Cloud lifetime effect

Lifetime hypotheses encapsulate three basic ideas

1. The number concentration of cloud droplets (Nd) increases with anthropogenic aerosol burden : Na ↑ Nd ↑ : basis for the albedo effect

less precipitation = cloud live longer, more radiative effect

2. The precipitation efficiency of shallow clouds decreases monotonically as a function of the concentration of cloud droplets

3. Cloud albedo is a strictly decreasing function of precipitation efficiency : provides the essential link to the radiative forcing of the climate system

No observation of cloud life time cycles

- low-albedo,- associated with regions of precipitation

- high-albedo,- is usually observed to be non-precipitating

Cloud droplets are on average smaller in the presence of more aerosol particles

Cloud fraction also tends to be larger, on average, in the presence of more aerosol particles.

As aerosol increases,

(1)precipitation decreases (2)precipitation increases.

Correlations between cloud amount and aerosol optical thickness as an evidence in support of a lifetime effect

1. Aerosol-cloud correlations (Measurement) : artefacts not reliable

2. can be explained by alternative ideas.

1. Artefact argument : Measurement artefacts that affects to correlation between aerosol optical depth and cloud amount

(1) Distinguish between cloud and aerosol

(2) Subvisible cloud (twilight zone)

(3) 3D radiative effects

Deficiencies in the data record• poor vertical resolution : to assess whether or not cloud and aerosol layers are intermingled (vertical location of aerosol layers w.r.t. cloud layer)

• Temporal resolution : causality

• Missing measurements of key variables (Humidity, precipitation)

Polar-orbiting satellitesHumidity

*RICO : aerosol, RH, cloud amount : positively correlated, rain more in high aerosol conditions (low level wind is stronger)

Effect of meteorology (cloud problem)

1. Because the meteorology and aerosol burden depend on the air-mass history 37

1. Because the meteorology and aerosol burden depend on the air-mass history 37

Effect of meteorology (cloud problem)

1. Because the meteorology and aerosol burden depend on the air-mass history 37

2. Because radiative effects of the aerosol can change the meteorological properties of the air mass where the aerosol is embedded

3. Because cloud processes exert a strong control on the aerosol38,39

(e.g., African dust? Warm and dry layer)

3. Because cloud processes exert a strong control on the aerosol38,39

3. Because cloud processes exert a strong control on the aerosol38,39

Indirect effect : 0 to -4 W/m2

• Lifetime effects alone : -0.3 ~ -1.3 W/m2

• Weaker cloud-mediated aerosol effects : Total aerosol forcing : -1.2 W/m2 (observation)

A buffered system

A change in one process in a complex system to be compensated for by the response of another is often described as a negative feedback on the system scale.

However, the concept of feedback implies that the output of a system modifies the input.

Because this is not the case in many of the examples we cite, we prefer to speak of bufferingbuffering, wherein different paths to a specific end buffer the system against disruptions to any particular path.

The response of a system to a forcing is weaker than would have been expected has internal mechanism-which absorb the impact of the forcing, and hence buffer the system

A buffered system

The effect of the anthropogenic aerosol on clouds and precipitation often proves difficult to establish.

WHY?

On the scale of individual cloud droplets (microphysical scale),

buffering is expected to reduce the strength of the coupling between the aerosol and cloud microstructure on one hand, and

between cloud micro- and macrostructure on the other.

e.g., 1) Activation of aerosol particles

3)Turbulence in the cloud

2) Precipitation susceptibility

modeling studies of cumulus-like drafts, which emphasize the importance of accretion process (that is, the collection of cloud droplets by precipitation embryo)(60), also result in weaker susceptibilities (beta <= 0.5) (61).

Recent work also suggests that the precipitation susceptibility of shallow clouds, β ( which provides an indication of the sensitivity of rain to changes in the cloud droplet number concentration), is less than predicted by simple scaling arguments.

• parameterization in large-scale models : beta=2 and imply a strong sensitivity of rain to droplet number (54).

• In the stratocumulus, observations suggest that beta=1 and, therefore, that the sensitivity of rain to droplet number is weaker (55-57).

Even this value may be an overestimate, as such measurements fail to account for the dependence of the aerosol on the precipitation, through the precipitation scavenging or washout of the cloud-active aerosol(58)

Recent analyses of satellite-based cloud radar (NASA’s CloudSat mission) data : weaker susceptibilities (0.1 < beta < 1.1) (59)

The inertial clustering of cloud droplets, which is associated with intense turbulence accelerations within clouds, may have a role in setting the rate of rain formation in warm clouds (62). Because the turbulence intensity depends on the cloud macrostructure , such processes further buffer the system against changes in cloud microstructure due to the changing aerosol.

the flux of liquid water through the cloud layer is critical to the growth rate of the layer(63-66) and, accounting for this process, can produce unexpected results (67)

For cumulus clouds: Less precipitation means more liquid is lofted to the cloud-top region, where it evaporates. The associated cooling destabilizes the environment, helpful to growth of deeper clouds (Fig.4)

Na ↑, smaller Nd ↑, less precipitation

Precipitation with aerosolPrecipitation with aerosol

Precipitation with aerosolPrecipitation with aerosol

However, deeper clouds produce more rain, which can more than compensate for the initial suppression of precipitation (61,66-stevens’ paper). In such cases, increases in the aerosol can be expected to produce more rain.

