Upper Tropospheric Humidity and Ice from
Meteorological Operational Sensors (UTH-MOS)
Stefan A. Bühler
Introduction
Upper tropospheric humidity (UTH) is a crucial parameter for the atmosphere's energy balance [Buehler, von Engeln, Brocard et al., 2004], but difficult to measure [Buehler and Courcoux, 2003]. The objective of the UTH-MOS project is to develop and interpret an upper tropospheric humidity climatology product from microwave data collected by the polar orbiting operational meteorological sensors of the AMSU-B type. Figure 1 shows an example of AMSU data and the derived UTH. For the correct interpretation of these data, the impact of cirrus clouds on the measurement in the microwave channels has to be well understood and taken into account in the retrieval. An auxiliary objective is to explore the potential of these data for deriving information on cloud ice particles. The research work in UTH-MOS is split between the modelling of the radiative transfer in the presence of cirrus clouds on one hand, and the actual data analysis and atmospheric research applications on the other hand.
The project will run until the end of 2005. Work so far has concentrated on radiative transfer modelling, developing the methods for the data evaluation, understanding the data properties, and validating the data by comparing to other satellite sensors and in-situ measurements.
Radiative transfer model development
The Atmospheric Radiative Transfer Simulator ARTS is an open source radiative transfer (RT) code developed jointly by the Institute of Environmental Physics, Bremen and the Department of Radio and Space Science, Chalmers University of Technology, Gothenburg. The code with documentation is freely available at:
http://www.sat.uni-bremen.de/arts/. It can simulate radiances for up, limb, and down looking instruments for a wide frequency range. An overview of the clear- sky version of ARTS is given by Buehler, Eriksson, Kuhn et al. [2004].
Particular attention was paid to ensure that the model correctly represents the absolute value of atmospheric absorption, not just the component that varies quickly with frequency. This requires the addition of absorption continua for water vapour, oxygen, and nitrogen [Kuhn et al., 2002].
As part of the UTH-MOS project, ARTS has been extended to include the effect of ice particles in cirrus clouds, which mainly interact with the radiation by scattering. The version with scattering is described by Emde et al. [2003], Emde et al. [2004], and Sreerekha et al. [2002]. The algorithm solving the radiative transfer problem uses a successive order of scattering approach in discrete ordinates. It can handle all four components of the Stokes vector in a 1D, 2D, or 3D spherical atmosphere.
Figure 1: Top: Radiances from channel 18 of the AMSU-B instrument on the NOAA-16 satellite (June 6, 2004). The unit is brightness temperature (BT) in Kelvin. This channel is most sensitive to the water vapour concentration at altitudes between 500 hPa (mid-latitude) and 350 hPa (tropics). Bottom: The retrieved upper tropospheric humidity (UTH) product. The unit is relative humidity with respect to ice. On this particular day a dry air extrusion ranged from the subtropics across Europe. (Figure by O. Lemke and V. Oommen.)
Model – data comparison
A necessary condition for accurate retrieval is that the forward model, in this case the RT model ARTS, accurately predicts the measurement from a given atmospheric state, and that all errors in RT model and measurement are well understood. One way to gain confidence in the RT model is to do comparisons to other RT models, as described by Melsheimer et al. [2004]. Another way is to compare directly AMSU measurements to simulated ones, if some in-situ data are available. Such a comparison, based on radiosondes, is described by Buehler, Kuvatov, John et al. [2004]. Figure 2 shows a comparison of different German radiosonde stations performed by the same method. The general level of
agreement is very satisfactory, but there are small differences between the
different radiosonde stations, pointing to potential problems in the humidity data of some stations. In the context of COST Action 723 these data are currently used for a systematic European radiosonde site intercomparison.
Figure 2: A scatter plot of AMSU radiance versus predicted radiance, based on radiosonde data. Coloured lines indicate linear fits for the different stations. (Figure by V. Oommen and M. Kuvatov.)
Upper tropospheric humidity retrieval
Although there is a straightforward relationship between atmospheric humidity content and temperature on the one hand, and top-of-the-atmosphere radiances, on the other hand, the task to invert this relationship and obtain humidity values from measured radiances is far from trivial. In principle, two approaches are possible, a variational profile retrieval [Eriksson, Jimenez, and Buehler, 2004], or a simpler statistical regression approach [Buehler and John, 2004; Jimenez et al., 2004].
