The Thermal Structure of Jupiter's Atmosphere: Analysis of the 7.7 Micron Region Using a Voigt Profile.
A sensitivity study has been conducted of methane transmission functions in the 7.7(mu) region under Jovian atmospheric conditions. This study examines the relative effects of variations in the transmission functions resulting from a number of different computational methods and evaluates the contributions of minor sources of opacity in Jupiter's atmosphere. It explores differences in computed radiances arising from these variations in the transmission functions. It also assesses the validity of Jovian thermal profiles obtained by inverting measured radiances using some of these transmission functions. A standard for the computation of the transmission functions is the line-by-line Voigt profile. However, when employed strictly, this profile consumes prohibitive amounts of computer time. A number of modifications were made to speed up the computations while introducing negligible error: The Voigt profile was replaced by the computationally faster line-by-line Lorentz profile for distances more than 0.092 cm('-1) from a given line center and for pressures greater than 600 mb; symmetry of shapes of lines about their centers was assumed; for a given line, whenever the optical depth became less than 10('-4) in its wings, the contribution was assumed to be zero; integration, which was performed layer-by-layer in the downward direction for 4 km thick layers, was terminated when the optical depth exceeded 9.22 (e('-9.22) = 10('-4)). A spectral resolution of 10('-3) cm('-1) was used in the computations. Transmissions computed at this resolution were averaged over 7 spectral intervals in the 7.7(mu) region such that (DELTA)(lamda)/(lamda) = 0.02. Radiances computed with this standard were compared to those calculated using the line-by-line Lorentz profile and the computationally rapid Ladenberg-Reiche, Random Exponential, and Inverse-Linear band models for the transmission functions. Radiances computed using the Lorentz shape either line-by-line or with a band model differed significantly from those computed with the Voigt profile indicating that these time-saving methods are unsuitable to the problem. The range of uncertainty in the current knowledge of the CH(,4)/H(,2) mixing ratio was tested and found to lead to a wide range of radiance differences. Two different sets of methane line data in the (nu)(,4) band near 7.7(mu) were also tried and yielded radiance differences almost as large as were obtained for the test in the uncertainty in mixing ratio. Opacities from gaseous ammonia and hydrogen were significant only at the long wavelength end of the 7.7(mu) region and these effects were nearly erased when an opaque cloud in Jupiter's zones was added near the 700 mb level. Allowances for an ammonia haze overlying the cloud was found to bring computed and measured radiances into good agreement for a value of 5.2 x 10('-1) g cm('-2) of solid ammonia, assuming a haze scale height of 1 km. Measured radiances were inverted by fitting a temperature profile to satisfy the observed radiances in the 4 spectral intervals on the long wavelength side of the 7.7(mu) region. Agreement was achieved between the resultant profile and one obtained from Orton (1977a) performed under similar conditions, indicating the viability of this method. A conspicuous feature of this result is a tropopause temperature of 114K and a temperature inversion in the stratosphere. When the profile was constrained to satisfy a fifth measured radiance near 7.7(mu), a stronger thermal inversion region developed in the lower stratosphere than in the previous case, and the middle to upper stratosphere was nearly isothermal, becoming far colder at high stratospheric altitudes than the result required to satisfy only 4 measured radiances. Constraining thermal profiles to satisfy 5 radiances, when a cloud was omitted from the opacity or the CH(,4)/H(,2) mixing ratio was lowered, the Jovian stratospheric temperature profiles were hardly affected, but differences arose in the troposphere. A widely different thermal profile was obtained throughout the Jovian atmosphere when radiances were inverted with the Random Exponential band model, indicating that despite its computational speed, this band model (and presumably others) is not accurate enough for inversion of radiances measured by instruments aboard Voyager and other future spacecraft sent to Jupiter for reliable thermal structure observations.
- Pub Date:
- December 1978
- Physics: Atmospheric Science, Physics: Astronomy and Astrophysics