Seasonal Variability in Surface Radiation and Energy Budgets Across Urban, Grass, and Cropland Sites in Nanjing, China

The rapid pace of urbanization dramatically alters surface radiation and energy budgets. These changes cause the urban heat island (UHI) effect, which refers to the microclimatic phenomenon that cities are typically hotter than the surrounding rural landscape. However, previous studies have largely overlooked seasonal variations, often focusing on single-season analyses. Moreover, previous studies often use a single rural site, but the rural landscape is diverse. To address these research gaps, this study examines differences in surface radiation and energy budgets among urban, grass, and cropland sites in Nanjing, China, using observational data from both the warm season and the winter season. Our findings reveal distinct seasonal patterns in surface radiation and energy budgets. During the warm season, urban areas exhibit the strongest ground heat flux during the day (heat storage) and at night (heat release); while the cropland has the highest latent heat flux among the three land types. These factors lead to the UHI effect particularly during nighttime. In contrast, winter data highlight reduced energy fluxes due to lower incoming solar radiation. However, the urban site still maintains higher nighttime temperatures due to the persistent influence of ground heat flux release and possibly anthropogenic heat flux. This research underscores the role of surface characteristics in affecting energy exchange and the seasonal variability of the UHI effect.