Understanding the influence of soil moisture on heatwave characteristics in the contiguous United States

In the context of global climate change, heatwaves are becoming increasingly significant because of the adverse impacts on human health and ecosystems. However, the quantification of heatwaves relies on different temperature metrics, and little is known about how the different types of heatwaves are affected by soil moisture. Using a set of observational datasets during the period 1981–2020, this study investigates the characteristics of warm-season heatwaves over the contiguous United States (CONUS) derived from three different temperature metrics (temperature, wet-bulb temperature, and equivalent temperature), and examines how different types of heatwaves are associated with soil moisture. Increasing trends of all types of heatwaves are observed in most parts of CONUS except for the central US, posing potential risks to human health. Due to limited evaporative cooling over dry soil, there is a substantial negative relationship between soil moisture and temperature-only heatwaves across the CONUS. Meanwhile, in some regions of the western and central CONUS, there is an evident positive relationship between soil moisture and humidity-included heatwaves, which represent the combined effects of temperature and humidity. The event-based analysis in Nebraska emphasizes that temperature-only heatwaves occur over relatively dry soil conditions, while humid heatwaves tend to occur over somewhat wet soil. Our results highlight the importance of considering different types of heatwaves and their relationship with soil moisture from the land-atmosphere coupling perspective, offering valuable insights for local and regional climate planning and mitigation.


Introduction
The prolonged periods of excessively hot weather hold immense importance in climate science and public health due to their potential to disrupt ecosystems, strain energy resources, and pose severe risks to human health and well-being (Wouters et al 2022).Globally, the frequency and intensity of heatwaves have shown a concerning upward trend during the past several decades.This raises worries that the broader impacts of climate change may worsen these extreme events, emphasizing the need to understand the underlying mechanisms of heatwave development and factors that contribute to heatwaves (i.e.Teuling et al 2010, Miralles et al 2012, Perkins and Alexander 2013, Dirmeyer et al 2014).A multivariate approach, such as the heatwave frequency and duration curve, has been employed to address extreme heat events by simultaneously considering both the intensity and duration of these heatwaves (Mazdiyasni et al 2019), but most of them are focused on traditional heatwave definitions.Heat stress, which is more commonly considered in public health studies, is a combination of temperature and humidity and is often overlooked in such assessments.
Heatwaves are often caused by the persistence of high-pressure systems, which trap warm air in a region for an extended period.The atmosphere prevents the vertical movement of air, inhibiting the dissipation of heat and leading to the accumulation of high temperatures at the surface.This stagnant atmospheric pattern can result in extended periods of extreme heat contributing to the development of conventional heatwaves.Under similar synoptic conditions, humid heatwaves often form due to the advection of hot and humid air from the tropical regions (Russo et al 2017), or moisture transport from the land surface through crops and irrigation (Li et al 2020, Souri et al 2020).Furthermore, unlike their conventional 'dry' counterparts, which are primarily characterized by elevated temperatures, humid heatwaves combine high temperatures with elevated humidity levels.This combination significantly impacts human and animal thermoregulatory mechanisms, posing substantial challenges to human health and agriculture productivity, demanding a focus to understand and address the challenges posed by this type of extreme heat events (Russo et al 2017, Schoof et al 2017, Wouters et al 2022).
Soil moisture plays a pivotal role in shaping local and regional climate conditions (Miralles et al 2012).Its influence extends beyond the ground, impacting temperature and humidity in the lower atmosphere, making it a fundamental element to scrutinize in the context of understanding the mechanism of heatwaves.Previous studies have highlighted the role of antecedent soil moisture deficits in the occurrence of heatwaves across different regions of the world (e.g.Kala et al 2015, Herold et al 2016, Hirsch et al 2019, Benson and Dirmeyer 2021).Those soil moisture deficits are often a result of accumulated rainfall deficits in the preceding months, which leads to a reduction in soil moisture and subsequently limits the process of evaporative cooling.Under certain atmospheric conditions, the reduction in evaporative cooling, which normally dissipates heat from the land surface, can worsen the intensity and longevity of heatwaves (Barriopedro et al 2023).The relationship between soil moisture and temperature can be further complicated by different landscapes and vegetation conditions (Teuling et al 2010, Anderson andVivoni 2016).Nonetheless, most of those studies are focused on conventional heatwaves.As high soil moisture can decrease temperature but increase humidity through enhanced evaporation, the competing effects of soil moisture on humid heatwaves are largely unknown.Incorporating humidity-temperature indices (e.g.wet-bulb temperature or equivalent temperature) would allow us to have a more comprehensive understanding of the mechanism of the combined effects on heatwaves.
This study aims to address critical gaps in characterizing different types of heatwaves and understanding their mechanism in the context of land-atmosphere interactions.Building upon previous insights linking soil moisture to heatwaves (e.g.Herold et al 2016), we examine heatwaves that are derived from three temperature variables: temperature, wet-bulb temperature, and equivalent temperatures, and examine the statistical relationship between soil moisture and heatwaves (or different temperature variables) at different temporal and spatial scales.In addition, an event-focused case study in Nebraska is conducted to further demonstrate the dynamics between soil moisture and heatwave development.

