Transcript Tephigrams
Tephigrams ENVI1400 : Lecture 8 • Tephigrams are thermodynamic diagrams – one of a range of such diagrams developed to help in the visual analysis of atmospheric profiles. • They have the property that equal areas on the diagram represent equal amounts of energy. ENVI 1400 : Meteorology and Forecasting 2 Tephigram Temperature (°C) Thermodynamic diagram showing the vertical structure of the atmosphere. Dewpoint temperature (°C) ENVI 1400 : Meteorology and Forecasting 3 Temperature (°C) Pressure (mb) Potential Temperature (°C) or ‘dry adiabat’ ENVI 1400 : Meteorology and Forecasting 4 Saturated Adiabat ENVI 1400 : Meteorology and Forecasting Saturation mixing ratio (g kg-1) 5 Potential Temperature • Much of the change in air temperature with altitude is due purely to the reduction in pressure. • It is often easier to work with a measure of temperature that accounts for this pressurerelated change in T, allowing us to focus on real differences in the energy content of the gas. The Potential Temperature is one such measure. • Potential temperature, (K) is defined as the temperature a parcel of air would have if moved adiabatically to a pressure level of 1000 mb. 1000 T P R Cp R/Cp = 0.286 for air T must be in Kelvin ENVI 1400 : Meteorology and Forecasting 6 Adiabatic Lifting • As a parcel of air is lifted, the pressure decreases & the parcel expands and cools at the dry adiabatic lapse rate. • As the parcel cools, the saturation mixing ratio decreases; when it equals the actual water vapour mixing ratio the parcel becomes saturated and condensation can occur. • The level at which saturation occurs is called the lifting condensation level. Saturation mixing ratio equal to actual water vapour mixing ratio of parcel Lifting condensation level Dew point at surface ENVI 1400 : Meteorology and Forecasting 7 • If the parcel continues to rise, it will cool further; the saturation mixing ratio decreases, and more water condenses out. • Condensation releases latent heat; this offsets some of the cooling due to lifting so that the saturated air parcel cools at a lower rate than dry air. • The saturated (or wet) adiabatic lapse rate is NOT constant, but depends upon both the temperature and pressure. ENVI 1400 : Meteorology and Forecasting 8 Stability Environmental Lapse Rate Dry adiabatic ascent of surface air parcel Environment warmer than lifted parcel stable If adiabatic ascent of a parcel of air results in a temperature less than the environmental temperature at any given level, then the air parcel will be more dense than the surrounding air, and will fall back towards its original level. Such conditions are described as (statically) stable. Similarly a parcel forced downward, under stable conditions will warm adiabatically to a temperature greater than the surrounding air, will be less dense, and will rise back towards its original level. ENVI 1400 : Meteorology and Forecasting 9 Dry Adiabatic Lapse Rate Environmental Lapse Rate Lifted air is warmer than environment unstable If adiabatically lifted air is warmer than the surrounding environment, it will be less dense, and therefore buoyant, and will continue to rise. Such conditions are described as statically unstable, or convective. This is common near the surface when heated by sunlight. ENVI 1400 : Meteorology and Forecasting 10 Theoretical maximum altitude to which parcel may overshoot Equal areas Equal areas on a tephigram represent equal amounts of energy. The buoyant potential energy available is represented by the area between the environmental temperature curve and the adiabatic lapse rate. As the parcel rises, this is converted to kinetic energy. The rising parcel may overshoot its level of neutral buoyancy by an amount that just uses up all the kinetic energy. ENVI 1400 : Meteorology and Forecasting 11 stable Absolute Stability Adiabatic lifting (dry & wet) never results in the air temperature exceeding that of the environment. Lifting can only take place if forced, and at the expense of using energy. This is sometimes called forced convection and may occur due to mechanical mixing of stable air in strong winds. Cloud is formed if air lifted above the lifting condensation level (LCL), but remains limited to extent of parcel lifted from below. LCL Temperature at surface Dew point at surface ENVI 1400 : Meteorology and Forecasting 12 Absolute Instability LCL unstable stable Cloud overshoots level of neutral stability Any adiabatic lifting results in air that is warmer than its environment, and thus in buoyant convection. The buoyancy force increases at the lifting condensation level due warming by the release of latent heat. Strong solar heating of the surface, or advection over a warmer surface often results in unstable, or convective, conditions in the boundary layer. Cumulus clouds frequently form in such conditions. Temperature at surface Dew point at surface ENVI 1400 : Meteorology and Forecasting 13 LCL Forced adiabatic lifting of an air parcel through a region of static stability such that wet adiabatic lifting succeeds in raising the temperature above the environmental temperature. At this point, the parcel becomes convectively unstable and continues to lift under its own buoyancy. stable unstable stable Conditional Instability Temperature at surface Dew point at surface ENVI 1400 : Meteorology and Forecasting 14 Convective Instability B' B A' LCL A The column of air A-B has a lapse rate less than the dry adiabatic lapse rate, and is thus stable. If the column is forced to lift adiabatically, the whole column cools. If the lower part of the column reaches saturation [A'], it starts to cool at the wet adiabatic lapse rate – if this is less than the lapse rate of the column A'-B‘, the column becomes unstable. This type of instability may occur during large scale lifting up frontal surfaces or flow over mountain ranges. ENVI 1400 : Meteorology and Forecasting 15 ENVI 1400 : Meteorology and Forecasting 16 ENVI 1400 : Meteorology and Forecasting 17 ENVI 1400 : Meteorology and Forecasting 18 ENVI 1400 : Meteorology and Forecasting 19