MTO 412E Physics of Growth of Cloud Droplets and Cloud and Precipitation Precipitation Too little : Drought • Condensation • Cool and Cold Clouds Growth of – Bergeron Process Cloud Droplets – Riming and Aggregation and Overview of • Warm Clouds and Overview of Too much : – Collision and Coalescenc Precipitation Flood • Precipitation and Precipitation types • Precipitation Measurement Condensation Condensation • Can lead to quick growth for small water • Condensation occurs when water vapor droplets. changes to a liquid. • Once radii of 20 microns is passed, the • For condensation to take place, the air must be saturated and there must be a surface on efficiency of condensation is reduced. which the vapor can condense. • By itself condensation would produce few • In the air above the ground, tiny hygroscopic raindrops. (water -absorbent) particles known as condensation nuclei serve as the surfaces on which water vapor can condense. Condensation onto water • When an air parcel is lifted and cooled, the vapour pressure eventually exceeds the SVP. • This condition is called supersaturated • Droplets cannot form from pure water vapour until very high supersaturations (> 5 times SVP) are reached • Particles are essential to provide surfaces on which condensation can start • These particles are called cloud condensation nuclei (CCN) **** 1 The Bergeron Process Bergeron Process • Ice crystals will gain water vapor from – Also called the ice crystal process neighboring water droplets and grow to a suitable size for precipitation. • Need: • Bergeron process most effective when ice to – Water Vapor droplet ratio is 1:100,000 – Ice crystals • Non -convective precipitating clouds most – Supercooled water drops in equilibrium likely use this process since the liquid water with the vapor content is lower than convective clouds. Saturation over ice Bergeron Process Saturation • There is a saturated vapour pressure over ice in the Vapor Pressure with respect to same way as over water Liquid Water • At any temperature below 0 oC, the SVP over ice is slightly less than the SVP over water Vapor Saturation • The difference is not constant - it is greatest at -12 Pressure Vapor Pressure o C with respect to • Ice crystals grow very fast if there are water Ice droplets present as well, because a low supersaturation over water is a high one over ice. **** 0C Temperature Bergeron Process Bergeron Process Below 0 C, the saturation vapor pressure with respect to ice is less than that with respect to a plane surface of liquid wate r Since the ice is now in a supersaturated environment, vapor at the same temperature. So, f or equilibrium there is a lower molecules will go to the ice crystal and the ice crystal will vapor pressure for ice than for liquid water. grow. Consider a supercooled liquid water drop next to an ice crystal: Key Point: If the liquid drop is in equilibrium with the environment, then the ice crystal will be in a supersaturated environment. 2 Bergeron Process Bergeron process Since the vapor molecules are going toward the ice crystal, the drop now finds itself in an unsaturated environment. The drop will begin to evaporate, thereby supplying more water • Freezing nuclei start the growth of ice vapor molecules for the ice crystal to acquire. crystals in supersaturated conditions and other water droplets are then attracted • Common in mid -latitudes (at higher altitudes) • Cloud seeding utilizes this process using silver iodide or dry ice (which are good freezing nuclei) Bergeron (ice crystal) process Bergeron Process • is operative in cold clouds • Depends on the co -existence of supercooled water and ice in the same cloud. • air that is saturated with respect to water is • Saturation vapor pressure over ice is less than that supersaturated with respect to ice, promoting over water at the same temperature. deposition (saturation vapor pressure lower • Leads to a continuous transfer, in which supercooled droplets surrender water vapor which over ice) is subsequently deposited onto the ice crystals. • Occurs in either mixed clouds or glaciated clouds Bergeron-Findesin -Wegner teorisi Bergeron Process 8 Water droplets and ice crystal are in equilibrium; water vapor molecules > liquid is saturation vapor pressure over water is Riming greater than it is over ice • Analogous to droplet collision • Riming is the collision of ice crystals and supercooled water droplets. • Causes rapid grow of ice crystals and acceleration of their fall speeds. Ice crystal growth accretion ice crystals collide with supercooled water droplets aggregation ice crystals collide with other ice crystals 4 Aggregation • The joining together of two ice crystals to Ice nuclei form a single larger crystal. • Deposition nuclei • Freezing nuclei • Contact nuclei “hygroscopic” Accretion Aggragation • Accretion (Riming ) - Ice crystals collide • Aggragation - Ice crystals collide and stick with supercooled droplets and freeze on together. contact to form graupel . • This process forms snow flakes. • Graupel may fracture forming many ice particles. • This is the process involved in the growth of hail. Droplet growth Droplet Growth • Many CCN are hygroscopic (they absorb water • Cloud droplets are 100X smaller than vapour) and deliquescent (they dissolve into the typical rain drops. water they have absorbed) – Cloud droplet - 20 m • eg salt from evaporation of sea spray – Rain drop - 2000 m • This lowers the saturated vapour pressure over the surface and enables condensation to occur at low • Droplets will stay the same size if they are supersaturations (typically 0.5%) at their equilibrium vapor pressure. **** 5 Buharlaşmada eğrilik etkisi... Curvature Effect • Curvature Effect - Droplets with a smaller radius have a higher equilibrium vapor pressure. • Air that is saturated w.r.t. a flat surface is unsaturated w.r.t. a curved surface. • For droplets to exist, the surrounding air must be supersaturated (RH >100%) Eriyik etkisi, küçük damlacıklar... Solute Effect • Solute effect - Equilibrium vapor pressure is less for a droplet with salt in solution. – Salt is hygroscopic and makes it harder for water to evaporate. • Water droplets are competing among themselves for available water vapor. – Increasing RH results in larger droplets. Terminal Velocity Growth of droplets • Smaller droplets can collide to form larger • Growth of droplets by condensation is rapid for droplets. small droplets (< 10 m) but slows down • Terminal velocity - Speed at which the • Smallest drizzle drops are ~ 200 m diameter - cannot grow these by water condensation alone force of air resistance balances the force of gravity for a falling droplet. • Larger droplets must grow by collision and combination – Larger droplets have a higher terminal velocity. • Collision growth is slow for small droplets, • Typical rain drop falls 600X faster than a increases with diameter typical cloud droplet. **** 6 Damlanın zamanla büyümesi condensation collision Collision and coalescence • is operative in warm clouds and the lower portions of cool clouds • larger droplets have a higher terminal The most important factor in the production velocity (drag =gravity) than smaller of raindrops is the cloud’s liquid water droplets, increasing the probability of content. collisions and coalescence Collision & Coalescence – contd. How surface area depends on the size • In clouds warmer than -15 °C(5 °F), collision between droplets play a significant role • Larger drops may form on larger condensation nuclei (salt particles or through random collision droplets; turbulent mixing between cloud and drier environment) • Amount of air resistance depends on the size of the drop and its rate of fall --- speed of falls increases until the air resistance = gravity – Terminal velocity – Larger drops means less evaporation also • Coalescence: Merging of droplets by collision • Forces that hold together tiny droplet together are so strong th at if the droplets collide with another droplet, they would not stick together 7 Collision & coalescence – contd. Cloud droplet rising & then falling through a warm cumulus cloud by growth and coalescence • Rising air currents slow the rate at which drops fall --- thick cloud with strong updrafts will maximize the time droplets spend in a cloud --- the bigger size droplets • When the fall velocity of the drop > updraft velocity, droplet slowly descends; when it reaches the bottom of the cloud, size ~ 5 mm --- typically occur in a rain shower originating in the warm, convective cumulus clouds • Factors in the production of raindrops – Cloud’s liquid water content (most important) – Range of droplet sizes, cloud thickness, updrafts of the cloud, electric charge of the droplets and the electric field in the cl oud Formation of Precipitation Formation of Precipitation How does a cloud cloud condensation nuclei (CCN) required droplet become a rain droplet (0.5 to several hygroscopic material that cloud droplets (~0.02 mm) mm)? form on Collision/coalescence - droplets supercooling occurs if no CCN are available collide with each other and enlargen , larger droplets less susceptible to evaporation due to the curvature effect (Figure 8.4) • Warm cloud all portions > 0 oC o Contributing factors • Cool cloud lower portions > 0 C upper portions < 0 oC o • Range of droplet sizes • Cold cloud all portions < 0 C • Cloud thickness • Mixed cloud -40oC < T < –20oC • Updrafts in cloud • Glaciated cloud T < -40oC • Electric charge of raindrops and cloud droplets “supercooled ” water T < 0 oC 8 Cb’nin Anatomisi Collision-coalescence