G2.14

G2.14    Lack of a comprehensive review of the uncertainty associated with MW absorption models used in MWR retrievals

Gap detailed description

Most common MWR retrieval methods are based on the theory of radiative transfer through the atmospheric medium. Thus, uncertainties in modelling the absorption/emission of microwave (MW) radiation by atmospheric gases and hydrometeors affect all the retrieval methods based on simulated MW radiances. Only retrieval methods based on historical datasets of MWR observations and simultaneous atmospheric soundings are not affected by absorption model uncertainties. Currently, the information on MW absorption model uncertainties are dispersed and not easily accessible. Most operational MWR operate in the 20-60 GHz range, where relevant absorption comes from water vapor, oxygen, and liquid water. A variety of models are available which combine the absorption of water vapor, oxygen, and liquid water, as well as other minor contributions. Absorption model uncertainties are currently estimated from the output difference of different models, while a more rigorous estimate is lacking. The intention is to address this gap within GAIA-CLIM.

Activities within GAIA-CLIM related to this gap

A review of the state-of-the-art of MW absorption models and the associated uncertainty is planned and has already started. The absorption model uncertainties need to be propagated through radiative transfer and inverse operator to estimate the total uncertainties affecting the retrieval methods.

Gap remedy(s)

Remedy #1

Specific remedy proposed

Modifications of absorption models are continuously proposed within the open literature based on laboratory data and MWR field observations. In addition, there have been some recent advances in this area, specially related to liquid water absorption, which are yet to be published. To fill this gap, and thus estimate the total uncertainties affecting the MWR retrievals, the following activities are needed: (i) a review of the state-of-the-art and the associated uncertainty of MW absorption models; (ii) propagation of absorption model uncertainties through radiative transfer and inverse operator.

Measurable outcome of success

The successful outcome is to produce rigorous estimates of MW forward model uncertainties in the 20-60 GHz band. An additional measure of success would be the usage of the estimated uncertainties in the retrieval methods exploited by the MWR user community.

Achievable outcomes

Technological / organizational viability: medium.  

The literature review has been started including several recently published papers on this topic. The rigorous literature review approach may highlight some scientific challenges that shall tentatively be addressed within GAIA-CLIM.

Indicative cost estimate: low (<1 million).

Relevance

The remedy will make estimates of MW forward model uncertainties available for the MWR user community. The availability of these uncertainties will also help in addressing another identified gap concerning MWR, specifically G2.16. In fact, the effective use of MW forward model uncertainties will provide better characterization of MWR temperature and humidity retrieval uncertainties.

Timebound

2 years.

Gap risks to non-resolution