Moisture and Stability Flashcards
Bouyancy force
Vector sum of upthrust and weight of parcel
Drives convection of convective clouds
Adiabatic process
No energy or mass enters or leaves the system. Many processes in the atmosphere are adiabatic since air is a poor conductor of heat and mixing is usually slow.
Adiabatic processes examples
Often in vertical motions
Ascent of dry convective plumes
Large scale lifting/subsidence
Non-adiabatic processes examples
Often near surface
Radiative heating/cooling
Surface heating/cooling
Removal of water from atmosphere by precipitation
Water added from evaporation of precipitation falling from above
Condensation or evaporation in an undilute airmass is ‘pseudoadiabatic’.
Lapse rate
-1 X vertical gradient of temperature.
Dry adiabatic lapse rate
Decrease of temperature with height due to decrease in pressure
Potential temperature
temperature a parcel would have if compressed adiabatically to 100 kPa
Vertical mixing within PBL is driven by solar heating of surface and makes potential temperature vertically uniform.
Atmospheric water
Troposphere contains nearly all water in the atmosphere, mostly in vapor form but some as droplets and ice-crystals.
mixing ratio in middle troposphere: 1 g /kg
mixing ratio in lower troposphere: 10 g/kg
Effects of atmospheric water on dynamics in troposphere
Convective processes partly driven by bouyancy from latent heat of phase changes. warming-ascent. cooling-decent.
Outside clouds moist air is less dense than dry air. This boosts the bouyancy of a parcel.
Latent heat
Released when water condensates or freezes and removed if water evaporates or melts. same amount of energy both ways.
Importance of water vapour
the most important greenhouse gas
Affect emission/absorbtion of radiation which affect climate.
Sources of water vapour
Evaporation from surface (requires sunlight). Evaporation of precipitation.
Sinks of water vapour
Precipitation. Condensation at surface (dew, frost)
Saturation
Equal rates of gain and loss of water molecules between liquid and vapour. When the actual vapour pressure is equal to the saturation vapour pressure. Condensation can start at this point. At surface of drop air is always saturated.
Saturated water vapour pressure
Hypothetical value of e at which saturation would occur.
Relative humidity
RH=e/es. In ambient air any value of RH is possible.
(saturation) Mixing ratio
(kg/kg) ratio of mass of water to original mass of dry air
Vapour density
(kg/m3) mass of water vapour per unit volume of moist air
Dew point depression
difference between temperature and dew point temperature
Specific humidity
(kg/kg) ratio of mass of mater vapour to total mass of moist air
Supersaturation
percentage excess of vapour pressure beyond saturated value
Factors constant during dry adiabatic ascent:
vapour mixing ratio and potential temperature
Factors decreasing during dry adiabatic ascent
temperature, saturated vapor pressure and saturated mixing ratio.
Factors constant during saturated adiabatic ascent
total water mixing ratio and equivalent potential temperature
Absolute stability
A parcel will always be colder and more dense than the environment and will therefore always return to it’s original position. The slope of the environmental lapse rate is steeper than both the dry and saturated adiabatic lapse rates.
Absolute instability
Parcel is always warmer and less dense than the environment and will be lifted. The dry adiabatic lapse rate and saturated lapse rate are steeper than that of the environment.
Conditional instability
A parcel is forces to lift above the LCL and LFC eventhough this is a stable part of the atmosphere. Above the LFC it is unstable so the parcel is lifted.
Level of free convection (LFC)
Where the temperature of the environment decreases faster than the moist adiabatic lapse rate of a parcel at the same level.
Lifted condensation level (LCL)
Where the dry lapse rate meets the mixing ratio from the surface. Here condensation can start. RH=100%
Convective instability
Unstable until parcel is lifted to LCL when it becomes unstable. This is because the environmental lapse rate is steeper than the dry lapse rate but not the wet.
CAPE (convective available potential energy)
Measure of intensity of vertical motions inside convective clouds. Work done by bouyancy force on parcel to lift it from one neutral bouyancy level (LFC) to another (EL). Area between environmental and moist lapse rate between LFC and EL is proportional to the potential energy.
Triggering CAPE
Parcel needs to be lifted to LFC for the CAPE to converted into kinetic energy.
CIN (convective inhibition)
Barrier which opposes the forced lifting needed to trigger the onset of any convection. Work needed to lift parcel to LFC. Area below LFC where environment is warmer than the parcel.
