G2.06

G2.06     Need of multi-wavelength Raman lidars

Gap detailed description

Raman lidars or multi-wavelength Raman lidars are undoubtedly the backbone of an aerosol global measurement infrastructure as they can provide quantitative range-resolved aerosol optical and microphysical properties. Whereas the detection of aerosol layers and their vertical extent requires only simple single wavelength backscatter lidars, the derivation of extinction coefficient profiles and a series of intensive aerosol properties requires advanced lidar concepts such as high-spectral resolution lidars (HSRL, Shipley et al., 1983) or Raman lidars (Ansmann et al., 1992). The retrieval of aerosol microphysical properties and mass concentration requires at least a one-wavelength Raman lidar, but the error affecting these estimations can be dramatically reduced if a multi-wavelength lidar systems is used.

This shows the relevance of having a large number of multi-wavelength lidar systems at the global scale; the relevance is also related to their potential role as anchor reference station for the study of the impact of aerosol on weather and climate and for the satellite validation. The availability of multi-wavelength Raman lidar measurements also ensures that  ground-based instruments can deliver wavelength conversion information for different aerosol and cloud types to relate the space-borne measurements performed by different satellite missons at different wavelengths (for example CALIPSO at 532 nm and the future EarthCARE mission at 355 nm).

Multi-wavelength Raman lidars could also be considered to be the future backbone of a larger network incorporating simpler lidar instruments and/or ceilometers, and so be able to have a more dense global spatial coverage. In this process it is very important to carefully assess the value of the retrieval of advanced lidar systems and to study if the coverage of the existing networks at the global scale is sufficient to carry out an accurate aerosol study.

Activities within GAIA-CLIM related to this gap

Some activities pertinent to this will be addressed in task 1.4 but the gap cannot be solved completely within the timeframe of GAIA-CLIM.

Gap remedy(s)

Remedy #1

Specific remedy proposed

The remedy is strongly related to the identification of the existing Raman lidar measuring aerosol properties at the global scale and then to the study of representativeness of the each station in the characterization of aerosol variability in the different vertical atmospheric regions. This study can allow the identification of those climatic regions where multi-wavelength Raman lidars are required.

Measurable outcome of success

This is obviously related to the establishment of multi-wavelength Raman lidars in the region where a lack of lidar instruments is identified by a study of representativeness of the existing measurements of aerosol properties. This study allows a rationalization of the required investments.

Achievable outcomes

Task 1.4 of GAIA-CLIM will partly deal with the achievable outcomes. It will provide an estimation of the aerosol variability at the continental or at the global scale giving also recommendations for the optimal design of an aerosol lidar network. This study will allow us to identify on a scientifically sound basis the prescient gaps in the current observing systems of aerosol optical properties. This will also allow us to provide recommendations for the expansion of existing networks.

Technological: The technology to provide a robust, compact and affordable solution already exists and has been launched on the market by a few companies; decreasing the current costs and improving the performance of these systems, while taking advantage of the expertise of the existing lidar networks and of their calibration facilities, this development should not represent a big technological challenge.

Organizational viability: Medium. Several institutions are investing in lidar systems and the available budget may cover the purchase or the implementation of multi-wavelength Raman lidars.

Indicative cost estimate: High (>5 million)/ medium (>1million).

This gap’s resolution also depends upon the funding plans of scientific institutions, agencies and Met Services who are encouraging the development of ceilometer/simple lidar networks but tend to neglect the need for a few reference Raman lidars. Costs are also strongly dependent upon the development of new robust low-cost solution available on the commercial market.

Relevance

On the basis of task 1.4 activities, recommendations on the improvements of the existing global lidar network to characterize aerosol optical and microphysical properties will be provided. However, a complete remedy for this gap is strongly related to the strategies of the international research institutions which are at present the key players in the deployment and the operation of Raman lidar measurements.

Timebound

Recommendations will be provided in the deliverable D1.9 expected in December 2017.

Low-cost commercial solutions may increase the number of Raman systems deployed at the global scale over the next 5-10 years. Future work will be addressed to assess this commercial solution using the WP2 work on the measurement traceability and the activities carried out at the ACTRIS-2 calibration center.

Gap risks to non-resolution