Module 9 Atmospheric Stability

MCEN 4131/ Preliminaries I will be gone next week, Mon-Thur Tonight is design night, 7:30ish, meet in classroom Next tues Tan and Nick will be in class to help you with your Projects - they are graduate students who took class

MCEN 4131/ Review Module 7 Educational Objectives Increased use of cars worldwide has altered the field of air pollution control The air ER is the actual air/fuel ratio divided by the stoichiometric air/fuel ratio. –For gasoline, the AFR is 14.7 fuel rich for ER < 1 –major pollutant emissions are CO, HCs fuel lean for ER > 1 –major pollutant emissions are NOx especially near ER = 1 The IC engine does not have complete combustion because of the temperature distribution within the cylinder, and the walls are cooler, quenching reactions Add-on technologies that control emissions are the catalytic converter and the carbon canister

MCEN 4131/ Learning Objectives for Today Module 8 Educational Objectives General circulation patterns –Coriolis force Stability and vertical mixing –Temperature gradient in atmosphere Lapse rate Temperature inversions

MCEN 4131/ Circulation of the Atmosphere Global circulation patterns due to –nonuniform heating of earth’s surface –Buoyancy (warm air rises) –Coriolis effect Nonuniform heating of earth’s surface –Greatest heating at equator –Air rises at equators, subsides at poles –Because of earth’s rotation, this pattern is broken up Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Wind profiles in lower atmosphere geostrophic layer –inviscid (viscous effects are negligible) –Wind profile determined by pressure gradient and coriolis effect planetary boundary layer –Effect of earth’s surface is important –Important in pollutant transport surface layer –Wind profile determined by surface drag and temperature gradient and pressure gradient Ekman layer –Wind profile determined by surface drag, pressure gradient and Coriolis m m Planetary boundary layer Ekman layer Surface layer Geostrophic layer Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Clicker Question This force results from the earth’s rotation and deflects air movement to the right in the N. hemisphere a. Friction force b. Coriolis force c. Rotational atmospheric force d. Centrifugal force

MCEN 4131/ Coriolis Forces Influences circulation in the geostrophic layer Think of wind blowing toward south in northern hemisphere –Surface velocity of earth increases toward equator –From earth, wind gains a velocity toward west Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Coriolis Cont’d Earth from above Earth from the side N N rotation Equator E E W W Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Ekman Spiral refers to winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary Happens within planetary boundary layer –Consequences: top of plumes can move in directions as much as 50 degrees from the bottom of the plume Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Clicker Question? The relationship between wind velocity and height in the atmosphere are described by which function? a. Exponential b. Logarithmic c. Power d. Linear Typically u 1 is measured at z 1 = 10 m. Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Temperature structure of the lower atmosphere Affects stability of troposphere Controls vertical air movement Disperses near-surface emissions Troposphere: T decreases with height –Warm air is less dense than cool air –Warm air under cool air results in vertical mixing Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Temperature of Atmosphere In the troposphere normally the temperature decreases as you go up in altitude Rate is on average 0.65 degrees C per 100 meters (called a lapse rate) This decrease in temperature helps to mix the air, dispersing pollutants Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Lapse Rate Consider stationary mass of air governed by pressure forces and gravity (ignore viscous effects) –Large distortable volume –Slowly exchanges heat and mass with surroundings –Pressure equilibrates rapidly –no energy is added or removed –Hydrostatics: -(dP/dz) = (MW a g/RT)P –Solve for dT/dz Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Adiabatic lapse rate Rate at which temperature of dry air changes with height in the atmosphere due to adiabatic expansion or compression Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Group clicker question If the lapse rate is equal to the dry adiabatic lapse rate, the stability condition is: a. Unstable b. Neutral c. Stable And what if the lapse rate is less than  d ? Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Atmospheric Stability Stable –buoyancy returns a parcel of air to its original position after it has been displaced upward or downward –Atmospheric lapse rate < adiabatic lapse rate –Atmosphere cools less rapidly with height than parcel –Vertical mixing suppressed Unstable –buoyancy increases the displacement of the parcel of air that has moved upward or downward –adiabatic lapse rate < atmospheric –Atmosphere cools more rapidly with height than parcel –Vertical mixing is promoted Neutral –the lapse rate is equal to the dry adiabatic lapse rate, parcel of air stays where it has been displaced –Adiabatic = atmospheric lapse rate Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Pasquill stability class Would like to predict atmospheric lapse rate from readily observable properties Pasquill (1961) introduced notion of stability class Based on 3 characteristics –Intensity of solar radiation –Near-surface wind speed –Extent of nighttime cloud cover Relationship of stability class to lapse rate Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Stability Classes Stability classLapse rate (C/100 m) A (extremely unstable)< -1.9 B (moderately unstable)-1.9 to -1.7 C (slightly unstable)-1.7 to -1.5 D (neutral)-1.5 to -0.5 E (slightly stable)-0.5 to 1.5 F (moderately stable)> 1.5 Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Temperature Inversions When there is cold air near the ground, and a layer of warmer air above Temperature profile as a function of height Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions

MCEN 4131/ When there is cold air near the ground, and a layer of warmer air above Clicker Question? Which of the following Inversions plays the most important role in cause smog problems? a.Subsidence b.Frontal c.Radiation And what about for wood-burning in the winter? Learning Objectives Circulation patterns Vertical mixing Lapse rate Temperature inversions