Clouds, Cloud Formation, and Stability

1. Clouds, Cloud Formation, and Stability Lab 6 October 12, 2009

2. Condensation Water vapor does not readily condense on its own Water has high surface tension Needs unreasonably high relative humidities or very cold temperatures (~-40oC) Cloud condensation nuclei are needed to aid condensation

3. Cloud Condensation Nuclei • CCN are described by the size of the particle

4. Cloud Condensation Nuclei Aerosol: a fine suspended solid or liquid particle in a gas Cloud droplets can form on both insoluble and soluble particles A particle that will serve as CCN is called “hygroscopic” or hydrophillic Vapor may condense at RH <100% A particle that will not serve as a CCN is called hydrophobic. Vapor usually will condense on these for RH >100%

5. CCN Sources are dust, volcanoes, factory smoke, forest fires, sea salt Over Ocean: 300-600 cm-3 Over land: 103 – 107 cm-3 More in urban areas, less in rural Aerosol concentrations decrease with height Very light, stay suspended for a long time

6. Cirriform Clouds Usually exist above 16,000 feet Generally thin, sometimes partially translucent Comprised of ice crystals Absorb longwave radiation, but are bright and reflective (have a high albedo) Rarely precipitate Virga Cirrus (Ci) Called “mares tails”

7. Cirrus

8. Stratiform clouds Characterized by a horizontally uniform base Forms in stable atmospheres May or may not precipitate May exist at any level Layered

9. Stratus

10. Nimbostratus

11. Cumuloform clouds Large in vertical extent May or may not precipitate Result from vertical motion Cumulus “fair weather cumulus” Cumulonimbus “anvil cloud”

12. “Fair weather” cumulus

13. Cumulonimbus

14. Other cloud types Mammatus Lenticular Kelvin-Helmholtz Cloud Streets Severe weather clouds

15. Mammatus clouds • Precipitation evaporates out of anvil • Evaporation cools the air and it sinks • If drops are large, mammatus will be long lived

16. Lenticular Clouds Stationary, lens-shaped clouds over mountains at high altitude Stable, moist air flows over mountain, creating a large scale standing wave Indicates region of turbulence

17. Kelvin-Hemholtz Waves Form when two parallel layers of air are moving at different speeds and in different directions Upper layer is usually faster Very short lived

18. Cloud Streets Form due to horizontal rolls in the atmosphere Also due to uneven surface heating Clouds form over updrafts in rolls Occurs more frequently over the ocean

19. Shelf and Roll Clouds Low, horizontal, wedge-like cloud Shelf: Attached to Parent Storm Roll: Removed from Parent Storm Formation is due to gust front from thunderstorms

20. Wall Cloud Associated with severe thunderstorms Indicates area of strongest updraft The strongest tornados form here

21. Satellite Imagery Visible imagery: essentially a black and white camera on a satellite. Measures brightness in the visible spectrum. Infrared imagery: measures infrared radiance from the object (ie, the surface or cloud top) it is pointed at. From blackbody theory, the temperature of the object can be found; since temperature changes with height, the cloud-top height can then be estimated.

22. Visible Satellite Pros- good at showing low clouds and fog- available in high spatial resolution Cons- only works in daylight- clouds can be confused with reflective features like snow- optically thin clouds like cirrus don’t show as well

23. IR Satellite Pros- available at all hours- provides an estimate of cloud-top height Cons- lower spatial resolution- low clouds don’t show because their temperatures are close to the surface temperature Color enhancement table often applied to bring out important temperatures Raw Enhanced

24. Clouds and Satellite Imagery • The bright, puffy areas in the visible image on the right are cumulus and cumulonimbus clouds (the cumulonimbus are fuzzier around the edges). Notice how the cloud tops over the Front Range are cold in the IR imagery

25. Cirrus in Visible vs. IR • Because cirrus are cold and optically thin (meaning the sun can be seen through the cloud), they are more easily seen in the IR than the visible

26. Low clouds/fog in visible vs. IR • Because low clouds are bright and warm, they are easily seen in the visible, but not the IR

27. Stability Where is the stable layer?

28. Stability Stable Equilibrium If the ball is displaced it will return to it’s original position Unstable Equilibrium If the ball is displaced it will accelerate away from the equilibrium point Neutral Equilibrium If the ball is displaced it will stay in it’s new location.

29. Stability In the atmosphere we can use the environmental temperature and dew point profile to determine the stability of a given sounding In an stable atmosphere, a displaced parcel will return to its original position In an unstable atmosphere, a displaced parcel will continue to move in the direction it was pushed

30. Conditions for Stability Absolutely Stable Absolutely Unstable Conditionally Unstable

31. Stable Atmosphere Vertical motion is suppressed This can be produced by an inversion, which can be caused by : Cooling of the surface at night Subsiding air (frequently associated with a ridge of high pressure) The tropopause is very stable due to the inversion caused by ozone in the stratosphere This means that storms cannot penetrate into the stratosphere

32. Unstable Atmosphere Buoyant parcels are accelerated upward As they rise and cool, they are still warmer than the environment since the environment is cooling faster than the adiabatic lapse rate Larger instabilities lead to larger updrafts Large updrafts lead to the formation of cumulonimbus clouds and thunderstorms

33. Examples Unstable Unstable

34. Lifting a Parcel

35. Sources of Lift 4 ways to lift a parcel to the LCL Frontal Boundary Orographic Convergence Convection

36. CAPE CAPE = Convective Available Potential Energy CAPE is the energy available to a rising parcel to accelerate it On a Skew-T, CAPE is proportional to the area between the parcel’s temperature and the environment’s when the parcel is warmer CAPE gives an upper limit on how high updraft speeds can get in a severe storm High values of CAPE are associated with the possibility of strong convection

37. CAPE

38. CIN CIN = Convective INhibition This is the energy that must be overcome in order to lift a parcel to its LFC On a Skew-T, CIN is proportional to the area between the parcel’s temperature and the environment’s when the parcel is colder Large values of CIN will prevent the formation of storms, but often the presence of some CIN can add strength to a storm if this energy is overcome

39. CAPE and CIN

40. More Uses for Skew-T’s Finding cloud levels Forecasting precipitation type

41. More Uses for Skew-T’s Finding cloud levels – useful for aviation Clouds are likely present at three layers on this diagram. Can you find them?

42. More Uses for Skew-T’s Forecasting precipitation type The 00C isotherm in this skew-T shows that the precipitation will fall through a layer which is above freezing, thus implying that freezing rain is possible