2 Path of air parcel on tephigram 1000900800700Lifting condensation level, LCLParcel initially at: 30°C, dew point 14°, pressure 1000 mb. We can see that this corresponds to w = 10 g kg-1As parcel ascends:T follows dry adiabaticMixing ratio is constantuntil saturation (T = TD)Then parcel follows saturated adiabatic
3 “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.700800LCL9001000TDTWTDry 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
4 Early morning tephigram over land TropopauseBoundary layer topRadiation inversion
5 Mixing out of radiation inversion – solar heating at the ground Cloud topCumulus layerLCL
6 Energy = area on tephigram Energy = area on tephigram. What happens if something pushes air parcel upwards?Convective available potential energy = area where Tparcel>TenvironmentLevel of free convectionConvective inhibition = area where Tparcel<Tenvironment
7 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 layerMechanical forcing often needed to overcome CIN – e.g. flow over mountain, sea breeze, cold frontAt other times large-scale forcing e.g. trough provides lift.
8 Orographic uplift – lift each point on tephigram by 50 mb Coloured dots and lines – initital and final positions of air parcelsGrey line – final temperature profileDark Grey – saturated segment (hill fog)CloudAt each point on the profile:No cloud Cloud forms
9 Potential Instability Grey line: temperature profile lifted 50 mbOrange line: SALR from uplifted surface airSlope of grey line > SALR so convectively unstable where saturated: deep thunderstorms would occur on this day over a small hillSaturation