Lorenz Martin

Temperature and humidity profile retrievals from ground-based microwave radiometers during TUC

Thermodynamic atmospheric profiles have been retrieved from ground-based microwave radiometers during the Temperature, hUmidity, and Cloud (TUC) profiling campaign. A variety of inversion methods is presented, in terms of requirements, advantages, and limitations. Results confirm the theoretical expectation that retrievals’ accuracy and resolution degrade steadily with height up to 3 km, then more rapidly. At higher levels the retrievals’ accuracy does not improve on that of a Numerical Weather Prediction model, which provides a background for the variational technique. Most retrieval methods produce a bias in the temperature profile above 1 km, which may be due to a bias in the absorption model used and/or observations at 51–54 GHz. Elevation scanning is shown to improve the accuracy and resolution of the retrievals in the boundary layer, but is limited by technical shortcomings. Zusammenfassung Thermodynamische atmospharische Profile wurden mit bodengestutzten Mikrowellenradiometern wahrend der Temperature, hUmidity, and Cloud (TUC) profiling Kampagne gemessen. Verschiedene Inversionsmethoden werden in Bezug auf Anforderungen, Vorteile und Einschrankungen vorgestellt. Die Resultate bestatigen die theoretische Erwartung, dass die Genauigkeit und die Auflosung der gemessenen Profile kontinuierlich bis 3 km Hohe schwach und daruber starker abnehmen. In den hoheren Schichten ist die Genauigkeit der Profile nicht besser als die des numerischen Wettervorhersagemodells, das die Hintergrundfelder fur das erorterte Variationsverfahren bereitstellt. Die meisten Inversionsmethoden fuhren zu systematischen Fehlern in den gemessenen Profilen oberhalb von 1 km, was auf systematische Fehler im verwendeten Absorptionsmodell und/oder bei der Messung der Helligkeitstemperatur zwischen 51 und 54 GHz hindeutet. Die zusatzliche Einbeziehung von Messungen unterschiedlicher Elevationswinkel verbessern die Genauigkeit und die Auflosung der abgeleiteten Profile in der planetaren Grenzschicht, wobei die Vorteile durch technische Unzulanglichkeiten eingeschrankt sind.

Validating clear air absorption models using ground-based microwave radiometers and vice-versa

Microwave radiometer observations are compared with various radiative transfer model calculations based on simultaneous radiosondes. This analysis uses observations from Payeme, Switzerland, in cloud free conditions during the Temperature Humidity and Cloud (TUC) experiment in winter 2003/04. The results show a systematic bias in the brightness temperatures measured by the Radiometrics profiler at 55-59 GHz, which has since been corrected in the control software. Observations at lower frequencies (22-30 GHz) in these cold conditions do not support recent proposed changes to the width of the 22.235 GHz water vapour line, although this is subject to the assumption of no residual bias in the radiosonde humidity. At intermediate frequencies (51-54 GHz), the absorption models produce large differences, which may be due to differences in oxygen line coupling and highlight the need for further laboratory measurements at low temperatures.

Middle Atmospheric Water Vapour Radiometer (MIAWARA): Validation and first results of the LAPBIAT Upper Tropospheric Lower Stratospheric Water Vapour Validation Project (LAUTLOS‐WAVVAP) campaign

[1] We present a validation study for the ground-based Middle Atmospheric Water Vapour Radiometer (MIAWARA) operating at 22 GHz. MIAWARA measures the water vapor profile in the range of 20–80 km. The validation was conducted in two phases at different geographical locations. During the first operational period the radiometer was operated at middle latitudes in Bern, Switzerland, and the measured water vapor profiles were compared with the HALOE satellite instrument. The agreement between HALOE and MIAWARA was for most altitudes better than 10%. In the second comparison phase, MIAWARA took part in the Lapland Atmosphere-Biosphere Facility (LAPBIAT) Upper Tropospheric Lower Stratospheric Water Vapour Validation Project (LAUTLOSWAVVAP) campaign in early 2004 in the subarctic region of northern Finland. During this campaign, different balloon sondes probed the water vapor content in the upper troposphere and lower stratosphere. The stratospheric water vapor profiles of the fluorescent hygrometer FLASH-B and the NOAA frost point hygrometer mirror in the range of 20–26 km were compared with the lowermost retrieval points of MIAWARA. The agreement between the balloon instruments and MIAWARA was better than 2% for a total number of 10 comparable flights. This showed the potential of MIAWARA in water vapor retrieval down to 20 km. In addition, the northern Finland MIAWARA profiles were compared with POAM III water vapor profiles. This comparison confirmed the good agreement with the other instruments, and the difference between MIAWARA and POAM was generally less than 8%. Finally, the tipping curve calibration was validated with tipping curve measurements of the All-Sky Multi Wavelength Radiometer (ASMUWARA) which was operated 10 months side by side with MIAWARA. The agreement of the tropospheric opacity derived from these tipping curves agree within 1%.

