Methane and carbon dioxide total columns over cloudy oceans measured by shortwave infrared satellite sounders D. Schepers, I. Aben, A. Butz, O.P. Hasekamp, J. Landgraf

GOSAT data coverage: Nadir-glint Typical spatial coverage of one 3-day repeat cycle of GOSAT observations Nadir modeGlint mode Spatial coverage of glint observations is limited and currently no Nadir ocean observations are not used for data interpretation. Science Question: Is it possible to retrieve CH 4 and CO 2 over cloudy ocean pixels?

RemoTeC retrieval approach The retrieval uses four spectral bands:

GOSAT physics retrieval (1) 4 Other elements of the state vector: profiles of CO 2 and CH 4 number densities H 2 O column, 2nd order polynomial for albedo, spectral shifts parameters. Single horizontally homogenous cloud layer parameterized by the total column droplet number density N cld Cloud height z cld (Gaussian height distr.), geometrical cloud thickness w cld 2 parameter γ distribution (r eff, v eff =0.05)

Sea surface albedo 5

GOSAT physics retrieval (2) 6 Additive radiance offset δI 1 in the NIR window to account for a non-linearity in the analogue electrical circuitry (Butz et al., 2011) Radiance scaling factors ψ n (n=2,3,4) for window 2-4. Synthetic Retrieval for five Measurement Scenarios: 1.Single cloud layer 2.Two cloud layers 3.Single cloud layer + dust-like aerosol 4.Single cloud layer + maritime cloud layer 5.Low cloud + cirrus + dust-like aerosol Because of the surface albedo reflection at the sea surface in neglected.

Retrieval error and forward error mitigation 7 CH 4 CO 2 Forward model errors due to single cloud layer assumption can be mitigated by radiance scaling factors ψ n (n=2,3,4) Scenario 5 (cloud+cirrus+aerosol) never converges

Averaging kernel CH 4 CO 2 X a from TM5 chemical transport model

Validation with TCCON 9 Lauder Validation with TCCON measurements at coast regions (Tsukuba, Caltech, Izana, Ascension, Darwin, Reunion, Wollongong, Lauder) Select GOSAT ocean soundings within a 5o radius around TCCON site Temporal co-registration ±5 hours A posteriori filtering Χ 2 <7 less than 20 iteration Cloud geometric width ω cld < 1km

Cloud parameters, Lauder cloud heightdroplet radiusoptical depth Initial values are z cld = 1km, r eff = 12.5μm, τ cld = 15, ω cld = 0.2km (but screening removes all ω cld > 1 km).

CH 4 and CO 2 time series, Lauder CH 4 CO 2 RemoTeCTCCON RemoTeCTCCON δCH 4 [%] δCO 2 [%]

CH 4 TCCON validation

CO2 TCCON validation 13

Conclusions 14  The GOSAT CH 4 and CO 2 column for nadir ocean observations is an interesting extension of the GOSAT data product to improve spatial coverage.  TCCON validation is aggregated by loose spatial co-registration and a priori contribution from models.  Overall, CH 4 validation is good, for CO 2 further improvement needed (e.g. better a posteriori quality screening).  Validation with ship campaign measurements would be very desirable.