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Vertical Transport and Mixing Program Home Page |
This is a first draft of a science plan for the Environmental Meteorology Program's (EMP) Vertical Transport and Mixing (VTMX) research program. Because it is a first draft, prepared even before the issuing of a request for proposals, it is necessarily incomplete at this stage. It is expected to evolve only slightly over the next few months, but will probably undergo more significant changes when successful proposals have been selected for funding and the team of investigators has been chosen. This plan provides background information and a discussion of the scope of the program, its anticipated elements and how they fit together, what "services" might be provided to investigators (principally, the availability of a data hub and some leg work by the lead scientist when experiments are being organized), some of the responsibilities of program participants, and how the VTMX program might be planned, administered, and reviewed.
The measurement and modeling of vertical transport and mixing processes in the lowest few kilometers of the atmosphere are problems of fundamental importance for which a fully satisfactory treatment has yet to be achieved. Important aspects of air quality modeling and weather forecasting are adversely affected by our inability to describe these processes adequately. Although a general theoretical understanding of many of the physical phenomena relevant to vertical transport and mixing processes exists, that understanding is incomplete, the representation of various phenomena in models is often poor, and the data needed to test those models are lacking. The upward and downward movements of air parcels in stable and residual layers of the atmosphere and the interactions between adjacent layers are particularly difficult processes to characterize, and significant difficulties also exist in describing the behavior of the atmosphere during morning and evening transition periods. Complications due to heterogeneous land surfaces and complex terrain further compromise our ability to treat vertical transport and mixing processes properly.
To address these issues, the Environmental Meteorology Program (EMP) has developed a program to investigate vertical transport and mixing (VTMX) processes in the lower atmosphere. The goals of the EMP/VTMX program are to
This program will concentrate on VTMX processes in stably stratified conditions, in conditions of weak or intermittent turbulence, and during morning and evening periods that mark transitions between stable and convective conditions. A complete characterization of a diurnal cycle of vertical transport and mixing may require consideration of fully developed mid-afternoon convective conditions, but convective boundary layers have been the subject of extensive previous studies and are relatively well understood.
Moreover, although progress in the treatment of VTMX processes would be useful in a wide variety of circumstances, there is particular interest in realizing these objectives for urban regions affected by adjacent elevated terrain (e.g., urban basins or valleys). In general, settings of this kind will, therefore, serve as a principal focus for this program. The choice of such sites will allow the investigation of phenomena such as the formation of inversions and cold pools and their destruction by surface heating or synoptic scouring, the formation and behavior of residual layers above the surface inversion, and the movement of pollutants or tracers within and through the multiple atmospheric layers that develop over a basin or valley.
Although there are logistical and scientific difficulties associated with studies in complex terrain, urban basins and valleys are an attractive option for an initial field study for a number of reasons. First, air quality problems are often significant in basin settings when synoptic and convective circulations are insufficient to remove pollutants from the region. In addition, stable boundary layers that form over sloping surfaces produce downslope winds with associated vertical wind shear and turbulence production. Turbulence production, wind fields, and turbulent fluxes are thus organized by the orography and may be treated and forecast somewhat more reliably than over flat terrain, where turbulence may be more intermittent and occurs primarily in bursts. Moreover, in the case of valleys and basins, the average vertical potential temperature gradients are less than over flat terrain because the cooling is distributed over a deeper layer by eddies that scale with valley or basin depth. Over flat terrain, inversions are typically 200 m deep with mean potential temperature gradients of 30 - 40 K/km; in valleys and basins, inversions are typically 500 m deep with potential temperature gradients of 10 - 25 K/km. This weaker stability may make initial studies of VTMX in stable conditions somewhat more tractable in models. Finally, because of the orographic influences that help regulate the basin atmosphere's characteristics, there is a reasonable expectation that instruments deployed in selected areas will be able to capture the principal flow features of the area on a more reproducible basis than might be true for flatter terrain.
Horizontal scales of interest for this program are on the order of a few hundred kilometers or less. Vertical scales will depend on the height of the daytime mixed layer and the elevation of any nearby terrain and will generally be on the order of a few kilometers or less.
