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i ICE NUCLEI : ~ ~ 1 COL~ISII)N ICE ICE CRYSTALS COALESCENCE MULTIPLICATION AtcRhlON ! GRAU~EL MELTING ! ACCRhlON \. * t FREEZING S~:~TI~LEET _jDR\OIPS HAILS~ONES SHEDDING ! ",,- SNOW HAIL COLD HAIN Ugllfte 2 b. The 60ftmation 06 J.>now, hail and Cold RMn 6ftom ic.e nuc.lu IN, in a c.loud 06 J.>upeftcooled dftopletJ.> . - 21 - 4. Stages of Precipitation of Formation The formation of precipitation can be schematically represented by Figure 2. There, the precipitation processes are separated into two categories: Warm (Figure 2a) and Cold Rain, hail and snow (Figure 2b). The Warm Rain formation is an all water and vapor process, the Cold Rain pro- cess involves the additional presence of ice particles. Both have one thing in common: First a cloud of water droplets needs to be formed (the possibility of ice fog formation is neglected since it occurs only in very limited regions of the world) by condensation of water vapor on aerosol particles (CCN). Over oceans there are 50-200 nuclei active per cm3, the figure for continental region is roughly 500-1000 cm-3. In Warm Rain, these droplets grow by condensation and, after reaching sizes of 20-40 ~m, the condensation is very inefficient to produce further growth but the probability of collisions with other droplets increases. In particular, some droplets will grow fast in a short time interval (a collection of another droplet is a dramatic event) while the maj ority may not grow at all. After collec- tion of a smaller droplet, the original larger droplet has substantially gained in size and thus also in velocity and collecting cross-section. Therefore, its chances to experience further collisions are much greater than before. Let us add another consideration. If the collision-collection process occurs in a raising (and thus cooling) air parcel - as is normally the case - then continuing super- saturation will prevail and its reduction will cause all droplets to grow from the gaseous phase. This will not affect the size of larger ones much but they can grow much faster because they collide with smaller droplets which have grown further by condensation. The result is dramatic as Kovetz and Olund (1969) and Leighton and Rogers (1974) showed. Reynolds (1876) had already speculated that growth of raindrops occurs by collec- tion, but it was not until Telford (1955) and Twomey (1964) that the use of the stochastic collection process lead to a much faster evolution to rain. Berry and Reinhardt's (1974) gives the best example of how rain evolves by coalescence (Figure 3). Langmuir (1948) speculated that raindrops do not grow without a limit. From observations in nature he set 6 mm as maximum diameter and implied that aerodynamic instability of this size would lead to breakup. It was Magarvey and Geldart (1962) who showed in experiments that collisions are the key contributing factor to breakup. But it was up to McTaggart-Cowan and List (1975), Lo\~ (1977) and List and Gillespie (1976) to demonstrate that the conceived maximum drop sizes in Warm Rain are reduced much further by breakup than it was ever believed before. It turned out through numerical models, which are based on laboratory breakup experiml~nts, that it is quite difficult in steady state and widespread rain to grow drops with sizes >2.5 rnm. This by the way is also in agreement with observations (Fujiwara, 1967).