Pre-monsoon Cyclones Research Articles

Spring tropical cyclones modulate near-surface isotopic compositions of atmospheric water vapour in Kathmandu, Nepal

Abstract. While westerlies are recognized as a significant moisture transport in Nepal during the pre-monsoon season, precipitation is also attributed to moisture from cyclones originating in the Bay of Bengal (BoB) or the Arabian Sea (AS). Tropical cyclones exhibit negative isotopic values in both precipitation and atmospheric water vapour; however, the factors influencing isotopic fractionation during tropical cyclones remain poorly understood. We present the results of continuous measurements of the isotopic composition of atmospheric water vapour (δ18Ov, δDv, and d-excessv) in Kathmandu from 7 May to 7 June 2021 during two pre-monsoon cyclones: cyclone Tauktae, formed over the Arabian Sea, and cyclone Yaas, formed over the Bay of Bengal. Our study reveals that tropical cyclones originating from the BoB and the AS during the pre-monsoon season modulate isotopic signals of near-surface atmospheric water vapour in Nepal. Comparing conditions before and after, we observed a significant depletion of δ18Ov and δDv during both cyclones, attributed to changes in moisture sources (local vs. marine). Convective activity plays a pivotal role in the variability of δ18Ov and δDv during both cyclones, confirmed by the spatial variations of outgoing longwave radiation (OLR) and regional precipitation during both cyclones. We also found a significant negative correlation between δ18Ov and/or δDv and rainfall amount along the trajectories during cyclone Tauktae, probably resulting from integrated upstream processes linked to the earlier Rayleigh distillation of water vapour via rainfall rather than local rainfall. The decrease in δ18Ov and/or δDv during cyclone Yaas is associated with the intensified convection and moisture convergence at the measurement site, while the lower cloud top temperatures (CTTs) and lower cloud top pressure (CTP) during intense convection contribute to higher d-excessv values during the final stage of cyclone Yaas. This characteristic is missing during cyclone Tauktae. Our results shed light on key processes governing the isotopic composition of atmospheric water vapour in Kathmandu with implications for the monsoon moisture transport and paleoclimate reconstructions of tropical cyclone activity.

Interdecadal variability of the pre-monsoon cyclone characteristics over the Bay of Bengal

The low-pressure systems intensified to cyclones prior to the onset of the Indian summer monsoon season over the North Indian Ocean are referred to as pre-monsoon season (PMS) cyclones. Climate change is amplifying the pre-monsoon cyclone landscape, fostering more frequent and intense storms with altered tracks, resulting in heightened risks for coastal communities and economies. This study investigates the interdecadal variations in tropical cyclone and key large-scale atmospheric parameters that influence the characteristics of cyclones, including track, frequency over the Bay of Bengal (BoB) during the pre-monsoon season from 60 years of data. The large-scale atmospheric parameters are analyzed by calculating climatological anomalies. It is noted that the frequency of cyclones making landfall over the eastern Indian coastal landmass has increased in the recent decade compared to the past five decades. Compared to the past fifty years, the percentage frequency has increased in the recent ten years by 50%. At the low level, stronger easterlies are dominant and upper-level jet streams shift to lower latitudes, indicating that the path of cyclones has shifted from the north (N)-northeast (NE) to the northwest (NW), i.e. towards the east Indian coastal landmass, over the recent decade. In contrast to the previous five decades, an unusual low-pressure region has emerged over the NW India and Pakistan regions, creating a favorable path for cyclones moving towards the Indian region in recent decade. Cyclones have been more intense in the recent decade than they were in the previous five decades, according to the rise in low- and mid-level specific humidity and temperature over the BoB.

