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Clouds and rain formation. Global water (hydrological) cycle Water Vapor Basics (names of different phase changes, latent heat) Two methods of achieving.

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Presentation on theme: "Clouds and rain formation. Global water (hydrological) cycle Water Vapor Basics (names of different phase changes, latent heat) Two methods of achieving."— Presentation transcript:

1 Clouds and rain formation

2 Global water (hydrological) cycle Water Vapor Basics (names of different phase changes, latent heat) Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds. Review of last lecture

3 The most common atmospheric circulation structure L H H L Heating Cooling or No Heating Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind Radiation Convection Latent/Sensible Conduction

4 Satellite observation of clouds NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites Now, cloud top height and optical depth. NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

5 Clouds Clouds consist of many tiny water droplets (~10 μm) and/or ice particles Most clouds form as air parcels are lifted and cooled to saturation Clouds have a wide range of height and thickness Clouds are instrumental to the Earth’s energy and moisture balances Level of saturation/condensation

6 1) rate of cooling of parcel (unsaturated v. saturated) unsaturated – dry adiabatic lapse rate (DALR) saturated – saturated adiabatic lapse rate (SALR) 2) rate of cooling of surrounding atmosphere environmental lapse rate (ELR): an overall decrease in air temperature with height ELR is measured by weather balloon. Long-term global average ELR = 0.65 o C/100m. Three possible outcomes: 1)absolutely unstable air 2)absolutely stable air 3)conditionally unstable air Static Stability and the Environmental Lapse Rate Static stability – refers to atmosphere’s susceptibility to being displaced Stability related to buoyancy  function of temperature The rate of cooling of a parcel relative to its surrounds determines its ‘stability’ of a parcel

7 Environment Parcel Environment Parcel Environment Parcel The three types of stability Absolutely Unstable Conditionally Unstable Absolutely Stable

8 1) Entrainment Turbulent mixing of ambient air into parcel Leads to evaporation along cloud boundaries Evaporation uses latent heat, cooling the cloud  reduces buoyancy Courtesy Russ Dickerson, U. Maryland What stops ‘unstable’ air masses from rising indefinitely ? 2) Encountering a layer of stable air (inversion) a rising parcel may reach a stable upper air environment the parcel cooling rate will exceed that of the ambient air the parcel will slowly cease ascension and come to rest at some equal temperature level three types: radiation, frontal, subsidence

9 1. Cloud top height/pressure 2. Cloud thickness (optical depth) 3. Cloud coverage When clouds comprise more than 9/10th of the sky = overcast When coverage is between 6/10th and 9/10th = broken When coverage is between 1/10th and 6/10th = scattered Cloud coverage less than 1/10th = clear Cloud Properties

10 NASA’s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate research

11 Clouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect). The net effect (cooling or warming) depends on the type of cloud In a changing climate, increases in high thin clouds would promote warming, while increases in low thick clouds would cause cooling Climate models have difficulties in simulating clouds, especially low thick clouds (stratocumulus) Conclusion: Clouds cause the largest uncertainty in model simulations of future climate. Why do clouds constitute a wildcard for climate change? Stronger warming effect Stronger cooling effect

12 Video: The climate wild card FKuJJU FKuJJU

13 Global distribution of precipitation

14 Precipitation formation - cloud drop growth Not all clouds precipitate due to their small sizes and slow fall rates Balance between gravity and frictional drag  eventually become equal to achieve terminal velocity V T, which is proportional to the square root of cloud drop radius V T =c r 0.5,where r is drop radius and c is a constant. For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up. Cloud drop growth is required for precipitation to form F gravity F drag

15 1.Collision Coalescence (warm clouds, T > 0 C, form rain) 2.Bergeron Process (cool/cold clouds, T < 0 C, form snow) Mechanisms for cloud drops to grow larger Cold CloudsCool Clouds

16 Process begins with larger collector drops which have higher terminal velocities Collector drops collide with smaller drops and merge with them (coalesce). Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining. If collector drop is too big: compressed air beneath falling drop forces small drops aside If collector drop is too small (same size as other drops) it will fall at same speed and no collision will occur So, collection efficiency is greatest when the size of collector drop is slightly larger than the size of the other drops After the collector drops become large, the larger one among them can serve as a “super- collector” to collide with other collector drops 1. Collision Coalescence: Growth in Warm Clouds

17 Determined by competition between surface tension and frictional drag. Frictional drag is larger at the bottom than at the top Small drop (<0.08in): frictional drag << surface tension  Sphere shape Medium-size drop (0.08in0.25in): frictional drag at bottom > surface tension  Split (The surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.) Maximum drop size of about 0.25in or 5 mm Raindrop shape and maximum size

18 Summary Formation of clouds: 3 types of stability. Two factors limiting the height of clouds. 3 cloud properties. 9 ISCCP cloud types. Why do clouds constitute a wildcard for climate change? Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius? Growth mechanisms for rain and snow Formation of rain: coalescence process (the collector is larger than the cloud droplets but not too large)

19 Works cited cloud-computing-is-run-on-actual-clouds/ cloud-computing-is-run-on-actual-clouds/ hotos.htm hotos.htm

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