# Presentation: 657505

## Aerosol effects on clouds, precipitation, and the organization of shallow cumulus convection

- Graham Feingold1
- With contributions from:
- Hongli Jiang1, Huiwen Xue1,2 Paquita Zuidema4, Bjorn Stevens3** **Amit Teller5, Zev Levin5
- 1NOAA Earth System Research Laboratory Boulder, Colorado
- 2Beijing University, P. R. China
- 3UCLA
- 4Univ of Miami
- 5Tel Aviv University, Israel
- 	Yoram Kaufman Workshop, May 30 – June 1 2007

## Outline

- Survey of some remote sensing observations of aerosol and clouds
- Conflicting results
- Precipitation in warm boundary layer clouds
- Aerosol-cloud-precipitation-_dynamical_ feedbacks
-  Cloud lifetimes
- Do enhanced aerosol concentrations increase cloud lifetime?
- Cloud Albedo
- Does the cloud brightening effect dominate in a dynamically evolving cloud field?

## 	More aerosol  more CCN  more drops  suppressed coalescence less rain  larger LWP, cloud fraction  longer lifetime

**The “Second Aerosol Indirect Effect”**

*A construct that needs to be revisited*

**Notes:**
Note that Albrecht 1989 discussed aerosol/precip effects on cloud fraction with only casual reference (once or twice) to lifetime. And yet curiously his paper is often referred to as the “lifetime effect”.

## Cloud Fraction increases with increasing aerosol

- (Kaufman et al., PNAS, 2005)
- Cloud Fractions: ~ 20% to 70%

- Increasing
- Aerosol Optical
- Thickness (AOT)

*1. Observations of Aerosol Effects on Cloud Fraction*

## Observations of Aerosol and cloud LWP, re, Albedo

**LWP **_**decreases**_** with aerosol**

**Albedo ~ constant**
- (competing effects)

**Drop size decreases with aerosol**

**Global low cloud satellite **
**measurements and reanalysis**

**Observations of Aerosol and cloud LWP, r****e****, Albedo**

**Notes:**
Albedo more dependent on LTS; high LTS is more stable – e.g. SCu regimes
Stress the importance of dynamics

## 	Correlation      Cause-and-effect

	Can we use models to understand the mechanisms

*	**Can we use models to understand the mechanisms*

**Notes:**
Note that Albrecht 1989 discussed aerosol/precip effects on cloud fraction with only casual reference (once or twice) to lifetime. And yet curiously his paper is often referred to as the “lifetime effect”.

## Modeling of Aerosol effects on stratocumulus in LES

**Wang, Wang, Feingold, 2003**

- LWP decreases
- with increasing aerosol:
*Evaporation-entrainment*
*feedback*

- Bulk microphysics

**LWC**

**Modeling of Aerosol effects on stratocumulus in LES**

**height**

**Also, Xue and Feingold 2006**
**for cumulus**

## Evaporation rates

**Evaporation rates**

- Timescale for condensation/evaporation:

- Small droplets evaporate faster than large ones

## Humidity of air above stratocumulus is important

- Ackerman et al. 2004

- >0
- <0

- ?

- LWP = liquid water path

## 2. Precipitation in warm boundary layer clouds:

*Aerosol-cloud-precipitation-dynamical feedbacks*

## Drizzling patches: open cells 

- Drizzling patches: open cells
- (POCS; pockets of open cells)

- Non drizzling closed cells

- E.g., Stevens et al. 2005

## Large Eddy Simulations

- Atlantic Trade Wind Experiment (ATEX, 1969) sounding
- Cumulus penetrating into stratus
- GCSS intercomparison case

## Large Eddy Simulation (LES) Bin-microphysics: activation, condensation, collision-coalescence,                             and sedimentation Fixed surface fluxes Radiation: simple function of LWP                       Domain size:  12.4 km  12.4 km  3.0 km Grid size:       x = y = 100 m, z = 20 m Time step :     t = 1 sRange of Aerosol inputs: clean (25/cc) to extremely polluted (2000/cc)				  (25, 50, 100, 500, 2000/cc)

**Numerical Model**

## Na=25/cc - precipitating

**N****a****=25/cc - precipitating**

- Red: convergence (surface)
- Blue: divergence (surface)
- Black: surface rainrate

- x-direction, m

- y-direction, m

- -6000

- 6000

- 6000

- Xue, Feingold, Stevens, JAS 2007, in press

## Na=25/cc

**N****a****=25/cc**

- Red: convergence (surface)
- Blue: Divergence (surface)
- Black: surface rainrate

- y-direction, m

- x-direction, m

- -6000

- 6000

- 6000

## Na=25/cc

**N****a****=25/cc**

- Red: convergence (surface)
- Blue: Divergence (surface)
- Black: surface rainrate

## Na=25/cc

**N****a****=25/cc**

**Precipitating **
**open**
**cells**

- New precip on
- convergence
- zone

## Na=100/cc (non precipitating)

**N****a****=100/cc (non precipitating)**

- Closed cell convection

- ~ no evolution with time
-      no cellular structure

- Xue, Feingold, Stevens, JAS 2007, in press

## Number of aerosol, mg-1

- Number of aerosol, mg-1

- LWP

- Cloud Fraction

**Notes:**
Why is TKE max under clean conditions? Is this effect of outflow? Precip generated downdrafts?
Why then is the max updraft lower? Is it because of sfc stabilization?
Tendency for water to linger longer because of slower evap outweighs the tendency for gravity to expedite it’s fall to surface.

