A New 1‐Hourly ERA5‐Based Atmosphere De‐Aliasing Product for GRACE, GRACE‐FO, and Future Gravity Missions

Abstract

The current state‐of‐the‐art of satellite gravity data processing makes use of de‐aliasing products to reduce high‐frequency mass anomalies. For example, the most recent official Atmosphere and Ocean De‐aliasing products (AOD1B‐RL06) are applied for the Gravity Recovery and Climate Experiment (GRACE) and GRACE‐Follow On (GRACE‐FO) missions. The temporal resolution of AOD1B‐RL06 is 3 h, and spectrally, they are computed up to degree and order 180. In this study, we explore a refined, e.g., geometrically, physically, and numerically improved, mass integration approach that is important for computing the atmosphere part of these products. Besides, the newly available ERA5 climate data are used to produce a new set of non‐tidal atmosphere de‐aliasing product (HUST‐ERA5) that is computed hourly up to degree and order 100, covering 2002 onwards. Despite an overall agreement with AOD1B‐RL06 (correlation≥0.99), considerable discrepancies still exist between HUST‐ERA5 and AOD1B‐RL06. The possible reasons are therefore analyzed, and we find the input climate data, sampling rate and integration method may result in a product difference of ∼0.3, ∼0.15 and ∼0.05 millimeter geoid height, respectively. The total differences between HUST‐ERA5 and AOD1B‐RL06 can lead to a mean variation of ∼7.34 nm00/s on the laser ranging interferometry (LRI) range‐rate residuals, for example, during January 2019, which is already close to the LRI precision. This impact is invisible for the GRACE (‐FO) gravity inversion because of the less accurate on‐board KBR (K‐band ranging) instrument, however, it will be non‐negligible and should be considered when the LRI completely replaces the KBR in the future gravity missions.

A Regionally Refined and Mass‐Consistent Atmospheric and Hydrological De‐Aliasing Product for GRACE, GRACE‐FO and Future Gravity Missions

“…Recent improvements on the atmospheric part of the AOD product included (a) homogenizing surface pressure time-series from different NWP models to a common reference orography, (b) moving from ERA-I to ERA5, (c) moving from 6-hourly resolution to 3-hourly resolution or even 1-hourly resolution, and (d) updating geometric, physical, and numerical approximations. Forootan et al (2013) and Yang et al (2021) showed that a more realistic geometrical and physical shape of the Earth, together with better approximation of latitude-and altitude-dependent gravity acceleration in the vertical integration process, leads to differences in VIM of few 0.01 mm in terms of geoid height without degree 0 and 1 coefficients, that is, less than 1 mm EWH. With AOD-RL06 the temporal resolution of the de-aliasing product was increased from 6-hourly data to 3-hourly data, which lead to differences in the geoid of about 0.1 mm averaged over one year (Yang et al, 2021), that is, few mm EWH.…”

“…It should be kept in mind, that atmospheric tides were not removed here. However, their impact can be assumed to be not significant for our analysis in the mid-latitudes (Yang et al, 2021). Shihora et al (2022) found, after careful removal of tidal signals, contribution of hourly temporal resolution versus 3-hourly resolution of about 5 mm and decided based on that analysis to keep the official AOD product at 3-hourly resolution.…”

“…With AOD-RL06 the temporal resolution of the de-aliasing product was increased from 6-hourly data to 3-hourly data, which lead to differences in the geoid of about 0.1 mm averaged over one year (Yang et al, 2021), that is, few mm EWH. Further increasing the temporal resolution to 1-hourly fields was assessed by Shihora et al (2022) and Yang et al (2021). Yang et al (2021) quantified the impact on the geoid to about 0.04 mm over Europe and up to 0.08 mm in equatorial regions again averaged over 1 year.…”

“…Shihora et al (2022) found, after careful removal of tidal signals, contribution of hourly temporal resolution versus 3-hourly resolution of about 5 mm and decided based on that analysis to keep the official AOD product at 3-hourly resolution. Likewise, Yang et al (2021) computed the impact from higher temporal resolution to reach values of about 0.2 mm geoid height (which corresponds to about 5 mm EWH) and suggested to test them as AOD product in the gravity field processing chain. Indeed, the impact from increasing the temporal resolution from 3hourly fields to 1-hourly fields on VIM is as large as the impact from updating the NWP model version from ERA-I to ERA5, and about 10 times as large as the impact from the refined vertical integration approach suggested by Yang et al (2021).…”

“…A number of studies (Forootan et al., 2013; Shihora et al., 2022; Yang et al., 2021) have compared the use of atmospheric fields from the IFS and the reanalyses ERA‐Interim (ERA‐I hereafter) and ERA5 for de‐aliasing. The spatial resolution of ECMWF‐IFS has changed over time and is about 9 km since 2016, while ERA‐I and ERA5 have a grid resolution of 79 and 31 km, respectively.…”