# Basic meteorological processes

 Date conversion 13.03.2017 Size 26,54 Kb.

## Objectives

• What is atmospheric thermodynamics?
• What are the variables of atmospheric thermodynamics?
• What is lapse rate?
• Explain the potential temperature.
• What is atmospheric stability and the various methods that define atmospheric stability?
• What is boundary layer development?
• What are the effects of meteorology on plume dispersion?
• What is wind velocity profile?
• What is wind rose diagram and what are the uses of it?
• Determination of mixing height.

## AIR POLLUTION METEOROLOGY

• Atmospheric thermodynamics
• Atmospheric stability
• Boundary layer development
• Effect of meteorology on plume dispersion

## ATMOSPHERE

• Pollution cloud is interpreted by the chemical composition and physical characteristics of the atmosphere
• Concentration of gases in the atmosphere varies from trace levels to very high levels
• Nitrogen and oxygen are the main constituents. Some constituents such as water vapor vary in space and time.
• Four major layers of earth’s atmosphere are:
• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere

## ATMOSPHERIC THERMODYNAMICS

• A parcel of air is defined using the state variables
• Three important state variables are density, pressure and temperature
• The units and dimensions for the state variables are
 Density (mass/volume) gm/cm3 ML-3 Pressure (Force/Area) N/m2 ( Pa ) ML-1T-2 Temperature o F, o R, o C, o K T
• Humidity is the fourth important variable that gives the amount of water vapor present in a sample of moist air

## EQUATION OF STATE

• Relationship between the three state variables may be written as:
• f ( P, ρ ,T) = 0
• For a perfect gas:
• P = ρ .R .T
• R is Specific gas constant
• R for dry air = 0.287 Joules / gm /oK
• R for water vapor = 0.461 Joules / gm /oK
• R for wet air is not constant and depend on mixing ratio

## Exercise

• Calculate the density of a gas with a molecular weight of 29 @ 1 atm (absolute) and 80 oF. Gas constant, R = 0.7302 ft3atm/lb-moleoR.

## Solution

• Absolute Temperature = 80 oF + 460 = 540 oR
• Density = P ( molecular weight) / RT
• Density = ( 1atm. )*(29 lb/lb mole) / ( 0.7302 ft3atm/lb-moleoR)*(540 oR)
• Density = 0.073546 lb/ ft3.

## Exercise

• Determine the pressure, both absolute and gauge, exerted at the bottom of the column of liquid 1 meter high, with density of 1000 kg / m3.

## Solution

• Step 1 :
• Pgauge = (density of liquid) * ( acceleration due to gravity) *(height of liquid column)
• Step 2 : Pabsolute = Pgauge + Patmospheric
• Pabsolute = 111.11 kPa

## LAWS OF THERMODYNAMICS

• First Law of Thermodynamics:
• This law is based on law of conservation of total energy.
• Heat added per unit mass = (Change in internal energy per unit mass) + (Work done by a unit mass)
• δH = δU+δW
• Second Law of Thermodynamics:
• This law can be stated as "no cyclic process exists having the transference of heat from a colder to hotter body as its sole effect"

## SPECIFIC HEAT

• Defined as the amount of heat needed to change the temperature of unit mass by 1oK.
• Cv = lim δQ
• δT→0 δT α = const
• Specific heat at constant pressure
• Cp = lim δQ
• δT→0 δT p = const
• Relationship between Cv and Cp is given by Carnot’s law:
• For perfect gas, Cp – Cv = R
• For dry air Cp = (7/2)*R (Perfect diatomic gas)
• Cv = (5/2)*R (Perfect diatomic gas)
• Ratio of Cp and Cv for dry air is 1.4
• Cpd = 1.003 joules/gm/o K ; Cvd = 0.717 joules/gm/o K

## PROCESSES IN THE ATMOSPHERE

• An air parcel follows several different paths when it moves from one point to another point in the atmosphere. These are:
• Isobaric change – constant pressure
• Isosteric change – constant volume
• Isothermal change – constant temperature
• Isentropic change – constant entropy (E)
• Adiabatic Process – δQ = 0 (no heat is added or
• removed )
• The adiabatic law is P. αγ = constant
• E =

## STATICS OF THE ATMOSPHERE

• Vertical variation of the parameters = ?
• Hydrostatic Equation:
• Pressure variation in a "motionless" atmosphere
• Pressure variation in an atmosphere:
• Relationship between pressure and elevation using gas law:

## STATICS OF THE ATMOSPHERE

• Integration of the above equation gives
• Using the initial condition Z=0, P = P0
• The above equation indicates that the variation of pressure depends on vertical profile of temperature.
• For iso-thermal atmosphere
• Therefore, pressure decreases exponentially with height at a ratio of 12.24 mb per 100m.