The second approach is computationally cheap and thus allows the analysis of large amounts of satellite data. It requires a set of atmospheric states and matching radiances, which could in principle be obtained by collecting in-situ measurement and correlated satellite measurements. However, it is in practice difficult to get enough global data this way. Another possibility is to use a
collection of atmospheric states covering the atmospheric variability, and use the radiative transfer model to generate matching radiances. This is the approach followed by us.
To give a flavour of the method’s capability, one can apply the method to an arbitrary AMSU overpass. Figure 1 shows AMSU radiances and derived UTH for a pass over Europe on June 6, 2004. The top plot shows the original
radiances, displayed as brightness temperatures in Kelvin, the bottom plot shows the derived UTH in relative humidity with respect to ice.
Conclusions and Outlook
Microwave data from polar orbiting meteorological sensors are well suited to study upper tropospheric humidity in climate applications. By the developed advanced radiative transfer algorithms, retrieval schemes, and validation efforts, the UTH-MOS project has paved the way for a wider use of these data. The remaining project time until the end of 2005 is planned to be used for applying the developed algorithms to the available data and for climatological studies with the derived datasets.
Below is a list of references. For those papers still in the review process or in press, preprints can be found at http://www.sat.uni-bremen.de/publications/.
References (Articles)
ERIKSSON,P.,M.EKSTROEM,C.MELSHEIMER AND S.A.BUEHLER, Efficient forward modelling by matrix representation of sensor responses, Int. J. Remote Sensing, submitted 2004.
JIMENEZ,C.,P.ERIKSSON,V.O.JOHN AND S.A.BUEHLER, A practical demonstration on the attainable retrieval precision for upper tropospheric humidity by AMSU, Atm. Chem. Phys., submitted 2004.
MELSHEIMER,C.,C.VERDES,S.A.BUEHLER,C.EMDE,P.ERIKSSON,D.G.FEIST,S.ICHIZAWA,V.O.JOHN,Y.KASAI,G.KOPP,N.
KOULEV,T.KUHN,O.LEMKE,S.OCHIAI,F.SCHREIER,T.R.SREEREKHA,M.SUZUKI,C.TAKAHASHI,S.TSUJIMARU AND J.
URBAN, Intercomparison of General Purpose Clear Sky Atmospheric Radiative Transfer Models for the Millimeter/Submillimeter Spectral Range, Radio Sci., submitted 2004.
EMDE,C.,S.A.BUEHLER,C.DAVIS,P.ERIKSSON,SREEREKHA T.R. AND C.TEICHMANN, A Polarized Discrete Ordinate Scattering Model for Simulations of Limb and Nadir Longwave Measurements in 1D/3D Spherical Atmospheres, J. Geophys. Res., submitted 2004.
BUEHLER,S.A. AND V.O.JOHN, A Simple Method to Relate Microwave Radiances to Upper Tropospheric Humidity, J. Geophys.
Res., submitted 2004.
BUEHLER,S.A.,A. VON ENGELN,E.BROCARD,V.O.JOHN,T.KUHN AND P.ERIKSSON, The Impact of Humidity and Temperature Variations on Clear-Sky Outgoing Longwave Radiation, J. Geophys. Res., submitted 2004.
DAVIS,C.,C.EMDE AND R.HARWOOD, A 3D Polarized Reversed Monte Carlo Radiative Transfer Model for mm and sub-mm Passive Remote Sensing in Cloudy Atmospheres, IEEE T. Geosci. Remote, submitted 2004.
HOEPFNER,MICHAEL AND CLAUDIA EMDE, Comparison of single and multiple scattering approaches for the simulation of limb- emission observations in the mid-IR, J. Quant. Spectrosc. Radiat. Transfer, in press, 2004.
VON ENGELN,A. AND G.NEDOLUHA, Retrieval of temperature and water vapor profiles from radio occultation refractivity and bending angle measurements using an Optimal Estimation approach: A simulation study, J. Geophys. Res., submitted 2003.
