Robert R. Conway

Seasonal variation of middle atmospheric CH4 and H2O with a new chemical‐dynamical model

A new zonally averaged, chemical-dynamical model of the middle atmosphere is used to study the processes which control the distributions and seasonal variability of CH4 and H2O. This model incorporates a nondiffusive, nondispersive advection scheme, a time-dependent linear model of planetary wave drag and horizontal mixing (Kyy), a new parameterization of gravity wave drag and vertical mixing (Kzz), and an explicit treatment of LTE (local thermodynamic equilibrium) and non-LTE IR cooling. Model chemistry is calculated using a Newton-Raphson iterative scheme, which allows consistent simulations of species with highly nonuniform chemical lifetimes. In this study we focus on the sensitivity of model CH4 and H2O to the magnitude of tropospheric latent heat release, planetary wave and gravity wave activity, and the methane oxidation rate. Model results show that in the tropical stratosphere their vertical distributions are strong functions of both the methane oxidation rate and the ascent rate, the latter driven by a combination of tropospheric latent heat release and atmospheric drag. At low latitudes HALOE observations and model results both show conservation of “potential H2” (2×CH4+H2O) below ∼50 km. However, the conservation of potential H2 from HALOE observations breaks down above ∼55 km, while the model shows conservation well into the middle mesosphere (∼70 km). This may suggest serious inadequacies in our understanding of the photochemistry of water vapor and mesospheric HOx, in particular those processes which control the partitioning of H2 and H2O. At high latitudes, H2O model/data comparisons suggest that horizontal mixing is important in determining the observed latitudinal gradient in mesospheric water vapor. We also find that inside the polar winter vortex, while the strength of tropical latent heat forcing and planetary wave drag influence the descent rate, both horizontal mixing and the methane photochemistry play important roles in determining the CH4 mixing ratio. Finally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H2O may be linked to larger values of Kzz in the southern winter mesosphere. This represents a key difference between mesospheric and stratospheric tracer transport. In the stratosphere, greater net unmixed descent in the southern hemisphere directly translates into lower tracer values relative to the northern hemisphere, while mesospheric tracer transport shows the opposite behavior.

Satellite observations of upper stratospheric and mesospheric OH: The HOxdilemma

We report the first observations of the vertical distribution of hydroxyl (OH) from the upper stratosphere to the mesopause. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 MAHRSI mission that were confined to altitudes above 50 km, namely that mesospheric OH densities are 25 to 35% lower than predicted by standard photochemical theory. However, the new observations show that below 50 km the OH density increases rapidly and at 43 km altitude it is larger than that expected from standard theory. This represents a serious dilemma for our understanding of odd-hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither standard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesosphere.

Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1

We describe the design, fabrication, and testing of a monolithic interferometer consisting entirely of optically contacted fused-silica optical elements that are assembled, adjusted, and permanently bonded in place. The interferometer is part of a spatial heterodyne spectrometer (SHS) [SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals)] that will be used for near-ultraviolet high-spectral-resolution limb imaging of OH solar resonance fluorescence from low Earth orbit aboard the satellite STPSat-1 scheduled for launch in 2006. The stability of the monolith coupled with the relaxed tolerances on optical quality and alignment inherent to SHS make this new instrument extremely robust and especially attractive for applications in harsh environments.

Middle Atmosphere High Resolution Spectrograph Investigation

The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) was developed specifically to measure the vertical density profiles of hydroxyl (OH) and nitric oxide (NO) in the middle atmosphere from space. MAHRSI was launched on its first flight in November 1994 on the CRISTA-SPAS satellite that was deployed and retrieved by the space shuttle. The instrument measured the radiance profiles of ultraviolet solar resonance fluorescence on the Earths limb with a spectral resolving power of 15,600 at a wavelength of 308 nm and 7200 at 215 nm. The instantaneous height of the field of view projected to the tangent point was about 300 m. OH limb radiance measurements were made between altitudes of 90 and 30 km, and each limb scan extended over a horizontal distance of 1200 km. For NO a limb scan extended between altitudes of 140 and 76 km and over a horizontal distance 700 km. Observations were made from 52°S latitude to 62°N latitude. The OH measurements have been inverted to provide the first global maps of the vertical distribution of OH between 90 and 50 km. The data show a detailed history of the morning formation of a strongly peaked layer of OH at an altitude of 68 km. This layer was produced by solar photodissociation of a thin layer of water vapor peaked at 65 km extending between 30°S and 35°N observed contemporaneously by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite. MAHRSI was successfully flown for a second time in August 1997 under conditions that extended the geographical coverage to 72°N latitude and local solar time coverage through the afternoon hours. This paper provides a detailed description of the experiment and instrumentation, of the algorithms used to reduce the spectral data and perform the inversions, and presents examples of key results from the 1994 flight.

PMCs and the water frost point in the Arctic summer mesosphere

In August, 1997 the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) obtained vertical profiles of OH number density and polar mesospheric cloud (PMC) brightness by scanning the limb up to 71° N while the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) obtained co-located vertical profiles of temperature. MAHRSI OH densities are converted to water vapor using a one-dimensional model that assumes photochemical equilibrium. By combining water vapor profiles with CRISTA temperatures we map the frost point both vertically and horizontally in the Arctic summer mesosphere. Our data show that supersaturation can exist between 80-87 km suggesting that growth of ice particles is limited to these altitudes. Additionally, we find that not only is supersaturation an insufficient condition for a PMC but also that PMCs can exist in apparently unsaturated air.

