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Seasonal Airmass Transport to the US
Big Bend, TX
Big Bend, TX
January
July
Prepared by: Rudolf B. Husar and Bret Schichtel
CAPITA ,Washington University, Saint Louis, Missouri 63130
Submitted to:
Angela Bandemehr
May 27, 2001
This report is also available as Word.doc (4 mb) and a Web page
Contents
• Introduction
• Transport Climatology of North America
• Back-Trajectory Calculations to the U.S.
– Methodology
– Seasonal back-trajectories to 15 receptors
• Transcontinental Transport Events: Dust and Smoke
• Summary
Introduction
•
Anthropogenic and natural pollutants generated in one country are transported
regularly to other countries, adding to their air quality burden.
•
On average, the intercontinental transport of pollutants represents small additions to
US pollution burdens, but under favorable emission and transport conditions it may
elevate pollutant levels for brief periods.
Goal of Work:
•
To illustrate the paths air masses take during transport to the United States
•
The current approach relies on the calculation of backward airmass histories from 15
receptor points in the US, located mostly at the boundaries.
•
The transport analyses was conducted over the entire calendar year 1999, aggregated
monthly to illustrate the seasonal pattern of transport to each location
•
This work is a spatial and temporal extension of the previous airmass history analysis
for Spring 1998.
The Climates of North America ( Based on Bryson and Hare, 1974)
•
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The dominant geographic features of N. America are the high Cordillera and the eastern Lowlands
The Cordillera, extensive system of mountain ranges stretching from Alaska , through British Columbia,
Sierra Nevada, the Coast and Cascade ranges, and the Rocky Mountains and the Sierra Madre in Mexico.
The Cordillera consists largely of a 1.5-2 km plateau with superimposed mountain ranges extruding to 23 km.
The Cordillera is a most significant obstacle to the zonal westerly and to the easterly trade winds
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East of the mountains, the plains allow
unobstructed path to great meridional
excursions: air sweeps southward from the
Arctic and northward from the tropics.
•
Cold and dry Arctic air, traveling always near
the surface, may reach central Mexico in a
few days, arriving there much colder than the
normal tropical air.
•
Warm and moist tropical air masses penetrate
northward to S. Canada, generally rising over
the cooler Arctic or Pacific air layers.
Transport Pathways
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Low level westerly winds impinging on
the Cordillera barrier are mostly
deflected, some pass through to the
Plains
At about 500 N the low-level westerly
zonal flow divides into northerly and
southerly branches along the western
slopes.
There are three main routes for the lowlevel westerlies to cross the Cordillera;
the most notable is the ColumbiaSnake-Wyoming Channel.
In Mexico, the southward deflected
westerlies usually do not cross the
Sierras.
Over the Gulf of Mexico, the low level
easterly the trade winds are usually
deflected northward.
South of the Yucatan, the trade winds
cross the continent and turn southward.
Features of Air Flow over North America
•
North America is under the influence of
Pacific, Arctic and Tropical air masses.
•
Between 300-500 the the strong westerlies
and a more broken mountain barrier allows
maximum eastward transit of Pacific air.
•
This ‘jet’ of westerly flow penetrating NAM
at mid-latitudes entrains and mixes with air
from the Arctic and the tropics.
•
This unique distribution of land, sea and
mountains produces a highly variable
weather: From one day to another, mild,
sunny air from the Rocky Mountains may
replace moist, warm, cloudy tropical air and
then give way to cold Arctic air.
Seasonal Airstream
Regions over N. America
NAVAIR,
1966
Streamlines are derived from monthly
average surface winds.
Airstream regions are separated from
each other by convergence zones
From October through January, the
strong zonal winds bring Pacific air
across the
January
July
April
October
Back-Trajectory Calculations to the U.S.
Methodology – Airmass Histories
• An airmass history is an estimate of the 3-D
transport pathway (trajectory) of an airmass prior to
arriving at a specific receptor location and arrival
time.
• Meteorological state variables, e.g. temperature and
humidity, are saved along the airmass trajectory.
• Multiple particles are used to simulate each airmass.
Horizontal and vertical mixing is included; particles
arriving at the same time to follow different
trajectories.
• Back trajectories incorporate the transport direction,
speed over source regions and dilution
The history of an airmass
arriving at Big Bend on 8/23/99
FNL Meteorological Data Archive
The FNL data is a product of the Global Data Assimilation System (GDAS), which uses
the Global spectral Medium Range Forecast model (MRF) to assimilate multiple sources
of measured data and forecast meteorology.
• 129 x 129 Polar Stereographic Grid with ~ 190 km
resolution.
• 12 vertical layers on constant pressure surfaces
from 1000 to 50 mbar
• 6 hour time increment
• Upper Air Data: 3-D winds, Temp, RH
• Surface Data includes: pressure, 10 meter winds,
2 meter Temp & RH, Momentum and heat flux
• Data is available from 1/97 to present.
Methodology: Airmass History Analysis
For details see: Springtime Airmass Transport Pathways to the US
Airmass history (Backtrajectory) Analysis
• Backtrajectories are aggregated by counting the hours each ‘particle’ resided in a grid cell.
Methodology –Residence Time Probability Field
• The grid level residence times hours are divided by the total time the airmasses reside over the entire domain and the
area of the grid cell.
• The resulting probability density function identifies the probability of an airmass traversing a given area prior to
impacting the receptor.
• The residence time probability fields are displayed as isopleth plots where the boundary of each shaded region is
along a line of constant probability.
• The units are arbitrary; colors indicate relative magnitudes of airmass residence over an area.
•The red shaded areas have the highest probability of airmass traversal and the light blue areas have the smallest
probability.
• The most probable pathways of airmass transport to the receptor are along the “ridges” of the isopleth plot.
The probable airmass pathways to the Seattle, WA
receptor site
Residence Time Analysis: A 2 Dimensional Approach
• The residence time analysis does not account for the height of the airmass, nor
does it account for removal processes.
• Air masses travelling above the planetary boundary layer cannot accumulate
surface level emissions in source regions; likewise, they cannot affect receptor
sites.
• Back trajectories tend to increase in height with increasing age
Seattle, WA Particle Height Distribution
Airmass History Database
• 15 receptor sites were placed primarily along
the United States border
• Ten day airmass histories were calculated
every two hours during all of 1999.
• 25 particles were used to simulate each
airmass trajectory
•Temperature, Relative Humidity, and Precipitation rate, were also saved out along
each trajectory.
•Airmass histories were calculated using the CAPITA Monte Carlo Model driven by the
FNL global meteorological data.
• This system was previously validated for hemispheric transport by simulating the
April 1998 Chinese Dust Event.
1. Aleutian Islands, AK
January
April
July
October
The Aleutian Islands are affected by air masses coming from all directions throughout
the year.
However, air masses affecting the Aleutian Islands appear to come preferentially from
the west.
1. Aleutian Islands, AK
January
April
July
October
The Aleutian Islands are affected by air masses coming from all directions throughout
the year.
However, air masses affecting the Aleutian Islands appear to come preferentially from
the west.
2. Point Barrow, AK
January
April
July
October
Pt. Barrow is affected strongly by air masses passing over the Arctic Ocean
throughout the year.
Transport of air masses from the southwest occurs- except during winter.
2. Point Barrow, AK
January
April
July
October
Pt. Barrow is affected strongly by air masses passing over the Arctic Ocean
throughout the year.
Transport of air masses from the southwest occurs- except during winter.
5. Seattle, WA
January
April
July
October
Seattle, WA is affected by air masses coming mainly from the west throughout the
year.
6. San Francisco, CA
January
April
July
October
San Francisco, CA is affected by air masses coming mainly from the west
throughout the year.
9. San Diego, CA
January
April
July
October
San Diego is affected by air masses coming mainly from the
Northwest throughout the year.
10. Big Bend, TX
January
April
July
October
There are large seasonal differences in the directions that air masses arriving in
Big Bend, TX have taken.
During winter and into spring, they come from the west and the northwest,while
during the summer, they come mainly from the east.
11. N. Minnesota, MN
January
April
July
October
Northern Minnesota is affected mainly by air masses coming from the north and the northwest
throughout the year.
During the summer, transport from the west and the south also occurs.
This site is close enough to Lake Superior so that their transport pathways are expected to be
similar.
12. St. Louis, MO
January
April
July
October
St. Louis, MO is affected by air masses coming from the north and northwest
throughout the year.
However, this pattern shifts so that St. Louis is more strongly affected by air
coming from the south during the warmer months.
13: Everglades, FL
January
July
April
October
Southern Florida is affected by air masses coming from the northwest during the cooler months of the year.
In contrast to the northern United States, southern Florida is strongly affected by air masses coming from
the east, especially during summer.
These air masses transport dust from North Africa to the southern United States.
14: Rochester, NY
January
July
April
October
Rochester, NY is affected by air masses coming from the north and northwest throughout the
year.
Transport from the south becomes more important during the summer.
Rochester is close enough to Lake Erie, so that their transport patterns are expected to be
relatively similar.
15: Burlington, VT
January
July
April
October
Burlington, VT is affected by air masses coming from the north and the northwest in
all seasons of the year.
During the summer, transport from the south increases in importance.
Intercontinental Transport Events: Dust
• Satellites observations provide convincing evidence for intercontinental transport of dust.