## Lapse Rate:

• Lapse Rate:
• Lapse rate is the rate of change of temperature with height
• Lapse rate is defined as Γ = -δT
• δz
• Value of  Γ varies throughout the atmosphere
• Potential Temperature:
• Concept of potential temperature is useful in comparing two air parcels at same temperatures and different pressures.

• θ

## ATMOSPHERE STABILITY

• The ability of the atmosphere to enhance or to resist atmospheric motions
• Influences the vertical movement of air.
• If the air parcels tend to sink back to their initial level after the lifting exerted on them stops, the atmosphere is stable.
• If the air parcels tend to rise vertically on their own, even when the lifting exerted on them stops, the atmosphere is unstable.
• If the air parcels tend to remain where they are after lifting stops, the atmosphere is neutral.

## ATMOSPHERIC STABILITY

• The stability depends on the ratio of suppression to generation of turbulence
• The stability at any given time will depend upon static stability ( related to change in temperature with height ), thermal turbulence ( caused by solar heating ), and mechanical turbulence (a function of wind speed and surface roughness).

## ATMOSPHERIC STABILITY

• Atmospheric stability can be determined using adiabatic lapse rate.
 Γ > Γd Unstable Γ = Γd Neutral Γ < Γd Stable
• Γ is environmental lapse rate
• Γd is dry adiabatic lapse rate (10c/100m) and dT/dZ = -10c /100 m

## ATMOSPHERIC STABILITY CLASSIFICATION

• Schemes to define atmospheric stability are:
• P- G Method
• P-G / NWS Method
• The STAR Method
• BNL Scheme
• Sigma Phi Method
• Sigma Omega Method
• Modified Sigma Theta Method
• NRC Temperature Difference Method
• Wind Speed ratio (UR) Method
• Radiation Index Method
• AERMOD Method (Stable and Convective cases)

## PASQUILL-GIFFORD STABILITY CATEGORIES

 Surface Wind Speed (m/s) Daytime Insolation Nighttime cloud cover Strong Moderate Slight Thinly overcast or 4/8 low cloud 3/8 < 2 A A - B B - - 2 - 3 A - B B C E F 3 - 5 B B - C C D E 5 - 6 C C - D D D D > 6 C D D D D
• Source: Met Monitoring Guide – Table 6.3

## SIGMA THETA STABILITY CLASSIFICATION

 CATEGORY PASQUILL CLASS SIGMA THETA (ST) EXTREME UNSTABLE A ST>=22.5 MODERATE UNSTABLE B 22.5>ST>=17.5 SLIGHTLY UNSTABLE C 17.5>ST>=12.5 NEUTRAL D 12.5>ST>=7.5 SLIGHTLY STABLE E 7.5>ST>= 3.8 MODERATE STABLE F 3.8>ST>=2.1 EXTREMELY STABLE G 2.1>ST
• Source: Atmospheric Stability – Methods & Measurements (NUMUG - Oct 2003)

## TEMPERATURE DIFFERENCE (∆T)

• Source: Regulatory guide; office of nuclear regulatory research- Table 1

## TURBULENCE

• Fluctuations in wind flow which have a frequency of more than 2 cycles/ hr
• Types of Turbulence
• Mechanical Turbulence
• Convective Turbulence
• Clear Air Turbulence
• Wake Turbulence

## Weather Station

• Home, Professional, and Live

## Weather Balloon

• Pressure, Temperature, Wind Speed, Wind Direction, & Humidity

## Use of Towers

• Velocity, Temperature, & Turbulence

## LOCAL CLIMATOLOGICAL DATA - TOLEDO

• Greatest snowfall – 73.1” (1997-1998)
• Least snowfall – 6.0” (1889-1890)
• Average number of days with a tenth of an inch or more snowfall – 27 days
 Annual 38.3” December 9.1” January 9.8” February 8.0” March 6.3”
• Snowfall
 Annual 49.6°F January 25.7°F July 73.2°F
• Temperature
 Annual 31.62” January 2.18” June 3.45”
• Precipitation

