Boris Khattatov

Simulating aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX

A system for simulating aerosols has been developed using a chemical transport model together with an assimilation of satellite aerosol retrievals. The methodology and model components are described in this paper, and the modeled distribution of aerosols for the Indian Ocean Experiment (INDOEX) is presented by Rasch et al. [this issue]. The system generated aerosol forecasts to guide deployment of ships and aircraft during INDOEX. The system consists of the Model of Atmospheric Transport and Chemistry (MATCH) combined with an assimilation package developed for applications in atmospheric chemistry. MATCH predicts the evolution of sulfate, carbonaceous, and mineral dust aerosols, and it diagnoses the distribution of sea salt aerosols. The model includes a detailed treatment of the sources, chemical transformation, transport, and deposition of the aerosol species. The aerosol forecasts involve a two-stage process. During the assimilation phase the total column aerosol optical depth (AOD) is estimated from the model aerosol fields. The model state is then adjusted to improve the agreement between the simulated AOD and satellite retrievals of AOD. During the subsequent integration phase the aerosol fields are evolved using meteorological fields from an external model. Comparison of the modeled AOD against estimates of the AOD from INDOEX Sun photometer data show that the differences in daily means are -0.03 ± 0.06. Although the initial application is limited to the Indian Ocean, the methodology could be extended to derive global aerosol analyses combining in situ and remotely sensed aerosol observations. Copyright 2001 by the American Geophysical Union.

Assimilation of satellite observations of long‐lived chemical species in global chemistry transport models

Use of data assimilation techniques such as optimal interpolation or the Kaiman filter in global chemistry transport models (CTM) is becoming more common. However, owing to high computational requirements, it is often difficult to apply these techniques to multidimensional models containing extensive photochemical schemes. We present a sequential assimilation approach developed for use with general global chemistry transport models. It allows fast assimilation and mapping of satellite observations and provides estimates of analysis errors. The suggested data assimilation scheme evolved from the one described by Levelt et al. [1998]. It is a variant of the suboptimal Kaiman filter and is based on ideas described by Menard et al. [2000] and Menard and Chang [200O]. One of the most important features of the developed scheme is its ability to routinely estimate variance of the analysis and to predict variance evolution in the model. The developed technique (or its variants) has been successfully interfaced with a number of different global models and used for assimilation of several types of measurements, including aerosol extinction ratios. Some of these experiments are described by Lamarque et al. [1999] and W. D. Collins et al. (Forecasting aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX, submitted to Journal of Geophysical Research, 2000, hereinafter referred to as Collins et al., submitted manuscript, 2000). We illustrate the method using assimilation of ozone observations made by the Upper Atmosphere Research Satellite/Microwave Limb Sounder in the three-dimensional chemistry transport model ROSE [Research for Ozone in the Stratosphere and its Evolution; Rose and Brasseur, 1989].

Assimilation of photochemically active species and a case analysis of UARS data

We present a short overview of applications of estimation theory in atmospheric chemistry and discuss some common methods of gridding and mapping of irregular satellite observations of chemical constituents. It is shown that these methods are unable to produce truly synoptic maps of short-lived photochemically active species due to insufficient temporal and spatial density of satellite observations. The only way to overcome this limitation is to supplement observations with prior independent information given, for instance, by atmospheric numerical models and/or climatologies. Objective approaches to combining such prior information with observations are commonly referred to as data assimilation. Mathematical basis of data assimilation known as optimal estimation equations is presented following Lorenc [1986]. Two particular techniques of data assimilation, the variational method and the extended Kalman filter, are briefly described, and their applications to time-dependent numerical photochemical models are discussed. We investigate validity of the linear approximation which is utilized in both methods, present time evolution of the linearization and covariance matrices, and discuss some of their properties. On the basis of ideas of Fisher and Lary [1995] we then employ a trajectory model and a photochemical box model for assimilation and mapping of the Upper Atmosphere Research Satellite (UARS) measurements of chemical species. The assimilation is performed using the variational technique and the extended Kalman filter, and results of both methods are presented and discussed.