Data
We use daily 2 m air temperature, precipitation, and humidity data from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) dataset, and surface soil moisture (SM surf ) data (top 10 cm) from the Global Land Evaporation Amsterdam Model (GLEAM) dataset.The PRISM dataset provides precipitation and temperature variables at a spatial resolution of 4 km over the CONUS (Daly et al 2021).It has been widely used in studies related to US hydroclimatic extremes and climate variability (e.g.Rupp et al 2022).On the other hand, the GLEAM dataset is a global resource that provides evapotranspiration and soil moisture at a spatial resolution of ∼28 km (Martens et al 2017).This dataset has been used in large-scale hydrological applications and investigations of land-atmosphere interactions (e.g.Schumacher et al 2019).Due to the different spatial resolutions of the datasets, the two datasets are regridded to a 1/8th degree spatial resolution before being used for further calculation of heatwave metrics and statistical analysis over the CONUS.
Additionally, to avoid bias from the choice of datasets, we also use atmospheric forcings and soil moisture provided by the National Climate Assessment-Land Data Assimilation System (NCA-LDAS) (Jasinski et al 2019).NCA-LDAS is an integrated terrestrial water analysis system that assimilates multiple remote sensing measurements of the terrestrial water cycle (Kumar et al 2019).

Methods
In the heatwave assessments, we incorporate wetbulb temperature (T wet−bulb ) and equivalent temperature (T equivalent ) in addition to the daily maximum temperature (T max ).This strategy is based on the understanding that heatwave complexity may not be fully captured by temperature alone because humidity significantly influences human discomfort, morbidity, and mortality during the heatwaves (Russo et al 2017).
The T wet−bulb is defined as the temperature of an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all laten heat being supplied by the parcel (American Meteorological Society (AMS) 2024).The daily T wet−bulb was calculated based on the daily mean temperature (T mean , • C) and relative humidity (RH, %) (Stull 2011).
RH was derived from the temperature and dewpoint of PRISM data.The T equivalent represents the temperature that a moist air parcel would reach if all its water vapor were condensed out under constant pressure and adiabatic conditions, meaning all released latent heat from condensation is utilized to warm the air (Camuffo 2014).The daily T equivalent was calculated using the approach by Schoof et al (2017) where L v is the latent heat of vaporization (J kg −1 ); q is specific humidity (kg kg −1 ), which is calculated based on temperature and dewpoint and atmospheric pressure; and C p is the specific heat of air at constant pressure (1005 J kg −1 K −1 ).Both T wet−bulb and T equivalent are moist heat metrics and have been widely used in humid heatwave assessment (Schoof et al 2017, Yu et al 2021).T equivalent is considered the linear combination of latent and sensible heat within an air parcel (Schoof et al 2017) and is similar 'heat index' (Matthews et al 2022).T wet−bulb measures the lowest temperature by adiabatic evaporation of water into the air and is often used in physiological studies.For instance, 35 • C T wet−bulb serves as a critical threshold for human survival (Lu and Romps 2023).Due to the different processes considered, T wet−bulb and T equivalent have different magnitudes and different sensitivities to temperature and humidity change.Heatwave can be defined using a relative threshold (e.g., the 90th percentile) or an absolute threshold (e.g.35 • C T wet−bulb ).To keep the definition consistent among the different temperature variables, heatwaves are identified as temperature (T max , T wet−bulb , or T equivalent ) exceeding the 90th percentile in the warm seasons from May to September (MJJAS) in 1981-2020.The 90th percentile of T wet−bulb for CONUS region during the study period ranges from −0.72 • C to 24.47 • C. Likewise, the 90th percentile of T equivalent ranges from 14.11 • C to 77.13 • C.Although the 90th percentile of T wet−bulb is considerably lower than the commonly defined survival threshold (35 • C), such a relative threshold defines the extremes of temperature distributions, and commonly used heatwave studies (e.g., Monteiro andCaballero 2019, Yu et al 2021).