EMDE,C.,S.A.BUEHLER,P.ERIKSSON AND T.R.SREEREKHA, The effect of cirrus clouds on microwave limb radiances, J. Atmos.
Res., in press, 2004.
BUEHLER,S.A.,P.ERIKSSON,T.KUHN,A. VON ENGELN AND C.VERDES, ARTS, the Atmospheric Radiative Transfer Simulator, J.
Quant. Spectrosc. Radiat. Transfer, in press, 2004.
ERIKSSON,P.,C.JIMENEZ AND S.A.BUEHLER, Qpack, a general tool for instrument simulation and retrieval work, J. Quant.
Spectrosc. Radiat. Transfer, in press, 2004.
BUEHLER,S.A.,M.KUVATOV,V.O.JOHN,U.LEITERER AND H.DIER, Comparison of Microwave Satellite Humidity Data and Radiosonde Profiles: A Case Study, J. Geophys. Res., 109, D13103, doi:10.1029/2004JD004605, 2004.
BUEHLER,S.A. AND N.COURCOUX, The Impact of Temperature Errors on Perceived Humidity Supersaturation, Geophys. Res.
Lett., 30(14), 1759, doi:10.1029/2003GL017691, 2003.
ERIKSSON,P.,C.JIMENEZ,D.MURTAGH,G.ELGERED,T.KUHN AND S.BUEHLER, Measurement of tropospheric/stratospheric transmission at 10-35 GHz for H2O retrieval in low Earth orbiting satellite links, Radio Sci., 38(4), 8069,
doi:10.1029/2002RS002638, 2003.
VON ENGELN,A.,G.NEDOLUHA,G.KIRCHENGAST AND S.A.BUEHLER, One-dimensional variational (1-D Var) retrieval of temperature, water vapor, and a reference pressure from radio occultation measurements: A sensitivity analysis, J. Geophys.
Res., 108(D11), ACL 7-1 to ACL 7-13, doi:10.1029/2002JD002908, 2003.
MIAO,J.,K.-P.JOHNSEN,S.A.BUEHLER AND A.KOKHANOVSKY, The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters, Atm. Chem. Phys., 3, 39-48, 2003.
SREEREKHA,T.R.,S.A.BUEHLER AND C.EMDE, A Simple New Radiative Transfer Model for Simulating the Effect of Cirrus Clouds in the Microwave Spectral Region, J. Quant. Spectrosc. Radiat. Transfer, 75, 611-624, 2002.
MIAO,J.,T.ROSE,K.KUENZI AND AND P.ZIMMERMANN, A Future Millimeter / Sub-Millimeter Radiometer for Satellite Observation of Ice Clouds, Int. J. Inf. Millim. Waves, 23(8), 1159-1170, August 2002.
KUHN,T.,A.BAUER,M.GODON,S.A.BUEHLER AND K.KUENZI, Water vapor continuum; absorption measurements at 350 GHz and model calculations, J. Quant. Spectrosc. Radiat. Transfer, 74(5), 545-562, 2002.
References (Theses)
BUEHLER,S.A., Remote Sensing of Atmospheric Composition for Climate Applications, Habilitationschrift, University of Bremen, June 2004.
KUHN,T., Atmospheric absorption models for the millimeter wave range, Logos Verlag Berlin, PhD thesis, University of Bremen, in press 2004.
BROCARD,E., Impact of Changing Tropospheric Humidity on Outgoing Longwave Radiation, Thesis, Postgraduateprogram Environmental Physics (PEP), University of Bremen, 2004.
MILLS,P., Chaotic Mixing of Water Vapour in the Upper Troposphere, Thesis, Postgraduateprogram Environmental Physics (PEP), University of Bremen, 2004.
COURCOUX,N., The statistical behaviour of humidity at mid-latitude in the troposphere, Thesis, Postgraduateprogram Environmental Physics (PEP), University of Bremen, 2003.
KUVATOV,M., Comparison of simulated radiances from radiosonde data and microwave sounding data for the validation of the forward model for water vapor retrieval, Thesis, Postgraduateprogram Environmental Physics (PEP), University of Bremen, 2002.
