are that:

  1. 1) A large increase in the kinetic energy of the total flow field and the nondivergent component of the flows occurs over the Arabian Sea about a week prior to the onset of monsoon rains over central India.

  2. 2) The field of differential heating moves during the onset period to a favorable position for the generation of eddy available potential energy and its release to eddy kinetic energy.

  3. 3) The release of the eddy available potential energy goes to enhance the kinetic energy of the divergent circulations.

  4. 4) The kinetic energy of the divergent circulation does not increase much with time. This energy is shown to he transferred rapidly to the nondivergent motion via a number of interaction functions. The orientation of the divergent flows is shown to be of prime importance in these transfers during the onset, active, break and revival periods of the monsoons.

Based on the above observational findings a series of numerical prediction experiments ire conducted to examine the sensitivity of the monsoon onset to initially imposed fields of differential heating. The results of some 96 h integrations seem to confirm the large sensitivity in the evolution to the field of heating. The results of time integrations also show that the kinetic energy of the monsoon circulations increases via the rapid increase of interactions among the irrotational and nondivergent modes.

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N. Ramanathan
,
K. Srinivasan
, and
B. V. Seshasayee

Abstract

In this study, a one-and-a-half-order ē–ε closure scheme is used to study the planetary boundary layer development over a full diurnal cycle using Wangara 33d-day observations as initial conditions. The simulated results are compared with a first-order closure model and higher-order closure model results. A scheme of this kind has the advantage of taking into account the history of turbulence state in terms of a prognostic equation for turbulence kinetic energy and provides a better basis for the representation of clouds. The results of the model simulations compare favorably with other investigators’ results.

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T. N. Krishnamurti
,
Philip Ardanuy
,
Y. Ramanathan
, and
Richard Pasch

Abstract

In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-level flow which is an update on the well-known Findlater analysis.

A number of calculations show that the horizontal shear of the monsoon current provides substantial energy during the evolution of the onset vortex. The flow satisfies the necessary condition for the existence of instability. Corresponding stability diagrams exhibit substantial growth rates around the period of the formation of the onset vortex. The scale of maximum growth rate is closely in correspondence with the scale of the onset vortex. Finally, the conversion from zonal to eddy kinetic energy is demonstrated via a simple prediction experiment with the conservation of absolute vorticity as a constraint. A reasonable simulation of the onset vortex is shown in this experiment.

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T. N. Krishnamurti
,
Y. Ramanathan
,
Hua-Lu Pan
,
Richard J. Pasch
, and
John Molinari

Abstract

Modeling of convective rainfall rates is a central problem in tropical meteorology. Toward numerical weather prediction efforts the semi-prognostic approach (i.e., a one time-step prediction of rainfall rates) provides a relevant test of cumulus parameterization methods. In this paper we compare five currently available cumulus parameterization schemes using the semi-prognostic approach. The calculated rainfall rates are compared with observed estimates provided in the recent publication of Hudlow and Patterson (1979). Among these the scheme proposed by Kuo (1974) provides the least root-mean-square error between the calculated and the observed estimates, slightly better than that of Arakawa and Schubert (1974), which was used by Lord (1978a). The simplicity of the approach holds promise for numerical weather prediction. Unlike some of the other schemes this method is not sensitive to and does not require computation of internal parameters such as profiles of cloud mass flux updrafts and downdrafts, detrainment of cloud matter and entrainment of environmental air. The present paper does not address the prognostic evolution and verification of the vertical distribution of temperature, humidity or momentum. These will be compared for the different methods in more detail separately.

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N. V. Koldunov
,
P. Kumar
,
R. Rasmussen
,
AL. Ramanathan
,
A. Nesje
,
M. Engelhardt
,
M. Tewari
,
A. Haensler
, and
D. Jacob
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