Research on and application of the operational storm surge ensemble forecast model in the Bay of Bengal

The coastal region of the Bay of Bengal (BoB) is one of the most vulnerable areas to storm surges caused by tropical cyclones (TCs). Historical data indicate that the BoB experiences two seasons in the annual cycle of TC formation, namely, the pre-monsoon season (April to May) and the post-monsoon season (October–December). The frequency of post-monsoon cyclones is significantly higher than that of pre-monsoon cyclones. Very severe cyclonic storms and stronger-intensity storms can cause disastrous storm surges. Based on the accuracy of current forecasts of the path, intensity and radius of maximum wind of TCs in the BoB, a total of 153 ensemble members are automatically generated according to the subjective deterministic forecast results of the TCs, basically covering all possible TCs. A graphics processing unit (GPU)-based parallel storm surge forecast model is established and validated for the coastal areas of the BoB. The speedup ratio of the model can exceed 300, which can meet the computational time requirements of large-scale ensemble numerical forecasts. For the 2020 Amphan cyclone, the costliest cyclone ever recorded in the North Indian Ocean (NIO), the numerical ensemble forecast model can provide rapid and accurate storm surge ensemble and numerical probability forecast products for the BoB. The probability of coastal flooding risk can be evaluated for the coastal areas of the BoB, enabling reasonable and valuable recommendations to be made to evacuate people and protect property near dangerous areas.

Observed tropical cyclone-driven cold wakes in the context of rapid warming of the Arabian Sea

ABSTRACT We present a detailed long-term (1997–2019) analysis of observed cyclone-induced surface cooling (or cold wake) in the Arabian Sea. Here, the analysis is performed for 33 cyclones that drove significant cooling at the sea surface in three different seasons: the pre-monsoon, monsoon and post-monsoon. Our study shows that cyclones can cool the sea surface up to 4.76° C after their passage, depending on the intensity, duration and other factors that contribute to cooling. The monsoon and pre-monsoon cyclones show stronger cooling, but the post-monsoon cyclones exhibit a longer duration (8–10 days) of cooling and slower recovery time (15 days). In general, the pre- and monsoon cyclones exhibit a strong positive correlation with the Latent Heat Flux, whereas the post-monsoon cyclones show a higher correlation with Ekman Pumping Velocity, wind stress and intensity of cyclones. The cold wake composite analysis for the El-Niño, La-Niña and Normal years shows that cyclone-induced cooling is similar in El-Niño and La-Niña years, and the cooling is more dominant during the negative Indian Ocean Dipole (IOD) than that in the positive IOD years. Co-occurrence of positive IOD and La Niña events has led to more intense cyclones in recent decades. The power dissipation index, accumulated cyclone energy and oceanic heat content also show an increasing trend in AS and favour rapid intensification of cyclones. Since the drop in SST normally impedes cyclones from intensification, our study is important and the findings of this study will aid in tropical cyclone predictions. In response to rapid warming of Indian Ocean in recent decades, extreme events such as cyclones are expected to increase in the context of climate change.

What Made the Sustained Intensification of Tropical Cyclone Fani in the Bay of Bengal? An Investigation Using Coupled Atmosphere–Ocean Model

The extremely severe tropical cyclone Fani (25 April–5 May 2019) unusually sustained high intensity for a prolonged duration over the Bay of Bengal (BoB). A regional coupled atmosphere–ocean model was used to investigate the atmospheric and oceanic conditions and processes responsible for the sustained intensification of the tropical cyclone (TC) Fani. The coupled model simulated the track and intensification/weakening stages of the cyclone reasonably well. A reduction in sea surface temperature (by −2 °C) and an increase in sea surface salinity due to cyclone-induced upwelling and inertial mixing was noticed in both observations and model. The passage of TC Fani over two geostrophic mesoscale warm-core eddies along the cyclone track was found to supply the necessary energy for the intensification of TC Fani. The sea surface height anomaly and tropical cyclone heat potential (TCHP) were higher during TC Fani than other pre-monsoon cyclones in the BoB. The anomalous TCHP in the warm-core eddy zones (i.e., in excess of >160 kJ cm−2) maintained the warm surface temperature and high air–sea heat fluxes. The air–sea latent heat flux and atmospheric wind shear were favourable for the intensification of the cyclone. The atmospheric moist static energy enhanced up to 360 kJ kg−1 with a deep vertical extension in the atmospheric column supporting the further intensification of TC Fani. Therefore, the unusual oceanic TCHP associated with mesoscale eddies, higher latent heat flux, and enhanced moist static energy in the atmosphere contributed to the sustained intensification of TC Fani for a prolonged period in the BoB.

