Uncertainties
A fundamental component of Urban-GEMMS is estimates of uncertainties in the emissions estimates being generated. Stakeholders need to be aware of these uncertainties in order to better interpret the results of the system. Analysis of uncertainties in each of the system elements, and the entire system, will be continuously carried out in the operational system and included in the results provided by the system. Examples of different approaches that will be used to characterize uncertainties in the system include the following:
Comparisons against observations. Meteorological modeling results will be compared against observations in the region, e.g., from ARL’s UrbanNet. The system’s overall emissions estimates will be evaluated by comparison against mobile- and aircraft-based measurements periodically carried out in the region. To this end, NOAA carries out mobile GHG measurements with a specially outfitted vehicle, known as the Air Resources Car, throughout the Washington DC-Baltimore region. Flight-based GHG measurement campaigns are carried out in the region in collaborations between NOAA, NIST, the University of Maryland, and other institutions.
Tracers of Opportunity. Uncertainties will also be characterized and quantified by carrying out so-called Tracers of Opportunity tests, in which the HYSPLIT transport and dispersion model driven by high-resolution meteorological model output is ground-truthed using sources with known emissions (e.g., power plants with measured CO2 emissions) and downwind CO2 measurements. In these tests, the ability of the model to link the known emissions with the known downwind concentrations — a fundamental source of uncertainty in the inversion system — will be evaluated and quantified.
Ensembles. Uncertainties in the system’s emissions estimates will also be characterized and quantified by carefully chosen ensembles of estimates, based on variations in inputs and simulation procedures that reflect the uncertainties in various system components. The spread of the differently calculated emissions estimates can help characterize the uncertainty in the overall inversion system.
Post-processing, publishing and distribution of results
Urban-GEMMS is currently configured to generate monthly emissions estimates, estimates for a given month are anticipated to be available within two months. In addition to overall regional emission estimates for a given month, spatially and temporally resolved estimates for each month will also be provided for the region. It is understood that uncertainties will generally increase with estimates made on smaller and smaller temporal or spatial scales. The ultimate resolution(s) provided by the system will be determined based on a consideration of stakeholder needs as well as an analysis of relative uncertainties. Once the estimates are complete for a given month, they will be provided on a public-facing website, including trends and graphical, tabular, and other summaries of the results. More detailed data products will also be available for download. The goal is to make the outputs as useful and relevant to stakeholders as possible. Focus groups and other mechanisms will be used during the development of the system — and after the system becomes operational — to ensure that this goal is met.
End-to-end system
An end-to-end system is being developed and implemented to carry out all of the operations and estimates of the system in an automated manner. When fully implemented, the end-to-end system will carry out all of the operations outlined above. Parallel computations will be employed where possible — in cases where more than one element can be carried out at a time — and serial computations will be used for elements that require the results of previous computations. Specific input and output formats will be used so that a given element can be changed if desired and the new component can be incorporated into the system, in a “plug and play” manner. Throughout the end-to-end system, numerous quality assurance/quality control checks will be performed on all elements to ensure proper operation, and to recognize if something needs to be corrected. The system will be designed to be as robust as possible so that estimates can be made reliably and sustainably moving forward.