Hendrik Elbern

Toward a Monitoring and Forecasting System For Atmospheric Composition: The GEMS Project

The Global and Regional Earth System Monitoring Using Satellite and In Situ Data (GEMS) project is combining the manifold expertise in atmospheric composition research and numerical weather prediction of 32 European institutes to build a comprehensive monitoring and forecasting system for greenhouse gases, reactive gases, aerosol, and regional air quality. The project is funded by the European Commission as part of the Global Monitoring of Environment and Security (GMES) framework. GEMS has extended the data assimilation system of the European Centre for Medium-Range Weather Forecasts (ECMWF) to include various tracers for which satellite observations exist. A chemical transport model has been coupled to this system to account for the atmospheric chemistry. The GEMS system provides lateral boundary conditions for a set of 10 regional air quality forecast models and global atmospheric fields for use in surface flux inversions for the greenhouse gases. Observations from both in situ and satellite sources are used as input, and the output products will serve users such as policy makers, environmental agencies, the science community, and providers of end-user services for air quality and health. This article provides an overview of GEMS and uses some recent results to illustrate the current status of the project. It is expected that GEMS will grow into a full operational service for the atmospheric component of GMES in the next decade. Part of this transition will be the merge with the Protocol Monitoring for the GMES Service Element: Atmosphere (PROMOTE) GMES project into the Monitoring of Atmospheric Composition and Climate (MACC) project.

Ozone episode analysis by four‐dimensional variational chemistry data assimilation

A chemical four-dimensional variational data assimilation system has been developed and applied for the study of an episode with enhanced summerly ozone levels. In this study the optimization parameters are the initial values of the chemical constituents. The case study focuses on an ozone episode over central Europe during August 1997. The associated observational database consists mainly of surface observations of ozone and nitrogen oxides, but also a limited number of ozone radiosonde records. The four-dimensional data assimilation algorithm is composed of the chemistry transport model, its adjoint model with the adjoint versions of the Regional Acid Deposition Model (RADM2) chemical mechanism, horizontal and vertical advection schemes, implicit vertical diffusion, and a limited memory quasi-newton minimization routine. The underlying model of the spatio-temporal data assimilation scheme is the comprehensive mesoscale-α EUropean Air pollution Dispersion model (EURAD), which is based on the RADM2 gas phase mechanism. On the basis of a 6 hours data assimilation interval, analyses of the chemical state of the atmosphere were obtained, where the skill is verified in two different ways: (1) improvements of forecasts subsequent to the assimilation procedure, and (2) validation of the analyses with observational data which is withheld from the variational data assimilation algorithm. It is demonstrated that significant improvements are achieved for short-term forecasts including the afternoon ozone peak abundances. The analysis skill is further corroborated by examinations with retained measurement data.

Variational data assimilation for tropospheric chemistry modeling

The method of variational adjoint data assimilation has been applied to assimilate chemistry observations into a comprehensive tropospheric gas phase model. The rationale of this method is to find the correct initial values for a subsequent atmospheric chemistry model run when observations scattered in time are available. The variational adjoint technique is esteemed to be a promising tool for future advanced meteorological forecasting. The stimulating experience gained with the application of four-dimensional variational data assimilation in this research area has motivated the attempt to apply the technique to air quality modeling and analysis of the chemical state of the atmosphere. The present study describes the development and application of the adjoint of the second-generation regional acid deposition model gas phase mechanism, which is used in the European air pollution dispersion model system. Performance results of the assimilation scheme using both model-generated data and real observations are presented for tropospheric conditions. In the former case it is demonstrated that time series of only few or even one measured key species convey sufficient information to improve considerably the analysis of unobserved species which are directly coupled with the observed species. In the latter case a Lagrangian approach is adopted where trajectory calculations between two comprehensively furnished measurement sites are carried out. The method allows us to analyze initial data for air pollution modeling even when only sparse observations are available. Besides remarkable improvements of the model performance by properly analyzed initial concentrations, it is shown that the adjoint algorithm offers the feasibility to estimate the sensitivity of ozone concentrations relative to its precursors.

