S. S. V. S. Ramakrishna

On the decreasing trend of the number of monsoon depressions in the Bay of Bengal

This study unravels the physical link between the weakening of the monsoon circulation and the decreasing trend in the frequency of monsoon depressions over the Bay of Bengal. Based on the analysis of the terms of Genesis Potential Index, an empirical index to quantify the relative contribution of large scale environmental variables responsible for the modulation of storms, it is shown here that the reduction in the mid-tropospheric relative humidity is the most important reason for the decrease in the number of monsoon depressions. The net reduction of relative humidity over the Bay of Bengal is primarily due to the decrease in the moisture flux convergence, which is attributed to the weakening of the low level jet, a characteristic feature of monsoon circulation. Further, the anomalous moisture convergence over the western equatorial Indian Ocean associated with the rapid warming of the sea surface, reduces the moisture advection into the Bay of Bengal and hence adversely affect the genesis/intensification of monsoon depressions. Hence, the reduction in the number of monsoon depression over the Bay of Bengal could be one of the manifestations of the differential rates in the observed warming trend of the Indian Ocean basin

Impact of Variational Data Assimilation for Simulating Tropical Cyclones over Bay of Bengal Using WRF-ARW

Tropical cyclones, one of the most destructive of all the natural disasters, are capable of causing loss of life and extensive damage to property. The Bay of Bengal is a potentially energetic region for the development of cyclonic storms and about 7% of the global annual tropical storms form over this region with two cyclone seasons in a year. Tropical cyclones have great socio-economic concern for the Indian subcontinent. Precise forecasting of tropical cyclone intensity and track are important for the countries bordering the Bay of Bengal, especially India, Bangladesh and Myanmar due to significant socio-economic impact. There has been remarkable improvement in forecasting of the tropical cyclones with the development of high resolution atmospheric models and the global forecasting systems such as the National Centers for Environmental Predictions (NCEP) Global Forecasting System (GFS). Assimilation of available observations has been considered to be very important for accurate description of initial conditions in numerical models (Park and Zupanski, 2003; Navon, 2009; Pu et al., 2009). In particular, assimilation methods like variational approach has the additional advantage of assimilating observations by satisfying model dynamic and thermodynamic constraints through a set of independent balance equations (in 3DVAR) (Courtier et al., 1998).

Moisture Budget of the Tropical Cyclones Formed over the Bay of Bengal: Role of Soil Moisture After Landfall

In the present study, the water budget of the Bay of Bengal tropical cyclones at varying intensities is analyzed. Results show that rainfall is not directly related to the intensities of tropical cyclones. A secondary peak in precipitation after landfall causes huge damage through floods and mud slides. The analysis of the water budget shows that the moisture flux convergence was the dominant term before landfall and contributes to 61% of the rainfall, while the remaining 39% is contributed by evaporation. After landfall, evaporation contributed 63% of the rainfall and 37% of rainfall was contributed by moisture flux convergence. The contribution of evaporation changed little with time in all the 12 case studies. Out of the 12 cyclones of varying intensities, seven cyclones either showed a secondary peak in precipitation or maintained a high rainfall over land. For the high rainfall over land, after the landfall, soil moisture was found to be important both in the observation and simulations of the Weather Research and Forecasting model. The predicted cyclone track errors are reduced in the model experiment with soil moisture, while the predicted cyclone intensity errors are less in the experiment without soil moisture. Accurate soil-moisture data are required for better prediction of cyclone track and their intensities.

On the relationship between the Indian summer monsoon rainfall and the EQUINOO in the CFSv2

Several recent studies have shown that positive (negative) phase of Equatorial Indian Ocean Oscillation (EQUINOO) is favourable (unfavourable) to the Indian summer monsoon. However, many ocean–atmosphere global coupled models, including the state-of-the-art Climate Forecast System (CFS) version 2 have difficulty in reproducing this link realistically. In this study, we analyze the retrospective forecasts by the CFS model for the period 1982–2010 with an objective to identify the reasons behind the failure of the model to simulate the observed links between Indian summer monsoon and EQUINOO. It is found that, in the model hindcasts, the rainfall in the core monsoon region was mainly due to westward propagating synoptic scale systems, that originated from the vicinity of the tropical convergence zone (TCZ). Our analysis shows that unlike in observations, in the CFS, majority of positive (negative) EQUINOO events are associated with El Niño (La Niña) events in the Pacific. In addition to this, there is a strong link between EQUINOO and Indian Ocean Dipole (IOD) in the model. We show that, during the negative phase of EQUINOO/IOD, northward propagating TCZs remained stationary over the Bay of Bengal for longer period compared to the positive phase of EQUINOO/IOD. As a result, compared to the positive phase of EQUINOO/IOD, during a negative phase of EQUINOO/IOD, more westward propagating synoptic scale systems originated from the vicinity of TCZ and moved on to the core monsoon region, which resulted in higher rainfall over this region in the CFS. We further show that frequent, though short-lived, westward propagating systems, generated near the vicinity of TCZ over the Bay moved onto the mainland were responsible for less number of break monsoon spells during the negative phase of EQUINOO/IOD in the model hindcasts. This study underlines the necessity for improving the skill of the coupled models, particularly CFS model, to simulate the links between EQUINOO/IOD and the Indian summer monsoon for reliable predictions of seasonal and intraseasonal variation of Indian summer monsoon rainfall.

Simulation of Pre-monsoon Cyclones of Two Contrasting Monsoon Years Using Mesoscale Model WRF (ARW)

Tropical cyclones are known for their devastation in tropical regions over the whole globe. The devastation is mainly due to high winds, intense rains and the associated storm surge. The life cycle of a tropical cyclone is dependent on a number of environmental factors (Gray, 1968) which are frequently present in the Inter Tropical Convergence Zone (ITCZ). These include a warm ocean surface (above 26° C) and several physical parameters contributing to a deep humid and unstable atmosphere. It is a well established fact that SST>26.6° C is a prerequisite for tropical cyclone formation in the Bay of Bengal. The formation process begins in an area of low pressure coinciding with vigorous convective cloud in the tropics between 5° and 22° latitude. Usually the cloud cluster drifts slowly towards the west as the convection increases and winds begin spiralling in towards the centre of the system. There are two cyclonic seasons in the North Indian Ocean (NIO), pre-monsoon (April and May) and post-monsoon (October and November). Tropical cyclones form in the NIO during pre-monsoon season of which most of these cyclones develop in the Bay of Bengal and hit the Bangladesh or Myanmar coast. Cyclonic disturbances that develop during this season have a high probability of reaching a severe cyclonic stage (Singh et al., 2000).