Calibration Set-up for Reference Radiosondes Meeting GRUAN Requirements

Abstract

Accurate and reliable weather observations are necessary for transport, industry and everyday living. Along with ground observations upper air observations provide data for forecasts and for climate change studies However, the quality of upper air measurements does not yet fulfil the requirements of climatologists [1] but improved methods and procedures are needed. To enhance the quality Global Climate Observing System (GCOS) has established GCOS Reference Upper-Air Network (GRUAN) comprising about 40 stations that will provide reference observation data for the global radiosonde station network. GRUAN has specified targets and their priority for measurements of all important parameters at upper troposphere and lower stratosphere [2]. Water vapour pressure is one of the first priority parameters with the uncertainty requirement of 2 % in terms of mixing ratio at the measuring range from 0.1 to 90000 ppm [3]. In order to meet the water vapour accuracy requirements set by GRUAN traceable calibrations for radiosondes are needed. Due to a short lifetime and a high calibration cost comparing to price of a sonde GRUAN aims to ensure radiosondes stability, traceability and uniformity by standards [4]. Regardless of that GRUAN requires calibration and traceability to SI for each radiosonde in order to be accepted to GRUAN [4]. This work presents a new apparatus for reference radiosonde calibrations meeting the GRUAN uncertainty requirements. The apparatus was designed to meet the requirements and still to be quick enough for practical calibration use. Applying a hybrid humidity generator method [5] with two saturators, the high accuracy of a single pressure generator and the short stabilisation time of a flow mixing generator are achieved in a single apparatus. The operation range covers dew-point temperatures from 183 K to 283 K and air temperatures from 183 K to 293 K. This paper presents the design of the apparatus with a preliminary uncertainty analysis.