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