B.5

We present the observation of the travelling ionospheric disturbance (TID) all around the world after the Hunga-Tonga Hunga-Ha'apai (HTHH) volcanic eruption on 15 January 2022.  This is based on the dual frequency K-band low-low tracking from the GRACE Follow-On satellites as well as the GNSS high-low tracking from a constellation of Spire Global CubeSats.  The latter complements the GRACE Follow-On measurements in terms of spatial sampling.  We discuss the measurements of global ionospheric disturbance and speed of the detected TID (~320 m/s) following the atmospheric Lamb waves initiated by the HTHH volcanic eruption.  Interestingly, we found 2-3 orders of magnitude reduction in electron density (“ionospheric hole”) at ~480 km altitude right after the first passage of the atmospheric Lamb wave and it lasted several hours before being recovered.  This characteristic is being observed consistently by the GRACE Follow-On and CubeSat measurements.  They are brand-new data that can be used to study less known high-altitude (>500 km) ionospheric processes.  We present our on-going analysis and modelling of these measurements.

How to cite: Han, S.-C., Mikesell, D., and Rolland, L.: The ionospheric hole detected by the GRACE Follow-On K-band ranging system after the 2022 Hunga-Tonga Hunga-Ha'apai volcanic eruption on 15 January 2022, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-60, https://doi.org/10.5194/gstm2022-60, 2022.

The polar motion (PM) changes comprehension is one of the major tasks in geodesy. In order to learn about the changes in PM caused by internal forces, it is necessary to perform detailed analyses of the so-called PM excitation functions of geophysical fluids layers - atmosphere, ocean and terrestrial hydrosphere with cryosphere. The impact of land hydrosphere is not well understood and the study of the Hydrological Angular Momentum (HAM) is still the main research topic in finding the agreement between observed geodetic changes in PM and geophysical ones. The study of different HAM excitations is possible through the use and analysis of temporal gravity models, which are constantly being developed by numerous research centres around the world.

The main aim of this study is to check the usefulness of ITSG daily gravity field models in determination of PM excitation at sub-seasonal time scales. To do so, we compare the equatorial components (χ1 and χ2) of the HAM calculated using the ITSG (The Institute of Geodesy at Graz University of Technology) daily gravity field models (ITSG-Grace2016, ITSG-Grace2018) with hydrological signal in the PM excitation (geodetic residuals) and HAM from LSDM (Hydrological Land Surface Discharge Model). Data from ITSG are the object of our work because they have a daily temporal resolution and allow us to determine oscillations with higher frequencies than the more commonly used monthly data. The period of our work was narrowed down to the years of the GRACE mission with the highest density of data, in order to avoid the impact of data gaps that can disrupt our short-term analyses. We limit our study to the period between 2004 and 2011 due to meaningful gaps in GRACE observations before and after this period.

The equatorial components of the PM excitation were calculated from the relationship between χ1, χ2 and the Stokes coefficients of degree-2, order-1 (∆C21, ∆S21) provided in the ITSG-Grace models. In this work, the HAM and SLAM (Sea-level Angular Momentum) series from the LSDM provided by GFZ (GeoForschungsZentrum in Potsdam) and the geodetic residuals, which are the differences between the geodetic angular momentum (GAM) and the sum of atmospheric and oceanic excitation, provided by Observatoire de Paris, were used for comparison with the PM excitation series determined from the ITSG-Grace models. In order to evaluate PM excitations obtained from ITSG-Grace models with series from other models, we determined short-period amplitude spectra and calculated correlation and root mean-square errors (RMSE). Our analyses confirm the existence of a sub-monthly signal in the HAM series determined from ITSG daily data. We observed a similar signal in LSDM-based HAM but with notably weaker amplitudes.

How to cite: Partyka, A., Nastula, J., Śliwińska, J., Kur, T., and Wińska, M.: Analysis of the ITSG-Grace daily models in the determination of polar motion excitation function, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-43, https://doi.org/10.5194/gstm2022-43, 2022.

Global Atmospheric re-analysis efforts aim at the realistic reproduction of the time-variable atmospheric state and associated diagnostic quantities by assimilating a wide range of different types of observations into a numerical model. Satellite gravimetry as realized with GRACE and GRACE-FO is on the one hand sensitive to the three-dimensional distribution of mass in the atmosphere and on the other hand also to (rapid) variations in terrestrial water storage, that largely represent the time-integrated effect of precipitation and evapotranspiration. By means of an ensemble of three global atmospheric reanalyses (ERA5, MERRA2, JRA-55) and two operational numerical weather prediction models (IFS from ECMWF, ICON from the German Meteorological Service) we will discuss the current level of agreement among the atmospheric models for (i) hydrostatic surface pressure, (ii) water vapor partial pressure, and (iii) atmospheric net-water fluxes obtained from summing precipitation and evapotranspiration. For time-scales between a few days and several weeks, we find that atmospheric net-water fluxes show the highest spread among the different models, thereby indicating a need for additional observations of both precipitation and/or evapotranspiration that are not yet routinely assimilated. We also find that water vapor partial pressure is quite uncertain, but note that tropospheric products from dense GNSS station networks are promising to constrain the moisture distribution. Finally, we demonstrate that atmospheric mass is already well constrained in weather models due to the abundant availability of in situ barometers at the Earth’s surface so that utilizing a priori information from global atmospheric model data in satellite gravity data processing via the Atmosphere and Ocean De-Aliasing Level-1B Model is very well justified. In terms of meteorologic applications of future satellite gravity missions currently under consideration by both NASA and ESA, we thus recommend to further refine methodologies aiming at isolating signals of atmospheric net-water fluxes out of the time-variable gravity signatures instead of attempting to map any direct signatures of the atmospheric mass distribution.