A heatwave event is defined as a period of at least 3 d with temperatures exceeding the 90th percentile.For simplicity, 'conventional heatwave' refers to

Heatwave metrics Definition
Heatwave magnitude The mean temperature across all days with temperature higher than the 90th percentile.

Heatwave amplitude
The highest temperature among all the days with temperature higher than the 90th percentile.

Heatwave frequency
The number of days with temperature higher than the 90th percentile.

Heatwave duration
The length of the longest heatwave event.

Heatwave number
Count of unique heatwave events that last 3 d or more.
the heatwave derived from T max , and 'humid heatwave' refers to the heatwave derived from T wet−bulb , or T equivalent .Following the methodology of Herold et al (2016), we characterize heatwave using five metrics: magnitude, amplitude, frequency, duration, and number.Table 1 shows the definition of different heatwave characteristics.

Temperatures
The warm-season climatology of four temperature variables including T mean , T max , T wet−bulb , and T equivalent is presented in figure 1.According to the 40 year climatology, the mean temperature is ranging from −15.40

Heatwave characteristics
The analysis of four heatwave metrics (magnitude, amplitude, duration, and number) derived from three temperature variables (T max 90, T wet−bulb 90, and T equivalent 90) reveals distinct patterns in their climatology (figure 2).Because heatwave frequency is based on the 90th percentile, the total number of days with temperature higher than the 90th percentile would be the same across all grid points.Therefore, heatwave frequency is not included here.The intensity-related heatwave elements (magnitude and amplitude) follow the climatological patterns of corresponding temperature variables.Intense heatwaves are experienced in the south and southeast when considering temperature only, and in the east and southeast when considering both temperature and humidity.For the duration of heatwaves, all three types of heatwaves show similar spatial patterns-prolonged heatwaves (up to 8 d) are mostly found in the southwest and southeast, and relatively short heatwaves (around 5 d) are found in the west coast, Northern Plains, northeast and Florida.Among different types of heatwaves, humid heatwave events show longer duration over much larger areas compared to conventional heatwaves.For the number of heatwave events during the investigated period, all types of heatwave events show a relatively higher occurrence (over two events per year) over the west, Central Plains, and the Midwest, except for conventional heatwaves showing a lower occurrence in the northern Plains.These comparisons suggest distinctive characteristics of heatwaves if using different temperature variables.The trends of the heatwave metrics over the CONUS from 1981 to 2020 are shown in figure 3.
When heatwaves are solely based on T max , the western US has witnessed an increasing trend in all heatwave metrics, while some regions in the central US exhibit a noticeable decline trend in heatwaves occurrences (figures 3(a)-(e)).When heatwaves are quantified using both temperature and humidity (e.g.T wet−bulb 90 and T equivalnet 90), there is a significant increase in heatwaves intensity and longevity in large areas of the US, including southwest, south and east, suggesting a growing risk of humid heatwaves in those regions, especially considering there is high population in those areas discussed above.

