Chapter 8

Modeling Component

J.T. Kiehl, P.J. Crutzen, M. Lawrence and P.J. Rasch

Scientific Objectives

To model:

* Transport of aerosols from the Indian subcontinent into the ITCZ

* Direct and indirect climate forcing of sulfate aerosols

* Solar radiation absorbed by the atmosphere and the sea surface

* Sulfur and sulfate chemical system

* Ozone production and destruction processes

* Distribution and radiative effects of mineral dust aerosols

Since INDOEX is focused on the Indian subcontinent region which spans a range of spatial scales, accomplishing these goals requires a hierarchy of models. The program for INDOEX includes models that span the spatial and temporal scales necessary to address the above goals. Regional models are required to resolve the relevant scales of the important processes (e.g., chemical transport). However, regional models are strongly dependent on boundary conditions. Boundary data of chemical concentrations and fluxes for the regional models can be obtained from larger scale global chemical transport models, but these models require assimilated data to characterize the winds and thermal fields. Finally, to quantify the climate effects (e.g., direct and indirect radiative forcing) of the predicted aerosols requires linking the predicted spatial and temporal aerosol distributions to a radiation model. Sensitivity analysis with these models can determine what properties, physical and chemical, determine the important aspects of climate forcing. The proposed research will create a modeling framework to meet these goals. The modeling group of INDOEX includes experts in chemical, transport and radiative modeling at three national facilities and one university. The proposed research is based on existing models developed over a number of years. The models involved are: the off-line Model of Atmospheric Transport and Chemistry (MATCH) that has been developed and applied within the CEPEX project [M. Lawrence, 1996, thesis in progress], and the global chemistry-climate model ECHAM that has been developed by the European Study of the Indirect and Direct Influences on Climate of Anthropogenic Trace Gas Emmission (SINDICATE) group [Roelofs and Lelieveld, 1995].

Meteorological Analyses

The chemical transport models will require accurate meteorological analyses (Chapter 7) to redistribute and remove chemical species and aerosols.

Global Chenical Modeling

The MPI contribution to INDOEX, in collaboration with NCAR (P. Rasch), will focus on simulations with two models: MATCH and CCM3 (NCAR-Community Climate Model, version 3). Both are global transport/ chemistry models: CCM is a general circulation model; MATCH is an off-line model and will be driven by analyses provided by the FSU group. There are also plans to develop a zoom version of MATCH. The photochemical package which has been developed for MATCH is relatively fast and focuses on consistency between temporal and spatial evolution of dynamical/ physical fields and the parameters used to drive the photochemistry, lightning (NOx emissions are based on information from the convection parameterization, and moist removal processes are based on predicted cloud water and precipitation fields). The photochemistry from MATCH is currently in the process of being built into the CCM, and a version with tropospheric photochemical fields (ozone and water vapor) coupled to the radiative transfer code should be available well before INDOEX. P. Rasch (NCAR) will improve the treatments of convective transports, cloud microphysics, scavenging and deposition processes in the CCM and MATCH models. The other global modeling effort,
led by J. Lelieveld, has been developed under the European SINDICATE project.

The global photochemical modeling contribution to INDOEX consists of pre-field phase planning support and post-field phase use of the data.

The modeling efforts will aid the planning of the field experiment phase of INDOEX. Preliminary indications with MATCH [M. Lawrence] are that this region is a strong sink for O3 and NOy, and further that it is also a powerful oxidizing basin, with loss rates of CO and CH4 being significantly higher than global average loss rates (with concomitant implications for rapid oxidation of other trace gases (e.g., hydrocarbons and reduced sulfur species). This implies that the region is being supplied with external sources of these species. We are currently making efforts to determine the horizontal origins of trace gases flowing into the region, as well as to examine the magnitude of inter-hemispheric and stratospheric-tropospheric exchange (STE) effective here. An important part of our contribution will be in classifying the anticipated regions of inflow and source strengths of various chemicals into the INDOEX region. This will aid in optimizing cruise- and flight-paths by directing them to regions where extremes (maxima, minima, and strong gradients) in photochemical distributions are anticipated (which can then be used to help evaluate the magnitudes of the sources, transformations, and sinks).

Preliminary findings from MATCH indicate substantial temporal and spatial variability in the distributions of some of the key photochemical species (O3, CO, etc.). This variability has significant implications for planning a careful observation campaign: for example, a single cruise/flight leaving one day later than another single cruise/flight - but following an identical spatial trajectory - might encounter markedly different photochemical distributions. Conclusions based on any single cruise/ flight might thus be somewhat unrepresentative of the mean photochemical distributions. We are currently attempting to statistically quantify the magnitude and temporal scale of the variability of photochemical species [M. Lawrence]. This effort may also include comparisons to satellite observations (e.g., Stratospheric Aerosol and Gas Experiment 2, SAGE2). These results will translate into information about how many passes over a single region may be necessary to establish statistical significance regarding the typical appearance of photochemical distributions along the trajectory, and will be vital in planning a well-conceived experiment.

