Vaisala RS92 Radiosonde Research Articles

Pressure and Temperature Differences between Vaisala RS80 and RS92 Radiosonde Systems

Abstract In several twin flight campaigns, Vaisala RS80 radiosonde systems report lower temperatures than Vaisala RS92 systems in the daytime. Simultaneous differences increase from less than 0.1 K at pressure altitudes below 100 hPa to 0.7 K at 10 hPa. Much of the difference can be explained by an overcorrection of the RS80 radiation error. At night, RS92 and RS80 sounding systems report very similar simultaneous temperatures throughout the atmosphere. Geopotential heights from RS92 pressure, temperature, and humidity data (pTU heights) are within 25 m of geopotential heights from the RS92 global positioning system data (GPS heights) from the ground up to about 70 hPa. At higher altitudes, RS92 sondes produced after July 2004 show nearly identical pTU and GPS heights, but other manufacturing batches show systematic differences, up to ±100 m near 10 hPa. RS80 sondes provide much less accurate pressure and geopotential height. On average, they give up to 1 hPa higher pressure and 20 m lower pTU heights than RS92 sondes in the troposphere, and lower pressures and larger heights in the stratosphere (e.g., by −0.4 hPa and +100 m near 10 hPa). Previous intercomparisons have found similar differences between the two sonde types. As expected from these simultaneous pressure and temperature differences, the transition from Vaisala RS80 to RS92 sondes at German radiosonde stations between 2003 and 2005 has produced artificial increases in stratospheric temperature records, particularly during daytime (1200 UTC), but due to the wrong RS80 pressures, also at night (0000 UTC). The spurious daytime temperature step reaches +0.3 ± 0.2 K at 50 hPa and +0.7 ± 0.4 K at 10 hPa, the nighttime step reaches +0.1 ± 0.1 K at 50 hPa and +0.35 ± 0.2 K at 10 hPa (2σ uncertainties). The mean difference between day- and nighttime temperatures (1200–0000 UTC) has increased as well, by 0.1 ± 0.06 K at 70 hPa and by 0.76 ± 0.16 K at 10 hPa. In the troposphere and at levels below 100 hPa no significant differences are found, although there are indications for higher daytime temperatures, possibly up to 0.1 K, from RS92 sondes. Results indicate that RS92 sondes are more accurate. Historic temperature records from RS80 sondes should be corrected for use in climate studies.

Correction for Radiation Dry Bias Found in RS92 Radiosonde Data during the MISMO Field Experiment

Atmospheric soundings using the Vaisala RS92 radiosonde were intensively conducted during the field experiment MISMO (Mirai Indian Ocean cruise for the Study of the MJO-convection Onset) in the central and eastern equatorial Indian Ocean from October to December 2006. By comparing the RS92 relative humidity data with that from the Meteolabor Snow White (SW) chilled-mirror dew/frost-point hygrometers launched on the same ship around the local noon time, the dry bias was found to increase significantly with height. In addition, it was also revealed that the dry bias had a clear diurnal variation with its maximum at local noon and near-zero at night from the comparison of precipitable water vapor (PWV) with that derived from the shipboard Global Positioning System (GPS) data. Therefore, the dry bias of the RS92 data could be attributed to a solar radiation-induced error that was recently discussed by V omel et al. (2007). In this study, we developed a correction scheme for the MISMO RS92 humidity data as a function of pressure and local time using SW data, and then confirmed its validity with GPS-derived PWV.

Radiation Dry Bias of the Vaisala RS92 Humidity Sensor

The comparison of simultaneous humidity measurements by the Vaisala RS92 radiosonde and by the Cryogenic Frostpoint Hygrometer (CFH) launched at Alajuela, Costa Rica, during July 2005 reveals a large solar radiation dry bias of the Vaisala RS92 humidity sensor and a minor temperature-dependent calibration error. For soundings launched at solar zenith angles between 10° and 30°, the average dry bias is on the order of 9% at the surface and increases to 50% at 15 km. A simple pressure- and temperature-dependent correction based on the comparison with the CFH can reduce this error to less than 7% at all altitudes up to 15.2 km, which is 700 m below the tropical tropopause. The correction does not depend on relative humidity, but is able to reproduce the relative humidity distribution observed by the CFH.