Download geostrophic wind

Typical
Cold
Front
•Convective precipitation
•50–100 mile wide
band along front
Typical
Warm
Front
•Clouds and precipitation
precede surface front
•Stratiform clouds starting
about 750 miles from front
•Stratiform precipitation starting
about 350 miles from front
“Overrunning”
OCCLUDED FRONTS
•Heavy precipitation along front
•Convective or stratiform
•Short duration
REVIEW OF KEY FORCES
Pressure gradient force (PGF)
• High to Low
• Perpendicular to the isobars
• Proportional to isobar spacing
Coriolis force (COR)
• Acts to right of parcel direction in N. Hemisphere
(and to left in S. Hemisphere)
• Maximum at poles, zero at equator
• Proportional to wind speed
(Chap. 8, pp. 210–214)
On the next slide, identify the forces associated
with the geostrophic wind acting on the air
parcel (box) over Arkansas. This is a 500 mb map.
PGF is from higher to lower pressure (or, in this case, heights).
For geostrophic flow, COR is equal (in magnitude) and opposite (in direction)
PGF
COR
COR acts to the right of the geostrophic wind in the Northern Hemisphere,
so the wind is WNW, parallel to the height contours.
PGF
Vgs
COR
Now identify the forces associated
with the gradient wind acting on the air
parcel (box) over Kentucky.
For flow around a curved path, the inward force (in this case PGF) is stronger
than the outward (COR). The resulting gradient wind flows parallel
to curved isobars (the wind goes around a curved path without crossing the contours).
PGF
Vgr
COR
Finally, identify the forces associated
with the surface wind acting on the air
parcel (box) over Lake Huron.
PGF
Friction
Vsfc
COR
This looks more complicated than it is. Friction slows the wind, thereby reducing
the magnitude of COR. PGF is unaffected. The resulting balance is
cross-isobaric flow TOWARD LOW PRESSURE. COR is always to the right of the
resulting wind (in the N. Hemisphere) and friction is always opposite the wind
direction. So more friction gives you more cross-isobaric flow and a weaker wind.