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7/29/2019 3 Meteorology and Air Quality
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 29
Meteorology and Air Quality
Processes that influence air quality
Sunlight
Horizontal dispersion
Vertical mixing Transport
Clouds and precipitation
Large scale to local scale
Global
Synoptic
Mesoscale
Urban
Neighborhood
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 30
Basic Weather
Aloft ridges and troughs Rising and sinking air
Surface highs and lows
Ridges, troughs, and temperature soundings Inversions
Stability
Mixing
Clouds and precipitation Winds
Synoptic scale
Meso- and local-scale
Transport (surface and aloft)
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 31
Aloft Ridges and Troughs (1 of 3) Mountains and valleys of warm and cool air
The height of the 500-mb pressure altitude depends
on the relative temperature of the column
500 mb
Very warm
column
Cool
column
Warm
column
Very cool
column
Surface
500 mb
RidgeRidge
Trough
Trough
Increasing
Height
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 32
Aloft Ridges and Troughs (2 of 3)
Waves (ridges and troughs) generally movewest to east
Winds generally travel faster around ridges and
slower around troughs
Areas of aloft convergence and divergence
Trough
Surface
500 mb
Wave movement
Fast wind
Ridge Slow
Fast
ConvergenceConvergence
Divergence
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 33
Aloft Ridges and Troughs (3 of 3)
Aloft divergence causes rising motion and asurface low
Aloft convergence causes sinking motion and asurface high
Surface pressure patterns are offset from aloftpatterns
Surface
500 mb
Trough
RidgeConvergence
Sinking
High
Pressure
Rising
Divergence
Low
Pressure
Sinking
Convergence
High
Pressure
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 34
Rising and Sinking Air
Sinking motion Warms the air
Creates stable conditions
Reduces vertical mixing
Creates clear skies Associated with poor air quality
Rising motion Cools the air
Creates unstable conditions Increases mixing
Causes cloud cover
Associated with good air quality
Tropopause
subsidence
top of boundary layer
updrafts
divergence
divergence
convergence
convergence
High LowStull (2000)
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Surface Highs and Lows
Relationship to Aloft Pattern (1 of 2)
500-mb heights on the afternoonof January 7, 2002 (00Z Jan 8)
Surface pressure on the afternoonof January 7, 2002 (00Z Jan 8)
5820 m
5700 m5580 m
1024 mb
1016 mb
1008 mb
Surface highRidge = Sinking =
High PM2.5 in Salt Lake City, Utah
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5400 m
5520 m
5640 m
5760 m
5280 m 1012 mb
1020 mb1012 mb
1004 mb
1012 mb
Surface lowTrough = Rising =
Low PM2.5 in Salt Lake City, Utah
500-mb heights on the afternoonof January 22, 2002 (00Z Jan 23)
Surface pressure on the afternoonof January 22, 2002 (00Z Jan 23)
Surface Highs and Lows
Relationship to Aloft Pattern (2 of 2)
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Ridges and Temperature Soundings
1200
2200
3200
4200
5200
6200
7200
8200
9200
-40 -30 -20 -10 0 10
Temperature(C)
HeightaboveSeaLevel(m)
Dry Adiabat
Ridge =
Sinking =Strong Inversion =
Poor Air Quality
TTd
Salt Lake City, Utah, temperature and dew point temperature sounding on January 7, 2002, at 0500 MST
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Troughs and Temperature SoundingsTrough =
Rising =
No Inversion =
Good Air Quality
1200
2200
3200
4200
5200
6200
-40 -30 -20 -10 0
Temperature (C)
HeightaboveSeaLevel(m
Dry Adiabat
TTd
Salt Lake City, Utah, temperature and dew point temperature sounding on January 22, 2002, at 0500 MST
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Inversions
Subsidence- Created by sinking air associated with ridges- Can limit daytime mixing depth and plays important role in
daytime pollutant concentrations
Nocturnal
Created by cooling ground at night Strongest with clear skies, light winds, and long nights
Can trap emissions, released during the overnight hours,close to the ground
Advection Created when warm air aloft moves over cooler air below
Can occur ahead of an approaching cold front
Can cause poor air quality, despite the lack of an aloft ridge
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Inversions and Mixing
Pollutants mix
into a large
volume resulting
in low pollution
levels
RL = Residual Layer
CBL = Convective Boundary Layer
NBL = Nocturnal Boundary Layer = Surface-based vertical mixing
= Surface-based mixing depth
Weak and high inversion
Sunrise Sunset
Height
Midnight
CBL
Temperature soundings
Inversion Breaks
NBLNBL
RL
Sunrise SunsetMidnight
Height
Pollutants mix
into a smaller
volume
resulting in
high pollutionlevels
Strong and low inversion
CBL RL
Inversion Holds
NBL NBL
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Mixing and Temperature Soundings
Estimating Mixing Height
Holzworth Method
Starting at the forecasted maximum temperature, follow thedry adiabat (dashed line) until it crosses the morning sounding. This is the
estimated peak mixing height for the day.
