Weather patterns modulate urban thermal field variance index variability : evidence from long-term landsat time series for temperate municipalities in Germany

Abstract

Thermal remote sensing is a valuable tool for assessing Surface Urban Heat Islands (SUHI). To quantify the SUHI intensity, the Urban Thermal Field Variance Index (UTFVI) is increasingly used as a proxy for urban heat distribution, e.g., by public authorities in Germany. The UTFVI is an ordinal-scaled metric that shows the spatial variability of LST in relation to the average LST of an area of interest. Numerous scientific studies utilize the UTFVI for detecting spatiotemporal increases in SUHI intensities attributed to Land Use/Land Cover (LULC) changes, such as rapid urbanization. However, UTFVI analyses often rely on only a few time steps over extended periods, ignoring the effects of weather patterns on actual UTFVI distributions. To address this research gap, this study investigates the influence of weather conditions of varying durations (up to 21 days) on seasonal UTFVI distributions in four Hessian municipalities (Germany) with less than 300,000 inhabitants and only minor LULC changes in the urban area over time. The analysis is based on more than 100 Landsat 4–9 Level‑2 datasets spanning a 40-year period. To reduce rural influences, only urban areas are considered. Results reveal high seasonal and intra-seasonal variability in UTFVI. Statistical tests (Friedman and Wilcoxon) show significant differences of UTFVI distributions even within one summer (2023). Spearman’s rank correlation coefficients indicate that spatial patterns of the UTFVI are influenced by the temperature intensity of preceding weather phases: short-term warming leads to an increase of the UTFVI categories indicating high LST levels, while they are less frequent during prolonged warmth. The presented study provides for the first time a comprehensive analysis of long-term UTFVI developments that focusses on factors altering the UTFVI which have not been investigated so far. These new insights support a better understanding and a more distinct interpretation of the UTFVI for potential users.