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 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: 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 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. 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 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 Thinly overcast or 4/8 low cloud
Source: Met Monitoring Guide – Table 6.3 SIGMA THETA STABILITY CLASSIFICATION Source: Atmospheric Stability – Methods & Measurements (NUMUG - Oct 2003) TEMPERATURE DIFFERENCE (∆T) 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 CONDITIONS OF TOLEDO Weather Station 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 National Weather Map National Air Quality Climate NATIONAL WEATHER MAP H – High Pressure Area L – Low Pressure Area 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 TBL USING ANALYTICAL SOLUTION EFFECTS OF METEOROLOGY ON PLUME DISPERSION 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. BEAUFORT SCALE This scale is helpful in getting an idea on the magnitude of wind speed from real life observations Leaves in constant motion
WIND ROSE DIAGRAM (WRD) 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 DETERMINATION OF MAXIMUM MIXING HEIGHT POWER PLANT PLUMES IN MICHIGAN POWER PLANT PLUMES IN MICHIGAN POWER PLANT PLUMES IN MICHIGAN Photo credit: Kimberly M. Coburn PROBLEMS 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: 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: http://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/active/01-023/01-023r1.pdf NOAA-National Climate Data Center
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