4D-variational data assimilation with an adjoint air quality model for emission analysis

Abstract The problem of analyzing the chemical state of the troposphere and the associated emission scenario on the basis of observations and model simulations is considered. The method applied is the four-dimensional variational data assimilation method (4D-var) which iteratively minimizes the misfit between modeled concentration levels and measurements. The overall model–observation discrepancy is measured in terms of a cost function, of which the gradient is calculated for subsequent minimization by adjoint modeling. The model applied is the University of Cologne EURopean Air pollution Dispersion model (EURAD) simulating the meso-alpha scale. The forward and adjoint components are Botts horizontal and vertical advection scheme (Bott, Mon. Wea. Rev. 117 (1989), 1006), implicit vertical diffusion, and the RADM2 gas phase chemistry. The basic feasibility of the adjoint modeling technique for emission rate assessment is demonstrated by identical twin experiments. The objective of the paper is to demonstrate the skill and limits of the 4D-var technique to analyze the emission rates of non-observed precursor constituents of ozone, when only ozone observations are available. It is shown that the space–time variational approach is able to analyze emission rates of NO directly. For volatile organic compounds (VOC), regularization techniques must be introduced, however.

Regional and Global Tropopause Fold Occurrence and Related Ozone Flux Across the Tropopause

This paper gives a synthesis of three algorithms to detect the presenceof tropopause folds from vertical ozone/radio-sounding profiles and frommeteorological analysis. Also an algorithm to identify injection ofstratospheric air into the lower troposphere fromozone/7beryllium time series is presented. Differences in theresults obtained from the algorithms are observed and discussed with respectto the criteria for fold detection and input data used. Spatial gradients inthe obtained folding frequencies are made evident on a global scale from thealgorithm based on meteorological analysis (Q-vector/potential vorticity)and probably also on a regional European scale from algorithms both basedmeteorological analyses and on ozone/PTU soundings. The observed seasonalvariation of folding occurrence is rather flat except during summer whenalso some differences appear between the algorithms. By combining thefolding frequencies with literature estimates of the cross-tropopause ozonetransfer in single folding events, an average stratospheric ozone influxinto the troposphere of 5.7 ± 1.3× 1010 mol.cm-2 s-1 is obtained for the Northern hemisphereand 12± 2.7× 1010 mol. cm-2s-1 for Western Europe. Potential additional contributions dueto other stratosphere-troposphere exchange processes than folds are not yetincluded in these estimates. Finally, the link between statistics fromozone/7beryllium data and folding statistics is brieflydiscussed.

Deep stratospheric intrusions: a statistical assessment with model guided analyses

Abstract A statistical assessment of deep intrusions of stratospheric air based on records of two adjacent mountain stations of the northern Alps at different altitudes is presented. Ten years recordings of beryllium activity, ozone concentrations, and relative humidity at the Zugspitze summit (2962 m a.s.l.), as well as ozone and relative humidity at the Wank summit (1776 m a.s.l., 15km distance) were analyzed. 195 stratospheric intrusion events could unambiguously be identified for the Zugspitze, whereas 85 intrusion events were found for the Wank. No event could be reliably identified at the valley floor station at Garmisch-Partenkirchen (740m a.s.l.). There is a pronounced seasonal cycle of the frequency of events showing highest activity during fall, winter, and spring, whereas low activity is found during summer. By assessing average events it was possible to infer the monthly mean enrichment rate of the lower tropospheric ozone concentration by deep stratospheric intrusions. It was found that at least 3% of the ozone burden is replaced every month on an annual average. Three events of moderate strength were taken to be further analyzed by mesoscale meteorological model simulations with subsequent trajectory studies. In two cases the intrusion of stratospheric air was induced by tropopause foldings. In the third case a cut-off low with an associated fold was responsible for the increased exchange. All three cases revealed that the ingress of stratospheric air observed at the mountain station is a non-local process induced more than 2000 km apart. Transport over these distances took about 2–4 days. Along the pathways through the tropopause the air parcels are shown to subside from the tip of the folds at 400–500 hPa down to about 700 hPa to reach the Zugspitze measurement station.

