Walyeldeen Godah Małgorzata Szelachowska Jan Kryński. Instytut Geodezji i Kartografii (IGiK), Warszawa Centrum Geodezji i Geodynamiki

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1 Porównanie globalnych modeli geopotencjału opracowanych na podstawie danych z misji GRACE oraz GOCE/GRACE z przyspieszeniem siły ciężkości pomierzonym w Obserwatorium Geodezyjno-Geofizycznym Borowa Góra Walyeldeen Godah Małgorzata Szelachowska Jan Kryński Instytut Geodezji i Kartografii (IGiK), Warszawa Centrum Geodezji i Geodynamiki

2 Contents 1) Objectives 2) Data used 3) Methodology 4) Results 5) Discussion 6) Conclusions

3 Objectives Objectives To compare the RL05 GRACE-based and GOCE/GRACE-based GGMs with time series of terrestrial gravity data determined using the A absolute gravimeter Long term (trend) gravity variations from satellite and terrestrial gravity data Short term (seasonal) gravity variations from satellite and terrestrial gravity data To study the suitability of using GRACE modelled time variable parameters for monitoring the temporal mass variations in the Earth system

4 Data used RL05 GRACE-based GGMs monthly solutions from the CSR and GFZ centers filtered using DDK1 (decorrelation) filter period 10/ /2015 GOCO05s: Gravity Observation Combination RL05 developed on the basis of GOCE, GRACE data + (SWARM, CHAMP, TerraSarX, Tandem-X and 6 SLR missions) SHC up to d/o 280 (first 100 SHC are time dependent) Absolute gravity data A absolute gravimeter measurements at three gravity stations (A_BG, BG_G2 and BG_EX2) quasi regular monthly 10/ /2015 accuracy 3-7 μgal GLDAS: The Global Land Data Assimilation System model sets of fully normalized SHC up to d/o 100 daily 0.25º 0.25º grids of GLDAS models

5 Methodology (1) Gravity disturbance g P from the A measurements g P where g P γ P g P gravity value at P measured with the A γ P normal gravity value at P first SHC d/o 100 of the GOCO05s are time dependent V ( t) where ( t t0 ) ( t t0 ) V0 Vt Vc cos 2 Vs sin2 T T V 0 the SHC at the reference epoch t 0 (01 January 2008) V t the drift parameter V c, V s coefficients of periodic terms T the annual period ( d) ( t t T 0 )

6 Gravity disturbance g from GGMs EWT Y R ρ Methodology (2) Equivalent water thickness (EWT) from GGMs and hydrological models with g where n n GM a ( r,, ) n 1 2 Y nm (, ) r n0 r m0 2n 1 n average ( r,, ) Y nm(, ) 3 n0 1 kn m0 nm (, ) ( C cosm S sin m) Pnm(sin) nm nm r, φ, λ geocentric coordinates of the computation point P R mean Earth's radius ρ average the average density of the Earth k n load Love numbers C P nm, (sin S ) fully normalized associated Legendre function nm nm dimensionless coefficients of degree n and order m GM product of the Newtonian gravitational constant G and the Earth s mass M a semi-major axis of the reference ellipsoid

7 Results (1) GRACE-based and GOCE/GRACE-based GGMs vs raw A measurements

8 Results (2) GRACE-based and GOCE/GRACE-based GGMs vs reduced A measurements using local hydrological (LH) effects

9 Results (3) GRACE-based and GOCE/GRACE-based GGMs vs smoothed A measurements using a moving average (MA) window size (3)

10 Results (4) GRACE-based and GOCE/GRACE-based GGMs vs reduced and smoothed A measurements using LH and MA window size (3)

11 Results (5) smoothed GRACE-based and GOCE/GRACE-based GGMs vs smoothed A measurements using the MA window size (3)

12 Results (6) smoothed GRACE-based and GOCE/GRACE-based GGMs vs reduced and smoothed A measurements using LH and MA window size (3)

13 Results (7) Estimated gravity variations trends from GRACE and GOCE/GRACE-based GGMs [µgal/year] Data From winter 2009 to winter 2011 ( ) From spring 2011 to spring 2015 ( ) raw smoothed (MA) raw smoothed (MA) CSR GFZ GOCO05s Estimated gravity variations trends from the measurements with the A [µgal/year] Data raw From winter 2009 to winter 2011 ( ) smoothed (MA) reduced (LH) reduced & smoothed (LH +MA) raw From spring 2011 to spring 2015 ( ) smoothed (MA) reduced (LH) reduced & smoothed (LH +MA) A_BG BG_EX BG_G

14 Results (8) GRACE-based and GOCE/GRACE-based GGMs vs the GLDAS hydrological models time series of δewt from CSR GFZ RL05 GRACE-based GGMs and GOCO05s GLDAS models truncated at d/o 90 trend of δewt [cm/year] Data From winter 2009 to winter 2011 From winter 2012 to winter 2015 ( ) ( ) CSR GFZ GOCO05s GLDAS

15 Discussion (1) Gravity variations o seasonal gravity variations from GRACE data are at the level of 3 8 µgal (cannot be reliably detected using the A measurements) o two linear trends (1) ascending trend from autumn 2008 to spring 2011 (2) descending trend from spring 2011 to summer/autumn 2015 observed in gravity variations obtained from the CSR and GFZ RL05 GRACEbased GGMs and the A measurements o gravity obtained from GOCO05s are monotonically ascending o the use of LH improve the fit between the estimated trends from the A measurements and the corresponding ones from RL05 GRACE-based GGMs

16 Discussion (2) Water mass variations o seasonal water mass variations pattern of δewt with maximum values in the spring months and minimum values in July-September o time series of δewt obtained from the CSR and GFZ RL05 GRACE-based GGMs as well as GLDAS hydrological models can be divided into two parts characterized with two different linear trends o δewt from the GOCO05s are monotonically ascending

17 Conclusions Time series of gravity obtained from the A measurements can be regarded as a valuable tool for the calibration/validation of the long term temporal gravity variations The A measurements seem not suitable for the monitoring of short term (seasonal) gravity variations main reason - insufficient accuracy (3 7 μgal) of the A measurements Inconsistency of spatial and temporal resolutions between GRACE-based GGMs and the A measurements is the limitation of this study For estimating temporal mass variations in the Earth system, the monthly RL05 GRACE-based GGMs are more suitable than time dependent GOCE/GRACEbased GGMs, e.g. GOCO05s GRACE data compared with GLDAS models in terms of EWT confirms the suitability of monthly RL05 GRACE-based GGMs to study the short term and long term temporal mass variations in the Earth system