• US Forecast

• Ozone

• Temperature

## NATIONAL WEATHER MAP

• A high pressure area forecasts clear skies.
• A low pressure area forecasts cloudiness and precipitation

## BOUNDARY LAYER DEVELOPMENT

• Thermal boundary Layer (TBL) development depends on two factors:
• Convectively produced turbulence
• Mechanically produced turbulence
• Development of TBL can be predicted by two distinct approaches:
• Theoretical approach
• Experimental studies

## BOUNDARY LAYER DEVELOPMENT

• Theoretical approach may be classified into three groups:
• Empirical formulae
• Analytical solutions
• Numerical models
• One layer models
• Higher order closure models

• Time
• Time
• Time
• Time

## EFFECTS OF METEOROLOGY ON PLUME DISPERSION

• Dispersion of emission into atmosphere depends on various meteorological factors.
• Height of thermal boundary layer is one of the important factors responsible for high ground level concentrations
• At 9 AM pollutants are pulled to the ground by convective eddies
• Spread of plume is restricted in vertical due to thermal boundary height at this time

## WIND VELOCITY

• A power law profile is used to describe the variation of wind speed with height in the surface boundary layer
• U = U1 (Z/Z1)p
• Where,
• U1 is the velocity at Z1 (usually 10 m)
• U is the velocity at height Z.
• The values of p are given in the following table.
 Stability Class Rural p Urban p Very Unstable 0.07 0.15 Neutral 0.15 0.25 Very Stable 0.55 0.30

## BEAUFORT SCALE

• This scale is helpful in getting an idea on the magnitude of wind speed from real life observations
 Atmospheric condition Wind speed Comments Calm < 1mph Smoke rises vertically Light breeze 5 mph Wind felt on face Gentle breeze 10 mph Leaves in constant motion Strong 25 mph Large branches in motion Violent storm 60 mph Wide spread damage

## WIND ROSE DIAGRAM (WRD)

• Wind Direction (%)
• Wind Speed (mph)

## WIND ROSE DIAGRAM (WRD)

• WRD provides the graphical summary of the frequency distribution of wind direction and wind speed over a period of time
• Steps to develop a wind rose diagram from hourly observations are:
• Analysis for wind direction
• Determination of frequency of wind in a given wind direction
• Analysis for mean wind speed
• Preparation of polar diagram

## Calculations for Wind Rose

• % Frequency =
• Number of observations * 100/Total Number of Observations
• Direction: N, NNE, ------------------------,NNW, Calm
• Wind speed: Calm, 1-3, 4-6, 7-10, -----------

## DETERMINATION OF MAXIMUM MIXING HEIGHT

• Steps to determine the maximum mixing height for a day are:
• Plot the temperature profile, if needed
• Plot the maximum surface temperature for the day on the graph for morning temperature profile
• Draw dry adiabatic line from a point of maximum surface temperature to a point where it intersects the morning temperature profile
• Read the corresponding height above ground at the point of intersection obtained. This is the maximum mixing height for the day

## POWER PLANT PLUMES IN MICHIGAN

• Monroe Power Plant

## POWER PLANT PLUMES IN MICHIGAN

• Trenton Channel

## POWER PLANT PLUMES IN MICHIGAN

• River Rouge Power Plant
• Photo credit:  Kimberly M. Coburn

## During an air pollution experiment the lapse rate was a constant at 1.1 °C per 100 m. If the atmosphere is assumed to behave as a perfect gas and the sea level temperature and pressure were 16 °C and 1 atm, at what altitude was the pressure one-third the sea level?

• During an air pollution experiment the lapse rate was a constant at 1.1 °C per 100 m. If the atmosphere is assumed to behave as a perfect gas and the sea level temperature and pressure were 16 °C and 1 atm, at what altitude was the pressure one-third the sea level?

## SOLUTION

• Step1:
• Step 2:
• Calculate Temperature
• Step 3:
• Substitute for temperature
• Step 4:
• Integrate between P = 1 and P = 0.333, and between z = 0, and z = z.
• Z = 7817.13m

## REFERENCES

• Met Monitoring Guide: http://www.webmet.com/met_monitoring/toc.html
• Regulatory Guide – office of nuclear regulatory research: