Initial Results of Altimetry Assimilation in POP2 Ocean Model

Transcription

1 Initial Results of Altimetry Assimilation in POP2 Ocean Model Svetlana Karol and Alicia R. Karspeck With thanks to the DART Group, CESM Software Development Group, Climate Modeling Development Group

2 POP-DART Assimilation System Model: POP2 ocean model at 1ºx1º horizontal resolution, with 60 levels, forced by the atmosphere state from data analysis in DART-CAM4 (Raeder et al., 2012). DA Method: Ensemble Adjustment Kalman Filter (EAKF, Anderson, 2009) of DART with 30 ens. members, daily data ingestion at 00:00 UT. Ocean Data: In situ temperature and salinity (XBT, MBT, CTD, drifters, floats, moorings, ocean stations) Altimetry Data: Delayed time merged all satellites Global Ocean Gridded Sea Level Anomalies, 0.25ºx0.25º, provided by AVISO, "Archiving, Validation and Interpretation of Satellite Oceanographic data ( Time Window for Assimilation Run: 06/ /2006

3 POP-DART Data Assimilation (DA) with the Ensemble Adjustment Kalman Filter (EAKF) observations: insitu T,S + SSH assimilated daily DART-EAKF DART 3D state 3D restart 30-members POP DATM Coupler 2D forcing from CAM4-DART assimilation Courtesy of A.Karspeck

4 Sea Level Anomalies (SLA) Data from AVISO AVISO daily data: 1,036,800 obs. To reduce computation time we chose one observation from 4º x 4º bin (at random for each day) to get a full coverage of data locations observations for one step of assimilation. Observational error: at SLA-based method (Oke & Sakov, 2008), standard deviation in 1ºx1º bin (POP2 resolution) from the AVISO SSH data collected in the bin. Sea Surface Height (SSH) = Sea Level Anomaly (SLA) + Mean Dynamic Topography (MDT) MDT Time Average SSH from POP-DART Temperature and Salinity Assimilation (C.Yan et al, 2015).

5 Maps of Sea Level Anomalies (SLA) for SLA with DA of Temp/Salinity/SSH SLA with DA of Temp/Salinity Change the sign of the anomalies Indian Ocean, West Pacific Ocean Increase of amplitude in the anomalies Southern Ocean, Southern Pacific Ocean SLA of POP2 AVISO-merged SLA Data assimilation of Temp/Salinity/SSH makes SLA closer to AVISO observations

6 Diagnostics of DART-POP Results in the Observation Space Sea Surface Height RMSE of Analysis (ANA) and Forecast (FST) Assimilation of Sea Level Anomalies (SLA) improves RMSE by 4% for Global Ocean: RMSE_FST = RMSE_ANA= And by 4-5% for other regions Results of [T,S,SSH]-DA and [T,S]-DA for FLOAT 300 m RMSE of FST of FLOAT Temperature from [T,S]-DA and [T,S,SSH]-DA [T,S,SSH]-DA reduced RMSE of Temperature (300m depth) by 3.2%. The difference between RMSE ANA and RMSE FST increased by 1% in [T,S,SSH]-DA Adding SSH to [T,S]-DA improves subsurface T,S estimates.

7 Comparison of Temperature RMSE: [T, S, SSH]-DA minus [T, S]-DA in three ocean layers: (0-100m, top; m, mid; m, bottom) m RMSE of [T, S, SSH]-DA (1.05º) is slightly higher (~1.9%) of RMSE of [T, S]-DA (1.03º). Different MDT induces significant variations in T@100m up to 5 in Nino 3.4, tropics (Segschneider et al, 2000, Storto et al, 2011) m RMSE of [T, S, SSH]-DA (0.92º) is decreased by 3.2% in comparison to RMSE of [T, S]-DA (0.95º) m RMSE of [T, S, SSH]-DA (0.40º) is decreased by 4.8% in comparison to RMSE of [T, S]- DA (0.42º).

8 Differences of RMSE Salinity: (T, S, SSH - T, S Assimilation) in three ocean layers m RMSE of [T, S, SSH]-DA (2.50 psu) is decreased by 0.8% in comparison to RMSE of [T, S]-DA (2.52 psu) m RMSE of [T, S, SSH]-DA (1.22 psu) is decreased by 3.17% in comparison to RMSE of [T, S]-DA (1.26 psu) m RMSE of [T, S, SSH]-DA (0.461 psu) is decreased by 0.65% in comparison to RMSE of [T,S]-DA (0.464 psu).

9 Global Profiles of RMSE for Temperature (right) and Salinity (left) RMS improve 0.02% NO ASSIM: 1.20 T, S ASSIM: T, S, SSH ASSIM: RMS improve 0.21% NO ASSIM: 2.72 T, S ASSIM: T, S, SSH ASSIM: Global RMSE profiles of T and S from [T, S, SSH]-DA were compared with no DA case NO ASSIM (identically forced ocean) and with [T, S]-DA (assimilation only with Temp-re and Salinity). Prior profiles are solid lines; posteriori profiles dashed lines. Both DA runs display significant decreases of RMSE relative to the No-Assim run. Some global degradations of RMSE in the upper layer (0-150m) ~0.1 and 0.18e-4 psu can be remarked in [T, S, SSH]-DA run.

10 Regional Profiles of RMSE Temperature (left) and Salinity (right): Labrador Sea (top) and Southern Ocean (bottom) RMS improve 10% NO ASSIM: 1.29 T, S ASSIM: 1.18 T, S, SSH ASSIM: 1.06 RMS improve 1.62% NO ASSIM: 2.59 T, S ASSIM: 2.30 T, S, SSH ASSIM: 2.26 RMS improve 6.40% NO ASSIM: 1.15 T, S ASSIM: 1.02 T, S, SSH ASSIM: 0.95 RMS improve 7.00% NO ASSIM: 1.81 T, S ASSIM: 1.45 T, S, SSH ASSIM: 1.35 [T,S,SSH]-DA improves T,S RMSE throughout the water column.

11 Concluding Remarks Sea Surface Height Assimilation (SSHA) have been implemented in the POP2 model with DART-ocean assimilation system. Initial SSHA results show positive impact of adding Altimetry data to the Temperature and Salinity data analysis in POP2. There are ocean basins with a significant (5-10%) decrease of RMSE (Temperature and Salinity) throughout the water column. The small degradation of simulations by the SSHA in the first 100m of upper Global Ocean needs future optimization & tests. Most of the improvement in POP2 predictions due to SSHA is under 100m. This indicates the Sea Surface Height influence on the thermocline and deeper ocean layers.

12 THANK YOU!

13 ADT- absolute dynamic topography MDT- mean dynamic topography SLA- sea level anomalies ADT = MDT A + SLA A = MDT B + SLA B (MDT B + (SLA B ) A ) + SLA A = MDT B +SLA B SLA A = SLA B (SLA B ) A Temporal mean over period A SLA - H Sea Level Anomalies (along-track) Satellite data from CNES Types of dataset: Ssalto/Duacs alongtrack multimission altimeter products. Contents: along-track sea surface heights computed with respect to a twenty-year mean. Condition of access : From May 2015 on, the Copernicus Marine and Environment Monitoring Service (CMEMS) is taking over the whole processing and distribution of those products (formerly distributed by Aviso+ with no changes in the scientific con-nt). Description: along-track multi-mission altimeter satellite product homogeneous with other satellites available in nearreal-time and in delayed-time.