The Chemical Weather

Environmental Context. Meteorological weather—temperature, pressure, wind direction—is familiar to all, and contrasts with meteorological climate in short-term (weather) versus long-term (climate) influence. From the atmospheric chemistry side, the focus has largely been on the chemical climate, the long-term mean concentrations of important trace gases and aerosols. An emerging new focus of study is the chemical weather—the tremendous short-term variability of the atmospheric chemical composition, resulting from the strong influence of meteorological variability, chemical complexity, and regionally and temporally varying emissions.

Evolution of the concentrations of trace species in an aircraft plume: Trajectory study

Permanent increase of the subsonic aviation flights and attempts to develop high-speed civil transport (HSCT) necessitate an assessment of their possible environmental impacts. To evaluate global aviation effects, it is important to know the role of the chemical transformations inside an aircraft plume taking into account heterogeneous reactions. The goal of this work is to model the principal physical and chemical processes occurring inside an aircraft exhaust plume. A new box model which calculates 41 relevant Ox, HOx, NOx, ClOx, BrOx, SOx, and HC species and includes the possible heterogeneous reactions on the combustion aerosol and ice contrail particles is proposed. The simplified descriptions of the aircraft plume dilution and ice contrail formation are described. The transformations inside the aircraft wake are presented for the trajectories of the exhaust product motions at the levels of 200 and 100 mbar calculated for the particular atmospheric conditions on April 25, 1986. The following problems are discussed: the ozone response at these altitudes, the oxidation rate of NOx and SO2, the role of the heterogeneous reactions on the combustion aerosol and contrail particles, and the possibility of ice and nitric acid trihydrate (NAT or HNO2•3H2O) particle formation inside the wake. The model results are in agreement with available experimental data for NO, NO2, HNO3, HNO2, and SO2. Analytical expressions are proposed to evaluate the oxidation rates of NOx, and SO2 in the aircraft wake. The local ozone response is small (between 0.6% at 200 mb and −0.1% at 100 mb). Possible future investigations are proposed.

PSAS and 4D-var data assimilation for chemical state analysis by urban and rural observation sites

Abstract Advanced data assimilation can be applied to retrieve as much information as possible from photooxidant measurements in the trophosphere in order to analyze the chemical state. Traditional assimilation methods have proved to be unsatisfactory due to their inability (i) to account for locally varying chemical scenarios, (ii) to exploit informations inherent in time series evolution, and (iii) to use observations measured at different times. The Physical—space Statistical Analysis System (PSAS) and a space—time data assimilation analysis are proposed aiming at the combination of both the four dimensional variational calculus (4D—var) and inhomogeneous error covariances allowing to focus on regional differences in emission rates an surface conditions. The University of Cologne, Eulerian mesoscale—α Chemistry Transport Model EURAD is part of a comprehensive 4D—var chemical data assimilation system which makes best use of available observations to provide skillful analyses. Special emphasis is placed on the locally varying representativeness of individual measurement sites.

Stratosphere—Troposphere Air Mass Exchange and Cross—Tropopause Fluxes of Ozone

Continuous air mass exchange is occurring between the stratosphere and troposphere. It is characterized by a mean meridional flow from the tropics towards the extratropics in the stratosphere and downward cross-tropopause mass flux at middle to higher latitudes. The paper is focusing on the latter part of the exchange system. Calculations of downward fluxes derived from dynamical arguments (downward control principle) are compared with estimates obtained from cross-tropopause fluxes of ozone and other tracers. Prominent features causing ozone intrusions are tropopause folds and cut-off lows. Model results, observations and weather statistics point towards rather large downward ozone fluxes due to intrusions (more than 10 x 1010 molecules cm -2 s -1). They should be accompanied by air mass fluxes at least 3 times as large as derived on the basis of the downward control principle or from the mean residual meridional circulation. The discrepancy between both air mass flux estimates is apparently due to assumptions about the structure of flows controlling troposphere-stratosphere exchange of air. Larger fluxes are an indication of comparatively short turnover times for the lowest stratosphere. Implications for upward cross-tropopause fluxes of tropospheric air and the ozone budget of the tropopause region are briefly addressed.