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Chemistry, Aerosols, and Dynamics Interactions

The foci of studies on chemistry, aerosols, and dynamics interactions are to examine the effect of physical and dynamical processes on chemical species and to study the effect of chemistry on aerosols and cloud condensation nuclei. Ongoing projects within the program include studies of the effects of boundary layer processes on chemical species distributions, cloud chemistry process studies, and prediction of chemical constituents on the cloud scale to mesoscale.

Effect of boundary layer processes on chemical species distributions

Segregation of Chemical Reactants in a Shallow Cumulus Boundary Layer— Simple Chemistry

Shallow cumulus clouds enhance vertical transport and mixing but scatter solar radiation, which initiates photochemistry in the atmosphere. Because chemical reactions may proceed more slowly if these boundary layer processes segregate reactants, studies of the covariance between two reacting chemical species are being performed by Mary Barth (joint with ACD), Jordi Vila-Geurau de Arellano (Wageningen University, The Netherlands), Si-Wan Kim (visitor, Seoul National University), and Edward Patton for the shallow cumulus boundary layer. A two reaction system, where species A and B produce C and C dissociates to form A and B, was simulated with two large-eddy simulation models, the NCAR LES and the Dutch LES. Preliminary results show that for a cloud fraction of 25-30% the segregation between A and B was significant in regions below cloud, but was negligible when averaged over the boundary layer. Further simulations are being performed to determine the sensitivity of these results to different amounts of cloud fraction.

Segregation of Chemical Reactants in a Shallow Cumulus Boundary Layer—
Complex Chemistry

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Figure 71. Intensity of segregation between isoprene and OH at (a) 1200 LT, (b) 1300 LT, (c) 1400 LT and (d) 1500 LT. Red lines with aqueous chemistry, blue lines with only gas chemistry. Gray lines are liquid water content (scaled at top of figure).

Shallow cumulus clouds provide locations for aqueous chemical reactions in addition to enhancing transport and mixing and scattering radiation. Barth, Kim, and Chin-Hoh Moeng used the NCAR LES coupled with gas and aqueous chemistry (over 50 species) to investigate the segregation between hydroxyl radical (OH) and isoprene, species which react to eventually produce ozone. Knowing the degree of segregation between reactants will help improve regional and global-scale chemistry simulations that assume the reactants are well-mixed; it will also improve interpretation of chemical species measurements in the atmospheric boundary layer. Results of a simulation with only gas-phase chemistry show that segregation of OH and isoprene reaches 30% in the cloud layer. Inside the cloud layer, the turbulent flux of isoprene is positive and the vertical gradient of the mean OH concentration is also positive. This leads to a major sink of the covariance between these species, and hence a larger segregation. When aqueous-phase chemistry is also included, the segregation between the two species reaches 40% in the cloud layer, further indicating the importance of cloud chemistry on the distribution of OH and isoprene. These LES results will continue to be analyzed to determine which chemical reactions are contributing to the covariance of isoprene and OH.

Lagrangian Dispersion Modeling in the CBL Using LES Fields

In collaboration with Chin-Hoh Moeng and Peter Sullivan, Jeffrey Weil (visitor, University of Colorado) used velocity fields from large-eddy simulations (LES) to drive a Lagrangian model of dispersion in the convective boundary layer (CBL). In this combined LES - Lagrangian stochastic model (LSM), passive “particles” were tracked with a velocity given by the sum of the filtered and random subfilter-scale (SFS) LES velocities. The SFS velocity was modeled using an adaptation of Thomson's (1987) LSM, in which the random forcing was based on the SFS fraction of the total turbulent kinetic energy (TKE). The modeled crosswind-integrated concentration (CWIC) fields were in good agreement with 1) surface-layer similarity theory for a surface source in the CBL, and 2) convection tank measurements of the CWIC from an elevated source in the CBL surface layer. The comparison included the modeled evolution of the vertical profile shape with downstream distance, which showed the attainment of an elevated CWIC maximum and vertically well-mixed CWIC far downstream, in agreement with the tank data. For the proposed model, the agreement with the data and theory was better than that found with an earlier model in which the SFS fraction of the TKE was assumed to be 1, and significantly better than a model that neglected the SFS velocities altogether.

Turbulent Dispersion of Scalars

The fumigation process, where an elevated source of a scalar is entrained into a growing convective boundary layer (CBL), is controlled by turbulent dispersion. By using a Lagrangian particle model, Jeffrey Weil (visitor, University of Colorado/CIRES), Si-Wan Kim (Seoul National University, Korea), C.-H. Moeng, and Mary Barth, examined the distribution of passive particles along the LES-generated turbulent velocity flow. Analysis of the simulations showed that fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. The translation from time to space coordinates (so that pollutants advected into a growing boundary layer can be studied) needs to be determined with the time-dependent mean boundary layer wind rather than the initial mean boundary layer wind because the mean boundary layer wind speed decreases as the simulation proceeds. They plan to improve the subgrid-scale velocity model to improve predictions of particle concentrations at the surface in a rapidly growing CBL.