Diurnal migrating tide as seen by the high‐resolution Doppler imager/UARS: 2. Monthly mean global zonal and vertical velocities, pressure, temperature, and inferred dissipation

Meridional diurnal tidal winds derived as described by Khattatov et al. [this issue] from the high-resolution Doppler imager (HRDI) data are used in the linearized tidal equations to solve for the diurnal tidal oscillations in the zonal and vertical velocity, pressure, temperature, and dissipation in the mesosphere and lower thermosphere. The resulting monthly mean latitude-altitude cross sections of the amplitudes of the tidal zonal, meridional, and vertical velocity, pressure, temperature, and Rayleigh friction and vertical diffusivity are presented for 12 months of the combined 1992/1993 year. The results show profound seasonal changes and interhemispherical differences in the calculated tidal amplitudes and dissipation. The derived monthly mean vertical diffusion coefficients are compared with the results of Garcia and Solomon [1985]. The Rayleigh friction coefficients and vertical diffusivities derived from HRDI measurements can potentially have important impact on modeling of the chemistry and composition of the mesosphere and lower thermosphere, in particular, on numerical modeling of atmospheric thermal tides.

Assimilation of MLS ozone measurements in the global three-dimensional chemistry transport model ROSE

A method for assimilating observations of ozone was implemented in the three-dimensional global stratospheric chemistry transport model ROSE. The model contains an extensive photochemical scheme which includes heterogeneous chemistry and uses temperature and wind fields from the UKMO (United Kingdom Meteorological Office) stratospheric analysis. Ozone measurements obtained by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite (UARS) were assimilated in the model using the sequential statistical interpolation approach. The analysis is performed using a time invariant background error covariance matrix that only includes horizontal covariances. Results from a sixty day simulation are presented and it is shown that assimilation of the MLS observations results in improved global three-dimensional distributions of ozone as seen from comparisons with MLS data not assimilated in the model. For further validation, the stratospheric total ozone fields computed from the analysis are compared with the TOVS total ozone measurements and it is shown that they agree within the uncertainty of the data.

Assimilation of Measurement of Air Pollution from Space (MAPS) CO in a global three-dimensional model

Observations of carbon monoxide (CO) by the Measurement of Air Pollution from Space (MAPS) instrument onboard the space shuttle were assimilated into a global three-dimensional chemistry-transport model using the Physical-space Statistical Analysis System approach. The assimilation considerably improved the calculated distribution of CO in the troposphere. On the global scale, the adjustment of the CO field resulting from the assimilation procedure was large at the beginning of the assimilation, suggesting discrepancies in the initial conditions of the model. As the model integration/assimilation progressed with time, transport caused the model to drift. This drift limited to a few days the “memory” of CO from its adjustment toward the observations. The assimilation of CO significantly influenced the distribution of other chemical species, even over the limited time periods (∼10 days) analyzed.

Diurnal migrating tide as seen by the high‐resolution Doppler imager/UARS: 1. Monthly mean global meridional winds

The high-resolution Doppler imager (HRDI) instrument on board the upper atmosphere research satellite measures global winds in the mesosphere and lower thermosphere on a day-to-day basis. The horizontal coverage of the HRDI data is excellent and provides a unique opportunity to study global-scale dynamic phenomena; however, the local time resolution and coverage are limited because of the nature of the satellite sampling. The lack of local time coverage makes conventional methods of data analysis (e.g., Fourier analysis) both difficult and erroneous. An original method of analysis, based on a numerical model of atmospheric thermal tides, is proposed and applied to the HRDI data. The tidal model is solved for the tidal oscillations in the meridional wind component. The simulated diurnal meridional tidal winds are used as a first guess in the analysis. The results of the model are adjusted to give meridional migrating tidal winds that have maximum consistency with HRDI measurements. This technique is used to derive monthly mean tidal oscillations of the meridional velocity. The derived tidal amplitudes show profound seasonal changes that seem to be consistent with gravity wave breaking theory. The results are compared with MF radar data. It is found that in the upper mesosphere and lower thermosphere, the tidal amplitudes obtained by HRDI can be bigger than those from MF radars by a factor of 2.

