Toward Improved Validation of Satellite Sea Surface Skin Temperature Measurements for Climate Research

Abstract

A poor validation strategy will compromise the quality of satellite-derived sea surface temperature (SST) products because confidence limits cannot be quantified. This paper addresses the question of how to provide the best operational strategy to validate satellite-derived skin sea surface temperature (SST skin ) measurements. High quality in situ observations obtained using different state-of-the-art infrared radiometer systems are used to characterize the relationship between the SST skin , the subsurface SST at depth (SST depth ), and the surface wind speed. Data are presented for different oceans and seasons. These data indicate that above a wind speed of approximately 6 m s Ϫ1 the relationship between the SST skin and SST depth , is well characterized for both day-and nighttime conditions by a cool bias of Ϫ0.17 Ϯ 0.07 K rms. At lower wind speeds, stratification of the upperocean layers during the day may complicate the relationship, while at night a cooler skin is normally observed. Based on these observations, a long-term global satellite SST skin validation strategy is proposed. Emphasis is placed on the use of autonomous, ship-of-opportunity radiometer systems for areas characterized by prevailing low-wind speed conditions. For areas characterized by higher wind speed regimes, well-calibrated, qualitycontrolled, ship and buoy SST depth observations, corrected for a cool skin bias, should also be used. It is foreseen that SST depth data will provide the majority of in situ validation data required for operational satellite SST validation. We test the strategy using SST skin observations from the Along Track Scanning Radiometer, which are shown to be accurate to approximately 0.2 K in the tropical Pacific Ocean, and using measurements from the Advanced Very High Resolution Radiometer. We note that this strategy provides for robust retrospective calibration and validation of satellite SST data and a means to compare and compile in a meaningful and consistent fashion similar datasets. A better understanding of the spatial and temporal variability of thermal stratification of the upper-ocean layers during low-wind speed conditions is fundamental to improvements in SST validation and development of multisensor satellite SST products.

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

“…At-sea measurements, of the skin surface temperature are usually lower than the temperature of the underlying water (cool skin effect), showing a dependence on wind speed (Fairall et al, 1996a;Minnett et al, 2011), and similar differences have been observed in freshwater environments (Cardenas et al, 2008). A warm near-surface layer can also appear on calm and sunny 30 days, without necessarily excluding the presence of a cool skin (Donlon et al, 1999). Under such conditions the thermal gradient can attain several degrees in the upper few meters of the water column (Ward, 2006).…”

“…Above 4 m s -1 , the two cases show a very similar behaviour, with average differences in cool skin effect and thickness of 0.002 ºC and 0.02 mm, respectively. For the higher wind speeds shear-induced turbulent heat exchange dominates, while for the lower wind speeds molecular and convective heat exchange 25 dominate (Fairall et al, 1996a;Donlon et al, 1999;Donlon et al, 2002). The simulated warm layer effect was more intense for the seawater case than for the freshwater case, especially for low wind speeds (Figure 4 b, d).…”

“…The 15 cool skin effect is almost always present and arises from the cooling of the first millimetres of the water column because of the joint action of longwave radiation, sensible and latent heat fluxes (Fairall et al, 1996a). Its magnitude is of -0.1 ºC to -0.5 ºC (Fairall et al, 1996a;Donlon et al, 2002).…”

“…Using data for the reservoir of Bimont, we applied the parameterizations obtained for the sea by Donlon et al (2002): 25 ∆ = −0.14 − 0.30exp(−0.27 10 ),…”

“…However, satellite-derived measurements correspond to the instant water temperature at the top of the surface (~10-20 µm 25 deep), known as skin temperature (Donlon et al, 1999;Kawai and Wada, 2007). Skin temperatures can differ from SWT because the thermal structure of the first meters of the water column is not uniform under all conditions.…”