Chapter 7 Precipitation Processes Chapter overview: Supersaturation and water availability Nucleation of liquid droplets and ice crystals Liquid droplet and ice growth by diffusion Collision and collection Precipitation Formation Precipitation types o Rain o Snow o Sleet and freezing rain o Hail Hydrometeors: Liquid and ice particles that form in the atmosphere Precipitation: Occurs when hydrometeors become large enough to fall to the Earth s surface How does precipitation form? Why do some clouds produce precipitation while other clouds do not? How does a cloud droplet grow to become raindrop? These are the questions we will address in this chapter.
The volume of a typical cloud droplet is one millionth the volume of a typical raindrop. Supersaturation and Water Availability Supersaturation: A condition in which there is more water vapor in the air than it can hold at equilibrium. How will the vapor pressure (e) compare to the saturation vapor pressure (e s ) when the air is supersaturated? What is the value of the relative humidity (RH) when the atmosphere is supersaturated? The supersaturation fraction (S) or supersaturation percentage (S%) is defined as: S = RH - 1 or S% = 100% x S = RH% - 100% The supersaturation fraction can also be found from the vapor pressure and saturation vapor pressure: S = e e s 1 or S r r s 1
How does the mixing ratio (r) and saturation mixing ratio (r s ), and total water mixing ratio (r T ) change as an initially unsaturated air parcel is lifted to and above its LCL? The excess water mixing ratio (r E ) is defined as: r E = r T - r s The excess water will increase above the LCL, as shown to the left. This excess water will condense onto liquid drops or deposit onto ice crystals such that: r E = r L + r i Why would the observed r E differ from the adiabatic value of r E? Note: Liquid cloud droplets can exist as supercooled water at temperatures between 0 and -40 deg C. The available supersaturation (S A ) for cloud droplet and ice crystal growth is: S A = r E r s = r T r s 1
As hydrometeors form and grow above the LCL how will the mixing ratio (r), total water mixing ratio (r T ), supersaturation (S), and available supersaturation (S A ) change? Nucleation of Liquid Droplets Nucleation: The creation of new droplets In the atmosphere nucleation occurs on tiny dust particles (called cloud condensation nuclei) in a process called heterogeneous nucleation. Aerosol: Any tiny solid or liquid particle suspended in the air. Cloud condensation nuclei (CCN): A subset of aerosol particles that can nucleate cloud droplets. What properties must an aerosol have in order to serve as a CCN? How does the number of condensation nuclei differ in air over land or the ocean? What impact does this have on the number and size of cloud droplets that form in a cloud over land or over the ocean? At typical excess water mixing ratios and CCN numbers found in the atmosphere cloud droplets will usually have radii of 2 to 50 µm. These cloud droplets are small enough that even a slight updraft can keep them suspended. The textbook discusses the details of how water vapor condenses onto CCN and how these droplets can grow from haze to cloud droplet sized particles, but we will not discuss these details in class.
Nucleation of Ice Crystals Why can cloud droplets exist in the liquid phase even when the temperature is below 0 C? What causes supercooled cloud droplets to freeze? Nucleation of ice crystals can occur by either homogenous or heterogeneous freezing. Homogenous freezing nucleation: The spontaneous freezing of supercooled liquid water droplets as temperature decreases to near -40 deg C. Below what temperature do all cloud droplets freeze? Heterogeneous freezing nucleation: The freezing of supercooled liquid water in the presence of a small dust particle (called an ice nucleus). There are three possible heterogeneous freezing mechanisms. Deposition nucleation: Water vapor is deposited (as ice) directly on a deposition nucleus. Immersion freezing: A freezing nucleus within a supercooled liquid droplet will cause the liquid to freeze when the droplet is cooled below a critical temperature that depends on the type of ice nucleus. Contact freezing: A supercooled droplet, with no embedded ice nuclei, freezes when an external ice nucleus comes in contact with the droplet. Only particles with a molecular structure that is similar to ice can serve as ice nuclei.
