Optimization of Instrumental Spectral Configurations for the Split-Window Method in the Context of the TRISHNA Mission

Abstract

We propose an original approach to optimize the TRISHNA instrument spectral configuration for the Split-Window (SW) method. First, we consider as input of end-to-end simulations an emissivity dataset that accounts for cavity effect within vegetation canopy. Second, we propose a bi-dimensional approach where both locations of TRISHNA SW channels, namely λ T IR3 c and λ T IR4 c , can slide within predefined spectral intervals. We report a large sensitivity to channel positions, with variations of RMSE on retrieved land surface temperature up to 3 K. Our bi-dimensional approach shows that this sensitivity is consistent with the underlying assumptions of the SW method. Indeed, two regions are observed in the (λ T IR3 c , λ T IR4 c ) space:(1) an unfavorable region corresponding to λ T IR3 c ≤ 10.0 µm, where large RMSE values are ascribed to large differences between emissivities in both SW channels, and (2) a favorable region corresponding to λ T IR3 c ≥ 10.3 µm, where differences between emissivities in both SW channels are small, and where RMSE values are driven by the differences between atmospheric transmittance in both SW channels. Overall, it is necessary to better account for the difference in surface emissivities between the two SW channels, whereas disregarding the cavity effect within vegetation canopy is not critical. Eventually, our bidimensional approach permits to define an optimal position for λ T IR3 c at 10.6 µm, which induces a larger robustness to uncertainties on channel positions. By applying our study on two structurally different SW formulations and addressing impacts of uncertainties on land surface emissivity and atmospheric water vapor content, we show that these results can be generalized to other SW formulations.

Optimizing TRISHNA TIR channels configuration for improved land surface temperature and emissivity measurements

“…From an operational viewpoint, the results of the current study do not contradict those reported by Vidal et al (2021) who addressed the sensitivity of the Split Window method to channel positions and bandwidths in the context of the TRISHNA mission. Overall, both studies converge toward a unique recommendation for the spectral configuration of the TRISHNA mission, with an overall error on surface temperature retrievals of 0.85 K. This is satisfactory in term of mission requirements ( 1.5 K), and is of the same order of magnitude as the precision requirements for further applications (0.8 K, cf Seguin et al, 1999;Hernandez-Baquero, 2000).…”

“…On the other hand, and as reported by Vidal et al (2021), the performance of the Split Window method is improved with the recommended configuration,…”

“…• For TIR3, a shift towards larger wavelengths would limit the impact of ozone absorption at 9.7 µm. Vidal et al (2021) reported an optimal position at 10.6 µm when dealing with the performance of the Split Window method in the context of the TRISHNA mission, and showed that this positioning of TIR3 was robust to the uncertainties on the parameters used in the SW formulations, such as emissivity and AWVC. We therefore recommend to shift T IR3 c at 10.6 µm.…”

“…• For TIR4, a shift towards lower wavelengths would attenuate the impact of water vapor absorption above 12 µm. Nonetheless, a limited overlap between TIR3 and TIR4 channel is required to ensure satisfactory performance of the Split Window method in the context of the TRISHNA mission (see Vidal et al, 2021). We therefore recommend to keep the original TIR4 position at 11.6 µm.…”

“…Using the same emissivity dataset, and in order to propose a spectral config-uration dedicated to the SW method for the TRISHNA mission, Vidal et al (2021) developed a bi-dimensional optimization approach that simultaneously moves spectral channels rather than using predefined configurations.…”