The Third Indirect Aerosol-Cloud Effect: Global Model Sensitivity and Restrictions Hans-F. Graf and Frank J. Nober EGS-AGU-ESF meeting Nice, April 2003

.... precipitation efficiency NASA VisibleEarth:

The Third Indirect Cloud Effect 1st: polluted (shallow) clouds are brighter 2nd: polluted (shallow) clouds live longer 3rd: polluted (deep) clouds have reduced rain rate, the vertical profile of latent heat release changes

Heating rates k/day in ECHAM4 Convective clouds Radiation Layered cloudsTurb. PBL heating

Cloud albedo Cloud opt depth Mixed cloud droplet + crystal concentr. + size precipi tation Aerosol mass, size, type Primar aerosol Precours gases CCNIN Warm cloud droplet + size distrib. Cold cloud crystal number + size distrib. Earth Radiation Budget Cloud fraction + life time Atmos. energy General circulation Hydrol. cycle

The third indirect effect of aerosols on clouds is a very complex and non-linear effect! Rain-suppression applies mainly to "warm" clouds (those not containing ice) If there is enough CAPE available (tropics) the air may rise and rain-production mechanisms involving ice will take over (however, ice formation also may be depressed) leading to intensified precipitation and lightning

Deep tropical convection (and latent heat released when rain is falling out) is the fuel for the general atmospheric circulation. Changes may immediately have global scale effects. So, there is a possibility that we have a direct influence of the microphysical processes in clouds on the global scale circulation. Again a model can help to test this hypothesis...

We use ECHAM4-T30 climate model with: Interactive S,OC, BC Prescribed GHG, SST, sea salt and mineral dust (Lohmann et al. 2000) Aerosol effect on cloud droplet number concentration parameterized for large scale clouds (Lohmann et al. 1999) These effects are included in a 15 year control run of the model, i.e. we actually use the experiment run of Lohmann et al. as our control.

In the original ECHAM model convective clouds form precipitation from the prognostic cloud water at a constant rate C if the depth of the cloud exceeds 150 hPa. We change this simplistic parameterization according to observations as: C e =0.25*C for CDNC > 750/cm 3 C e = 0 for CDNC > 1000/cm 3 There is no effect at temperatures below 263 K! I.e. freezing is not affected.

We use two kinds of analysis methods: 1.A single time step analysis, where we investigate the instantaneous effects of aerosols on cloud microphysics and energetics (production of CAPE or heating rates) without the full interaction with circulation. We call this the Primary Forcing. 2.A fully coupled model result at time scales of a month or season, where we include all feedbacks of the model system. We call this the Effective Forcing.

Aerosol effects in a single column of ECHAM4 At a single time step Cloud water enhanced Heating rate reduced Precipitation formation reduced control

# Aerosol # concentration, 900 hPa, * 10 3 Convective precipitation Single time step reduced released latent heat anomaly, mean over 10 days APRIL model results

DJF JJA Total convective precipitation (isolines) and anomalies (col.) in mm/month MAM SON

Velocity potential (200 – 900 hPa), contour lines control, shading absolute anomaly in experiment (m 2 /s*10 5 ) DJFMAM JJASON Negative values mean large scale rising motion (active branches) Extension of Walker cell to central Pacific Enhanced subsidence

Anomaly of geopotential 500 hPa in gpm, shading indicates local t-test 80 and 90% DJF MAM JJASON

So far convective clouds are treated very simplistic (mass flux scheme) in the ECHAM GCM: Only one „mean“ cloud Dynamic and microphysical details only rudimentary Vertical velocity and cloud coverage not directly available Therefore only the „potential“ Third Indirect Effect estimated!

New scheme including the interaction between different cloud types ( see Poster by Nober&Graf Today and Thursday ) improves the situation: Cloud spectra develop Detailed treatment of microphysics Dynamics (vertical velocities (Transport!!)) explicitely Cloud coverage explicitely

Precipitation Formation Process Berry (Continental Cloud) Berry (Intermediate Cloud) Kessler Berry (Maritime Cloud)

Mass flux [kg/m2/s]Heating rate [K/s] Height [m] Results for maritime case, intermediate pollution and continental case

Maritime Mean case Continental Cloud coverage conv. clouds New sc clouds

Conclusions Inclusion of the effect of aerosols on the formation of warm rain in deep convective clouds leads to: Significant instantaneous changes in latent heat forcing Significant changes in precipitation patterns, but not of the global mean precipitation Part of the reduced convective rain is filled up by large scale rain Shifts in the patterns of velocity potential and, therefore, global circulation New cloud types may develop under the influence of aerosols Must treat convection in GCMs more sophisticated, including microphysics...

The latent heat released by precipitation formed in deep convection in the tropics is the main fuel for the general circulation of the atmosphere. In the tropics also biomass burning is abundant – so we might expect some impact of the smoke on deep convective clouds, and via the latent heat release also on global atmospheric circulation. To test this hypothesis, we use a climate model, ECHAM4...

Single time step reduced released latent heat anomaly, Column mean over 10 days in W/m 2 January July April October

The number of cloud condensation nuclei at cloud base determines the number of cloud droplets that are formed. Q ccn =1/dt * max(N a * w/(w + aN a ); 0) w = w conv TKE If there are few CCN (like in maritime clouds) fewer droplets form which can easily grow to precipitation size, If there are lots of CCN (like in polluted air) many cloud droplets form which are competing for the existing water vapour. They will not grow fast enough to produce rain without freezing. Observations of Lahav and Rosenfeld (2000) show that coalescence effectivity decreases rapidly between cloud droplet concentrations of 700 and 800 per cm3. not in deep convective clouds. These processes have so far only been introduced in stratiform clouds in global climate models (e.g. Lohmann et al. 1999, Roeloffs et al. 1998), but not in deep convective clouds.

N cn ~ cm -3 Even deep clouds do not precipitateCloud droplets remain small

Cloud albedo Cloud opt depth Mixed cloud droplet + crystal concentr. + size precipi tation Aerosol mass, size, type Primar aerosol Precours gases CCNIN Warm cloud droplet + size distrib. Cold cloud crystal number + size distrib. Earth Radiation Budget Cloud fraction + life time

Even shallow clouds precipitateCloud droplets are big N cn ~500 cm -3 Even deep clouds do not precipitateCloud droplets remain small N cn ~ cm -3 Rosenfeld et al. Mult.

Polluted deep convective clouds may produce intense rainfall and lightning

Total precipitation in control (isolines) and exp. anomalies (col.) in mm/month DJFMAM JJASON