Introduction to Cloud Dynamics

Introduction to Cloud Dynamics

Condensation in the Atmosphere The atmosphere contains a mixture of dry air and a variable amount of water vapor (0-4% or 0-30 g/kg) An air parcel is said to be saturated with liquid water when sufficient numbers of molecules exist so that in the presence of a plane surface of pure water at the air temperature, equal fluxes of molecules are escaping from the liquid surface as are entering the liquid surface from the air borne vapor. Generally we ignore the short period when air borne water

droplets are too small to be considered a plane surface of pure water and assume that if the air exceeds saturation, there is a net flux of droplets into the liquid phase, maintaining the vapor content at saturation or 100% relative humidity. Variation of Saturation with Temperature Tetens formula for computing saturation vapor pressure (over

plane surface of pure water) is: esl (T ) a(T 273.16) es =6.1078exp T b water

ice a 17.2693882 21.8745584 b 35.86 7.66 rsl =0.611 esl , p esl

rsl =0.611 esl p esl esi (T ) curved function

How does atmosphere form cloud? e > esl (T ) I. Adiabatic A. Increase e

a) Evaporation - can only increase RH to 100%! It CANNOT form a cloud by itself! dqliq dt B. (e es (T ))

Decrease T a) Expansional cooling b) Conduction with cold surface in parcel, ie ice hydrometeor

How does atmosphere form cloud? e > esl (T ) II. Diabatic A. Increase e

a) B. Evaporation - still cannot moisten a parcel because an adjacent parcel must have temperature only infintesimally different Decrease T a)

Radiational Cooling i. b) Tends to occur only at interface between: moist and dry layer between cloud and clear air

Conduction with cold surface i. c) Molecular diffusion of heat very inefficient, especially when diffusing a cool layer upward Parcel Mixing

i. ii. Due to curvature of saturation variation with temperature Mix two subsaturated parcels to achieve super saturation and the formation of cloud droplets Cloud formed by breath on cold day Mixing over a relatively warm lake

on a cold day Fogs Fog is a cloud in contact with the ground The reasons for fog formation mirror all the ways that saturation can be achiweved, i.e. Radiation (radiation fog, ground fog) Conduction (sea, advection fogs) Mixing is still involved

Mixing (steam fog, frontal fog, advection fog) Expansional cooling (upslope fog) Fog Types Advection Fogs Sea Fog advection of warm moist air over a cold sea surface leads to mixing of warm moist and conduction cooled air producing saturation and fog Advection of warm moist air over cold land surface leads to mixing of warm moist and conduction cooled air producing

saturation and fog (e.g. warm air advection over a snow cover) Land and sea breeze fog Tropical air fog Ice fog Snow fog Role of Dew Cooling of the surface causes moisture of the air in contact with the surface to be deposited as dew This causes a net downward transport of moisture

into the ground and the formation of a dew point inversion The dew point inversion may inhibit fog formation However, once the sun rises and the surface warms, the dew acts as a reservoir of water to allow fog to persist for several hours. Role of Droplet Settling Small liquid droplets settle very slowly Settling depletes liquid water content at top

of fog and increases it below This weakens radiative divergence at the top Hence low CCN contents produce more settling (larger droplets) and lower water contents Radiation Fog 1. Radiation cools the surface, surface air cools by conduction

2. Radiation divergence across top of moist layer cools the air above, destabilizing air above 3. Static instability at layer top causes turbulence to overturn air, mixing cold air from below, forming saturation 4. Once cloud layer forms, radiational cooling at top of fog layer is greatly enhanced, further increasing overturning and increasing fog water content Valley Fog

1. 2. 3. 4. 5. 6. 7. Nocturnal radiation cooling along side walls produces sinking motion along sidewalls

Dew deposition at the surface creates a dew point inversion at the surface Converging cold and somewhat dry air flows over the valley force upward motion and deepen the inversion About 3 h before fog formation, mountain wind forms, providing continuity for the downslope flow, but restricting upward motion in valley center. Cooling is then capped to low and mid levels of the valley by the strengthening inversion Radiation cooling at the top of the inversion layer leads

to the formation of a thin cloud layer The thin cloud layer enhances radiation divergence and deepens to the surface Marine Fog Differs from Radiation fog:

Radiation does not rapidly affect surface temperature Less CCN- more drizzle (Giant salt nuclei) Moisture flux up Heat flux down Results of model experiments show: Case 1: upward moisture flux, downward heat flux , ie cold water/warm air promotes fog Case 2: upward heat and moisture flux, ie fog if air

above is cold and moist Fog Produced By Marine Stratus Lowering Radiational cooling lowers base of stratus cloud: Fog Streets Marine Stratocumulus

Exist over Large spans of the eastern Pacific, eastern Atlantic and western Indian Oceans These are upwelling regions of cool water so air naturally near saturation in marine PBL These are also regions of large scale subsidence aloft Dynamics of Marine

Stratocumulus Subsidence Drying Aloft Moistening from the cool ocean surface Radiation divergence at top of marine PBL Entrainment of low theta-e but high theta

air from above PBL inversion Other Factors: Drizzle: Weakens radiation divergence Shear: enhances entrainment

Recently Viewed Presentations

  • GVHD LUNGS - UC San Diego School of Medicine

    GVHD LUNGS - UC San Diego School of Medicine

    Oral lesions budesonide topical Crackles- chest CT: mosaic perfusion Flovent 44 2 puffs BID Cyclosporine 2-4 years after 2nd transplant ( Patient is 4-6 years of age) Asymptomatic PFT FVC 94 pre FEV1 68 post FEV1 74 FEV1/FVC 62 TLC...
  • Hospitality Classifications - Sacramento State

    Hospitality Classifications - Sacramento State

    Hospitality Classifications According to the AH&LA (American Hotel and Lodging Association - formerly the AHMA) there are several, not necessarily exclusive, categories of hotel and lodging property types.
  • Voting systems in elections - Sociology - Home

    Voting systems in elections - Sociology - Home

    Single transferable vote (STV) system. Advantages. Tends to produce proportional representation better than most other systems. Allows voters to vote for more than one candidate eg may not want to cast all votes for one party. Disadvantages. Complex system to...
  • Présentation PowerPoint

    Présentation PowerPoint

    Promotion des équipements solaires Synthèse de l'étude de faisabilité
  • Day 12 - Ch. 5 - Mercury and Venus

    Day 12 - Ch. 5 - Mercury and Venus

    Mercury has no atmosphere, just like our moon. Because there is no atmosphere to trap heat, the night side of Mercury gets very cold (100 K or -280oF). The day side of Mercury gets up to 700 K (or 800oF)....
  • Impact Evaluation Measuring Impact: Click to edit Master

    Impact Evaluation Measuring Impact: Click to edit Master

    Note: slides by Sebastian Martinez, Christel Vermeersch and Paul Gertler. The content of this presentation reflects the views of the authors and not necessarily those of the World Bank.
  • Les médecins inspecteurs de santé publique

    Les médecins inspecteurs de santé publique

    Les médecins inspecteurs de santé publique Tentative d '(auto) portrait question parfois embarrassante: Maman c'est quoi un MISP ? les Médecins Inspecteurs de Santé Publique forment un corps de fonctionnaires de l'Etat Statut - missions recrutement - formation - avancement...
  • November 9 and 10 - West Ada School District

    November 9 and 10 - West Ada School District

    Two individuals with the same race or ethnicity also have many differences in their DNA—so many differences that a tall Caucasian could have more DNA in common with a tall Asian than a short Caucasian! ... "We see sexist jokes,...