NATS 101 Lecture 13 Precipitation Processes

Supplemental References for Today’s Lecture Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN ) Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN )

Review: Vertical Stability Rising and sinking unsaturated (clear) air Temp changes at DAR of 10 o C/km Dew Point (DP) changes at rate of 2 o C/km Rising and sinking saturated (cloudy) air Latent Heating Mitigates Adia. Cooling Temp and DP cool at MAR of 6 o C/km Water Vapor Condenses into Liquid

Review: Vertical Stability Vertical Stability Determined by ELR Conditionally Unstable (MAR < ELR < DAR) Temp Difference between Environmental Air and Air Parcel, and the Depth of Conditionally Instability Controls Vertical Extent and Severity of Cumulus

Conditionally Unstable: Lower Rock Ahrens, Fig 5.7

Environmental Lapse Rate (ELR) ELR is the Temp change with height that is recorded by a weather balloon ELR is absolutely unstable in a thin layer just above the ground on hot, sunny days Ahrens, Meteorology Today 5th Ed. ELR is 6.5 o C/km, on average, and thus is conditionally unstable! 6.0 o C/km 10.0 o C/km 6.5 o C/km

Lapse Rates and Cumulus Types The ELR and depth of unstable layer modulates the type of Cu. As depth increases, the vertical extent of Cu generally increases. As temp difference between the air parcel and the environment increases, the updraft speed and severity of Cb typically increase. Ahrens, Meteorology Today 5th Ed.

Cloud Droplets to Raindrops A raindrop is 10 6 bigger than a cloud droplet Several days are needed for condensation alone to grow raindrops Yet, raindrops can form from cloud droplets in a less than one hour What processes account for such rapid growth? 10 6 bigger Ahrens, Fig. 5.15

Terminal Fall Speeds (upward suspension velocity) Small-Large RaindropsCloud Droplets-DrizzleCCN

small raindrop Area swept is smaller than area of drop Collision-Coalescence Big water drops fall faster than small drops As big drops fall, they collide with smaller drops Some of the smaller drops stick to the big drops Collision-Coalescence Drops can grow by this process in warm clouds with no ice Occurs in warm tropical clouds Collection Efficiency 10-50%

Warm Cloud Precipitation As cloud droplet ascends, it grows larger by collision-coalescence Cloud droplet reaches the height where the updraft speed equals terminal fall speed As drop falls, it grows by collision-coalescence to size of a large raindrop Ahrens, Fig Updraft (5 m/s)

Mixed Water-Ice Clouds Clouds that rise above freezing level contain mixture of water-ice Mixed region exists where Temps > -40 o C Only ice crystals exist where Temps < -40 o C Mid-latitude clouds are generally mixed Ahrens, Fig glaciated region

SVP over Liquid and Ice SVP over ice is less than over water because sublimation takes more energy than evaporation If water surface is not flat, but instead curves like a cloud drop, then the SVP difference is even larger So at equilibrium, more vapor resides over cloud droplets than ice crystals Ahrens, Meteorology Today 5th Ed.

SVP near Droplets and Ice SVP is higher over supercooled water drops than ice Ahrens, Fig. 5.18

Ice Crystal Process Since SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice Ice crystals grow at the expense of water drops, which freeze on contact As the ice crystals grow, they begin to fall Ahrens, Fig Effect maximized around -15 o C

Accretion-Aggregation Process Accretion (Riming) Aggregation Supercooled water droplets will freeze on contact with ice ice crystal Small ice particles will adhere to ice crystals snowflake Splintering Ahrens, Fig Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process

Summary: Key Concepts Condensation acts too slow to produce rain Several days required for condensation Clouds produce rain in less than 1 hour Warm clouds (no ice) Collision-Coalescence Process Cold clouds (with ice) Ice Crystal Process Accretion-Splintering-Aggregation

Examples of Precipitation Types

Definitions of Liquid Precipitation Williams, The Weather Book, p73

Temp Profiles for Precipitation Snow - Temp colder than 0 o C everywhere (generally speaking!) Sleet - Melting aloft, deep freezing layer near ground Freezing Rain - Melting aloft, shallow freezing layer at ground Rain - Deep layer of warmer than 0 o C near ground Ahrens, Meteorology Today 5th Ed.

Weather Conditions Associated with Precipitation Types Gedzelman, The Science and Wonders of the Atmosphere

Radar Estimates of Precipitation Danielson et al Radar emits pulses of EM radiation of wavelength between 3-10 cm Pulse reflects off raindrops, dust, bugs, chaff, etc. Distance from radar and intensity of precipitation can be determined from radar reflectivity Object size can be determined from amplitude of return pulse. Larger objects are more reflective. Reflections from objects farther away take longer to return.

Doppler Radar Danielson et al Doppler can detect motion toward or away from radar by the frequency of the return beam Higher - toward radar Lower - away from radar Doppler effect explains why pitch of whistle changes as a train approaches then moves away Frequency of return beam changes when reflective object is moving either toward or away from radar. Velocity can be determined from frequency shift. Lower frequency Higher frequency

Summary: Key Concepts Precipitation can take many forms Drizzle-Rain-Glazing-Sleet-Snow-Hail Depending on specific weather conditions Radar used to sense precipitation remotely Location-Rate-Type (liquid v. frozen) Cloud drops with short wavelength pulse Wind component toward and from radar

Assignment for Next Lecture Topic – Atmospheric Pressure Reading - Ahrens pg Problems - 6.1, 6.7, 6.8