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1. Introduction Soil moisture (SM) is an essential component of the Earth system. It affects the variability of the coupled energy (latent and sensible heat fluxes) and water fluxes (runoff and evapotranspiration) by modifying the partitioning of water and energy across the land–atmosphere interface ( Seneviratne et al. 2010 ). The effects of SM on evapotranspiration also impact temperature variability and may intrigue persistent heatwaves ( Fischer et al. 2007 ; Hirschi et al. 2011
1. Introduction Soil moisture (SM) is an essential component of the Earth system. It affects the variability of the coupled energy (latent and sensible heat fluxes) and water fluxes (runoff and evapotranspiration) by modifying the partitioning of water and energy across the land–atmosphere interface ( Seneviratne et al. 2010 ). The effects of SM on evapotranspiration also impact temperature variability and may intrigue persistent heatwaves ( Fischer et al. 2007 ; Hirschi et al. 2011
–atmospheric feedbacks (e.g., Dirmeyer 1994 ; Hong and Kalnay 2000 ; Karl 1983 ; Myoung and Nielsen-Gammon 2010 ; Oglesby 1991 ; Oglesby and Erickson 1989 ; Rind 1982 ; Schubert et al. 2004 ; Zhuang et al. 2020 ), though most of these studies focus on local land–atmospheric feedbacks. Recently, numerical model experiments have shown remote land impacts on atmospheric circulation (e.g., Koster et al. 2016 ). Erfanian and Fu (2019) have evaluated the moisture budget over the Great Plains and found that
–atmospheric feedbacks (e.g., Dirmeyer 1994 ; Hong and Kalnay 2000 ; Karl 1983 ; Myoung and Nielsen-Gammon 2010 ; Oglesby 1991 ; Oglesby and Erickson 1989 ; Rind 1982 ; Schubert et al. 2004 ; Zhuang et al. 2020 ), though most of these studies focus on local land–atmospheric feedbacks. Recently, numerical model experiments have shown remote land impacts on atmospheric circulation (e.g., Koster et al. 2016 ). Erfanian and Fu (2019) have evaluated the moisture budget over the Great Plains and found that
1. Introduction Droughts that develop on time scales of several weeks to a few months, known as flash droughts, have been receiving increased attention by the scientific community (e.g., Svoboda et al. 2002 ; Otkin et al. 2018 ; Pendergrass et al. 2020 ). Characterized by a lack of precipitation, enhanced evapotranspiration, and a rapid decline in soil moisture, flash droughts can disrupt agricultural production, deplete water resources, and impair natural ecosystems and vegetation ( Smith
1. Introduction Droughts that develop on time scales of several weeks to a few months, known as flash droughts, have been receiving increased attention by the scientific community (e.g., Svoboda et al. 2002 ; Otkin et al. 2018 ; Pendergrass et al. 2020 ). Characterized by a lack of precipitation, enhanced evapotranspiration, and a rapid decline in soil moisture, flash droughts can disrupt agricultural production, deplete water resources, and impair natural ecosystems and vegetation ( Smith
and/or to describe a different type of drought. For example, the SPI is commonly used to represent meteorological drought, while the PDSI is commonly used to represent agricultural and hydrological drought ( Quiring 2009 ). Soil moisture, reflecting the water content of soil, can also be used to represent different types of drought. Previous research has shown that soil moisture can be used to describe meteorological drought conditions. For example, Sims et al. (2002) compared the SPI and PDSI
and/or to describe a different type of drought. For example, the SPI is commonly used to represent meteorological drought, while the PDSI is commonly used to represent agricultural and hydrological drought ( Quiring 2009 ). Soil moisture, reflecting the water content of soil, can also be used to represent different types of drought. Previous research has shown that soil moisture can be used to describe meteorological drought conditions. For example, Sims et al. (2002) compared the SPI and PDSI
, agricultural drought refers to a period of time with a deficit in soil moisture (SM), which could consequently result in crop failure. With the purpose of monitoring agricultural drought, several indices have been proposed based on a combination of temperature, precipitation, evapotranspiration (ET), and soil moisture ( Ahmadalipour et al. 2017 ; Vicente-Serrano et al. 2010 ; Mu et al. 2013 ; Almamalachy et al. 2020 ). Satellite data imagery for drought monitoring can be applied for either gathering
