The earth’s average temperature has been rising since the late 1970s (IPCC, 2021). This has lead to an increase in the intensity and frequency of heatwaves over many parts of the world including India resulting in a loss of lives and agricultural productivity. Recent heatwaves over the Northern Hemisphere, have set new temperature records in multiple locations around the world (Kornhuber et al., 2019; Leach et al., 2019; NOAA, 2018; Toreti et al., 2019; Vogel et al., 2019, 2020). For instance, the 2022 heatwave over India and Pakistan was an extreme weather event which resulted in the hottest March in India since 1901 (IMD, 2022). The hot season arrived unusually early in 2022 and affected a large part of northwest India and Pakistan. The heatwave along with a scanty rainfall (-63%) of normal (https://imdpune.gov.in/Climate_summary_Pre_Monsoon_MAM_2022.pdf) resulted in drought-like conditions over these regions. In the recent summer seasons, excess mortality, crop losses, wildfires, and damage to infrastructure were reported (Vogel et al., 2019), along with this, labour productivity is reduced during heatwaves (Dunne et al., 2013), especially in the fields of agriculture and construction, which can lead to high economic losses (Orlov et al., 2019). In the United States, extreme heatwaves are responsible for more deaths annually than cyclones/hurricanes, lightning, tornadoes, floods, and earthquakes combined (Luber & McGeehin, 2008). Many studies have investigated anthropogenic influences on the frequency and/or severity of heatwaves on a global scale (Kishore et al., 2022; Otto et al., 2012; Miralles et al., 2014; Quesada et al., 2012). Some studies also show a strong influence of north Pacific Ocean variability on Indian summer heatwaves (Hari et al., 2022). Priyankar et al, (2022) in their study have carried out a day-to-day analysis and concluded that cities are becoming hotter in response to the heat trapped near the earth’s surface and pointed out some causes like rapid urbanisation using concrete structures, decreasing green covers and energy intensity activities. The frequency of heat stress and heatwaves is majorly experienced (Priyankar et al., 2022) in urban cities like Delhi, Mumbai, Chennai and Kolkata during the summer seasons.
The definition of a heatwave is helpful in studying how the magnitude, duration and spatial extent of a heatwave have changed over time. A heatwave is a period of abnormally high temperatures that occurs during the summer season in India. As per the India Meteorological Department (IMD) when the maximum temperature crosses at least 40°C in the plains and 30℃ in the hilly regions and shows an increase of at least 4.5–6.5℃ (> 6.5) above normal temperature, then it can be classified as a heatwave (severe heatwave). With slight variations in the characteristics, many studies elaborated on heatwaves (Satyanarayana and Rao, 2020; Della-Marta et al., 2007; Srivastava et al., 2009; Ganguly et al., 2009). One of the earliest studies on heatwaves over India by Raghavan (1966) reported that the regions of West Bengal, Bihar plains, Madhya Pradesh, East and West Uttar Pradesh and Punjab are the most vulnerable due to the persistence and duration of heatwaves. A few studies have tried to understand the nature and impacts of heatwaves in India (Raghavan, 1966; Bedekar et al., 1974; Subbaramayya and Surya Rao, 1976; Pai et al., 2013; Kothawale et al., 2010), but they were all restricted to individual case studies and limitation of spatial sparsity of temperature observations. Pai et al. (2004) in their study have reported the regions of heatwave vulnerability in coastal Andhra Pradesh (AP), Orissa, East Madhya Pradesh and Chhattisgarh. Kothawale et al. (2010) reported that abnormal maximum temperatures during May often lead to heatwave conditions over India and exposure to dry winds during this period may result in casualties. It has been established in the literature that anomalies in atmospheric circulation, soil moisture, temperature, and sea surface temperatures are related to the formation and intensity of extreme heatwaves (Perkins, 2015; Alghamdi and Harrington, 2019; Zhang et al., 2019). Some studies have also analyzed another factor i.e, the weakening of the polar jet stream caused by global warming as a possible reason for an increased probability of the occurrence of stationary weather, resulting in heavy rainfalls or heatwaves (Broennimann et al. 2009; Coumou et al. 2015; Mann, 2019). The jet stream is one of the most important factors for determining weather in near mid-latitude regions such as Asia, Europe, and North America. The combination of global warming and population growth in already warm cities in India is one of the primary drivers of increased temperatures. The Urban Heat Island (UHI) also elevates temperatures within cities, which gets amplified during the heatwaves (García 2022). It occurs when cities replace the natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. Several studies have also attempted to estimate the frequency, intensity and causes of heatwaves in the current climate change scenario (Meehl and Tebaldi, 2004; IPCC et al., 2007; Ross et al., 2018; Rohini et al., 2019; Chen et al., 2019) due to the resulting socio-economic impacts (Rusticucci and Vargas, 2002; Poumadere et al., 2005; IPCC et al., 2007).