How to cite: Balidakis, K., Dobslaw, H., Zus, F., Eicker, A., Dill, R., and Wickert, J.: Prospects of Satellite Gravimetry to Monitor Atmospheric Mass, Water Vapor Partial Pressure, and/or Atmospheric Net-Water Fluxes, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-23, https://doi.org/10.5194/gstm2022-23, 2022.

GRACE and GRACE-FO missions have provided gravity fields water mass changes within the Earth's surface since 2002.  This gravity fields are post-processed to infer water mass changes between Ocean and land storage compartments. Unfortunately, most storage compartments (lakes, groundwater, glaciers…) are too small to be resolved given the current spatial resolution of gravimetry missions, resulting in Leakage and Gibbs effects.This effects generates spurious signals that makes gravimetry-based Land water storage (LWS) changes estimates difficult to attribute and to interpret at individual basin and regional scales.
Here, we combine gravimetry-based water mass change data with independent satellite observations to derive refined estimates of the water mass changes. The combination consists in including observations from  satellite altimetry and high resolution visible imagery of glacier (Hugonnet et al., 2021) and lake (Cretaux et al., 2016) mass changes in the conversion process from gravity L2 data to water mass changes. The combination is done for all regions of the world on a monthly basis.
This approach allows to reduce the uncertainty in LWS changes at interannual to decadal time scales, and to derive glacier-free estimates of TWS in the endorheic basins and the exorheic basins.
We find that for the period from 2002 to 2021, the total LWS trend of 0.30±0.26 mm SLE/yr is mainly due to a mass loss in endorheic basins LWS of 0.24±0.13 mm SLE/yr. Over the same period, exorheic basins control interannual variability (2-5 years) but present a non-significative trend of 0.06±0.14 mm SLE/yr.

How to cite: Blazquez, A., Meyssignac, B., Berthier, E., Longuevergne, L., and Cretaux, J. F.: Combining space gravimetry and other satellite observations to reduce spatial uncertainties, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-73, https://doi.org/10.5194/gstm2022-73, 2022.

An inherent problem of satellite gravimetry is the limited spatial resolution of space-borne sensors observing various functionals of the Earth's gravity field. Due to the limited amount of sensor data available to calculate monthly gravity fields and the typically applied global mathematical basis functions, the problem is particularly severe for mass change applications from GRACE and GRACE-FO. That leads to apparent mass loss or gain in regions by mass signals from neighbouring regions and is called the spatial leakage effect.

The spatial leakage problem was identified well before GRACE's launch (see Wahr et al., 1998), and numerous methods have been proposed to assess and mitigate the adverse effects of spatial leakage. Available approaches can be grouped into

• estimates obtained from numerical models of the most prominent surface mass processes;
• data-driven approaches that only rely on the geodetic data themselves;
• methodologies based on (non-geodetic) satellite remote sensing data; and
• forward-modelling approaches, where a certain (time-invariant) geometry of surface mass change is assumed a priori and a limited set of (time-variable) parameters are estimated to fit the (spatially much smoother) gravity field data in a least-squares sense.

Despite the various approaches available, we note that spatial leakage is treated very differently for individual surface mass change applications and across the various international research institutions. Sometimes, leakage is readily corrected before providing mass anomalies to non-geodetic users. Other groups only provide some leakage approximation to inform users about this systematic error source. And in (increasingly rare) cases, leakage is not even treated at all. In order to consolidate approaches across the geodetic community, we propose to establish an IAG Working Group aiming to

• collect different leakage mitigation methods;
• develop a common and independent validation approach; and
• recommend best practices in communicating spatial leakage to non-geodetic users of surface mass change data derived from satellite gravimetry.

This contribution will summarize the current status of our ideas in order to invite all interested colleagues to contribute to this initiative.

How to cite: Boergens, E., Flechtner, F., Jäggi, A., Güntner, A., Dahle, C., and Dobslaw, H.: Towards a new IAG Working Group on Spatial Leakage Mitigation in Satellite Gravimetry, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-21, https://doi.org/10.5194/gstm2022-21, 2022.