• Dust from the Gobi and Taklamakan Deserts in Asia and from North
Africa is transported routinely transported to to North America.
The Asian Dust Event of April 1998
Mongolia
China
Korea
On April 19, 1998 a major dust storm occurred over the Gobi Desert
The dust cloud was seen by SeaWiFS, TOMS, GMS, AVHRR satellites
The transport of the dust cloud was followed on-line by an an ad-hoc international
group
Trans-Pacific Dust
Transport
It took about 4 days for the
dust cloud to traverse the
Pacific Ocean, at an altitude
of about 4 km
As the dust approached N.
America, it subsided to the
ground
Asian Dust Cloud over N. America
Reg. Avg. PM10
100 mg/m3
Hourly PM10
On April 27, the dust cloud arrived in North America.
Regional average PM10 concentrations increased to 65 mg/m3
In Washington State, PM10 concentrations briefly exceeded 100
mg/m3
Smoke from Central American Fires
May 14, 98
During a ten-day period, May 7-17, 1998, smoke from numerous widespread fires in
Central America drifted northward and caused severe perturbation of the atmospheric
environment over parts of Eastern North America.
A draft paper describes the impact of the Central American smoke on the on the
atmospheric environment of Eastern North America.
Smoke from Central American Fires
• Fire locations detected by the Defense Meteorological
Satellite Program (DMSP) sensor.
• The sensor detects low levels of visible night at night
• Satellite image of color
SeaWiFS data, contours of
TOMS satellite data (green)
and surface extinction
coefficient, Bext
• The smoke plume extends
from Guatemala to Hudson
May in Canada
• The Bext values indicate
that the smoke is present at
the surface
May 15, 98
Smoke Aerosol and Ozone During the Smoke Episode – Inverse Relationship
Extinction Coefficient (visibility)
Surface Ozone
The surface ozone is generally depressed under the smoke cloud
Summary
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Air masses reaching the boundaries of the United States arrive from different
directions
However, each receptor location has a climatologically well defined seasonal
pattern of air mass history
Alaska is affected by air masses coming mainly from the west
Northern California, Oregon and Washington are affected mainly by air masses
coming from the west throughout the year.
Air masses arriving at the above locations have the highest probability of
passing over Asia during the ten days prior to arrival.
Southern California is affected by air masses coming from the west northwest.
The central United States is affected by air masses coming mainly from the
northwest during the cool months and from the south during the summer.
The southeastern United States is affected by air masses coming from the north
in the cold season and from the southeast during the warm season.
The northeastern United States is affected by air masses coming from the
Arctic, the Pacific Ocean and the Tropics
The above transport patterns are consistent with the known climatological
regimes of N America
Appendix I: Source Impact of Pollution and Dust/Smoke Events
• Two key measures of source impacts are on the
concentration and dosage at the receptor
• Both depend on the source strength as well as the
atmospheric transmission probability
• Long-term, average pollution emission rates are
relatively low (say E=1) compared to dust/smoke events
(E = 100) but they are continuous (L= 1), whereas
emissions causing the dust/smoke events are intermittent
(L=0.01)
• Dust/smoke events produce high short-term
concentration peaks at the receptor that are easily
detectable.
• Over longer periods, the effects of long-range transport
of pollutants are difficult to detect because the receptor
concentrations are low.
• However, the long-term dosage from the two types of
sources may be similar.
Example Concentration/Dosage Calculation
The impact of the emission from source i, Ei, on the
concentration at receptor j, Cj , is determined by
the transmission probability, Tij :Cj = Tij Ei
The dosage is the integral of the concentration over
the time length, Li, Dj = Li Cj
Emission Rate:
Transmission:
Emission Length:
Cj = 1 x 1 = 1
Dj = 1 x 1 x 1 = 1
Pollution
Ei = 1
Tij = 1
Li = 1
Dust or Smoke Event:
Emission Rate:
Ei = 100
Transmission:
Tij = 1
Emission Length:
Li = 0.01
Cj = 100 x 1 = 100
Dj = 100 x 1 x 0.01 = 1
Appendix II: SatelliteMeasured Surface Winds
• The surface wind over the
ocean surface is being
monitored by the SeaWind
Sensor on QuickScat.
• The surface winds are
animated at JPL to show
the surface flow pattern
Pacific
SeaWind
01/01/27
SeaWind
01/04/22
• Typical animations
(QuickTime MOV format)
– 01/01/27
– 01/04/22
– 01/05/15
SeaWind
01/05/15
Satellite-Measured Surface
Winds
SeaWind
01/01/26
Atlantic
• Typical animations
(QuickTime MOV format)
SeaWind
01/03/26
– 01/01/28
– 01/04/22
– 01/05/22
SeaWind
01/05/22