Shifting seasonality of cyclones and western boundary current interactions in Bay of Bengal as observed during Amphan and Fani

In recent years, the seasonal patterns of Tropical Cyclones (TC) in the Bay of Bengal have been shifting. While tropical depressions have been common in March–May (spring), they typically have been relatively weaker than the TCs during October–December. Here we show that the spatial pattern of recent warming trends during the last two decades in the southwestern Bay has allowed for stronger springtime pre-monsoon cyclones such as Amphan (May 2020, Super Cyclone) and Fani (April–May 2019, Extremely Severe Cyclone). The tracks of the pre-monsoon cyclones shifted westward, concurrent with an increasing rate of warming. This shift allowed both Fani and Amphan tracks to cross the northeastward warm Western Boundary Current (WBC) and associated warm anti-cyclonic eddies, while the weaker Viyaru (April 2013, Cyclonic Storm) did not interact with the WBC. A quantitative model linking the available along-track heat potential to cyclone’s intensity is developed to understand the impact of the WBC on cyclone intensification. The influence of the warming WBC and associated anti-cyclonic eddies will likely result in much stronger springtime TCs becoming relatively common in the future.

Impact of assimilation of SCATSAT-1 data on coupled ocean-atmospheric simulations of tropical cyclones over Bay of Bengal

This study examines the impact of assimilation of the surface winds obtained from SCATterometer SATellite-1 (SCATSAT-1) in predicting the tropical cyclones over the Bay of Bengal using a coupled ocean-atmospheric model. Three sets of numerical experiments are conducted for six cyclones during post-monsoon (VARDAH, GAJA, PHETHAI) and pre-monsoon cyclones (MORA, FANI, AMPHAN). The first experiment, ‘CONTROL’, is conducted with the Weather Research and Forecasting - Ocean Mixed Layer (WRF-OML) model initialized using Global Forecasting System analysis and ocean initial conditions obtained from the HYbrid Coordinated Ocean Model (HYCOM) model. The second experiment, ‘PREPBUFR’, is conducted by assimilating the National Center for Environmental Prediction (NCEP) prepared BUFR observations with the WRF-OML model and three-dimensional variational assimilation method. Further, the 'SCATSAT' experiment is conducted as PREPBUFR, but additionally, the SCATSAT-1 surface wind vectors are assimilated. Our results of the simulated tracks from three experiments suggest that CONTROL and PREPBUFR simulations exhibit faster translation speed and more track deviations than the India Meteorological Department (IMD) observations. The realistic representation of low-level cyclonic vortex through the assimilation of SCATSAT-1 winds seems to produce positive feedback to both track and intensity, producing a significant improvement in predicting intensity and marginal enhancement on simulation of track and translation speed. The assimilation of winds further improved the representation of different life cycles of the storms as seen in IMD. The analysis of air-sea parameters, in terms of mixed layer deepening, sea surface temperatures, and air-sea flux exchanges, suggests that the response of air-sea feedback is strong in SCATSAT compared to PREPBUFR and CONTROL. Overall, the assimilation of SCATSAT surface winds improved the WRF-OML performance on the prediction of track and intensity, upper ocean response, primary and secondary circulations of tropical cyclones, and the rainfall distributions.

