Atmospheric Chemistry and Physics Discussions
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2007
10.5194/acpd-7-15025-2007
http://www.atmos-chem-phys-discuss.net/7/15025/2007/
http://www.atmos-chem-phys-discuss.net/7/15025/2007/acpd-7-15025-2007.html
http://www.atmos-chem-phys-discuss.net/7/15025/2007/acpd-7-15025-2007.pdf
15025
15065
2007-10-19
The daytime mixing layer observed by radiosonde, profiler, and lidar during MILAGRO
W. J. Shaw
will.shaw@pnl.gov
M. S. Pekour
R. L. Coulter
T. J. Martin
J. T. Walters
Pacific Northwest National Laboratory, Richland, Washington, USA
Argonne National Laboratory, Argonne, Illinois, USA
University of Alabama in Huntsville, Huntsville, Alabama, USA
During the MILAGRO campaign centered in the Mexico City area, Pacific
Northwest National Laboratory (PNNL) and Argonne National Laboratory (ANL)
operated atmospheric profiling systems at Veracruz and at two locations on
the Central Mexican Plateau in the region around Mexico City. These systems
included radiosondes, wind profilers, a sodar, and an aerosol backscatter
lidar. An additional wind profiler was operated by the University of Alabama
in Huntsville (UAH) at the Mexican Petroleum Institue (IMP) near the center
of Mexico City. Because of the opportunity afforded by collocation of
profilers, radiosondes, and a lidar, and because of the importance of
boundary layer depth for aerosol properties, we have carried out a comparison
of mixing layer depth as determined independently from these three types of
measurement systems during the campaign. We have then used results of this
comparison and additional measurements to develop a detailed description of
the daily structure and evolution of the boundary layer on the Central
Mexican Plateau during MILAGRO.
<br><br>
Our analysis indicates that the profilers were more consistently successful
in establishing the mixing layer depth during the daytime. The boundary
layer growth was similar at the three locations, although the mixing layer
tended to be slightly deeper in the afternoon in central Mexico City. The
sodar showed that convection began about an hour after sunrise. Maximum
daily mixing layer depths always reached 2000 m a.g.l. and frequently extended
to 4000 m. The rate and variability of mixing layer growth was essentially
the same as that observed during the IMADA-AVER campaign in the same season
in 1997. This growth did not seem to be related to whether deep convection
was reported on a given day.
<br><br>
Wind speeds within the boundary layer exhibited a daily low-altitude maximum
in the late afternoon with lighter winds aloft, consistent with previous
reports of diurnal regional circulations. Norte events, which produced high
winds at Veracruz, did not appreciably modulate the winds on the plateau.
Finally, despite the typically dry conditions at the surface, radiosonde
profiles showed that relative humidity often exceeded 50% in the early
morning and in the upper part of the boundary layer. This suggests that
aerosol particles would have experienced hygroscopic growth within the
boundary layer on many days.
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