## ATEX

**ATEX**

**More efficient evap**
**(polluted)**

- 25/cc

- 100/cc

- 2000/cc

- cf

*q**l*(domain)

*q**l*(cloud)

*r**e*

*drizzle*

*cond/evap*

## 3. Aerosol Effects on Cloud Lifetime

- Houston, Aug/Sept 2006: Photo from CIRPAS Twin Otter

*3. Aerosol Effects on Cloud Lifetime*

## Modeling Aerosol Effects on the Lifetime of Warm Cumulus

| Model | Domain | Resolution | t; T | Other |
| --- | --- | --- | --- | --- |
| RAMS-LES
(NOAA)
3-D | 6.4 x 6.4 km | 100 x100 x 50m | 2s;
8h | - Bin physics
- surface model
 radiation (cloud/
                  aerosol)
SMOCC continental sounding (Brazil) |
| UCLA-LES
3-D | 6.4 x 6.4 km | 100 x100 x 40m | 1.5s;
 6h | - Bin physics
- Simple surface/  radiation models
BOMEX Trade-Cu sounding |
| Tel Aviv Univ. 2D; x,z |  | 40 x 40 m | 2s;
1.5h
 |  Single cloud
 Bin physics
BOMEX, Kogan,  Med. Sea soundings |

**Modeling Aerosol Effects on the Lifetime of Warm Cumulus**

## Analysis of Cloud Lifetimes (LES)

**BOMEX**

**Analysis of Cloud Lifetimes (LES)**

**Threshold:**
**LWP > 20 gm****-2**
**1-min **
**resolution**
**6-8 h simulations**

## Aerosol Effects on Cloud Lifetime

- Based on analysis of _100s of individual clouds_ using object oriented
- cloud tracking tools

- Jiang, Xue, Teller, Feingold, Levin: GRL 2006

**Aerosol Effects on Cloud Lifetime**

## LES simulations spawned from common dynamical point at 2-h

- BOMEX Trade Cumulus clouds

## Buoyancy gradient and entrainment

- Zhao and Austin 2005
- LES output

- Contours: buoyancy
- Arrows: wind vectors

- Strength of horizontal buoyancy gradient contributes to vorticity.

**Notes:**
The vortical circulation is directly related to the horizontal buoyancy gradient. 
 This horizontal buoyancy gradient produces a net torque, which tends to rotate the flow during its ascent 
 The vortical circulation drives the large scale entraining eddies and wraps environmental air into the rotating center.

## Continental 

**Continental **
**Cumulus**

**Trade**
**Cumulus**

**Continental**
**Cumulus **
**(single 2-D**
**cloud simulations;**
**Tel Aviv Univ.)**

- buoyancy

- vorticity

- dB/dz

- Entrainment:
- dB/dz>0;
- Detrainment:
- dB/dz<0

- Polluted
- Clean

**Normalized Cloud Depth**

## More aerosol  more drops  less coalescence less rain 

- 				vs.
- More aerosol  more drops  smaller drops  _more efficient evaporation_

## 4. Aerosol Effects on Cloud Albedo for 

*Dynamically Evolving Cloud Systems*

## BOMEX trade Cumulus 

**BOMEX trade Cumulus **
**simulations**

- Cloud fraction

- Cloud optical depth

- LWP cloud

- LWP domain

- Aerosol conc, mg-1

## Susceptibility calculations based on 3-D RTM for LES 

- simulated cloud fields

- Const LWP

**Notes:**
Black: Monte Carlo
Grey ICA
Dotted: S=A(1-A)/3N

## Susceptibility calculations based on 3-D RTM for LES 

- simulated cloud fields

- Enhancement

- Reduction

- ATEX

- Const LWP

**Notes:**
Black: Monte Carlo
Grey ICA
Dotted: S=A(1-A)/3N

## Summary

- Observations of aerosol effects on clouds are ambiguous; modeling attempts to show why
- Aerosol effects on precipitation generate significant changes to cloud morphology
- Simulated transition from closed to open cell convection
- Opposing effects of precip and evaporation
- Aerosol does not appear to modify shallow cumulus cloud lifetime in our large eddy simulations
- Faster evaporation in polluted cases  entrainment feedback that dilutes clouds
- Aerosol effects on albedo
- Cases where effects counter one another (BOMEX)
- Case where effects work in unison (closed to open cell transition - ATEX)