Atmospheric mixing processes derived from aircraft observations in the Tropics
Jim McQuaid, Doug Parker & Mat Evans (NCAS/York)
The composition of the atmosphere is perturbed by the emission of compounds typically from the surface. This perturbation is central to issues such as air quality degradation, climate change, ecosytem damage, reduction in visibility etc. Eventually these emissions mix through the whole atmosphere and are removed by chemical and physical processes. Overall, for chemical species it is a balance between spatial dispersion and the lifetime of the compound of interest. This project will investigate and interconnect the key processes which control the mixing of trace gases in the tropical atmosphere. In particular, the study will explore the mixing and transport pathways of sets of trace gases, of importance to the global climate, which are emitted over tropical forests. Mixing is achieved through a raft of meteorological processes that occur on scales ranging from a couple of metres (microscale), through tens of kilometres (mesoscale) to thousands of kilometres (global scale).
Thus this project pulls together two key areas of atmospheric science, meteorology which governs the mixing mechanisms and the chemical which continues to occur during the dilution of emissions into the bulk atmosphere. Leeds was the lead institute of the large African Monsoon Multidisciplinary Analysis (AMMA) project in West Africa in 2006. A vast amount of data was collected during the campaign and this project will further exploit this most impressive dataset. The project will unravel the various processes that lead to the distribution of the emissions from the tropical forests in the AMMA region into the larger regional scale.
The rate of mixing can have a critical impact upon the potential for further and longer-range transport due to the higher wind speeds in the free troposphere. Through careful examination of key relationships between observed species we aim to reveal the controlling mechanisms under different conditions. In particular, we will explore conditions ranging from convectively driven circulations, including cumulonimbus events above tropical forests, to nocturnal jet formation.
- Figure 1. The AMMA flight region
In this project, observations of both chemical and physical properties made on-board the UK146 research (www.faam.ac.uk) aircraft during the AMMA project in West Africa in 2006 (Figure 1) will be used to assess the relative importance of different meteorological processes in this mixing. The initial emphasis will be on the small scale processes that lift compounds from the forest canopy into the boundary layer and the lower free troposphere but eventually the emphasis will be on constructing a framework for this mixing against which models can be tested.
Understanding the controlling mechanisms across the various atmospheric scales is still something which atmospheric scientists still have something of an incomplete handle upon. Leeds is excellently positioned to exploit such datasets from projects such as AMMA, the combination of observational expertise (understanding instrumental constraints), coupled to the meteorological skills available in ICAS as well as the additional strand to this research program through access to powerful and well recognized chemical models including GEOS-CHEM brought to the table by Mat Evans. This interdisciplinary approach pulls together an internationally recognized team to lead this project. The development of techniques to decipher the relative importance of the different mixing processes is a key challenge, not just for this project but also for atmospheric science and in particular airborne observations. Instrument development is a continual process and a number of instruments on the 146 aircraft have been upgraded since the AMMA project and these now give access to the sorts of data rates that are needed for robust flux estimates to be generated, this project will develop the tools for this to be done and this will have significant output to the community and be a highly valued output. There is no one else in the UK looking at these processes as a whole package. This project will provide an excellent training experience for the student and equip them with a highly sought after skill in understanding the various processes at work in mixing in the atmosphere.
- Figure 2: Comparison of MEGAN vegetation field and in-situ observations of isoprene
Furthermore, in the absence of direct observations, schemes such as the Model of Emissions of Gases and Aerosols from Nature (MEGAN) are often used to initialise emission fields in numerical models. During AMMA the comparison between the observations of isoprene, the dominant biogenic hydrocarbon, showed an excellent fit to the vegetation map used by MEGAN (Figure2). Knowledge of the relative importance of the different processes which mix the emissions into the boundary layer and beyond are as important to have a good robust understanding of as are the surface emission inventory.
Program of work:
- Year one – Development of numerical methods (determination of temporal scales of sampled data and relationship across measured turbulence parameters and application to existing dataset (AMMA). Familiarisation with the instrument characteristics used in the data collection.
- Year two - Sensitivity testing of analysis in different regimes (nocturnal / daytime / convective etc). Comparison with other evidence (the sensitivity analysis alone has excellent potential for published output)
- Year three – Evaluation of model results and preparation of thesis.
Research Training:
It will be a requirement that the student takes a full part in the NERC-approved PhD training programme in the School of Earth and Environment at Leeds. Graduate students at Leeds follow 2 training programs. These are given by faculty of external University staff and cover generic skills (preparing publications, reading papers efficiently, time management, giving presentations), technical skills (online resources) and where necessary specialist skills (Fortran, IDL, Matlab). The University Staff & Departmental Development Unit (SDDU) run courses aimed at graduates. The courses cover generic skills, the courses are also open to staff. Students are encouraged to attend these where relevant. SDDU also runs a number of courses specifically tailored for first year graduates. The student will be a member of ICAS which is a large (>100 researchers) and very vibrant research community within the school.
References and further reading:
Garcia-Carreras, L; Parker, DJ; Taylor, CM; Reeves, CE; Murphy, JG (2010) Impact of mesoscale vegetation heterogeneities on the dynamical and thermodynamic properties of the planetary boundary layer, J GEOPHYS RES-ATMOS, 115, . doi:10.1029/2009JD012811
Karl, T. G., Spirig, C., Rinne, J., Stroud, C., Prevost, P., Greenberg, J., Fall, R., and Guenther, A.: Virtual disjunct eddy covariance measurements of organic compound fluxes from a subalpine forest using proton transfer reaction mass spectrometry, Atmos. Chem. Phys., 2, 279-291, doi:10.5194/acp-2-279-2002, 2002.
Jobson, B. T., D. D. Parrish, P. Goldan, W. Kuster, F. C. Fehsenfeld, D. R. Blake, N. J. Blake, and H. Niki (1998), Spatial and temporal variability of nonmethane hydrocarbon mixing ratios and their relation to photochemical lifetime, J. Geophys. Res., 103(D11), 13,557–13,567, doi:10.1029/97JD01715.
Garcia-Carreras, Luis, Douglas J. Parker, John H. Marsham, 2011: What is the Mechanism for the Modification of Convective Cloud Distributions by Land Surface–Induced Flows?. J. Atmos. Sci., 68, 619–634. doi: 10.1175/2010JAS3604.1