Tropical cyclones (TCs) are one of nature's most devastating occurrences and have the potential to cause severe damage over a wide section of the coastline (Asnani et al. 2005; Pattanayak et al. 2008). Property losses have significantly risen due to land-falling cyclones. As coastal populations continue to expand, the instability in the wind field of a land dropping cyclone has become evident both horizontally and vertically. As development continues along the cyclone-prone coastlines of large high-rise buildings, an understanding of the features of the vertical wind profile is sorely required. It is necessary to establish a regional weather forecasting system in order to provide early warnings and reduce calamities. As a result, precise forecasting of cyclone track and severity is critical to minimising human deaths and property damage (Reddy et al. 2014). The mean wind profile in the tropical cyclone boundary layer is vital to understand the overall dynamics of tropical cyclones because it reflects air-sea momentum transfers that are critical in tropical cyclone weather forecasting (Tse et al. 2013). The intensity of TCs has been particularly high over the Bay of Bengal, south-west India, during the post-monsoon period of October and November. These TCs grows northwestward, towards India's east coast, over the southern and central Bay of Bengal, as well as the Andaman Sea, and frequently receive between 15°N and 18°N, affecting Bangladesh's coast (Kumar et al. 2014). During the post-monsoon season, there was an increase in the frequency of extremely violent cyclonic storms across the Arabian Sea (Murakami et al. 2017).
The number of cyclones created over the Bay of Bengal was 3-4 times more than that over the Arabian Sea, according to (Obasi et al. 1997). Although cyclones are uncommon in the Arabian Sea, it does produce powerful tropical cyclones (Bhatla et al. 2020). Several research has been conducted to document the reaction of ocean dynamics in the creation of TCs, as well as to comprehend the relevant air-sea interaction processes (Stramma et al. 1986, Lyulyukin et al. 2015, Lyulyukin et al. 2017, Zaitseva et al. 2018). The wind speed and the differential in moisture content between the air and the saturation mixing ratio at the sea's surface determine how much energy is transferred from the ocean to the atmosphere in the form of moisture (Chinthalu et al. 2018). The tropical cyclone boundary-layer (TCBL) has the least well-observed component of the storm (Jelesnianski et al. 1993). However, the boundary layer is responsible for cyclone communication with ocean by receiving heat and moisture, and transmitting momentum in the form of oceanic current and waves (Giammanco et al. 2012).
The Sound Detection and Ranging (Sodar), a remote sensing instrument is commonly used to study the atmospheric boundary layer (ABL) and also for regional scale observation of wind turbulence. The transfer and exchange of energy, mass, momentum, and moisture from the lower to the upper layers is governed by boundary layer dynamics. Sodar is suitable to study the lower part of the ABL (Kumar et al, 2011, Kallistratova et al. 2004). In addition to Sodar measurements, other measurements will be helped to visualize three dimensional structure of wind profile. Because terrain roughness and complexity influence the mean wind profile in the approaching direction, it is expected to change under the influence of the sea fetch (Cheynet et al. 2020).
A series of 11 sensitivity experiments were conducted on various physical parameterization schemes of the Weather Research Forecast (WRF-ARW core) model to predict the track and intensity of tropical cyclone Nargis, which formed over the Bay of Bengal and hit Myanmar on May 2, 2008, causing widespread human and economic losses (Raju et al. 2011). Using archival tropical storm track data from the Joint Typhoon Warning Center, a rigorous analysis was undertaken to generate the most likely (synthetic) cyclone track (JTWC) from Sahoo et al. 2017. They found that post-monsoon cyclones occur more frequently than pre-monsoon cyclones. While plotting the route of tropical storms, it is also observed that the path of tropical cyclones abruptly alters during their active life span. According to historical statistics, the main cyclone seasons in the South Indian Ocean are May-July and September-December, with substantial storm occurrences in April and August (Nair et al. 2018). From 2014 to 2018, (Hendricks et al. 2019) published a summary of scientific advances on tropical cyclone intensity change. Improved understanding of the role of vertical wind shear (VWS) and its impact on convection, surface fluxes, ocean eddies, dry/dusty air intrusions, eyewall replacement cycles (ERCs), spiral rainband dynamics, eyewall instability and inner-core mixing, and the mechanisms by which TCs intensify have all resulted from research on intensity change.
The present study attempts to investigate the characteristics of wind during the cyclonic storm Nisarga. A short description of the measuring equipment, Sodar, is given in the second section of this paper. This study presents data of wind speed and direction in the atmospheric boundary layer during the Nisarg cyclone that occurred over the Arabian Sea. We use continuous wind measurements located at a low-latitude station of Atigre, Kolhapur (16.69° N, 74.24° E) which described in detail in the experimental details and data section.