Relation between soil moisture and heatwaves
Previous studies have suggested possible roles of soil moisture in heatwave development through landatmosphere interactions.Here we investigate the statistical relationship between soil moisture and temperature/heatwave metrics at a seasonal scale.Figure 4 shows the correlation between summertime (June-August) heatwave metrics and SM surf .First, there is a significant negative relationship between T max -based heatwave and SM surf across the central and eastern CONUS (figures 4(a)-(e)).This suggests that areas experiencing lower SM surf levels tend to have more intense and more frequent heatwaves.If interpreting this as the impacts of soil moisture on heatwave, during temperature-only heatwaves, soil with lower moisture content and limited evaporation can heat up more readily, leading to increased sensible heat flux from the surface to the atmosphere.This can potentially intensify the warming of the lower atmosphere, contributing to the elevated temperatures characteristics of such heatwave events.In contrast, the correlation between the T wet−bulb 90 heatwave metrics and SM surf reveals a slightly different picture (figures 4(f)-(j)).As there are much less significant negative correlations compared to conventional heatwaves, certain areas in the eastern US still exhibit this relationship.Meanwhile, there is a positive relationship in the southwestern US, particularly in California (figure 4(j)).This positive correlation implies that in regions with positive soil moisture anomalies, T wet−bulb 90 tends to increase.Furthermore, when considering the relationship between the heatwave metrics based on T equivalnet 90 and SM surf , more areas of the southern and central US display a positive correlation, suggesting that areas experiencing higher T equivalnet 90 during heatwaves tend to have higher SM surf levels.Meanwhile, negative correlation is still found in the Pennsylvania and Ohio regions (figures 4(k)-(o)).The coupling of soil moisture and temperature involves a shift in surface energy partitioning between latent heat and sensible heat fluxes, leading to temperature increases when there are enhanced sensible fluxes over the dry soil (Liu et al 2020).In regions with wetter soil, the higher moisture content boosts evaporation, leading to latent heating.Although enhanced latent heat flux reduces air temperature, this excess evaporation would increase air humidity, potentially contributing to enhanced humid heatwave conditions (Matthews et al 2022).
It should be noted that the correlation analysis of the heatwave and soil moisture during the same season only reveals the mutual relationship between the two variables but does not necessarily suggest their causality.In other words, a negative correlation between soil moisture and heatwave also can be explained by the enhanced evaporation during heatwaves drying out the soil.Therefore, here we examine the lag correlation between spring soil moisture and following summer heatwaves to further explore how soil moisture potentially influences heatwave development in the following season (figure 5).Overall, the inter-seasonal relationship is generally less significant compared to the same-season analyses (figure 4).This suggests a relatively short soil moisture memory or lack of influence of spring soil moisture on summer heatwaves in some regions, such as large areas of the southwestern and northern US.Nonetheless, the general patterns of positive and negative relationships continue.In limited regions of the southern, southwestern and central US, there is positive correlation between spring soil moisture and summer humid heatwaves, indicating the potential of longer soil moisture memory to enhance or prolong summer humid heatwave in those areas.This could be attributed to the moisture-retaining capacity of the soil, which can contribute to elevated humidity levels during the summer months potentially worsening humid heatwave conditions.For conventional heatwaves, areas with negative correlations (such as the southern and mountainous US) indicate that lower spring soil moisture levels may have a positive effect on summer heatwaves.The lag relationship between spring soil moisture and summer heatwaves suggests that soil moisture conditions in the preceding season  can have a visible impact on the characteristics of summer heatwaves (Herold et al 2016).
To further confirm the influence of soil moisture on heatwaves, we examine the relationship between soil moisture and temperature (figure 6).
Temperature alone (T max ) is significantly negatively correlated with soil moisture in most areas of the US during the summer (figure 6(a)).The significant correlation is absent in large areas if including the seasonal lag between T max and SM surf .This is also confirmed by the strong correlation between the T max and SM surf for the same month (e.g. in June, figure S1), and relationship gets weaker when we consider the SM surf from the previous month (e.g.soil moisture in May and temperature in June, figure S2).Compared to the seasonal lag, more areas (especially in the western US) show significant relationship between antecedent surface soil moisture and temperature with a 1 month lag, suggesting that the 'memory' of soil moisture in those regions may last for over a month and cannot exert impacts on temperature at a seasonal scale.
For T wet−bulb and T equivalent (figures 6(b), (c), (e) and (f)), we notice that the previously observed negative relationship becomes less pronounced, and instead, a positive relationship emerges.Nevertheless, similar to the heatwave metrics, both the relationships are less significant in many areas of the US compared to T max .Because T wet−bulb and T equivalent are the combination of temperature and humidity, we also examine the relationship between soil moisture and humidity (figure S3).Overall, high soil moisture corresponds to high relative humidity or specific humidity during the same season.Such significant positive relationship is evident across the US.This does not necessarily suggest the causality from wet soil to high humidity, because humid conditions (e.g.rainy days) also lead to high soil moisture.However, the positive relationship between spring soil moisture and summer humidity in some regions over the Rockies, Great Plains, and southeast may suggest the influence of positive soil moisture anomalies would exert influences on the humidity conditions in following summer, either through the direct impacts on water vapor by enhanced evaporation or indirect processes by enhanced precipitation.

