W. Kouker

Three-dimensional model simulations of SF6 with mesospheric chemistry

Multiannual integrations with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) have been performed using meteorological analyses of vorticity and divergence up to 10 hPa to analyze the influence of a simplified SF 6 mesospheric chemistry on estimation of mean age of air and to compare profiles of SF 6 mixing ratios observed in the stratosphere with model simulations. The chemical degradation scheme includes electron attachment of SF 6 and subsequent reactions of SF 6 - , such as photodetachment and charge transfer with ozone. Several combinations of reaction rate constants and electron profiles have been tested. Good agreement with observations is found for inert SF 6 transport. However, when mesospheric loss is inclnded in the model, significant deviations are found for polar winter observations above 25 km. Chemical loss by electron attachment without reactions yielding SF 6 again is not compatible with observations. The atmospheric lifetime of SF 6 spans 400 to 10,000 years, depending on the assumed loss mechanism and the value for the electron density in the stratosphere.

NOy partitioning and budget and its correlation with N2O in the Arctic vortex and in summer midlatitudes in 1997

Vertical profiles of the most important species of nocturnal total reactive nitrogen (NO y = NO 2 + HNO 3 + CIONO 2 + 2 N 2 O 5 + HO 2 NO 2 ) together with its source gas N 2 O were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B) instrument inside the late winter arctic vortex from Kiruna (Sweden, 68°N) on 24 March 1997 and in summer midlatitudes from Gap (France, 44°N) on 2 July 1997. The measured data were compared to calculations performed with the three-dimensional chemistry transport model (CTM) Karlsruhe Simulation model of the Middle Atmosphere (KASIMA). The results show that in the late winter arctic vortex most of the available nitrogen and chlorine is in the form of HNO 3 and CIONO 2 , respectively. An anomalous N 2 O-NO y correlation observed in March 1997 appears to be caused to a large extent by quasi-horizontal mixing of air masses across the vortex edge. However, near 20 km some denitrification of ∼1.5 to 2 ppbv NO y could be observed. The N 2 O profile measured in July 1997 indicates remnants of polar vortex air and is not reproduced by the CTM at the same location. However, the profile shapes of the individual compounds of the NO y family as well as the NO x /NO y ratio are reproduced fairly well by the model.

Streamers observed by the CRISTA experiment and simulated in the KASIMA model

Data from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) showed three narrow streamers of air with tropical mixing ratios of HNO3 and N2O pointing from the tropics toward middle latitudes in the middle stratosphere on November 6, 1994. By means of the mechanistic prognostic model, the diagnostic chemical transport model (CTM) and the combined nudged model, which are all versions of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the hypothesis is checked of whether these streamers are due to adiabatic transport processes on a timescale of days. Whereas the prognostic model reproduces the northern hemisphere streamers only qualitatively in their position, the CTM and the nudged model show a good agreement between their simulated tracer structures and the observed streamers. Because of the clear streamer signal in the nudged model compared to the CTM, its data are used for the investigation of isentropic tracer deformations. They show that the northern hemisphere streamers are mainly built by adiabatic transport on a timescale of days. Rossby wave breaking plays a role in the dissolution of the streamers. In the southern hemisphere, the production of Ertels potential vorticity (EPV) and the net heating rate is large, and the observed streamers are therefore not reproduced in the EPV. Moreover, the isentropic deformations of the EPV due to the horizontal flow are that strong during a minor warming in the end of October that the reproduction of the southern hemisphere streamer by means of artificial tracers fails.

The influence of the OH + NO2+ M reaction on the NOy, partitioning in the late Arctic winter 1992/1993 as studied with KASIMA

The northern hemispheric winter 1992/1993 is simulated with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA). The model is a combination of an off-line model using analyses from the European Centre for Medium Range Weather Forecasts up to a pressure altitude of 10 hPa and a mechanistic prognostic model on top with an entire altitude range from 10 up to 120 km. The chemistry scheme included in the model represents the gas phase chemistry as well as heterogeneous reactions on polar stratospheric clouds and on liquid sulfate aerosols. We compare model results with global measurements performed by the instruments of the Upper Atmosphere Research Satellite. The model is able to simulate the global distribution of source gases and long-lived species as well as reactive species in good agreement with the observations. The most significant discrepancies occur inside the vortex where the calculated ozone mixing ratio is overestimated. Below the pressure altitude of 30 hPa inside the vortex the calculated NO2 mixing ratio is underestimated compared to the measurement. A sensitivity study with a new recommendation of the OH + NO2 + M rate constant using the full form of the falloff function for the pressure dependence has been performed. The mixing ratios of the NOy species differ by only about 5% due to the new recommended rate constant inside the vortex with no significant effect on the denoxification.

The vertical distribution of ClO at Ny‐Ålesund during March 1997

Results of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) are compared with vertical ClO profiles measured by the groundbased Millimeter Wave Radiometer MIRA2 inside the vortex during March 1997 at Ny-Alesund. The influence of the OH + ClO and HO 2 + ClO reaction branching ratio and of the absorption cross section of Cl 2 O 2 on the calculated mixing ratios of ClO and ozone has been investigated. In the upper stratosphere the ClO mixing ratio is reduced by 90% by using a minor channel of the OH + ClO reaction with a branching ratio of 0.07. A temperature dependent minor channel of the HO 2 + ClO reaction reduces the upper stratospheric ClO mixing ratio by 22%. Different absorption spectra of Cl 2 O 2 alter the ClO mixing ratios up to 12% at noon at 20 km. This causes differences of 15% in the ozone loss during winter.