The focus of INDOEX will be on aerosols. Of critical importance to aerosol microphysics is the presence of aerosol precursors. These precursors are provided from oxidation of SO2 and DMS by OH, O3(aq) and H2O2(aq). Though we do not yet have an aerosol microphysics parameterization coupled with our photochemistry module, we can readily provide temporal and spatial distributions of the anticipated source strengths of various precursors by inclusion of reduced sulfur species emissions and the initial oxidation step (based on our predicted photochemical fields); such work is currently underway [M. Lawrence]. Prior knowledge of the anticipated distributions of aerosol precursors, in particular in terms of north-south gradients, will aid in planning paths of cruises/flights which are aimed at evaluating the processes which lead from reduced sulfur to aerosol formation.

Following the intensive field phase of INDOEX, our studies will be broken into three major components: 1) model validation; 2) assisting in interpreting the data; and 3) experiments (emissions scenarios).

Global photochemical models are an indispensable tool in improving our understanding of atmospheric chemistry and climate. In order to improve confidence in using these models for such studies, we must validate these models against measurements of distributions of trace gases. This task is at present made difficult by the sparsity of such observations, particularly in poorly explored regions such as the Indian Ocean. INDOEX will provide a great opportunity to validate model-predicted fields against data gathered on a concentrated scale and long-term basis, and will help indicate which processes are apparently being poorly represented, and which are most important to focus on in future research.

Using the FSU analysis we will test the predicted location and magnitudes of the photochemical species inflow regions. This will also provide an opportunity for evaluation of differences between CCM3 climatology and a single observed situation. The potential for additional information regarding inter-hemispheric and STE is a particularly exciting aspect, given their importance and our recognition of their typically poor representation in the models. The adequacy of our current understanding of important photochemical reactions ("conventional model chemistry") will be tested in our ability to predict the evolution of ozone mixing ratios in polluted air masses being transported southward from the Indian sub-continent, indicating whether additional photochemical reactions of significance may be absent from our models. Photolysis rates and reaction rates employed in the model will be put under scrutiny by comparison to observations of various ratios of photochemical species (e.g., NO/NO2, NOx/NOy, NOy/O3, as well as CO/NOy and CO/O3 along a trajectory with strong pollutant gradients, etc.). If simultaneous vertical profiles of NO, NOx, and O3 are made in cloudy regions, it may provide an opportunity to evaluate the effects of clouds on NO2 photolysis rates, a process which is at present relatively poorly understood, and for which efforts are underway to develop better representations for our models [J. Landgraf, thesis in progress]. An important component of model validation is the evaluation of the quality of scavenging schemes by comparing nitrate and sulfate concentrations measured in rainwater with the model predictions; at present, wet scavenging processes are one of the major weaknesses and sources of disagreement between global models (c.f. World Climate Research Program Scavenging Workshop, Cambridge, August 1995, Chair: P. Rasch).

Model studies will play a paramount role in interpreting and understanding the observations. Estimate of the quality of emissions inventories currently available for India, southeast Asia, and Africa (biomass burning, in particular) will be gained from an evaluation of the consistency of model fields versus observations, especially during reproducible meteorological events which indicate outflow from the continents. Information regarding interhemispheric and STE gathered during the field phase can be employed in the models to better understand the implications of these data for photochemical species distributions.

As indicated previously, the focus of INDOEX is on aerosols, and of critical importance to modeling aerosol microphysics is the knowledge of aerosol precursors, which requires accurate estimates of the OH-, O3(aq) and H2O3(aq) concentrations. We will provide distributions of oxidants and aerosol precursors to aerosol modelers. INDOEX will provide us with an opportunity to contrast the roles of DMS and SO2 in providing aerosol-forming species such as H2SO4. P. Rasch will characterize the origins and properties of sulfate aerosols over the INDOEX region using the (relatively) simple sulfur cycle now present in the CCM and MATCH. The CCM and MATCH (using a variety of meteorological driving datasets) provide one with a variety of 'pictures' of the processes driving the photochemistry, and thus the uncertainty in our understanding of those processes. In addition, they are able to partition sulfate at a particular location into 'flavors' in terms of its point of origin (i.e., from the Indian subcontinent, southeast Asia, or Africa) and the photochemical pathway (i.e., arising from gas phase or aqueous phase chemistry). Thus, we can identify the spatial and temporal distributions of aerosols over INDOEX, where the aerosol came from, and its photochemical pathway of formation. The model can also be used to identify the uncertainties arising from different meteorological forcing, and uncertainties in the emissions scenarios. A knowledge of the frequency of occurrence of aerosols in a particular region, and the source region and photochemical pathway, with some statement about uncertainties due to meteorology and emissions will help in identifying interesting regions for investigation, and perhaps ancillary observations useful for confirming or denying the most likely picture of sulfur aerosol formation in INDOEX. Statements characterizing the role of scavenging and convection may suggest particular flight strategies.

A final role in understanding the observations is the evaluation of their implications for global chemistry/climate. We will contribute to this by performing simulations with on-line coupled troposphere ozone fields. Information from the field campaign could be employed towards determining what is needed to force the models to reproduce observations, and what implications this might have on a global scale.

