On the hotspot-Sun angular offset in urban thermal anisotropy

Abstract

The hotspot-sun angular offset (the spherical circular angle between hotspot and sun positions, termed ΔCA) in urban thermal anisotropy (UTA) plays a pivotal role in advancing remote sensing of urban climates. However, its diurnal and monthly variations across scenarios influenced by sun position, urban morphology, and thermal inertia remain largely unknown. Here we filled this knowledge gap based on 1049 UTA scenarios incorporating 12 urban surface models using both computer simulations and in-situ measurements. Our findings reveal ΔCA varies from 0° to 50° (predominantly 10° – 30°). Surface thermal inertia, urban morphology (i.e., building-street aspect ratio), and sun position collectively drive ΔCA variation, with their relative contributions exhibiting significant diurnal and monthly variabilities. In general, ΔCA is most pronounced around noon with relatively low thermal inertia (279–739 W·s^1/2·m⁻¹·K⁻¹) and low aspect ratio values (< 1.5), leading to high ΔCA values exceeding 30°. Monthly ΔCA variations shows a bimodal pattern, with peaks (ΔCA is ∼25°) in April and August and a trough (ΔCA is ∼15°) from May to July around the summer solstice when the solar zenith angle (SZA) is relatively small. Hourly ΔCA variations exhibits sinusoidal variation during daytime, also characterized by a noon trough around the summer solstice. Troughs in diurnal/annual cycles are associated with small SZA (< 20°) and high sunlit roof/ground temperatures that ensure hotspot-sun adjacency. Our results indicate pronounced ΔCAs under scenarios with lower thermal inertia, lower aspect ratio, and midday periods in May to August. Our findings could facilitate designing kernel-driven models for simulating UTA.