Boundary Layer Clouds. Intertropiccal Convergence Zone (ITCZ) Trade cumulus Transition Stratus and...

Boundary Layer Clouds

Intertropiccal Convergence Zone (ITCZ)

Trade cumulus

Transition

Stratus and stratocumulus

subsidence

Trade wind inversion

St & Sc

St &

Sc

SGP Low cloud coverage (ceilometer& MPL): 27.8% (Lazarus et al. 2000)

Cooling effect Warming effect

NASA: The Earth Radiation Budget Experiment (ERBE)

It measures the energy budget at the top of the atmosphere.

Energy budget at the top of atmosphere (TOA)

Fictitiousclimate system

Incoming solar radiation 340 W/m2

Reflected SW radiation Q1= 50 W/m2

Emitted LW radiation F1= 270 W/m2

shortwave cloud forcingdQ=Q1-Q=-50 W/m2 (cooling)

longwave cloud forcingdF=F1-F=30 W/m2 (warming)

Present climate system

Incoming solar radiation 340 W/m2

Reflected SW radiation Q= 100 W/m2

Emitted LW radiation F= 240 W/m2No clouds with clouds

SW cloud forcing = clear-sky SW radiation – full-sky SW radiation

LW cloud forcing = clear-sky LW radiation – full-sky LW radiation

Net cloud forcing (CRF) = SW cloud forcing + LW cloud forcing

Current climate: CRF = -20 W/m2 (cooling)

Direct radiative forcing due to doubled CO2, G = 4 W/m2

feedback cloud negative 0

feedback cloud zero 0 G

CRF

feedback cloud positive 0

But this does not mean clouds will damp global warming! The impact of clouds on global warming depends on how the net cloud forcing changes as climate changes.

Cloud radiative effects depend on cloud distribution, height, and optical properties.

gT

cT

cg TT

cT aT

ac TT

Low cloud High cloud

SW cloud forcing dominates LW cloud forcing dominates

2 W/m4 (-20) - 16- CRF

e.g. If the net cloud forcing changes from -20 W/m2 to -16 W/m2 due to doubling CO2, the change of net cloud forcing will add to the direct CO2 forcing. The global warming will be amplified by a fact of 2.

In GCMs, clouds are not resolved and have to be parameterized empirically in terms of resolved variables.

water vapor (WV) cloud surface albedo lapse rate (LR) WV+LR ALL

Radiation

Turbulence

Microphysics

Surface Processes

LS Forcing

Issues

Cloud evolution and maintenance.

Cloudiness .

Radiative and microphysical properties.

Cloud entraining processes and cloudmass transport. Cloud mesoscale organizations.

Cloud-aerosol-drizzle interactions

Mesoscale cellularconvection (MCC)

Pockets ofopen cell (POCS)

Variations of MCCs and POCs are much larger than the individual variations within the structures (Jensen et al. 2008)

Aerosol feedback

Direct aerosol effect: scattering, reflecting, and absorbing solar radiation by particles.

Primary indirect aerosol effect (Primary Twomey effect): cloud reflectivity is enhanced due to the increased concentrations of cloud droplets caused by anthropogenic cloud condensation nuclei (CNN).

Secondary indirect aerosol effect (Second Twomey effect):

1. Greater concentrations of smaller droplets in polluted clouds reduce cloud precipitation efficiency by restricting coalescence and result in increased cloud cover, thicknesses, and lifetime.

2. Changed precipitation pattern could further affect CCN distribution and the coupling between diabatic processes and cloud dynamics.

GCM/

NWP

CRMS

LES OBS

PAR

Parameterization Development and Testing Strategy

Hi-Res simulation s and 3-D Observations

Traditional LES: idealized initial profiles and prescribed horizontalhomogeneous large-scale forcings.

Representativeness of clean cloud cases?

Clouds

Liquid water mixing ratio airdrymass

liquidmasswl

Liquid water density of clouds

airdryofvolume

liquidmassl

airll w

Cloud droplet distribution

Number density N (D):the number of droplets per nit volume (concentration) in an interval D + ΔD

36

Ddropletaofmass l

36

)( iiil DDNL Liquid water content

Variables that are useful for cloud research

tls rrrr ,,,Mixing ratio, saturated mixing ratio, liquid water mixing ratio, total mixing ratio

)608.01( lv rr

rTC

Le

p

v

lTCL

l rp

sTCL

es rp

v

Equivalent potential temperature

Liquid water potential temperature

Saturated quivalent potential temperature

Instrumentation

Latest version W-band (95 GHz)cloud radar

Millimeter Wave Cloud Radar (35 GHz)

Vaisala Ceilometer

X-band scanning ARM precipitation radar

Mechanisms of maintaining cloud-topped boundary layer

1. Surface forcing

2. Cloud top radiative cooling

3. Cloud top evaporative cooling

Cloud parameterization

1. Cloud fraction parameterization

cc

ecc

ecccu

)1(

),ww)(1(M

,)(MwF

zM

M1

)(z

model plume Entraining

c

c

cc

1. How to close the system?2. How to determine entrainment and detrainment rates?

Shallow cumulus parameterization: Mass-flux approach

Stratocumulus parameterization

Cloud top entrainment parameterization

Eddy viscosity

A1, A2: empirical coefficients. V: turbulent velocity scale. ΔF: cloud-top radiative flux divergence. ΔB: buoyance jump across the inversion.