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 mode Glint 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 CH4 and CO2 over cloudy ocean pixels?

RemoTeC retrieval approach

The retrieval uses four spectral bands:

4

GOSAT physics retrieval (1)

Other elements of the state vector:• profiles of CO2 and CH4 number densities • H2O column, 2nd order polynomial for albedo, spectral shifts parameters.

• Single horizontally homogenous cloud layer parameterized by

• the total column droplet number density Ncld

• Cloud height zcld (Gaussian height distr.), • geometrical cloud thickness wcld

• 2 parameter γ distribution (reff, veff=0.05)

5

Sea surface albedo

6

GOSAT physics retrieval (2)

• Additive radiance offset δI1 in the NIR window to account for a non-linearity in the analogue electrical circuitry (Butz et al., 2011)

• Radiance scaling factors ψn (n=2,3,4) for window 2-4.

Synthetic Retrieval for five Measurement Scenarios:1. Single cloud layer2. Two cloud layers3. Single cloud layer + dust-like aerosol4. Single cloud layer + maritime cloud layer5. Low cloud + cirrus + dust-like aerosol

Because of the surface albedo reflection at the sea surface in neglected.

7

Retrieval error and forward error mitigation

CH4 CO2

Forward model errors due to single cloud layer assumption can be mitigated by radiance scaling factors ψn (n=2,3,4)

Scenario 5 (cloud+cirrus+aerosol) never converges

Averaging kernel

CH4 CO2

Xa from TM5 chemical transport model

9

Validation with TCCON

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• Χ2<7• less than 20 iteration• Cloud geometric width ωcld< 1km

Cloud parameters, Lauder

cloud height droplet radius optical depth

Initial values are zcld = 1km, reff = 12.5μm, τcld= 15, ωcld = 0.2km (but screening removes all ωcld > 1 km).

CH4 and CO2 time series, Lauder

CH4

CO2

RemoTeC TCCON

RemoTeC TCCON

δCH4 [%]

δCO2 [%]

CH4 TCCON validation

13

CO2 TCCON validation

14

Conclusions

The GOSAT CH4 and CO2 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, CH4 validation is good, for CO2 further improvement needed (e.g. better a posteriori quality screening) .

Validation with ship campaign measurements would be very desirable.