Omega equation

The omega equation is of great importance in meteorology and atmospheric physics. It is a partial differential equation for the vertical velocity, ω, which is defined as a Lagrangian rate of change of pressure with time, that is, $\omega = \frac{dp}{dt}$.
The equation reads:

$\sigma\nabla^2_H\omega + f^2\frac{\partial^2\omega}{\partial p^2} = f \frac{\partial}{\partial p} \left[ \mathbf{V}_g \cdot \nabla_H (\zeta_g + f) \right] - \nabla^2_H \left( \mathbf{V}_g\cdot\nabla_H \frac{\partial \phi}{\partial p}\right)$

(1)

where f is the Coriolis parameter, σ is the static stability, $\mathbf{V}_g$ is the geostrophic velocity vector, ζ_g is the geostrophic relative vorticity, φ is the geopotential, $\nabla^2_H$ is the horizontal Laplacian operator and $\nabla_H$ is the horizontal del operator ^[1].

Derivation

The derivation of the ω equation is based on the vorticity equation and the thermodynamic equation. The vorticity equation for a frictionless atmosphere may be written as:

$\frac{\partial \xi}{\partial t} + V \cdot \nabla\eta - f \frac{\partial \omega}{\partial p} = \left( \xi \frac{\partial \omega}{\partial p} - \omega \frac{\partial \xi}{\partial p} \right) + k \cdot \nabla\omega \times \frac{\partial V}{\partial p}$

(2)

Here ξ is the relative vorticity, V the horizontal wind velocity vector, whose components in the x and y directions are u and v respectively, η the absolute vorticity, f the Coriolis parameter, $\omega = \frac{dp}{dt}$ the individual rate of change of pressure p. k is the unit vertical vector, $\nabla$ is the isobaric Del (grad) operator, $\left( \xi \frac{\partial \omega}{\partial p} - \omega \frac{\partial \xi}{\partial p} \right)$ is the vertical advection of vorticity and $k \cdot \nabla\omega \times \frac{\partial V}{\partial p}$ represents the transformation of horizontal vorticity into vertical vorticity ^[2].

The thermodynamic equation may be written as:

$\frac{\partial}{\partial t} \left( - \frac{\partial Z}{\partial p} \right) + V \cdot \nabla \left( - \frac{\partial Z}{\partial p} \right) - k\omega = \frac{R}{C_p \cdot g} \cdot \frac{q}{p}$

(3)

where $k \equiv \left( \frac{\partial Z}{\partial p}\right) \frac{\partial}{\partial p} \ln\theta$, in which q is the supply of heat per unit-time and mass, C_pthe specific heat of dry air, R the gas constant for dry air, θ is the potential temperature and ϕ is geopotential (gZ).

The ω equation (1) is then obtained from equation (2) and (3) by substituting values:

$\xi = \frac{g}{f}\nabla^2 Z$

and

$\hat k \cdot \nabla\omega \times \frac{\partial V}{\partial p} = \frac{\partial \omega}{\partial y}\frac{\partial u}{\partial p} - \frac{\partial \omega}{\partial x}\frac{\partial v}{\partial p}$

into (2), which gives:

$\frac{\partial}{\partial t}\left(\frac{g}{f}\nabla^2 Z \right) + V \cdot \nabla\eta - f \frac{\partial \omega}{\partial p} = \left(\xi \frac{\partial \omega}{\partial p } - \omega \frac{\partial \xi}{\partial p} \right) + \left(\frac{\partial \omega}{\partial x}\frac{\partial v}{\partial p}\right)$

(4)

Differentiating (4) with respect to p gives:

$\frac{g}{f}\frac{\partial}{\partial t} \nabla^2 \left(\frac{\partial Z}{\partial p} \right) + \frac{\partial}{\partial p} (V \cdot \nabla\eta) - f \frac{\partial^2 \omega}{\partial p^2} - \frac{\partial f}{\partial p}\frac{\partial \omega}{\partial p} = \frac{\partial}{\partial p}\left(\xi \frac{\partial \omega}{\partial p } - \omega \frac{\partial \xi}{\partial p} \right) + \frac{\partial}{\partial p} \left(\frac{\partial \omega}{\partial y} \cdot \frac{\partial u}{\partial p} - \frac{\partial \omega}{\partial x}\cdot \frac{\partial v}{\partial p}\right)$

