Solar Farm as an Ideal Test Bed for Satellite Surface Emissivity and Temperature Retrieval Algorithms

Motivated by a previous study of using the MODIS observations to quantify changes in surface spectral reflectances caused by six solar farms in the southwest United States, here we used a similar method to study the longwave effects, namely changes of surface emissivities and land surface temperature (LST), of the six solar farms. Two MODIS surface products were examined: one relying on generalized split-window algorithm while assuming emissivities from land cover classification (MYD11A2), the other based on Temperature Emissivity Separation algorithm that is capable of dynamically retrieve emissivities (MYD21A2). Both products suggest that, compared to adjacent regions without changes before and after solar farm constructions, the solar farm sites have reduced outgoing radiances over the MODIS infrared window channels. Such reduction in upward longwave radiation is consistent with previous in-situ and satellite observations. MYD11A2 reports constant emissivities before and after solar farm constructions because its land type classification algorithm is not aware of the presence of solar farms. The estimated daytime LST is reduced by ~2K after the construction of the solar farms and nighttime LST by ~0.5K. Meanwhile, MYD21A2 shows a consistent decrease in Band 31 (10.78-11.28 µm) emissivity by ~0.01, and little or inconsistent changes in the emissivity of the other two window channels. Correspondingly, the LST decreases in MYD21A2 is not as large as those in MYD11A2. However, laboratory and in-situ measurements indicate the longwave emissivity of solar panels can be as low as 0.83, considerably smaller than the retrieved surface emissivity (0.95) by MODIS. Based on back-envelope calculation and simulation, we discussed the possible ways to reconcile the MODIS results with such laboratory and in-situ measurements. If an emissivity of 0.83 is adopted as the true value, the surface temperature of the solar farm should be adjusted by around +5 K. We argued that the measurements before and after solar farm constructions form a set of strict tests for surface emissivity retrieval algorithms.