G2.09    Continuous water vapour profiles from Raman lidars limited during daytime

Gap detailed description

Raman lidars have been shown to provide high resolution measurements in several experiments, but these measurements are typically restricted to night time only, as Raman scattering is a weak physical process and the high solar background radiation during the day tends to mask these signals. During daytime, a few water vapour Raman lidars have already proven to be able to measure water vapour up to 3-4 km above ground level. Only DIAL systems can do better, but they do worse in the UT/LS. Most of the water vapour Raman lidar systems are not operated during daytime and this generates a discontinuity in the water vapour monitoring in the troposphere in a climatological sense.

The use of commercial systems, Raman lidar or DIAL, designed to operate on a continuous basis, can improve the gap but with moderate to high costs, though their performance needs to be carefully assessed in advance.

Synergy with other techniques, like passive microwave radiometry, provides an alternative solution to obtaining a profile of atmospheric water vapour during daytime over the entire investigated atmospheric column: this could partially address this gap but this synergetic solution requires the elaboration of new and more accurate algorithms to fully exploit the potential of the combined datasets.

Activities within GAIA-CLIM related to this gap

GAIA-CLIM does not have any specific activity to address this gap.

Gap remedy(s)

Remedy #1 

Specific remedy proposed

The ACTRIS-2 and HD(CP)² projects are working on this aspect and before April 2017 both should provide results and the assessment of the real performances of this synergetic solution. Technological improvements of lidar techniques for measuring water vapour are also expected but over the mid and long term.

Measurable outcome of success

Successful comparison with other ground based measurements techniques (e.g. Radiosondes) showing the capability of lidar - microwave radiometer synergy during daytime operations. Technological improvements to the current lidar technology must be validated again radiosoundings as well.

Achievable outcomes

Retrieval algorithms exploiting the synergy between lidar and microwave radiometer are under elaboration to improve daytime water vapour profiling capabilities.

Technological / organizational viability: High / medium

Indicative cost estimate: low (<1 million).  The cost of technological improvements in the Raman lidar or DIAL systems to improve daytime performance in the troposphere must be quantified once implemented or available on the market.


The proposed remedy is the only chance at the moment to improve daytime water vapour profiling capabilities.


Synergetic retrieval shall be available by end of ACTRIS project (2019).

Gap risks to non-resolution

Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

Missing monitoring of water vapour such that it is measured only during night time conditions


Diurnal water vapour variability in the troposphere will rely on radiosoundings only; temporal resolution of available data for OSSE and satellite validation will be limited.

Lower performances of the algorithm retrieving the site atmospheric state best estimate.


Lower vertical and temporal resolution of the lidar profiling used to retrieve the site atmospheric state best estimate.


Work package: