Abstract
Radiosondes are important for calibrating satellite sensors and assessing sounding retrievals. Vaisala RS41 radiosondes have mostly replaced RS92 in the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) and the conventional network. This study assesses RS41 and RS92 upper tropospheric humidity (UTH) accuracy by comparing with Infrared Atmospheric Sounding Interferometer (IASI) upper tropospheric water vapor absorption spectrum measurements. Using single RS41 and RS92 soundings at three GRUAN and DOE Atmospheric Radiation Measurement (ARM) sites and dual RS92/RS41 launches at three additional GRUAN sites, collocated with cloud-free IASI radiances (OBS), we compute Line-by-Line Radiative Transfer Model radiances for radiosonde profiles (CAL). We analyze OBS-CAL differences from 2015 to 2020, for daytime, nighttime, and dusk/dawn separately if data is available, for standard (STD) RS92 and RS41 processing, and RS92 GRUAN Data Processing (GDP; RS41 GDP is in development). We find that daytime RS41 (even without GDP) has ~1% smaller UTH errors than GDP RS92. RS41 may still have a dry bias of 1–1.5% for both daytime and nighttime, and a similar error for nighttime RS92 GDP, while standard RS92 may have a dry bias of 3–4%. These sonde humidity biases are probably upper limits since “cloud-free” scenes could still be cloud contaminated. Radiances computed from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses match better than radiosondes with IASI measurements, perhaps because ECMWF assimilates IASI measurements. Relative differences between RS41 STD and RS92 GDP, or between radiosondes and ECMWF humidity profiles obtained from the radiance analysis, are consistent with their differences obtained directly from the RH measurements.
Highlights
Tropical Western Pacific (TWP) Atmospheric Radiation Measurement (ARM) site, by comparing with observed Infrared Atmospheric Sounding Interferometer (IASI) radiances. We adopt their approach to understand upper tropospheric humidity (UTH) accuracy, and we extend their study to several GRUAN and ARM sites and compare Vaisala RS41 with
Dual launches of RS92 and RS41 radiosondes at Lauder, Lindenberg, and Payerne were made at synoptic times
Fewer cases are analyzed at Lauder and Lindenberg than totals shown in Table 1 due to insufficient night or dusk/dawn collocations for reasonable statistical analysis
Summary
Balloon-borne radiosonde (or “sonde”) observations (RAOBs) are critical in numerical weather prediction (NWP), data assimilation and forecasting, satellite data calibration/validation (cal/val), and upper air climate change detection. Vaisala RS92 was a major sonde type in the global operational upper air network and a reference sonde in the Global. Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) [1]. Vaisala RS41 includes new sensor technologies aimed at improving measurement accuracy for temperature, humidity and other variables throughout the atmosphere. These include a heated humidity sensor to prevent dew or frost formation in clouds and a separate temperature sensor attached to the humidity sensor. Characterizing the RS41 measurement improvement and accuracy is key to the GRUAN RS92-to-RS41 transition management program
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