Assessment of the Accuracy of Retrieval of the Caspian Sea Surface Temperature from the MODIS Data Based on SVPB Drifter Measurements

This study is devoted to assessing the accuracy of determining the Caspian Sea surface temperature (SST) using MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data. The authors used in situ temperature measurements obtained from Surface Velocity Program Barometer (SVPB) drifters deployed in the Caspian Sea in 2006 – 2007 and 2008 as part of the MACE Project. For comparison, daily MODIS LST products with a spatial resolution of 1 km, available on the Google Earth Engine platform, were used. Products obtained using the Split-Window (MOD11A1 and MYD11A1) and Temperature Emissivity Separation (TES) algorithms (MOD21A1D, MYD21A1D, MOD21A1N, MYD21A1N) for day and night surveys were analyzed. The results showed a significant linear relationship between the MODIS satellite data and drifter measurements for all considered products. For the products based on the Split-Window Algorithm, the correlation coefficients were 0.94 – 0.96, and the determination coefficients were 0.73 – 0.78. The average error for daytime data was -1.23 – -1.52 °C, and for nighttime data -2.14 – -2.16 °C. It was noted that for daytime data of the MOD11A1 and MYD11A1 products, the deviation increases with increasing water temperature. The products based on the TES algorithm showed higher accuracy. For daytime products MOD21A1D and MYD21A1D, the correlation coefficient was 0.91 – 0.93, the determination coefficient was 0.83 – 0.84, and the average error was 0.37 – 0.64 °C. For the nighttime MOD21A1N and MYD21A1N products, the correlation coefficient was 0.96, the determination coefficient was 0.91 – 0.92, and the average error was 0.03 – 0.25 °C. The MODIS data were also compared with Landsat Level-2 data. The determination coefficient was 0.92 in both cases (using MODIS Split-Window and TES products). On average, the Landsat temperature was higher by 0.5 °C when compared with the MYD11A1 and MOD11A1 products (daytime) and lower by 0.85 °C when compared with the MYD21A1D and MOD21A1D products. Overall, the study showed that MODIS products demonstrate high accuracy in reproducing the Caspian Sea surface temperature, with products using the TES algorithm having higher accuracy compared to products based on the Split-Window algorithm.