Water vapor is the most important greenhouse gas in Earth's atmosphere. It also represents a major positive feedback factor in climate warming as tropospheric water vapor concentration increases with temperature. Stratospheric water vapor has a significant role as well; recent studies even suggest that the observed flattening of the global warming trend since about 2000 may be caused by changes in stratospheric water vapor concentrations (Solomon et al., 2010). At the same time, unfortunately, atmospheric humidity and especially the extremely low humidity levels of the stratosphere are very difficult to measure accurately. New instruments to enable frequent, accurate and cost-effective measurements of upper tropospheric and stratospheric humidity are badly needed. The Vaisala climate reference radiosonde program therefore focuses initially on humidity.
The first prototype of the reference radiosonde, Vaisala RR01, is based on the current suite of Vaisala Radiosonde RS92 operational sensors and the Vaisala DRYCAP®, a new capacitive sensor capable of measuring extremely low humidity levels. The highly sensitive sensor material can be applied for humidity measurement in the range -30 to -90 °C frostpoint temperature, thus supplementing well the Humicap sensor used in the standard Vaisala radiosondes. The DRYCAP® sensor module connects electrically to the RS92 add-on sensor interface. It is worth mentioning that as a part of the reference radiosonde program, the technical details and message protocol of the interface are being made public by Vaisala. This will allow, in addition to Vaisala's own sensors, third-party sensor modules also to be connected to RS92.
Field testing of the RR01 prototype began in December 2009. Since the RR01 has participated in the LAPBIAT2 upper-air humidity measurement campaign (Sodankylä, Finland, January-March 2010) and the Scientific Sounding Instrument experiment of the WMO 2010 Radiosonde Intercomparison (Yangjiang, China, July 2010). Also, the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, has started evaluating the RR01. Consistent performance and excellent repeatability observed to date in these tests demonstrate the promise of the DRYCAP® sensor as a new water vapor measurement technology for the upper troposphere and lower stratosphere. After finalizing the design and verifying the performance, Vaisala intends to make the RR01 available for wider use during 2011.
In addition to developing precise reference instruments, it is imperative to guarantee the continuity of observation datasets - both for standard and reference instruments. Accordingly, Vaisala has established a public, web-based metadata database that provides RS92 radiosonde-related information which affects the interpretation of climatological time series; similar information is provided for the climate reference radiosonde.
Reference: Solomon, Susan, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford and Gian-Kasper Plattner, 2010: Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming. Science, 327:1219 - 1223.