Geography Reference
In-Depth Information
Fig. 6.12
Schematic 500-hPa height contours (solid lines), isotherms (dashed lines), and vertical
motion field (w>0 dash-dot lines, w<0 dotted lines) for a developing synoptic-scale
system. Upward motion occurs where vorticity decreases moving left to right along an
isotherm, and downward motion occurs where vorticity decreases moving left to right along
an isotherm.
motion, we again consider an idealized developing baroclinic system. Figure 6.12
indicates schematically the geopotential contours, isotherms, and vertical motion
field at 500 hPa for such a system. Due to the westward tilt of the system with
height, the 500-hPa geopotential field leads the isotherm pattern. Because the
thermal wind is parallel to the isotherms, the right side of (6.36) can be estimated
from the change of absolute vorticity along the isotherms. At 500 hPa the positive
relative vorticity advection is a
maximum
above the surface low, whereas negative
relative vorticity advection is strongest above the surface high. Thus, for a short-
wave system where relative vorticity advection is larger than the planetary vorticity
advection, the pattern of vertical motion at 500 hPa forced by the advection of
vorticity by the thermal wind has
f
∂z
·∇
ζ
g
+
∂
V
g
< 0 east of the 500-hPa ridge
> 0 east of the 500-hPa trough
w
∝
−
Thus, there is rising motion above the surface low and subsidence above the sur-
face high. This pattern of vertical motion is in fact just what is required to produce
the thickness tendencies in the 500- to 1000-hPa layer above the surface highs
and lows. For example, above the surface low there is positive vorticity associated
with negative geopotential deviations, as vorticity is proportional to the Laplacian
of geopotential. Increasing vorticity thus implies a falling geopotential (χ<0).
Hence the 500- to 1000-hPa thickness is decreasing in that region. Because hori-
zontal temperature advection is small above the center of the surface low, the only
