Wet tropospheric correction comparison GPD V2.0 (FCUP) and GPD V1.1 corrections for ERS-1, ERS-2, Envisat, Jason-1, Jason- 2, TOPEX/Poseidon Synthesis of all the RRDP analyses Lionel Zawadzki, Michael Ablain (CLS) Synthesis of all the RRDP analyses

Introduction: We will observe and analyse the impact of the new GPD wet troposphere correction V2.0 by comparison to its previous version V1.1 for climate applications for JA1, JA2, T/P, ERS-1, ERS- 2, Envisat. In order to determine the impact of the new wet troposphere correction in terms of climate applications and temporal scales, we will try in this study to indicate for each impact detected if it’s a positive (+) or a negative (-) impact : No impact detected Low impact Significant impact Synthesis of all the RRDP analyses

Climate Applications Temporal ScalesT/PJason-1Jason-2ERS-1ERS-2Envisat Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) + Periodic Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi- annual Signals MesoscaleSignals < 2 months + Selection of GPD V2.0 / SL_cci (ref V1.1) ??? ? ?? Wet Troposphere Corrections GPD V2.0 / V1.1

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version.  Performances at crossovers are equivalent and the analysis of MSL shows no degradation nor improvement over this period.  There is however a strong impact on the long term evolution at global scale TOPEX/Poseidon Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months Wet tropospheric correction comparison for TOPEX/Poseidon mission

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version.  Performances at crossovers are equivalent and the analysis of MSL shows no degradation nor improvement over this period.  There is however a strong impact on the long term evolution at global scale  The bug over 2008 in the GPD_V1.1 induced by the bug in the “enhancement products” seems attenuated but is still present. This implies a 1.5mm bias on the Global MSL in SL_cci product over Jason-1 Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) + Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months Wet tropospheric correction comparison for Jason-1 mission

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version.  Performances at crossovers are equivalent and the analysis of MSL shows no degradation nor improvement over this period.  There is however a strong impact on the long term evolution at global scale Jason-2 Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months Wet tropospheric correction comparison for Jason-2 mission

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version.  Performances at crossovers are equivalent and the analysis of MSL shows no degradation nor improvement over this brief period.  Main impacts are on the long-term evolutions, at global and regional scales. ERS-1 Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months Wet tropospheric correction comparison for ERS-1 mission

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version.  Performances at crossovers are equivalent and the analysis of MSL shows no degradation nor improvement over this period. The improvements are mainly in the Tropical Band ERS-2 Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months Wet tropospheric correction comparison for ERS-2 mission

To conclude: GPD_V2.0 correction extension shows performances equivalent to its previous version except for mesoscale where it shows improvements  The analysis of MSL shows no degradation nor improvement over this period.  There are however low impacts are on the long-term evolutions, at global and regional scales and strong positive impacts on mesoscale. ENVISAT Climate Applications Temporal Scales Round Robin Data Package (RRDP) GPD_V2.0 Versus GPD_V1.1 Global Mean Sea Level Long-term evolution (trend) Inter annual signals (> 1 year) Annual and semi-annual Signals Regional Mean Sea Level Long-term evolution (trend) Annual and semi-annual Signals Mesoscale Signals < 2 months + Wet tropospheric correction comparison for ENVISAT mission

Complementary work has been performed to analyze the impact of GPDV2.0 on the SL_cci product long term evolutions. Comparison between GPDV2.0 and GPDV1.1 on the whole period shows a “parabolic” impact: positive over the first decade and negative afterwards Complementary analyses Fig: Temporal evolution of global mean differences between GPDV2.0 and GPDV1.1 WTCs concatenated for TOPEX/Jason-1/Jason-2

A comparison to the Wet Troposphere Correction computed with ERA-Interim has been performed for TOPEX/Poseidon, Jason-1, Jason-2, ERS-2, ERS-2 and Envisat. This study is based on the paper Legeais et al. (2014): New characterization of radiometer wet troposphere correction errors for the altimeter sea level estimations thanks to the ERA-Interim reanalysis. A conclusion of this paper is that the WTC derived from ERA-Interim is the most- adapted model reference to perform this quality assessment, particularly over Complementary analyses Analyzing the temporal evolution of the differences between the modeled and the instrumental WTC, Legeais et al. (2014) highlights a ‘parabolic’ signal for the TP/J1/J2 time series as well as for the E1/E2/EN one.

Complementary analyses WTC Difference Trend (mm/yr) Period: Period: ERA-Radiometer (enhancement product) ERA-GPD V ERA-GPD V Fig: Temporal evolution of global mean differences between ERA-interim-based WTC and Radiometer, GPDV1.0, GPDV2.0 WTCs concatenated for TOPEX/Jason-1/Jason-2

Complementary analyses Fig: Temporal evolution of global mean differences between ERA-interim-based WTC and Radiometer, GPDV1.0, GPDV2.0 WTCs concatenated for TOPEX/Jason-1/Jason-2 WTC Difference Trend (mm/yr) Period: Period: ERA-Radiometer (enhancement product) ERA-GPD V1.1 (correction of 1mm bias cycle 228 Jason-1) ERA-GPD V2.0 (correction of 0.8mm bias cycle 228 Jason-1) Same plot: If we correct empirically for the ~1mm bias in Jason-1 (from cycle 228 onwards) in GPDV1 and GPDV2, we see that: GPDV1 has actually a trend close to the radiometer correction over whereas GPDV2 is closer to the ERA WTC

Complementary analyses Fig: Temporal evolution of global mean differences between ERA-interim-based WTC and Radiometer, GPDV1.0, GPDV2.0 WTCs concatenated for ERS-1/ERS-2/ENVISAT WTC Difference Trend (mm/yr) Period: Period: ERA-Radiometer (RA2-MWR v2.1b) ERA-GPD V ERA-GPD V

Complementary analyses Fig: Temporal evolution of global mean differences between ERA-interim-based WTC and Radiometer, GPDV1.0, GPDV2.0 WTCs concatenated for TOPEX/Jason-1/Jason-2 WTC Difference Trend (mm/yr) Period: Period: ERA-Radiometer (enhancement product) GPD V1.1-Radiometer (enhancement product) GPD V2.0-Radiometer (enhancement product) 0.1 The bias in Jason-1 (from cycle 228 onwards) in GPDV1 and GPDV2 is clearly visible

Complementary analyses Fig: Temporal evolution of global mean differences between ERA-interim-based WTC and Radiometer, GPDV1.0, GPDV2.0 WTCs concatenated for ERS-1/ERS-2/ENVISAT WTC Difference Trend (mm/yr) Period: Period: ERA-Radiometer (RA2-MWR v2.1b) GPD V1.1-Radiometer (RA2- MWR v2.1b) GPD V2.0-Radiometer (RA2- MWR v2.1b)

Complementary analyses In conclusion, GPD V2.0 induces a very significant impact on the long-term evolution of the Global MSL. A priori, this impact is induced by the use of SSM/I data for intercalibration in the correction, which was not performed in GPD V1.1 The comparison to a WTC based on ERA-Interim shows that GPD V2.0 tends to be closer to this model than GPD V1.1 was for the first decade. However, SSM/I data are also used in ERA-Interim, implying the corrections are not independent, therefore this comparison is not necessarily sufficient to assess the reality of this drift.. The potential impact on SL_cci V2.0 would be significant on long-term evolutions. Since SL_cci products are designed for climate studies, this choice and its impact must be duly justified. However, as of today, we have no element indicating that this change will have a positive impact on the product’s quality.