Enhanced solar radiation diffusion in the urban environment: A case study over Houston, Texas

Although solar radiation has been considered in assessing human sensation of heat extremes in urban climate studies, the spatiotemporal variation in the ratio of diffuse horizontal irradiance to global horizontal irradiance, termed as Diffuse Fraction (k_d), remains unquantified. The importance of this variable to human thermal comfort estimation and photovoltaic energy motivates this study. Using the high-resolution (2 km, hourly) satellite-derived solar radiation flux data from the National Solar Radiation Database (NSRDB), we examine the mean k_d over southeast Texas for the years 2019–2022. We identify a robust urban-rural contrast in k_d around the metropolitan area of Houston. The enhancement of k_d is strongest during the warm season (June to September) and during the local afternoon (1 pm to 4 pm). Furthermore, we examine the relationship between the k_d pattern and multiple potential contributing factors, such as surface albedo (as a proxy of urban-rural land characteristics), cloud frequency, aerosols, and ozone. Among all factors, cloud frequency shows a strong, positive spatiotemporal correlation with the mean k_d. Satellite-derived cloud frequency itself demonstrates a statistically significant enhancement over the Houston urban area. Future studies can assess whether this phenomenon exists in other cities and to what extent it impacts human heat sensation.