Infrared and Sub-millimetre Observing Conditions on the Antarctic Plateau

Marton G. Hidas, Michael G. Burton, Matthew A. Chamberlain, John W.V. Storey, PASA, 17 (3), 260.
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Title/Abstract Page: Infrared and Sub-millimetre Observing
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MODTRAN Modelling

The atmospheric modelling program MODTRAN 4.0 (AFRL/VSBM) was used to determine the flux from the sky at zenith as a function of wavelength. MODTRAN calculates the transmission and emission of the atmosphere at wavelengths from the optical to the sub-millimetre. It uses various standard atmospheric models, based on common geographic locations. It also allows the user to define an atmospheric profile with any specified set of parameters.

The MODTRAN package also includes several parameters to define aerosol profiles in the atmosphere. The most important of these parameters are the type of aerosol and the ``visibility''. There are several types of aerosol available, based on common aerosol mixtures that are found in most terrestrial conditions, eg. rural, urban and maritime. Of all the aerosol types available in MODTRAN, ``radiative fog'' was found to produce a mid-IR emission spectrum that was most similar to that measured (Chamberlain et al. 2000). The second main aerosol parameter, visibility, is then used to define the amount of the aerosol in the atmosphere.

Figure 1: Atmospheric profiles showing the pressure, temperature and relative humidity as a function of altitude above the South Pole. These profiles were used to investigate the fluxes across the IR and sub-mm windows at the site. The model is from Chamberlain et al. (2000), based on radio-sonde balloon measurements. The two dotted lines represent doubling and halving the relative humidity. Ice-level is at 2,900m.
\begin{figure} \begin{center} \psfig{file=profiles.ps,width=15cm}\end{center}\end{figure}

Data from meteorological balloon launches from the South Pole were used to define the pressure, temperature and relative humidity profiles of our models (see CMDL/NOAA). The profiles for other atmospheric constituents (O3, CH4, N2O, CO, CO2, O2, NO, SO2, NO2, NH3, HNO3) were set to those from MODTRAN's standard ``subarctic winter'' model, which was closest to the conditions at the South Pole. However, these have a negligible effect on the sky emission in the infrared windows discussed here (MODTRAN allows the contributions of individual species to the total emission to be examined).

Fig. 1 shows the pressure, temperature and relative humidity recorded on August 11 1998, a typical example of a clear day. The strong temperature inversion in the lowest hundred metres is clearly defined, the temperature rising by $30^{\circ}$C. Note that while the relative humidity may seem high the absolute level of water content is low because of the extreme cold. The total column of precipitable water vapour for this day is 164$\mu $m. These profiles were assumed to be representative of good observing conditions. Our aim was to determine the effects of water content and aerosol content on the sky emissivity during such times. Also shown in Fig. 1 are the profiles when the relative humidity was both doubled and halved, corresponding to total columns of 324$\mu $m and 82$\mu $m respectively (the column has not doubled for the first case since the humidity actually saturates at 100% for part of the column). As described later, these two models were used to examine how water vapour content effects the brightness of sky emission.

Figure 2: The measured near-IR sky spectrum (from 1.5-2.5$\mu $m and 2.9-4.1$\mu $m, Phillips et al. 1999) and mid-IR sky spectrum (5-14$\mu $m, Chamberlain et al. 2000) at the South Pole. Overplotted, with a dashed line, is the model spectrum calculated by MODTRAN using the measured atmospheric profile of Fig. 1 (corresponding to 164$\mu $m of precipitable H2O), plus an aerosol visibility of 100km. The model fails below 2.3$\mu $m because of the neglect of airglow emission.
\begin{figure} \begin{center} \psfig{file=data_model.ps,width=15cm}\end{center}\end{figure}

In Fig. 2 are shown measured near-IR (1995) and mid-IR (1998) sky fluxes at the South Pole (from Phillips et al. 1999 and Chamberlain et al. 2000), overlaid with a model spectrum from MODTRAN, calculated for the atmospheric profile of Fig. 1. In addition, a ``radiative fog'' aerosol layer with visibility equal to 100km was included. While not a perfect fit, the model provides a good explanation (see Chamberlain et al. 2000) for the overall flux level and features in the spectrum across the mid-IR. The success of this model was used as a basis for further investigation of the effects of water vapour and aerosols on the complete spectrum from 2.4 to 500$\mu $m. It fails below 2.3$\mu $m because airglow emission is not included.

Next Section: The effect of temperature
Title/Abstract Page: Infrared and Sub-millimetre Observing
Previous Section: Introduction
Contents Page: Volume 17, Number 3

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