Tropical cyclogenesis identification using eddy detection technique for the Bay of Bengal Basin

The study deals with formative stages and lead detection time of tropical cyclogenesis using Okubo-Weiss Parameter, a proven robust eddy detection technique used for tropical cyclones that developed over the north Indian Ocean basin. Specifically, it aims to identify the initial traces of upper atmospheric genesis and understand the spatio-temporal evolution of pre-cyclonic eddy vortices in the atmospheric column using high-resolution WRF model outputs simulated for different cyclones. A coarser resolution grid of 27 km was used for identification purpose and a finer resolution of 9 km used to evaluate the characteristics of eddy vortices. Case studies were performed with four post-monsoon severe cyclones viz.; Phailin(2013), Vardah(2013), Gaja(2018), Madi(2013) and two pre-monsoon cyclones Mora(2017) and Aila(2009) that developed over this region. Study reveals a good skill in genesis prediction with a minimum of four days (~ 90 h) lead time in all cases irrespective of pre- and post-monsoon seasons. Factors that favor cyclogenesis during pre-monsoon season (except relative humidity) contribute to early detection of pre-cyclonic vortices in the atmosphere. Initiation mechanism of tropical cyclogenesis occurs at upper atmospheric levels and also detected at higher lead time for pre-monsoon cases unlike the post-monsoon cases. A lower relative vorticity at 850 hPa in pre-monsoon seasons is a major factor that initiates pre-cyclonic vortices at relatively higher levels compared to post-monsoon cyclones. Study also investigated the behavior of eddies in the atmospheric column for a non-developing case and compared these findings against developing cases. The study signifies that this technique is quite promising and has potential for early detection of tropical cyclogenesis in the atmospheric column prior to satellite detection over ocean surface.

Role of warm ocean conditions and the MJO in the genesis and intensification of extremely severe cyclone Fani

Cyclone Fani, in April 2019, was the strongest pre-monsoon cyclone to form in the Bay of Bengal after 1994. It underwent rapid intensification and intensified quickly to an extremely severe cyclone. It maintained a wind speed of ≥ 51 m s−1 (≥ 100 knots) for a record time period of 36 h. The total lifespan of the cyclone was double than the climatological lifespan. Also, the duration of the cyclone in its extremely severe phase and the accumulated cyclone energy were significantly larger than the climatological records for the pre-monsoon season. In the current study, we investigate the ocean-atmospheric conditions that led to its genesis, rapid intensification and long lifespan. Our analysis shows that the Madden Julian Oscillation and anomalous high sea surface temperatures provided conducive dynamic and thermodynamic conditions for the genesis of cyclone Fani, despite forming very close to the equator where cyclogenesis is generally unlikely. Further, favourable ocean subsurface conditions and the presence of a warm core eddy in the region led to its rapid intensification to an extremely severe cyclone. A large area of warm ocean surface and subsurface temperatures aided the cyclone to maintain very high wind speed for a record time period. The vital role of the ocean surface and the subsurface in the genesis and the intensification highlights the need to efficiently incorporate ocean initial conditions (surface and sub-surface) and ocean–atmosphere coupling in the operational cyclone forecasting framework.

Landfall season is critical to the impact of a cyclone on a monsoon-regulated tropical coastal lagoon

Cyclones can produce a wide variety of short-term and long-term ecological impacts on coastal lagoons depending on cyclone's physical-meteorological characteristics and the lagoon's geographic, geomorphic, and bathymetric characteristics. Here, we theorized that in monsoon regulated tropical coastal lagoons, another important factor that could determine the impact of a cyclone is the landfall season or time of the year with reference to the monsoon season. We analyzed the impact of two cyclones which made landfall near Chilika, Asia's largest brackish water lagoon in different seasons, Cyclone Fani and Titli before and after the monsoon season. We compared field measured and satellite-derived water quality parameters including nutrient, salinity, water temperature, transparency, Chlorophyll-a (Chl-a), total suspended matter (TSM), and colored dissolved organic matter (CDOM) before and after the cyclones. We found that although both the cyclones were of similar intensities, after their land interaction, their impact on the lagoon's water quality was contrasting. The post-monsoon cyclone produced a substantial increase in total nitrogen (TN) and total phosphorous (TP), a large drop in salinity, CDOM, and Chl-a. In contrast, after the pre-monsoon cyclone, TN and TP did not show any such hike, no substantial change in salinity and CDOM either, and only a slight increase in Chl-a was observed. We found that the controlling factor in determining the impact of a cyclone is the rate and duration of freshwater discharge to the lagoon, which is normally a strong pulse for pre-monsoon and a continued high flow for post-monsoon cyclones. We conclude that the antecedent conditions of the lagoon and the watershed at the time of a cyclone's landfall is a key criterion in determining the impact. The combined use of satellite data and field data was proved critical to capture the overall impact of cyclones on the hydrological characteristics of the monsoon-regulated coastal lagoon.