ASMUWARA, a ground-based radiometer system for tropospheric monitoring

ASMUWARA, the All-Sky MUlti WAvelength RAdiometer, is a new ground-based and automatically operating radiometer system designed for tropospheric monitoring. ASMUWARA has ten channels in the microwave and infrared range and is able to observe the sky in all directions with an angular resolution of 9°. No radome is used to allow an optimum view quality at all wavelengths. The purpose of ASMUWARA is to retrieve temperature and humidity profiles, maps of integrated water vapour and liquid water, and additional cloud properties. The construction and characteritics of ASMUWARA are described. Special emphasis is put on the calibration loads, the calibration method, and the correction of antenna beamwidth effects.

Tropospheric water and temperature retrieval for ASMUWARA

The retrieval of tropospheric water and temperature with the ground-based and automatically operating radiometer system ASMUWARA (All-Sky MUlti WAvelength RAdiometer) is described. This instrument operates simultaneously at microwave and IR channels. Integrated water vapour (IWV) and integrated liquid water (ILW) are retrieved with a newly developed linear algorithm to an accuracy of 0.014 kgm -2 (ILW) and 0.41 kgm -2 (IWV), thanks to the inclusion of a channel at 151 GHz. These measurements are made for the whole hemisphere and therefore provide information about the spatial distribution of water in the troposphere. With an optimal estimation algorithm, tropospheric temperature and humidity profiles are retrieved. The results are quasi bias free with a mean error of less than 2.5 K for the temperature (less than 1 K in the lowest km above ground), and less than 1 gm -3 for the humidity. Examples of all measurements are shown.

Intercomparison of integrated water vapour measurements

Measurements of tropospheric integrated water vapour (IWV) made with two microwave radiometers (ASMUWARA, TP/WVP-3000), GPS, and radiosondes (SRS 400) during the Temperature, hUmidity, and Cloud (TUC) profiling campaign under mid-latitude conditions in Payerne, Switzerland, in winter 2003/2004 are compared. All methods provide robust IWV retrievals in clear sky and cloudy situations. The mean difference between radiometric and radiosonde IWV is less than 0,15 kgm -2 being not significant with respect to the standard deviation and to the theoretical accuracy. The GPS IWV measurements have a persistent significant dry bias of approx: 0,5 kgm -2 with respect to radiometers and radiosondes. The different temporal and spatial resolutions of the instruments were found to have a strong influence on the standard deviation. A characteristic diurnal cycle of the GPS and radiometric IWV was observed.

Survey of Intercomparisons of Water Vapour Measurements

The survey begins with a discussion of the necessity of intercomparisons. Then the history of intercomparisons is surveyed over the time interval from 1773 to 1976. Past scientific works on water vapour intercomparisons are often forgotten. However it can be motivating and helpful if we know more about the roots of intercomparisons and hygrometers. The number of intercomparisons of water vapour measurements strongly increased in the period 1977 to 2010. It is impossible to give details about the recent achievements within the present chapter. On the other hand it is not fair to pick out a few articles for a detailed discussion. This is the reason why we give an overview of intercomparisons since 1977 by means of tables of literature references. Hopefully the reference tables can introduce intercomparison studies to you which you otherwise would have overlooked or forgotten. We send our regrets to all authors which we forgot to mention in this survey chapter. You may inform us since your articles could be referenced in an update of the survey chapter in a few years. The survey is strongly supported by the new interactive Water Vapour Literature Database which is a practical work tool of the water vapour community: http://www.watervapour.org.