Line Radiative Transfer Model Research Articles

Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

Abstract. In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1–2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5–1 K) and moistening (3%–12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.

The effect of a strong external radiation field on protostellar envelopes in Orion

We discuss the effects of an enhanced interstellar radiation field (ISRF) on the observables of protostellar cores in the Orion cloud region. Dust radiative transfer is used to constrain the envelope physical structure by reproducing SCUBA 850 µm emission. Previously reported 13 CO, C 17 O and H2CO line observations are reproduced through detailed Monte Carlo line radiative transfer models. It is found that the 13 CO line emission is marginally optically thick and sensitive to the physical conditions in the outer envelope. An increased temperature in this region is needed in order to reproduce the 13 CO line strengths and it is suggested to be caused by a strong heating from the exterior, corresponding to an ISRF in Orion 10 3 times stronger than the “standard” ISRF. The typical temperatures in the outer envelope are higher than the desorption temperature for CO. The C 17 O emission is less sensitive to this increased temperature but rather traces the bulk envelope material. The data are only fit by a model where CO is depleted, except in the inner and outermost regions where the temperature increases above 30–40 K. The fact that the temperatures do not drop below ≈25 K in any of the envelopes whereas a significant fraction of CO is frozen-out suggest that the interstellar radiation field has changed through the evolution of the cores. The H2CO lines are successfully reproduced in the model of an increased ISRF with constant abundances of 3–5 × 10 −10 .

Constraints on Jupiter's hydrogen corona from Galileo UVS observations

During Galileo orbits C10 and E11, the Ultraviolet Spectrometer (UVS) obtained H Lyα scans at low latitudes across the disk of Jupiter from a phase angle of 90–109° and a distance of 18–21 jovian radii. Jupiter's nightside Lyα brightness was about 0.5kR, approximately half as bright as interplanetary space. From the nightside to the dayside, the Lyα brightness rose to a peak of ∼15kR near the sub-solar limb and then dropped sharply to interplanetary brightness levels. On orbit C10 three disk scans were obtained (near 30°N latitude, 20°S latitude, and along the equator), while a single equatorial scan was made on orbit E11. The steep falloff in Lyα brightness at the sub-solar point provides a strong constraint on the distribution of atomic hydrogen in the upper atmosphere. All disk scans are reasonably well fit by a resonance line radiative transfer model which assumes a total H column density of ∼1017cm−2 and a temperature profile consistent with the Galileo probe results with an exospheric temperature of 940K. However, we find that the inclusion of a small amount of “hot” hydrogen, with an abundance described by a Chapman profile, can substantially improve the fit to the UVS limb scan data. Exactly such a hot hydrogen component is expected in the upper atmosphere of Jupiter, and has been previously invoked to explain observations of the jovian Lyα line profile made from Earth orbit. The best fits to the UVS data occur with a hot H column density that is 0.1% of the ambient H column, with an effective temperature of 25,000K and a topside scale height of 1000km (in our empirical representation, the hot H scale height is unrelated to the hot H temperature). Other combinations of column abundance and the effective temperature can also provide good fits to the data. As long as the hot H is located above the homopause, there is no strong dependence on the height of the Chapman profile.

Frontiers of Radiative Transfer

Detailed radiative transfer calculations are an essential tool for the analysis of observations related to fragmented molecular cloud cores, protobinaries, and circumbinary and circumstellar disks. In this contribution, recent progress in line and continuum radiative transfer models will be reviewed.

A pseudo three‐dimensional resonance line radiative transfer model with overlapping lines

A description of a resonance line Feautrier radiative transfer (RT) code which allows for coupling between closely spaced emission lines is presented. This code has been used to model closely spaced H and D Lyman α 1215.67 Å and 1215.34 Å emission lines, respectively, in the Jovian thermosphrre. There is no direct interaction between these lines, but coupling and energy transfer may occur as the Voigt broadened lines overlap. The RT method is pseudo three‐dimensional in that a spherical shell atmosphere is accounted for in direct solar beam scattering but with one‐dimensional (1‐D) plane‐parallel multiple scattering. In addition, allowance is made for a varying solar zenith angle along the line of sight in calculating intensities. Using a model atmosphere for thermospheric D and H produced by a 1‐D photochemical‐diffusion model, we present results illustrating the relative importance of including coupling of H and D Lyman α emission lines in radiative transfer calculations for both the disk and limb of Jupiter. These results suggest that the spectral contrast between the H and D Lyman α features is reduced in long limb path geometries and that the disk measurement provides better detection possibilities.