View
215
Download
0
Tags:
Embed Size (px)
Citation preview
Using satellite-bourne instruments to diagnose the
indirect effectA review of the capabilities and
previous studies
What is the aerosol indirect effect?Definition:
The effect of aerosols upon the radiative balance of Earth via their interaction with clouds
Indirect Effect Forcing Estimates
Aerosol Indirect Effect Forcing (Wm-2)
Cloud brightening -0.5 to -1.9
Cloud lifetime effect -0.3 to -1.4
Cloud evaporation +0.1 to -0.5
Mixed-phase effects ?
Surface energy budget -1.8 to -4 (at surface)(Lohmann and Feichter, ACP 2005)
Role of Aerosol in Cloud Formation
• Aerosol of diameter D>0.02μm can act as cloud condensation nuclei (CCN)
• Most CCN have D<1 μm• Higher super-saturations
required to activate smaller CCN
• Aerosol hygroscopicity important for potential as CCN
Important for indirect effect
Detecting the indirect effect
• Need information on…– Aerosol at cloud base: number, size
distribution, composition– Cloud properties: droplet number, size
distribution, liquid water content (LWP)– Meteorological condtions: RH, updraft v
• Meteorological conditions less necessary if enough measurements taken
Aerosol Detection - Scattering
Size parameter: μ±ητ∆
For x<<1 Rayleigh scattering (Qsc ~ λ-4)
x ~ 1 Mie scattering (Qsc complex)
x>>1 Geometric scattering (Qsc 2)
For most CCN 0.5<x<10 in visible spectrum
D
x
Useful parameters
• Optical depth
• Albedo
• Effective radius
• Angstrom exponent
• Liquid water path
Pros and cons of remote sensing
• Pros– Global coverage– Long-term measurements
• Cons– Considerable post-processing of measurements– Less detail of aerosol and clouds than in-situ– Low measurement frequency per location (~days)
MODIS instrument
• General info
MODIS – Useful Products
• MODIS cloud products– Cloud fraction– Cloud top pressure– Optical depth– Liquid water content
• MODIS aerosol products– Optical depth– Fine-coarse mode fraction
MODIS Aerosol Retrieval
• Assumes bi-modal log-normal distribution
• Observed radiance compared to several modelled radiances
• Optical properties and relative ratio of modes, η, retrieved (fine/coarse ratio)
Important for indirect effect
MODIS Aerosol Retrieval
Limitations
• Sun glint on water source of error
• Retrieval over land has substantial error
• Relies upon cloud screening
• Assumed that all aerosol in a mode has same optical properties
MODIS Cloud Retrieval
• Both visible and near-IR bands used for determining Reff and optical depth
• Observed reflectances compared to lookup table of the reflection function R(τc,re, θ0,θ,φ) use to determine τc and re
• Error ∆τc< 30% after Rayleigh scattering correction
Τc<1 - transparent
Τc~40 eg. Cumulus
Τc<100 eg. cumulonimbus
MISR Instrument
Validation – MODIS Cloud
• Effective radius determined within ~3um for radii 5-15um
Validation – MODIS Aerosol
Over ocean… Over land…
∆τ = ±0.05 ±0.2 τ∆τ = ±0.03 ±0.05 τ
Validation – MISR Aerosol
Cross Comparison
Studies using satellite instruments
• Info on satellite instruments
Schwartz study
• AVHRR Cloud optical depth, LWP and Reff
• Modelled (sulphate) aerosol transport
Region of study
Nakajima Study
Kaufman Study
Less aerosol, less cloud
Increased drop size with less aerosol
1st IE?
2nd IE?
Estimated TOA Forcing
W/m2
Further Research
• Need improved aerosol data
• GLOMAP - Detailed aerosol information – can estimate CCN
• Analyse cloud properties w.r.t. CCN and composition
Further Research
• Diagnose how cloud drop number and LWP is affected by aerosol parameters
• Perform cross-comparison of GLOMAP and satellite-retrieved aerosol