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A geostrophic meteorological model is one in which the horizontal wind vector in the equations defining the model is replaced everywhere by its geostrophic approximation. In a quasigeostrophic model the horizontal wind vector in some instances is replaced by the geostrophic approximation but not in others. In the standard quasigeostrophic model it is also assumed that the Coriolis parameter f is constant except where its derivative ∂f/∂y=β is taken. (Justification 1) This is known as the betaplane approximation.
In the following ∇ represents the horizontal gradient operator. Symbols in red represent vectors and symbols i,j and k represent unit vectors.
The assumption of constant Coriolis parameter f means that the geostrophic wind approximation used in the model is
where f_{0} is the value of the Coriolis parameter at midlatitude of the study area. Thus the geostrophic wind used in the quasigeostrophic model is an approximation to the exact geostrophic wind. (Since the geostrophic wind is used as an approximation of the true wind, the quasigeostrophic wind may or may not be a worse approximation of the true wind than is the exact geostrophic wind.)
The Eulerian acceleration operator is
but in the quasigeostrophic model this is replaced by
The horizontal momentum vector equation is:
The RHS of the above equation is
But ∇φ= f_{0}k×V_{g} so the above reduces upon expansion to
The last term in the above, βy(k×V_{a}), is of a low order of magnitude than the other terms and may be neglected. Justification 2
When the Eulerian acceleration DV/Dt is replaced by the geostrophic acceleration D_{g}V_{g}/dt the momentum equation is then
In pressure coordinates the continuity equation is
But ∇·V_{g} = 0. ( Justification 3) This means that the continuity equation for the quasigeostrophic model reduces to
The general thermodynamic equation is
where S_{p} is the stability parameter and is equal to T(∂ln(θ)/∂p).
The temperature field T is decomposed into two components: the barotropic field T_{0}(p) which is a function of pressure alone and the deviational field T_{d}(x,y,p,t). The magnitude of the gradient of T_{d} with respect to pressure is much smaller than the magnitude of the gradient of T_{0} with respect to pressure. (Justification 3)
The stability parameter S_{p} may be expressed as
where σ = (RT/p)(∂ln(θ)/∂p).
The potential temperature θ is also decomposed into its barotropic and deviational components and, because of the assumption concerning the gradients,
Since ∇T_{0}=0, the thermodynamic equation for the quasigeostrophic model reduces to
Justification 1: The justification for treating the Coriolis parameter f as a constant
On a sphere f=2Ωsin(φ) so
where a is the radius of the Earth. Thus
where L is the scale of a meteorological disturbance.
Since cot(φ) is on the order of unity, for the midlatitudes this means that the deviation of f from a constant is on the order of (L/a), which is a lower order of magnitude than unity. Hence the Coriolis parameter can be reasonably approximated at the midlatitudes as a constant.
The ratio of the third term to the first term is equal to (βy/f_{0}) which from the previous justification is a lower order of magnitude than unity.
The ratio of the third term to the second term is equal to V_{a}/V_{g}, which is a lower order of magnitude than unity.
The quasigeostrophic wind vector V_{g} is (1/f_{0})k×∇φ. A straight forward computation gives
The divergence of any vector V=ui+vj is (∂u/∂x)+(∂v/∂y), therefore
because of the equality of the cross derivatives.
An alternate version of the above justification starts with the generic formula for the divergence of a vector cross product:
For
The term in the bracket, ∇×∇Φ, is alway the zero vector because the curl of a gradient always vanishes. The first term in the bracket is also the zero vector if the local vertical unit vector k is treated as a constant vector. In spherical coordinates this vector is not constant in direction although in rectangular coordinates it is.
Thus
The assumption that dT_{0}/dp >> dT_{d}/dp is based upon the notion that T_{0} includes all of the sytematic dependence of temperature upon pressure and thus T_{d} cannot contain any systematic pressure dependence.
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