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Modelling surface structure and temperature of relevance to remote sensing of cities Soux, Cristian Andres

Abstract

The current increase in the use of remote sensors necessitates a closer investigation into the nature of what these sensors view. This is particularly true over urban areas where the well developed three-dimensional surface structure creates anisotropic surface radiative emissions. This study presents a numerical model to interpret and predict surface facet view factors and remotely-sensed radiative surface emissions from urban areas. The model that is developed (S3MOD) is able to create a simplified urban surface containing a repeating pattern of buildings, streets and alleys at any azimuth and geographic location. A remote sensor can then be located and oriented over a full range of possible inputs from below canopy level to near satellite height. Surface facet temperatures can be either input directly or evaluated using the Mills (1997) UCL energy balance model. S3MOD is then able to calculate surface view factors and sensor apparent temperatures. S3MOD is validated against measurements taken during a field campaign in Vancouver, B.C. The geometric validation cannot be completed using measured values due to uncertainty in the accuracy of those measurements. A theoretically based approach is employed which reveals very good agreement exists between modelled values and theory. The radiative validation is conducted using measured sensor apparent temperatures and with a sensor specific EFOV weighting function, provides good agreement between modelled and measured values. The validated model is used to investigate a number of hypothetical remote sensing scenarios. The first of these results indicates that for a specified sensor location and orientation and over a given surface structure, a critical height exists above which surface view factors do not change appreciably. In addition, it is found that sensors at different elevations but viewing the same surface area (i.e. the higher sensor has a smaller IFOV) do not have the same surface view factors. The domain size of the model must be increased to further expand the range of sensor heights over which the model works effectively. The final modelling exercise attempts to find the location and orientation where a sensor would sample surface facets in proportion to their contribution to the complete surface area for a specified urban surface type. The results of this final scenario suggest that for sensors located at five times building height, an extreme off-nadir angle is necessary to correctly sample wall facets. Further work is required to determine if this ideal sensor setup exists for some of the surface types tested.

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