Cloud Chemistry Process Studies

The Importance of the Cloud Drop Representation on Cloud Chemistry

Previous studies have shown that the representation of cloud drop microphysics affects the amount of sulfate produced by aqueous chemistry because aqueous sulfur chemistry depends on pH and pH varies with drop size. Because other aqueous chemical reactions (e.g., those involving hydro peroxy radical and formic acid) also depend on pH, M. Barth is using a cloud parcel model coupled with gas and aqueous chemistry to investigate the importance of the model’s cloud drop representation on cloud photochemistry. M. Barth’s study focused on ozone, hydrogen peroxide, formaldehyde and formic acid under remote and moderately polluted chemical and aerosol conditions. Results of the study showed that formaldehyde and formic acid are sensitive to the cloud drop representation, while ozone and hydrogen peroxide are not.

Modeling for Field Experiment Analysis

As a collaboration with Jorgen Jensen (ATD), the cloud parcel model coupled with chemistry will be used as part of the Rain in Cumulus over the Ocean (RICO) project. Results of the model will be evaluated with field measurements of sulfur dioxide, hydrogen peroxide, and ozone, and will also be used to estimate the age of a cumulus parcel, which is one of the research objectives of RICO.

WRF-AqChem, a coupled meteorology and multi-phase chemistry model

Development and Initial Results of Model

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Figure 72.
Cross-section of the total (gas + cloud water + rain + ice + snow + hail) mixing ratio of carbon monoxide (CO) and of formaldehyde (CH_2O). Both species are found in high concentration near the surface and lower concentration above the boundary layer. Carbon monoxide, an insoluble species, is primarily transported to the upper troposphere, while CH_2O, a soluble and reactive species, has a fraction reacted or precipitated to the ground.

M. Barth, William Skamarock, and Si-Wan Kim implemented a simple gas and aqueous-phase chemistry mechanism into the WRF model to begin investigations on the effect of convection on the chemical environment. Convection plays an important role in transporting pollutants from the surface to the upper troposphere, scavenging soluble species and depositing much of the soluble species to the surface, and converting species chemically in the cloud drops. Simulations of the 10 July 1996 STERAO storm were performed to provide a way to evaluate the modeled chemistry results with observations. Sensitivities of the chemical species distributions to the cloud microphysics representation are currently being investigated. This WRF-AqChem model will continued to be developed by incorporating the aqueous chemistry modules into the community WRF-Chem model, by incorporating lightning production of NOx and interactions between gas and ice phase species. M. Barth and W. Skamarock also plan to continue collaborations with John Worden and Kevin Bowman (both at JPL) to examine the feasibility of using convective-scale cloud chemistry modeling with satellite data retrieval of ozone and carbon monoxide.

Intercomparison of Convective Cloud Chemistry Models

M. Barth designed and led an intercomparison study for convective cloud chemistry models as part of the 6th International Cloud Modeling Workshop, Cloud Chemistry Case. http://box.mmm.ucar.edu/individual/barth/TracerTransportDeepConvection.html. An intercomparison of these models will show how well-reputed models perform when simulating the same storm, giving a range of results that agree reasonably with observations. The intercomparison case focused on the 10 July 1996 STERAO storm for which observations of carbon monoxide, ozone, and nitrogen oxides (NOx)are available. Barth and Si-Wan Kim contributed results from the WRF-AqChem model. These results are being compared to those from Chien Wang (Massachusetts Institute of Technology), Jean-Pierre Pinty and Celine Mari (Laboratoire d’Aerologie, Toulouse, France), Ann Fridlind (NASA-Ames Research Center), Vlado Spiridonov (Hydrometeorological Institute. Skopje, Macedonia), Maud Leriche and Sylvie Cautenet (Laboratoire de Meteororologie Physique, Clermont-Ferrand, France), and Ken Pickering, Lesley Ott (U. Maryland) and Georgiy Stenchikov (Rutgers U.). Passive tracer transport show similar results among the models and agree fairly well with observations. The intercomparison of NOx produced from lightning and of the soluble species, nitric acid, hydrogen peroxide and formaldehyde, is currently being pursued.

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Figure 73. Vertical cross-sections of carbon monoxide (CO) across the storm anvil. Observations are from Skamarock et al. (2003). Results of WRF-AqChem are provided by Barth and Kim, results of C. Wang are provided by Chien Wang (MIT), results of DHARMA are provided by Ann Fridlind (NASA-Ames), and results of Meso-NH are provided by Jean-Pierre Pinty (Obs. Midi-Pyrenees, Toulouse, France). Cloud particle conentration of 0.1 per liter is marked by the solid black line, indicating the cross-sectional area of the anvil. Although anvil sizes vary between simulations, all model results show CO mixing ratios of 100 ppbv or greater in the anvil as is also found in the observations.

 


End of Research Narrative

 

 

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National Center for Atmospheric Research University Corporation for Atmospheric Research National Science Foundation Annual Scientific Report - Home Atmospheric Chemistry Division Advanced Studies Program Atmospheric Chemistry Division Climate and Global Dynamics Division Environmental and Societal Impacts Group High Altitude Observatory Mesoscale & Microscale Meteorological Division Research Applications Program National Center for Atmospheric Research Scientific Computing Division