Thermodynamics Review [?] Entropy & thermodynamic potentials Hydrostatic equilibrium & buoyancy Stability [dry & moist adiabatic]

Transcription

1 Thermodynamics Review [?] Entropy & thermodynamic potentials Hydrostatic equilibrium & buoyancy Stability [dry & moist adiabatic]

2 Entropy 1. (Thermodynamics) a thermodynamic quantity that changes in a reversible process by an amount equal to the heat absorbed or emitted divided by the thermodynamic temperature. It is measured in joules per kelvin [J/K]. 2. (Statistical Physics) a statistical measure of the disorder of a closed system expressed by S = k B log W + c where W is the probability that a particular state of the system exists, and c is an arbitrary constant 3. (General Physics) lack of pattern or organization; disorder 4. (Electronics & Computer Science / Communications & Information) a measure of the efficiency of a system, such as a code or language, in transmitting information

3 1st Law of Thermodynamics & Thermodynamic Identity du = d 'q! d 'w The notation dʹ denotes an inexact differential, i.e., an integral over the quantity depends on the path taken. s(u,{x i })! ds =!s $ # & "!u% {x i }!!s $ # & = 1 "!u%{x i } T! du +!s $ # & "!x i % " du = Tds!T!s % $ ' #!x i &! du = Tds +T!s $ # & " d" % u u,{x n(i } u,{x n'i } dx i dx i d" = Tds ' pd" = d 'q! d 'w (1) 1st Law Energy conservation: the change in energy of a system du must equal the heat added to/removed from the system dʹ q and the work done on/by the system dʹ w. (2) Definition of temperature [from entropy s, which is written as a function of u and a set of other variables {x i } ] Expanding s as a linear function of it s first partial derivatives and rearranging, using (2) leads to (3) the thermodynamic identity. Here, pressure p is related to the partial derivative of s w.r.t. system volume.

4 Aside: Thermodynamic Potentials Potential: generalized force F i multiplied by a generalized displacement d i : Internal energy (U): " = F i d i U = U(S,V, { N i }) = TS " pv + µ i N i Helmholtz free energy (A): A = A(T,V, { N i }) = U " TS Gibbs free energy (G): G = G(T, p, { N i }) = U " TS + pv Enthalpy (H): H = H(S, p, { N i }) = U + pv Landau or Grand Potential (Ω): " = "(T,V,{ µ i }) = U # TS # µ i N i Chemical potential (µ i ): force related to particle exchange (of species i) between systems Euler s homogeneous function theorem allows U to be written in terms of TS, pv, T,p,µ i are intensive variables: they are unchanged if 2 or more identical systems are combined. S,V,N i [and, e.g., U] are extensive, scaling with system size. Legendre transforms applied to obtain other potentials Properties of potentials can be exploited for different purposes, e.g., Gibbs is useful for processes in T-p space. Maxwell relations: equivalence of mixed partial derivatives

5 Hydrostatic equilibrium & buoyancy Newton s 2nd Law of Motion in the z direction:! F = m! a z Consider first the case with equal environmental and parcel densities, i.e., " e = " b = " ("#x#y#z) d 2 #z dt 2 = $("#x#y#z)g + p(z)#x#y $ p(z + #z)#x#y In equilibrium, and applying limit δz 0, 0 = "#g " $p $z Consider now : z+δz z y δx p(z+δz) " e # " b (" b #x#y#z) d 2 #z = $(" dt 2 b #x#y#z)g + p(z)#x#y $ p(z + #z)#x#y " d 2 #z = $g + 1 &p dt 2 % b &z = $g ' 1$ % * e ), - B Buoyancy ( + % b ρ b x p(z) δy g ρ e

6 Relating buoyancy and entropy Defining the specific volume α as " = 1 #, we can rewrite the buoyancy as: $ B = "g 1" # e & % # b ' $ ) = "g 1" * b & ( % * e ' $ ) = g * b "* e ' & ) = g +* ( % ( * * e where " # " e and "# $ # b %# e Consider α=α(s,p). Then, at constant pressure: d! = "! "s p ds = "T " p s ds α is equivalent to specific enthalpy, h. The second equality here thus follows from the Maxwell relations for mixed second partial derivatives of h. Thus, B = g!!! = g ""T % $ '! #" p & s ds = ( "T "z s ds = )ds Use the hydrostatic relationship to covert derivative to a function of z.

