Douglas A. Rotman

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

Assessing future nitrogen deposition and carbon cycle feedback using a multimodel approach: Analysis of nitrogen deposition

n this study, we present the results of nitrogen deposition on land from a set of 29 simulations from six different tropospheric chemistry models pertaining to present-day and 2100 conditions. Nitrogen deposition refers here to the deposition (wet and dry) of all nitrogen-containing gas phase chemical species resulting from NOx (NO + NO2) emissions. We show that under the assumed IPCC SRES A2 scenario the global annual average nitrogen deposition over land is expected to increase by a factor of ∼2.5, mostly because of the increase in nitrogen emissions. This will significantly expand the areas with annual average deposition exceeding 1 gN/m2/year. Using the results from all models, we have documented the strong linear relationship between models on the fraction of the nitrogen emissions that is deposited, regardless of the emissions (present day or 2100). On average, approximately 70% of the emitted nitrogen is deposited over the landmasses. For present-day conditions the results from this study suggest that the deposition over land ranges between 25 and 40 Tg(N)/year. By 2100, under the A2 scenario, the deposition over the continents is expected to range between 60 and 100 Tg(N)/year. Over forests the deposition is expected to increase from 10 Tg(N)/year to 20 Tg(N)/year. In 2100 the nitrogen deposition changes from changes in the climate account for much less than the changes from increased nitrogen emissions.

The chemical and radiative effects of the Mount Pinatubo eruption

The eruption of Mount Pinatubo introduced large amounts of sulfur-containing particles into the stratosphere. Stratospheric ozone measured by ozonesondes and satellites is significantly lower following the June 1991 eruption and throughout 1992 and 1993. To clarify the mechanisms leading to effects on stratospheric ozone, time-dependent stratospheric aerosol and gas experiment II (SAGE II) and cryogenic limb array elaton spectrometer (CLAES) aerosol optical extinction data and SAGE II surface area density are used as parameters in a two-dimensional (2-D) zonally averaged chemical radiative transport model. The model was integrated with time from before the eruption through December 1993. The modeled impact on global ozone results from increased rates of heterogeneous reactions on sulfate aerosols and from the increased radiative heating and scattering caused by these aerosols. The models dynamical response to changes in forcing (from changes in radiatively active trace gas concentrations and from aerosol heating) is treated in one of three ways: (1) the stratospheric temperature is perturbed, with fixed seasonal circulation, (2) the circulation is perturbed, with fixed seasonal temperature, or (3) both circulation and temperature are unperturbed, when investigating only the impact of Mount Pinatubo increased aerosol surface area density (SAD) and aerosol scattering of actinic solar radiation, When the aerosol heating is allowed to modify the temperature distribution, the maximum change calculated in equatorial column ozone is −1.6%. The calculated equatorial temperature change and peak local ozone change in October 1991 are +6 K and −4%, respectively. When aerosol heating perturbs the circulation in the model, the maximum change in equatorial column ozone is −6%. Increased heterogeneous processing on sulfate aerosols is calculated to have changed equatorial column ozone in late 1991 by −1.5%. Global column ozone in the model in 1992 and 1993 changed by −2.8% and −2.4%, respectively. The relationship of ozone-controlling processes in the lower stratosphere is altered as well; HOx becomes the most important catalytic cycle, followed by ClOx and NOx. This is driven by significant changes in trace gas concentrations. In October 1991, lower stratospheric, equatorial NOx decreased by 40%, ClOx increased by 60%, and HOx increased by 25%. When the effect of heterogeneous chemical processing on sulfate aerosols is combined with aerosol heating, modifying either circulation or temperature, dramatically different ozone fingerprints with time and latitude are predicted. Model-derived changes in the equatorial region in column ozone best represented the observed data when perturbed circulation was combined with heterogeneous chemical effects. However, at high latitudes, the increased ozone production from the strengthening of the mean circulation tends to cancel the heterogeneous reduction of ozone. This is not in good agreement with observed data, especially in 1992 and 1993. When the circulation is held fixed and the temperature allowed to change, and heterogeneous chemical effects are included, the equatorial ozone decrease predicted was too small for 1991. However, the mid- to high-latitude decrease in 1992 and 1993 is in better agreement with observed data.

