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Published July 22, 2016 | Published
Journal Article Open

Los Angeles megacity: a high-resolution land–atmosphere modelling system for urban CO_2 emissions


Megacities are major sources of anthropogenic fossil fuel CO_2 (FFCO_2) emissions. The spatial extents of these large urban systems cover areas of 10000 km^2 or more with complex topography and changing landscapes. We present a high-resolution land–atmosphere modelling system for urban CO_2 emissions over the Los Angeles (LA) megacity area. The Weather Research and Forecasting (WRF)-Chem model was coupled to a very high-resolution FFCO_2 emission product, Hestia-LA, to simulate atmospheric CO_2 concentrations across the LA megacity at spatial resolutions as fine as  ∼ 1 km. We evaluated multiple WRF configurations, selecting one that minimized errors in wind speed, wind direction, and boundary layer height as evaluated by its performance against meteorological data collected during the CalNex-LA campaign (May–June 2010). Our results show no significant difference between moderate-resolution (4 km) and high-resolution (1.3 km) simulations when evaluated against surface meteorological data, but the high-resolution configurations better resolved planetary boundary layer heights and vertical gradients in the horizontal mean winds. We coupled our WRF configuration with the Vulcan 2.2 (10 km resolution) and Hestia-LA (1.3 km resolution) fossil fuel CO_2 emission products to evaluate the impact of the spatial resolution of the CO_2 emission products and the meteorological transport model on the representation of spatiotemporal variability in simulated atmospheric CO_2 concentrations. We find that high spatial resolution in the fossil fuel CO_2 emissions is more important than in the atmospheric model to capture CO_2 concentration variability across the LA megacity. Finally, we present a novel approach that employs simultaneous correlations of the simulated atmospheric CO_2 fields to qualitatively evaluate the greenhouse gas measurement network over the LA megacity. Spatial correlations in the atmospheric CO_2 fields reflect the coverage of individual measurement sites when a statistically significant number of sites observe emissions from a specific source or location. We conclude that elevated atmospheric CO_2 concentrations over the LA megacity are composed of multiple fine-scale plumes rather than a single homogenous urban dome. Furthermore, we conclude that FFCO_2 emissions monitoring in the LA megacity requires FFCO_2 emissions modelling with  ∼ 1 km resolution because coarser-resolution emissions modelling tends to overestimate the observational constraints on the emissions estimates.

Additional Information

© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 15 Feb 2016 – Discussion started: 21 Mar 2016 – Revised: 22 Jun 2016 – Accepted: 04 Jul 2016 – Published: 22 Jul 2016. A portion of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The Megacities Carbon Project is sponsored in part by the National Institute of Standards and Technology (NIST). Sally Newman acknowledges funding from the Caltech/JPL President & Director's Research and Development Fund. Kevin R. Gurney thanks NIST grant 70NANB14H321. Ravan Ahmadov was supported by the US Weather Research Program within the NOAA/OAR Office of Weather and Air Quality. Seongeun Jeong and Marc L. Fischer acknowledge the support by the Laboratory Directed Research and Development Program, Office of Science, of the US Department of Energy under contract no. DE-AC02-05CH11231. Thanks to W. Angevine at NOAA for radar wind profiler data, K. Aikin at NOAA for Aircraft WP-3D data, and B. Lefer at University of Houston for ceilometer data. Edited by: R. Cohen. Reviewed by: I. Super and one anonymous referee. Author contributions. Sha Feng and Thomas Lauvaux designed the model experiments, evaluated the model performance, and developed the assessment of the measuring network; Sally Newman provided the calibrated CO_2 measurements and support for the model evaluations. Preeti Rao, Risa Patarasuk, Darragh O'Keeffe, Jianhua Huang, Yang Song, and Kevin R. Gurney developed and prepared the Vulcan and Hestia emission products; Ravan Ahmadov contributed to the development of the WRF-VPRM model and relevant guidance; Aijun Deng provided quality control for the observations from the National Weather Stations; Liza I. Díaz-Isaac tested PBL algorithms; Seongeun Jeong and Marc L. Fischer provided the background CO_2 concentration for the LA megacity (region); Riley M. Duren, Christoph Gerbig, Zhijin Li, Charles E. Miller, Stanley P. Sander, Kam W. Wong, and Yuk L. Yung provided comments and discussion on the results of the study. Data availability: The model output can be accessed by request (sfeng@psu.edu). Both the Vulcan and Hestia fossil fuel CO_2 emissions data products can be accessed by request (kevin.gurney@asu.edu). Access and information about National Weather Service data can be found at www.weather.gov. Access and information about CalNex data can be found at http://www.esrl.noaa.gov/csd/groups/csd7/measurements/2010calnex/.

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