Key Points:• ECOSTRESS is a state-of-the-art combination of thermal bands, spatial and temporal resolutions, and measurement accuracy and precision • Data from 82 eddy covariance sites were coalesced concurrently with the first year of ECOSTRESS for Stage 1 validation • Clear-sky ET from ECOSTRESS compared well against a wide range of eddy Abstract The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was launched to the International Space Station on 29 June 2018 by the National Aeronautics and Space Administration (NASA). The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as Level-3 (L3) latent heat flux (LE) data products. These data are generated from the Level-2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear-sky ET product (L3_ET_PT-JPL, Version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear-sky instantaneous/time of overpass: r 2 = 0.88; overall bias = 8%; normalized root-mean-square error, RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are overrepresented. The 70-m-high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1-km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data.

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There are no direct methods to evaluate calculated soil heat flux (SHF) at the surface (G0). Instead, validation and cross evaluation of methods for calculating G0 usually rely on the conventional calorimetric method or the degree of the surface energy balance closure. However, there is uncertainty in the calorimetric method itself, and factors apart from G0 also contribute to nonclosure of the surface energy balance. Here we used a novel approach to evaluate nine different methods for calculating SHF, including the calorimetric method and methods based on analytical solutions of the heat diffusion equation. The SHF (Gz) measured by a self‐calibrating SHF plate at a depth of z = 5 cm below the surface (hereafter Gm_5cm) was deployed as a reference. Each SHF calculation method was assessed by comparing the calculated Gz at the same depth (hereafter Gc_5cm) with Gm_5cm. The calorimetric method and simple measurement method performed best in determining Gc_5cm but still underestimated Gm_5cm by 19% during the daytime. Possible causes for this underestimation include errors and uncertainties in SHF measurements and soil thermal properties, as well as the phase lag between Gc_5cm and Gm_5cm. Our results indicate that the calorimetric method achieves the most accurate SHF estimates if self‐calibrating SHF plates are deployed at two depths (e.g., 5 cm and 10 cm), soil temperature and water content measurements are made in a few depths between the two plates, and soil thermal properties are accurately quantified.

The effects of large eddies on turbulence structures and flux transport were studied using data collected over a flat cotton field during the Energy Balance Experiment 2000 in the San Joaquin Valley of California in August 2000. Flux convergence (FC; larger fluxes at 8.7 m than 2.7 m) and divergence (FD) in latent heat flux (LE) were observed in a disturbed, unstable atmospheric surface layer, and their magnitudes largely departed from the prediction of Monin‐Obukhov similarity theory. From our wavelet analysis, it was identified that large eddies affected turbulence structures, scalar distribution, and flux transport differently at 8.7 m and 2.7 m under the FC and FD conditions. Using the ensemble empirical mode decomposition, time series data were decomposed into large eddies and small‐scale background turbulence, the time‐domain characteristics of large eddies were examined, and the flux contribution by large eddies was also determined quantitatively. The results suggest that large eddies over the frequency range of 0.002 Hz < f < 0.02 Hz (predominantly 300–400 m) enhanced the vertical velocity spectra more significantly at 8.7 m than 2.7 m, leading to an increased magnitude of the cospectra and thus LE at 8.7 m. In the FD case, however, these large eddies were not present and even suppressed in the vertical velocity spectra at 8.7 m. Consequently, the cospectra divergence over the low‐frequency ranges primarily caused the LE divergence. This work implies that large eddies may either improve or degrade the surface energy balance closure by increasing or decreasing turbulent fluxes, respectively.

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