David E. Siskind

Ground‐based microwave observations of ozone in the upper stratosphere and mesosphere

A 9-month-long series of mesurements of ozone in the upper stratosphere and mesosphere is reported. The measurements are presented as monthly averages of profiles in blocks of roughly 20 min local time and as night-to-day ratios. An error analysis predicts accuracies of 5-26% for the monthly profiles and 2.5-9% for the ratios. The data are compared to historical data from Solar Mesosphere Explorer (SME) and limb infrared monitor of the stratosphere (LIMS), and it is shown how to remove the effect of different vertical resolution from the comparisons. The microwave data typically agree to better than 10% with SMF and nighttime LIMS ozone at all altitudes below the 0.1-mbar surface. Comparison of the microwave night-to-day ratio with the corresponding ratio from LIMS suggests that nonlocal thermodynamic equilibrium effects in the LIMS daytime data exceed 10% at all pressures less than or equal to 1 mbar.

Increases in middle atmospheric water vapor as observed by the Halogen Occultation Experiment and the ground‐based Water Vapor Millimeter‐Wave Spectrometer from 1991 to 1997

Water vapor measurements made by the Halogen Occultation Experiment (HALOE) from 1991 to 1997 are compared with ground-based observations by the Water Vapor Millimeter-wave Spectrometers (WVMS) taken from 1992 to 1997 at Table Mountain, California (34.4°N, 242.3°E), and at Lauder, New Zealand (45.0°S, 169.7°E). The HALOE measurements show that an upward trend in middle atmospheric water vapor is present at all latitudes. The average trend in the HALOE water vapor retrievals at all latitudes in the 40–60 km range is 0.129 ppmv/yr, while the average trend observed by the WVMS instruments in this altitude range is 0.148 ppmv/yr. This trend is occurring below the altitude where changes in Lyman α associated with solar cycle variations should produce a significant increase in water vapor during this period, and is much larger than the ∼0.02 ppmv/yr trend in water vapor associated with increases in methane entering the stratosphere. In addition to the water vapor increase, HALOE measurements show that there is a temporal decrease in methane at altitudes between 40 and 70 km. This indicates an increase in the conversion of the available methane to water vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of ∼0.1 K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.

An assessment of southern hemisphere stratospheric NOx enhancements due to transport from the upper atmosphere

Data from the Halogen Occultation Experiment (HALOE) are used to evaluate the contribution of upper atmospheric NOx to the stratospheric polar vortex. Using CH4 and potential vorticity as tracers, an isolated region of enhanced NOx is shown to occur in the Southern Hemisphere (SH) polar vortex almost every spring from 1991–1996. The magnitude of this enhancement varies according to the Ap auroral activity index. Up to half of the NOx in the mid-stratospheric SH polar vortex may be due to particle precipitation. The peak enhancement occurred in 1991 with a magnitude of 3–5% of the NOy, source due to N2O oxidation.

Global and temporal distribution of meteoric smoke: A two-dimensional simulation study

[1] Meteoric material entering Earth’s atmosphere ablates in the mesosphere and is then expected to recondense into tiny so-called ‘‘smoke particles.’’ These particles are thought to be of great importance for middle atmosphere phenomena like noctilucent clouds, polar mesospheric summer echoes, metal layers, and heterogeneous chemistry. Commonly used one-dimensional (1-D) meteoric smoke profiles refer to average global conditions and yield of the order of a thousand nanometer sized particles per cubic centimeter at the mesopause, independent of latitude and time of year. Using the first two-dimensional model of both coagulation and transport of meteoric material we here show that such profiles are too simplistic, and that the distribution of smoke particles indeed is dependent on both latitude and season. The reason is that the atmospheric circulation, which cannot be properly handled by 1-D models, efficiently transports the particles to the winter hemisphere and down into the polar vortex. Using the assumptions commonly used in 1-D studies results in number densities of nanometer sized particles of around 4000 cm �3 at the winter pole, while very few particles remain at the Arctic summer mesopause. If smoke particles are the only nucleation kernel for ice in the mesosphere this would imply that there could only be of the order of 100 or less ice particles cm �3 at the Arctic summer mesopause. This is much less than the ice number densities expected for the formation of ice phenomena (noctilucent clouds and polar mesospheric summer echoes) that commonly occur in this region. However, we find that especially the uncertainty of the amount of material that is deposited in Earth’s atmosphere imposes a large error bar on this number, which may allow for number densities up to 1000 cm � 3 near the polar summer mesopause. This efficient transport of meteoric material to the winter hemisphere and down into the polar vortex results in higher concentrations of meteoric material in the Arctic winter stratosphere than previously thought. This is of potential importance for the formation of the so-called stratospheric condensation nuclei layer and for stratospheric nucleation processes.

