Investigation of the Aerosol Indirect Effect on Ice Cloudsand its Climatic Impact Using A-Train Satellite Data and a GCM

Yu Gu1, Jonathan H. Jiang2, Hui Su2, and K. N. Liou1

1Department of Atmospheric and Oceanic Sciences 2Jet Propulsion Laboratory, California Institute of Technology and Joint Institute for Regional Earth System Science and Engineering 4800 Oak Grove Drive, Pasadena, California, 91109 University of California, Los Angeles, Los Angeles, California

UCLA AGCM

Physical Parameterizations Planetary boundary layer processes: Suarez et al. (1983), Li et al. (1999, 2001) Cumulus Convection: Prognostic Arakawa-Schubert (Pan and Randall 1998), with downdrafts (Cheng and Arakawa 1997) Radiation: Harshvardhan et al. (1987) (Control run) Fu and Liou (1992, 1993), Gu et al. (2003) Prognostic Cloud Water/Ice: Kohler (1999) + Fractional clouds/Cloud overlap (Gu et al. 2003) Gravity Water Drag: Kim and Arakawa (1995)

Dynamics Horizontal Finite Difference Scheme: Arakawa and Lamb (1981) Resolution: 5 longitude x 4 latitude Vertical Finite Difference Scheme: Suarez and Arakawa (1983) Resolution (top at 1 hPa): 15 layers Time integration: Leapfrog, Matsuno

Surface Conditions Prescribed sea surface temperatures (Rayner et al. 1995), albedo, ground wetness, and surface roughness (Dorman and Sellars 1989)

Parameterizations for Ice Number

Summary

GCM Simulation Results

Parameters Polluted (AOT=0.5) Clean (AOT=0.2) Polluted-Clean Precipitation

(mm/day) 2.831 2.855 -0.024

OLR (W/m2) 212.839 213.775 -0.936 Flux at SFC (W/m2) 142.264 143.423 -1.159

Flux at TOA (W/m2)

207.332 208.313 -0.981

Cloud Cover (%) 63.470 63.213 0.257 SFC Air

Temperature (K) 281.440 281.746 -0.306

None of the De parameterizations accounted for the distinction

between “polluted” and “clean” clouds

A conventional approach has been to prescribe a mean effective ice crystal size in GCMs (e.g., Köhler 1999; Ho et al. 1998; Gu et al. 2003)Use IWC and/or temperature produced from GCMs to determine a mean effective ice crystal size (Kristjánsson et al. 2005; Gu and Liou 2006; Ou and Liou 1995; Ou et al. 1995; McFarquhar et al. 2003; Liou et al. 2008).

Model Description

Parameterizations for Re

Relate ice nucleation and number to aerosol concentration on the basis of explicit microphysics modeling, laboratory studies, as well as theoretical considerations (e.g., Diehl and Mitra 1998; karcher and Lohmann, 2002, 2003; Riemer et al. 2004; Liu and Penner 2005; Karcher et al. 2006)Mean effective ice crystal size calculated from ice mass and number for radiation calculations

large uncertainties in the parameterization of ice microphysics processes

and requirement of significant computational efforts

Empirical Re, IWC, and AOT Relation

Using least-squares fitting, we obtained an empirical formula for Re as a function of IWC and AOT. This function broadly captures the variation of Re with IWC and AOT.

Satellite Observations

=1.436, =0.5858, =0.282, =8.09, Re0=56.0293, IWC0=1.3838, =1.11

Precipitation

OLR

Cloud Cover Surface Solar Flux

Surface Air Temperature

Global July Mean

Aerosol First Indirect Effect

Change in cloud droplet/ice particle numbers associated with increase in aerosol number concentrations

Cloud particle mean effective size

Cloud radiative forcing

Use of A-Train Satellite DataInvestigation of the aerosol indirect effect on ice clouds have been limited due primarily to the lack of accurate global-scale observations.

New data from the NASA’s A-Train constellation makes it possible to examine the aerosol-cloud interaction in a more comprehensive way that can lead to improved physical understanding of this interaction.

The A-Train is a constellation of 6 satellites spaced a few minutes apart from each other and so their collective observations can be used to construct high-definition three dimensional images of the earth’s atmosphere and surface.

Polluted Clouds (Aerosol 1st Indirect Effect):Less OLRLess solar radiation reaching the surface More reflected solar radiation at TOAIncreased cloud coverReduced precipitationColder surface air temperature

GCM simulations show that the global averaged OLR and net solar flux at TOA are smaller in polluted case, illustrating more reflected solar flux and trapped OLR due to smaller De. Global radiative forcing produced by the aerosol 1st indirect effect is about 0.94 W/m2 for IR and -0.98 W/m2 for solar radiation. Surface solar flux is also reduced, resulting in colder surface air temperature.Reduced precipitation and increased cloud cover are found globally in response to the aerosol 1st indirect effect. Changes in the precipitation pattern show that, due to the aerosol 1 st indirect effect, reduced precipitation is found in the regions where polluted clouds mostly occur.

Left Fig: Satellite observations indicate Re increases with IWC but decreases with AOT Right: an empirical model derived from fitting of satellite data, as described below.

1AOT

AOT

1IWCIWC

RR

0

0e

e

β

γ

α

This empirical relationship of Re with IWC and AOT can serve as a first-order parameterization of the first indirect effect of aerosols on ice clouds for application to climate models.