1. Precipitation and Intro to Radar ATS 351 Lecture 7 October 19, 2009

2. Droplet Formation Recall the two types of nucleation Homogeneous Nucleation Water molecules come together to form a cloud droplet Heterogeneous Nucleation Requires a cloud condensation Nuclei (CCN)‏

3. Heterogeneous Nucleation

4. Droplet Growth Once a cloud droplet forms, there are 2 ways it can grow into precipitation Growth by condensation Growth by Collision and Coalescence Growth by condensation Very slow process Growth by Collision and Coalescence More realistic mechanism

5. Collision and Coalescence Coalescence occurs in clouds with tops warmer than 5°F (- 15°C)‏ The greater the speed of the falling droplet, the more air molecules the drop encounters Important factors for droplet growth High liquid water content within the cloud Strong and consistent updrafts Large range of cloud droplet sizes Vertically thick cloud Terminal velocity Droplet electric charge and cloud electric field

6. Collision and Coalescence

8. Homogeneous nucleation of ice Freezing of pure water Enough molecules in the droplet must join together in a rigid pattern to form an ice embryo The smaller the amount of pure water, the lower the temperature at which water freezes Supercooled droplets Water droplets existing at temperatures below freezing 1 ice crystal to 10 6 liquid droplets at -10°C Homogeneous nucleation (freezing) occurs at temperatures of – 40°C Vapor deposition From vapor to solid Not likely to be sufficient in our atmosphere

9. Ice nuclei Ice crystals (IN) form in subfreezing air on particles called ice nuclei Ice nuclei are rare; only 1 out of 10 million aerosols is an effective ice nuclei Fewer sources than CCN Desert and arid regions: silicate particle (dominant)‏ Clay particles: for temperatures between –10 and –20°C Volcanic emissions Combustion products Bacteria Oceans are NOT good sources of IN

10. IN requirements Insolubility If soluble, cannot maintain molecular structure requirement for ice Size Must be comparable, or larger than, that of a critical ice embryo (typically 0.1 microns)‏ Chemical bond Must have similar hydrogen bonds to that of ice available at its surface Crystallographic Similar lattice structure to that of ice (hexagonal)‏ Active Site Pits and steps in their surfaces

11. Growth mechanisms Vapor deposition Saturation vapor pressure over water greater than over ice Temperature affects saturation vapor pressure over ice the same way that it affects saturation vapor pressure over liquid When ice and liquid coexist in cloud, water vapor evaporates from drop and flows toward ice to maintain equilibrium Ice crystals continuously grow at the water droplet’s expense The process of precipitation formation in cold parts of clouds by ice crystal diffusional growth at the expense of liquid water droplets is known as Bergeron process

13. Growth mechanisms Diffusional growth alone not sufficient for precipitation formation Accretion/Riming Ice crystals collide with supercooled droplets, which freeze upon impact Forms graupel (snow pellets)‏ May fracture or split as falls, producing more ice crystals Growth mechanisms

14. Graupel from Accretion

15. Accretion of ice from ocean spray

16. Growth mechanisms Aggregation Collision of ice crystals with each other and sticking together Clump of ice crystals referred to as a snowflake Common in temperatures near freezing where there may be some liquid water on the surface of the crystal Differing temperatures can cause aggregates to grow into different shapes

19. Precipitation Types Rain - drop greater than 0.5 mm Rarely larger than mm because collisions break them up What is the shape of a raindrop? Drizzle - < 0.5 mm Usually from stratus Snow - small ice of many forms Fallstreaks (like virga, but from cirrus)‏ Flurries (no accumulation)‏ Snow squalls Blizzard - winds > 30 kts

20. Precipitation Types Sleet - tiny ice pellets formed from refreezing of rain drops Translucent (unlike graupel), < 5 mm Freezing rain/drizzle - freezes upon contact with the surface Can be extremely damaging Knocks out power Pulls down tree branches Both are common along warm fronts

21. Damage from freezing rain

22. Precipitation Types Virga - any precipitation that evaporates before hitting the surface

23. Graupel Ice crystals falls through cloud, accumulating supercooled water droplets that freeze upon impact. Thus, graupel is an example of growth by accretion/riming. Creates many tiny air spaces These air bubbles act to keep the density low and scatter light, making the particle opaque When ice particle accumulates heavy coating of rime, it’s called graupel

24. Hail An extreme example of growth by accretion Hailstones form when either graupel particles or large frozen drops grow by collecting copious amounts of supercooled water Graupel and hail stones carried upward in cloud by strong updrafts and fall back downward on outer edge of cloud where updraft is weaker Hail continues to grow through updrafts until it’s so large that it eventually falls out bottom of cloud

25. Hail growth As hailstone collects supercooled drops which freeze on surface, latent heat released, warming the surface of the hailstone Dry Growth At low growth rates (caused by lower liquid water contents), this heat dissipates into surrounding air, keeping surface of stone well below freezing and all accreted water is frozen Wet Growth If a hailstone collects supercooled drops beyond a critical rate or if the cloud water content is greater than a certain value, latent heat release will warm surface to 0°C Prevents all accreted water from freezing Surface of hailstone covered by layer of liquid water

26. Hail layers Alternating dark and light layers Wet growth solubility of air increases with decreasing temperature so little air dissolved in ice during wet growth Ice appears clear Dry growth Hailstone temperature close to environmental temperature so at cold temperatures, large amount of air dissolved Ice appears opaque

28. Hail Descriptors Size (inches) ‏ Name 0.25Pea 0.75Quarter 1.00Golf Ball 1.75Tennis Ball 2.50Baseball 2.75Grapefruit 4.00Giant > 4.00 Ruler measured

29. RAdio Detection And Ranging Transmits a microwave into the atmosphere and measures the return power – 10, 5, 3 cm typical Size chosen depends on use

30. Transmitter Transmit/Receive Switch ReceiverDisplay Antenna

31. Pulse of microwave energy sent out (emitted from antenna to parabolic dish reflector), dish focuses energy into beam Beam travels through atmosphere If the beam hits an object, then some of the energy is reflected back to the radar Return power measured Data processed to a visual display

32. Radar measures the intensity of the returned signal, the frequency of the returned signal, and the elapsed time from the transmission of the pulse Energy beam travels at the speed of light – Knowledge of the elapsed time allows the computation of the distance from the radar site Frequency uses doppler shift to determine movement

33. Only a fraction of the emitted energy gets returned from reflection amplified and measured in decibels (dbz), Reflectivity – 1dbz = 10 log(p 2 /p 1 )‏ – p 2 = power received at radar, varies Reflectivity is dependent upon the size of the object In meteorology, the objects are precipitation particles

34. The return power (reflected beam) is dependent on the number of particles present, and the size of the particles Particle diameter^6 dependence Number^1 dependence Larger drops lead to larger reflectivities Reflectivity mostly based on particle size

35. Drizzle: 20 - 25 dBz Light rain: 25 – 35 dBz Heavy Rain: 35 – 50 dBz Thunderstorm Heaviest Rainfall: >50 dBz Light Snowfall: 15 – 25 dBz Heavy Snowfall: 25 – 35 dBz

36. How much rain falls to the surface in a given hour R=inches/hour a and b are constants Higher reflectivity generally corresponds to higher rainfall rates