Two‐dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with Halogen Occultation Experiment data

A two-dimensional chemical transport model has been used to examine the physical processes governing the transport of high levels of thermospheric nitric oxide (NO) downward into the middle atmosphere. Three different facets of this transport are studied. The first facet involves diffusion from the thermosphere to the summertime mesopause region. The second facet involves downward advection by the mean meridional circulation in the wintertime mesosphere and the effects of planetary wave mixing on the latitudinal gradient of NO. The third facet is the residual amount of NO deposited in the springtime upper stratosphere and its senstivity to the magnitude and duration of the unmixed descent which occurred the previous winter. Comparison of the model with observations by the Halogen Occultation Experiment (HALOE) suggest the following: (1) A clear auroral enhancement in summertime NO exists at 89 km. Model calculations suggest this results from both in situ ionization and dissociation of N2 as well as downward diffusion from the thermosphere above 100 km. (2) Using HALOE CH4 observations as a tracer, enhanced NO in the wintertime mesosphere is seen to be transported to latitudes as far equatorward as 30°–40°. The model is in good agreement with these observations when planetary wave mixing is included. Without this mixing, the enhanced NO remains confined to high latitudes that are not observed by HALOE in winter. (3) The model overestimates the net NO deposited into the upper stratosphere. This appears to be related to the model springtime warming being delayed relative to the real atmosphere. Inclusion of an additional source of drag in the polar stratosphere in late winter yields better agreement with observations.

A new calculation of nitric oxide photolysis in the stratosphere, mesosphere, and lower thermosphere

Photodissociation of nitric oxide in the middle and upper atmosphere is examined using a line-by-line approach to describe absorption in the NO δ bands and O2 Schumann-Runge bands. The new analysis of O2 absorption results in greater transmission of ultraviolet radiation in the Schumann-Runge (5–0) band in comparison with previous studies, leading to increased rates for photolysis of nitric oxide in the δ(0–0) band. Reduced transmission in the O2 (9–0) and (10–0) Schumann-Runge bands produces smaller photolysis rates for the NO δ(1–0) band. Absorption in strong lines of the NO δ bands is shown to make a nonnegligible contribution to atmospheric opacity at wavelengths which are important for NO photodissociation. Representative distributions of nitric oxide are used to quantify possible changes in the NO photolysis rate over the course of a solar cycle. As a result of changes in the NO abundance in the thermosphere, modulation of the photolysis frequency at lower altitudes may be opposite in phase to variations in the solar irradiance. For solar zenith angles greater than 60°, photolysis rates at altitudes below 100 km may be smaller during solar maximum compared to solar minimum. A method is described which enables rapid calculation of NO photolysis frequencies, allowing also for effects of varying opacity by nitric oxide.

Stratospheric NOx enhancements in the Southern Hemisphere Vortex in winter/spring of 2000

POAM III data show unusually large increases in stratospheric NO2 throughout the late winter and spring at high southern latitudes during 2000. Using HALOE CH4 data as a tracer of vertical descent, we conclude that excess NOx was created by particle impacts in the upper atmosphere and descended in the polar vortex during the winter. We speculate that these NOx enhancements were due to the solar proton event that occurred on 14–15 July 2000, and show that they caused reductions of up to ∼45% in middle stratospheric ozone mixing ratios. Comparison of HALOE and POAM data in 2000 to data from 1991–1999 suggests that the 2000 NOx enhancements were the largest ever documented by satellite in the southern hemisphere middle stratosphere. Also, based on H2O data, we conclude that NOx-enriched air observed in the south polar vortex from 1991–1999 originated in the mesosphere, not the thermosphere as is often assumed.

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.

An intercomparison of model ozone deficits in the upper stratosphere and mesosphere from two data sets

We have compared a diurnal photochemical model of ozone with nighttime data from the limb infrared monitor of the stratosphere (LIMS) and ground-based microwave observations. Consistent with previous studies, the model underpredicts the observations by about 10–30%. This agreement is strong confirmation that the model ozone deficit is not simply an artifact of observational error since it is unlikely to occur for two completely different ozone data sets. We have also examined the seasonal, altitudinal, and diurnal morphology of the ozone deficit. Both comparisons show a deficit that peaks in the upper stratosphere (2–3 mbar) and goes through a minimum in the lower mesosphere from 1.0 to 0.4 mbar. At lower pressures (<0.2 mbar) the deficit appears to increase again. The seasonal variation of the deficit is less consistent. The deficit with respect to the LIMS data is least in winter while with respect to the microwave data, the deficit shows little seasonal variation. Finally, the night-to-day ratio in our model is in generally good agreement with that seen in the microwave experiment. Increasing the rate coefficient for the reaction O + O2 + M → O3 + M improves the fit, while a very large (50%) decrease in the HOx catalytic cycle is not consistent with our observations. Increasing the atomic oxygen recombination rate also improves the overall agreement with both data sets; however, a residual discrepancy still remains. There appears to be no single chemical parameter which, when modified, can simultaneously resolve both the stratospheric and mesospheric ozone deficits.

A climatology of nitric oxide in the mesosphere and thermosphere

Abstract Global measurements of nitric oxide (NO) in the earths upper atmosphere have now been obtained by two satellite experiments during the declining phases of the last two solar cycles. In the 1980s, NO was measured by the Solar Mesosphere Explorer while in the 1990s, NO has been observed by the Halogen Occultation Experiment (HALOE) on board the UARS satellite. The SME data cover the altitude range from 95 to 160 km and extend from pole to pole. The HALOE data used here cover the altitude range from 50 to 125 km and extend from 70N to 70S. Both datasets show a well defined decrease in NO during the decline of solar activity. Also, large perturbations due to auroral activity are seen at middle and high latitudes. In addition, the HALOE data show large increases in the high latitude winter mesosphere which are associated with downward transport from the thermosphere. This paper presents a reference NO model which is based upon the two datasets and which covers a wide range of solar, geomagnetic and seasonal conditions.