Population Exposure to Amphan-Scale Cyclones Under Future Climates

Background: Climate change impacts are felt disproportionately in developing countries, and in particular Southern Asia experiences the most damaging hydrometeorological events in the world, with loss of life from past cyclones in the hundreds of thousands. Despite this, the Bay of Bengal cyclone basin receives far less research attention than many of the others around the world. Here, we study the historical and future impacts of Super Cyclone Amphan, which made landfall in May 2020, bringing storm surges of 2-4 meters to coastlines of India and Bangladesh. Methods: In this modelling study, we combine projections of sea level rise from the Coupled Model Intercomparison Projection, phase 6 (CMIP6), with estimates of storm surge using a dynamic storm surge model. Sampling the spectrum of possible sea level rises, we consider a low, medium and high scenario, based on projections in 2100. We then feed these into a flood inundation model to simulate storm surge-induced flooding, had Cyclone Amphan occurred in these future worlds. Finally, we consider the change in future population growth and urbanisation, thereby calculating the change in population exposure to these future flooding events. Our approach is that of the extreme event attribution community, but projecting into the future rather than interrogating the past. Findings: We find that in 2100, the local sea level rise in the Bay of Bengal during the pre-monsoon cyclone season is between 0.32-0.84 m, depending on which emissions scenario is followed. If a Cyclone Amphan-scale storm surge occurred on top of that sea level rise, the future population of both India and Bangladesh will be more exposed, with India showing >200% increased exposure to extreme (>3 m) and moderate (>1 m) flooding under a high emissions scenario, and Bangladesh showing ~60-80% increased exposure to the same scenarios. The majority of this change in both countries comes from sea level rise rather than population changes, and in Bangladesh the future population change contributes negatively to the change in exposure, as more citizens migrate further inshore. However, if we follow an emissions scenario consistent with meeting the upper Paris Agreement climate goal, we project very little change in exposure. Interpretation: There is an urgent need to reduce carbon emissions to net zero, to prevent the negative impacts of climate change. By far the majority of cyclone research has been undertaken for countries such as America and Japan, with less resilient countries such as those in South Asia, which are more sensitive to changes in climate, seeing far less attention. With Cyclone Amphan occurring at the height of the COVID-19 crisis, we highlight how the risk was compounded and recommend that future climate risk assessments explicitly account for these potential non-linearities. Funding Statement: The main funding is from the Natural Environment Research Council.Declaration of Interests: The authors declare no competing interests.

Cyclone Fani: the tug-of-war between regional warming and anthropogenic aerosol effects

