Research Article Understanding the characteristics of rapid intensity changes of Tropical Cyclones over North Indian Ocean Raghu Nadimpalli1 · Shyama Mohanty1 · Nishant Pathak1 · Krishna K. Osuri2 · U. C. Mohanty1 · Somoshree Chatterjee1 Received: 31 August 2020 / Accepted: 21 December 2020 © The Author(s) 2021 OPEN Abstract North Indian Ocean (NIO), which comprises of Bay of Bengal (BoB) and Arabian Sea (AS) basins, is one of the highly poten- tial regions for Tropical Cyclones (TCs) in the world. Signifcant improvements have been achieved in the prediction of the movement of TCs, since the last decade. However, the prediction of sudden intensity changes becomes a challeng- ing task for the research and operational meteorologists. Hence, the present study focuses on fnding the climatological characteristics of such intensity changes over NIO regions. Rapid Intensifcation (RI) is defned as the 24-h maximum sustained surface wind speed rate equal to 30 knots (15.4 ms−1). The results suggest that the TCs formed over the NIO basin are both seasonal and basin sensitive. Since 2000, a signifcant trend is observed in RI TCs over the basin. At least one among three cyclones getting intensifed is of RI category. More number of RI cases have been identifed in the BoB basin than the AS. The post-monsoon season holds more RI and rapid decay cases, with 63% and 90% contribution. Most of the TCs are attaining RI onset in their initial stage. Further, India is receiving more landfalling RI TCs, followed by Bangladesh and Oman. The east coast of India, Tamil Nadu, and Andhra Pradesh are the most vulnerable states to these RI TCs. The cyclogenesis locations associated with RI cases hold higher moisture, and sea surface temperature as compared to the Non-RI cases. Keywords Tropical cyclones · Rapid intensifcation · North Indian ocean 1 Introduction magnitude of > 34 knots or more [10]. In the NIO, BoB reports the highest number of tropical cyclones, which is North Indian Ocean (NIO) is a highly potential region for almost fve times higher than the AS [25]. Past reports con- Tropical Cyclones (TCs). The Indian coastline is a prone frmed that the most deadly TCs and highest catastrophe area to TCs and is the worst afected region globally, hav- and death tolls occurred in the NIO basin, especially over ing a coastline of ~ 7516 km [12]. Thirteen coastal states/ the BoB [10]. UTs encompass 84 coastal districts afected by cyclones With the advancements in the dense observational [12]. About 7% of the global TCs are formed over the NIO network such as in-situ, satellite, radar, and other remote basin, comprised of the Bay of Bengal (BoB) and Arabian sensed platforms, the TC movement’s prediction accuracy Sea (AS) [10]. The NIO coastal countries are much popu- has been improved from the last few decades [12]. Sig- lated and thus more concerned [10]. Generally, NIO experi- nifcant improvements have been achieved in the predic- ences ~ 5–6 systems intensifed to TC with sustained wind tion of the track of TC since the previous few years due * Raghu Nadimpalli, [email protected] | 1School of Earth Ocean and Climate Sciences, Indian Institute of Technology Bhubaneswar, Argul, Odisha 752050, India. 2Department of Earth and Atmospheric Sciences, National Institute of Technology Rourkela, Rourkela, Odisha 769008, India. SN Applied Sciences (2021) 3:68 | https://doi.org/10.1007/s42452-020-03995-2 Vol.:(0123456789) Research Article SN Applied Sciences (2021) 3:68 | https://doi.org/10.1007/s42452-020-03995-2 to the advancements in numerical weather prediction and RD stages of the TCs. Further, the European Centre models and data assimilation techniques over the NIO for Medium-Range Weather Forecast (ECMWF) Reanaly- basin [6, 7, 18, 20, 21, 23], 2015, 2017, [14–16],Mohanty sis 5th Generation (ERA5) data at a spatial resolution of et al., 2015,2019, [2]. However, the intensity prediction is 0.25° × 0.25° degree and temporal resolution of 1 h for the still a challenging task for the operational and research 1982- 2018 period is used to analyze the characteristics community due to their intensity changes such as rapid associated with TCs. intensifcation (RI)1 and rapid decay (RD)2 over the basin [11, 26],[27]. The operational intensity forecast error for the intense TCs (wind speed > 48 knots) is considerably large 3 Results and discussions over the NIO region [12]. Recent studies have addressed the improving predic- 3.1 Cyclogenesis tion of intensity changes through various methods such as the use of high-resolution modeling systems [1, 3, 6, A TC is termed as an anti-clockwise non-frontal synoptic- 22], vortex in Bhalachandran itialization [2], data assimila- scale low-pressure circulation system over tropical