The dry adiabatic rate is the rate at which an unsaturated air parcel cools as it rises.
It is defined as -9.8C per km.
Uncertainty in mixing height estimates can be caused by changes in aloft
temperatures or errors in predicted maximum temperatures.
Forecasted
max. temp.
Dry
adiabat
Estimatedmixing height
2000 m
1000 m
1500 m
500 m
T
Estimatedmixingheight
Forecasted
max. temp.
Dry
adiabat
2000 m
1000 m
1500 m
500 m
T
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Stability Beyond Inversions
A measure of the ability of an air parcel to rise Inversions create stable conditions
Stable conditions in a temperature profile can existwithout an inversion
Forecasted
max. temp.
Estimatedmixing height
Dry adiabat
2000 m
1000 m
1500 m
500 m
T
Forecasted
max. temp.
Unlimitedmixing
Dry adiabat
2000 m
1000 m
1500 m
500 m
T
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Clouds and Precipitation (1 of 2)
Clouds form when the air becomes saturated Adding water vapor
Cooling air
Many processes add water vapor or cool air Rising motion
Trough
Daytime heating
Cold front undercutting warm air (or vice versa)
Orographic
Air in contact with cooler surface
Air moving over water
Others
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Clouds and Precipitation (2 of 2)
Clouds and fog can increase the conversionof sulfur dioxide to sulfate from 1% per hourto 50% per hour
Important in the East where one-half of PM2.5 issulfate
Clouds reduce ozone photochemistry
Precipitation removes PM10 but has littledirect impact on PM2.5
Convective clouds can vent pollution from theboundary layer under stable conditions
Inhibits heating and ability to break inversion(mid and high clouds)
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Winds
Horizontal dispersion and transport
Synoptic scale Winds are driven by large high- and low-pressure systems
Meso- and local-scale Land/sea or lake breeze
Mountain/valley
Terrain forced
Diurnal cycles
Foster stagnation and recirculation
Local flows are often difficult for weather models to predict butcan be predicted by forecasters with knowledge of the area
Surface vs. boundary layer Transport at different levels
Mixing during the day
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Winds PM2.5 Variation with Synoptic Pattern
Polar High (PH)
Pre-cold Front (Pre-CF)
HL
H
Extended High (EH)
H
Back of High (BH)
H
L
Low
PM2.5
High
PM2.5
High
PM2.5
Moderate
PM2.5
Lines of constant surface pressureLine of constant 500-mb height
General synoptic surface flow
Adapted from Comrie and Yarnal (1992)
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Winds Mesoscale (1 of 2)
Surface winds on July 18, 1991, at (a) 0600 CDT and (b) 1500 CDT. Peak ozone
concentrations on this day were about 170 ppb. (Dye et al., 1995)
Land Breeze Lake Breeze
Lake
Michigan
Lake
Michigan
WarmCool Hot Warm
500-mb heights on July 18, 1991
5880 m
Surface pattern on July 18, 1991
1016 mb
Synoptic Flow
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Winds Mesoscale (2 of 2)
With a strong inversion and the
surrounding mountains, surfacewinds are decoupled from the aloft
winds. The strong inversion and light
surface winds allow for high PM2.5concentrations.
Trapped particulate matter isvisible in the Salt Lake Valley
as viewed from the Wasatch
foothills.
Univ. of Utah
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Transport
2-km satellite image from 1235 EST on 7/7/02
Source: NASA
The 24-hr average PM2.5 concentrationin Boston on 7/7/02 was 62.7 g/m3
Backward trajectoryending at 0600 EST on 7/7/02
Source: NOAA HYSPLIT
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Sinking Motion
Warms, Dries, and Stabilizes
Creates
Temperature
Inversion
Reduces Vertical
Mixing
Surface High
Local Flows and/or Light
Winds, Possible Transport
Stagnation/Recirculation
Ridge of High Pressure
Poor Air Quality
Clear Skies
Photochemistry
Summary Meteorology Associated with Poor AQ
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Rising Motion
Cools, Moistens, and Destabilizes
No
Temperature
Inversion
Enhances Vertical
Mixing
Surface Low
Moderate to
Strong Winds
(Transport)
Horizontal Dispersion
Trough of Low Pressure
Cloudy Skies
Good Air Quality
Reduces Photochemistry
(but may enhance PM2.5 chemistry)
Summary Meteorology Associated with Good AQ
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February 2003 Short Course on Air Quality Forecasting: Meteorology and Air Quality 53
Summary Importance of Variables
Variable PM Ozone CO
Inversion Strength High High High
Surface Temp. Medium High High
Humidity High Medium Low
Aloft Pattern Medium High Medium
Cloud Cover Low High Medium
Wind Speed High Medium High
Persistence High High HighImportance of each variable varies by region and season
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Summary
Next step
Meteorological Products andExamples
Questions
Processes that influence air quality Sunlight
Horizontal dispersion
Vertical mixing Transport
Clouds and Precipitation
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