Note that the scientific focus of the program will be on vertical transport and mixing in urban basins or valleys as a whole, or significant portions thereof, and not on detailed descriptions of much smaller-scale urban processes (e.g., street canyon scales). To the extent that studies of small-scale processes may be essential for realizing the primary programmatic goals, they may be legitimate areas of inquiry, but they are anticipated to be, at most, a minor component of this program. Similarly, this program will not, in general, focus on synoptic-scale vertical transport, although it is also realized, or course, that processes involving larger scales may have to be taken into account for a full understanding of basin- or valley-scale ones.
Given the considerations described above - the programmatic scope (vertical transport and mixing), geographic focus (urban basins or valleys), and areas of emphasis (stable conditions, conditions of weak or intermittent turbulence, and morning and evening transition periods) - some examples of scientific questions that may be addressed in the EMP/VTMX program are:
These questions should be taken as examples of the kind of studies that may be pursued in this program. They are not intended to be all-inclusive or definitive, and it is anticipated that other significant questions will be identified during the course of this program.
In approaching the problem of vertical transport and mixing, four principal program elements are envisioned: critical analyses of existing and anticipated "external" data sets to extract information pertinent to VTMX processes and their representation in models; field experiments and subsequent analysis of the collected data to increase our understanding of VTMX processes and our ability to describe them in models; the development and testing of new observational and analysis techniques for measurements of key features of VTMX processes; and the development, improvement, and application of numerical and conceptual modeling approaches.
There are a number of sets of "external" observational data, i.e., data collected by other groups, that can be used to examine aspects of vertical transport in heterogeneous terrain on scales of tens to hundreds of kilometers. Examples include the 1992 perfluorocarbon tracer data from Project MOHAVE, data obtained from the Southern California Ozone Study (SCOS97-NARSTO), and data from the anticipated Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study and the Cooperative Atmospheric/Surface Exchange Study (CASES) in 1999. In many cases, data from studies such as these are only partially analyzed due to time pressures and limited funds. In addition, although the original focus of many of these studies was not on VTMX processes, a further analysis of the data collected in each may be used to advance our understanding of such processes. It may also provide insight on how to improve the design of future field studies.
The use of external observational sets may provide a cost effective way to supplement the information anticipated from the field experiments that will form a critical part of this program. At the same time, efforts should be made to explicitly relate this supplementary information and the insight gained from it to the understanding or interpretation of phenomena to be studied in the VTMX program's field and modeling studies.
Field experiments in other programs have generally not focused on VTMX processes so that data obtained from them are often not well-suited for detailed analyses of these phenomena. For this program, multiple experiments designed explicitly to investigate selected vertical transport or exchange mechanisms will be needed to provide the information necessary for detailed studies of those mechanisms. Complete plans for an experimental campaign, the analysis of the resulting data, and the development of revised parameterizations of vertical transport and mixing will depend on the level of funding in the program, the mix of participants, the state of measurement technology when the field campaigns occur, and the resources of program participants or collaborating groups that can be marshaled for the study.
The range of instruments that might be brought into play includes aircraft, radar wind profilers, radio acoustic sounding systems for measurements of virtual temperatures, sodars, Doppler lidars, various turbulence profilers, scanning aerosol backscatter lidars, ozone differential absorption lidars (DIAL), chemically specific DIAL lidars, tracers, and balloon- or kite-borne turbulence measurement platforms. Other instruments potentially useful to this program are being or may be developed over the next several years. The experimental design will be driven by the need for detailed measurements of the vertical profiles of the wind, temperature, turbulence structure, and pollutant or tracer concentrations in and above the boundary layer overlying a basin or valley and its surroundings.
