Ground Urban Heat Island: Strengthening the Connection Between Spaceborne Thermal Observations and Urban Heat Risk Management

As urbanization progresses under a changing climate, urban populations face increasing threats from chronically higher heat exposures and more frequent extreme heat events. Understanding the complex urban thermal exposure patterns becomes crucial for effective heat risk management. The spatial advantage of satellite thermal observations positions surface urban heat islands (SUHI) as a primary measure for such applications at the city scale. However, satellite-inherent biases pose considerable uncertainties. To improve the representation of human-relevant heat exposure, this study proposes a simple but effective satellite-based measure- ground urban heat island (GUHI), focusing solely on radiant temperatures from urban ground elements. Leveraging ECOSTRESS land surface temperature product and radiation-based statistical downscaling, diurnally representative GUHIs were evaluated over NYC. The findings reveal that overall GUHI is consistently warmer than SUHI diurnally. However, GUHI exhibits complex spatial contrasts with SUHI, primarily influenced by vegetation coverage. Various indicators associated with urban structures and materials were examined, showing important but dissimilar roles in shaping the spatial dynamics of GUHI and SUHI. This study highlights the value of satellite thermal observations compared to air temperature while addressing uncertainties in widely adopted practices of using them. By improving the depiction of human-related urban heat patterns from Earth observations, this research offers valuable insight and more reliable measures to address the urgent requirements for urban heat risk management globally. As cities grow under a changing climate, people in urban areas face more heat, putting them at risk. To protect them, it's crucial to understand where and when heat is most intense in cities. Direct satellite measurements help show surface urban heat islands (SUHI) that are widely used for mapping hot spots to identify higher heat risks and vulnerable communities. However, this method can be biased. This study suggests a new way to measure heat in cities using satellites that only consider the heat coming from the ground in urban areas, called ground urban heat islands (GUHI). By using data from the ECOSTRESS aboard the International Space Station and statistical approaches, we looked at how hot urban ground was throughout the day in New York City. We found that overall, GUHI was consistently hotter than SUHI throughout the day. However, heat patterns depicted by GUHI were influenced by urban surface properties in complex ways and were often different from SUHI at the local scale. This research shows that using satellite data to measure heat can give us better insights than air temperature for spatial applications, and this study offers more reliable ways to support heat risk management in cities worldwide. Ground urban heat island (GUHI) is proposed to improve satellite thermal observations applied for heat risk management GUHIs are warmer and show complex spatial contrasts with surface urban heat islands Urban materials and structures contribute differently to diurnal dynamics of GUHIs