G4.08

G4.08    Estimates of uncertainties in ocean surface emissivity models.

Gap detailed description

Ocean surface emissivity models are used to estimate ocean surface emissivity based on ocean surface wind fields, temperature and salinity. Several have been developed over the last two decades to support the assimilation of microwave imager data at operational NWP centres and to support applications based on retrievals of the ECV’s listed above from satellite-based microwave imager observations.  These models lack traceable estimates of the uncertainties associated with the computed emissivities in the 10-250 GHz range.  Improved uncertainties associated with emissivity estimates could be developed through targeted campaigns using, for example, airborne radiometers.

Activities within GAIA-CLIM related to this gap

WP4 involves the assessment of at least two microwave imager instruments (AMSR-2 and FY-3C MWRI).  Work carried out during Year 1 of GAIA-CLIM has already highlighted an issue with inconsistencies in various ocean surface emissivity models, which significantly effects the values of simulated radiances computed from NWP fields.  Absolute validation of the various models is currently lacking and hence it is likely that this will remain a significant contribution to the uncertainty in the NWP simulated brightness temperatures. Comparison of the FASTEM model (versions 3 and 6) with other emissivity models will be carried out during Years 2 and 3.

Gap remedy

Remedy #1

Specific remedy proposed

Inter-comparison of available emissivity models.

Measurable outcome of success

Documented quantitative model inter-comparison:   intercomparisons of non-traceable estimates, in this case outputs from independent ocean surface emissivity models, in themselves will not constitute a validation of any individual estimate.  For example, independent estimates can be biased in the same sense.  In many cases, however, such an intercomparison yields useful insights into the mechanisms, processes and parameterisations that give rise to biases.  This approach thus constitutes a useful first step in the validation of (in this case) ocean surface emissivity estimates.  The measureable output of success therefore, for this activity, will be a documented quantitative comparison of FASTEM (various versions) with another, independent, emissivity model, for a realistic sample of global ocean surface conditions.

Achievable outcomes

Technological / organizational viability: Medium

Indicative cost estimate: low-medium (€1M).  The work undertaken as part of WP4 will amount to 3-6 man. months of effort (€50K).  This will be the start of a longer process of emissivity model intercomparison, development and ultimately validation.  Longer term effort to be ~36-100 man-months (~ €360K – 1M).

Relevance

The solution proposed here is fully aligned with the requirement (to establish traceable uncertainties for NWP fields and radiances calculated from them).

Timebound

The remedy (#1) proposed here -  an intercomparison of ocean surface emissivity models- will be started within the timeframe of GAIA-CLIM WP4, but is expected to take longer than the GAIA-CLIM timeframe.

Remedy #2

Specific remedy proposed

Airborne measurements, using traceably calibrated radiometers, combined with traceable in-situ measurements, or traceable estimates from models, of ocean temperature, surface wind-speed and salinity. Airborne campaigns to validate emissivity models in the region (10-250 GHz) using traceable airborne radiometry at these frequencies.  The determination of emissivity will be reliant on sufficiently accurate co-located estimates (from models) or in-situ measurements, of ocean surface skin temperature, salinity and ocean surface wind speed.

Measurable outcome of success

Documented, quantitative, evaluation of ocean surface emissivity models with respect to measurements of ocean surface emissivity obtained during airborne campaigns, for a globally representative range of ocean surface wind speeds, temperatures and salinity. Peer reviewed.

Achievable outcomes

Technological / organizational viability: Medium.  Airborne radiometry capability exists in Europe and the US.

Indicative cost estimate: medium (>1 million Euros).  Airborne campaigns cost ~€250K per 3-week campaign.  Several would be required to sample the full range of ocean surface wind speed and ocean surface temperature conditions.

Relevance

The solution proposed here is fully aligned with the requirement (to establish traceable uncertainties for NWP fields and radiances calculated from them).

Timebound

Remedy #2 is a longer term aspiration, that could be completed in a 2-5-year timescale, resources permitting.

Gap risks to non-resolution