Aerosol properties in a cloudy world (from MODIS and CALIOP) Alexander Marshak (GSFC) Bob Cahalan (GSFC), Tamas Varnai (UMBC), Guoyong Wen, Weidong Yang (USRA)

Is it a problem? Climate studies (e.g., aerosol indirect effect) demand a precise separation of clear and cloudy air; Remote sensing retrievals of aerosol properties near clouds are not problem free; Excluding aerosols retrieved near clouds underestimates aerosol radiative forcing while including them may overestimate the forcing. from MODIS: 60% of all clear sky pixels are located 5 km or less from all clouds from CALIPSO: 50% of all clear sky pixels are located 5 km or less from low clouds (e.g., Twohy et al., 2009)

What happens to aerosol in the vicinity of clouds? All observations show that aerosols seem to grow near clouds or (to be safer) “most satellite observations show a positive correlation between retrieved AOT and cloud cover”. However, it is not clear yet how much grows comes from “real” microphysics, e.g. increased hydroscopic aerosol particles, or other in-cloud processes. “artificial” effects, e.g. cloud contamination (sub-pixel clouds), extra illumination from clouds (a clear pixel in the vicinity of clouds) Both “artificial” effects may significantly overestimate AOT.

ASTER image over ARM SGP: MODIS retrievals ASTER image ~ 80 by 80 km Cloud Optical ThicknessAerosol Optical Thickness CERES SW fluxes Advanced Spaceborne Thermal Emission and Reflection Radiometer

Contributors to the enhanced reflectance of cloud-free areas near clouds Possible contributors: More/larger aerosol (e.g., swelling) Undetected cloud particles Instrumental limitations (e.g., latency) 3D radiative processes

Simple two-layer model to correct for cloud enhancement: ΔR R cor = R MODIS - ΔR( τ Rayleigh, F NB up ) Need this CERES provides this - RT model (τ, CF, r e ) - Input from CERES and MODIS - Correlated-k for BB

Application to aerosol retrievals over ARM SGP 0.47 µm AOT at 0.47 µm Corrected AOT at 0.47 µm

Application to aerosol retrievals over ARM SGP 0.66 µm AOT at 0.66 µm Corrected AOT at 0.66 µm

MODIS vs. CALIPSO MODIS: 3D effects present CALIPSO: no 3D effects However, both are not free from cloud contamination

CALIPSO Vertically integrated median backscatter at 532 nm Following Tackett and Di Giloramo (2009), we studied global night data over ocean Sep 15 – Oct 14, 2008 from Varnai and Marshak, RSL, 2011

CALIPSO Color Ratio from Varnai and Marshak, RSL, 2011 Wavelength Scattering cloud air molecules

Fraction of cloud-free vertical profiles Far from clouds (> 5km) CALIPSO (ColorRatio vs. Backscat close to and far from clouds) Global night data over ocean

Fraction of cloud-free vertical profiles Close to clouds (< 5km) CALIPSO (ColorRatio vs. Backscat close to and far from clouds) Global night data over ocean

CALIPSO Sep 15 – Oct 14, 2008; Global night data over ocean Increases occur below cloud top Cloud top

CALIPSO CAD (cloud–aerosol discrimination) - β 532 is always bigger near clouds; - β 532 decreases with the increase of confidence level; - the difference in β 532 between the two zones decreases with higher confidence level Higher confidence level all clear pixels away from clouds Yang et al. Atm. Res. 2011

CALIPSO Effect of the distance between clouds Normalized backscatter The transition zone is shorter for (a) for high confidence level data, and (b) relatively dry air 3 km 11 km 35 km

Three dust regions in North Atlantic Map of Dust Optical Depth. Based on the CALIPSO L2 oceanic night-time data from 06/07/2007 to 07/07/2007. Pixel resolution is 2 0 X 2 0.

Three dust regions in North Atlantic

Color ratio and backscatter are constant but depolarization ratio increases with altitude

Low and High Dust vs. Distance to Clouds Low dust increases near clouds while high dust is constant.

Synergy of MODIS and CALIPSO MODISCALIPSO

Synergy of MODIS and CALIPSO 61 km Wide Field Camera (WFC) Spectral range: nm IFOV: 125 m Swath: 61 km Joint histogram MODIS & WFC reflectances are very similar; some differences are due to clouds drifting with the wind MODIS/Aqua is 72 sec ahead of CALIPSO

Synergy of MODIS and CALIPSO 61 km Wide Field Camera (WFC) Spectral range: nm IFOV: 125 m Swath: 61 km CALIOP 532 nm backscatter integrated up to 3 km altitude using MODIS and CALIPSO cloud masks

Near Cloud Enhancement from MODIS and CALIPSO CALIPSO, nmMODIS, nm Comparison of median backscatter/reflectance within 5 km of and farther than 5 km from clouds

Summary Clouds are surrounded by a wide transition zone of enhanced particle size and light scattering. Transition zones need to be considered in studies of aerosols and aerosol-cloud interactions. In passive RS observations, the 3D radiative processes play an important role in creating clear sky reflectance enhancements near clouds and need to be accounted and corrected. Synergy of passive and active remote sensing helps to better interpretation and improving MODIS retrievals of aerosol properties near clouds.

Reflectance difference from values at 10 km for different COD at 0.47 & 2.13 m m from Varnai and Marshak, GRL, 2009 t cloud : t cloud : 7-13 t cloud : 2-7 t cloud : 0-2

Asymmetry in cloud-induced enhancement

from Varnai and Marshak, GRL, 2009 MODIS data 0.47 µm 0.86 µm R 3D (0.47 µm) R 3D -R 1D (0.47 µm) simulations from Frank EvansR 3D (0.47 µm)

Conceptual model to account for the cloud enhancement (at 0.47  m) aerosol or molecular Amazon clouds molecular (82%) + aerosol (15%) + surface (3%) MODIS sensor surface ARM clouds molecular (76%) + aerosol (8%) + surface (16%)

Assumption for a simple model Molecular scattering is the main source for the enhancement in the vicinity of clouds thus we retrieve larger AOT at shorter wavelengths We call it the “apparent bluing of aerosols near clouds”, i.e. enhanced retrievals of small size aerosols. Wavelength Scattering cloud air molecules

How to account for the 3D cloud effect on aerosols? The enhancement ΔR is defined as the diff. between the two radiances: one is reflected from a broken cloud field with the scattering Rayleigh layer above it and one is reflected from the same broken cloud field but with the Rayleigh layer having extinction but no scattering Broken cloud layer Rayleigh layer from Marshak et al., JGR, 2008

Three dust regions in North Atlantic