Using satellite-bourne instruments to diagnose the indirect effect A 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 EffectForcing (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 (Q sc ~ λ -4 ) x ~ 1 Mie scattering (Q sc complex) x>>1 Geometric scattering (Q sc  2) For most CCN 0.5<x<10 in visible spectrum

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 R eff and optical depth Observed reflectances compared to lookup table of the reflection function R(τ c,r e, θ 0,θ,φ) use to determine τ c and r e Error ∆τ c < 30% after Rayleigh scattering correction

Τ c <1 - transparent Τ c ~40 eg. Cumulus Τ c <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 1 st IE? 2 nd IE?

Estimated TOA Forcing W/m 2

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