Analysis of extreme weather events over Satara district of western Maharashtra using RClimDex model

Introduction

There is a general consensus within the climate community that, any change in the frequency or severity of extreme climate events would have profound impacts on nature, human society and natural ecosystems. It is thus, very important to analyse extreme events. Extreme weather describes unusual weather events that are at the extremes of the historical distribution for a given area. Extreme weather includes unexpected, unusual, severe or unseasonal weather. The major forms of extreme weather include heat waves, cold waves, heavy precipitation and storm events. Increase in the frequency and intensity of extreme weather events for a particular region indicates change in climatic conditions of that region. Western Maharashtra majorly has a tropical monsoon climate, which is characterized by hot summers and cool winters and by high rainfall during monsoon season (Koppen climate classification: Aw). Western Maharashtra region extending in Satara district experiences moderate type of climate. Some part of the region experiences heavy rainfall while other part of same district witness very less intense rain due to topography of region. Most of the rain is brought by the southwest monsoon winds during the summer and about 87% of rainfalls during the monsoon months. The topography of the region makes the study of extreme weather events necessary to analyse historical data on climatic factors using proper statistical tools to enable the creation of location-specific technologies and adaptation strategies. Also, the micro level analysis plays crucial role in understanding region's climatic characteristics which in turn helps in formulating location-specific developmental approaches.

Materials and methods

This chapter deals with description of study area, data collected for the study and their sources and methodologies adopted for research work of "Analysis of extreme weather events of Western Maharashtra region using RClimDex model" using temperature and precipitation data from 1982-2022.

Location and Extent

Western Maharashtra is the region adjacent to Western Ghats between the Godavari River and Krishna River, a part of Deccan Plateau. It is bordered by the Konkan region to West and the states of Karnataka and Goa to South. Satara is district of Western Maharashtra which is situated such that it has Western Ghats to west and Deccan Plateau to the east. The study area experiences tropical monsoon climate. The low rainfall is observed over rain shadow region which is to the east of Western Ghats. With the exception of the western hilly region, Satara district experiences 49 rainy days annually on average. Similar to the quantity of rainfall, the Ghats region experiences the greatest number of rainy days, while the eastern border region experiences the least. While Mahabaleshwar on the hills has on an average 119 rainy days in a year, Mhaswad near the eastern border has only 30 rainy days in a year. At Mahabaleshwar the highest rainfall in 24 hours ever recorded was 458.5 mm in 1896. In the plains of the district, the highest rainfall in 24 hours at any station was 330.3 mm at Mhaswad in 1961. At Satara the highest maximum temperature ever recorded was 41.4°C. (Climate of Maharashtra, GOI, IMD, Pune).

