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S. S. Vaidya and S. S. Singh

Abstract

The sensitivity of the prediction of a monsoon depression to the adjustment parameters in a Betts–Miller scheme of deep convection is examined and an optimum parameter set is identified for the monsoon depression. For this purpose, a number of experiments have been carried out with a limited area model by assigning different values to the adjustment parameters, namely, the saturation pressure departure, the stability weight, and the adjustment time period. When one parameter is varied, the other two are kept constant. Results indicate that the depression track is sensitive to all three adjustment parameters. The upper-tropospheric temperature is sensitive to the stability weight and the rainfall rates are sensitive to the saturation pressure departure values. The rainfall shows minor sensitivity to the stability weight and the adjustment time period. A set of adjustment parameters that produced the best forecasts is taken as the optimum parameter set for the monsoon depression.

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S. S. Vaidya and S. S. Singh

Abstract

The performance of the Betts–Miller–Janjic scheme of convection has been investigated for prediction of the Indian monsoons. For this purpose a limited area numerical weather prediction model with two schemes of convection, one with the Betts–Miller scheme and other with the Betts–Miller–Janjic scheme, is run for five cases of monsoon depression that made landfall over the Indian coast. The results from the two schemes are compared.

Detailed analyses of mean sea level pressure, wind, and rainfall have shown that the Betts–Miller–Janjic scheme has considerably improved the rainfall prediction over the Indian landmass and improvement is also seen in the mean sea level pressure fields and cyclonic circulation associated with the depression at the 850-hPa level. The forecast results are further verified by computing the root-mean-square errors, and the difference in the skill scores between the two model runs are tested for their statistical significance. It is found that the Betts–Miller–Janjic scheme has a statistically significant effect on the model skill beyond 24 h, with maximum impact on mean sea level pressure and geopotential height.

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Martin S. Singh

Abstract

The role of planetary rotation in limiting the extent of the cross-equatorial solsticial Hadley cell (SHC) is investigated using idealized simulations with an aquaplanet general circulation model run under perpetual-solstice conditions. Consistent with previous studies that include a seasonal cycle, the SHC extent increases with decreasing rotation rate, and it occupies the entire globe for sufficiently low planetary rotation rates. A simple theory for the summer-hemisphere extent of the SHC is constructed in which it is assumed that the SHC occupies regions for which a hypothetical radiative–convective equilibrium state is physically unattainable. The theory predicts that the SHC extends farther into the summer hemisphere as the rotation rate is decreased, qualitatively reproducing the behavior of the simulations, but it generally underestimates the simulated SHC extent. A diagnostic theory for the summer-hemisphere SHC extent is then developed based on the assumptions of slantwise convective neutrality and conservation of angular momentum within the Hadley cell. The theory relates the structure of the SHC in the summer hemisphere to the distribution of boundary layer entropy in the dynamically equilibrated simulations. The resultant diagnostic for the SHC extent generalizes the convective quasi-equilibrium-based constraint of Privé and Plumb, in which the position of rain belts is related to maxima in the low-level entropy distribution.

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Martin S. Singh and Zhiming Kuang

Abstract

The influence of eddy momentum fluxes on the equinoctial Hadley circulation is explored using idealized simulations on an equatorial beta plane in which the sea surface temperature (SST) distribution is fixed. By comparing simulations run in a wide-domain configuration, in which large-scale eddies are present, to simulations in which the model domain is too narrow to permit baroclinic instability, the role of large-scale eddies in determining the characteristics of the Hadley circulation is elucidated. The simulations also include an explicit representation of deep convection, allowing for an evaluation of the influence of convective momentum transport on the zonal-mean circulation.

The simulated eddy momentum fluxes are much weaker in the narrow-domain configuration than in the wide-domain case, and convective momentum transport is found to be of secondary importance. As a result, many characteristics of the narrow-domain Hadley circulation are well described by axisymmetric theory and differ from those of the wide-domain case. Nevertheless, the strength of the Hadley circulation is similar irrespective of the domain width. The sensitivity of this result to the strength of the eddy forcing is investigated using narrow-domain simulations forced by artificial sinks of zonal momentum. As the magnitude of the momentum sink increases, the Hadley circulation strengthens, but the increase is relatively modest except at very strong forcing magnitudes. The results suggest that the fixed-SST boundary condition places a strong thermodynamic constraint on the Hadley circulation strength and that one should consider the energy budget as well as the angular momentum budget in order to fully understand the influence of large-scale eddies on the zonal-mean circulation in the tropics.

