Seasonal Airmass Transport to the US

Prepared by: Rudolf B. Husar and Bret Schichtel

CAPITA ,Washington University, Saint Louis, Missouri 63130

Submitted to: Angela BandemehrMay 27, 2001

This report is also available as Word.doc (4 mb) and a Web page

Big Bend, TX

July

Big Bend, TX

January

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)

• 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 2-3 km.

• The Cordillera is a most significant obstacle to the zonal westerly and to the easterly trade winds

• 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

• 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 low-level westerlies to cross the Cordillera; the most notable is the Columbia-Snake-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

January

OctoberJuly

April

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

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

• Backtrajectories are aggregated by counting the hours each ‘particle’ resided in a grid cell.

Airmass history (Backtrajectory) Analysis

Methodology –Residence Time Probability Field

The probable airmass pathways to the Seattle, WA receptor site

• 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.

Residence Time Analysis: A 2 Dimensional Approach

Seattle, WA Particle Height Distribution

• 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

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

Seattle, WA is affected by air masses coming mainly from the west throughout the year.

6. San Francisco, CA

July

April

October

January

San Francisco, CA is affected by air masses coming mainly from the west throughout the year.

9. San Diego, CA

July

April

October

January

San Diego is affected by air masses coming mainly from the Northwest throughout the year.

10. Big Bend, TX

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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

July

April

October

January

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 inter-continental 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

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

China

Mongolia

Korea

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

On April 27, the dust cloud arrived in North America.

Regional average PM10 concentrations increased to 65 g/m3

In Washington State, PM10 concentrations briefly exceeded 100 g/m3

Reg. Avg. PM10

100 g/m3

Hourly PM10

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.

May 15, 98

• Fire locations detected by the Defense Meteorological Satellite Program (DMSP) sensor.

• The sensor detects low levels of visible night at night

Smoke from Central American Fires

• 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

Smoke Aerosol and Ozone During the Smoke Episode – Inverse Relationship

The surface ozone is generally depressed under the smoke cloud

Extinction Coefficient (visibility)

Surface Ozone

Summary

• 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

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

Pollution

Emission Rate: Ei = 1

Transmission: Tij = 1

Emission Length: Li = 1

Cj = 1 x 1 = 1

Dj = 1 x 1 x 1 = 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 

• 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.

Appendix II: Satellite-Measured 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

• Typical animations (QuickTime MOV format)– 01/01/27– 01/04/22– 01/05/15

SeaWind01/01/27

SeaWind01/04/22

SeaWind01/05/15

Atlantic

• Typical animations (QuickTime MOV format)– 01/01/28

– 01/04/22

– 01/05/22

Satellite-Measured Surface Winds

SeaWind01/01/26

SeaWind01/03/26

SeaWind01/05/22