This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 640276.
G2.08 Need for a metrologically rigorous approach to long - term water vapour measurements from Raman lidars in the troposphere and UT/LS
Submitted by Anna Mikalsen on 9 June, 2017 - 16:13
Gap abstract:
One of the paramount needs for developing long-term ECV datasets for atmospheric monitoring is to calibrate measurements using SI traceable standards. For water vapour measured with the Raman lidar technique, a solution is represented by the calibration of water vapour profiles using reference calibration lamps, which are traceable to NMIs standards. Another critical issue to ensure continuous water vapour Raman lidar measurement is due to the weakness of the Raman backscattering from water vapour molecules. 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, but despite this, only a few of them are operated on a continuous basis. Technological improvements or the effective integration with other techniques needs to be pursued.
Part I Gap description
Primary gap type:
- Knowledge of uncertainty budget and calibration
Secondary gap type:
- Spatiotemporal coverage
- Vertical domain and/or vertical resolution
ECVs impacted:
- Water vapour
User category/Application area impacted:
- Climate research (research groups working on development, validation and improvement of ECV Climate Data Records)
Non-satellite instrument techniques involved:
- Lidar
Detailed description:
A long-term data set for monitoring atmospheric water vapour using lidar techniques requires the calibration of Raman lidar water vapour profiles that vary randomly around some mean value (often addressed as a calibration constant that depends only on the instrument setup) and does not involve step jumps of unknown magnitude. Such step jumps in calibration increase the time required to detect atmospheric trends, which is already typically measured in decades [Weatherhead et. al., 1998; Boers and Meijgaard, 2009]. For this reason, it is important to carefully examine any calibration technique developed for ensuring stable and long-term calibrations. Absolute and relative, but also hybrid calibration methods have been developed. More recently, reference calibration lamps, which are traceable to NMIs standards, have proven to be robust for absolute calibration of water vapour Raman lidar to reduce systematic uncertainties and may represent a common reference for all the available systems.
Another challenge for Raman lidars to ensure the collection of water vapour long-term measurements for climate applications is to improve their daytime observing capability. Raman lidars have been shown to provide high resolution water vapour 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 at night compared to Raman lidar. 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 mitigate the gap but with moderate to high costs, though their performance needs to be carefully assessed in advance. Further technological improvements of lidar techniques for measuring water vapour are also expected but over the mid and long term.
In addition, the improvement of synergy of water vapour Raman lidar with other measurement techniques represents an alternative solution upon which to invest. For example, the ACTRIS-2 and HD(CP) projects are working on this aspect to provide users with a synergetic lidar-
Operational space missions or space instruments impacted:
- MetOp-SG
- Microwave nadir
- Infrared nadir
- Passive sensors
- GNSS-RO
Validation aspects addressed:
- Geophysical product (Level 2 product)
Gap status after GAIA-CLIM:
- GAIA-CLIM has partly closed this gap
GAIA-CLIM has contributed to addressing this gap under activities associated with the metrological characterisation of instrumentation.
Part II Benefits to resolution and risks to non-resolution
| Identified benefit | User category/Application area benefitted | Probability of benefit being realised | Impacts |
|---|---|---|---|
Harmonization of water vapour measurements and reduction of biases in the satellite validation |
|
| Improved capability to detect signals of climate change |
Continuous monitoring of water vapor in the troposphere and in the UT/LS in support of satellite validation and assimilation models |
|
| Improved weather and climate forecasts. |
| Identified risk | User category/Application area at risk | Probability of risk being realised | Impacts |
|---|---|---|---|
Lack of harmonization between water vapor Raman lidars globally. |
|
| Inhomogeneities affecting water CDR in the troposphere and stratosphere to detect a signal of climate change. |
Bias and lower performances in the intercomparison or in the retrieval of atmospheric state estimates from sensors synergy. |
|
| Biased, lower vertical and temporal resolution of atmospheric best estimate profile; partially compensated by potential sensor intercalibration. |
Measurement and temporal biases affecting datasets used for satellite validation. |
|
| Limited quality and temporal resolution of lidar water vapour reference measurements available data for OSSE and satellite validation. |
Part III Gap remedies
Gap remedies:
Remedy 1: Synergy between water vapour Raman lidar and other measurement techniques.
