G4.09

G4.09     Estimates of uncertainties in land surface microwave emissivity atlases.

Gap detailed description

Microwave imagers are highly surface sensitive, particularly in dry atmospheric conditions, to emission from the surface and so the validation of microwave imaging instruments over land requires independent, well-characterised values of emissivity with traceable uncertainty estimates. Land surface emissivity atlases in the microwave region (10-250 GHz) have been developed in recent years and these are widely used as starting points for dynamic (i.e.  those updated on a daily timescale or less) retrievals of land surface emissivity within retrieval and assimilation schemes. Such atlases can provide a good estimate of the emissivity in most surface conditions, particularly for un-changing environments such as rainforests and deserts. However, for some conditions, particularly snow and ice, the emissivity exhibits significant temporal and (in the case of snow and ice) spectral variability and atlas values based on climatological averages are then less accurate. For this reason, we suggest both the development of dynamic land emissivity atlases (e.g. daily-monthly maps of emissivity) in the short-term as well as the development of emissivity models in the long-term, particularly for snow and ice-covered surfaces. Both should be validated against experimental campaigns with traceable measurements, using, for example, well calibrated airborne radiometers, in order to obtain uncertainty estimates for these emissivity values.

Activities within GAIA-CLIM related to this gap

None.

Gap remedies

Remedy #1

Step 1:  Development of dynamic land surface emissivity atlases in the MW, derived (for example) from dynamic estimates of spectral emissivity at window channel frequencies from NWP systems (see Remedy #2 below).

Step 2: Airborne measurements, using traceably calibrated radiometers, combined with traceable in-situ measurements (or model-based estimates where the accuracy has been previously determined) of land surface temperature over a diverse range of land surface environments and over several seasons.

Specific remedy proposed

Airborne campaigns to validate emissivity atlases, and models, in the region (10-250 GHz). These should sample a diverse range of surface types and characterize the seasonal cycle for those surface types which exhibit a strong seasonal dependence (or are suspected of doing so).

Measurable outcome of success

Documented, quantitative, evaluation of land surface emissivity atlases (and models) with respect to measurements of land surface emissivity obtained during airborne campaigns, for a globally representative range of land surfaces. Peer reviewed.

Achievable outcomes

Technological / organizational viability: Medium

Indicative cost estimate: medium (>1 million Euros)

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

Remedy #2

·        Establishing dynamic atlases of land surface emissivity, building on existing ‘climatological’ atlases, but using operational data assimilation systems to derive optimal estimates of land surface emissivity from satellite observations.

·      Conducting inter-comparisons of independent land surface emissivity atlases, both dynamic and climatological.

·      Ground-based radiometer measurements (e.g. J.-C. Calvet et. al., 2011) over a range of surface types and measuring for a range of frequencies using traceably calibrated radiometers, and supported with traceable in-situ measurements.

Specific remedy proposed

·         Establish dynamic land surface emissivity atlases.

·         Inter-comparison of available emissivity models.

·       Development of emissivity models over a wide range of frequencies (10 – 250 GHz), that rely on remotely-sensed parameters and/or atlases of land surface characteristics and are validated with ground-based or airborne radiometer measurements for different surface types

Measurable outcome of success

Documented, quantitative, evaluation of land surface emissivity atlases and models with respect to measurements of land surface emissivity obtained during airborne campaigns, for a globally representative range of surfaces. Peer reviewed

Achievable outcomes

Technological / organizational viability: Medium

Indicative cost estimate: medium (>1 million Euros)

Relevance

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

Timebound

These remedies are longer term aspirations, that could be completed in a 2-5 year timescale, resources permitting.

Gap risks to non-resolution