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Transcript of Sea Surface Salinity under rain cells: SMOS satellite and in-situ drifters observations J. Boutin 1,...
Sea Surface Salinity under rain cells: SMOS satellite and in-situ drifters observations
J. Boutin1, N. Martin1, G. Reverdin1,S. Morisset1, X. Yin1, L. Centurioni2 and N. Reul3
1 LOCEAN, Sorbone Universités, UPMC/CNRS/IRD/MNHN, Paris, France2SIO, La Jolla, CA, USA
3IFREMER, Toulon, France
Boutin et al., 2014
The impact of rain on SMOS SSS?
SMOS SSS lower than ARGO optimal Interpolated SSS maps in rainy regions (e.g. ITCZ, SPCZ..) : what part of this difference explainable by rain stratification/intermittency?
SMOS – ARGO SSS in tropical Pacific 0.1 fresher and more variablethan in subtrop Atlantic; if SMOS rainy measurements are removed, std_diff in ITCZ and SPURS becomes the same =>rain effect in ITCZ
Ascending + Descending orbits
Mean(std) SMOS-ARGO SSS-0.12 (0.46) 45°S-45°N -0.02 (0.25) Subtrop. Atlantic-0.15 (0.30) N.E. Trop. Pacific
SMOS - ARGO (Jul-Sep 2010)SMOS SSS averaged within +/-50km & +/- 5days around ARGO SSS
32 3932
39
Sargo
Ssm
os
Boutin et al., Ocean Science, 2013
SalinityWell Mixed
Salinity
Dep
th
Stratified with Rain
Dep
th
Salinity
Dep
th
Stratified with Evaporation
Near-Surface Salinity Schematic Diagram for L-band Radiometer
Rain-inducedfresh layer Evaporation-
inducedsalty layer
Satellite L-band radiometric salinity at depth range from 1 to 10 cm
Salinity measured by in situ sensors/platforms at depth below 1 m
Schematic Diagram made by the SISS working group
Boutin et al., EGU 2012
Vertical gradients 15cm & 45 cm depth as seen by surface drifters17 events SVP-BS / Surplas
Vertical gradients 15cm & 45 cm depth as seen by surface drifters17 events SVP-BS / Surplas
SURPLAS tied to a SVP-BS drifter (CAROLS2010 cruise, Gulf of Biscay)
Reverdin et al. JGR 2012
SVP
Surplas
French and Spanish SSS drifters 2007-2012
http://www.locean-ipsl.upmc.fr/smos/drifters/
Freshening event +4h
Question
How reliable is the rain induced SSS variability measured by SMOS?
Can we confidently use satellite SSS for studying the influence of rain on sea surface (~1cm) salinity?
SMOS SSS (color) & SSM/I rain rate (isolines)
Train-Tsmos =0.5h
Boutin et al. JGR 2014
Boutin et al., 2013
SATELLITE
SMOS (Soil Moisture and Ocean Salinity) SSS
ESA v5 reprocessing (available since 2010)
SSS at 1cm depth ; ~43km resolution (s~0.6) or monthly 100km averaged
(CATDS-CEC/LOCEAN_v2013 product available at www.catds.fr) ; moderate wind speed (3-
12m s-1)
Rain Rate: - RemSS: www.remss.com ; SSM/I; AMSRE; TMI; WindSat: 0.25° resolution within
[-30mn;+15mn] from SMOS SSS - TRMM 3B42 rain rates (within [-2hr;+1hr] from ARGO to identify‘ARGO rainfree’)
IN SITU SSS
ARGO INDIVIDUAL PROFILES
‘SSS’ between 10m and 4m depth; Colocation with SMOS within +/-5days, +/-50km CORIOLIS
GDAAC: http://www.coriolis.eu.org
ARGO + TSG OPTIMAL INTERPOLATED SSS MAPS (ISAS)
Monthly maps from In-Situ Analysis System v6 http://wwz.ifremer.fr/lpo/SO-Argo/Products/Global-Ocean-T-S
SEA SURFACE AUTONOMOUS DRIFTER SSS
Upper S at 45cm depth; Pacific Gyre drifterhttp://www.locean-ipsl.upmc.fr/smos/drifters
Data & Methods
Rain effect -The closest rainy ARGO-SMOS colocation case
SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38
SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34
0
50
100
150
200
Dep
th(m
)
a) ARGO profile on 11/8 20:00 UTC
32.5 33.0 33.5 34.0 34.5 35.0SSS
2
ARGO
AMSRE 11 Aug. 20:55 ~ARGO profile
<SSSsmos 10d,100km>
ISAS
Boutin et al, 2013, Ocean ScienceRain Rate (mm/hr)0 2
SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38
SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34
0
50
100
150
200
Dep
th(m
)
a) ARGO profile on 11/8 20:00 UTC
32.5 33.0 33.5 34.0 34.5 35.0SSS
2
ARGO
AMSRE 11 Aug. 20:55 ~ARGO profile
SSMIs F17 11 Aug. 13:40 ~SMOS 1st pass <SSSsmos 10d,100km>
ISAS
Boutin et al, 2013, Ocean ScienceRain Rate (mm/hr)0
Effect of rain on ARGO & SMOS (The closest colocated case)
Norain
10 days after...