Validation of the models will be performed on the basis of actual meteorological calculations. Analyzed meteorological fields will be used to simulate tracer transports in the off-line MATCH model and to dynamically adjust (nudge) the ECMWF-Hamburg (ECHAM) model. Chemistry simulations can thus directly be compared with in situ measurements from ships and aircraft. Further, satellite measurements of total ozone and tropospheric ozone (Total Ozone Mapping Spectrometer (TOMS) and Global Ozone Monitoring Experiment, GOME) will be used for model validation. Preliminary model simulations of the INDOEX region predict steep ozone gradients between southern Asia and the equatorial Indian Ocean (e.g., tropospheric ozone varies from 16 to 36 DU in the INDOEX region) which can be resolved from satellite observations. In addition to chemical measurements, information about clouds will be used to compare with model results. An ongoing project addresses convective cloud retrieval from AVHRR measurements. This algorithm will be applied to the INDOEX region to develop cumulonimbus statistics that can be compared to model results.

Regional Chemical Modeling

In order to study problems controlled by long spatial and temporal scales, MATCH has historically been used as a global model. There are inevitably compromises which must be made in this mode of operation: the resolution and the sophistication of the representation of processes are strongly constrained by the computational expense of the model. Therefore MATCH has historically been run using relatively low resolution (200-300 km resolution) with quite simple physical formulations (parameterizations).

The dataset described in Chapter 7 will provide us with the opportunity to explore the sensitivity of the process representations at substantially higher (about 50 km) resolution, with a consequent increase in computational expense. We anticipate the modification of MATCH to make it work as a 'regional model' which can be used to focus on the INDOEX region for shorter periods of time. Global simulations at a lower resolution will be used as initial and boundary conditions for the regional simulations. By restricting the domain of interest we gain in computation cost, with the benefit of a substantial improvement in the representation of transport, and the reduction in errors in fields used to drive other parameterizations within the model. A good example of this gain is in the potential for improvements to processes associated with convection. Convection is thought to be responsible for: 1) the direct transport of trace species from near the surface to the upper troposphere; 2) the downward transport of tracers of stratospheric origin (NOy and O3) in regions surrounding overshooting convective turrets; and 3) is integrally involved in the production of NOx by lightning. Running MATCH in both a global and regional mode provides a uniform environment to explore such sensitivities with a minimum in computational expense.

Mineral Dist Aerosols

A substantial contribution to the total aerosol forcing of the INDOEX region is due to mineral dust aerosols. Recent model studies [Tegen and Fung, 1994 and 1995; Tegen et al., 1995] indicate that wind blown mineral dust can effect both the shortwave and longwave radiative budget in this region. Furthermore, mineral aerosols may provide metals and alkalinity which enhance SO2 oxidation and provide a surface for heterogeneous loss of reactive nitrogen (via N2O5 loss and HNO3 uptake). Thus it is important to incorporate these aerosols in our model studies. Kiehl (NCAR, Climate and Global Dynamics) and Zender (NCAR, Advanced Study Program) are incorporating these aerosols into the CCM. This parameterization (which follows the approach of Tegen) will also be incorporated into the global version of MATCH. Rasch and Lawrence plan to develop a zoom version of MATCH, and the mineral dust formulation will also be incorporated in this version of MATCH when it is available. Using this version of MATCH with the high resolution analyses provided by the FSU group, we will be able to compare modeled dust distribution with the observational data obtained from Prospero. Validation of the spatial distribution of mineral dust can also be carried out with satellite observations.

Climate Forcing of Aerosols and Ozone

One of the primary goals of INDOEX is to quantify the aerosol radiative forcing in the INDOEX region. The modeling contribution to this goal is to use the aerosol distributions produced by the global and regional models in conjunction with radiative transfer models to calculate the radiative forcing by the various aerosol types (e.g., sulfates versus mineral dust). Both direct and indirect aerosol forcing effects will be calculated. These calculations require more than an aerosol mixing ratio. The radiative forcing is determined by the extinction optical depth, single scattering albedo, and asymmetry parameter [Kiehl and Briegleb, 1993, Kiehl and Rodhe, 1995]. These optical properties are determined by the aerosol microphysical properties, especially the aerosol size distribution and the refractive index of the aerosol material. We will use INDOEX observational data to constrain these optical properties, then carry out forcing calculations for the combined aerosols and the individual aerosol types. We are aware of the problems associated with combining different aerosol types (internal versus external mixing rules) and will determine which mixing rule is more appropriate for the aerosols in the INDOEX region. Calculated radiative forcings will be compared with surface, in situ and satellite observed radiative fluxes, from which we will gain a deeper understanding of the manner in which aerosols affect the global climate.

The climate forcing by anthropogenic O3 formation in the troposphere is relatively strong if the O3 increase occurs near the tropical tropopause. The free tropospheric O3 budget over the Indian Ocean is to a large extent controlled by transport from higher latitudes. Substantial amounts of ozone from the stratosphere are introduced into the troposphere. Subsequently, equatorward transport of O3 occurs through the middle and lower troposphere. The chemical lifetimes of O3 near the surface in the tropics is less than a week, so that a significant part of it is destroyed before it reaches the ITCZ. Radiative calculations will be carried out to estimate the forcing due to tropospheric ozone.