A matter of scale

(1)

CLOUD PHYSICS

HANNA PAWŁOWSKA

Introduction

(2)

What is a cloud?

• Clouds are turbulent flows

• Clouds are multiphase flows

• Clouds are a collection of hydrometeors

• Clouds are a means to modulate electromagnetic radiation

• Clouds are a source of heat

• Clouds are a crucial

component to circulations

The GOES-13 satellite captured a "full-disk image" of North and South America in an image created December 30 2010 at 1445 UTC (9:45 a.m. EST), as the world

awaits the new year.

Credit: NOAA/NASA GOES Project

(3)

E. Bodenschatzet al. Science 2010; 327:970-971

A matter of scale

MIXING

P^ARTICLE T^RACKS

E^NTRAINMENT MICROPHYSICS

CLOUDS

W /C

100 ^µm 1 cm

1 m

100 m

1 km

(4)

Why are we interested in clouds?

Radiation

Microphysics Thermodynamics Dynamics

1. Clouds couple to the water cycle because they are the vessels in which precipitation develops.

2. Clouds couple to the radiative balance because they interact strongly with both short and long-wave radiation.

(5)

Radiation; Energy budget

(6)

Clear sky

Sea surface Shortwave Radiation

incoming shortwave radiation

most shortwave radiation absorbed by the surface

Longwave Radiation

longwave radiation

emitted by surface longwave radiation emitted by the atmosphere

longwave radiation emitted by the atmosphere longwave radiation

emitted by surface reflected

shortwave radiation

~10% of incoming shortwave radiation is reflected by the ocean surface

The atmosphere absorbs longwave radiation and re-emits longwave radiation

(7)

Cloudy sky

only 70% of shortwave radiation absorbed by the surface

~20% of incoming shortwave radiation is reflected by the clouds

clouds reflect

shortwave radiation

reflected shortwave radiation

longwave radiation

emitted by surface longwave radiation emitted by the atmosphere

longwave radiation emitted by the atmosphere longwave radiation

emitted by surface

The atmosphere absorbs longwave radiation and re-emits longwave radiation

(8)

Cloudy sky

~20% of incoming shortwave radiation is reflected by the clouds

longwave radiation

emitted by surface longwave radiation emitted by clouds

longwave radiation emitted by clouds longwave radiation

emitted by surface clouds reflect

shortwave radiation

reflected shortwave radiation

clouds absorb and emit longwave radiation

only 70% of shortwave radiation absorbed by the surface

the atmosphere absorbs and re-emits longwave radiation

(9)

Clouds may have a warming or cooling influence depending on their altitude, type, and when they form.

Clouds reflect sunlight back into space, which causes cooling.

They can also absorb heat that radiates from the Earth's surface, preventing it from freely escaping to space.

One of the biggest sources of uncertainty in computer models that predict future climate is how clouds influence the climate system and how their role might

change as the climate warms.

(10)

Low clouds – cooling effect

strong albedo

infrared emission comparable to the surface emission

cloud temperature comparable to the surface temperature

+

-

=

(11)

High clouds – warming effect

weak albedo

weak infrared emission

cloud temperature much lower than the surface temperature

=

-

(12)

The Earth’s energy balance

TOA

T_a

T_s

S* AS* S*(1-A)

A – planetary albedo

𝛼_a – absorptivity atmosphere

S*(1-A) - 𝛼_a S* B(T_s)

𝜏_a – transmissivity atmosphere 𝜏_aB(T_s)

B’(T_a) B’(T_a)

+

=

= -

(13)

Impact of clouds on the radiation balance

TOA

T_a

T_s

S*(1-A_s)

A_s – surface albedo 𝜏_a – transmissivity atmosphere [𝜏_aB(T_s)+B’(T_a)]

c – cloud amount A_c – cloud albedo

(1-c) c S*(1-A_c) c [𝜏_aB(T_c)+B’’(T_a)] (1-c)

(14)

Impact of clouds on the radiation balance at the top of the atmosphere

TOA

T_a

T_s

S*(1-A_s)

A_s – surface albedo 𝜏_a – transmissivity atmosphere [𝜏_aB(T_s)+B’(T_a)]

c – cloud amount A_c – cloud albedo

(1-c) c S*(1-A_c) c [𝜏_aB(T_c)+B’’(T_a)] (1-c)

F

_SW

F

_LW

F

^↓

= F

_SW^↓

− F

_LW^↑

c ∂F

^↓

≡ F

^↓

− F

^↓

( ) c = 0

Forcing:

(15)

Surface albedo

cloud albedo ➛ microphysics cloud amount ➛ dynamics

estimation of cloud radiative forcing requires:

(16)

Albedo

Albedo (latin: albedo, meaning 'whiteness') is the measure

of the diffuse reflection of solar

radiation out of the total solar radiation received by an astronomical body (e.g.

a planet like Earth).

