On the Accuracy of Vaisala RS41 versus RS92 Upper-Air Temperature and Humidity Observations

Balloon-borne radiosonde observations (raobs) play a critical role in upper-air climate change detection, numerical weather prediction (NWP) data assimilation and forecasting, and satellite data calibration/validation. Common with other long-term observing systems, upper-air instrumentation changes over time. Vaisala RS41 has gradually replaced Vaisala RS92 in the past several years, becoming the major sonde type across the Global Reference Upper Air Network (GRUAN) sites as well as much of the global upper-air network.

The temperature observation accuracy of Vaisala RS92 versus RS41 global radiosonde soundings is assessed using two different reference data sources. First, NWP model outputs are used as a transfer medium to produce relative RS92 and RS41 comparisons by analyzing observation minus NWP model background(OB–BG) and observation minus analysis (OB–AN) differences using the NOAA Climate Forecast System Reanalysis (CFSR) and the operational European Centre for Medium-Range Weather Forecasts (ECMWF) model. Second, GPS radio occultation (GPSRO) dry temperature profiles are directly compared with radiosondes, using GPSRO data from the University Corporation for Atmospheric Research (UCAR) Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) and EUMETSAT Radio Occultation Meteorology (ROM) Satellite Application Facility (SAF).

The humidity observation accuracy of Vaisala RS92 versus RS41 radiosonde soundings is assessed in the upper tropospheric water vapor absorption spectrums (1400 – 900 cm-1). This was achieved through comparing the radiance computed from the radiosonde profiles using radiative transfer modelling (RTM) with collocated Infrared Atmospheric Sounding Interferometer (IASI) measurements, reference sensor measurements in the Global Space-based Inter-Calibration System (GSCIS). Different radiosonde launch configurations, including RS92-RS41 dual launches (both radiosondes attached to the same balloon) and single launches are used for the assessment. Those launches were made at GRUAN sites, Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) sites, and satellite overpass synchronized field campaigns.