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Use of the Tephigram. 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: 25 °C, 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

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path of air parcel on tephigram
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: 25°C, 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

Cloud

Lifting condensation level, LCL = 780 mb

normand s theorem
“Normand’s Theorem”

Wet bulb potential temperature, θW 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

early morning tephigram over land
Early morning tephigram over land

No cloud

Tropopause

Boundary layer top

Lifting Condensation Level = 930 mb

Radiation inversion

mixing out of radiation inversion solar heating at the ground
Mixing out of radiation inversion – solar heating at the ground

Cloud top

Cumulus layer

LCL in the afternoon

Surface warms from 18°C to 23°C

energy area on tephigram what happens if something pushes air parcel upwards
Energy = area on tephigram. What happens if something pushes air parcel upwards?

Convective available potential energy = area where Tparcel>Tenvironment

Level of free convection

Convective inhibition = area where Tparcel<Tenvironment

deep convection
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.
orographic uplift lift each point on tephigram by 50 mb
Orographic uplift – lift each point on tephigram by 50 mb

Initial profile. We will use the four coloured altitudes as examples

orographic uplift lift each point on tephigram by 50 mb9
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

orographic uplift lift each point on tephigram by 50 mb10
Orographic uplift – lift each point on tephigram by 50 mb

Lift the next point through 50 mb. It does reach saturation

50 mb

orographic uplift lift each point on tephigram by 50 mb11
Orographic uplift – lift each point on tephigram by 50 mb

Lift the next two points through 50 mb.

50 mb

orographic uplift lift each point on tephigram by 50 mb12
Orographic uplift – lift each point on tephigram by 50 mb

Join up the dots to get the new temperature profile.

Hill Cloud

potential instability
Potential Instability

Grey line: temperature profile lifted 50 mb

Orange line: SALR from uplifted surface air

Slope of grey line > SALR so convectively unstable where saturated: deep thunderstorms would occur on this day over a small hill

Saturation