A polar stratospheric cloud parameterization for the global modeling initiative three‐dimensional model and its response to stratospheric aircraft

We describe a new parameterization of polar stratospheric clouds (PSCs) which was written for and incorporated into the three-dimensional (3-D) chemistry and transport model (CTM) developed for NASAs Atmospheric Effects of Aviation Project (AEAP) by the Global Modeling Initiative (GMI). The parameterization was designed to respond to changes in NOy and H2O produced by high-speed civilian transport (HSCT) emissions. The parameterization predicts surface area densities (SADs) of both Type 1 and Type 2 PSCs for use in heterogeneous chemistry calculations. Type 1 PSCs are assumed to have a supercooled ternary sulfate (STS) composition, and Type 2 PSCs are treated as water ice with a coexisting nitric acid trihydrate (NAT) phase. Sedimentation is treated by assuming that the PSC particles obey lognormal size distributions, resulting in a realistic mass flux of condensed phase H2O and HNO3. We examine a simulation of the Southern Hemisphere high-latitude lower stratosphere winter and spring seasons driven by temperature and wind fields from a modified version of the National Center for Atmospheric Research (NCAR) Middle Atmosphere Community Climate Model Version 2 (MACCM2). Predicted PSC SADs and median radii for both Type 1 and Type 2 PSCs are consistent with observations. Gas phase HNO3 and H2O concentrations in the high-latitude lower stratosphere qualitatively agree with Cryogenic Limb Array Etalon Spectrometer (CLAES) HNO3 and Microwave Limb Sounder (MLS) H2O observations. The residual denitrification and dehydration of the model polar vortex after polar winter compares well with atmospheric trace molecule spectroscopy (ATMOS) observations taken during November 1994. When the NOx and H2O emissions of a standard 500-aircraft HSCT fleet with a NOx emission index of 5 are added, NOx and H2O concentrations in the Southern Hemisphere polar vortex before winter increase by up to 3%. This results in earlier onset of PSC formation, denitrification, and dehydration. Active Cly increases and produces small (∼1%) decreases in lower stratospheric vortex O3 concentrations during the spring relative to the HSCT-free run.

Shock wave effects on a turbulent flow

A parametric study is done to investigate the change in a turbulent flow field caused by the passage of a shock wave. Two parameters are studied: the initial turbulent kinetic energy and the shock wave strength or density jump. A random or turbulent flow field is initiated within a two‐dimensional box. Euler’s equations are then solved using a second‐order accurate Godunov shock capturing method to calculate the change in turbulent structure and flow field parameters caused by the passage of a shock wave through the turbulent field. Two fields were analyzed, a random density field and a random velocity field. The passage of a shock through the random density field caused density and pressure variations that compare very well with experiments. Results of the shock passage through the random velocity field show that the shock causes an amplification in the turbulent kinetic energy of about 2 on a per unit mass basis. Furthermore, the length scale of the turbulent field behind the shock is smaller than that ...

An evaluation of upper troposphere NO x with two models

Upper tropospheric NOx controls, in part, the distribution of ozone in this greenhouse sensitive region of the atmosphere. Many factors control NOx in this region. As a result it is difficult to assess uncertainties in anthropogenic perturbations to NO from aircraft, for example, without understanding the role of the other major NOx sources in the upper troposphere. These include in situ sources (lightning, aircraft), convection from the surface (biomass burning, fossil fuels, soils), stratospheric intrusions, and photochemical recycling from HNO3. This work examines the separate contribution to upper tropospheric “primary” NOx from each source category and uses two different chemical transport models (CTMs) to represent a range of possible atmospheric transport. Because aircraft emissions are tied to particular pressure altitudes, it is important to understand whether those emissions are placed in the model stratosphere or troposphere and to assess whether the models can adequately differentiate stratospheric air from tropospheric air. We examine these issues by defining a point-by-point “tracer tropopause” in order to differentiate stratosphere from troposphere in terms of NOx perturbations. Both models predict similar zonal average peak enhancements of primary NOx due to aircraft (≈10–20 parts per trillion by volume (pptv) in both January and July); however, the placement of this peak is primarily in a region of large stratospheric influence in one model and centered near the level evaluated as the tracer tropopause in the second. Below the tracer tropopause, both models show negligible NOx derived directly from the stratospheric source. Also, they predict a typically low background of 1-20 pptv NOx when tropospheric HNO3 is constrained to be 100 pptv of HNO3. The two models calculate large differences in the total background NOx (defined as the source of NOx from lightning + stratosphere + surface + HNO3) when using identical loss frequencies for NOx. This difference is primarily due to differing treatments of vertical transport. An improved diagnosis of this transport that is relevant to NOx requires either measurements of a surface-based tracer with a substantially shorter lifetime than 222Rn or diagnosis and mapping of tracer correlations with different source signatures. Because of differences in transport by the two models we cannot constrain the source of NOx from lightning through comparison of average model concentrations with observations of NOx.