oceans tion [16, 19]. However, these studies are mostly limited to with organized convection in the northern hemisphere the individual TCs or very few cases. Moreover, due to the [10]. There is no universally accepted defnition for tropi- increasing temperatures globally, TCs pose a severe haz- cal cyclogenesis. According to Ritchie and Holland [24], ard to the coastal regimes [10], [9]. The BoB has become cyclogenesis is defned as the sequence of physical pro- more conducive to TC intensifcation due to increased cesses such as a warm core, cyclonic vortex with high upper ocean heat content and sea surface temperature wind speed near the surface. Later, [17] defned genesis (SST) [10]. as when the Maximum Sustained Wind (MSW) reaches the The best possible way to approach this problem is to depression stage. Hence, in this present study, we consid- analyze the history of tropical cyclones in the NIO basin. ered the depression stage as the cyclogenesis. Figure 1 Therefore, it is crucial to understand if, and why, there are shows the cyclogenesis points of TCs formed during the observed upward trends in TC intensifcation rates. It pro- pre-monsoon and post-monsoon season, respectively. The vides an opportunity to understand the behavior of their grid size is 0.5° × 0.5° for the period 1945–2018. From the pathways, intensity, and a possible way to evaluate the fgure, it can be seen that the number of grid points with cyclone hazard. Accurate TC intensity prediction is vital to cyclogenesis is signifcantly large for the BoB as compared forecasting because early guidance of the strength of a TC to the AS during both the seasons of the pre-monsoon and its rainfall distribution can save lives and reduce prop- and post-monsoon. During the post-monsoon season, the erty damages and is a research interest. So, an attempt has spatial distribution of cyclogenesis grid points is more con- been made to analyze the climatological characteristics of centrated than that of the pre-monsoon and has an exten- TCs, which helps in understanding the RI and RD nature sion of 100°E longitude. Thus, the post-monsoon season of TCs. receives more TCs over the BoB basin than pre-monsoon season. A grid maximum of 2 cyclogenesis points lies and falls under the BoB basin, shows that the BoB region is 2 Data and methodology more conducting than the AS region. Latitude-wise and longitude-wise, genesis point dis- In this study, the International Best Track Archive for tributions during the pre-monsoon and post-monsoon Climate Stewardship (IBTrACS) data [8] have been con- seasons are also analyzed. A few latitude and longitudinal sidered for getting cyclogenesis in the BoB and the AS regions have embraced more frequency of cyclogenesis regions 1945–2018. Besides, high temporal resolution (at than others. During the pre-monsoon season, 68 cases out 3hourly) best track archival from the Regional Specialized of 103 are formed between 82° and 92° E longitudes. Simi- Meteorological Centre (RSMC), published by the India larly, the latitudes 9°N–15°N favor the genesis of 74 cases Meteorological Department (IMD), for the period 1982 out of a total of 103 in the pre-monsoon season. However, to 2018 (37 years) [available at www.rsmcn ewdel hi.imd. the latitudinal belt 11°N–13°N prefers the most. Therefore, gov.in] has been considered to identify the accurate RI it can be concluded that the active TC cyclogenesis region ranges between 82° and 92°E and 9° and 15°N during the pre-monsoon season. During the post-monsoon season, longitudes from 84°E to 90°E hold 51% of total cases (193 1 Rapid intensifcation is defned as a decrease of 30 knots intensity change in maximum sustained wind in a 24 h period [22]. cases out of 374), and the longitudes from 84°E to 86°E 2 Rapid Decay is defned as a decrease of 25 knots intensity change have a maximum number of cases, i.e., 62. In the case of in maximum sustained wind in a 24 h period [5]. latitudes, 76% of TCs are formed between 7°N and 13°N. Vol:.(1234567890) SN Applied Sciences (2021) 3:68 | https://doi.org/10.1007/s42452-020-03995-2 Research Article Fig. 1 Spatial distribution of cyclogenesis over NIO basin during a Pre-monsoon and b Post-monsoon seasons. c Longitudinal distribution of cyclogenesis for both the seasons. d same as c but represents latitudinal distribution The latitudes of 9°N to 11°N hold maximum cases, i.e., 91. 3.2 Rapid intensity changes of TCs During the post-monsoon season, the most active region for cyclogenesis lies in the area 82°E–90°E and 7°N–17°N. The cyclogenesis of RI and Non-RI cases have been plot- This analysis also concludes that the BoB basin is more ted in Fig. 2a, b, respectively. From the fgure, it is seen vulnerable to TC formation than the AS.