Before Cyclone Amphan took place in 2020, Cyclone Fani (May 2019) is the strongest pre-monsoon cyclone in the Bay of Bengal (BOB) since 1991, killing 90 people in eastern India and Bangladesh while causing US$1.81 billion of damages. Fani developed during a period of high concentration of anthropogenic aerosols in the BOB with abnormally high sea surface temperature (SST), thereby presenting an opportunity to understand the compound effects of atmospheric aerosols and regional climate warming on a tropical cyclone. A quantitative attribution analysis was conducted using the Weather Research and Forecasting model with chemistry (WRF-Chem) run at the convection-permitting (4 km) grid spacing, accompanied by an ensemble of coarser-resolution simulations to quantify the uncertainty. The removal of post-1990 trends in the tropospheric variables and SST from WRF-Chem’s initial conditions (IC) and boundary conditions (BC, including the lateral and lower boundary conditions) resulted in a reduction of cyclone precipitation by about 51% during the 5 d of April 28-May 2. The removal of tropospheric warming shows approximately twice as strong an effect on Fani (39% reduction in precipitation) as that of SST warming (22% reduction). When aerosol’s direct and indirect effects were removed from the simulations, i.e., no aerosol influence on radiation and cloud microphysics, Fani initially strengthened but later weakened, as measured by geopotential height and precipitation amounts. These results suggest that aerosol and its interaction with the atmosphere acted to mitigate the strengthening effect of anthropogenic warming on Fani, but was not strong enough to entirely counteract it. Although the ensemble of coarser simulations appears to overestimate Cyclone Fani in terms of precipitation, the direction of the effects is in agreement with that obtained from the 4 km simulations. Given the increasing anthropogenic aerosols in the BOB, future attribution studies using more sophisticated dynamical aerosol models on BOB tropical cyclones are urged.

Differential Upper Ocean Response Depicted in Moored Buoy Observations during the Pre-Monsoon Cyclone <i>Viyaru</i>

Differential Upper Ocean Response Depicted in Moored Buoy Observations during the Pre-Monsoon Cyclone <i>Viyaru</i>

Turbulence of tropical cyclone ‘Fani’ in the Bay of Bengal and Indian subcontinent

The present study is focused on the movement and impact of extremely severe tropical cyclone ‘Fani,’ which developed in the Bay of Bengal near the Equator during April 2019 and caused a severe impact on landmass due to high wind speed (max.180 km/h) and extreme precipitation (150–300 mm/day). The multi-satellite-based study exhibited that the cyclone remained in the oceans for a longer duration (7 days; 70% of its life span) since its formation, providing a substantial strength and moisture to travel a very long trajectory (2136 km on the ocean and 1040 km on land). The high-intensive Fani cyclone had severely affected a large area in the eastern Indian subcontinent (primarily Odisha, West Bengal, Assam in India and Bangladesh). The precipitation pattern over the landmass was estimated using temporal tropical rainfall measurement mission (TRMM) satellite dataset. The TRMM-based study exhibited a very high-intensity precipitation (>300 mm/day) in the Bay of Bengal, whereas high- (150–300 mm/day) to low-intensity precipitation (< 50 mm/day) was observed in the coastal Andhra Pradesh, Odisha and West Bengal due to the influence of extremely severe cyclonic storm during the landfall and its progression on the landmass. The tropical cyclone significantly affected the agricultural land (128,019 km2; 57% of total area) and least affected the built-up land (2758 km2; 1.2% of total area) in the region. Nevertheless, the cyclonic impact on built-up land was extensive as it devastated human life, property and assets, with the high-speed wind, torrential rain and extensive flooding. The premonsoon cyclone is a sporadic history in BoB, and Fani was one of the most influential cyclones of the region. The study provides the nature of the 10th most severe cyclone in the Indian subcontinent in the last 52 years, through its movement, wind direction and impact in terms of heavy rainfall and on land use/land cover in the region.

A coupled model analyses on the interaction between oceanic eddies and tropical cyclones over the Bay of Bengal

