An investigation of the sub-grid variability of trace gases and aerosols for global climate modeling Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Washington, USA
Received: 30 Mar 2010 – Published in Atmos. Chem. Phys. Discuss.: 23 Apr 2010Abstract. One fundamental property and limitation of grid based models is their
inability to identify spatial details smaller than the grid cell size. While
decades of work have gone into developing sub-grid treatments for clouds and
land surface processes in climate models, the quantitative understanding of
sub-grid processes and variability for aerosols and their precursors is much
poorer. In this study, WRF-Chem is used to simulate the trace gases and
aerosols over central Mexico during the 2006 MILAGRO field campaign, with
multiple spatial resolutions and emission/terrain scenarios. Our analysis
focuses on quantifying the sub-grid variability (SGV) of trace gases and
aerosols within a typical global climate model grid cell, i.e. 75×75 km2.
Revised: 10 Jul 2010 – Accepted: 17 Jul 2010 – Published: 29 Jul 2010
Our results suggest that a simulation with 3-km horizontal grid spacing
adequately reproduces the overall transport and mixing of trace gases and
aerosols downwind of Mexico City, while 75-km horizontal grid spacing is
insufficient to represent local emission and terrain-induced flows along the
mountain ridge, subsequently affecting the transport and mixing of plumes
from nearby sources. Therefore, the coarse model grid cell average may not
correctly represent aerosol properties measured over polluted areas.
Probability density functions (PDFs) for trace gases and aerosols show that
secondary trace gases and aerosols, such as O3, sulfate, ammonium, and
nitrate, are more likely to have a relatively uniform probability
distribution (i.e. smaller SGV) over a narrow range of concentration values.
Mostly inert and long-lived trace gases and aerosols, such as CO and BC, are
more likely to have broad and skewed distributions (i.e. larger SGV) over
polluted regions. Over remote areas, all trace gases and aerosols are more
uniformly distributed compared to polluted areas. Both CO and O3 SGV
vertical profiles are nearly constant within the PBL during daytime,
indicating that trace gases are very efficiently transported and mixed
vertically by turbulence. But, simulated horizontal variability indicates
that trace gases and aerosols are not well mixed horizontally in the PBL.
During nighttime the SGV for trace gases is maximum at the surface, and
quickly decreases with height. Unlike the trace gases, the SGV of BC and
secondary aerosols reaches a maximum at the PBL top during the day. The SGV
decreases with distance away from the polluted urban area, has a more rapid
decrease for long-lived trace gases and aerosols than for secondary ones,
and is greater during daytime than nighttime.
The SGV of trace gases and aerosols is generally larger than for
meteorological quantities. Emissions can account for up to 50% of the SGV
over urban areas such as Mexico City during daytime for less-reactive trace
gases and aerosols, such as CO and BC. The impact of emission spatial
variability on SGV decays with altitude in the PBL and is insignificant in
the free troposphere. The emission variability affects SGV more
significantly during daytime (rather than nighttime) and over urban (rather
than rural or remote) areas. The terrain, through its impact on
meteorological fields such as wind and the PBL structure, affects dispersion
and transport of trace gases and aerosols and their SGV.
Citation: Qian, Y., Gustafson Jr., W. I., and Fast, J. D.: An investigation of the sub-grid variability of trace gases and aerosols for global climate modeling, Atmos. Chem. Phys., 10, 6917-6946, doi:10.5194/acp-10-6917-2010, 2010.