G109: Weather and ClimateClouds & PrecipitationClouds & PrecipitationTopics1. Importance of Clouds2. Achieving Saturation3. Cloud Classification4. Cloud Formation5. Precipitation Formationa. Bergeron Processb. Collision-Coalescence Process6. Precipitation Types7. FogReadingsA&B: Ch.5 (p. 148-151), Ch.6 (p. 157-160,169-184), Ch.7 (p. 189-202)CD Tutorial: PrecipitationG109: Weather and Climate 9: Clouds & PrecipitationImportance of Clouds• For weather & climate Radiative Properties• Block K↓→• Absorb and re-radiate L↑→ Moisture content – Precipitation•• For weather forecastingG109: Weather and Climate 9: Clouds & PrecipitationAchieving Saturation• Formation of clouds, fog, or dew, requires: Condensation: water vapor (gas) ⇒ liquid i.e., Saturation• Air can become saturated by any one of three processes:G109: Weather and Climate 9: Clouds & PrecipitationAchieving Saturation1. Radiation cooling – during evening hours dew, fog2. Cooling via – most common Cooled adiabatically (DALR if unsaturated)0200400600800100012008 101214161820Temperature (oC)Height (m LCLDALRSALR If parcel rises high enough, cools sufficiently → lifting condensation level Above this point air continues to rise but now at SALR SALR<DALR → does not cool as rapidlyG109: Weather and Climate 9: Clouds & PrecipitationAchieving Saturation• Mechanisms that lift aira. Orographic Uplift: b. Frontal wedging: • uplift of • due to air ρ differences c. Convergence: d. Localized convection: G109: Weather and Climate 9: Clouds & PrecipitationStability and Cloud Type• Different types of clouds form depending on the stability• Unstable – air tendency to rise¾ May get • Stable - air tendency to resist upward movement Air forcibly lifted by other means e.g. front, mountains¾ Precipitation, if any, isG109: Weather and Climate 9: Clouds & PrecipitationCloud Classification• Initial classification of clouds had four basic categories (Howard, 1803)1. Cumulus• “heaped”• Clouds with vertical development2. Stratus• “layered”3. Cirrus• “curl of hair”• High, thin wispy ice clouds4. Nimbus • “rain”• http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxmlG109: Weather and Climate 9: Clouds & PrecipitationClouds Classification• Current classification includes 10 cloud types, grouped by height:1. High Clouds: Cirrus Cirrostratus Cirrocumulus• Above 6000 m → predominantly 2. Middle Clouds: Altostratus Altocumulus• 2000-6000 m → mix of 3. Low Clouds: Stratus Nimbostratus Stratocumulus• Below 2000 m → mostly 4. Clouds of extensive vertical development: Cumulus Cumulonimbus• Base of cloud is low extend to high altitudes (>6000+ m)G109: Weather and Climate 9: Clouds & PrecipitationCloud ClassificationG109: Weather and Climate 9: Clouds & PrecipitationG109: Weather and Climate 9: Clouds & PrecipitationCloud Formation• Requires condensation (gas ⇒ liquid)• Air must be saturated: cooled to Tdewor water added• Must be a surface on to which water condenses What surface is in the atmosphere? Cloud Condensation Nuclei (CCN or CN)•• Normally large numbers in the atmosphere If CCN are absent, can have G109: Weather and Climate 9: Clouds & PrecipitationCloud Formation• Air saturated with water + CCN →Condensation• When condensation takes place Initial growth of cloud droplets very rapid Growth rate diminishes quickly because H2O availability decreases Even in very moist air - growth of these cloud droplets by additional moisture is slow Also: cloud droplets are so tiny, they remain suspended in the airG109: Weather and Climate 9: Clouds & PrecipitationPrecipitation Formation• Rain drops: large enough to fall• Takes about cloud droplets to form 1 rain drop• But…→Need a mechanism to merge cloud droplets into rain drops1. 2. G109: Weather and Climate 9: Clouds & Precipitation1. Bergeron Process Occurs in • All or part of cloud < 0°C• i.e., Precipitation Formation Pure water suspended in air:• Doesn't freeze until -40°C• Below 0 °C, it is liquid waterG109: Weather and Climate 9: Clouds & Precipitation1. Bergeron Process Supercooled water will freeze if it:• Is agitated sufficiently• Comes into contact with Freezing Nuclei Freezing Nuclei (FN): solid particles with ice or crystalline structure Sparse in the atmosphere FN generally not active until T < -10°CPrecipitation FormationLiquid and ice crystals co-exist -10 to -20°C Clouds primarily supercooled H2O 0 to -10oCClouds generally composed entirely of ice crystals<-20oC Phase of Water in CloudsTemperatureG109: Weather and Climate 9: Clouds & Precipitation1. Bergeron Process Growth of ice crystals in the Bergeron process, relies on differences in saturation vapor pressure:es ice< es water Ice crystals grow  Requires a mix of both liquid water and ice• i.e., clouds between 0 and -40oCPrecipitation FormationG109: Weather and Climate 9: Clouds & PrecipitationPrecipitation Formation1. Bergeron Process Ice crystals grow at the expense of liquid water• Ice crystals grow• Fall….collide with supercooled droplets• Break…forming more FN• Grow….• Lifted by vertical ascent of the air G109: Weather and Climate 9: Clouds & Precipitation2. Collision – Coalescence Process Occurs in Precipitation Formation Giant condensation nuclei • Hygroscopic particles - sea salt Larger particles fall faster - collide with smaller (slower) particles• May coalesce• Millions of collisions ⇒ produces something large enough to fall to the surface without completely evaporating• Need clouds with great vertical extent• Need abundant moistureG109: Weather and Climate 9: Clouds & PrecipitationPrecipitation Formation2. Collision – Coalescence Process If collector drop is much larger than drops below it, then collision efficiency will be lowG109: Weather and Climate 9: Clouds & PrecipitationPrecipitation Formation2. Collision – Coalescence Process Raindrops may break up if• Large size (> 5 mm) when falling 30 km hour-1 Surface tension - holds the drop together • Surpassed by the drag imposed by the airG109: Weather and Climate 9: Clouds & PrecipitationForms of Precipitation• Will precipitation be liquid or solid?• Rain Liquid water formed in warm clouds