Use of the Tephigram Path of air parcel

Use of the Tephigram

Path of air parcel on tephigram Parcel initially at: 30°C, dew point 14°, pressure 1000 mb. 700 We can see that this corresponds to w = 10 g kg-1 As parcel ascends: T follows dry adiabatic Mixing ratio is constant until saturation (T = TD) Then parcel follows saturated adiabatic 800 900 1000 Lifting condensation level, LCL

“Normand’s Theorem” Project the saturated adiabatic to 1000 mb. Wet bulb potential temperature, θW is used to label saturated adiabats (value of T where adiabat crosses 1000 mb) 700 It is colder than the (unsaturated) 800 temperature because water is evaporated into the air to keep it 900 saturated, and the energy comes from the air. 1000 This is the definition of the wet bulb temperature. LCL TD TW T 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 Tropopause Boundary layer top Radiation inversion

Mixing out of radiation inversion – solar heating at the ground Cloud top Cumulus layer LCL

Energy = area on tephigram. What happens if something pushes air parcel upwards? Convective available potential energy = area where Tparcel>Tenvironment Convective inhibition = area where Tparcel<Tenvironment Level of free 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 Coloured dots and lines – initital and final positions of air parcels Grey line – final temperature profile Dark Grey – saturated segment (hill fog) At each point on the profile: No cloud Cloud forms Cloud

Potential Instability Grey line: temperature profile lifted 50 mb Orange line: SALR from uplifted surface air Saturation Slope of grey line > SALR so convectively unstable where saturated: deep thunderstorms would occur on this day over a small hill