Fred N. Alyea

A Three-Dimensional Dynamical-Chemical Model of Atmospheric Ozone

Abstract A three-year integration of a global three-dimensional model including dynamics and simple photo- chemistry is used to predict ozone. Distributions of NO3 and odd hydrogen deduced by McConnell and McElroy are used to incorporate in a simple way the chemical effect of these species. Good agreement with observation is obtained for stratospheric motion patterns, meridional circulations, ozone density as a function of height and latitude, eddy transports of ozone, surface destruction of ozone, and correlations of ozone with other variables. The annual cycle of columnar ozone in high latitudes is present, but at a smaller amplitude than observed. Vertical transport of ozone downward from the main generation level at 30 km is accomplished primarily by small-scale eddy diffusion between 20 and 30 km and again near the ground; large-scale vertical transport dominates inbetween. The model predicts a secondary maximum in ozone mixingg ratio at 45 km somewhat equatorward of the winter-polar-night zone. This...

Preliminary calculations concerning the maintenance of the zonal mean ozone distribution in the Northern Hemisphere

Results from a three-dimensional photochemical-dynamical model of ozone are presented and a qualitative description of the maintenance of the ozone distribution and its seasonal variations below 1 mb is given. The transition between photochemical and transport control of the ozone distribution is emphasized. Between 1 and 10 mb, transport by the eddies seems to play only a minor role at mid-latitudes in producing the observed ozone distribution despite the zero correlation between ozone and temperature which occurs in that region. In the lower stratosphere, mean and eddy contributions to ozone change generally strongly offset one another. The buildup and decay of the springtime ozone maximum is discussed. Emphasis is given to the mechanism of ozone transport by the mid-latitude eddies, which play an important role in the springtime accumulation of ozone.

Meteorological constraints on tropospheric halocarbon and nitrous oxide destructions by siliceous land surfaces

Abstract A sink for tropospheric halocarbons (and nitrous oxide) by photodissociation of molecules adsorbed on silicates has recently been proposed. We assume that the necessary conditions for this to occur may be found on dry desert surfaces. Using typical desert meteorological flow characteristics, the adsorption/photochemical removal efficiencies for the worlds desert regions that would be necessary to significantly influence tropospheric levels of these species are estimated. Based upon these calculations, it is concluded that the proposed destruction mechanism may be capable of providing an effective sink in the troposphere for halocarbons and nitrous oxide.

Atmospheric methane: A global three-dimensional model study

We have developed a three-dimensional model to study global methane distributions and the transport of methane. The model includes transports, chemical reactions, and global methane sources. The calculations provide a fairly good simulation of the observed latitudinal gradient of methane and its global mean distribution in the stratosphere. The model results also suggest that rice paddies in Southeast Asia and wetlands in the high latitudes of North America can contribute to required sources for maximum of methane. The simulation also models the positive vertical gradients of methane in the troposphere of the southern hemisphere, which was observed by Fraser et al. (1984). The model analyses show that this positive gradient is the result of the interhemispheric transport of methane at upper levels of the troposphere.

Three-dimensional simulations of atmospheric methyl chloroform : effect of an ocean sink

A global three-dimensional chemical tracer model of the distribution and seasonal cycles of the surface concentration of CH3CCl3 is compared with surface observations from the Atmospheric Lifetime Experiment (ALE) for the years 1980–1985. Two-dimensional OH distributions calculated by a photochemical model are empirically adjusted from observed trends in the global average and the interhemispheric ratio of methyl chloroform. The effects of the recently discovered ocean sink for methyl chloroform were investigated. The model simulates the 5-year record of observations made at the five ALE sampling sites to generally within ±5% of the observed mean. The calculated average global lifetime of methyl chloroform is 5.7 ± 0.3 years. The estimated global mean OH concentration is 6.5 ± 0.4 × 105 cm−3. However, the inclusion of the ocean sink does not significantly improve the simulation of the observed interhemispheric gradient of methyl chloroform. Atmospheric transport dominates the simulated CH3CCl3 seasonal cycle throughout the northern hemisphere but is less important in the southern hemisphere.