Assimilation of carbon monoxide measured from satellite in a three‐dimensional chemistry‐transport model

Carbon monoxide measurements are obtained from the analysis of the spectra provided by the Interferometric Monitor for Greenhouse Gases (IMG) instrument, which flew on board the Japanese ADEOS satellite. The averaging kernel function of the instrument, which provides the sensitivity of retrieved CO to the vertical atmospheric layers, is calculated. A sequential assimilation approach is used to incorporate this CO data set, along with a detailed associated error budget, into a global three-dimensional chemistry-transport model (MOZART version 2). We show how data assimilation allows one to highlight the differences between modeled and observed CO global distribution. Surface CO mixing ratios computed after assimilation of total columns provided by the IMG instrument are compared with the National Oceanic and Atmospheric Administration (NOAA) - Climate Monitoring and Diagnostics Laboratory (CMDL) in situ measurements, and a good agreement is found between the two data sets.

Sequential assimilation of stratospheric chemical observations in a three‐dimensional model

We describe a technique to assimilate chemical observations in a three-dimensional (3-D) chemical transport model (CTM). The method uses the established sequential technique of Khattatov et al. [2000], but here, it is applied simultaneously to many observed species. Following the assimilation, care is taken to preserve compact correlations between all modeled long-lived tracers and the total abundance of reactive families (e.g., inorganic chlorine). This way, the observations of long-lived tracers and family members constrain many other species in the model. In this paper, we apply the technique to the assimilation of O 3 , CH 4 , H 2 O, and HCl from the Halogen Occultation Experiment (HALOE) in 1992. Despite the poor coverage of HALOE, the assimilation of species with long photochemical lifetimes is a useful global constraint on the model. Results of the assimilation model have been tested by comparison with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) profiles of O 3 , CH 4 , H 2 O, HCl, and N 2 O. Direct comparison of the assimilated species shows that the assimilation model performs better in reproducing the independent observations. Comparison of the nonassimilated species (N 2 O) shows that assimilation has generally improved the comparison, especially in the midlatitude lower stratosphere.

Evaluation of 2001 springtime CO transport over West Africa using MOPITT CO measurements assimilated in a global chemistry transport model

The global chemistry and transport model MOCAGE (Modèle de Chimie Atmosphèrique à Grande Echelle) is used to investigate the contribution of transport to the carbon monoxide (CO) distribution over West Africa during spring 2001. It is constrained with the CO profiles provided by the Measurements Of Pollution In The Troposphere (MOPITT) instrument through a sequential assimilation technique based on a suboptimal Kalman filter. The improvement of troposphericCOdistribution fromMOCAGEis evaluated by comparing the model results (with and without assimilation) with the MOPITT CO concentrations observed during the analysed period (between 2001 March 15 to 2001 April 30), and also with independent in situ CMDL and TRACE-P observations. The initial overestimation in high CO emissions areas (Africa, SE Asia andNWcoast of South America) is considerably reduced by using the MOPITT CO assimilation. We analysed the assimilated CO for a period of three successive 15 d periods in terms of average fields overWest Africa and contributions to the CO budget of transport and chemical sources. It is found that the horizontal and vertical CO distributions are strongly dependent on the characteristics of the large-scale flows during spring, marked by the onset of the low-level southerly monsoon flow and the gradual increase of the well-known African and tropical easterly jets at middle and upper levels, respectively. Total transport by the mean flow (horizontal plus vertical advection) is important in the CO budget since it mostly compensates the local sink or source generated by chemical reactions and small-scale processes. The major source of CO is concentrated in the lower troposphere (1000–800 hPa) mainly due to convergent low-level flow advecting CO from surrounding regions and surface emissions (biomass burning). Vertical transport removes 70% of this low-level COand redistributes it in the middle troposphere (800–400 hPa) where chemical reactions and horizontal exports contribute to the loss of CO. A lesser proportion is transported upwards into upper troposphere, and then horizontally, out of the considered domain.