92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 9:30 AM
Towards Enhanced Accuracy in Upper-Air Observations—Progress Report on the Vaisala Reference Radiosonde Program
Room 239 (New Orleans Convention Center )
Heikki Turtiainen, Vaisala Oyj, Helsinki, Finland; and H. Jauhiainen, J. Lentonen, P. Survo, V. P. Viitanen, and W. F. Dabberdt

In 2009, Vaisala launched a program to develop an accurate radiosonde for climate studies, especially targeting the needs of the GCOS Reference Upper Air Network (GRUAN). The program's first focus is to improve upper-air measurements of water vapor, the most important greenhouse gas in earth's atmosphere.

Thus far the program, carried out in close collaboration with the international scientific community, has produced a prototype of the operational reference radiosonde, Vaisala RR01. RR01 is built around the DRYCAP® humidity sensor, an advanced capacitive sensor capable of measuring extremely low humidity levels in upper troposphere and lower stratosphere. Vaisala RS92 radiosonde technology is used for pressure, temperature and tropospheric humidity measurements and GPS wind finding.

During 2009-2010 the prototype was extensively tested together with research partners, as well as in several field campaigns. In most of these tests RR01 demonstrated consistent performance, low noise and excellent repeatability, but also some technical challenges were recognized. First, in conditions with very humid lower troposphere "moisture contamination", or formation of ice on the sensor's shield or nearby surfaces tends to cause measurement errors. Secondly, in the stratosphere systematically +1.5 … +2°C higher frost point temperatures were observed when compared to CFH, a cryogenic frost point hygrometer used as a reference.

In 2011, the work has concentrated in removing this small but consistent bias. The error seems to be caused by differences between the ambient conditions in calibration and in actual flight: in flight the sensor signal is noisier and temperature gradient across the sensor assembly larger, both causing slight positive bias. Removing these effects with enhanced filtering algorithms and correction for temperature gradient effects result in significantly improved accuracy and better consistency with the CFH.

In the next phase of the program, Vaisala intends to improve the performance of RR01 in humid conditions. The final target is a reference radiosonde that is considerably easier to operate and less expensive than the current reference grade instruments, thus enabling more frequent climatological soundings.

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