I know that geostrophic flow means straight wind flow that is balanced by the pressure gradient and Coriolis forces. But what do quasi-geostrophic and ageostrophic mean specifically?
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2$\begingroup$ Quasi- just means not really. We don't have proper geostrophic flow because we have friction. $\endgroup$– gerrit ♦Apr 17, 2014 at 21:23
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2$\begingroup$ to be slightly pedantic, this isn't always about wind. One can have geostrophic ocean currents too. Not sure if it's worth adding an ocean-currents tag... $\endgroup$– Semidiurnal SimonApr 17, 2014 at 22:21
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$\begingroup$ @SimonW Alternatively, one could remove the wind-tag. $\endgroup$– Torbjørn T.Apr 17, 2014 at 22:26
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2$\begingroup$ @gerrit there is a bit more to quasi-geostrophic than neglecting friction. $\endgroup$– caseyApr 18, 2014 at 2:43
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1 Answer
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Ageostrophic winds are merely the component of the actual wind that is not geostrophic. In other words, given the actual wind ($\mathbf v$) and the geostrophic wind ($\mathbf v_g$), the ageostrophic wind ($\mathbf v_a$) is the vector difference between them. The ageostrophic wind represents friction and and other effects. This, for example, is responsible for surface wind crossing isobars rather than following them.
$$\mathbf v_a =\mathbf v - \mathbf v_g$$
Quasigeostrophic refers to a modified set of equations that are make a number of assumptions, approximations and neglecting terms due to scaling. The equations are only valid for Rossby numbers much less than 1 (Ro << 1)
Some of these include:
- neglect advection by $\mathbf v_a$
- neglect advection by vertical velocity
- neglect the time tendency of $\mathbf v_a$
- neglect advection of $\mathbf v_a$ by $\mathbf v_g$
- replace $f$ with the beta plane approximation ($f = f_0 + \beta y$)
- neglect friction
- horizontally constant static stability ($\sigma$)
Starting with the frictionless horizontal momentum equations you end up with a number of "Q-G" equations: a thermodynamic energy equation, a vorticity equation and the omega equation. The Q-G omega equation:
$$\left( \nabla^2_p + \dfrac{f^2_0}{\sigma}\dfrac{\partial^2}{\partial p^2} \right)\omega = - \dfrac{f_0}{\sigma} \dfrac{\partial}{\partial p} \left[ - \mathbf v_g \cdot \mathbf \nabla_p(\zeta_g + f) \right] + \dfrac{R}{\sigma p}\left[ -\nabla^2_p(- \mathbf v_g \cdot \mathbf \nabla_p T) \right]$$
The first term on the RHS vertical change in advection of geostrophic absolute vorticity by the geostrophic wind. Positive vorticity advection increasing with height results in upward vertical motion. Negative vorticity advection increasing with height results in downward vertical motion. The second term on the RHS is related to temperature advection by the geostrophic wind. Cool air advection (CAA) correlates with upward vertical motion.
This is the traditional form of the equation and other forms exist to aid in diagnosing vertical motion with specific variables (e.g. the Sutcliffe-Trenberth recasts the equation using the thermal wind and Hoskins et al. (1978) defines the equation in terms of $\vec Q$, "Q vectors").
There isn't much NWP utility to the Q-G equations with todays computers, but they are good for diagnosing vertical motion in hand map analysis.
(will add the QG chi equation here)
Further reading:
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4$\begingroup$ More info on the quasigeostrophic equations and the abbreviated derivation of QG equations. $\endgroup$– DrewP84Apr 18, 2014 at 3:34
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$\begingroup$ Do v_g and v_a correspond to the non-divergent and irrotational component in the Helmholtz decomposition? $\endgroup$– JasonApr 12, 2018 at 6:47