What type of particles act as ice nuclei in the atmosphere? How does the number of ice nuclei and cloud condensation nuclei compare? The shape of ice crystal that forms from deposition varies depending on the temperature and the supersaturation (or vapor density excess) as shown to the left. As these ice particles fall through a cloud they will encounter different temperatures and supersaturations and can acquire more complex shapes. Liquid Droplet Growth by Diffusion In a supersaturated environment water vapor will diffuse towards a cloud droplet causing the cloud droplet to grow. The rate of water vapor diffusion depends on the supersaturation (or mixing ratio) gradient. Based on the figure above will the small or large droplet grow more quickly by diffusion of water vapor?
Due to differences in droplet growth by diffusion for different sized droplets a cloud that contained only droplets growing by diffusion would have nearly all the same size cloud droplets (monodisperse). Ice Crystal Growth by Diffusion Cold cloud: A cloud in which the temperature is below freezing at some locations in the cloud and cloud droplets may be either liquid or ice. How does the equilibrium vapor pressure differ over a liquid cloud droplet and an ice crystal? The saturation vapor pressure over a water surface is greater than the saturation vapor pressure over an ice surface.
What impact does the difference in saturation vapor pressure over water and ice have on evaporation or condensation occurring over cloud droplets and ice crystals? The process of ice crystals growing at the expense of cloud droplets is called the Wegener - Bergeron - Findeisen (WBF) process (or cold cloud process). At what temperature is the WBF process at a maximum? Collision and Collection Terminal velocity: The maximum fall speed of an object that occurs when gravity is balanced by frictional drag. How does the fall speed of hydrometeor change as its size increases? In a cloud with many different sized hydrometeors the different fall speeds will result in hydrometeors colliding with each other. We need to consider collisions between: - just cloud droplets - just ice crystals - a mixture of ice crystals and cloud droplets
Coalescence: The merging of two liquid cloud droplets. Warm cloud: A cloud in which the temperature is above freezing everywhere and all cloud droplets are liquid. Collision and coalescence is the only way that cloud droplets can grow to raindrop size in a warm cloud. Aggregation: The process in which ice particles collide and stick together. Aggregation is favored for dendrite shaped ice crystals and when the temperature of the ice is warmer than -5 deg C. Accretion (riming): The growth of ice crystals by collision, collection, and instant freezing of supercooled liquid droplets. Graupel: A hydrometeor that has become so heavily rimed that the shape (habit) of the original ice crystal is completely covered.
Precipitation Formation Warm Clouds What factors enhance the collision and coalescence process in a warm cloud? A range of cloud droplet sizes The cloud s liquid water content Cloud thickness Updraft Electric charge of droplets and electric field in the cloud What factors create a broad distribution of cloud droplet sizes? Which type of cloud would you expect to produce larger raindrops - a thin, warm stratus cloud with a weak updraft or a warm, cumulus cloud with a moderate updraft? Cold Clouds The WBF process allows the small number of ice crystals to grow into larger ice particles in cold clouds with supercooled water. These large, falling ice crystals can then grow further through aggregation and riming. Does this frozen precipitation (graupel or snowflakes) always reach the ground in a frozen state?
For both warm and cold clouds the thicker the cloud is the larger the size of raindrops produced and the heavier the precipitation rate. Precipitation Types Rain Rain: Falling drop of liquid water with a diameter greater than 0.5 mm Drizzle: Falling drop of liquid water with a diameter less than 0.5 mm Virga: Falling precipitation that evaporates before reaching the ground Snow Snow: Falling ice crystals or snowflakes. Blizzard: Low visibility (less than or equal to 0.25 miles) due to falling or blowing snow with wind speeds greater or equal to 35 mph for at least 3 hours. Sleet and Freezing Rain Sleet: A raindrop that has frozen before reaching the ground. What atmospheric conditions lead to the formation of sleet?
Freezing rain: Rain that freezes upon striking a solid object, such as the ground. What is the difference between sleet and freezing rain? Rime: Small supercooled fog or cloud droplets that freeze as white or milky granular ice upon striking an object whose temperature is below 0 C. Ice storm: A storm in which there is a substantial accumulation of freezing rain. What difference in atmospheric conditions lead to the formation of rain, freezing rain, sleet, or snow? Hail Hail: Pieces of ice, that are either transparent or partially opaque, ranging in size from that of small peas to that of golf balls or larger. How does hail form? What causes hail to eventually fall to the ground? How does hail differ from graupel?
Precipitation Distribution