Laboratory characterisation of the radiation temperature error of radiosondes and its application to the GRUAN data processing for the Vaisala RS41

Abstract

Abstract. The paper presents the Simulator for Investigation of Solar Temperature Error of Radiosondes (SISTER), a setup that was developed to quantify the solar heating of the temperature sensor of radiosondes under laboratory conditions by recreating as closely as possible the atmospheric and illumination conditions that are encountered during a daytime radiosounding ascent. SISTER controls the pressure (3 to 1020 hPa) and ventilation speed of the air inside the wind-tunnel-like setup to simulate the conditions between the surface and 35 km altitude, to determine the dependence of the radiation temperature error on the irradiance and the convective cooling. The radiosonde is mounted inside a quartz tube, while the complete sensor boom is illuminated by an external light source to include the conductive heat transfer between sensor and boom. A special feature of SISTER is that the radiosonde is rotated around its axis to imitate the spinning of the radiosonde in flight. The characterisation of the radiation temperature error is performed for various pressures, ventilation speeds, and illumination angles, yielding a 2D parameterisation of the radiation error for each illumination angle, with an uncertainty smaller than 0.2 K (k=2) for typical ascend speeds. This parameterisation is applied in the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) processing for radiosonde data, which relies on the extensive characterisation of the sensor properties to produce a traceable reference data product which is free of manufacturer-dependent effects. The GRUAN radiation correction model combines the laboratory characterisation with model calculations of the actual radiation field during the sounding to estimate the correction profile. In the second part of this paper it is described how this procedure was applied in the development of the GRUAN data product for the Vaisala RS41 radiosonde (version 1, RS41-GDP.1). The magnitude of the averaged correction profile increases gradually from 0.1 K at the surface to approximately 0.8 K at 35 km altitude. Comparisons between sounding data (N=154) that were GRUAN-processed and Vaisala-processed reveal that the daytime differences (GRUAN−Vaisala) are smaller than +0.1 K in the troposphere and increase above the tropopause steadily with altitude to +0.35 K at 35 km. These differences are just within the limits of the combined uncertainties (with coverage factor k=2) of both data products, meaning that the GRUAN processing and the Vaisala processing are in agreement.

Laboratory characterisation and intercomparison sounding test of dual thermistor radiosondes for radiation correction

“…Using the setup, the uncertainty of the GDP of RS41 is evaluated to be about 0.3 • C (k = 2) at 35 km. It is also found that the daytime GRUAN profile is 0.35 • C warmer than the manufacturer's at 35 km (von Rohden et al, 2022); however, the surrounding temperature, which also affects the radiation correction, cannot be changed. Furthermore, an upper-air simulator (UAS) was developed by the Korea Research Institute of Standards and Science (KRISS) to similarly evaluate radiosondes (Lee et al, 2020).…”

“…The uncertainty of the GDP of RS92 during daytime was gradually increased from 0.2 • C at the surface to 0.6 • C at 30 km with the coverage factor k = 2 (Dirksen et al, 2014). Recently, the same group built a new simulator to investigate the solar temperature error of radiosondes (SISTER) and derived a new GDP algorithm for the Vaisala RS41 radiosonde (von Rohden et al, 2022). The setup can control the irradiance, air pressure, ventilation, sensor rotation and tilting of the light incident angle.…”

“…An alternative approach comprises the simulation of solar irradiance with appropriate cloud scenarios, surface albedo and solar angle (Key and Schweiger, 1998). From the perspective of the radiation correction uncertainty, the SI traceability of the simulated irradiance is incomplete when the sky is clear or cloudy because the simulated irradiance is constructed from the average of clear and cloudy sky cases (von Rohden et al, 2022). This results in the increase of the radiation correction uncertainty in the troposphere.…”

Radiation correction and uncertainty evaluation of RS41 temperature sensors by using an upper-air simulator

“…It was previously observed that the temperature rise of the RS92 was initially fast due to the small thermal mass of the sensor and subsequently slow (Dirksen et al, 2014). More recently, the temperature of the RS41 oscillated when the radiosonde was rotating under irradiation (von Rohden et al, 2022). These observations are attributed to the fact that the heating of the sensor boom with a comparably large area is coupled to the heating of the temperature sensor.…”

“…Recently, the same group conducted experiments by using a new laboratory set-up including a wind tunnel with various functionalities and improved uncertainties in processing the GRUAN data for the Vaisala RS41 sensors (von Rohden et al, 2022). However, these experiments were conducted at room temperature, and thus, the influence of the ambient temperature on the radiation error was not investigated.…”