Atmospheric_Thermodynamics_III

Atmospheric_Thermodynamics_III - Atmospheric Thermodynamics...

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Atmospheric Thermodynamics – III Adiabatic Processes Leila M. V. Carvalho Dept. Geography, UCSB
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Review We learned that there are processes that occur at constant pressure, others at constant volume (no work done or received) We also learned that in other situations volume and pressure may change We defined the concept of Enthalpy (sensible heat) h=CpdT, which is the sensible heat transferred at constant pressure.
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Consider a dry atmosphere (no clouds) Pressure Heat Parcel move and can change pressure as it rises or sinks. A few physical concepts to remember: dp d α - = Φ dp dh dq - = dT c dh p = ) ( ) ( Φ + = Φ + = T c d h d dq p IF dq=0 (no heat gain or lost ) Variation Geopotential First Law of Thermodyn. Enthalpy constant T c p = Φ + ) ( Dry Static Energy: Conservative
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Adiabatic Processes If a material undergoes a change in its physical state (e.g., pressure, volume, or temperature) without any heat being added to it or withdrawn from it, the change is said to be ADIABATIC dq=0
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Suppose an Isothermal contraction F=pA State A State B Every state of the gas between A and B will be represented by the red line above connecting A to B
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Example of Adiabatic Compression State C State A During the adiabatic compression, the internal energy increases because: dq = du + pdα and dq=0 , so du= - pdα Compression: < 0 du > 0 In conclusion, Temperature should rise: TC> TB and pC> pB
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Examples where Adiabatic Compression takes place n the center of High pressure systems (low levels)
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Local scale: Santana winds Sundowner winds Adiabatic compression TC TA
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Concept of an Air parcel Molecular mixing is important only within a centimeter of the surface At intermediary levels (up to ~ 105km) all mixing in the vertical is accomplished by the exchange of macroscale “air parcels” with horizontal dimensions ranging from millimeters to the scale of the Earth itself
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Infinitesimal parcels Thermally insulated from its environment (temperature changes adiabatically as it rises or sinks. It is always in hydrostatic Equilibrium Moves slowly enough that the macroscopic kinetic energy of the air parcel is a negligible dq=0 g -(1/ρ)dp/dz
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Dry adiabatic Lapse rate Is the rate of change of temperature with height of a parcel of dry air satisfying the conditions described in the previous slide T=9 oC T=8 oC T=7 oC ) ( ) ( Φ + = Φ + = T c d h d dq p ) ( 0 Φ + = = T c d dq p Dividing through by dz and using (20) dT c dp gdz d p - = - = Φ α 1 1 1 1 2 8 . 9 0098 . 0 1004 81 . 9 - - - - - = = = Γ = = - km K m K kg K J s m c g dz dT d p Remember: variations in oC or K are equivalent
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Important Note 7oC 8oC 9oC T=9 oC T=8 oC T=7 oC Γd is the rate of change of temperature following a parcel of dry air that is being raised (or lowered) adiabatically in the atmosphere. The environmental lapse rate (the one that is measure by a
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This note was uploaded on 12/28/2011 for the course GEOG 226 taught by Professor Leila during the Fall '09 term at UCSB.

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Atmospheric_Thermodynamics_III - Atmospheric Thermodynamics...

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