The TOMS imagery clearly show an aerosol plume impacting the west coast of
North America beginning on March 21st.
The question is: What is its composition? The NAAPS
analyses of dust optical depth
capture the features seen in the TOMS imagery between 30 and 50N.
(The TOMS data are valid for around noon LOCAL TIME.
We have combined data from NAAPS for
00, 06, 12, and 18Z in the appropriate longitudinal ranges
to produce the anaylsis shown in the loop.)
By reversing the animation, you can follow the
plume back to the Gobi on the 16th.
A dust storm was generated in the Gobi on the
15th (see Observations and Dynamical Forcing, below).
So we could conclude that the aerosol plume is Gobi desert dust.
This case is similar to the
April 15-25, 1998 case though weaker.
However, mixed in with the dust is sulfate aerosol. This isn't evident from these
plots because the sulfate optical depth is plotted before (or underneath) the dust
optical depths.) The two aerosols are intermixed
because the cold front that generates the dust over the Gobi, sweeps across
Chinese SO2 sources. The dust optical depths are about 5X higher than the
sulfate values, so it is still most likely that the aerosol impacting N. America
in this case is desert dust.
The un-quality-assured AERONET data for San Nicolas Island show an increase in optical depth on the
24th, but the extinction is not neutral, indicating it it not dust.
A further complicating factor is the
Mystery Plume
seen in the TOMS imagery as a bright spot over
southern China and northern Viet Nam for most of this month.
NOAA/OSEI has
produced an AVHRR image showing forest fire smoke plumes along the Burma/China
border.
Local biomass sources might be suggested, but the surface observations show very few smoke
reports for the same period.
The visible GMS imagery show a layer of low cloud and there are numerous fog
reports in the area.
It is likely that the TOMS aerosol residual is amplified by the bright cloud
albedo beneath the biomass smoke.
Sources in India and the Indochina Peninsula are likely.
6-hourly NOGAPS China Loop
: 15/00Z - 15/06Z, weak wave at 500 mb (upper-left) on largely zonal flow,
amplified at 700mb (upper-right), leads to high winds near 40-45N, 100-105E
and friction velocities in excess of the threshold (red shading in lower-right).