Primary gap remedy type:
Laboratory
Secondary gap remedy type:
Laboratory
Proposed remedy description:
The synergy of water vapour Raman lidar with other measurement techniques, like GPS/GNSS, optical and microwave radiometry, etc., provides complementary information on the water vapour structure to constrain, extend or simply improve the quality of the information provided by lidar. In particular, synergy with passive microwave radiometers provides an robust solution to obtaining a low resolution profile of atmospheric water vapour during daytime also above the atmospheric altitude covered by lidar enabling the characterization of the entire atmospheric column: This could partially address this gap but this synergetic solution requires the development of new and more accurate algorithms to fully exploit the potential of the combined datasets. It also requires the co-location of these synergistic measurement techniques in close enough geographical proximity to be usable in this manner.
Relevance:
Continuous measurements of water vapour observations with high spatial (vertical) and temporal resolution are needed to achieve a comprehensive understanding of the role of water vapour on climate at regional and global scales as well as to estimate its impact on OLR = outgoing long- wave radiation (OLR) at top of atmosphere. The availability of water vapour profiles in both cloud and clear sky conditions would largely enhance several activities related to the study of climate, to satellite retrievals, and radiative transfer modelling. Furthermore, the synergetic approach to improve water vapour measurement continuity is at present the only chance to improve daytime water vapour profiling capabilities.
Measurable outcome of success:
Success of any kind of synergetic products or joint retrieval performed using Raman lidar and microwave radiometry (or other measurement techniques) shall be assessed by using the data in the input data stream of the mesoscale models or by validating the water vapour model output profiles. Alternatively, a comparison with radiosounding profiles from Reference networks (i.e. GRUAN) can represent another good way to assess the added values of this higher-level products though in this case the difference in the representativeness of the two different products (lidar+other vs radiosonde) must be quantified and taken in account.
Expected viability for the outcome of success:
- Medium
- High
Scale of work:
- Individually
- Single institution
Time bound to remedy:
- Less than 5 years
Indicative cost estimate (investment):
- Low cost (< 1 million)
Indicative cost estimate (exploitation):
- No
Potential actors:
- Academia, individual research institutes
- SMEs/industry
- National measurement institutes
Remedy 2: Verification and further deployment of the GAIA-CLIM approach to metrological characterisation to Raman Lidar measurements
Primary gap remedy type:
Laboratory
Proposed remedy description:
Work within GAIA-CLIM has advanced the metrological characterisation of raman lidar water vapour products. Verification of the results generated in GAIA-CLIM is required prior to broad-scale adoption of the traceable measurement and processing approach by networks. At the same time work is required to improve the temporal coverage of measurements to increase their utility with a particular focus on advancing daytime measurements.
Relevance:
For water vapour lidar calibration, the proposed remedy will dramatically improve the traceability of water vapour Raman lidar measurements and data consistency globally, and will help to manage changes in the system.
Measurable outcome of success:
Success would be, for example, if long term comparison between Raman lidar water vapour measurements and another traceable reference measurement technique (e.g. GRUAN radiosondes) would be compared over long term showing a reduction in the lidar calibration uncertainty using absolute techniques as well as the added value of synergetic lidar-radiometer products during daytime operations. Evidences of this improvement have been reported in literature but comparisons over long time periods have not been reported yet.
Expected viability for the outcome of success:
- High
Scale of work:
- Individually
- Single institution
Time bound to remedy:
- Less than 5 years
Indicative cost estimate (investment):
- Low cost (< 1 million)
Potential actors:
- Academia, individual research institutes
- SMEs/industry
- National measurement institutes
References:
- Boers, R., van Meijgaard, E., 2009. What are the demands on an observational program to detect trends in upper tropospheric water vapor anticipated in the 21st century? Geophys. Res. Lett. 36, L19806.
- Weatherhead, E. C., and coauthors, Factors affecting the detection of trends: Statistical considerations and applications to environmental data. J. Geophys. Res., 103, 17 149–17 161, doi:10.1029/98JD00995, 1998.