Monday 7 October 2013, Class #15

Transcription

1 Monday 7 October 2013, Class #15 Concepts for Today (Basics for Thermodynamics) Weather versus climate Lapse Rate (Adiabatic Lapse Rate) Ideal Gas Law Adiabatic Processes Potential Temperature Hydrostatic Law Scale Height 1

2 Papers for this week 1. Stevens et al. Nature Geosciences 2012, An update on Earth s energy balance in light of the latest global observations 2. Why reducing emissions may not be enough? and Climate Report puts geoengineering in the spotlight Pair of volunteers for each paper (set for 2). You will lead the discussion. You can give us all a quiz. You should look up additional material as needed. Prepare about 30 minutes with your co-volunteer. This will be a 50% of your paper presentation grade. Wednesday first 5 minutes, a local nature advocate Friday AM, Marie Thom on writing press releases 2

3 * New post on history of this diagram no contrails Black Carbon TS, IPCC 2013

4 What is the difference between weather and climate? Climate is what we expect and weather is what we get. -Lazarus Long from R. Heinlein s Time Enough for Love Climate tells you what clothes to buy, but weather tells you what clothes to wear. - Anonymous middle school student, [

5 What is Climate? Synthesis of weather in a particular region Monthly averages Climate influences how life develops in an area Life is sensitive to climate events Temperature & Precipitation History of Climate Science - S. Weart book web page Peixoto & Oort, 1992 Interconnected Components of Climate 5

6 Sea ice September Minimum NASA Goddard 6

7 Importance of Sea Ice? Ruddiman,

8 Lots of Warm Water goes into the Arctic (Atlantic Water) 8

9 Monthly Temperature Extremes for Fairbanks # years 42 JAN FEB MA R APR MAY JUN JUL AUG SEP OCT NOV DEC Alltime record MAX MIN swingleydev.com/weather 9

10 Define Climate Variability, & Climate Change Climate Variability is the variations in the mean state and other statistics (e.g. standard deviations, extremes) on all temporal and spatial scales beyond that of individual weather events. Variability may be due to - Natural internal processes (natural variability) - Variations in natural and external (anthropogenic) forcing -One year to the next (interannual variability) ex: ENSO -One decade to the next (decadal variability) ex: PDO -Multiple decades (multi-decadal variability) ex: slow ocean circulation Climate Change Definition 1:Statistically significant change in the mean or variability of the climate that persists for an extended period (decades or more) due to natural or human induced effects (chemical composition or land use). (IPCC - scientists) Definition 2: UNFCCC - Change attributed directly or indirectly to human activity which is in addition to natural variability. 10

11 Temperature Profile in Atmosphere Most recognized variable Global average surface of earth 288 K, 15 C, or 59 F Coldest -128 F in Antarctica to warmest of wmo.asu.edu (extreme) Lapse rate - temperature decrease with height Layers of Atmosphere - Troposphere - Stratosphere - Mesosphere [Neelin 2011] 11

12 Concepts 1.Ideal Gas Law 2.First Law of Thermodynamics 3.Adiabatic process 4.Potential Temperature (Poisson s law) 5.Adiabatic Lapse Rate 6. Hydrostatic Relationship 7. Scale Height 12

13 Ideal Gas Law Starting point for describing atmospheric behavior is the ideal gas law! pv = nr * T single gas, n=moles, R* universal constant p = ρrt Equivalent form used for gas mixture like air p pressure, ρ density, R gas constant, T temperature T=constant then, p goes down, then density goes down, gas expands Sample problem if there is time. 13

14 Pressure - Volume Diagram constant temperature no heat added to system 14

15 Thermodynamic Energy Equation c v DT Dt + p Dα Dt = J Term 1 Term 2 Heating rate of system due to diabatic processes cv is specific heat, T temperature, p pressure, α specific volume (1/ρ), J - diabatic heating (add heat): diabatic: radiation, conduction, latent heat release term 1: Internal energy term 2: Rate of work by the fluid system, convert thermal energy to fluid motions (solar heating moves the atmospheric air) 15

16 Adiabatic Processes c v DT Dt + p Dα Dt = 0 c v dt + pdα = 0 no heat added to system, J=0 Differential form of this equation Another form of c dt αdp = 0 p thermodynamic equation under adiabatic conditions Integrate these two to get Poisson s equations: T c v α R = cons tant T c p p R = cons tant Evolution of a state variable during an adiabatic process 16

17 Potential Temperature Potential temperature Θ is defined as that temperature the system would have if compressed or expanded adiabatically to a reference pressure of po=1000 hpa. T c p p R = cons tant T c p p R = Θ c p p ο R Θ = Τ p ο p R c p Many atmospheric motions are adiabatic since expansion work is much faster than heat transfer (less than a day vs multiple days) ==> potential temperature is conserved Air parcel displaced vertically under adiabatic conditions will experience a temperature decrease (P decrease, θ the same), adiabatic warming 17

18 Surfaces of constant Θ [Salby 2012] 18

19 Dry Adiabatic Lapse Rate Γ = dt dz = g c p = 9.8 K/km atmospheric stability 6.5 K/km is the average global lapse rate Moisture is ignored here and if there was moist adiabatic processes included then this decrease with height would be smaller. Explain 19

20 Hydrostatic Balance in Atmosphere, Pressure balances Gravity Pressure profile in the atmosphere Hartmann,

21 Hydrostatic & Scale Height g = 1 ρ dp dz Force = mass times acceleration Atmosphere at rest has downward force due to gravity at all times. Board picture, 6 replace density using ideal gas law p = ρrt dp p dz = g RT = dz H Assume isothermal T=constant then H=constant, get pressure distribution. p = p s e z H H = RT g Scale height is the height at which pressure drops off by factor 1/e. H is scale height about 7.6 km 21

22 Atmospheric Humidity: Rapidly decreases upward & poleward Humidity is amount of water vapor in air. - mixing ratio, w (g/kg), mass vapor over mass dry air (w) - specific humidity, q (g/kg), mass vapor over total mass air - relative humidity %=100*w/w s Hartmann,

23 Mean Annual Specific Humidity Peixoto & Oort,

24 Seasonal Amplitude of Specific Humidity Peixoto & Oort,

25 Relative Humidity at 850 mb % Peixoto & Oort,

26 Summary & Homework Introduction to terminology for Thermodynamics. Exposure to the basic equations Homework: Due Monday October If the atmosphere warmed up by 5 C, would the atmospheric pressure at 5 km above sea level increase or decrease and by about how much? (On board) 2. Explain why the North Polar temperature inversion is present in winter but not in summer. 3. Explain why the environmental lapse rate is less than the adiabatic one. 26