It is dimensionless and measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation)

to 1 (corresponding to a body that reflects all incident radiation).

Cloud albedo

(17)

First estimates of mean cloud albedo were overestimated

Figure 1.6: The contribution of different components of the shortwave radiative fluxes to the total reflected radiation, as derived from early studies on the Earth energy budget and modern satellite observations. The white boxes indicate that most, but not all,

atmospheric reflection of solar radiation is associated with clouds.

(18)

Yet our models poorly reproduce

variability in e.g. the planetary albedo

ERBE

Earth Radiation Budget Experiment (NASA)

The mean geographical distribution of correlation between monthly mean albedo in ERBE and 20

(19)

Siebesma, A., Bony, S., Jakob, C., & Stevens, B. (Eds.). (2020).Cloudsand Climate: ClimateScience'sGreatestChallenge. Cambridge University Press.

HIGH

MID

LOW

altocumulus (Ac)

altostratus (As)

nimbostratus (Ns)

stratocumulus (Sc)

stratus (St) cirrus (Ci)

cirrocumulus (Cc)

cirrostratus (Cs)

cumulonimbus (Cb)

cumulus (Cu)

(20)

Siebesma, A., Bony, S., Jakob, C., & Stevens, B. (Eds.). (2020).Cloudsand Climate: ClimateScience'sGreatestChallenge. Cambridge University Press.

22 / 12

HIGH

MID

LOW

altocumulus (Ac)

altostratus (As)

nimbostratus (Ns)

stratocumulus (Sc)

stratus (St) cirrus (Ci)

cirrocumulus (Cc)

cirrostratus (Cs)

cumulonimbus (Cb)

cumulus (Cu)

26 / 28

17 / 34 9 / 19

C l ou d a m ou n t ( % ) ov e r lan d / oc e a n

(21)

(22)

Cloud Amount

Annual mean total cloud cover (%) averaged over 1983-2009. Data were obtained from the International Satellite Cloud Climatology Project (ISCCP).

(23)

Microphysics

A cloud is an aggregate of cloud droplets or ice crystals, or a combination of both, suspended in air.

For a cloud to be visible,

the cloud particles need to exist in a sufficiently large concentration.

(24)

A matter of scale

MIXING

P^ARTICLE T^RACKS

E^NTRAINMENT MICROPHYSICS

CLOUDS

W /C

100 ^µm 1 cm

1 m

100 m

1 km

MICROPHYSICS

(25)

Shapes and appearance of clouds are influenced by the cloud microstructure and its interaction with the radiation.

Further than just an aesthetic influence on clouds, the details of the microphysical properties of the cloud affect its future development and its interaction with the environment.

➛radiation

Interactions with radiation can lead to variations in absorption, emission and scattering of heat and light and, as a result, have far reaching consequences for global energy budgets.

➛dynamics

Thermal interactions through redistribution of heat and moisture can modify both the local behaviour of cloudsand have cumulative effects on large scale atmospheric circulations.

(26)

➛chemistry

Chemical interactions result in removal, generation and transformation of aerosols and gases.

➛electricity

Interaction with the electrosphere leads to the redistribution of electric charge and the dramatic discharge in the form of lightning strikes.

Each interaction is highly dependent on the shape, size, number and material phase of the collective particles within the cloud.

(27)

A link between microphysics and radiation

Clouds formed in clean air have low aerosol concentration, scatter less light and are less reflective.

Clouds formed in polluted air have with aerosol concentration, scatter more light and aremore reflective.

(28)

Ship tracks

This schematic illustration shows how aerosols from ship exhaust can result in a larger number of droplets in a given volume of cloud, but with smaller droplet sizes. Such changes can alter the onset of precipitation

(29)

Ship tracks

Visible ship tracks in the Northern Pacific, on March 4th 2009.

https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=3667

Over the Atlantic Ocean close to Spain and Portugal on 16 January 2018.

http://www.esa.int/spaceinimages/Images/2018/02/Atl antic_ship_tracks

(30)

Dynamics

(31)

The large-scale circulation

Clouds are not just a visual expressions of the state of the atmosphere, or a collection of small droplets that produce considerable rain, rather, they are a crucial component of the dynamics of the atmosphere as a whole.

(32)

The large-scale circulation

(33)

Key processes controlling boundary layer clouds

Large decks of stratocumulus are particularly interesting to studies aerosol effects on cloud albedo.

Radiative cooling at the top drives turbulence.

(34)

Stratocumulus

(35)

Clouds vs. Climate Change

(36)

(37)

Fig. SPM5 form Summary for Policymakers https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_SPM_FINAL.pdf

(38)