"Lindenberg Meteorological Observatory - Richard Aßmann Observatory
Trend of relative humidity within the upper troposphere over Lindenberg

Humidity, in form of water vapor or clouds, influences all atmospheric radiative processes within the spectral range of solar and terrestrial radiation. Furthermore, water operates in the atmosphere by means of its chemical properties and its ability to wash out aerosols. Therefore, determination of atmospheric humidity within gaseous, liquid and solid state has got a significant interest in aerology, similar to measurement of temperature, wind, and pressure. Different measures of atmospheric moisture have been established in the field of meteorology, like vapor pressure, vapor density, absolute humidity, mixing ratio, specific humidity, temperature of dew point, and relative humidity. By means of temperature and/or pressure these quantities can be converted into each other. Relative humidity has special significance for radiation balance and energy budget of the atmosphere. Furthermore the relative humidity is measured directly (in situ) by radiosounding. For these reasons the relative humidity will be considered here as measure of atmospheric moisture.

For measurement of humidity within the history of aerology, primarily tree different sensor types have been used:
hairhygrometer: for kites and tethered balloons, sondes LANG and FREIBERG; up to 1971
goldbeaterskin: russian sondes RKS and MARZ; 1971 to 1992
polymersensor: radiosondes RS80, RS90, RS92 from Finland; from 1992

Measuring relative humidity at temperatures below zero is a notorious problem. Examinations by SODEN and LANZANTE [1] show that there are large systematical differences between the specific sensor types (comparison of satellite data and data of radiosondes in upper troposphere). For this reason and due to the noticed "break" in the Lindenberg data set within measurements of humidity on change of measuring instruments, Lindenberg Observatory has tried to improve humidity measurements with operational radiosondes since 1995. Since 1999 precision humidity radiosondes have been launched once a week using the Lindenberg reference sonde (FN-method, modified RS90 radiosondes, [2]). Precision humidity radiosondes using the FN-method reach accuracy in measurement of relative humidity of 1 % within the atmosphere [2]. An adjustment method for measurement of humidity with RS80-A-humicap sondes has been developed as a result of a comparison of RS80 routine sondes and the precision humidity radio-sondes. The adjustment method for RS80 humidity uses the following steps[2]:
ground check adjustment for 100 % relative humidity
lag correction (dependent on humidity gradient and temperature)
temperature dependent adjustment
deletion of measured data for relative humidity in case of icing of humidity sensor

By analysing the gradient of relative humidity within similar measuring sensors and by determining the regression line, previous measurements have been amended. It was possible to homogenize the time series of water vapor dispersion as the most important greenhouse gas in the atmosphere over Lindenberg. The determined amendments produce a data quality that allows an evaluation of humidity profile measurements. For Lindenberg, a first draft of the time series of relative humidity for more than 45 years has been created.



Graphic


For the 300 hPa level reliable values of atmospheric humidity are available from the early sixties on. The figure shows the relative humidity at the pressure level 300 hPa. The figure indicates a decrease of humidity of about 4 % within the last 45 years in the upper troposphere over Lindenberg.


References:

1 Soden, B.J. and J.R. Lanzante; 1996: An assessment of satellite and radiosonde climatologies of upper-tropospheric water vapor. J. Climate, 9, 1235-1250.

2 Leiterer, U., H. Dier, D. Nagel, T. Naebert, D. Althausen, K. Franke, A. Kats, F. Wagner; 2004: A Correction Method for RS80-A Humicap Profiles and their Validation by Lidar Backscattering Profiles in Tropical Cirrus Clouds. Journal of Atmospheric and Oceanic Technology (JTECH), Vol. 22, No. 1, 18-29, 2005.

3 U. Leiterer, H. Dier, W. Adam, 2001: Aerologischer Schichtaufbau der Atmosphäre und Trends über Lindenberg Klimastatusbericht 2000. Deutscher Wetterdienst Offenbach, ISBN 3-88148-368-3, 7-14"
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Type of sensor: : hairhygrometer (up to 1971); goldbeaterskin: 1971 to 1992; Polymer (from 1992); since 2000 online correction