Because unlimited resources are never available for experimental campaigns, it will be necessary to balance carefully what questions are to be addressed in a particular campaign, what measurements are required to provide the necessary data, what instruments are available to provide those measurements, and what the cost is for their deployment and the subsequent data analysis. As a starting point for the first VTMX field campaign, the goal will be to measure, characterize, and understand the full three-dimensional dynamics of transport and diffusion in a selected urban basin or valley during the night, morning and evening transition periods, or under other conditions of stable stratification and weak turbulence. This is an extremely ambitious undertaking, encompassing a host of processes such as gravity and KH waves, jets, slope flows, flux divergence, interactions of local and synoptic flows, radiative and turbulent cooling, etc. In practice, sensible limitations on the scope of research implied by such a goal will be necessary and only a selected set of phenomena can be addressed in any given field campaign.
The design of a strategy for any particular field measurement campaign may well require considerable innovation and ingenuity. Various VTMX mechanisms are likely to be coupled and attempts to study a particular one in isolation from others may be unwarranted and unwise. Standard approaches for measuring vertical profiles of winds, aerosols, etc. will undoubtedly be necessary and useful but may not be sufficient to provide the data needed for real progress in understanding some VTMX processes. Given the complexity of the flow phenomena likely to be encountered, considerable thought will be required not only on what measurements are possible but also what measurements are truly useful.
The design of a field experiment must also be a coordinated effort between modelers and experimentalists, between those who obtain the data and those who subsequently analyze it. These groups may be one and the same or they may be quite distinct. Although it may seem obvious it is nonetheless important to bear in mind that resources should not be devoted to collecting data that nobody uses. Conversely, modelers should be aware of what data are likely to become available and they will need to work with experimentalists to help define the measurement strategy for each experiment.
A considerable effort will be made to coordinate major EMP/VTMX field experiments with field studies being conducted under the auspices of other programs. There are clearly significant benefits to be had if the resources of multiple groups can be concentrated in a single study area.
It is anticipated that most of the observational efforts in this program will take place in the context of major, multi-investigator, cooperative field campaigns. There may also be some benefit from more limited measurements by individuals or smaller groups of investigators for special purposes. In some cases, for example, useful data might be collected locally (i.e., near a participating institution) or in concert with another program's field campaigns at nominal cost. It is not anticipated, however, that such activities will be a major component of this program.
Tracers have the potential to be an extremely useful tool in the study of VTMX processes but their use may require that considerable conceptual, technical, and logistical difficulties be overcome. Ideally, tracers should be inexpensive, easy to release in a controlled manner from point, line, or area sources, detectable in extremely low concentrations, and able to be measured inexpensively and readily over extended volumes of the atmosphere with good spatial and temporal resolution. In practice, one or more of these properties will not be realized and allowances for various limitations will have to be made.
Tracers of opportunity suitable for the study of VTMX, i.e., naturally occurring tracers that are found in the atmosphere, may take a variety of forms, including aerosols, carbon monoxide, ozone, radon, and water vapor. These have the obvious advantage of requiring no apparatus or cost for their distribution and release but have the corresponding disadvantage of allowing no control of the source term. For some tracers of this type, e.g., aerosols and ozone, remote sensors have been developed that allow at least two-dimensional sampling of the tracer. If the sensor is mounted on an airplane or a mobile ground platform, some limited three-dimensional sampling is also possible. Other naturally occurring tracers may require in situ sampling, although the samplers may themselves be mobile.
Tracers that are deliberately released, such as perfluorocarbons, offer the considerable potential advantage of control over the source term. By using multiple tracers that can be distinguished from each other it is possible to examine the contributions of multiple sources or a single source at various times. Detection of such tracers is now relatively straightforward, although improvements continue to be made and there is considerable scope for the development of novel detector platforms. The technology for remote sensing of such tracers on the spatial and temporal scales useful for the VTMX studies envisioned for this program is not generally available at this time, and significant progress in this area could be extremely valuable.
