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ATS/ESS 452: Synoptic Meteorology
Wednesday 09/10/2014
• Quiz!
• (Short?) Weather Discussion
• Continue Review Material
• Geostrophic Wind
• Continuity
• Vorticity
Synoptic Scale Governing Equations
Frames of reference:
• Eulerian – object is fixed in space; it doesn’t move; changes due to air moving
through the object; Referred to as the local derivative in equations
• Lagrangian – object is allowed to move with the air; Referred to as the total
derivative in equations
• What is cold air advection (CAA)? What is warm air advection (WAA)?
• Cold air advection is the process in which the wind blows from a region of cold air to a
region of warmer air
• Warm air advection is the process in which the wind blows from a region of warm air
to a region of cooler air
CAA
WAA
Temperature Advection
Geostrophic Wind
**A scale analysis of the *horizontal* governing equations reveals that only the pressure
gradient force and Coriolis are important on the synoptic scale
The resulting balance between these terms is called geostrophic balance, which yields
the geostrophic wind
**At the synoptic scale, in the Northern hemisphere, the geostrophic wind blows parallel
to isobars (isoheights) with low pressure (low heights) to the left.
Geostrophic balance allows the governing equations to be simplified considerably and
allows the development of quasi-geostrophic theory (chapter 2)
Notice how wind in this 500mb map is flowing west to east, generally
paralleling the isoheights  geostrophic balance
Geostrophic balance does not always apply  surface
Vertical Momentum Equation
dw
1 Dp
=+ fucosj - g + FZ
dt
r Dz
Vertical
acceleration
Vertical
PGF
Coriolis
gravity
friction
Which terms are left if you perform a scale analysis on this equation?
All that remain are the vertical PGF and gravity.
The balance between these two terms is called hydrostatic balance.
**Synoptic scale motions are largely in hydrostatic balance in the vertical direction
If we are to assume hydrostatic balance, then are we assuming no vertical air
motion (i.e. no vertical acceleration)?
NO… they’re just very small compared to the vertical PGF and gravity
Continuity Equation
• This relation is based on the conservation of mass (in a closed system, mass will not
change).
• The kinematic method uses the continuity equation to estimate vertical motion
- Not very accurate
- Errors in actual wind observations will lead to very large errors in the vertical
motion estimate
• Convergence of winds at the surface leads to vertical motion
• What do you think convergence aloft leads to? Divergence of winds aloft?
(sinking air)
(rising air)
• Being able to correctly diagnose and forecast divergence and convergence
aloft is an important component of this course