Satellite measurements of hydroxyl in the mesosphere

The global distribution of hydroxyl (OH) in the middle atmosphere was recently measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) on a satellite deployed and retrieved by the space shuttle. During 75 orbits, MAHRSI acquired 1800 daytime limb scans of the OH ultraviolet solar resonance fluorescence intensity. Each limb scan extends over the altitude region from 30 to 90 km and across 10° of latitude between 53°S and 63°N. OH number densities were retrieved using a Twomey regularization scheme constrained by the smoothness of the retrieved profile. Results provide a detailed description of the diurnal variation of mesospheric OH. Midmorning OH densities had a well defined peak of about 6 ×106 cm³ near 70 km, a broad minimum centered near 64 km, and rose to about 1 × 107 cm³ at 50 km. This profile is in substantial disagreement with photochemical model predictions [Summers et al., this issue]. The observations are compared with the two previous measurements.

Mesospheric HOx photochemistry: Constraints from recent satellite measurements of OH and H2O

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd-hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H 2 O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HO x species. Models of OH using standard recommended HO x chemical rate coefficients are found to substantially overpredict OH between 50-65 km. Proposed modifications to HO x chemistry that produce lower OH and higher HO 2 and O 3 , yield better agreement in the lower mesosphere but worsen the agreement at the observed OH peak near ∼70 km. We find that neither standard HO x chemistry, nor models incorporating proposed HO x modifications, adequately reproduce the observed OH density profile over the entire 50-75 km altitude range for any of the observed early to mid morning local times.

A resolution of the N2 Carroll‐Yoshino (c4′ ‐ X) band problem in the Earth's atmosphere

In the study of UV airglow from the Earths atmosphere, the N2 Carroll-Yoshino (CY) c4′ 1Σu+ - X 1Σg+(0,0) and (0,1) Rydberg band emissions near 958 A and 980 A, respectively, are found to be weak relative to the c4′ (0) excitation rate. This result is surprising because laboratory measurements show that CY(0,0) and CY(0,1) are the brightest N2 emission features between 910-1010 A even under optically thick conditions [Zipf and McLaughlin, 1978]. In order to investigate the cause of this weak emission quantitatively, we have developed a resonant fluorescent scattering model for CY(0,0) and CY(0,1). The model is intended to be comprehensive, including multiple scattering, extinction, branching, escape to space, predissociation, and temperature effects. Results show CY(0,0) photons are radiatively trapped and undergo resonant fluorescent scattering accompanied by substantial loss in the atmosphere. Indeed, the model predicts weak CY(0,0) intensities, consistent with observations. We find that the most important loss processes for the CY(0,ν″) system in the Earths dayglow are predissociation and branching to CY(0,1) followed by absorption by the overlapping, 100% predissociated Bridge-Hopfield I (BH I) b1Πu(2) - X1Σg+(0) band. Near solar minimum, model CY (0,1) and (0,2) dayglow zenith intensities between 160-170 km range between 4-9 R and 0.5-1.5 R, respectively, where the lower number assumes 16.5% predissociation of the c4′(0) state and the higher number assumes 1% predissociation. These intensities are all consistent with observations reported by Morrison et al. [1990]. For the Earths aurora, model CY(0,1) and (0,2) intensities averaged between 88°-96° from the zenith at 118.5 km range between 60-180 R and 150-390 R, respectively, and CY(0,2) intensities at 170 km range between 200-360 R. These results are consistent with upper limits from Feldman and Gentieu (1982) if the probability of c4′ (0) predissociation is at least 9%. We also present qualitative arguments to explain the relatively bright CY(0,ν″) emission on Titan and Triton.

Satellite observations of the oi 1304, 1356 and 1641 Å dayglow and the abundance of atomic oxygen in the thermosphere

Abstract The far ultraviolet spectrometer experiment on the S3-4 satellite made nadir-viewing observations of the atomic oxygen dayglow during the spring of 1978. Comparison of 27 orbits of OI 1304 A (3P-3S), 1356 A (3p-5S), and 1641 A (1D-3S) intensities shows important orbit-to-orbit variations. For four selected orbits of data, airglow intensities have been computed using the O abundance predicted by the MSIS-83 model atmosphere for the appropriate geophysical conditions. In general, the MSIS-83 empirical model reproduces the airglow rather well. To match the absolute magnitude of the airglow, the computed photoelectron flux was increased by 40%, also the observed 1641 1304 intensity ratio required a branching ratio of 2.4 × 10−6. Reductions in the airglow during geomagnetically disturbed conditions suggest decreases in the O concentration which can be observed remotely.

Mission studies the composition of Earth's middle atmosphere

Remote sensing of the middle atmosphere from Earth orbit has provided a wealth of two- and three-dimensional images over the last 15 years. Although these images have been powerful tools for testing chemical-dynamical models, they lacked high-horizontal spatial resolution. In contrast, high-altitude aircraft experiments provided high spatial resolution along the aircraft flight path at the expense of limited area coverage. The aircraft observations made during the Antarctic and Arctic ozone campaigns proved invaluable in unraveling the respective roles of dynamics and chemistry in the formation of the ozone hole during the austral spring. The importance of high-resolution easurements is further suggested by the predictions of increasingly detailed three-dimensional models which show that trace gases are distributed with spatial structures on many scales and that these structures can form clear signatures of specific dynamical activity.