Comparison of Meisei RS2-91 Rawinsondes and Vaisala RS92-SGP Radiosondes at Tateno for the Data Continuity for Climatic Data Analysis

Observation instruments are commonly upgraded because of technological advances and the convenience of the observation agency. However, great care is necessary when changing instruments to ensure data continuity for climatic data analysis. The Tateno upper-air observation station of the Japan Meteorological Agency replaced the Meisei RS2-91 type rawinsondes with Vaisala RS92-SGP type GPSsondes in December 2009. We carried out a total of 115 simultaneous dual launches for four seasons to investigate any differences in performance. The simultaneous sensor comparison results showed that Vaisala RS92-SGP temperature was 0.1–0.4K higher than Meisei RS2-91 temperature above the 100 hPa layer in night time observations; and that Meisei RS2-91 temperature was ~0.1K higher above the 30 hPa layer in day time observations. Vaisala RS92-SGP relative humidity was ~5% lower, particularly under humid conditions and in autumn. Vaisala RS92-SGP pressure was ~0.5 hPa higher in the stratosphere. We also made pressure-level comparisons for temperature and relative humidity. Furthermore, comparison results are shown for precipitable water vapor measurements taken with a collocated GPS receiver for a sensitivity analysis on the number of dual soundings and for a reanalysis of upper-air temperature trends for 1956–2010, taking the three instrumental change events into consideration. weather analysis, data assimilation in weather forecasting and climate monitoring. The Japan Meteorological Agency (JMA) operates 16 stations in Japan and at the Showa station in Antarctica for upper-air observations, one of which is the Aerological Observatory in Tateno, Ibaraki Prefecture. The Tateno station is placed at 35°3′N 140°8′E and an elevation of 25.2m. Corresponding author: Eriko Kobayashi: Aerological Obser vatory/JMA, Nagamine 1-2, Tsukuba-shi, Ibaraki 305-0052, Japan. E-mail: eriko-kobayashi@met.kishou. go.jp Journal of the Meteorological Society of Japan Vol. 90, No. 6 924 The 16 Japanese stations used Meisei RS2-91 type rawinsondes for observations from 1992 until 2009, when they were replaced with the Vaisala RS92-SGP type GPSsonde at Tateno and 10 other stations. Four other stations use the Meisei RS-06G type GPSsonde, and the Chichijima station uses the Sippican LMS6 type GPSsonde. The Tateno station contributes to the GCOS Reference Upper Air Network (GRUAN, Seidel et al. 2009) established by the World Meteorological Organization (WMO), Intergovernmental Oceanographic Commission, United Nations Environment Programme and International Council for Science. Precise upper-air observations for the monitoring of long-term climate change is one of the GRUAN’s purposes, and documenting changes in instruments and their properties is important in climate change research so that an instrument performance change does not introduce an incorrect climatic trend (Steinbrecht et al. 2008). Therefore, verification of data continuity is particularly important when observation equipment is replaced, or when equipment manufacturers change their equipment or data processing method. JMA has made replacement of radiosondes to accomplish upper-air observations with higher accuracy, and they made comparison observations of radiosondes during past equipment changes (Aerological Division of JMA 1983, Sakota et al. 1999). Therefore, we made a series of comparison launches of the Meisei RS2-91 sonde (hereafter Meisei-91) with the Vaisala RS92-SGP sonde (hereafter Vaisala-RS92) at Tateno, deploying both instruments at once in order to verify data continuity. The replacement of radiosondes at Tateno in 2009 was the first such case among the GRUAN sites, and the verification of data continuity at Tateno could provide a model for management of instrument changes within the GRUAN. The remainder of this paper is organized as follows. Section 2 describes the methods of dual launches and data analysis, including the explanation of two comparison methods, i.e., simultaneous sensor comparison and pressure-level comparison. Section 3 shows the results from the two comparison methods. Section 4 discusses precipitable water vapor comparison measurements with a collocated GPS receiver (4.1), sensitivity analysis on the number of dual soundings (4.2), and reanalysis of upper-air temperature trends for 1956–2010 by considering three instrumental change events (4.3). Finally, Section 5 lists the main conclusions. 2. Methods of dual launches and data analysis 2.1 Flight configuration and flight schedule The Meisei-91 and Vaisala-RS92 sondes (JMA 2004; Vaisala 2006) characteristics are listed in Table 1, and the photographs of the temperature and humidity sensors for both radiosondes are shown in Fig. 1. The flight configurations of a dual launch are as follows (Fig. 2). The two radiosondes were attached to the ends of a rig consisting of a 2-m bamboo pole to avoid radio interference and were launched with a balloon. The weight of the balloon was 1200 g (2000 g balloons for winter in order to obtain the data at the highest possible altitude), and the rig was suspended on a string 30 m long (60 m when the 2000 g balloons were used). The Table 1. Types of sensors for the Meisei-91 and Vaisala-RS92 radiosondes and their manufacturer specifications (*relative humidity data at temperatures below –40°C were not used).