Heatwave cases in Nebraska
The previous section presents the statistical relationship between soil moisture and heatwave metrics from a climatological perspective.Here we examine soil moisture conditions prior to and during the heatwaves in Nebraska to further explore the influence of soil moisture on heatwave from the event-based perspective.We identify individual state-wide heatwave events for each of the three temperature variables: T max 90 (14 events), T wet−bulb 90 (25 events), and T equivalent 90 (20 events).A state-wide heatwave event is defined as a consecutive period of three or more days, during which temperatures over 75% of grid cells in the state exceed the 90th percentile.Among the three types of heatwave events, majority of the identified events are independent because different temperature variables are used.The averaged values of T max and SM surf anomalies during heatwave events (figure 7) indicate that T max -based heatwaves are associated with relatively dry soil conditions (−0.035 m 3 m −3 below normal).One week before the heatwaves, there has been already considerable dry anomalies (−0.021 m 3 m −3 below normal) in central and eastern Nebraska while temperature warm anomalies are less evident, indicating that dry soil can potentially contribute to the development of conventional heatwaves.
Conversely, the T wet−bulb -based humid heatwave events show a different evolution in soil moisture dynamics.During the events, soil moisture is slightly drier than normal.However, a week before the heatwave, there is higher than normal soil moisture in northern Nebraska, suggesting wet soil may help the development of humid heatwaves through increased humidity.Moreover, for the-based humid heatwave events, there are wet conditions (+0.005 m 3 m −3 above normal) throughout the events, indicating there is a stronger need for humidity to considerably elevate T equivalent to satisfy the requirement of heatwave identification.This also agrees with our results in figures 1 and 2 that T equivalent is more dominantly contributed by humidity.
Overall, the relationships between soil moisture and heatwave in Nebraska from the event-based analysis agree with the results from statistical analysis in the previous section, and further demonstrate that soil moisture conditions do not uniformly affect different types of heatwaves.

