G. D. Carver

Accuracy of the isentropic trajectories calculated for the EASOE Campaign

During EASOE 10 day backward trajectories ending on most measurement sites were calculated on 6 isentropic levels every day. The temporal and spatial coarseness of the ECMWF analyses used cause significant errors in the trajectories. These interpolation errors are largest in the direction along the trajectories. The difference between two trajectories of the Polar Vortex Balloon Experiment and calculated trajectories is 8 and 12% of the trajectory length after 1.83 and 2.14 days. This discrepancy is not explainable by interpolation errors alone.

Forest fire plumes over the North Atlantic: p‐TOMCAT model simulations with aircraft and satellite measurements from the ITOP/ICARTT campaign

[1] Intercontinental Transport of Ozone and Precursors (ITOP) (part of International Consortium for Atmospheric Research on Transport and Transformation (ICARTT)) was an intense research effort to measure long-range transport of pollution across the North Atlantic and its impact on O3 production. During the aircraft campaign plumes were encountered containing large concentrations of CO plus other tracers and aerosols from forest fires in Alaska and Canada. A chemical transport model, p-TOMCAT, and new biomass burning emissions inventories are used to study the emissions long-range transport and their impact on the troposphere O3 budget. The fire plume structure is modeled well over long distances until it encounters convection over Europe. The CO values within the simulated plumes closely match aircraft measurements near North America and over the Atlantic and have good agreement with MOPITT CO data. O3 and NOx values were initially too great in the model plumes. However, by including additional vertical mixing of O3 above the fires, and using a lower NO2/CO emission ratio (0.008) for boreal fires, O3 concentrations are reduced closer to aircraft measurements, with NO2 closer to SCIAMACHY data. Too little PAN is produced within the simulated plumes, and our VOC scheme’s simplicity may be another reason for O3 and NOx modeldata discrepancies. In the p-TOMCAT simulations the fire emissions lead to increased tropospheric O3 over North America, the north Atlantic and western Europe from photochemical production and transport. The increased O3 over the Northern Hemisphere in the simulations reaches a peak in July 2004 in the range 2.0 to 6.2 Tg over a baseline of about 150 Tg.

Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era

We know from the ice record that the concentration of atmospheric methane, [CH4], at the Last Glacial Maximum (LGM) was roughly half that in the pre-industrial era (PI), buthow much of the difference was source-driven, and how much was sink-driven, remains uncertain. Recent developments include: a higher estimate of the LGM-PI change in methane emissions from wetlands―the dominant, natural methane source; and the possible recycling of OH consumed in isoprene oxidation―the principal methane sink. Here, in view of these developments, we use an atmospheric chemistry-transport model to re-examine the main factors affecting OH during this period: changes in air temperature and emissions of non-methane volatile organic compounds from vegetation. We find that their net effect was negligible(with and without an OH recycling mechanism), implyingthe change in [CH4] was almost entirely source driven―a conclusion that, though subject to significant uncertainties,can be reconciled with recent methane source estimates.

Comparison of cross-tropopause transport and ozone in the upper troposphere and lower stratosphere region

Wintertime tracer transport in the atmospheric “middle world” is analyzed for five winters using three different advection methods, namely Eulerian advection, reverse domain-filling trajectory calculations, and contour advection with a diagnosis of filaments. In all approaches, isentropic motion is assumed. The results are compared to synopses of ozone derived from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) airborne ozone measurement program. Tracer and ozone data are cast into the “reduced” space spanned by equivalent latitude, potential temperature, and time. In this representation some anomalies of MOZAIC ozone are mirrored in corresponding features of stratospheric tracer mass mixing ratios. Eulerian and reverse domain-filling trajectory studies give similar results, suggesting that numerical diffusion only plays a minor role. Tracer transport out of the stratosphere minimizes in the height range of 340 to 350 K. The trajectory studies reveal preferred regions of isentropic stratosphere-troposphere exchange associated with the positions of storm tracks. These asymmetries are similar to asymmetries in the correlation of potential vorticity (PV) and ozone found in the MOZAIC data. Zonal asymmetries are also found in the occurrence of filamentary structure in the contour advection simulations with filamentation maximizing downstream of the North Atlantic storm track. Hemispheric- and seasonal-mean filamentation activity is strongly correlated with stratosphere-troposphere exchange discerned from the Lagrangian transport experiments. In both signals the winter of 1997/1998 shows the lowest activity of the five winters analyzed here. A drop in activity in the winter of 1995/1996 may, however, be related to the introduction of a more advanced data assimilation scheme to the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis cycle. This change may also be partly responsible for a somewhat improved correlation of PV and MOZAIC ozone at high equivalent latitudes after the winter of 1994/1995.

The impact of local surface changes in Borneo on atmospheric composition at wider spatial scales: Coastal processes, land-use change and air quality

We present results from the OP3 campaign in Sabah during 2008 that allow us to study the impact of local emission changes over Borneo on atmospheric composition at the regional and wider scale. OP3 constituent data provide an important constraint on model performance. Treatment of boundary layer processes is highlighted as an important area of model uncertainty. Model studies of land-use change confirm earlier work, indicating that further changes to intensive oil palm agriculture in South East Asia, and the tropics in general, could have important impacts on air quality, with the biggest factor being the concomitant changes in NOx emissions. With the model scenarios used here, local increases in ozone of around 50 per cent could occur. We also report measurements of short-lived brominated compounds around Sabah suggesting that oceanic (and, especially, coastal) emission sources dominate locally. The concentration of bromine in short-lived halocarbons measured at the surface during OP3 amounted to about 7 ppt, setting an upper limit on the amount of these species that can reach the lower stratosphere.