(5)

Taking the Laplacian ($\nabla^2$) of (3) gives:

$\nabla^2 \left(-\frac{\partial Z}{\partial p} \right) + \nabla^2 V \cdot \nabla \left(-\frac{\partial Z}{\partial p} \right) - \nabla^2 k \omega = \frac{R}{C_p \cdot g} \cdot \frac{\nabla^2 q}{p}$

(6)

Adding (5) and (6), simplifying and substituting gk = σ, gives:

$\nabla^2\omega + \frac{f^2}{\sigma} \frac{\partial^2\omega}{\partial p^2} = \frac{1}{\sigma} \left[ \frac{\partial}{\partial p} J(\phi,\eta) + \frac{1}{f}\nabla^2 J \left(\phi, -\frac{\partial \phi}{\partial p} \right) \right] - \frac{f}{\sigma} \frac{\partial}{\partial p} \left( \frac{\partial \omega}{\partial y} \cdot \frac{\partial u}{\partial p} - \frac{\partial \omega}{\partial x} \cdot \frac{\partial v}{\partial p} \right) - \frac{f}{\sigma} \frac{\partial}{\partial p} \left( \xi \frac{\partial \omega}{\partial p} - \omega \frac{\partial \xi}{\partial p} \right) \frac{R \cdot \nabla^2 q}{C_p \cdot S \cdot p}$

(7)

Equation (7) is now a linear differential equation in ω, such that it can be split into two part, namely ω₁ and ω₂, such that:

$\nabla^2\omega_1 + \frac{f^2}{\sigma} \frac{\partial^2\omega_1}{\partial p^2} =\frac{1}{\sigma} \left[ \frac{\partial}{\partial p} J(\phi,\eta) + \frac{1}{f}\nabla^2 J \left(\phi, -\frac{\partial \phi}{\partial p} \right) \right] - \frac{f}{\sigma} \frac{\partial}{\partial p} \left( \frac{\partial \omega}{\partial y} \cdot \frac{\partial u}{\partial p} - \frac{\partial \omega}{\partial x} \cdot \frac{\partial v}{\partial p} \right) - \frac{f}{\sigma} \frac{\partial}{\partial p} \left( \xi \frac{\partial \omega}{\partial p} - \omega \frac{\partial \xi}{\partial p} \right)$

(8)

and

$\nabla^2\omega_2 + \frac{f^2}{\sigma} \frac{\partial^2\omega_2}{\partial p^2} =\frac{R \cdot \nabla^2 q}{C_p \cdot \sigma \cdot p}$

(9)

where ω₁ is the vertical velocity due to the mean baroclinicity in the atmosphere and ω₂ is the vertical velocity due to the non-adiabatic heating, which includes the latent heat of condensation, sensible heat radiation, etc. (Singh & Rathor, 1974).

Interpretation

Physically, the omega equation combines the effects of vertical differential of geostrophic absolute vorticity advection (first term on the right-hand side) and three-dimensional Laplacian of thickness thermal advection (second term on the right-hand side) and determines the resulting vertical motion (as expressed by the dependent variable ω.)

The above equation is used by meteorologists and operational weather forecasters to assess development from synoptic charts. In rather simple terms, positive vorticity advection (or PVA for short) and no thermal advection results in a negative ω, that is, ascending motion. Similarly, warm advection (or WA for short) also results in a negative ω corresponding to ascending motion. Negative vorticity advection (NVA) or cold advection (CA) both result in a positive ω corresponding to descending motion.

References

1. ^ Holton, J.R., 1992, An Introduction to Dynamic Meteorology Academic Press, 166-175
2. ^ Singh & Rathor, 1974, Reduction of the Complete Omega Equation to the Simplest Form, Pure and Applied Geophysics, 112, 219-223

Links

American Meteorological Society definition