1.1. Heatwaves over India
1.1.1 Climatology of Heatwaves over India
Climatologically, in India the pre-monsoon months of May and June are the hottest months however, heatwaves are observed during the months of April to June and may extend up to the month of July. Heatwaves during this period may be enhanced by scanty rainfall and delayed southwest monsoons. The country’s hazard atlas launched by the IMD in 2021 does not include the month of March while providing the data for heatwaves over India (https://imdpune.gov.in/hazardatlas/heatnew.html) and has provided figures on heatwaves for April to July only. This is an indicator that the heatwaves which occurred in March 2022 were anomalous. Casualties due to heatwaves have also increased in recent years (De and Mukhopadhyay, 1998 and Mazdiyasni et al., 2017) in India due to the abnormally high temperatures from March to June over northern and north-western parts of the country (Kothawale et al., 2010). Twelve of India’s 16 warmest years have occurred since 2004. Figure 1 shows the mean Tmax averaged over the period of 1981–2010 along with the frequency of days with Tmax exceeding 40, 45 and 47°C. It is seen from the figure that large parts of India experienced Tmax > 40°C for over a period of 90 days (Fig. 1b). Northwest and central India experienced a significant number of days with Tmax > 45℃ (Fig. 1c). Also, Fig. 1d shows that over parts of northwest and north central India Tmax exceeded 47℃ for a few days.
In general, heatwaves occur most commonly during the summer months of March to June with their intensity and frequency peaking in the month of May. The most heatwave-prone areas known as the core heat zones (CHZ) are Rajasthan, Punjab, Haryana, Chhattisgarh, Delhi, west Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Odisha, Vidarbha in Maharashtra, parts of Gangetic West Bengal, coastal Andhra Pradesh and Telangana. Generally, heatwave spells in May are longer than the ones occurring in April and June mainly because of the absence of WD-induced rainfall over northern India. The regions in the extreme north namely; Himachal Pradesh, Kashmir, Ladakh, northeast and southwest Karnataka, Kerala and Goa are less prone to heatwaves.
1.1.2 Synoptic features during the 2022 heatwaves in India
Anti-cyclonic circulations are a part of Indian climatology as they form time to time, particularly during the change of seasons. Anticyclones cause hot and dry weather by sinking winds around high-pressure systems in the atmosphere. Figure 5 (a,b) is showing the incursion of dry air from north west India for 14 and 15th May 2022. Normally, an anticyclonic circulation forms over western India around mid-April due to which spring transits into summer. However, in 2022 the first instance of an anti-cyclonic circulation over India was observed during mid-March which strengthened further towards the end of March. There are episodes of light rainfall and thundershowers, typical for this time of the year, due to the active western disturbances (WDs) that bring rain from the Mediterranean over northern India. The presence of anticyclones over western parts of India in March and the absence of WDs triggered early and extreme heatwaves in 2022. Though northwest India experienced four WDs between March and April 2022 they were not strong enough to cause a significant change in the weather. There was also no significant pre-monsoon activity upto the 20th of April, which compounded the severity of the successive heatwave events.
1.1.3. Heat waves in India during 2022
India witnessed an unprecedented heatwave spell in 2022 that began in early March. The country reported 280 heatwave days (Fig. 2a & b) from March 11 to May 18, 2022, the highest in the last 12 years, this is almost double what the country experienced in 2012, the second-highest heatwave year in the past decade. According to IMD, in 2022, India recorded its hottest March and severe heatwave conditions were consistently reported over large parts of India since the beginning of the month. During this year the maximum temperature in west Rajasthan and Vidarbha in Maharashtra remained between 40oC and 45oC throughout the season (MAM). The month of March marks the seasonal transition from winter to summer and experienced two heatwave spells. There have been at least 26 heatwave days in the month of March (Table-1).
