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The model


The simulation of the atmospheric transmission is essential for all astronomy and middle atmospheric applications. In astronomy, the knowledge of atmospheric transmission at a given frequency is mandatory to asses the quality of an existing or future observatory site. It is also required to calculate the system noise temperature in case the opacity is not independently determined. Remote observations of the Earth's middle atmosphere require to model accurately the observed emission spectra. 'Forward models' are used to describe the radiative transfer, spectroscopy, and instrument characteristics and to compute weighting functions with respect to the searched atmospheric quantities. A versatile forward and inversion model for the millimetre and submillimetre wavelengths range, used in many aeronomy and some astronomy applications, is the MOLIERE-5 code (Microwave Observation LIne Estimation and REtrieval). See Schneider (2003), Urban et al. (2004), and Kasai (2006) for its use for the analysis of ground-based and airborne microwave observations, and Urban et al. (2005) for applications with respect to space-borne missions. MOLIERE-5 has also been used for prediction/feasibility studies of future satellite projects for the exploration of the Earth and Mars atmospheres Urban et al. (2000), Urban et al. (2005b).

A detailed mathematical description of the MOLIERE-5 forward and inversion model and the underlying principles is provided by Urban et al. (2004). Designed for a variety of applications, the here relevant MOLIERE-5 forward model comprises modules for spectroscopy, radiative transfer, and instrument characteristics. Important features of the absorption coefficient module are the line-by-line calculation as well as the implemented H2O, O2, N2, and CO2 continuum models Borysow and Frommhold (1986), Clough et al. (1989,2004), Liebe (1993), Rosenkranz (1993), and Pardo et al. (2001).

The radiative transfer module allows for calculations in different geometries such as limb and nadir sounding from orbiting platforms as well as up-looking observations of ground-based or airborne sensors. A spherically stratified (1-D) emitting and absorbing (non-scattering) atmosphere in local thermodynamical equilibrium is assumed, i.e. the source function is given by Planck's function. The geometrical radiation path is corrected for the effect of refraction. Weighting functions, required for inversions, are calculated by differentiating the radiative transfer equation analytically after discretisation. The radiative transfer model is supplemented by a sensitivity module for estimating the contribution to the spectrum of each catalogue line at its centre frequency, enabling the model to effectively filter large spectral data bases for relevant spectral lines.

Several independent modules permit accurate simulation of instrument characteristics such as the antenna field-of-view, the sideband response of a heterodyne receiver, as well as the spectrometer bandwidth and resolution. Frequency switched observations may also be modelled. These features, however, were not used for the forward-models presented here.

Since water vapour is the main atmospheric absorber at (sub)-mm wavelengths, and in order to provide a common base for inter-comparison with results from other models, we present the results for different values of precipitable water vapour (pwv). Considering the altitude of the site, we produced different H2O vertical profiles by scaling the tropospheric part of the U.S. standard profile accordingly (i.e. so that the integrated H2O column above the site corresponds to a certain pwv). The typical scale height of H2O is 2 km.

Specific input parameters for the models

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J.Boissoles, C. Boulet, R.H.Tipping, et al., 2003, JQSRT, 1-4, 505
A.Borysow, L.Frommhold, 1986, AJ, 311, 1043
S.A.Clough, F.X.Kneizys, R.W. Davies, 1989, Atmos. Res., 23, 229
S.A.Clough et al., MT-CKD2004:
P.Eriksson, 1999, Technical report No. 355, Chalmers Univ. of Technology, Goteburg, Sweden
Y.Kasai, C.Takahashi, J.Urban et al., 2006,I3ETGRS, 44, 3, 676
H.J.Liebe, G.A.Hufford, M.G.Cotton, 1993, 52nd Meeting of the Elec. Wave Propagation Panel
J.R.Pardo, J.Cernicharo, E.Serabyn, 2001,I3ETAP, 49, 12, 1683
J.R.Pardo, E.Serabyn, M.C.Wiedner, 2005, JQSRT, 96, 537
P.Rosenkranz, 1993, Atmospheric Remote Sensing by Microwave Radiometry, Wiley Series in Remote Sensing, New York
N.Schneider, O.Lezeaux, La Noe, J.Urban, et al., 2003,JGR, 108, D17, 4540
J.Urban, K. Kuellmann, K.Kuenzi et al., 2000, Chemistry and Radiation Changes in the Ozone layer, Nato Science Series C, 233, 557
J.Urban, P.Baron, N.Lautie, K.Dassas, N.Schneider et al., 2004,JQSRT, 83, 3-4, 529
J.Urban, N.Lautie, E.Le Flochmoen et al., 2005a, JGR, 110, D14307
J.Urban, K.Dassas, P.Ricaud, F.Forget, 2005b, AO, 44, 2438

Spectral line data base

NIST: National Institute of Standards and Technology catalog

HITRAN: High-resolution transmission molecular absorption database

CDMS: Cologne Database for Molecular Spectroscopy

Jet Propulsion Laboratory (JPL) catalog

Verdandi mm- and submm database

Molecular Spectroscopic Constants Database

LAMDA: Leiden Atomic and Molecular Database

Web-based transmission plotters

CSO Atmospheric Transmission Plotter

Atmospheric Models and Radiative Transfer (RT)


ATM: Atmospheric Transmission at Microwaves

AM: Atmospheric Model

Atmospheric and Environmental Research RT working group

ARTS: Atmospheric Radiative Transfer Simulator