SSSsmos on 11/8 at 13h39= 32.9+/-0.2; N=38
SSSsmos on 16/8 at 13h44 = 33.4 +/-0.3; N=34
0
50
100
150
200
Dep
th(m
)
a) ARGO profile on 11/8 20:00 UTC
32.5 33.0 33.5 34.0 34.5 35.0SSS
0
ARGO
<SSSsmos 10d,100km>
ISAS
SSSsmos on 21/8 at13h49= 33.6+/-0.2; N=37
SSSsmos on 24/8 at13h33= 33.3+/-0.6; N=34
Dep
th(m
)
b) ARGO profile on 22/08 6:52 UTC
32.5 33.0 33.5 34.0 34.5 35.0SSS
0
50
100
150
200
0.7 0 Rain Rate (mm/hr) 2
0 2Rain Rate (mm/hr)
AMSRE 22 Aug. 9:40 ~ARGO profile
SSMI F15 24 Aug. 13:12 ~SMOS 2nd pass
ARGO
Norain
The impact of rain on SMOS SSSSMOS SSS - ARGO_rainfree[-2hr;+1hr] SSSSMOS SSS- ARGO SSS versus satellite RRTropical Pacific 5N-15N (July-Sept 2010)
-0.18 pss/ mm/hrr = -0.5
Boutin et al, JGR, 2014
In SW Pac :-0.22 pss/mm/hrr = -0.5
The impact of rain on SMOS SSS ~-0.2pss/mm/hr : Roughness, Atmosphere or Salinity effect?
SMOS SSS retrieved with 5m/s error on prior wind speed (instead of 2m/s error)
More on SMOS retrieved wind speed: Yin et al. RSE 2013
=> Roughness contribution is at the limit of detection by SMOS
~ -0.01pss/mm/hr
Atmospheric contribution Rayleigh approximation (e.g., Peichl
et al., 2004; Wentz, 2005)~ - 0.03pss/mm/hr
=> Fresher 1cm SSS linked to rain:
at least -0.14 pss/mm/hr
-0.01 pss/ mm/hrLimit of significance
Error on a priori WS=5m/s
Error on a priori WS=2m/s
Boutin et al, JGR, 2014Aquarius (atm-rouhness corrected )=> ~-0.12pss/mm/hr @7m/s, Meissner
The impact of rain on SMOS SSSSMOS SSS spatial variations
SMOS SSS (color) & SSM/I rain rate (isolines)
Train-Tsmos =0.5h
Rainy SSS – ‘Rain-free’ SSS ~ -0.19 RR ‘Rain-free’ SSS: average of SSS colocated with RR=0 in a radius of 150km around rainy pixels
Slope=-0.19pss/mm/hrr=-0.66
Rain
y –
‘rain
-free
’ SSS
Rain Rate (mm/hr)26 August 2012
Similar dependency as in SMOS-ARGO comparisons:After roughness & atmospheric effect correction => SSSrain – SSSrain_free : at least -0.15 pss/mm/hr How does this compare with S45cm drifters observations?
Trajectories of drifters deployed and validated since 2010
Þ Identification of 470 ‘freshening’ (>0.4 signature) eventsÞ Colocations with 6 satellites (WindSat, TMI, SSMIs, AMSRE), at
+/-15mn => 25 matchups
The impact of rain on in situ SSS at 45cm depth
Thanks to LEGOS, LPO, ICM and SPURS-US colleagues for drifters deployments
Example of SSS decrease observed by drifter and colocated with RR within 15mn
SSM/I F16 RR21:06
-0.6
The impact of rain on in situ SSS at 45cm depth
SSS decrease between local maximum and SSS minimum
SSS decrease observed by drifter and colocated with RR within 15mn: a complicated case
SSM/I F17 RR19:24
-0.6
Sref
Smin
SSM/I F16 RR19:36
19:30
SSS decrease between local maximum and SSS minimum
Large variability of RR within 12mn and in most cases only one satellite RR pass => in order to smooth the RR variability, we take RR averaged over 9 pixels around the drifter
The impact of rain on in situ SSS at 45cm depth
Wind Speed >15m/s
Wind Speed < 3m/s
RR averaged over ~75km (mm/hr)
The impact of rain on drifter SSS at 45cm depth
Loca
l S45
cm d
ecre
ase
Moderate wind speed (3-12m/s)SSSmin-SSSref= -0.21 (+/-0.14) RRr=-0.6N=21
O Subtropics• Tropics
Summary• Validating satellite SSS under rain cell is a big issue : difficult to colocate in
situ & satellite measurements (S, RR, U...) at relevant time/space scales
• At moderate wind speed (~7m/s), decrease of SSS associated with microwave rain rates estimated to be:• AQUARIUS (150km resolution, S1cm ) , ~0.12pss/mm/hr (Meissner et al. 2014)• SMOS (40km resolution, S1cm ) , > ~0.14 pss/mm/hr (This work) • Drifters (ponctual , S45cm ) , ~0.21 pss/mm/hr (This work)
=> the rain impact seems to decrease when increasing the pixel size – qualitatively expected from a spatially heterogeneous process like rain (NB: not only stratification matters - drifters S are deeper than satellite S!) - Is it quantitatively reasonnable? SPURS 2 experiment should help constraining the S vertical and horizontal variability and the RR horizontal variability within a satellite pixel • N.B.: the monthly difference SMOS-ISAS SSS cannot be fully explained
applying a -0.2pss/mm/hr correction on monthly SMOS SSS (locally <0.2 correction) explains less than 40% of the observed SMOS-ISAS SSS : ISAS smoothing/relaxation to climatology, SMOS artefacts (RFIs, islands ...)....