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Photodissociation of nitric oxide in the middle and upper atmosphere is examined using a line-by-line approach to describe absorption in the NO 5 bands and 02 Schumann-Runge bands. The new analysis of 02 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 5(0-0) band. Reduced transmission in the 02 (9-0) and (10-0) Schumann-Runge bands produces smaller photolysis rates for the NO 5(1-0) band. Absorption in strong lines of the NO 5 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 ø, 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. Reactions (1) and (2) represent a net loss of two NO molecules. This sink is the dominant loss process for N Oxin the upper stratosphere, mesosphere, and lower themnosphere. Early estimates of the photolysis frequency of nitric oxide, JNO, were made by Bates [1954], Strobet et at. [1970], and $trobet [1971], although the first quantitative calculation to explicitly account for the NO rotational line structure was that of Ciestik and Nicotet [1973]. They demonstrated that the dominant contribution was due to predissociation in the (0-0) and (1-0) delta bands (C-X). Weaker absorption, occurring primarily in the beta bands (B-X, v' > 6) and gamma bands (A-X, v' • 3), was expected to play a minor role in determining the overall photolysis frequency in the stratosphere and mesosphere. The magnitude of JNO at zero optical depth was estimated to be greater than I x 10 -5 s -•. It was then shown by Frederick and Hudson [1979a] (hereinafter referred to as FH79) that values for the oscilla-Paper number 93JD02007. 0148-0227 / 93 / 93 J D-0200 7505.00 for strengths used by Ciestik and Nicotet [1973] were likely to have been over a factor of 2 too large. FH79 calculated revised values of Jr•o which were later parameterized by Allen and Frederick [1982] in a form suitable for photochemical models. Nicolet and Ciestik [1980] (hereinafter referred to as NC80) also obtained reduced estimates of JNO using smaller values for the oscillator strengths. An analytic approximation of their results was presented by Nicolet [1979]. Finally, Frederick et al. [1983] showed that NO absorption in the thermosphere could provide sufficient opacity in the cores of strong lines to reduce the photolysis frequency in the strato...

Abstract. We report the discovery of a layer of enhanced water vapor in the Arctic summer mesosphere that was made utilizing two new techniques for remotely determining water vapor abundances. The first utilizes Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) OH measurements as a proxy for water vapor. The second is a reanalysis of Halogen Occultation Experiment (HALOE) water vapor data with a technique to simultaneously determine polar mesospheric cloud (PMC) ice particle extinction along with the water vapor abundance. These results reveal a narrow layer of enhanced water vapor centered between 82-84 km altitude and coincident with PMCs, that exhibits water vapor mixing ratios of 10-15 ppmv. This indicates that a higher degree of supersaturation is present in the PMC region, and that PMCs are thus more efficient at sequestering total water (both ice particles and vapor) within the layer, than previously believed.

[1] A variety of spaceborne experiments have observed polar mesospheric clouds (PMC) since the late 20th century. Many of these experiments are on satellites in Sunsynchronous orbits and therefore allow observations only at fixed local times (LT). Temperature oscillations over the diurnal cycle are an important source of PMC variability. In order to quantify long-term natural or anthropogenic changes in PMCs, it is therefore essential to understand their variation over the diurnal cycle. To this end, we employ a prototype global numerical weather prediction system that assimilates satellite temperature and water vapor observations from the ground to ∼90 km altitude. We assemble the resulting 6 hourly high-altitude meteorological assimilation fields from June 2007 in both LT and latitude and use them to drive a one-dimensional PMC formation model with cosmic smoke serving as nucleation sites. We find that there is a migrating diurnal temperature tide at 69°N with a variation of ±4 K at 83 km, which controls the variation of PMC total ice water content (IWC) over the diurnal cycle. The calculated IWC is normalized to observations at 2300 LT by the Solar Occultation for Ice Experiment and allowed to vary with temperature over the diurnal cycle. We find that the IWC at 69°N has a single maximum between 0700 and 0800 LT and a minimum between 1900 and 2200 LT and varies by at least a factor of 5. The calculated variation of IWC with LT is substantially larger at 57°N, with a single prominent peak near 0500 LT.Citation: Stevens, M. H., et al. (2010), Tidally induced variations of polar mesospheric cloud altitudes and ice water content using a data assimilation system,

We compare simulations of mesospheric tracer descent in the winter and spring of 2009 with two versions of the Whole Atmosphere Community Climate Model (WACCM), both with specified dynamics. One is constrained with data from the Modern‐Era Retrospective Analysis for Research and Applications which extends from 0 to 50 km; the other uses the Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude (NOGAPS‐ALPHA) which extends up to 92 km. By comparison with Solar Occultation for Ice Experiment data we show that constraining WACCM to NOGAPS‐ALPHA yields a dramatic improvement in the simulated descent of enhanced nitric oxide (NO) and very low methane (CH4). We suggest that constraining to NOGAPS‐ALPHA compensates for an underestimate of nonorographic gravity wave drag in WACCM. Other possibilities, such as missing energetic particle precipitation or underestimated eddy diffusion, are less likely for the Arctic winter and spring of 2009.

[1] We investigate the solar cycle modulation of the quasi-biennial oscillation (QBO) in stratospheric zonal winds and its impact on stratospheric ozone with an updated version of the zonally averaged CHEM2D middle atmosphere model. We find that the duration of the westerly QBO phase at solar maximum is 3 months shorter than at solar minimum, a more robust result than in an earlier CHEM2D study due to reduced Rayleigh friction drag in the present version of the model. The modeled solar cycle ozone response, determined via multiple linear regression, is compared with observational estimates from the combined Solar Backscattered Ultraviolet (SBUV/2) data set for the period 1979-2003. We find that a model simulation including imposed solar UV variations, the zonal wind QBO, and an imposed 11-year variation in planetary wave 1 amplitude produces a lower stratospheric ozone response of $2.5% between 0 and 20°S and an upper stratospheric ozone response of $1% between 45 and 55 km, in good agreement with the SBUV-derived ozone response. This simulation also produces an (enhancement/reduction) in the (lower/upper) stratospheric temperature response at low latitudes compared to the effects of solar UV variations alone, which are consistent with model vertical velocity anomalies produced by the solar-modulated QBO and imposed changes in planetary wave forcing.