Lecture 7. Science A-30 February 21, 2008 Air may be forced to move up or down in the atmosphere by mechanical forces (wind blowing over an obstacle,

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1 Lecture 7. Science A-30 February 21, 2008 Air may be forced to move up or down in the atmosphere by mechanical forces (wind blowing over an obstacle, like a mountain) or by buoyancy forces. Air that is forced to move up does work on the atmosphere. If no heat is added or removed, the work that is done comes exclusively from thermal energy of the molecules in the air. Air moving up "adiabatically" cools at a rate of 9.8 K/km. Air that is forced to move down has work done on it by the atmosphere. If no heat is added or removed, the work that is done goes exclusively into thermal energy of the molecules in the air which warms at a rate of 9.8 K/km. If the change in atmospheric temperature with altitude is less negative than 9.8 K/km, the atmosphere is stable because an air parcel that starts to move vertically will be pushed by buoyancy forces back to where it started. { The parcel will be colder [more dense] than the environment [the surrounding atmosphere] above where the parcel started, and warmer below where it started. Heating of the ground be the sun in the day time makes air parcels buoyant. They rise until their temperature is the same as the atmosphere. The "atmospheric mixed layer" ["Planetary Boundary Layer"] develops in the lowest 1-3 km in a few hours. As parcels cool when they rise in the atmosphere, water vapor may condense if the parcel has sufficient water content. Condensation adds heat to an air parcel, tending to make the lapse rate less negative than 9.8K/km, and the parcel is less dense/more buoyant than a dry parcel.

2 Critical reading of the scientific literature Function of an abstract it is like an executive summary (not an intro, or a simple summary). It must make the important points and give the important results. May readers may only view the abstract and in fact that may be all that they get! What we need to be able to find out when we read an abstract, or a scientific paper: The authors were trying to: answer what questions or test what hypotheses? Example: is ice cover in the Arctic Ocean changing? What did they actually do? Example: they obtained satellite and submarine data for ice cover in the Arctic Ocean from an archive, and analyzed the data statistically. What were their major results? Example: ice cover in the Arctic Ocean has gotten thinner, and the extent coverage in early fall has declined, in the last 50 years. What conclusions did they draw from these results? Example: the reduction in ice cover shows that the North polar climate has been warming rapidly for the last ~50 years; this will likely amplify future warming, and maybe bring a wetter climate near the Arctic Ocean.

3 Z (km) Pressure and work on a vertically-displaced air parcel P2,Z2 P1,Z2 P1,Z1 P2,Z2 The parcel does work on the atm expanding from P1->P2 at Z2 P1,Z2 P1,Z1 Altitude (km) mesosphere stratosphere troposphere P (bar) T (K) The change in temperature with altitude in the atmosphere. The example is from 30 degrees north latitude in summer.

4 -mc p T = P V (basic energy balance) V P = - P V (Boyle s law) =>> - mc p T = - V P P = - g ρ Z (Barometric law) =>>- mc p T = (Vρ) g Z ρv = m = mass of parcel We see that for an air parcel moving vertically in a hydrostatic atmosphere (barometric law applies), - c p T = g Z T / z = -g/c p = o K/km This change in temperature with altitude is called the "adiabatic lapse rate". c p = 1005 J/kg/K; g = 9.8 m s -2 =>> - g / c p = 9.8 x 10-3 K/m or 9.8 K/km.

5 The concept of atmospheric stability The atmosphere is stable if cannot move up or down spontaneously. A parcel that is displaced a little distance vertically tends to move back to where it started. The atmosphere is unstable if tends to move up or down spontaneously. A parcel that is displaced a little distance vertically tends to accelerate in the direction is was moved. The atmosphere is neutral ("neutrally stable") if it neither tends to move back to where it started, nor does it accelerate. Forces are balanced on both of these spheres. But one is stable, and the other unstable. The environment determines if the sphere (or an atmospheric air parcel) is stable, neutral, or unstable. The sphere is neutrally stable on a flat surface.

6 Altitude (km) B unstable adibatic A stable Ambient lapse rates and parcel temperature changes. Schematic diagram showing stable (Curve A) and unstable (Curve B) temperature profiles for the ambient atmosphere, compared to the lapse rate followed by a parcel subjected to adiabatic vertical displacement T (Kelvin)

7 Heating of the ground and dry convection : Diagram of the lapse rate just before sunrise (stable) and in mid-afternoon (neutral or slightly unstable). Dry convection driven by solar heating stirs the lower atmosphere, creating the planetary boundary layer or mixed layer during daytime over land.

8 Atmospheric temperature and dewpoint for a typical summer day shows the "planetary boundary layer" or "atmospheric mixed layer", that develops as the sun heats the ground in the daytime. This graph is drawn from actual data obtained by Harvard's Forest and Atmosphere Studies group during an experiment (code name "COBRA") over North Dakota in August, 2000.

9 What you see Puffy little clouds, called fair weather cumulus, occurring over land on a typical afternoon. The lapse rate in the mixed layer is approximately adiabatic, and air parcels heated near the ground are buoyant. Each little cloud represents the top of a buoyant plume. (Photograph courtesy University of Illinois Cloud Catalog).

10 Moist pseudo-adiabatic lapse rate Air is heated by release of latent heat when water condenses: T will decline less rapidly than the dry adiabat Temperature (C) Pressure (Mb) Ambient T (->35) km km Dry Adb Γ= -g/(c p + λ w/ T ) λ= latent heat of vaporization (J/kg); w/ T=change in spec humidity/k

11 Vapor Pressure of Water P sat = A exp [B( 1/ /T)] A=6.11 mbar, B= 5308K. A=water vapor pressure at 0C. The pressure of H 2 O vapor in equilibrium with liquid water. Clausius- Clapeyron relation. Water vapor pressure versus T.

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14 Convective cloud over Amazonia 3 Z km 2 latent heat release 1 0 T dew Temperature K cloud base T dew = T air [Photo: S. Wofsy, Manaus, Brazil, 1987.]

15 DIURNAL CYCLE OF SURFACE HEATING/COOLING: ventilation of urban pollution z Subsidence inversion MIDDAY 1 km Γ Mixing depth NIGHT 0 MORNING T NIGHT MORNING AFTERNOON

16 SUBSIDENCE INVERSION typically 2 km altitude

17 Lecture 7. Science A-30 February 21, 2008 Air may be forced to move up or down in the atmosphere by mechanical forces (wind blowing over an obstacle, like a mountain) or by buoyancy forces. Air that is forced to move up does work on the atmosphere. If no heat is added or removed, the work that is done comes exclusively from thermal energy of the molecules in the air. Air moving up "adiabatically" cools at a rate of 9.8 K/km. Air that is forced to move down has work done on it by the atmosphere. If no heat is added or removed, the work that is done goes exclusively into thermal energy of the molecules in the air which warms at a rate of 9.8 K/km. If the change in atmospheric temperature with altitude is less negative than 9.8 K/km, the atmosphere is stable because an air parcel that starts to move vertically will be pushed by buoyancy forces back to where it started. { The parcel will be colder [more dense] than the environment [the surrounding atmosphere] above where the parcel started, and warmer below where it started. Heating of the ground be the sun in the day time makes air parcels buoyant. They rise until their temperature is the same as the atmosphere. The "atmospheric mixed layer" ["Planetary Boundary Layer"] develops in the lowest 1-3 km in a few hours. As parcels cool when they rise in the atmosphere, water vapor may condense if the parcel has sufficient water content. Condensation adds heat to an air parcel, tending to make the lapse rate less negative than 9.8K/km, and the parcel is less dense/more buoyant than a dry parcel.