B. Prasad Kumar

Tsunami travel time prediction using neural networks

The present work reports the development of a nonlinear technique based on artificial neural network (ANN) for prediction of tsunami travel time in the Indian Ocean. The expected times of arrival (ETA) computation involved 250 representative coastal stations encompassing 35 countries. A travel time model is developed using ANN approach. The ANN model uses non-linear regression where a Multi-layer Perceptron (MLP) is used to tackle the non-linearity in the computed ETA. The back-propagation feed forward type network is used for training the system using the resilient back-propagation algorithm. The model demonstrates a high degree of correlation, proving its robustness in development of a real-time tsunami warning system for Indian Ocean.

Intensity of tropical cyclones during pre- and post-monsoon seasons in relation to accumulated tropical cyclone heat potential over Bay of Bengal

The aim of the present study is to understand the impact of oceanic heat potential in relation to the intensity of tropical cyclones (TC) in the Bay of Bengal during the pre-monsoon (April–May) and post-monsoon (October–November) cyclones for the period 2006–2010. To accomplish this, the two-layer gravity model (TLGM) is employed to estimate daily tropical cyclone heat potential (TCHP) utilizing satellite altimeter data, satellite sea surface temperature (SST), and a high-resolution comprehensive ocean atlas developed for Indian Ocean, subsequently validated with in situ ARGO profiles. Accumulated TCHP (ATCHP) is estimated from genesis to the maximum intensity of cyclone in terms of minimum central pressure along their track of all the cyclones for the study period using TLGM generated TCHP and six-hourly National Centre for Environmental Prediction Climate Forecast System Reanalysis data. Similarly, accumulated sea surface heat content (ASSHC) is estimated using satellite SST. In this study, the relationship between ATCHP and ASSHC with the central pressure (CP) which is a function of TC intensity is developed. Results reveal a distinct relationship between ATCHP and CP during both the seasons. Interestingly, it is seen that requirement of higher ATCHP during pre-monsoon cyclones is required to attain higher intensity compared to post-monsoon cyclones. It is mainly attributed to the presence of thick barrier layer (BL) resulting in higher enthalpy fluxes during post-monsoon period, where as such BL is non-existent during pre-monsoon period.

Response of upper ocean during passage of MALA cyclone utilizing ARGO data

In the present study an attempt has been made to study the response of the upper ocean atmospheric interactions during the passage of a very severe cyclonic storm (VSCS) ‘MALA’ formed over the Bay of Bengal (BoB) on 24 April 2006. Deepening of mixed layer depth (MLD), weakening of barrier layer thickness (BLT) associated with a deeper 26 °C isotherm level (D26) is observed after the MALA passage. Tropical cyclone heat potential (TCHP) and depth averaged temperature (T100) exhibit a good degree of correlation for higher values. The passage of MALA cyclone also resulted in cooling the sea surface temperature (SST) by 4–5 °C. The findings suggest that turbulent and diapycnal mixing are responsible for cooler SSTs. Turbulent air–sea fluxes are analyzed using Objectively Analyzed air–sea Fluxes (OAFlux) daily products. During the mature stage of MALA higher latent heat flux (LHF), sensible heat flux (SHF), and enthalpy (LHF + SHF) are observed in the right side of this extreme event.

Numerical Simulation of Typhoon Wind Forcing in the Korean Seas Using a Spectral Wave Model

Abstract This study investigates the application of the wave model (WAM) to simulate the generation and propagation of typhoon waves in Korean seas. The model solves the energy balance equation for wave growth based on wind energy input, and simulates spatial and time evolution of wave spectra. Although WAM has been extensively validated and used in various global and regional wave forecasts, its application to the simulation of typhoon waves has not been investigated thoroughly. Crucial to the application of WAM in typhoon wave modeling is the specification of accurate wind input data and adequate resolution of the wind structure. This study compares and analyzes two different wind fields for the same event, viz., simulated wind field from a storm model and blended global QSCAT/NCEP winds. The simulation experiment was run for 4 days as Typhoon Olga (1999) approached the west coast of Korea. The results are compared with shallow water buoy observations as the typhoon was approaching landfall. The simulated significant wave heights in the open ocean were approximately 8.6 m, which gradually decreased as the typhoon approached shore. Olga was an intense typhoon, and its compact wind structure provides a unique test case to examine the required directional, frequency, and spatial resolution in WAM for modeling typhoon generated waves.

