Time-Continuous Hemispherical Urban Surface Temperatures

Traditional methods for remote sensing of urban surface temperatures (Tsurf) are subject to a suite of temporal and geometric biases. The effect of these biases on our ability to characterize the true geometric and temporal nature of urban Tsurf is currently unknown, but is certainly nontrivial. To quantify and overcome these biases, we present a method to retrieve time-continuous hemispherical radiometric urban Tsurf (Them, r) from broadband upwelling longwave radiation measured via pyrgeometer. By sampling the surface hemispherically, this measure is postulated to be more representative of the complex, three-dimensional structure of the urban surface than those from traditional remote sensors that usually have a narrow nadir or oblique viewing angle. The method uses a sensor view model in conjunction with a radiative transfer code to correct for atmospheric effects in three-dimensions using in situ profiles of air temperature and humidity along with information about surface structure. A practical parameterization is also included. Using the method, an eight-month climatology of Them, r is retrieved for Basel, Switzerland. Results show the importance of a robust, geometrically representative atmospheric correction routine to remove confounding atmospheric effects and to foster inter-site, inter-method, and inter-instrument comparison. In addition, over a month-long summertime intensive observation period, Them, r was compared to Tsurf retrieved from nadir (Tplan) and complete (Tcomp) perspectives of the surface. Large differences were observed between Tcomp, Them, r, and Tplan, with differences between Tplan and Tcomp of up to 8 K under clear-sky viewing conditions, which are the cases when satellite-based observations are available. In general, Them, r provides a better approximation to Tcomp than Tplan, particularly under clear-sky conditions. The magnitude of differences in remote sensed Tsurf based on sensor-surface-sun geometry varies significantly based on time of day and synoptic conditions and prompts further investigation of methodological and instrument bias in remote sensed urban surface temperature records.