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Stability & Cloud Development

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Stability & Cloud Development Stability & Cloud Development Chapter 6 Part 2 Reversible/Irreversible • Reversible adiabatic process- no precipitation falls and leaves parcel- parcel can return to original temperature and pressure • Irreversible pseudoadiabatic process- precipitation fall out and parcel cannot return to it’s original temperature and pressure (warms) Irreversible pseudoadiabatic process - precipitation falls out and ppgparcel cannot return to it’s original temperature and pressure (warms) 3500 Sinking parcel Precipitation becomes starts 3000 unsaturated (cloud dissipates)- dry 2500 lapse rate )) 2000 sinking T 1500 rising T eight (m HH 1000 500 Condensation 0 occurs- wet 0 10203040 lapse rate Temperature (C) Stability • Determine stability by comparing temperature of rising parcel to surroundings - if colder than surroundings , more dense, parcel sinks toward original position- stable, if warmer, less dense, continues to rise-unstable • Need to know temperature of parcel and it’s environment (surrounding air) at different heights in atmosphere • Environmental lapse rate is lapse rate of atmosphere- can bdtbe determi ned db by radi osond ddtfe data from wea ther balloons • If environmental lapse rate is less than wet adiabatic rate→absolutely stable • Greater that dry adiabatic rate→ absolutely unstable • Between wet & dry adiabatic → conditionally unstable Absolutely stable atmosphere- environmental lapse rate less than moist adiabatic rate Absolutely unstable atmosphere- lapse rate greatthdter than dry a dibtitdiabatic rate Conditionally unstable - stable if rising air not sa turat tded, unst tblifiiable if rising a ir sa tura tdted Summary of stability conditions Stable air • Stable air resists vertical motion - if clouds form from forced lifting, flat tops and bases • Atmosphere becomes more stable if air aloft warms or air below cools • Su bsidence inv ersion – sinking air w arms at dry adiabatic lapse rate, air below often cooler- very common Pacific Coast in summer, with cool marine layer below- high air pollution potential • Cooling in lower layers - radiational cooling; cold air a dvec tion; a ir mov ing over co ld sur face • Cold air drainage into valleys • Invers ion layer ac ts as lid to preven t ver tica l mixing • Neutral stability → environmental lapse rate = dry adiabatic rate – would occur in well mixed atmosphere such as windy conditions or much of lower atmosphere iftin sunny afternoons • Stability changes diurnally and varies with height • Unstable- usually only very near ground during intense solar radiation- air quickly rises and mixes resulting in near neutral stability • Conditionally unstable- between wet & dry adiabatic lapse rates- commonly the case • Average tropospheric lapse rate ≈665.5 º C per Km , typically near or greater than the wet adiabatic rate • Show example radiosonde temperature profiles 0z, 12z Adiabatic Charts • Adiabatic charts show how various atmospheric variables changgg,,ye with height: P, T, humidity The right white line is T profile. The left line is the Td. The pressure lines are plotted horizontally in blue and are also on an inverse log scale. The concept of Skew T means that the temperature itlttdtillbtlfftthihttis not plotted vertically but angles off to the right at a 45 degree angle. The temperature lines of the Skew T are in blue. The green lines are called dry adiabats. The light blue dashed lines are saturation adiabats. The yellow dashed lines are lines of constant mixing ratio. Morning sounding shows ground-based inversion Afternoon sounding shows near dry adiabatic lapse rate to 5000 m Causes of Instability • Cooling aloft- winds bringing in colder air (cold air advection) – radiational cooling from clouds or air • Warming offf surface air- daytime solar heating of the surface (ground); warm air advection by winds; air moving over warm surface • Lifting layer of air- top cools more than bottom, becomes less stable- often conditionally unstable and clouds may form • Convective instability- if lower layer portion moist, upper layer portion dry, lower layer becomes saturated and cools at wet adiabatic rate, upper portion at dry rate, upper portion becomes much cooler and get instability- often associated with thunderstorms and tornadoes Cumulus clouds forming Condensation begins Rising air cooling to dewpoint Rising, cooling, and expanding air Hot unstable air Mixing tends to steepen lapse rate Cloud Development • Clouds develop as an air parcel rises and cools below the dew point. • Usually a trigger or process is need to initiate the rise of an air parcel. Cloud Development • Convection – Differential land surface heating creates areas of high surface temperature. – Air above warm land surface heats, forming a ‘bubble’ of warm air that rises or convection. – Cloud base forms at level of free convection. Development of Cumulus Clouds Stability influences Cumulus Clouds Cloud Development • Topography – Orographic uplift – Orographic clouds – Windward, leeward, rain shadow – Lenticular clouds Orographic Uplift Wave clouds from mountains Mountain Wave Clouds Lenticular Clouds Cloud Development • Changing cloud forms – Stratus clouds can change to cumulus clouds if the top of the cloud cools and the bottom of the cloud warms. – Alto cumulus castellanus: towers on alto stratus – If moist stable air without clouds is mixed or stirred it can form stratocumulus clouds. Billow clouds from wind shear Stratocumulus Formation.
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