Response of upper ocean and impact of barrier layer on Sidr cyclone induced sea surface cooling

In the present study an attempt has been made to investigate the impact of salinity stratification on the SST during the tropical cyclone (TC) passage. In this context, a severe post monsoon cyclone, Sidr, (Category 4) that developed over the south-eastern Bay of Bengal (BoB) during 11–16 November, 2007 was chosen as a case study. Pre-existence of a thick barrier layer (BL), temperature inversions and a higher effective oceanic layer for cyclogenesis (EOLC) were noticed along the path of the Sidr cyclone. The analysis of available Argo floats along the Sidr cyclone track also revealed less cooling during as well as after its passage as was reported from satellite derived SST. The role of BL on Sidr induced sea surface cooling was investigated using a diagnostic mixed layer model. Model results also depict the reduced sea surface cooling during the passage of Sidr. This is attributed to the presence of BL which results in the inhibition of the entrainment of cool thermocline water into the shallow mixed layer. Climatological as well as in situ observations of tropical cyclone heat potential (TCHP) and EOLC shows that the Sidr cyclone propagated towards the regions of higher EOLC.

TSUNAMI TRAVEL TIME COMPUTATION AND SKILL ASSESSMENT FOR THE 26 DECEMBER 2004 EVENT IN THE INDIAN OCEAN

Tsunami waves are considered the most dangerous natural hazard affecting the population of the world living near the coastal belts. With the increasing intensity of economic exploitation of coasts there is also an increase in socio-economic consequences resulting from the hazardous action of tsunami waves generated from submarine seismic activity and other causes. On 26 December 2004, the countries within the vicinity of East Indian Ocean experienced the most devastating tsunami in recorded history. This tsunami was triggered by an earthquake of magnitude 9.0 on the Richter scale at 3.4°N, 95.7°E off the coast of Sumatra in the Indonesian Archipelago at 06:29 hrs IST (00:59 hrs GMT). As of now (September, 2005), the only Tsunami Warning System (TWS) that is in existence is the one for the Pacific Ocean, which began in the late 1940s. Following the recent disastrous tsunami of 26 December 2004 in the Indian Ocean, the nations around the Indian Ocean rim are now working together to establish a tsunami warning system which should become operational in the near future. One of the most basic information that an Indian Ocean tsunami warning center should have at its disposal, is information on tsunami travel times to various coastal locations surrounding the Indian Ocean rim, as well to several island locations. Devoid of this information, no ETAs (expected times of arrival) can be included in the real-time tsunami warnings. The importance of ETA for tsunami warning system motivated the computation of arrival times comprising 250 representative coastal locations from 35 countries, showing the feasibility of developing a TWS in a relatively short time-span. Numerical accuracy in computating arrival times for this energetic event has been verified from in situ tide gauge data and satellite track data from Jason-I and Topex/Poseidon in the Indian Ocean and also from coastal stations off South Africa. The expected outcome of this work is to develop a widely distributed tsunami travel time (TTT) atlas which can serve as a valuable information database to reduce warning time in the event of tsunamis in the Indian Ocean and promote awareness among the population dwelling in the littoral belts of the South-Asian countries.