The Bay of Bengal (BOB) region of Indian Ocean is affected by numerous tropical cyclones during pre- and post-monsoon seasons when various eddies are generated in the central and western bay. Here, numerical simulations of few tropical cyclones (Aila, Laila, Phailin, Hudhud and Madi) that occurred in different seasons are carried out using an ocean atmosphere coupled numerical model, consisting of Weather Research and Forecasting (WRF) and Regional Ocean Modeling Systems (ROMS), to analyse the influence of eddies on tropical cyclone intensifications over the bay. The model is able to track the tropical cyclones with less error, while the central pressure and wind speed are underestimated. Also, the coupling process steer the cyclones to the observed track compared to standalone WRF model. The variations in simulated sea surface temperature and salinity are in agreement with the satellite and in situ measurements. During the passage of Phailin and Hudhud over the eddies, the cyclones intensified almost twice of their initial central pressure and wind (before interacting with eddies), which the model captured very well. The eddy feedback factor, which estimate the influence of warm eddies on tropical cyclones, is about 152% for Phailin and 90% for Hudhud. In the case of the pre-monsoon cyclones, Aila and Laila, the eddy feedback factor is estimated up to 36% and 21%, respectively. Madi cyclone lost its intensification up to 80% because of its interaction with a cold eddy. These predictions are comparable with that of the observations during the cyclone passage. Also it is noted that the warm eddy regions resulted in the maximum transfer of latent heat resulting in strong intensifications of the cyclones. Our study reveals that the eddies play an important role in the intensification and dissipation of BOB cyclones and understanding their nature can help in estimating the track and intensity of cyclones.

A numerical study of hypothetical storm surge and coastal inundation for AILA cyclone in the Bay of Bengal

The head Bay region bordering the Bay of Bengal is highly vulnerable to tropical cyclones. Catastrophic risks from storm surge and associated inundation are quite high due to high population density in coastal areas, socio-economic conditions, and shallow bathymetry. It features the world’s largest deltaic system comprising of ‘Sunderbans’ bordered by West Bengal and Bangladesh. In a geomorphologic sense, the head Bay region is a low-lying belt comprising several barrier islands and river drainage systems, numerous tidal creeks, and mud flats having a high risk for widespread inundation. In addition, the high tidal range together with low-lying topography leads to high risk and vulnerability from storm surge inundation. During May 2009, a severe cyclonic storm Aila struck West Bengal causing enormous destruction to life and property along coastal belts of West Bengal and Bangladesh. It was the strongest pre-monsoon cyclone in the past two decades that had landfall in West Bengal. This work reports on a numerical study for hypothetical storm surge and associated inundation from Aila using the ADCIRC model. The study covers a comprehensive qualitative analysis on water level elevation and onshore inundation for West Bengal and Bangladesh regions. The estimated peak storm surge was about 4 m in the Sunderban region that propagated into all major riverine systems, inundating the river banks as well the inland areas. Numerical simulations indicate an average inland penetration distance of 350 m with a maximum of 600 m at various coastal locations in West Bengal and Bangladesh. The study emphasizes the need and importance of inundation modeling system required for emergency preparedness and disaster management.

Assessment on historical cyclone tracks in the Bay of Bengal, east coast of India

The Bay of Bengal (BoB) basin in the North Indian Ocean reports the highest number of tropical cyclones, and the frequency is much higher almost five times compared with the Arabian Sea (AS). Semi-enclosed nature of this basin in conjunction with its funnel shape steers the cyclone pathway striking the land. Historical reports signify that most deadly cyclones with highest catastrophe and death tolls occurred in this basin. The four maritime states located in the east coast of India are highly vulnerable to tropical cyclones. Hence, there is a need to understand the risk factors attributed from extreme winds, storm surge, and associated coastal inundation. Historical data signify higher frequency of post-monsoon cyclones compared with pre-monsoon cyclones in the BoB. This study aims to develop a synthetic track or the most probable cyclone track for each state located along the east coast of India. The synthetic track is a general guidance for numerical models, with utility to evaluate and assess the risk factors along coastal belts. This study also deals with aspects on frequency and cyclogenesis locations in the BoB based on analysis from historical data. Results for decadal scale variability signify that in the present decade, the eastern BoB is quite conducive for cyclogenesis. In this region, oceanic thermal field exhibits a warming trend, and found to extend up to water depth of 600 m. In addition, this study also evaluates the energy metrics such as Power Dissipation Index (PDI), and Accumulated Cyclone Energy (ACE) for tropical cyclones that occurred during past four decades in the BoB. The results signify that PDI for tropical cyclones in the present decade is six times higher when compared with the past. The study has practical applications for mapping coastal vulnerability in a changing climate. Tropical cyclones are associated with devastation resulting in enormous loss of life and property during its landfall. In a broad perspective, lack of disaster management pre- paredness would contribute to high vulnerability. The gen- esis of a cyclone usually referred as 'cyclogenesis' is the development stage or strengthening of a cyclonic circula- tion in the atmosphere during the initial stage of its forma- tion. Tropical cyclones form over warm ocean surface, and favourable conditions are sea-surface temperature (SST) greater than 26 ∘ C; low magnitude of vertical wind shear; larger low-level vorticity; and higher mid-troposphere rel- ative humidity. In global oceans, the western part of the North Pacific reported the maximum number of tropical cyclones (average of 26 per year) followed by eastern North Pacific (average of 17 per year). The South Indian Ocean and North Atlantic has an average of 10 per year, while regions in the North Indian Ocean (NIO) have an average of five cyclones per year (Niyas et al., 2009). Notably, the NIO alone accounts for about 7% of global tropical cyclone activity, with a frequency about five

