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

CAAWAA

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 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 Vertical acceleration Vertical PGF Coriolis gravity friction

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