However, for ice-bearing systems the complexity of microphysical processes leads to the possibility of a much richer set of behaviors (69-72).

Variants of this idea have also been shown to invigorate convection and precipitation in ice clouds (by making more lofted condensate available for freezing)

↑ Precipitating cloud

Precipitation with aerosolPrecipitation with aerosol

Cloudness with aerosol: (Weak precipitation)

Fine-scale simulations that resolve the turbulent dynamics of stratocumulus show

shallow cumulus :Na↑, D ↓ increase rate of droplet evaporation reduces cloudness.

that the evaporation of drizzle just below the cloud base can destabilize the sub-cloud layer and enhance the mixing of moisture into the cloud, thus increasing cloudiness

precipitation may enhance cloudness

Cloudness with precipitation

For weakly precipitating clouds, larger droplets have more thermal inertia and thus linger in sub saturated environments, leading to effectively more cloud in the presence of precipitation (61,82)

the longevity of cloud systems is closely tied to the development of precipitation and the cold pools it produces (80).

RICO : cloud activity organized along the outflow of cold air

associated with showers from previous showers from previous convectionconvection, which suggests that precipitation creates clouds (32,81)

I need a fig!!

Radiative processes

e.g.: absorbing aerosol

Aerosol layer coexist with cloud layer.Could may be suppressed by radiative heating (stabilization) (83,84).

an increase in aerosol amount may reduce cloudness, hence reinforcing the concept of a well-buffered system

cool the surface, further stabilizing the layer and reducing cloudiness (85).

dry out?

Will not be buffered for non-absorbing aerosol (not from the paper)

Rather, our argument is that the sensitivity of clouds and precipitation to changes in the aerosol is weaker than implied by simple arguments and is regime dependent or state dependent.

This is not to say that lifetime effects are entirely without merit.

Pockets of open cells (Fig 2) provide dramatic support for the conventional wisdom that precipitation reduces cloud amount (13).

This capacity of the system to respond differentially to change in the aerosol buffers the global system, by endowing it with the ability to offset positive responses within some regimes with negative responses in others

1. The possibility of significant, but regime- (or regionally) specific (86,87) response is one reason to intensify search efforts-even if the diversity of responses and regimes makes it likely that lifetime effects are negligible (or at least not discernible given current approaches) on global scales.

2. Another compelling reason for intensifying our effects to understand lifetime effects is because doing so can help solve the cloud problem, namely that of relating the statistics of cloud field to their meteorological environment.

Because clouds and precipitation and the effect that the aerosol has on them are almost certainly regime dependent,

The most fruitful approach is to work through the issues regime by regime.

future work must identify how the trace of such interactions depends on the state of the system, and improve the representation of such cloud regimes in models.

Given our present understanding, certain cloud regimes can already be identified as worthy of closer scrutiny.

because of its role in the hydrological cycle and because a pronounced susceptibility of rainfall to aerosol infusions.

shallow maritime clouds (including trade wind cumulus and stratocumulus),

tropical deep convection over land and

stratiform cloud regimes in the polar regions.

These include

Trade-wind cumulus are critical because they prevail over the global ocean

Stratocumulus are important because they carry a pronounced signature of changing aerosol, and thus could regulate the planetary albedo.

because they may mediate interactions over a part of the earth system that appears to be unusually sensitive to the changing climate.

over ocean??

1. It offers methodological advantages.

- well suited to fine-scale models, which are capable of resolving (rather than parameterizing) the many of interactions ranging from the cloud microscale to the cloud macroscale.

Advantage of regime-centered studies

2. It makes more straightforward to augment and enrich current and future space-based measurements.

Advantage of regime-centered studies

3. Offers the opportunity to take advantage of novel experimental strategies.

Absorbing aerosol

(1) Activation of aerosol particles(2) Precipitation susceptibility (3) 3)Turbulence in the cloud

Precipitation with aerosolPrecipitation with aerosol

Synoptic conditions by sc

Photographs of the collapse of clouds in shafts of precipitation that readily form in aerosol-poor environments are common

Apparent transition from low-albedo shallow-cumulus cloud regimes to high-albedo stratocumulus cloud regimes as a result of aerosol infusions from passing ships.

5 minutes

Photos from Barbados; Courtesy Joe Prospero

Observations of the indication of a lifetime effect

Cloud droplets are on average smaller in the presence of more aerosol particles

Cloud fraction also tends to be larger, on average, in the presence of more aerosol particles.

No observation of cloud life time cycles

Indirect effect : 0 to -4 W/m2

Lifetime effects alone : -0.3 ~ -1.3 W/m2 3.

• Weaker cloud-mediated aerosol effects : Total aerosol forcing : -1.2 W/m2 (observation)