, agricultural drought refers to a period of time with a deficit in soil moisture (SM), which could consequently result in crop failure. With the purpose of monitoring agricultural drought, several indices have been proposed based on a combination of temperature, precipitation, evapotranspiration (ET), and soil moisture ( Ahmadalipour et al. 2017 ; Vicente-Serrano et al. 2010 ; Mu et al. 2013 ; Almamalachy et al. 2020 ). Satellite data imagery for drought monitoring can be applied for either gathering
1. Introduction Useful predictability of deterministic weather forecasts is usually no more than 2 weeks, limited by the sensitivity to the atmospheric initial state, while longer memory from ocean heat content plays a dominant role in the climate predictability on seasonal and longer time scales (e.g., Lorenz 1963 , 1975 ; Shukla 1985 ; Lorenz 1993 ). There is a gap between the two time scales of weather and climate predictions, where inertia in the land surface, such as soil moisture
1. Introduction Useful predictability of deterministic weather forecasts is usually no more than 2 weeks, limited by the sensitivity to the atmospheric initial state, while longer memory from ocean heat content plays a dominant role in the climate predictability on seasonal and longer time scales (e.g., Lorenz 1963 , 1975 ; Shukla 1985 ; Lorenz 1993 ). There is a gap between the two time scales of weather and climate predictions, where inertia in the land surface, such as soil moisture
. 2019 ). Regional land surface states (e.g., soil moisture, snow cover, vegetation properties, etc.) also contribute to drought severity and development (e.g., Higgins et al. 1998 ; Schubert et al. 2007 ; Koster et al. 2017 ). In particular, positive feedbacks between land and atmosphere can exacerbate or prolong dry anomalies, playing a role in maintaining droughts (e.g., Durre et al. 2000 ; Fischer et al. 2007 ; Koster et al. 2009 ; Kam et al. 2014 ; Dirmeyer et al. 2015 ; Fernando et al
. 2019 ). Regional land surface states (e.g., soil moisture, snow cover, vegetation properties, etc.) also contribute to drought severity and development (e.g., Higgins et al. 1998 ; Schubert et al. 2007 ; Koster et al. 2017 ). In particular, positive feedbacks between land and atmosphere can exacerbate or prolong dry anomalies, playing a role in maintaining droughts (e.g., Durre et al. 2000 ; Fischer et al. 2007 ; Koster et al. 2009 ; Kam et al. 2014 ; Dirmeyer et al. 2015 ; Fernando et al
soil moisture (SM), and/or runoff deficits, usually for 6 months or longer ( Svoboda et al. 2002 ). Flash droughts have much shorter durations—typically a few weeks. Furthermore, while conventional droughts develop slowly, a key feature of flash droughts is their rapid onset and intensification ( Pendergrass et al. 2020 ). Mo and Lettenmaier (2015 , 2016) studied flash droughts over the United States, and classified them into two categories based on their forcings: heat wave flash drought ( Mo
soil moisture (SM), and/or runoff deficits, usually for 6 months or longer ( Svoboda et al. 2002 ). Flash droughts have much shorter durations—typically a few weeks. Furthermore, while conventional droughts develop slowly, a key feature of flash droughts is their rapid onset and intensification ( Pendergrass et al. 2020 ). Mo and Lettenmaier (2015 , 2016) studied flash droughts over the United States, and classified them into two categories based on their forcings: heat wave flash drought ( Mo
DO&T refers to a change over time it brings in higher-frequency components of the climate system that may be highly random in time and difficult to predict. In contrast, drought, as measured by soil moisture, integrates precipitation less evapotranspiration (ET) plus runoff ( R ) over time and, hence, records lower-frequency components of the climate system and can be quite closely related to SST variations ( Schubert et al. 2004a , b ; Seager et al. 2005 ; Herweijer et al. 2006 ; Seager and
DO&T refers to a change over time it brings in higher-frequency components of the climate system that may be highly random in time and difficult to predict. In contrast, drought, as measured by soil moisture, integrates precipitation less evapotranspiration (ET) plus runoff ( R ) over time and, hence, records lower-frequency components of the climate system and can be quite closely related to SST variations ( Schubert et al. 2004a , b ; Seager et al. 2005 ; Herweijer et al. 2006 ; Seager and