7 Dry adiabatic lapse rate Adiabatic parcel + ideal gas law [+hydrostatic eq] Tds = 0 = du + pd! p" = R d T For an ideal gas, the internal energy is a function of T only. [R d " 287Jkg #1 K #1 ] z leads to the dry adiabatic lapse rate γ d : % " d = # $T ( ' * & $z ) ds= 0 = g c p +10K /km [c p "1004Jkg #1 K #1 ] z 0 +δz T(z 0 +δz) T e (z 0 +δz) From the 1D force equation in the vertical [again applying the ideal gas law]: dw dt = g " e # " = g T # T e " T e T(z 0 + "z) # T e (z 0 ) $ % d "z z 0 T(z 0 ) =T e (z 0 ) T e (z 0 ) dw dt = g"z(# e $ # d ) /T e T e (z 0 + "z) # T e (z 0 ) $ % e "z

8 (Dry) Static Stability w = d"z dt # d 2 "z dt 2 & $ g % e $ % d ( ' T e ) + "z = 0 * d 2 "z + N 2 (z)"z = 0 N(z) = "g # " # e d dt 2 T e (z) N is known as the Brunt- V äisälä frequency. If γ e < γ d, N 2 (z) > 0, so solutions to the perturbation equation are oscillatory (sinusoids). In this case, the equilibrium is stable. If γ e > γ d, N 2 (z) < 0, so solutions to the perturbation equation are exponential (hyperbolic sine/cosine). In this case, the equilibrium is unstable. Height Slope: -γ Stable d Slope: Unstable Temperature -γ e

9 Let s re-evaluate radiative equilibrium Emanuel, 2005 Pure radiative equilibrium is in fact unstable for conditions in the troposphere [but is reasonable in the stratosphere]. Tropospheric γ d significantly exceeds the observed lapse rate (~6.5K/km) Need to account for vertical heat transport via convection [more shortly and later]

10 Moist atmosphere Ideal gas law for dry air (denoted d ) and water vapor (denoted v ): p d " d = R d T e" v = R v T p = p d + e # % $ R d R v = m v " 18 m d 28.9 = & ( ' The total pressure of moist (dry +vapor) air [Dalton s Law of Partial Pressures]: The density of moist air is: " = " d + " v = p $ & R d T 1# e ' ) % p( Virtual temperature (T v ): temperature to which dry air must be raised to have the same density as moist the same pressure # T v = T% 1" e "1 & ( $ p' Specific humidity (q): q = " v " = 0.622e p # 0.378e $ e p Using the definitions of q and T v, the equation of state of moist air is approximately: p " #R d T( q)

11 Saturation Consider the relative humidity (rh): rh = e e s Here, the saturation vapor pressure (e s ) is the maximum vapor pressure attainable at a given temperature The Clausius-Clapeyron equation governs the temperature dependence of e s for two-phase equilibrium: de s T = L T(" 2 #" 2 ) If 1 denotes the condensed phase (solid or liquid) and 2 the gaseous phase, α 2 >>α 1. From prior definitions and after integration: $ e s (T) "exp #0.622 L ' & ) % R d T(

12 Adiabatic process in a moist atmosphere Consider first the potential temperature for dry air, θ, which is the temperature a parcel of air would have if displaced, adiabatically and reversibly, to a reference pressure p 0 [typically, 1000 mb]: # " = T p & 0 % ( $ p ' ) ;) = R d c p [To derive: apply 1st law for an adiabatic process (ds=0), use definition of internal energy and ideal gas law, and integrate.] The (dry) entropy can be expressed in terms of θ:!s d = c p ln! d!s d = 0 " d! = 0 Note: the Δ notation indicates entropy is defined up to an arbitrary constant, i.e., Δs d =s d -constant. Now, for an adiabatic upward displacement of saturated air, condensation will occur, leading to a release of latent heat in the amount -Ldq s. The entropy change associated with this release is dδs c =(-Ldq s )/T. Equating dδs d and dδs c and integrating gives the equivalent potential temperature θ e : c p dln" = # L cdq s T % $ #d L cq s ( ' * & T ) # " e = " exp Lq & s % ( $ c p T'

13 Moist Stability Starting with the vertical acceleration and using the definition of potential temperature, it can be shown that: d 2 "z dt 2 + N 2 (z)"z = 0 Thus: "# "z > 0 Stable "# "z < 0 Unstable d 2 "z dt + % g $# ( ' *"z = 0 2 &# $z ) Equivalent potential temperature can be used to evaluate stability for a moist atmosphere; by analogy, a moist adiabatic lapse γ m rate can be defined. A region of conditional instability emerges, for γ m < γ e < γ d. For unsaturated air, this region is stable; for saturated air, it s unstable. Height Conditionally Unstable m=-γ d Absolutely Unstable m=-γ m Absolutely Stable Temperature

14 Overview of convection [more later] Convection is an important source of atmospheric heating associated with H 2 O phase changes. Vertical motion field associated with convection is an important control on the space-time behavior of H 2 O vapor, other tracers, aerosols, and clouds. Global hydrologic cycle balance implies balance between global mean precipitation and evaporation rates [~3 mm day -1 ] although significant regional [sub-global] heterogeneity exists. Evaporation [aka latent heating] is an important component of the surface energy budget.

15 Convective adjustment Eliminates atmospheric column instability Environmental conditions that favor instability: Cooling aloft via cold air advection or longwave radiative cooling Warming of surface via warm air advection or solar heating Lifting of air mass through low-level convergence