Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: D. R. Sikka x
  • Refine by Access: All Content x
Clear All Modify Search
D. R. Sikka and Sulochana Gadgil

Abstract

An investigation is presented of the daily variation of the maximum cloud zone (MCZ) and the 7W mb trough in the Northern Hemisphere over the Indian longitudes 70–90°E during April–October for 1973–77. It is found that during June–September there are two favorable locations for a MCZ over these longitudes–on a majority of days the MCZ is present in the monsoon zone north of 15°N, and often a secondary MCZ occurs in the equatorial region (0–10°N). The monsoon MCZ gets established by northward movement of the MCZ occurring over the equatorial Indian ocean in April and May. The secondary MCZ appears intermittently, and is characterized by long spells of persistence only when the monsoon MCZ is absent. In each of the seasons studied, the MCZ temporarily disappeared from the mean summer monsoon location (15–28°N) about four weeks after it was established near the beginning of July. It is reestablished by the northward movement of the secondary MCZ, which becomes active during the absence of the monsoon MCZ, in a manner strikingly similar to that observed in the spring to summer transition. A break in monsoon conditions prevails just prior to the temporary disappearance of the monsoon MCZ. Thus we conclude that the monsoon MCZ cannot survive for longer than a month without reestablishment by the secondary MCZ. Possible underlying mechanisms are also discussed.

Full access
D. R. Sikka and K. Raghavan

Abstract

No abstract available.

Full access
Y. Ramanatnan, Padma Kulkarni, and D. R. Sikka

Abstract

The value of a meteorological parameter at a grid point can be derived from Fourier analysis of data at station observations around the grid point. But the inhomogeneity of data distribution does not warrant such an elaborate and rigorous approach. Cressman's method which is shown as a simplified version of local-point Fourier analysis is found to be more feasible from rms error statistics of wind field analyses by both methods.

Full access
Y. Ramanathan, Padma Kulkarni, and D. R. Sikka

Abstract

The statistical macrostructure of the wind field at 500 mb in the Indian region has been studied for the purpose of deriving useful information regarding scan length, scan area and weighting function for Cressman's method of objective analysis.

Full access
S. V. Singh, R. H. Kripalani, and D. R. Sikka

Abstract

The Madden-Julian oscillations are quite prominent over the Indian monsoon region and they are related with the large-scale active-break phases of the monsoon. These oscillations, however, show considerable inter-annual variability in period and intensity. In this study interannual variability of these oscillations has been studied by using daily rainfall data of 365 stations for 80 years (1901–80). It is found that the intensity of these oscillations is not related with the overall performance of the monsoon and the El Niño–Southern Oscillation phenomenon.

Full access
S. S. Singh, A. A. Kulkarni, and D. R. Sikka

Abstract

Application of a dynamic initialization scheme for balancing initial wind and pressure fields for a one-level primitive equation model in the Indian region has been investigated. For this purpose, the model equations are integrated forward and backward around the initial time following the Euler backward time-difference scheme without restoration of any variable. For comparison, the initial wind-pressure balance has also been constructed from the observed horizontal motion field by a hierarchy of models of increasing complexity, using the geostrophic relation, the linear balance equation and the nonlinear balance equation. Furthermore, the 48 h forecasts are prepared using the balanced initial data derived from the static nonlinear balance equation and the dynamic initialization scheme. The forecast results from both initialization schemes are compared and discussed. The results obtained from the dynamic initialization scheme are found to be either slightly superior or comparable to those based on the static initialization scheme.

Full access
Someshwar Das, U. C. Mohanty, Ajit Tyagi, D. R. Sikka, P. V. Joseph, L. S. Rathore, Arjumand Habib, Saraju K. Baidya, Kinzang Sonam, and Abhijit Sarkar

This article describes a unique field experiment on Severe Thunderstorm Observations and Regional Modeling (STORM) jointly undertaken by eight South Asian countries. Several pilot field experiments have been conducted so far, and the results are analyzed. The field experiments will continue through 2016.

The STORM program was originally conceived for understanding the severe thunderstorms known as nor'westers that affect West Bengal and the northeastern parts of India during the pre-monsoon season. The nor'westers cause loss of human lives and damage to properties worth millions of dollars annually. Since the neighboring South Asian countries are also affected by thunderstorms, the STORM program is expanded to cover the South Asian countries under the South Asian Association for Regional Cooperation (SAARC). It covers all the SAARC countries (Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka) in three phases. Some of the science plans (monitoring the life cycle of nor'westers/severe thunderstorms and their three-dimensional structure) designed to understand the interrelationship among dynamics, cloud microphysics, and electrical properties in the thunderstorm environment are new to severe weather research. This paper describes the general setting of the field experiment and discusses preliminary results based on the pilot field data. Typical lengths and the intensity of squall lines, the speed of movements, and cloud-top temperatures and their heights are discussed based on the pilot field data. The SAARC STORM program will complement the Severe Weather Forecast Demonstration Project (SWFDP) of the WMO. It should also generate large-scale interest for fueling research among the scientific community and broaden the perspectives of operational meteorologists and researchers.

Full access
K.-M. Lau, V. Ramanathan, G.-X. Wu, Z. Li, S. C. Tsay, C. Hsu, R. Sikka, B. Holben, D. Lu, G. Tartari, M. Chin, P. Koudelova, H. Chen, Y. Ma, J. Huang, K. Taniguchi, and R. Zhang

Aerosol- and moonsoon-related droughts and floods are two of the most serious environmental hazards confronting more than 60% of the population of the world living in the Asian monsoon countries. In recent years, thanks to improved satellite and in situ observations, and better models, great strides have been made in aerosol and monsoon research, respectively. There is now a growing body of evidence suggesting that interaction of aerosol forcing with monsoon dynamics may alter the redistribution of energy in the atmosphere and at the Earth s surface, thereby influencing monsoon water cycle and climate. In this article, the authors describe the scientific rationale and challenges for an integrated approach to study the interactions between aerosol and monsoon water cycle dynamics. A Joint Aerosol-Monsoon Experiment (JAMEX) is proposed for 2007–11, with enhanced observations of the physical and chemical properties, sources and sinks, and long-range transport of aerosols, in conjunction with meteorological and oceanographic observations in the Indo-Pacific continental and oceanic regions. JAMEX will leverage on coordination among many ongoing and planned national research programs on aerosols and monsoons in China, India, Japan, Nepal, Italy, and the United States, as well as international research programs of the World Climate Research Program (WCRP) and the World Meteorological Organization (WMO).

Full access
G. S. Bhat, S. Gadgil, P. V. Hareesh Kumar, S. R. Kalsi, P. Madhusoodanan, V. S. N. Murty, C. V. K. Prasada Rao, V. Ramesh Babu, L. V. G. Rao, R. R. Rao, M. Ravichandran, K. G. Reddy, P. Sanjeeva Rao, D. Sengupta, D. R. Sikka, J. Swain, and P. N. Vinayachandran

The first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during July–August 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented.

During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CAPE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2–3 kg−1 following convection, and recovered in a time period of 1–2 days. The surface wind speed was generally higher than 8 m s−1.

The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

Full access