Geography Reference
In-Depth Information
200
300
400
500
θ
600
θ
e
*
700
800
LFC
θ
e
900
1000
320
340
360
380
400
Temperature (K)
Fig. 9.10
Schematic sounding for a conditionally unstable environment characteristic of Midwestern
North America thunderstorm conditions showing the vertical profiles of potential temper-
ature, θ , equivalent potential temperature, θ
e
, and the equivalent temperature, θ
e
,ofa
hypothetically saturated atmosphere with the same temperature profile. Dotted line shows
θ
e
for a nonentraining parcel raised from the surface. Arrow denotes the LFC for the parcel.
to saturation. The parcel will thus become buoyant when raised to the level z
0
if
θ
e
(z
0
−
δz)>θ
e
(z
0
), for then the parcel temperature will exceed the temperature
of the environment at z
0
. From Fig. 9.10 we see that θ
e
for a parcel raised from
about 960 hPa will intersect the θ
e
curve near 850 hPa, whereas a parcel raised from
any level much above 850 hPa will not intersect θ
e
no matter how far it is forced to
ascend. It is for this reason that low-level convergence is usually required to initiate
convective overturning over the oceans. Only air near the surface has a sufficiently
high value of θ
e
to become buoyant when it is forcibly raised. Convection over
continental regions, however, can be initiated without significant boundary layer
convergence, as strong surface heating can produce positive parcel buoyancy all the
way to the surface. Sustained deep convection, however, requires mean low-level
moisture convergence.
