boundary layer clouds. intertropiccal convergence zone (itcz) trade cumulus transition stratus and...
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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.
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