Use of the Tephigram

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Use of the Tephigram
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Path of air parcel on tephigram
Consider a parcel of air lifted from the surface,
e.g. by flow over a hill
Parcel initially at 25C, dew point 8.5,
pressure 1000 mb. We can see that this
corresponds to w 7 g kg-1 As parcel ascends T
follows dry adiabatic Mixing ratio is
constant until saturation (T TD) Then parcel
follows saturated adiabatic
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Normands Theorem
Wet bulb potential temperature, ?W is used to
label saturated adiabats (value of T where
adiabat crosses 1000 mb)
Project the saturated adiabatic to 1000 mb. It
is colder than the (unsaturated) temperature
because water is evaporated into the air to keep
it saturated, and the energy comes from the
air. This is the definition of the wet bulb
temperature.
Cloud
LCL
TW
T
TD
Dry adiabatic through the temperature, the mixing
ratio line through the dew point, and the
saturated adiabatic through the wet bulb
temperature, all meet at the LCL
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Early morning tephigram over land
No cloud
Tropopause
Boundary layer top
Lifting Condensation Level 930 mb
Radiation inversion
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Mixing out of radiation inversion solar heating
at the ground
Cloud top
Cumulus layer
LCL in the afternoon
Surface warms from 18C to 23C
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Energy area on tephigram. What happens if
something pushes air parcel upwards?
Convective available potential energy area
where TparcelgtTenvironment
Level of free convection
Convective inhibition area where
TparcelltTenvironment
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Deep convection

• Develops when CIN is small and CAPE is large
• Need some CIN otherwise there is no build-up of
  energy in the boundary layer
• Mechanical forcing often needed to overcome CIN
  e.g. flow over mountain, sea breeze, cold front
• At other times large-scale forcing e.g. trough
  provides lift.

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Orographic uplift lift each point on tephigram
by 50 mb
Initial profile. We will use the four coloured
altitudes as examples
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Orographic uplift lift each point on tephigram
by 50 mb
Lift the bottom point through 50 mb. It does not
reach the LCL and remains unsaturated
50 mb
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Orographic uplift lift each point on tephigram
by 50 mb
Lift the next point through 50 mb. It does reach
saturation
50 mb
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Orographic uplift lift each point on tephigram
by 50 mb
Lift the next two points through 50 mb.
50 mb
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Orographic uplift lift each point on tephigram
by 50 mb
Join up the dots to get the new temperature
profile.
Hill Cloud
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Potential Instability
Grey line temperature profile lifted 50 mb
Orange line SALR from uplifted surface air
Slope of grey line gt SALR so convectively
unstable where saturated deep thunderstorms
would occur on this day over a small hill
Saturation