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S. V. Singh and R. H. Kripalani

Abstract

The potential predictability of the lower tropospheric circulation and the rainfall over India during the peak summer monsoon season (July–August) is studied by analyzing the signal-to-noise ratio. Daily 700-mb heights, mean sea level pressure anomaly and rainfall at 220 stations for 21, 30 and 19 years, respectively, are used to represent the circulation and rainfall fields. The predictability of the circulation fields in general increases with decreasing latitude but is low over the area normally occupied by the monsoon trough. The potential predictability of rainfall is about 50% over the major parts of the country.

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S. V. Singh and R. H. Kripalani

Abstract

No abstract available.

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Ashok Kumar, Parvinder Maini, and S. V. Singh

Abstract

An operational system for forecasting probability of precipitation (PoP) and yes/no forecast over 10 stations during monsoon season is developed. A perfect prog method (PPM) approach is followed for statistical interpretation of numerical weather prediction products. PPM model equations are developed by using analysis data obtained from the European Centre for Medium-Range Weather Forecasts for a period of 6 yr (1985–90). PoP forecasts are obtained from these equations by using global T-80 model output, which was installed at the National Centre for Medium Range Weather Forecasting in 1993. Results of verification study conducted during the monsoon season of 1995 covering various aspects of forecast skill and quality are also described.

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K. D. Prasad and S. V. Singh

Abstract

Three selected parameters have been analyzed for the spatial and temporal relationships with the Indian monsoon rainfall. These parameters are (i) the subtropical ridge position at 500 hPa over India in April, (ii) January–April Darwin surface pressure tendency, and (iii) January and February Northern Hemispheric surface air temperature. Multiple regression equations have been developed for forecasting monsoon rainfall on bimonthly to seasonal scales and on subdivisional to all-India scales. All equations have been verified by independent data.

We obtain positive skill in forecasting the seasonal rainfall of not only all of India but also of its three large subregions and meteorological subdivisions lying in west-central parts of the country. Also, the skill is generally better for the forecast of rainfall for the latter half of the monsoon season than the whole season.

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S. V. Singh and R. H. Kripalani

Abstract

Extended empirical orthogonal function (EEOF) analysis has been employed to study linear relationships among the mean sea level pressure, 700 mb height and rainfall over India, and their low-frequency sequential evolution during the peak summer monsoon months. The interrelationships between these fields are strongest over central India and, while the rainfall activity is colocated with the corresponding changes in the 700 mb heights, it is displaced southward with respect to the pressure changes. The first two EEOF's of all the three fields (averaged over 5 or 7 days) show that the dominant low-frequency sequential evolution is associated with north and northeastward movement of the anomaly centers with a recurrence period of about 40 days. In addition, the presence of a westward moving wave in sea level pressure anomalies located roughly near 15°N latitude is revealed by the third EEOF.

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Martin S. Singh and Paul A. O’Gorman

Abstract

Many features of the general circulation of the atmosphere shift upward in response to warming in simulations of climate change with both general circulation models (GCMs) and cloud-system-resolving models. The importance of the upward shift is well known, but its physical basis and the extent to which it occurs coherently across variables are not well understood. A transformation is derived here that shows how an upward shift of a solution to the moist primitive equations gives a new approximate solution with higher tropospheric temperatures. According to the transformation, all variables shift upward with warming but with an additional modification to the temperature and a general weakening of the pressure velocity. The applicability of the vertical-shift transformation is explored using a hierarchy of models from adiabatic parcel ascents to comprehensive GCMs. The transformation is found to capture many features of the response to climate change in simulations with an idealized GCM, including the mid- and upper-tropospheric changes in lapse rate, relative humidity, and meridional wind. The transformation is less accurate when applied to simulations with more realistic GCMs, but it nonetheless captures some important features. Deviations from the simulated response are primarily due to the surface boundary conditions, which do not necessarily conform to the transformation, especially in the case of the zonal winds. The results allow for a physical interpretation of the upward shift in terms of the governing equations and suggest that it may be thought of as a coherent response of the general circulation of the mid- and upper troposphere.

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