For tracer studies in stable conditions or in conditions with weak or intermittent turbulence, there are significant technical and logistical difficulties in designing a useful sampling strategy. Sampling in the vertical direction will be particularly problematic, with the use of kites, tethered balloons, and airplanes all being subject to deployment restrictions, possibly severe, over urban areas. A full three-dimensional, quantitative sampling of tracer throughout a valley or basin atmosphere is unlikely to be feasible and a much more limited and focused approach will be required. One possibility is to use tracers for studies in an hypothesis-driven mode in which the effects of a VTMX process on tracer distributions from one or more sources is predicted for a limited volume and a sampling strategy is then designed to test that prediction. An attempt can then be made to detect a unique signature of a particular process. Thus, one might "tag" a region in an urban area and deploy samplers in the adjacent elevated terrain to identify suspected preferred transport paths. Alternatively, tracers might be released by an airplane in a layer above a surface inversion and searched for with a network of ground samplers below to infer diffusion rates through a stagnant surface layer.
Other, novel approaches may also be possible. For example, tracers of opportunity may accumulate in stable pools in low lying terrain during the night and be subsequently released the following morning. By measuring the evolution of vertical profiles of such tracers during the morning transition period, it may be possible to extract useful information on the ventilation rates and breakup mechanisms of cold pools.
It is anticipated that a major field measurement campaign will be conducted as part of this program approximately every second or third year. The initial field experiment will most likely occur during the fall (probably October) of 2000 or the winter of 2001, and will last nominally three to four weeks. Likely candidate sites include Salt Lake City and Phoenix. To avoid additional complications arising from sea breezes and other coastal effects, and to avoid duplication of studies carried out by other agencies, cities in coastal areas are not likely candidates for studies in this program even if they are situated in areas with nearby significant topographic features. A final determination of dates and location for a field campaign will be made late in the summer of 1999 when the program participants have been identified. Prior to that, preliminary site studies and discussions with potential collaborators from other agencies will be carried out under the direction of the lead scientist for the VTMX program. This information will be made available to the program participants before a final site decision is made.
The advent of remote sensors for probing the properties of the lower atmosphere has revolutionized boundary-layer meteorology studies. New generations of in situ devices for measuring turbulence that are light enough to be lifted by balloons or kites are also being developed. The development of new instruments designed to observe winds, temperatures, turbulence, radiative fluxes, and pollutants continuously in the lowest few kilometers of the atmosphere may enable us to achieve accurate measurements of many of the quantities necessary to improve descriptions of vertical transport and mixing. The integration of information from a variety of sensors such as sodars, lidars, and radar profilers can also be combined to provide information not readily extracted from a single instrument alone. Because different sensors respond to turbulent fluctuations of different variables - humidity, aerosol concentration, etc., complementary pictures of the atmospheric state can be obtained. In addition, it is possible to relate higher order turbulence moments calculated by some numerical models to quantities directly measured by some remote sensing instruments.
Even with such progress, however, standard instruments and techniques are still limited in their ability to measure quantities required for significant advances in our understanding of VTMX processes. Thus, novel approaches for interpreting remote sensing data, combining results from a variety of instrument platforms, and relating these data to quantities that can be calculated in numerical models will be areas of research encouraged in this program.
With the possible exception of investments in tracer technology, it is not anticipated that this research program will support significant efforts in instrument development per se. Such work is more properly and efficiently carried out in other programs. However, to the extent that the use of a specific instrument might provide crucial measurements for field experiments, or that these experiments might provide an opportunity to test instrument technologies developed under other programs, then modest support for such activities could provide significant returns and would be seriously considered. Alternatively, this program could provide the occasion for learning how best to deploy a given instrument or how to interpret and analyze data acquired by it. An example of this type of support was the DOE funding for the deployment of an infrared lidar, developed by NOAA, in a field experiment in the early 1980s. That occasion provided a clear demonstration of the power and versatility of the lidar in providing continuous, high resolution, remotely-sensed wind data that had been previously unattainable.
Tracers are expected to be an important tool in the study of vertical transport and mixing in field measurement programs. Especially useful advances in this program may be made if technologies for improved detection of inert tracers that may be released in a controlled manner, such as perfluorocarbons, can be developed. New or improved in situ detectors that can be carried on aircraft, balloons, or kites would allow vertical profiles of tracer concentrations to be obtained over depths of hundreds or even thousands of meters. In addition to in situ detectors, the development of an instrument capable of remote sensing of tracers to elevations of several kilometers and with a vertical resolution on the order of tens of meters would be extremely valuable; if such as instrument could be operated in a scanning mode with horizontal ranges of 10 km or more, its value would be still greater.
