Clouds & Precipitation

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Kerry George Rogers
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1 Readings A&B: Ch.5 (p ), Ch.6 (p , ), Ch.7 (p ) CD Tutorial: Precipitation Clouds & Precipitation Topics 1. Importance of Clouds 2. Achieving Saturation 3. Cloud Classification 4. Cloud Formation 5. Precipitation Formation a. Bergeron Process b. Collision-Coalescence Process 6. Precipitation Types 7. Fog G109: Weather and Climate Importance of Clouds For weather & climate Radiative Properties Block K Absorb and re-radiate L Moisture content Precipitation For weather forecasting

2 Achieving 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: Achieving Saturation 1. Radiation cooling during evening hours dew, fog 2. Cooling via most common Cooled adiabatically (DALR if unsaturated) If parcel rises high enough, cools 1200 sufficiently lifting condensation level 600 Above this point air 400 continues to rise but 200 now at SALR 0 SALR<DALR does not cool as rapidly Height (m SALR DALR Temperature ( o C) LCL

3 Achieving Saturation Mechanisms that lift air a. Orographic Uplift: b. Frontal wedging: uplift of due to air ρ differences c. Convergence: d. Localized convection: Stability and Cloud Type Different types of clouds form depending on the stability Unstable air tendency to rise May get Stable - air tendency to resist upward movement Air forcibly lifted by other means e.g. front, mountains Precipitation, if any, is

4 Cloud Classification Initial classification of clouds had four basic categories (Howard, 1803) 1. Cumulus heaped Clouds with vertical development 2. Stratus layered 3. Cirrus curl of hair High, thin wispy ice clouds 4. Nimbus rain Clouds 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 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)

5 Cloud Classification

6 Cloud Formation Requires condensation (gas liquid) Air must be saturated: cooled to T dew or 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 Cloud Formation Air saturated with water + CCN Condensation When condensation takes place Initial growth of cloud droplets very rapid Growth rate diminishes quickly because H 2 O 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 air

7 Precipitation 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 drops Precipitation Formation 1. Bergeron Process Occurs in All or part of cloud < 0 C i.e., Pure water suspended in air: Doesn't freeze until -40 C Below 0 C, it is liquid water

8 1. Bergeron Process Precipitation Formation 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 C Temperature <-20 o C -10 to -20 C 0 to -10 o C Phase of Water in Clouds Clouds generally composed entirely of ice crystals Liquid and ice crystals co-exist Clouds primarily supercooled H 2 O Precipitation Formation 1. Bergeron Process Growth of ice crystals in the Bergeron process, relies on differences in saturation vapor pressure: e s ice < e s water Ice crystals grow Requires a mix of both liquid water and ice i.e., clouds between 0 and -40 o C

9 1. Bergeron Process Precipitation Formation 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 Precipitation Formation 2. Collision Coalescence Process Occurs in 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 moisture

10 Precipitation Formation 2. Collision Coalescence Process If collector drop is much larger than drops below it, then collision efficiency will be low Precipitation Formation 2. 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 air

11 Forms of Precipitation Will precipitation be liquid or solid? Rain Liquid water formed in warm clouds (Collision- Coalescence process) Ice crystals from cold clouds (Bergeron process) that melt completely before reaching surface Snow Ice crystals formed by the Bergeron process that do not melt before reaching surface Form (plate/column/needle-like) depends on history: T, humidity Forms of Precipitation Sleet Small clear translucent particles of ice Begins as ice crystal (Bergeron process) Melts as it passes through warm layer of air Then re-freezes in the air above the surface

12 Forms of Precipitation Freezing Rain (glaze) Process and vertical T similar to that for sleet BUT subfreezing air near the ground is not thick enough to allow raindrops to freeze Rain becomes supercooled Freezes when collides with solid objects Forms of Precipitation Hail Concentric shells of different densities and degrees of opaqueness Form in cumulonimbus clouds Clouds with vertical extent & strong updrafts Abundant supply of supercooled H 2 0 Size depends on strength of updrafts length of path through the cloud

13 Fog Fog Air is Depth: ~1 meter to tens of meters Visibility < 1 km Formation different to cloud Method of formation is used to distinguish types a. Radiation Fog b. Advection Fog c. Upslope Fog d. Evaporation Fog Fog 1. Radiation Fog Cause: Forms in a shallow layer of moist air Surface cools rapidly via L Air above surface cools Air becomes saturated Fog forms Most likely to form: Valley bottoms cool, moist air drains down

14 Fog 2. Advection Fog Cause: Surface must be sufficiently cool to reduce the T to T dew Involves Surface H 2 0 near the coast is colder than surface water offshore Wind: Will not dissipate fog Brings in more moisture Common along coastlines of lakes or oceans e.g., San Francisco in summer 3. Upslope Fog: Cause: Fog Air gradually If sufficient cooling fog

15 4. Evaporation Fog: Cause: Fog Mix two air parcels together Air becomes saturated because saturation vapor pressure increases rapidly with T Examples Breath on a cold day Steam fog above a heated swimming pool Fog: Geographic Distribution in U.S.

Lecture 7a: Cloud Development and Forms Why Clouds Form?

Lecture 7a: Cloud Development and Forms Why Clouds Form? Clouds form when air rises and becomes saturated in response to adiabatic cooling. Why Clouds Form Cloud Types (from The Blue Planet ) Four Ways