Nighttime Cool Skin Effect Observed from Infrared SST Autonomous Radiometer (ISAR) and Depth Temperatures

The nighttime ocean cool skin signal ΔT [defined as skin sea surface temperature (SSTskin) minus depth SST (SSTdepth)] is investigated using 103 days of matchups between shipborne Infrared SST Autonomous Radiometer (ISAR) SSTskin and water intake SSTdepth at ~7.1–9.9-m depths, in oceans around Australia. Before data analysis, strict quality control of ISAR SSTskin data is conducted and possible diurnal warming contamination is carefully minimized. The statistical distribution of ΔT, and its dependencies on wind speed, heat flux, etc., are consistent with previous findings. The overall average ΔT value is −0.23 K. It is observed that the magnitude of the cool skin signal increases after midnight and a coolest skin offset (with an average value of −0.36 K) is found at around dawn. The dependency of ΔT on SST conditions is observed. Direct warm skin events are discovered when the net heat flux direction is from the atmosphere to the ocean, which is more likely to occur at high latitudes when the air is very humid and warmer than the SST. In addition, several cool skin models are validated: one widely used physical model performs best and can capture most skin-effect trends and details; the empirical models only reflect the basic features of the observed ΔT values. If the user cannot apply the physical model (due to, e.g., the algorithm complexity or missing inputs), then the empirical parameterization in the form proposed in a 2002 study can be used. However, we recommend using a new set of parameters, calculated in this study, based on much more representative dataset, and with more rigorous quality control.

LakeSST: Lake Skin Surface Temperature in French inland water bodies for 1999–2016 from Landsat archives

The spatial and temporal coverage of the Landsat satellite imagery make it an ideal resource for the monitoring of water temperature over large territories at a moderate spatial and temporal scale at a low cost. We used Landsat 5 and Landsat 7 archive images to create the Lake Skin Surface Temperature (LakeSST) data set, which contains skin water surface temperature data for 442 French water bodies (natural lakes, reservoirs, ponds, gravel pit lakes and quarry lakes) for the period 1999–2016. We assessed the quality of the satellite temperature measurements by comparing them to in situ measurements and taking into account the cool skin and warm layer effects. To estimate these effects and to investigate the theoretical differences between the freshwater and seawater cases, we adapted the COARE 3.0 algorithm to the freshwater environment. We also estimated the warm layer effect using in situ data. At the reservoir of Bimont, the estimated cool skin effect was about −0.3 and −0.6 °C most of time, while the warm layer effect at 0.55 m was negligible on average, but could occasionally attain several degrees, and a cool layer was often observed in the night. The overall RMSE of the satellite-derived temperature measurements was about 1.2 °C, similar to other applications of satellite images to estimate freshwater surface temperatures. The LakeSST data can be used for studies on the temporal evolution of lake water temperature and for geographical studies of temperature patterns. The LakeSST data are available at https://doi.org/10.5281/zenodo.1193745.

LakeSST: Lake Skin Surface Temperatures in French inland water bodies for 1999–2016 from Landsat archives

The spatial and temporal coverage of the Landsat satellite imagery make it an ideal resource for studies on the long term evolution of lake surface temperature and for geographical studies of temperature patterns. The Lake Skin Surface Temperature (LakeSST) data set contains skin surface temperature data for 442 French water bodies (natural lakes, reservoirs, ponds, gravel pit lakes and quarry lakes) for the period 1999–2016 obtained from the thermal band of Landsat 5 and Landsat 7 archive images. The skin temperature measured by satellites differs slightly from water temperature in the first meters of the water column because of cool skin and warm layer effects. Nevertheless surface temperature parameterizations originally developed for the sea can be used to adjust LakeSST to commonly used lake water temperature, e.g. surface temperature or temperature of the first 1~2 m. Moreover, theoretically small differences are to be expected between the freshwater and seawater case for low wind speeds. In fact, at the reservoir of Bimont, the estimated cool skin effect was about −0.3 ºC and −0.6 ºC most of time, while the warm layer effect at 0.55 m was negligible in average, but could occasionally attain several degrees and a cool layer was often observed in the night. The overall accuracy of the satellite-derived temperature measurements was about 1.5 ºC, similar to other applications of satellite images to estimate freshwater surface temperatures. The LakeSST data are available at https://doi.org/10.5281/zenodo.1041746.