Studies on the tropospheric and stratospheric water vapor measurements for climate monitoring

Atmospheric water vapor plays a critical role in the climate system because it acts as a medium for heat exchange and transport, and because it is linked to the formation of clouds and precipitation. It is also the most dominant greenhouse gas. Thus, it is important to monitor and understand long-term variability in atmospheric water vapor in the upper troposphere and lower stratosphere.Routine radiosonde observations provide the longest record of upper-air conditions. However, it is known that the original radiosonde record contains various errors and inhomogeneity because of changes in instrumentation. In this study, the author investigates the measurement uncertainty of the Meisei RS-06G. Comparisons of relative humidity (RH) measurements from the RS-06G radiosonde and from a chilled mirror hygrometer revealed that the RS-06G RH shows a stepwise change of ~3% RH at 0 °C (drying when air temperature decreasing). This is due to a discontinuous correction factor in the processing software that compensates for the temperature dependence of the RH sensor. Using the results from chamber experiments, the author develops a new temperature-dependence (T-D) correction scheme to resolve the artificial stepwise change at 0 °C. Because the RS-06G radiosonde is a successor to the Meisei’s previous radiosonde, RS-01G and RS2-91, on which the same RH sensor material had been installed since July 1999, the new T-D correction should be applied to the data obtained by these radiosondes as well. Here, the author applies the new correction into the data at Japan Metrological Agency’s Sapporo and Tateno stations. Because the present (original) T-D correction has only been applied only since February 2003, the time series of RH at both stations show apparent large downward trends between 1999 and 2009. The new T-D correction is found to result in a much smaller (mostly negligible) downward RH trend at Sapporo and almost no trend at Tateno.Because the operational radiosonde observation is designed to obtain the lower tropospheric water vapor for weather forecasting, the RH sensors on operational radiosondes have very poor response in and above the upper troposphere. To measure water vapor precisely in the upper troposphere and stratosphere, special research-quality instruments are required. For example, the Cryogenic Frostpoint Hygrometer (CFH) is regarded as a reference instrument for balloon-borne water vapor observation. However, the CFH needs cryogen material for the observation, and the cryogen material has a strong greenhouse effect. Meanwhile, the Meteolabor Snow White is a Peltier-based chilled mirror hygrometer, which needs no cryogen. Although the operation is simple and environmentally-friendly, it is known that the Snow White cannot measure the stratospheric water vapor even within the cooling capability. In this study, a Peltier-based digitally-controlled chilled-mirror hygrometer has been developed to measure atmospheric water vapor accurately. The developed instrument is environmentally-friendly in nature because this instrument does not use a cryogenic material in addition to a merit of easy-to handle. Also, this instrument has an advanced feedback controller and algorithm to maintain the condensate on the mirror because of the digital circuit. Since January 2011, the author has conducted nine test flights to evaluate the performance. The results showed that the developed instrument can measure water vapor from the surface to the lower stratosphere (~25km). The results of simultaneous measurements with the CFH showed that the frost point temperature from the developed instrument is consistent with that from CFH within ~0.5 K throughout the troposphere.Also, several chamber experiments were conducted to investigate the behavior of chilled-mirror hygrometers. The observation of the condensate by a microscope shows that more and smaller ice crystals are formed on the mirror at lower temperatures. Because the evaporation/condensation rates depend on the particle size as well as water vapor content in the ambient air, the response time and the stability of chilled-mirror hygrometers are considered to depend on the particle size on the mirror, water vapor concentration, and the value of the feedback gain to maintain the constant condensate amount. Based on the experimental results, the author discusses the measurement uncertainty of developed chilled-mirror hygrometers.