The impact of satellite zenith angle on atmospheric sounding information content

A concern in satellite sounding retrieval is that the quality of soundings is expected to degrade with increasing satellite zenith angle (SZA). Larger SZAs (≳ 40°) introduce various negative factors that can contribute to degraded sounding quality, such as cloud contamination, representative errors, surface emissivity uncertainties, and observational noise. However, a theoretical analysis reveals that Jacobians experience higher peaks with larger SZAs, even for the window channels that are sensitive to low-level moisture. Consequently, a greater number of channels become sensitive to the troposphere, where enhanced information content is anticipated. Shannon information content analysis of the Cross-track Infrared Sounder's longwave infrared (LWIR) channels corroborates the increase in information content throughout the troposphere with rising SZAs, most significantly in the lower troposphere. Meanwhile, midwave infrared (MWIR) channels exhibit increased information content primarily in the upper troposphere. Evaluation of Fengyun-3E sounding retrievals demonstrates that root mean square errors (RMSEs) decline with increasing SZA across the troposphere, reaching a minimum around 50°. These findings strongly support the notion of increased sounding information content with larger SZAs assuming other factors are accounted for a priori. However, beyond 50°, negative factors mentioned before begin to dominate, causing RMSEs to rise swiftly. Yet, at an SZA of 60°, RMSEs align with those at nadir. These outcomes have significant implications for forthcoming geostationary sounding products from the Infrared Sounder (IRS) and the Geostationary Extended Observations Sounder (GXS). Geostationary sounding products are anticipated to exhibit quality comparable to or better than nadir soundings when SZA remains under 60°. Products with larger SZAs should be flagged. Additionally, this study emphasizes the irreplaceable role of LWIR channels in comparison to MWIR channels for temperature and moisture soundings.