Discussion
In this study, we examine the characteristics of heatwaves across the CONUS using three different temperature variables and explore the role of soil moisture in different types of heatwaves.The examination of heatwave characteristics based on different temperature variables unveiled intriguing patterns.The decreasing trend in conventional heatwaves over the central US is consistent with the observed cooling of summer temperature extremes (Mueller et al 2016), which can be associated with the Pacific decadal mode (Wang et al 2009), increasing anthropogenic aerosol emissions (Banerjee et al 2017), and intensified agriculture (Mueller et al 2016).The inclusion of humidity in temperature calculations led to an increase in humid heatwaves over broader areas, suggesting humidity played a significant role in the humid heatwaves (Russo et al 2017).An overall negative relationship is found between soil moisture and conventional (temperature-only) heatwaves, while a positive relationship between soil moisture and humid heatwaves.
Although the GLEAM-based soil moisture product has been widely used in studies of climate extremes and land-atmosphere interactions (e.g.Miralles et al 2014, Lian et al 2020, Joy and Satheesan 2024), we acknowledge the potential limitations of GLEAM-based soil moisture.For instance, Martens et al (2017) indicate the possibility of unrealistic high-frequency fluctuations in GLEAM soil moisture estimates due to representativeness mismatches or noise in the observed satellite signal.Gevaert et al (2018) find that soil moisture-temperature coupling strength in GLEAM is relatively low compared to other gridded analysis datasets.Therefore, our analysis is further conducted with using the NCA-LDAS dataset from 1981 to 2015 (figure S4), emphasizing the robustness of our findings.
We specifically emphasize the significance of humid heatwaves, which are often inadequately considered in heatwave assessments despite their substantial adverse effects on human well-being.High humidity levels during heatwaves impede the body's ability to cool down through sweat evaporation, increasing the risk of heat-related illnesses (Russo et al 2017, Wouters et al 2022).These conditions can be life-threatening, especially for vulnerable populations like the elderly, children, and individuals with preexisting health conditions.Moreover, ignoring the role of humidity and soil moisture may underestimate the severity of heatwaves, potentially leading to inadequate preparedness and response measures to heatwave risks.For instance, irrigation has been frequently proposed as a climate mitigation strategy due to its agricultural benefits and cooling effect (Seneviratne et al 2018).As irrigation could also increase heat stress through elevating humidity, its impacts on humid heatwaves should not be overlooked (Krakauer et al 2020).
Additionally, this study only presents the potential link between soil moisture, temperature, and heatwave development.However, it is important to note that the impacts of soil moisture on heatwave are closely regulated by land-atmosphere coupling, which can be determined by the regime, whether it is atmosphere-driven or land-driven (Hirsch et al 2019), and the soil moisture breakpoint where the regime shift may occur (Benson and Dirmeyer 2021).Therefore, further investigation is needed to understand the spatio-temporal complexity of heatwave development within a land-atmosphere coupling framework needs to be further investigated.

Conclusion
Due to the competing impacts of soil moisture on temperature and humidity, it is important to investigate the role of soil moisture in conventional heatwaves and humid heatwaves.In this study, warmseason heatwaves are quantified based on three temperature variables (temperature, wet-bulb temperature, and equivalent temperature).The 40 year climatology of heatwave metrics (magnitude, amplitude, duration, and number) reveals distinct patterns over the CONUS among the different temperature variables.Overall, there are increasing trends in heatwaves across CONUS during the past 40 years, except for the conventional heatwaves that only consider temperature alone showing an evident decreasing trend in intensity and longevity over the central US.Based on multiple observation-based datasets, we develop the relationship between soil moisture and different types of heatwaves (temperature) at different temporal and spatial scales.The negative correlation between conventional heatwaves and soil moisture, especially across the central and eastern US, highlights the potential of soil moisture deficits to intensify temperature and conventional heatwaves.Conversely, a positive correlation observed in southern and central US for humid heatwaves indicates the intricate interplay between soil moisture and humidity, influencing the severity and duration of humid heatwave events.Comparing T wet−bulb and T equivalnet -based humid heatwaves, there is stronger positive relationship between soil moisture and T equivalnet -based heatwaves, suggesting that T equivalnet and T equivalnet -based humid heatwaves are more dominantly contributed by humidity instead of temperature.Our event-based analysis for the heatwaves in Nebraska further demonstrates the relationship between soil moisture and temperatures during different types of heatwaves.Our findings highlight the importance of incorporating humidity in heatwave assessments and the potential role of soil moisture in heatwaves.A process-focused analysis is needed in future work to further investigate the physical mechanism of soil moisture-atmosphere interactions and the synoptic-scale atmospheric processes in conventional and humid heatwave development.

