| NATS 101 Lecture 13 Precipitation Processes |
| Supplemental References for TodayÕs Lecture |
| Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6) | |
| Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6) |
| Review: Vertical Stability |
| Rising and sinking unsaturated (clear) air | |
| Temp changes at DAR of 10oC/km | |
| Dew Point (DP) changes at rate of 2oC/km | |
| Rising and sinking saturated (cloudy) air | |
| Latent Heating Mitigates Adia. Cooling | |
| Temp and DP cool at MAR of 6oC/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 |
| Environmental Lapse Rate (ELR) |
| Cloud Droplets to Raindrops |
| A raindrop is 106 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? |
| Terminal Fall Speeds (upward suspension velocity) |
| 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 |
| 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 |
| Mixed Water-Ice Clouds |
| Clouds that rise above freezing level contain mixture of water-ice | |
| Mixed region exists where Temps > -40oC | |
| Only ice crystals exist where Temps < -40oC | |
| Mid-latitude clouds are generally mixed |
| 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 |
| SVP near Droplets and Ice |
| 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 |
| Accretion-Aggregation 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 |
| Temp Profiles for Precipitation |
| 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 141-148 | |
| Problems - 6.1, 6.7, 6.8 |