Thermodynamic Energy Equation
The temperature tendency is
∂T u∂T v∂T w∂T dT ∂t = − ∂x − ∂y − ∂z + dt (1)
where dT/dt is the individual derivative of temperature. This temperature change experienced by the air parcel itself, dT/dt can be viewed as a "source/sink" term. The first law of thermodynamics allows us to quantify € this "source" term. The term ∂T/∂z is the environmental lapse rate, Γ.
The time derivative of the first law of thermodynamics is
dT g 1 dq = − w + (2) dt c p c p dt
where q is sensible heating or cooling. For adiabatic processes, the far right o -1 hand term is 0. The term – g/cp is 1 C 100 m and is the dry adiabatic lapse rate Γ . € Substitute (2) into (1)
g dq ∂T = −u∂T − v∂T − w∂T − w + 1 (3) ∂t ∂x ∂y ∂z c p c p dt
The definition of the adiabatic and environmental lapse rates:
€ g dT ∂T Γd = = − ,Γ = − (4) c p dz ∂z
can now be substituted into (3). This substitution gives the Temperature Tendency Equation
€ ∂T ∂T ∂T 1 dq = −u − v − w(Γd − Γ) + (5) ∂t ∂x ∂y c p dt
€
Notice that the third right hand term replaces the vertical temperature advection with a term that modifies that vertical temperature advection by adiabatic warming or cooling.
Substitution of the hydrostatic equation into (3) gives the Thermodynamic Energy Equation
% ( dq ∂T = −u∂T − v∂T +ω' dT −∂T * + 1 (6) ∂t ∂x ∂y & dp ∂p) c p dt
as is given as equation (4.3.4) in Bluestein except with the term a/cp for dT/dp. € Static Stability Parameter
The relationship between the modified vertical temperature advection in the rectangular (x,y,z) and isobaric (z, y, p) can be obtained by etting (5) equal to (6) gives:
# dT ∂T & −w(Γd − Γ) = ω% − ( (7a) $ dp ∂ p '
Remember that the relation between omega and vertical velocity is
� ≈ −���
which can be inserted into (7a) to give
$ dT ∂T ' w(Γd − Γ) ≈ −ρgw& − ) (7b) % dp ∂ p (
Rearranging terms gives the static stability parameter
2 (Γd − Γ) # dT ∂T & = % − ( = σ (8) ρg $ dp ∂ p '
For a stable atmosphere, the dry adiabatic lapse rate always exceeds the environmental lapse rate, and the static stability parameter is > 0.
By substitution of (8), Equation (6) may now be rewritten
∂T = −u∂T − v∂T +ωσ + 1 dq (9) ∂t ∂x ∂y cp dt
Brunt-Vaisala Frequency
In ERTH 260, we discussed various ways of visualizing hydrostatic stability. In that series of discussions, we derived Poisson’s Relation, which is
R "1000 % C p Θ = T $ ' (10) # p &
Taking the natural log of both sides gives