Figure 1 .
Figure 1.Climatology of different temperature variables (a): daily average temperature Tmean; (b): daily maximum temperature Tmax; (c): daily wet-bulb temperature T wet−bulb ; and (d): daily equivalent temperature T equivalent ) in • C during the warm season (May-September) in 1981-2020.

Figure 4 .
Figure 4. Correlation coefficient between summer heatwave metrics (magnitude, amplitude, frequency, duration, and number) and soil moisture.Black contours indicate areas with correlations that are statistically significant (p ⩽ 0.05).

Figure 5 .
Figure 5. Correlation coefficient between summer heatwave metrics (magnitude, amplitude, frequency, duration, and number) and spring soil moisture.Black contours indicate areas with correlations that are statistically significant (p ⩽ 0.05).

Figure 7 .
Figure 7. Temperature and soil moisture anomalies during state-wide heatwaves (lower map) and the preceding week (upper map) in Nebraska.Over 40 years, the mean number of state-wide heatwave events lasting 3 d or more is 14, 25, and 20 for Tmax90, T wet−bulb 90 and T equivalent 90, respectively.

American
Meteorological Society (AMS) 2024 Glossary of Meteorology (available at: https://glossary.ametsoc.org/wiki/Equivalent_temperature) (Accessed May 2024) Anderson C A and Vivoni E R 2016 Impact of land surface states within the flux footprint on daytime land-atmosphere coupling in two semiarid ecosystems of the Southwestern US Water Resour.Res.52 4785-800 Banerjee A, Polvani L M and Fyfe J C 2017 The United States 'warming hole': quantifying the forced aerosol response given large internal variability Geophys.Res.Lett.44 1928-37 Barriopedro D, García-Herrera R, Ordóñez C, Miralles D G and Salcedo-Sanz S 2023 Heat waves: physical understanding and scientific challenges Rev. Geophys.61 e2022RG000780 Benson D O and Dirmeyer P A 2021 Characterizing the relationship between temperature and soil moisture extremes and their role in the exacerbation of heat waves over the contiguous United States J. Clim.34 2175-87 Bieri C A, Dominguez F and Lawrence D M 2021 Impacts of large-scale soil moisture anomalies on the hydroclimate of southeastern South America J. Hydrometerol.22 657-69 Camuffo D 2014 Parameters to describe air masses and vertical motions Microclimate for Cultural Heritage 2nd edn, ed D Camuffo (Elsevier) pp 119-30 Daly C et al 2021 Challenges in observation-based mapping of daily precipitation across the conterminous United States J. Atmos.Ocean.Technol.38 1979-92 Dirmeyer P A, Wang Z, Mbuh M J and Norton H E 2014 Intensified land surface control on boundary layer growth in a changing climate Geophys.Res.Lett.41 1290-4 Gevaert A I, Miralles D G, de Jeu R A, Schellekens J and Dolman A J 2018 Soil moisture-temperature coupling in a set of land surface models J. Geophys.Res.Atmos.123 1481-98 Herold N, Kala J and Alexander L V 2016 The influence of soil moisture deficits on Australian heatwaves Environ.Res.Lett.11 064003 Hirsch A L et al 2019 Amplification of Australian heatwaves via local land-atmosphere coupling J. Geophys.Res.Atmos.124 13625-47 Jasinski M F et al 2019 NCA-LDAS: overview and analysis of hydrologic trends for the national climate assessment J. Hydrometerol.20 1595-617