Observations of an atmospheric chemical equator and its implications for the tropical warm pool region

This paper reports observations of a tropospheric chemical equator in the Western Pacific region during the Austral monsoon season, separating the polluted Northern Hemisphere from the cleaner Southern Hemisphere. Measurements of carbon monoxide, ozone, aerosol size/composition, and non-methane hydrocarbons were made from aircraft, flying north from Darwin, Australia as part of the Aerosol and Chemical Transport In tropical conVEction (ACTIVE) campaign. A chemical equator, defined as a sharp gradient in the chemical background, was found not to be coincident with the Intertropical Convergence Zone during this period. A pronounced interfacial region was identified between 8.5 and 10 degrees S, where tracer mixing ratios increased rapidly within the boundary layer, e.g. CO from 40 ppbv to 160 ppbv within 0.5 degrees latitude (50 km), with inhibited inter-hemispheric mixing. These measurements are discussed in context using a combination of meteorological and Earth-observing satellite imagery, back trajectory analysis and chemical model data with the conclusion that air flowing into and subsequently uplifted by the active convection of the Tropical Warm Pool (TWP) region in the Western Pacific is likely to be highly polluted, and will perturb the composition of the Tropical Tropopause Layer. The main source of CO and other pollutants within the TWP region is expected to be biomass burning, with extensive fires in North Sumatra and Thailand during this period. The sharp gradient in composition at the chemical equator seen here results from extensive burning to the north, contrasting with pristine maritime air advected from the Southern Indian Ocean by a strong land-based cyclone over the Northern Territory of Australia.

Three‐dimensional chemical forecasting: A methodology

During the recent ASHOE (Airborne Southern Hemisphere Ozone Experiment) and SESAME (Second European Stratospheric Arctic and Middle-Latitude Experiment) field campaigns we produced a number of 10-day chemical forecasts of the stratosphere to aid in flight planning and interpretation of the observations. A general circulation model was integrated at high resolution to produce a 10-day meteorological forecast. These forecast fields were then used to specify the circulation in an off-line three-dimensional chemical transport model to produce a medium-range chemical forecast of the stratosphere. The chemical model has a detailed stratospheric chemistry scheme, including heterogeneous reactions on polar stratospheric clouds and sulfate aerosols. The NASA ER-2 high-altitude research aircraft made in situ measurements of chemical tracers in the lower stratosphere during the ASHOE campaign, which took place in four deployments from March to October 1994. This paper presents examples from the chemical forecasts made during the second and fourth deployments. We show that the ability to indicate the likely composition of the atmosphere is an important improvement over the standard meteorological forecasts, especially in air perturbed by chemical processing on polar stratospheric clouds. Of particular interest was the ability to forecast the chemical composition of filaments, which had been eroded from the vortex edge. Furthermore, immediate post-flight comparison between observed data and synoptic model data gave useful chemical interpretation of the observations.

Quantification of filaments penetrating the subtropical barrier

A method based on contour advection is introduced that aims to quantify the formation of filaments both equatorward and poleward of the subtropical barrier. It is applied to diagnosed potential vorticity fields for every day of January and February in 1997 and 1998. An isosurface of modified potential vorticity is found to represent the region of largest isentropic gradients of potential vorticity and is hence assumed as the tropopause. Tropopause-penetrating structures (“filaments”) forming in contour-advected potential vorticity fields are identified, and statistics of the abundance of such structures are derived. Filamentation as measured by this method exhibits a large temporal variability on the scale of days to weeks. For example, tropospheric cutoff systems developing in the wake of Rossby wave breaking events cause strong filamentation to occur in the lowermost stratosphere. The timescales governing filamentation are a function of altitude, reflecting the differing types and amplitudes of waves inducing filamentation at different isentropic levels. Zonal asymmetries arise as Rossby waves favorably break near the end of the North Atlantic storm track. A difference in the intensities of filamentation in January and February of 1997 and 1998 suggests that the relatively low values of ozone in the lowermost stratosphere observed in 1997 are related to increased filamentation-induced stratosphere-troposphere exchange, compared to 1998. For example, around 30% more filaments are found in the hemispheric and bimonthly mean at the 330 K isentropic surface in January and February of 1997 than during the same months of 1998. Single events can represent most of the filamentation occurring in a month, and hence interannual variability in the frequency of filamentation can be considerable.

Some case studies of chlorine activation during the EASOE Campaign

A combination of meteorological data and measurements of the CFCs is used to assess the chlorine activation in January 1992 during the EASOE campaign. Large activation can be expected when the coldest region occurs inside the polar vortex, as during early January. In mid January activation on the edge of the vortex, where the available chlorine is lower, should also have occurred. We consider the subsequent fate of this activated air, a small fraction of which clearly mixes into middle latitudes. A situation where extra-vortex air mixes into the vortex is also considered.

A case study in forecasting the stratospheric vortex during EASOE

Different high resolution versions of the UGAMP GCM are used to forecast the position of the stratospheric vortex for a period in January 1992. The sensitivity of these forecasts to changes in the vertical resolution, gravity wave drag parametrization scheme and horizontal diffusion are presented. The forecasts are compared to the ECMWF 5 day forecast and ECMWF analyses. For this case period, all runs produce a good 5 day forecast. Increased vertical resolution and reduced horizontal diffusion improve the 7 day forecast. The forecasts are also sensitive to the parametrization of gravity wave drag.