3 Meteorology and Air Quality

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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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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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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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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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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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



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



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



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



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



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



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



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



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)



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



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)



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



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



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



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



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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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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3 Meteorology and Air Quality