Time-Continuous Hemispherical Urban Surface Temperatures

Abstract

Abstract:Traditional methods for remote sensing of urban surface temperatures (T surf ) 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 T surf is currently unknown, but is certainly nontrivial. To quantify and overcome these biases, we present a method to retrieve time-continuous hemispherical radiometric urban T surf (T hem, 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 T hem, 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, T hem, r was compared to T surf retrieved from nadir (T plan ) and complete (T comp ) perspectives of the surface. Large differences were observed between T comp , T hem, r , and T plan , with differences between T plan and T comp of up to 8 K under clear-sky viewing conditions, which are the cases when satellite-based observations are available. In general, T hem, r provides a better approximation to T comp than T plan , particularly under clear-sky conditions. The magnitude of differences in remote sensed T surf 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.

Assessing the impact of urban geometry on surface urban heat island using complete and nadir temperatures

“…The difference between T c and T r has been addressed in several studies (Voogt and Oke, 1997; Adderley et al ., 2015; Allen et al ., 2018; Jiang et al ., 2018), which documented the large difference between complete and radiometric surface temperature. This study compared the evaluation of SUHII using either nadir‐viewed radiometric or complete surface temperature based on satellite thermal images and high‐resolution airborne images.…”

“…The complete surface temperature may be more useful than the radiometric surface temperature to map SUHI intensity, but it has been rarely used. The difference between complete and radiometric surface temperature can reach 10 K (Voogt and Oke, 1997; Allen et al ., 2018; Jiang et al ., 2018). This would lead to large differences between SUHI maps generated with either surface temperature.…”

The Effect of Building Facades on Outdoor Microclimate—Reflectance Recovery from Terrestrial Multispectral Images Using a Robust Empirical Line Method

“…Since the microclimate impacts of urban vertical surfaces are spatially dependent [54,[73][74][75][76] and may be significantly influenced by microstructure heterogeneity [77] the ability to compute the spatial and geometric distribution of reflectance at sub-facet scale (<10 m) is desirable [78][79][80][81][82]. Satellite, aerial and ground-based remote sensing technologies permit increasing spatial, spectral, radiometric and temporal resolution with greater spatial coverage [48,83,84]. Ground-based imaging sensors are lightweight and mobile enabling in-canyon observations of surfaces with relative operational simplicity [85].…”

A semi-empirical method for estimating complete surface temperature from radiometric surface temperature, a study in Hong Kong city

“…Voogt and Oke (1997) compared sensible heat fluxes calculated from air temperature and different surface temperatures, and their results showed that the complete surface temperature should be used for the estimation of sensible heat flux in urban areas. In order to consider its significant implications in urban climate research, researchers have attempted to calculate 𝑇 𝑐 from remote sensing or field measurements (Allen et al, 2018;Jiang et al, 2018;Voogt and Oke, 1997). Voogt and Oke (1997) calculated 𝑇 𝑐 based on field measurement by a thermal camera data and digitizing building data from high-resolution (1:2500) aerial photography.…”

“…Their results showed that there is a significant difference between 𝑇 𝑐 and the nadir T r . Allen et al (2018) calculated 𝑇 𝑐 from the hemispherical T r measured by pyrgeometers and results showed that the difference between 𝑇 𝑐 and the temperature observed from a nadir view is up to 8 K under clear-sky viewing conditions. Jiang et al (2018) estimated 𝑇 𝑐 from directional radiometric temperature based on simulated data from urban micro-climate model and remote sensing observation model, without ancillary ground-based data.…”