Parameterization of rain induced surface roughness and its validation study using a third generation wave model

The effect of raindrops striking water surface and their role in modifying the prevailing sea-surface roughness is investigated. The work presents a new theoretical formulation developed to study rain-induced stress on sea-surface based on dimensional analysis. Rain parameters include drop size, rain intensity and rain duration. The influences of these rain parameters on young and mature waves were studied separately under varying wind speeds, rain intensity and rain duration. Contrary to popular belief that rain only attenuates surface waves, this study also points out rain duration under certain condition can contribute to wave growth at high wind speeds. Strong winds in conjunction with high rain intensity enhance the horizontal stress component on the sea-surface, leading to wave growth. Previous studies based on laboratory experiments and dimensional analysis do not account for rain duration when attempting to parameterize sea-surface roughness. This study signifies the importance of rain duration as an important parameter modifying sea-surface roughness. Qualitative as well quantitative support for the developed formulation is established through critical validation with reports of several researchers and satellite measurements for an extreme cyclonic event in the Indian Ocean. Based on skill assessment, it is suggested that the present formulation is superior to prior studies. Numerical experiments and validation performed by incorporating in state-of-art WAM wave model show the importance of treating rain-induced surface roughness as an essential pre-requisite for ocean wave modeling studies.

Bottom Boundary Layer Characteristics in the Hooghly Estuary Under Combined Wave-Current Action

The development of a theoretical model for estimating bottom boundary layer characteristics in the Hooghly estuary, located in the east coast of India, under combined effects of waves and currents is reported. Three numerical models, viz a depth averaged hydrodynamic model, SWAN wave model, and bottom boundary layer model, were integrated. In the bottom boundary layer parameters, maximum bottom stress, effective friction factor, and near-bed velocity both during ebb and flood phases of the tidal forcing are investigated and validated for the Haldia channel. The close match seen from results signifies applicability of this model for entire Hooghly basin.

TSUNAMI EARLY WARNING SYSTEM — AN INDIAN OCEAN PERSPECTIVE

On 26th December 2004, the countries within the vicinity of East Indian Ocean experienced the most devastating tsunami in recorded history. This tsunami was triggered by an earthquake of magnitude 9.0 on the Richter scale at 3.4°N, 95.7°E off the coast of Sumatra in the Indonesian Archipelago at 06:29 hrs IST (00:59 hrs GMT). One of the most basic information that any tsunami warning center should have at its disposal, is information on Tsunami Travel Times (TTT) to various coastal locations surrounding the Indian Ocean rim, as well as to several island locations. Devoid of this information, no ETAs (expected times of arrival) can be included in the real-time tsunami warnings. The work describes on development of a comprehensive TTT atlas providing ETAs to various coastal destinations in the Indian Ocean rim. This Atlas was first released on the first anniversary of the Indian Ocean Tsunami and was dedicated to the victims. Application of soft computing tools like Artificial Neural Network (ANN) for prediction of ETA can be immensely useful in a real-time mode. The major advantage of using ANN in a real-time tsunami travel time prediction is its high merit in producing ETA at a much faster time and also simultaneously preserving the consistency of prediction. Overall, it can be mentioned that modern technology can prevent or help in minimizing the loss of life and property provided we integrate all essential components in the warning system and put it to the best possible use.

A comparative study of air-sea exchange coefficients and turbulent fluxes over Indian sub-continent and Korean peninsula

In this article, we describe the variation of air-sea exchange coefficients and air-sea interface fluxes over the East Asian marginal seas surrounding the Korean peninsula and compare them with the similar estimates reported for the tropical Indian Ocean. Surface layer meteorological observations for a period of about five years obtained from five oceanic buoys in the adjoining seas of Korean peninsula form the database for this study. Depending on the stability of the atmosphere, buoy data is classified into three categories - unstable, neutral and stable data. For unstable conditions, sensible and latent heat flux show good correlation with the wind speed, whereas it is not so for the neutral and stable condition. Irrespective of the stability of the atmosphere, momentum flux always shows a steady dependence on the varying wind speed. Sensible and latent heat fluxes also show good correlation with the difference between sea surface temperature and air temperature. Unlike the linear regression between the exchange coefficients and wind speeds reported for the Indian Ocean, we suggest second order and exponential fits for these exchange coefficients, which give better representation of their wind speed dependence. The results presented in this article form very useful input to the coupled ocean atmospheric models and the oceanic wave models, hence significant.