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Michael B. McElroy, Nien Dak Sze, and Yuk Ling Yung


Carbon monoxide, produced in the Venus atmosphere by photolysis of CO2, is removed mainly by reaction with OH. The radical OH is formed in part by photolysis of H2O2, in part by reaction of 0 with H02. Photolysis of HCl provides a major source of H radicals near the visible clouds of Venus and plays a major role in the overall photochemistry. The mixing ratio of 02 is estimated to be approximately 10−7, about a factor of 10 less than a recent observational upper limit reported by Traub and Carleton. A detailed model, which accounts for the photochemical stability of Venus CO2, is presented and discussed.

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Malcolm K. W. Ko, Nien Dak Sze, Mikhail Livshits, Michael B. McElroy, and John A. Pyle


A two-dimensional zonal-mean model with parameterized dynamics and an advanced photochemical scheme is used to simulate the stratospheric distributions of atmospheric trace gases including ozone. The model calculates the distributions of 37 species that are photochemically coupled via 140 reactions with rate data from WMO/NASA. A full diurnal treatment is used to calculate the diurnal variations of the short-lived species and the diurnal mean of the production/loss rates for the long-lived species. The calculated concentrations are compared with a wide range of observations with emphasis on the seasonal and latitudinal features. In this work, no post hoc adjustment of the dynamical parameters has been attempted to improve agreement with observations.

In general, the model results are in good agreement with observations, although several discrepancies are noted. Rather than focusing on any individual species, we look for systematic agreements and discrepancies between model and observations for a wide range of species. The model appears to successfully simulate the major features of the mixing ratio surfaces for the long-lived species. However, at the equatorial region, the model tends to underestimate the concentrations of upward diffusing species (e.g., CFCs, CH4, N2O) and overestimate the column abundances of the downward diffusing species (HNO3, HCl, O3). These discrepancies are systematically examined and their implications for transport parameterization assessed.

The model successfully simulates the general latitudinal and seasonal behavior of the local concentration and column abundance of O3. Apart from the overestimation of the column abundances at the equator, the model also underestimates its seasonal contrast at high latitudes. There are difficulties in explaining the observed low concentrations of NO2 in winter at high latitudes. It is shown that errors in the simulation of NO2 concentration in these regions can significantly affect the calculated seasonal and latitudinal behavior of the column abundance of ozone in the middle and high latitudes.

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Hans R. Schneider, Malcolm K. W. Ko, Nien Dak Sze, Guang-Yu Shi, and Wei-Chyung Wang


The effect of eddy diffusion in an interactive two-dimensional model of the stratosphere is reexamined. The model consists of a primitive equation dynamics module, a simplified HOx ozone model and a full radiative transfer scheme. The diabatic/residual circulation in the model stratosphere is maintained by the following processes: 1) nonlocal forcing resulting from dissipation in the parameterized model troposphere and frictional drag at mesospheric levels, 2) mechanical damping within the stratosphere itself, and 3) potential vorticity flux due to large scale waves. The net effect of each process is discussed in terms of the efficiency of the induced circulation in transporting ozone from the equatorial lower stratosphere to high latitude regions. The same eddy diffusion coefficients are used to parameterize the flux of quasi-geostrophic potential vorticity and diffusion in the tracer transport equation. It is shown that the ozone distributions generated with the interactive two-dimensional model are very sensitive to the choice of values for the friction and the eddy diffusion coefficients. The strength of the circulation increases with the mechanical damping and Kyy. At the same time, larger diffusion in the tracer transport equation reduces the equator to pole transport (Holton 1986). Depending on the amount of friction assumed in the stratosphere, increasing eddy diffusion can lead to an increase as well as a decrease in the net transport. It is shown that reasonable latitudinal gradients of ozone can be obtained by using small values for the mechanical damping [≈1/(100 days)] and Kyy (order 104 m2 s−1) for the mid- and high-latitude stratosphere.

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