Metrology for Essential Climate Variables
High-quality observations are possible only if they are based on a sustained traceability to SI and with uncertainties associated to the measured essential climate variables... read more
Starting from the experience achieved during the previous JRP ENV07 MeteoMet on the traceability to SI of some Essential Climate Variables (ECVs) and the evaluation of calibration uncertainties, this joint research project, JRP (ENV58 MeteoMet2) aims to extend the investigations to other variables and at evaluating further aspects and contributions to the uncertainty.
The aim is to make a further step towards the final goal: the evaluation of the overall measurements uncertainties for the quantities involved in the meteorological observations and climate change evaluations. The improvement of quality of ECVs recorded data through the inclusion of measurement uncertainty budget will bring to possible strategies for the reduction of the uncertainty.
This JRP, which aims to develop metrological traceability for the measurement of main ECVs defined by GCOS1 , is structured in 3 work packages each covering a different area of observation: Air, Sea and Land. The ECVs considered are: water vapour in upper-air and surface atmosphere, surface and deep sea temperature, salinity, air temperature, precipitation, albedo, permafrost temperature and soil moisture.
As stated by GCOS “Long-term, high-quality and uninterrupted observations of the atmosphere, land and ocean are vital for all countries, as their economies and societies become increasingly affected by climate variability and change”. High-quality observations are possible only if they are based on a sustained traceability to SI and with uncertainties associated to the measured ECVs.
Air humidity is a key parameter in climate processes. A big challenge, for humidity sensors, is the wide dynamic range with a factor of more than 10000 of water content in the atmosphere. Reliable measurements require sensitive and fast responding sensors to quantify dynamic changes characterizing the phenomenon. Radiosondes sensors calibration protocols need to be improved and validated, to cover real conditions.
Two of the key oceanic ECVs, for monitoring and understanding decadal changes in heat content and transport, are temperature and salinity. A comprehensive study of the effect of the main quantities of influence (water pressure and temperature) on thermometers and salinometers is needed, for reducing measurement uncertainty, and validating their characterization.
For land ECVs consistent measurement uncertainties calculations need complete knowledge of the measurement system: its intrinsic behaviour, other parameters of influence, siting etc.
WMO/CIMO2 notices the “importance of instrument intercomparison as a tool to improve the data traceability and the uncertainty calculation and to improve operational and maintenance procedures”. Although at present meteorological calibration laboratory intercomparisons are not satisfactory to guarantee full traceability and data comparability, and a well-defined protocol is missing.
Though the permafrost temperature is classified as a parameter to investigate climate changes, at present few or none are the measurement procedures reporting fully detailed uncertainty budget.
Calibration and measurements standards for precipitation and soil moisture are not yet well enough developed to solve differences between laboratory setup and natural conditions.
To investigate the performance of humidity sensors in real working conditions, reference systems will be developed to validate reference instruments and test facilities for radiosonde and airborne sensors. Two types of sensors for fast changing ECVs will be improved: a microwave hygrometer, and a reference free space instrument for temperature and humidity detection. To improve the reliability of humidity measurements a facility to quantify the enhancement factors for measurements under atmospheric conditions will be built.
A comprehensive study of the pressure dependence of the most widespread deep-sea thermometers will be carried out, as well as an analysis of the temperature-resistance linearization model. A facility for determining temperature and pressure effects on a new generation of absolute salinometers – based on the measurement of seawater refraction index – will be manufactured. Optic-fibers based on Bragg grating will be conceived, studied and metrologically characterized as distributed temperature sensors for sea-surface and sea-profile temperature traceable measurements.
Metrological procedures to evaluate intrinsic characteristics of air thermometers and humidity sensors plus radiation shield will be developed. The effect of influence parameters, such as: sensors siting, rain, and albedo on air temperature measurements, will be analyzed.
Procedures for comparison of laboratories and instruments will be defined in order to give consistency and coherence at the meteorological measurements taken in different places.
Procedures for traceable dynamic calibrations of hygrometers for the air humidity near Earth surface will be developed. New fast step change humidity generators will be manufactured and validated using traceable spectroscopic methods like as TDLAS3 or CRDS4.
A laboratory facility will be constructed to carry out a metrological comparison of the two current methods to measure permafrost temperature (buried or in pipe thermometers along the well drilled in the soil) and to develop calibration procedure, measurements technique, and uncertainty evaluation methods associated with the best technique.
The project will deliver advances to an international science area of critical importance to future global sustainability. The dissemination and impact will be based on the network of collaborators, established during the previous JRP ENV07 MeteoMet. The vision of this JRP is to establish a permanent collaboration between metrology and meteorology communities, for the benefit of the future generations of climatologists. The impact effort is demonstrated also by the involvement of key international stakeholders such as GRUAN5, ISTI6, IAPWS7, main manufacturers and several others.
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