Precipitation Processes

• sufficient moisture - relative humidity of at least 70% to begin the process with hygroscopic nuclei


• lifting mechanism - cooling to cause increased relative humidity within the parcel


• additional process(es) - to grow cloud particles to a size (mass) sufficient to overcome the upward motion

• Cloud Drops - water drops that are suspended in the cloud updraft. Size range: several $\mu$m to ~400 $\mu$m. Typical size : ~10 $\mu$m


• Raindrops - water droplets large enough (having sufficient mass) so that they can fall towards the surface (through the updraft). Size range : few hundred $\mu$m to ~3 mm. typical size is ~1 mm.
  
  Note: drizzle drops are considered a sub-category of raindrops and have a typical size ~250 $\mu$m.


• Ice Crystals - relatively pure crystalline ice (or frozen form of water) in hexagonal shape. Size range: ~30 $\mu$m to ~300 $\mu$m.


• Snowflakes - crystalline ice, either a single large ice crystal or a an aggregate of ice crystal large enough (having sufficient mass) to fall through the cloud's updraft. Size range: ~300 $\mu$m to ~3 cm.


• Rimed Crystal or Graupel - ice crystals and super-cooler cloud droplets collide and the droplets freeze on the surface of the ice crystal. This process is called "riming."
  
  Rimed-crystal results if the crystal shape is maintained because only a small amount of riming has occurred.
  
  Graupel results if the process continues so that the original crystal shape is no longer recognized (the hydrometeor is now spherical, conical, or pear-shaped). The diameter of graupel is less than 5 mm. Graupel is also known as "soft hail" or "snow pellet."


• Hail - graupel having a diameter of 5 mm or greater.
  
  The largest recorded diameter of hail is 20 cm (about the diameter of a volleyball) and weighed 0.88 kg (~1.94 pounds). The thunderstorm that produced the hailstone occurred on July 23, 2010 at Vivian, South Dakota.


• Water and ice distribution in a cumulus cloud


• Percentage of wet clouds

• One major sources of CCN is sulfate aerosol ( $\mbox{SO}_4^{2^-}$ and methanesulfonic acid droplets ). These sulfate aerosols form partly from the dimethyl sulfide (DMS) produced by phytoplankton in the open ocean. Large algal blooms in ocean surface waters occur in a wide range of latitudes and contribute considerable DMS into the atmosphere to act as nuclei.


• Important effects
  
  
  • curvature (-)
    
    Evaporation rates per unit area of pure water over a curved surface, such as those associated with cloud droplets, are greater than those over a flat surface. Consequently, the condesation rate $R_c$ must increase by increased vapor pressure for the droplet to survive, let alone grow. Further details on this effect are given here.
  
  
  • solute (+)
    
    The evaporation rates $R_e$ of water molecules in solution are less than those of pure water, resulting in the need for a lesser vapor pressure for the rate of condensation $R_c$ to equal $R_e$. Cloud condesation nuclei (CCN) dissolving to facilitate droplet growth at RH less than 100% are known as ``hygroscopic'' CCN.


• Kohler Curves - quantifying the competing effects


• This condensation process is adequate for growth to around 20 $\mu$m.


• Further growth under this process would require relative humidities of greater than 100%


• Not rapid or efficient enough to account for growth to precipitable size (one million times more mass)

• collision-coalescence


• ice crystal process

• wide spectrum of cloud drop diameters
  
      -   terminal (fall) velocity depends on the droplet mass
  
      -   results in varying relative velocities
  
      -   which, in turn, results in colliding droplets


• received abundant study by cloud physicists


• collision efficiency
  
      -   depends on the radii of the collector drop and the cloud droplet, as well as a collision cross-section parameter
          (Wallace and Hobbs, page 255)
  
      -   The collision efficiency, $E$, for a pair of droplets with the collector radius $a_1$, and the collectee with radius $a_2$ is defined as
  

          $E = 1 - \frac{\pi y_c^2}{\pi(a_1 + a_2)^2}$

          where $y_c$ is called the critical initial offset, the horizontal distance between the vertical lines running through the center of the drops. (P.K. Wang, page 253)
  
      -   wake effect ($\uparrow$)
  
      -   turbulent effect ($\uparrow$ or $\leftrightarrow$ ?)


• coalescence efficiency
  
  
  • colliding drops will either rebound, coalesce (combine masses), or coalesce and breakup
  
  
  • range of efficiency : 0 - 70%, generally better for large drop small drop collisions
  
  
  • efficiency influences:
    
        -   surface tension ($\downarrow$)
    
        -   collision angle ($\leftrightarrow$)
    
        -   electric charge ($\leftrightarrow$)
    
        -   cloud thickness ($\uparrow$)
    
        -   updraft strength ($\uparrow$)
    
        -   spectrum width ($\uparrow$)

Small raindrops (less than 1mm in diameter) are spherical, like a round ball. A sphere is the shape that requires the least amount of energy for the drop to hold itself together.

As drops grow bigger than a millimeter or so, they start to become flat along their bottom edge as they fall, due to the resistance of air flowing around the drop. By the time a drop reaches 2-3 mm in diameter, it looks more like a hamburger bun than a sphere. Drops bigger than about 6 mm in diameter are relatively rare because the air resistance tends to cause the drops to breakup as they fall.

• homogeneous nucleation (cloud temperature less than freezing)
  
      -   molecules must join to form an ice embryo
  
      -   warmer embryos have thermal agitations large enough to weaken and break apart the ice embryos
  
      -   cooler temps reduce the thermal agitations, embryos more likely to grow
  
      -   larger droplets increase the chances of embryo growth
  
      -   only larger droplets freeze by homogeneous nucleation at temperatures greater than -40 degrees C


• heterogeneous nucleation
  
      -   ice nuclei - particles on which liquid water or water vapor will freeze
  
      -   the colder the environment, the more plentiful they are
  
      -   areosols with a physical structure similar to that of an ice crystal lattice are better ice nuclei
  
          *  clay mineral particles (kaolinite) (at cooler temps -10 to -37 C), bacteria and leaf debris from decaying leaves (-1 to -10 C) appear to be the most common forms of ice nuclei aerosols
  
      -   much less plentiful than condensation nuclei
  
      -   Different ice crystal nucleation processes
  
        *   condensation nucleation : condensation forms on a cloud condensation nucleus, and the condensation freezes once the temperature of the drop decreases to a sufficiently cool value
  
        *   immersion freezing : condensation forms on a cloud condensation nucleus, but the ice nucleation occurs around another areosol immersed in the droplet
  
        *   deposition nucleation : ice forms on the nucleous as vapor changes directly to ice
  
        *   contact nucleation : supercooled droplets freeze on contact with an ice nucleus
                  -   recent studies show that almost any crystal-shaped particle can act as a contact nuclei, thought by some researchers to be the dominant process in ice crystal formation

• Diffusion
  
      -   ice crystals grow at the expense of SC droplets     (Bergeron)
  
      -   SC droplets outnumber ice crystals by 100,000 or 1,000,000 to 1
  
      -   difference in saturation vapor pressures : $e_s^{water} - e_s^{ice}$


• Aggregation
  
      -   dominant process in colder, layered clouds
  
      -   ice crystals collide with ice crystals, welded by small spot of liquid water
  
  
• Accretion (Riming)
  
      -   dominant process in warmer, higher moisture clouds
  
      -   ice crystals collide with SC droplets, droplets freeze on contact to form rimed ice crystals
  
      -   rimed crystals may warm further, loosing much of their crystalline shape, to form graupel

• snow


• sleet


• freezing rain


• rain


• snow pellets