Table 1: Characteristics of Heat Wave/Severe Heatwave in March 2022
Met sub-divisions affected
|
Spell Periods
|
Maximum Temperature
range and Departure
|
1st spell of the season-11-21 March 2022
|
Saurashtra and Kutch
|
11-18 March
|
40-43°C with +5° to +9°C
|
Gujarat Region
|
14-18 March
|
40-43°C with +5° to +9°C
|
Konkan-Goa
|
11-13 March
|
40-43°C with +5° to +9°C
|
West Madhya Pradesh
|
16-22 March
|
40-43°C with +5° to +9°C
|
Vidarbha
|
17-18 March
|
40-43°C with +5° to +9°C
|
Lower ridges of the western Himalayan region - Jammu Division, Himachal Pradesh & Uttarakhand
|
16-21 March
|
32-37°C with +8° to +11°C
|
2nd spell of the season 26-31 March 2022
|
Saurashtra and Kutch
|
26-31 March
|
40-43°C with +5° to +9°C
|
South Haryana and Delhi
|
29-31 March
|
40-43°C with +5° to +9°C
|
Rajasthan
|
27-31 March
|
40-43°C with +5° to +9°C
|
Madhya Pradesh
|
27-31 March
|
40-43°C with +5° to +9°C
|
Southwest UP and East Uttar Pradesh
|
29-31 March
|
40-43°C with +5° to +9°C
|
Lower ridges of the western Himalayan region -Jammu
Division, HP & Uttarakhand
|
27-31 March
|
33-37°C with +6° to +9°C
|
The first spell lasted from March 11th to 19th and affected south Saurashtra, northern Konkan and Madhya Maharashtra, Rajasthan, Delhi, Punjab, Himachal Pradesh, Jammu, west Madhya Pradesh, West Uttar Pradesh and interior Odisha. In the second spell which commenced on March 27, severe heatwave conditions developed over various meteorological subdivisions i.e. west Rajasthan and adjoining areas of Gujarat along with west Madhya Pradesh. It further extended to east Rajasthan, east Madhya Pradesh, south Haryana, Delhi and Southern parts of Uttar Pradesh by 29th March and this spell continued for more than a week. During this period the Tmax was recorded in the range of 40-42oC over most of the stations in these states, with a few stations mainly over West Rajasthan and Madhya Pradesh recording a temperature in the range of 42-43oC. Besides this, persistent heatwave to severe heatwave conditions was observed over northern plains, central India and parts of the western Himalayan region with maximum temperature in the range of 25-33oC (33-39oC) at higher (lower) ridges which is about 6 to 9oC above normal. After a brief respite of a few days, the third spell of heatwave started on April 17. This was relatively shorter and only lasted till 20th April and was mainly restricted to regions of Delhi, Rajasthan, east Uttar Pradesh, and Bihar. The fourth spell of heatwave started over Kutch-Saurashtra and Rajasthan regions. Another spell can be seen in Fig. 4 from 11th -17th May 2022 over North West India to central India.
1.4 Why object-based Verification?
A lot of emphasis has been given to predicting heatwave conditions over India by using deterministic NWP models in recent times (Gouda et al., 2017; Singh et al., 2020, 2016). These models are able to predict the increase in the Tmax over the Indian land region with reasonable success. However, these models, even with all the improvements in the model physics and resolution, face a problem in accurately predicting extreme events at higher lead times (Zhang et al., 2019). This is mainly due to the uncertainties present in the initial conditions or the model parametrizations. This forecast may have spatial and temporal errors which will result in poorer verification results when the forecasts are verified on a grid-to-grid basis. These forecasts suffer through a ‘double penalty’ (Zingerle and Nurmi, 2008). For this reason, spatial verification methods are used which do not use a grid-to-grid comparison but compare the forecasted and observed objects/features. In the current study, we have carried out the spatial verification of Tmax forecasts obtained from the NCMRWF Unified Model (NCUM) using the method for object-based evaluation (MODE) which is a features-based/object-based verification approach. This study will determine ‘how well the forecasts capture the spatial patterns and assess the skill of forecasts in predicting localized and episodic phenomena of heatwaves as compared to the traditional grid-to-grid approaches. This technique will be used to obtain statistical information based on differences in the features of forecast and observed objects such as shape, size, coverage areas, displacement, orientation, and intensity. Using “object-based” techniques the objects are the Tmax areas, regions of heatwaves or extreme heatwaves that can be compared to one another in a meaningful way. More details about these methods are presented in section 2.2.
Spatial (Object-based) verification methods are able to provide information which may be helpful in quantifying the skill of an NWP system, particularly for discrete events like areas of heavy precipitation or extreme temperature (Davis et al. 2006b; 2009; Ebert and Gallus 2009; Johnson et al. 2013; Clark et al. 2014; Wolff et al. 2014; Bytheway and Kummerow 2015 and Mohandas and Singh 2015). These techniques have proved to be well suited to the verification of convection-allowing model (CAM) thunderstorm forecasts (e.g. Kain et al. 2013; Cai and Dumais 2015; Sobash et al. 2016; Potvin et al. 2019; Duda and Turner 2021, and Chen et al. 2022), including predictions of convection initiation (Burghardt et al. 2014; Burlingame et al. 2017), storm mode (Pinto et al. 2015; Johnson et al. 2020), and meso-cyclone occurrence (e.g. Clark et al. 2013, 2014; Skinner et al. 2016; Stratman and Brewster 2017). A recent study quantified the warn-on-forecast system (Skinner et al. 2018 and Guerra et al. 2022) accuracy by storm age using object-based verification. Some other studies also used object-based verification for atmospheric rivers (Nardi et al. 2018, DeFlorio et al. 2018; Shields et al. 2018, DeHaan et al. 2021). However, not many studies have considered spatial verification methods for the verification of heatwaves from global NWP models.
This manuscript is divided into the following sections: Data and model descriptions are presented in section 2. Results and discussions based on them are presented in section 3 followed by the salient conclusions derived from this study which are presented in section 4.