Study of Weak Intensity Cyclones over Bay of Bengal Using WRF Model

Numerical simulations of four weak cyclonic storms [two cases of pre-monsoon cyclones: Laila (2010), Aila (2009) and two cases of post-monsoon cyclones: Jal (2010), SCS (2003)] are carried out using WRF-ARW mesoscale model. Betts-Miller-Janjic (BMJ) as cumulus parameterization (CP) scheme, Yonsei University(YSU) planetary boundary layer (PBL) scheme and WRF single moment 6 class (WSM6) microphysics (MP) scheme is kept same for all the cyclone cases. Three two-way interactive nested domains [60 km,20 kmand6.6 km] are used with initial and boundary conditions from NCEP Final Analysis data. The model integration is performed to evaluate the track, landfall time and position as well as intensity in terms of Central Sea Level Pressure (CSLP) and Maximum Surface Wind speed (MSW) of the storm. The track and landfall (time and position) of almost all cyclones are well predicted by the model (except for SCS cyclone case) which may be because of the accurate presentation of the steering flow by CP scheme. Irrespective of season, the intensity is overestimated in all the cases of cyclone, mainly because of the lower tropospheric and mid-tropospheric parameters are overestimated. YSU PBL scheme used here is responsible for the deep convection in and above PBL. Concentration of frozen hydrometeors at the mid-tropospheric levels and thus the latent heat released during auto conversion of hydrometeors is also responsible for overestimation of intensity.

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.

Tropical cyclone frequency in the north Indian Ocean in relation to southern oscillation phenomenon

The present paper deals with the influence of Southern Oscillation (SO) on the frequency of tropical cyclones in the north Indian Ocean. The results show that during the negative phase of SO the frequency of tropical cyclones and depressions over the Bay of Bengal and the Arabian Sea diminishes in May which is most important pre-monsoon cyclone month. The correlation coefficient between the frequency of cyclones and depressions and the Southern Oscillation Index (SOI) is +0.3 which is significant at 99% level. Post-monsoon cyclone frequency in the Bay of Bengal during November shows a significant positive correlation with SOl implying that it also decreases during the negative phase of SO. Thus there is a reduction in the tropical cyclone frequency over the Bay of Bengal during both intense cyclone months May and November in EI-Nino/Southern Oscillation (ENSO) epochs. Therefore it would not be correct to say that ENSO has no impact on the cyclogenesis in the north Indian Ocean. It is true that ENSO has no significant impact on the frequency of cyclones in the Arabian Sea. ENSO also seems to affect the rate of intensification of depressions to cyclone stage. The rate of intensification increases in May and diminishes in November in the north Indian Ocean during ENSO. The results are based on the analysis of monthly frequencies of tropical cyclones and depressions and SOI for the 100 year period from 1891-1990.