Results and Discussion

Using the RClimDex model, extreme weather events were assessed. The significance of the variations that arise from the RClimDex model's computation was investigated more thoroughly. The p-value and a comparison of the estimate and error slopes were used to test the significance. This test was used to calculate the magnitude of trends. If the p-value is less than 0.05, significant changes will be identified. Significant changes were recognized when p-value was less than 0.05 and slope estimate was larger than the slope error value. The data revealed that there do not exist any frost days or ice days in Satara district (Table 3). Over the period 1982-2022, the number of cold days (FD15) decreased significantly while the (TXn) coldest daily maximum air temperature for Satara district increased significantly. Over the study period, along with the rise in the number of extremely hot days (SU38), there has also been a significant annual increase in the number of summer days (SU25). Annual number of summer days for 1993 was highest 73 days whereas for the year 1982 it was less than 25 days. The decadal trend for SU38 increased from 42 days in 2000 to 60 days in 2010. This showed significant increase in SU38 over decades. New Mark et al (2006) [16] observed similar results when studying daily temperature (maximum and minimum) and precipitation data from 14 southern and western African countries from 1961 to 2000. The number of cool nights (TN10p) over Satara district during the period of 1982-2022 has found to be (p=0) decreasing significantly whereas the temperature index namely warm nights (TN90p) was found to be significantly increasing in Satara district (p=0). The decadal trend of number of cool days (TX10p) followed non-significant decreasing trend with p=0.772, while the number of warm days (TX90p) followed non-significant increasing trend with p=0.617. Ladislaus et al. (2017) [14] obtained similar results. The number of cool nights decreased from 24 days in 1983 to four in 2002, 2009 and five in 2014. This indicated significant decrease in number of cool nights over decades in Pune district. However, it was observed that number of warm nights increased from four in 2000 to twenty-six in 2010 year which indicated significant increasing trend of TN90p. The warmest daily maximum temperature (TXx) showed significant decreasing trend over Satara district whereas the monthly minimum value of daily minimum temperature (TNn) have showed non-significant decreasing trend. The study conducted by Kalita et al (2020) [11] on maximum and minimum temperatures in the Eastern state of Meghalaya revealed a positive trend for all months except July and September, as well as a similar increasing trend for maximum temperature. The decadal trend for the warmest daily minimum temperature (TNx) has been observed to have a non-significant increasing trend over time in the Satara district. The Satara district's diurnal temperature range (DTR) demonstrated highly significant declining trends over the 1982-2022 period. There was most significant increase in number of tropical nights (TR20) and hot nights (TR23) from 1982-2022. Over the period 1982-2022, the temperature index, cold spell duration indicator (CSDI), showed a very significant decreasing trend, whereas the warm spell duration indicator (WSDI) showed a non-significant increasing trend. Brown et al (2010) [3] found similar result. The consecutive dry days (CDD) where maximum number of consecutive days with RR<1mm was found to be increasing over period of 1982-2022 but non-significantly (p=0.826) whereas the consecutive wet days (CWD) where maximum number of consecutive days with RR≥1mm was found to be significantly increasing over the period of 1982-2022 for Satara district (p=0.008). Significant increase in CWD and non-significant increase in CDD implies that the considerable variation exists in the frequency and intensity of rainfall over the different parts of the study area. The annual number of very wet days (R95p) where annual total precipitation when RR>95 th percentile, extremely wet days (R99p), simple daily intensity index (SDII) and annual total wet-day precipitation (PRCPTOT) all indicated significantly increasing trend over the period of 1982-2022 for Satara district with p=0.001; p=0.002 and p=0.026 and p=0.001 respectively. Rao K.K et al (2014) [18] observed similar trends. Due to circulation changes and presence of Western Ghats over the region majorly even though the studied precipitation indices showed increasing trends the distribution of rainfall over the study region plays major role due to which there exists heaviest rainfall area as well as rain shadow region in study area. Out of nine precipitation indices 7 indices showed significantly positive (increasing) trend whereas only two indices showed positive trend but non-significantly (Table 04). The investigation revealed that decadal trend in precipitation indices is consistently positive since 1982.

Satara District

Conclusion

In case of Satara district the indices such as number of cold days (FD15), number of cool nights (TN10p), cold spell duration indicator (CSDI) and Diurnal Temperature Range (DTR) have indicated significantly decreasing trends over the period of 1982-2022 whereas temperature indices like number of very hot days (SU38), annual count of number of tropical nights (TR20), number of hot nights (TR23) and number of warm nights (TN90p) have showed significant increasing trend over Satara district for the period of 1982-2022. Out of nine precipitation indices 7 indices showed significantly positive (increasing) trend whereas only two indices showed positive trend but nonsignificantly (Table 04). The investigation revealed that decadal trend in precipitation indices is consistently positive since 1982.

Acknowledgement

AcknowledgementI place my deep sense of gratitude to Department of Agriculture, Government of Maharashtra, India Meteorological Department, Pune and Department of Agricultural Meteorology, College of Agriculture, Pune for their valuable help, meticulous guidance and cooperation.

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