The Global Modeling Initiative assessment model: Application to high-speed civil transport perturbation

(3-D) chemical transport model (CTM) was applied to assess the impact of a fleet of high-speed civil transports (HSCTs) on abundances of stratospheric ozone, total inorganic nitrogen (NOv), and H20. This model is specifically designed to incorporate a diversity of approaches to chemical and physical processes related to the stratosphere in a single computing framework, facilitating the analysis of model component differences, modeling intercomparison and comparison with data. A proposed HSCT fleet scenario was adopted, in which the aircraft cruise in the lower stratosphere, emitting nitrogen oxides (NOx) and water (H20). The model calculated an HSCT-induced change in Northern and Southern Hemisphere total column ozone of +0.2% and +0.05%, respectively. This change is the result of a balance between an increase in local ozone below approximately 25 km and a decrease above this altitude. When compared to available NOy observations, we find that the model consistently underestimates lower stratospheric NO v. This discrepancy is consistent with the model bias toward less negative ozone impact, when cohapared to results from other models. Additional analysis also indicates that for an HSCT assessment it is equally important for a model to accurately represent the lower stratospheric concentrations of ozone and H20. The GMI model yields good agreement in comparisons to ozone data for present-day conditions, while H20 is constrained by climatology as much as possible; thus no further biases would be expected from these comparisons. Uncertainties due to discrepancies in the calculated age of air compared to that derived from measurements, and of the impact of emissions on heterogeneous and polar chemistry, are difficult to evaluate at this point.

A High Performance Chemical Kinetics Algorithm for 3-D Atmospheric Models

The ability to visually interact with and guide a running simulation can greatly enhance understanding of both the nature of a numerical model and the characteristics of the underlying physical process. This article will discuss the development of a distributed, interactive modeling system capable of providing responsive visual feedback to user input. The system is built around a discrete element soil model being used to simulate the behavior of soil masses under large deformations. In designing this application to be interactively responsive to the user for soil masses of several hundred thousand particles, a range of technical problems were encountered, including the computational burden of the discrete element method (OEM), real-time visualization of large particle masses, and communication of data over large distances between heterogeneous resources using a variety of networks. This article will highlight how each of those obstacles was addressed and discuss the effectiveness of the final system.Atmospheric chemistry transport (CT) models are vital in performing research on atmospheric chemical change. Even with the enormous computing capability delivered by massively parallel systems, many extended three-dimensional global CT simulations are not computationally feasible. The major obstacle in an atmospheric CT model is the nonlinear ordinary differential equation (ODE) system describing the chemical kinetics in the model. These ODE systems are usually stiff and can account for a significant portion of the total CPU time required to run the model. In this report, we describe a simple explicit algorithm useful in treating chemical ODE systems. This algorithm is one of a growing number of preconditioned time-stepping procedures based on dynamic iteration. In this study, the algorithm is compared with an established, general-purpose implementation of the common backward differentiation formulas. It is shown to be a viable choice for the chemical kinetics in a full 3-D atmospheric CT model across architectural platforms and with no special implementation.

A Greenhouse-Gas Information System: Monitoring and Validating Emissions Reporting and Mitigation

This study and report focus on attributes of a greenhouse-gas information system (GHGIS) needed to support MRVV the need for a system that meets specifications derived from imposed requirements; the need for rigorous calibration, verification, and validation (CVV the need to develop and adopt an uncertainty-quantification (UQ) regimen for all measurement and modeling data; and the requirement that GHGIS products can be subjected to third-party questioning and scientific scrutiny. This report examines and assesses presently available capabilities that could contribute to a future GHGIS. These capabilities include sensors and measurement technologies; data analysis and data uncertainty quantification (UQ) practices and methods; and model-based data-inversion practices, methods, and their associated UQ. The report further examines the need for traceable calibration, verification, and validation processes and attached metadata; differences between present science-/research-oriented needs and those that would be required for an operational GHGIS; the development, operation, and maintenance of a GHGIS missions-operations center (GMOC); and the complex systems engineering and integration that would be required to develop, operate, and evolve a future GHGIS.