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Clouds and Climate A. Pier Siebesma KNMI What are clouds? How do they form? Cloud climatology Clouds and Radiation Clouds in Climate models Cloud Climate.

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Presentation on theme: "Clouds and Climate A. Pier Siebesma KNMI What are clouds? How do they form? Cloud climatology Clouds and Radiation Clouds in Climate models Cloud Climate."— Presentation transcript:

1 Clouds and Climate A. Pier Siebesma KNMI What are clouds? How do they form? Cloud climatology Clouds and Radiation Clouds in Climate models Cloud Climate Feedback

2 klimaat cursus KNMI October What is a Cloud? “just” water ! ; 12:00 Amsterdam

3 klimaat cursus KNMI October How do you build a cloud ?

4 klimaat cursus KNMI October Water vapour is the building material for clouds nameSymbol Units DefinitionNear surface values Atmospheri c column spec. humidity q v [g/kg]amount of water vapour in 1kg dry air 10 g/kg20 kg/m2 Saturation spec. hum. q s [g/kg]Max. amount of water vapour in 1kg dry air 15 g/kg Liquid water q l [g/kg]amount of liquid water in 1kg dry air 1 g/kg100 g/m2

5 klimaat cursus KNMI October Saturation specific humidity q sat increases exponential with temperature (Clausius-Clayperon) Because of the presence of Cloud Condensation Nuclei (CCN’s) in the atmosphere condensation takes place if q v > q sat (p,T) Usually through cooling that results from rising motion. CCN’s are hygroscopic aerosols (salt, dust, etc)

6 klimaat cursus KNMI October Rising air cools with 1 K per 100m ……….. Until it becomes so cold that it starts to condensate… And a cloud is born!!!

7 klimaat cursus KNMI October What makes air to rise?

8 klimaat cursus KNMI October Orography Lenticularis above Mount Etna seen from Taormina, Sicily Italy.

9 klimaat cursus KNMI October Convection The sun heats the soil so that…….. Thermals are formed…. that rise because of buoyancy…. And a cloud forms as a wig on top of an invisible man :30 Amsterdam: cumulus humilis or “fair weather” cumulus

10 klimaat cursus KNMI October Condensational Heating allows cumulus to grow into a more mature stage: cumulus mediocris Humidity condensates into cloud water….. And produces latent heat Which serves as onboard fuel that allows the cloud to rise further….. With ~5 m/s…. Until the cloud is stopped by a temperature inversion Amsterdam: cumulus mediocris. 15:30 Wolken basis (~1km) Wolken top (~3 km)

11 klimaat cursus KNMI October Lifting condensation level (LCL) Level of free convection (LFC) Level of neutral buoyancy (LNB) “Level of zero kinetic energy” Mean profile height well mixed layer Inversion non-well mixed layer Poor man’s cloud model: adiabatic ascent

12 klimaat cursus KNMI October Mean profile height Horizontal Variability and Correlation But what if the cloud breaks through the inversion?????????

13 klimaat cursus KNMI October Then the cumulus can reach the stage of a socalled cumulonimbus With vertical velocities over 10m/s Up to a height of 5~15 km So that the water becomes ice which gives the fluffy appearance of the top of the cloud and strong precipation is on the way Amsterdam: cumulonimbus. Wolken top (5~8 km) ijs Moist Convection occurs all over the globe but is predominant in the tropics and over the subtropical oceans.

14 klimaat cursus KNMI October Convection in the (sub)tropics Hadley circulation }

15 klimaat cursus KNMI October Cartoon of Hadley Circulation Subsidence ~0.5 cm /s 10 m/s inversion Cloud base ~500m Tropopause 10km Stratocumulus Interaction with radiation Shallow Convective Clouds No precipitation Vertical turbulent transport No net latent heat production Fuel Supply Hadley Circulation Deep Convective Clouds Precipitation Vertical turbulent transport Net latent heat production Engine Hadley Circulation

16 klimaat cursus KNMI October Stratocumulus

17 klimaat cursus KNMI October Trade Wind Cumulus

18 klimaat cursus KNMI October Deep Convective Towers

19 klimaat cursus KNMI October Cirrus

20 klimaat cursus KNMI October Large Scale Lifting through fronts Occuring at mid-latitudes } }

21 klimaat cursus KNMI October Summary of all types

22 klimaat cursus KNMI October Global Distribution of Clouds Determined by: Main circulation Systems Availability of Moisture ISCCP Data set ISCCP globe63.1 NH59.3 SH68.7 Tropics61.2 Global cloud cover climatology ( ):

23 klimaat cursus KNMI October Why are clouds relevant for climate ? They strongly affect the radiative fluxes throughout the atmosphere

24 klimaat cursus KNMI October Radiative Effects of Clouds in the Climate System 2 main effects: Shortwave Reflection (cooling) “umbrella effect” Longwave Emission (warming) “blanket effect”

25 klimaat cursus KNMI October Shortwave Cloud Radiative Forcing (ERBE JJA87) SWCRF = S obs – S clear ; mainly by solid (BL) cloud decks cooling in subtropics and extratropics Global cooling effect: ~-48 W/m2

26 klimaat cursus KNMI October Longwave Cloud Radiative Forcing (TOA ERBE JJA87) LWCRF = L obs – L clear ; mainly by high clouds Heating in tropics and extratropics Global warming effect: ~31 W/m2

27 klimaat cursus KNMI October Total Cloud Radiative Forcing (ERBE JJA87) Total effect Overall cooling subtropics Concentrated in subtropics and extratropics Longwave and shortwave effects almost cancel in tropics (W/m 2)

28 klimaat cursus KNMI October Why are clouds relevant for climate ? They strongly affect the radiative fluxes throughout the atmosphere Condensation processes in clouds are a dominant source of energy and influence directly the large scale dynamics and cause precipitation.