It is expected that a data hub will be established to archive results from field experiments. Program participants will be expected to make their data available on this hub for use by others in a timely fashion during and after the completion of each experiment. Both "raw" and reprocessed (quality controlled) data will be placed on the hub as they become available. More specific procedures will be determined to accommodate the requirements of individual instruments in specific experiments.
Parameterizations of VTMX are often based on assumptions about turbulence that are not applicable in all circumstances or on results of simulations that have been "tuned" to match a particular data set. In many cases the choice of parameter values is left to the individual investigator for subsequent modeling studies. Numerical models are particularly prone to failure as the atmosphere becomes more stable and in areas where topographic and thermal forcing are all significant. New conceptual or numerical approaches may then be required to effect significant improvements in model performance.
Full three-dimensional primitive equation atmospheric models, simpler one-dimensional boundary-layer models, and simplified conceptual models all provide useful ways of representing and understanding the dynamics of the atmosphere; Eulerian and Lagrangian dispersion models can be used to study the movements of pollutants and tracers in response to those dynamics. There is a need not only for further developments in these areas but also for more systematic testing and evaluation of the parameterizations and assumptions in these models that affect VTMX processes. Whenever possible, such testing should be based on data and not simply on model vs. model comparisons.
Substantial advances in numerical and conceptual models have also been made in recent years, spurred in large part by the ready availability of powerful workstations and the growing accessibility of parallel computing platforms. Large eddy simulations (LES) and Full Turbulence Simulations (FTS) of fluid flows enable numerical experiments to be conducted that are capable of providing new insights into VTMX processes. Even so, much remains to be done. Neither LES nor FTS methods are yet suitable for many real-world studies, but if applied carefully their results may be used to develop insights into how to model real-world problems. Results of such studies will then need to be generalized and incorporated in more widely used mesoscale codes.
Even as experimentalists should have some idea of how their data will be used, so, too, must modelers remain or become cognizant of what data are or will become available to test the performance of their models. Thus, they will be expected to assist in the design of field measurement campaigns. Modelers will also be expected to critically and openly evaluate the performance of the codes they use. Models that must be tuned to each data set to achieve acceptable agreement are of limited use in advancing our understanding and treatment of VTMX processes. It is hoped that the data collected during the life of this program will provide a basis for a rigorous and thorough testing of numerical and conceptual models of VTMX processes and that modelers participating in this and other programs will avail themselves of this opportunity.
The VTMX study will be the principal activity of the Environmental Meteorology Program in DOE's Environmental Sciences Division of the Office of Biological and Environmental Research. Three main structural elements are anticipated for the VTMX study: scientists from participating DOE National Laboratories; scientists from universities, other government laboratories, and the private sector; and an oversight group composed of the EMP Program Manager, a lead scientist chosen by the Program Manager, and a group of three to five external reviewers of the program's activities and directions.
Participating scientists from both DOE and non-DOE organizations will be chosen on the basis of peer-reviewed proposals and programmatic needs. They will be expected to participate actively in the measurement, modeling, and analysis aspects of the program. The importance of collaboration among funded investigators cannot be overemphasized. Although independent investigations are anticipated in this program, it is important to keep the programmatic scope (vertical transport and mixing), geographic focus (urban basins or valleys), and areas of emphasis (stable conditions, conditions of weak or intermittent turbulence, and morning and evening transition periods) in mind when pursuing a course of investigation. In addition, efforts will be made to encourage scientists funded by other agencies to participate in field experiments and to share data and results with researchers in this program. An annual meeting of program participants and other interested parties is anticipated, and investigators funded under this program should plan to attend. Relatively brief reports to DOE may be required annually, but it is expected that the principal method of reporting scientific findings will be through publications in the refereed scientific literature.