An evaluation using 14C and N2O simulations of three‐dimensional transport driven by United Kingdom Meteorological Office and Goddard Space Flight Center assimilated winds

We have used two different sets of assimilated input wind fields (United Kingdom Meteorological Office (UKMO), NASA) to drive the transport in a three-dimensional global chemical transport model. In a first experiment the distribution of carbon 14 after nuclear bomb testing in the early 1960s was simulated. The second, more comprehensive experiment consists of a comparison of simulated nitrous oxide (N 2 O) distributions for both input wind fields against CLAES satellite measurements. Both experiments show that the NASA winds are producing too much upward transport in the equatorial area. By analyzing the Eulerian-mean conservation equation for zonal mean mixing ratios, we were able to show that these model differences are initially caused by differences in the mean residual circulations in the two different wind field sets but that because the latitudinal gradients remain steeper in the NASA results, differences in isentropic mixing rates are also evident. The NASA residual mean circulation in the equatorial lower stratosphere in the model is higher than the UKMO residual mean circulation by about a factor of 2. However, there is also evidence that isentropic mixing rates are too large in the subtropics in both models, and there is evidence from the N 2 O comparisons of overly large amplitudes of wavenumbers 4 to 7 in the tropics and subtropics in the models. Filtering the incoming wind fields to eliminate wavenumbers larger than 10 resulted in only minor improvements in the model comparisons against CLAES observations. Because of the significant discrepancies between the model simulations, using either one of the wind field sets, and the observations, we recommend care in using these assimilated winds for long-term model studies in general. Both wind field sets, however, produced good simulations of strong intrusions of tropical air into midlatitudes of the upper stratosphere in September 1992.

Predictability and Spectral Truncation

Abstract A quasi-geostrophic, two-level baroclinic model with a fixed heating function and simple friction represented in spectral form was integrated with climatological initial conditions for 30 days. Three separate integrations were performed changing only the spectral truncation. Two of the experiments had 10 degrees of freedom in latitude with 16 and 20 planetary waves, while the third had 11 degrees of latitudinal freedom and 18 planetary waves. Comparison of the integration results indicates that the increase in latitudinal resolution caused pronounced changes in the predicted variables whereas the increase in the number of planetary waves had a negligible effect on the distribution of variables over the integration period.

The Impact of Stratospheric Variability on Measurement Programs for Minor Constituents

Abstract The scales of temporal and spatial variations in stratospheric minor species concentrations and dynamical quantities are briefly reviewed. Despite the fact that the stratosphere is often considered to he a very quiescent region of the atmosphere, it exhibits transient and generally unpredictable episodes of intense activity. The resultant necessity for some degree of coordination between different observers in their choice of locations and times for measurements of minor constituents is emphasized. We provide the specific suggestion of one or more short “International Stratospheric Periods” whose duration would depend upon the time of year. During these short periods, as many observers as possible would perform an integrated series of observations designed to optimize the usefulness of their data to atmospheric scientists.

A general circulation model of stratospheric ozone

The paper reports on a general circulation model being developed with the ultimate objective of assessing the effect of SST operations on the atmospheric ozone distribution. The model variables are represented in the spectral domain using 79 spherical harmonics and 26 vertical levels between the ground and 70 km. Initial calculations have attempted to simulate the seasonal variations of the unperturbed global ozone distribution. The model shows the presence of a westerly stratospheric jet in the winter hemisphere with amplitudes similar to those observed in the polar night jet. Easterlies dominate in the summer hemisphere. The model also shows ozone to be transported polewards and downwards. Furthermore the annual cycle in ozone concentrations at mid-latitudes has been simulated. Preliminary comparisons are also made with other features of the dynamics and the ozone distribution in the stratosphere.

Meteorological control of lower stratospheric minor species variations: An observational example

Abstract Lower stratospheric air trajectories entering the region over Alaska at the ~ 125 mb level during late May, 1975 indicate a substantial shift in the geographical source regions for the air masses present during that time. This shift coincides with an approx 25% decrease in the observed halocarbon mixing ratios at the 125 mb level as determined from a daily sequence of halocarbon profiles. Since the halocarbon species measured are essentially chemically inactive at this level, the observed variation is linked to the changing meteorological pattern.