29 klimaat cursus KNMI October Latent Heating by Convection

30 klimaat cursus KNMI October Why are clouds relevant for climate ? Clouds are a common atmospheric feature and cover large parts of the globe They strongly affect the radiative fluxes throughout the atmosphere Condensation processes in clouds are a dominant source of energy and influence directly the large scale dynamics and cause precipitation. Clouds connected to convective processes transport heat, moisture, momentum and other atmospheric constituents over large distances in the vertical (Remember the Hadley Circulation!!)

31 klimaat cursus KNMI October CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types

32 klimaat cursus KNMI October CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically)

33 klimaat cursus KNMI October km Virtually all cloud systems are not resolved by present-day “state of the art” global atmospheric models.

34 klimaat cursus KNMI October CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically) Clouds are the result of complex interactions of a large number of processes, e.g., Convection and turbulentie vertical motion microphysics

35 klimaat cursus KNMI October CLOUDS in GCM’s : What are the problems? There is a huge variety of cloud types Many of the observed clouds and especially the processes within them are of subgrid-scale size (both horizontally and vertically) Clouds are the result of complex interactions of a large number of processes, e.g., moist convection turbulence Large scale vertical motion Microphysics Many of these processes are still poorly understood The major cloud effect (radiative) is determined by many different cloud parameters.

36 klimaat cursus KNMI October Cloud-radiation interaction Cloud macrophysics Cloud microphysics Cloud fraction Cloud top and base height Amount of condensate In-cloud conden- sate distribution Phase of condensate Cloud particle size Cloud particle shape

37 klimaat cursus KNMI October Clouds in GCM’s: How? Main variables: Cloud fraction, a; Cloud condensate (cloud water and/or ice), l. Assume that cloud fills a model layer entirely in the vertical, hence cloud fraction refers to horizontal cover. Diagnostic approach Prognostic approach

38 klimaat cursus KNMI October Cloud-radiation interaction Cloud macrophysics Cloud microphysics Cloud fraction Cloud top and base height Amount of condensate In-cloud conden- sate distribution Phase of condensate Cloud particle size Cloud particle shape

39 klimaat cursus KNMI October Indirect Aerosol Effects Cloud 1 Cloud 2 Higher aerosol concentration => more CCN’s => smaller droplet sizes Consequences: 1)Enhanced Cloud albedo => cooling effect (Twomey Effect) 2)Reduced precipitation => longer lifetime clouds => cooling effect

40 klimaat cursus KNMI October IPCC

41 klimaat cursus KNMI October How well are GCM’s doing in the present climate? Global Cloud Cover

42 klimaat cursus KNMI October ECMWF Model climate - Cloud fraction JJA 1987 T63L31TCC Model - ISCCP Underestimation of Scu-decks ; Overestimation cc in ITCZ and Arctics

43 klimaat cursus KNMI October AMIP-intercomparison (10 years) OLR Cloud cover How well are GCM’s doing for present climate?

44 klimaat cursus KNMI October How well are GCM’s doing for future climate? G : direct radiative forcing due to CO2 doubling: 4 W/m2  CRF : change of the effects that clouds exert on the radiative fluxes of the top of the atmosphere Cloud feedback factor :  CRF / G : 0 no feedback >0 positive feedback <0 negative feedback

45 klimaat cursus KNMI October How well are GCM’s doing for future climate? Cess et al: 1990, 1996 Cloud climate feedback Is the largest uncertainty for future climate predictions!!!

46 klimaat cursus KNMI October Which clouds are responsible for this uncertainty? Bin clouds according to the mid-tropospheric vertical velocity Bony and Dufresne 2005

47 klimaat cursus KNMI October

48 klimaat cursus KNMI October Future Directions The new generation satellites will constrain: Water vapour content (with vertical resolution) Condensed water content (with vertical resolution Radiative properties Allows for the design of more accurate cloud and convection schemes and constrained radiation schemes in GCM’s and reduce the uncertainty in cloud representation in these models and hence cloud climate feedback!!!

49 klimaat cursus KNMI October Model Development Large Eddy Simulation Models + Observations Single Column Model version Implementation In full 3d model Methodology:

50 klimaat cursus KNMI October \TURK_ATrain_Movie_ avi

51 klimaat cursus KNMI October

52 Cloud albedo bias Plane parallel cloud Scu Cloud albedo Liquid water path (LWP) x x a a(LWP) < a(LWP)

53 klimaat cursus KNMI October : : : :00

54 klimaat cursus KNMI October /70’s Condensation (non- convective) Radiation effects Prescribed Albedo MicrophysicsNo 70/80’s a = f(RH) l prescribed No 80/90’s l prognostic a = f(RH) Simple Bulk microphysics ECMWF l prognostic a prognostic Complex Bulk microphysics Statistical Cloud Schemes a: cloud fraction l: cloud condensate A Brief History


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