The lead scientist, in consultation with the DOE Program Manager, will be responsible for coordinating the activities of the individual participants, helping to establish the scientific directions of the program, leading the planning and preparation for major program activities such as field experiments, workshops, proposal solicitations, and reviews, and carrying out related research to support these activities. A panel of three to five external reviewers, i.e., individuals not receiving funding to carry out VTMX research in the EMP, will be invited to attend the annual workshops for program participants and provide feedback on the quality of the work being conducted, make suggestions for modifications in programmatic scope and direction, and advise the Program Manager and lead scientist on any other aspects of the EMP that they feel may be relevant. They may also be asked to attend more formal program reviews that will be conducted at less frequent intervals (e.g., every three years) to provide similar feedback and comments. Membership on the oversight group will be for a minimum of three years, with new members phased in over a period of time to avoid sudden large turnover.
One of the most significant advantages that this program has to offer is the long term nature of its commitment to a set of focused scientific objectives to meet a recognized national need. This is a characteristic that is often difficult to achieve in other programs, especially if they are organized about only one or two experimental campaigns or have a lifetime of only a few years. The success and impact of this program will depend to a large degree, however, on the working relationships that can be developed with other programs and the individual investigators in them Accordingly, a significant and continuing effort will be made to identify and collaborate with participants in other major studies.
Several opportunities for collaboration suggest themselves at this time. The first opportunity lies with DOE's Atmospheric Chemistry Program. There is now widespread acknowledgment of the crucial importance of meteorological processes, including VTMX processes, in trying to understand a range of problems in atmospheric chemistry. There is also considerable interest among some of the ACP scientists in forging closer links with meteorologists. These could take the form of conducting collocated field experiments, incorporation of improved treatments of VTMX processes in atmospheric chemistry models, workshops, etc. The main emphasis for VTMX program participants, of course, will be on the meteorological aspects of the problems but there appear to be numerous possibilities for joint efforts. A second, similar opportunity exists to develop close working ties with NARSTO scientists.
Additional opportunities may also arise. Some features of the research envisioned in the VTMX program relate closely to areas of research that various Prospectus Development Teams have identified as potentially important to the U.S. Weather Research Program. Previous air quality research programs have been supported by state agencies, the U.S. Environmental Protection Agency, NOAA, the National Parks Service, and other groups, and new and continuing efforts may be expected. Such studies will be carefully considered for the role they may play in helping to advance the objectives of the EMP/VTMX program and for the impact that the EMP can make in those studies. Still other candidates for possible collaboration are the Atmospheric Boundary Layer Experiment (ABLE) project and the U.S. Army Research Office (ARO). ARO has a long history of interest in stable boundary layers while ABLE is actively developing collaborations with investigators in CASES, whose current focus of interest is stable boundary-layer processes over relatively flat terrain.
In years in which major field campaigns are carried out, some modest supplementary funding may be available to cover the increased costs associated with field work. Supplementary funds might be used to offset some of the costs for transportation of equipment, site preparation and installation costs, and per diem allowances for personnel needed to operate instruments.. In exceptional circumstances, funds might be available to cover the full cost of rental or deployment of one or more specialized instruments. The amount available for supplemental funding will depend upon the resources available to the program and the level at which individual projects are funded. At this stage it is not possible to specify a particular amount but totals greater than $250K in a given year are unlikely and the actual amount may well be considerably smaller. Prospective investigators who anticipate the need for additional support of this type should request in their proposals the level of additional funding desired and describe the reasons for the request.
EMP field campaigns may also include the use of the DOE G-1 Research Aircraft Facility. This facility is normally provided without additional charges to an individual project except those required to install and operate instruments that an individual investigator might require. Costs for fuel, pilots, hangar space, etc., are not paid by a project. Note, however, that there are typically multiple requests for the use of the G-1 from several projects and its availability may be limited.
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Contact: J. Christopher Doran (509) 372-6149, e-mail: christopher.doran@pnl.gov
Last updated: October 19, 1998