KEYWORDS: Clouds, Thermal modeling, Data modeling, Humidity, Solar radiation models, Thermal effects, Solar radiation, Shortwaves, Atmospheric modeling, Temperature metrology
Total sky irradiance onto the Earth's surface includes contributions from solar (or shortwave) radiation as well as thermal (longwave) radiation. Whereas shortwave downwelling is only present during daylight hours, thermal downwelling radiation is present throughout the day and night. Sky thermal irradiance on the Earth's surface has been described in other references as a function of surface ambient temperature and relative humidity. In this study, we show that with the introduction of low overcast clouds (altitude less than 2km and 100% cloud cover), thermal downwelling sky irradiance increases 34%. A comprehensive model was developed to compute the thermal downwelling sky irradiance as a function of temperature, relative humidity, cloud height, and percent cloud cover. Based on ground truth measurements collected in Reston, Virginia, we propose coefficients to model the total thermal downwelling irradiance including cloud effects with an operational error of 9.7%.
Comparison of the components and the overall fidelity of infrared synthetic image generation models with truth data and imagery is a crucial part of determining model validity and identifying areas in which improvements can be made. The Rochester Institute of Technology's Digital Imaging and Remote Sensing Image Generation Model, DIRSIG, was validated in the midwave infrared (MWIR) and longwave infrared (LWIR) regions using measured meteorological, material, and radiometric data. Error propagation techniques clearly defined areas where improvements to the model could be made (e.g., inclusion of clouds). An overall comparison of truth and synthetic images yields rms errors of as low as 1.8 degree(s)C for actual temperature, and 5 degree(s)C (LWIR) and 6 degree(s)C (MWIR) for apparent temperatures. Analysis of rank order correlation statistic shows a very high correlation between brightness rank for object in the truth and DIRSIG images for most times of day.
The Digital Imaging and Remote Sensing laboratory's Image Generation model combines computer aided design, ray tracing techniques, radiometric principles, and thermodynamic models to create synthetic imagery. The model emphasizes rigorous radiometric solutions that account for spectral reflectance effects, angular emissivities, atmospheric transmission and upwelled and downwelled sky radiance. This paper describes enhancements to the radiometric portion of the code that permits inclusion of variations with azimuth of downwelling sky radiance, solution of the radiometric propagation models using specific radiosonde data including adjustments for the time of day, and the incorporation of background effects from objects adjacent to the target. Simulated scenes are presented that show how these enhancements produce imagery that more closely match observed phenomena. In particular, the importance of properly modeled sky radiance is shown both for low altitude oblique imagery where the sky is directly observed and for near nadir imagery where reflected sky radiance is important.
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