Chapter 7 – Precipitation Processes Understanding Weather and Climate Aguado and Burt ATMO 1300 Precipitation ATMO 1300 1 Why Cloud Droplets Don’t Fall • Gravity vs. Updraft – Terminal Velocity – The final speed obtained by an object falling through the atmosphere. A balance between gravity and the frictional forces (drag) on the object. ATMO 1300 How Large is Large Enough? ATMO 1300 2 Growth of Cloud Droplets • Condensation • Warm Clouds – Collision and Coalescence • Cool and Cold Clouds – Bergeron Process – Riming and Aggregation ATMO 1300 Condensation • Can lead to quick growth for small water droplets. • Once radii of 20 microns is passed, the efficiency of condensation is reduced. • By itself condensation would produce few raindrops. ATMO 1300 3 Collision • Collisions between drops – Dependent on absolute size of the collector drop and relative size of the droplets below. – Most efficient when droplets are slightly smaller than the collector drop. ATMO 1300 Coalescence • When droplets stick together after colliding. • Coalescence efficiencies are usually assumed to be near 100% ATMO 1300 4 Bergeron Process • Depends on the co-existence of supercooled water and ice in the same cloud. • Saturation vapor pressure over ice is less than that over water at the same temperature. • Leads to a continuous transfer, in which supercooled droplets surrender water vapor which is subsequently deposited onto the ice crystals. ATMO 1300 Bergeron Process ATMO 1300 5 Riming • 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. ATMO 1300 Aggregation • The joining together of two ice crystals to form a single larger crystal. ATMO 1300 6 Types of Precipitation •Snow •Rain •Graupel •Hail • Sleet • Freezing Rain ATMO 1300 Snow • Results from the growth of ice crystals through deposition, riming and aggregation. • A snowflake’s structure depends on the temperature and moisture conditions that exist when the crystal is formed. – Warmer conditions provide for riming and a wet snow pack – Colder conditions provide for less adhesion (riming) and a “powder” ATMO 1300 7 US Snowfall ATMO 1300 Raindrop Shape • Not tear drop shaped! ATMO 1300 8 US Precipitation ATMO 1300 Graupel • When riming coats an ice crystal and its 6- sided sharp edges are lost into a milky- white spongy texture. • Graupel can fall to the ground or remain in the cloud providing a nucleus for hail. ATMO 1300 9 Hail • Ice pellets forms in roughly concentric layers. • Formed by a repetitive sequence, where ice falls into a region with liquid droplets and provides a liquid coating, upon lifting the ice crystal higher the coating freezes and the process restarts. (Figure 6-13 in text book) ATMO 1300 Hail ATMO 1300 10 Sleet • Raindrops freeze in the air while falling to the surface. • Must have a layer in the lowest portion of the atmosphere with a temperature below 0°C. • Requires an inversion aloft. ATMO 1300 Sleet ATMO 1300 11 Freezing Rain • Light rain or drizzle of supercooled water droplets fall through air at or slightly below 0°C. It freezes when it hits the ground. ATMO 1300 Measuring Rainfall • Raingauge – A instrument which collects the rain and enables its measurement – Tipping Rain Gauges – Weighing-Bucket Gauges ATMO 1300 12 Measurement Errors • Rain gauges only represent a point measurement. •Errors: – Wind generated turbulence can deflect precipitation from entering the gauge – Splashing – Snow drifts ATMO 1300 Snow Measurement • Measure depth • Water equivalency – depth of water which would result if all of the snow was melted. • Conversion factors – 10:1 • Snow pillows – big mattresses filled with antifreeze liquid and connected to a pressure regulator. ATMO 1300 13 Measuring Precipitation by Radar • Intensity of backscattered radiation to the radar indicates the size of the particles (droplets, snow flakes, gruapel or hailstones) ATMO 1300 Radar Estimates ATMO 1300 14.