Elisa Sena 1 , Allison McComiskey 2 , Graham Feingold 2 - - PowerPoint PPT Presentation

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Elisa Sena 1 , Allison McComiskey 2 , Graham Feingold 2 (elisats@if.usp.br) 1 University of So Paulo May/2016 2 NOAA-ESRL Aerosol-cloud interactions Less reflective More reflective clouds clouds (few large drops) (many small drops)

SLIDE 1

Elisa Sena1, Allison McComiskey2, Graham Feingold2 (elisats@if.usp.br)

1 University of São Paulo 2 NOAA-ESRL

May/2016

SLIDE 2

Aerosol-cloud interactions

Aerosol conc., Na Drop conc., Nd Cloud optical depth, τc Drop effective radius, re Cloud albedo, Ac Cloud Condensation Nuclei conc., CCN All else equal (Liquid water path, LWP)

Robert Simmon

More reflective clouds (many small drops) Less reflective clouds (few large drops)

SLIDE 3

Changes in Cloud Microphysics

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But… Goal

Clarify how aerosol and macroscopic cloud properties impact the cloud radiative forcing. Cloud Albedo Change Aerosol, macroscopic cloud properties and dynamics are interconnected LWP Adjustments ???

SLIDE 4

Methodology

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14-years of coincident ground-based

measurements of clouds, aerosol and meteorological properties from SGP ARM deployment.

Measurements at 1-minute resolution.
Low non-drizzling clouds (ice crystals

and precipitation avoided).

SLIDE 5

Properties analyzed

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Indicates how well-mixed the boundary layer is. RELATIVE CLOUD RADIATIVE EFFECT DECOUPLING INDEX PROXY FOR TURBULENCE AEROSOL INDEX w0: mean vertical velocity at the cloud base. Proxy for CCN. Non-dimensional measure for the surface cloud radiative effect. LOWER TROPOSPHERIC STABILITY Related to the strength

the capping inversion. θ is the potential temperature.

SLIDE 6

Microphysical responses

CCN concentration

Previous approaches vs. New approach

McComiskey et al., 2009

SW Radiative responses

Drop size (µm) Cloud Radiative Effect (rCRE)

vs.

Liquid water path (g/m2) Unperturbed Cloud Drop Conc. (constant LWP)

ACI

SLIDE 7

How do different properties influence the rCRE?

At Fixed LWP: Weak trends with Ai in both directions.

Sena et al., ACPD, 2016

Southern Great Plains (SGP)

Aerosol: Ai τc rCRE vs. LWP rCRE Liquid water path (g/m2) rCRE Liquid water path (g/m2)

SLIDE 8

How do different properties influence the rCRE?

At Fixed LWP: Weak trends with Ai in both directions. 2 cloud regimes:

Low fc; High w’2
High fc; Low w’2

Sena et al., ACPD, 2016

Southern Great Plains (SGP)

Aerosol: Ai τc Turbulence: w’2 fc rCRE vs. LWP rCRE Liquid water path (g/m2) rCRE Liquid water path (g/m2) rCRE rCRE

SLIDE 9

How do different properties influence the rCRE?

At Fixed LWP: Weak trends with Ai in both directions. 2 cloud regimes:

Low fc; High w’2
High fc; Low w’2

Southern Great Plains (SGP)

Aerosol: Ai τc Decoupling: Di LTS Turbulence: w’2 fc rCRE vs. LWP rCRE Liquid water path (g/m2) rCRE rCRE rCRE rCRE rCRE Liquid water path (g/m2)

SLIDE 10

Aerosol vs. LWP signals on rCRE

At least 25 observations per day. N = 323 days

Mean: 0.00 + 0.02 Mean: 0.46 + 0.02

50% 50% 10% 90%

SGP SGP

ρrCRE,Ai ρrCRE,LWP

Correlation rCRE, Ai Correlation rCRE, LWP

Distributions of daily correlations

SLIDE 11

Aerosol vs. LWP signals on rCRE

At least 25 observations per day. N = 323 days

Mean: 0.00 + 0.02 Mean: 0.46 + 0.02

50% 50% 10% 90%

SGP SGP

ρrCRE,Ai ρrCRE,LWP

Correlation rCRE, Ai Correlation rCRE, LWP

Distributions of daily correlations

SLIDE 12

Jan-09-2006

Case study 1: Positive correlation, ρrCRE,Ai = 0.75

Strong positive correlation between rCRE, τc and LWP.

Time UTC (hour)

SLIDE 13

Jan-09-2006

Case study 1: Positive correlation, ρrCRE,Ai = 0.75

Strong positive correlation between rCRE, τc and LWP.

Time UTC (hour) Aerosol Index

Negative slopes, as expected.
Large variance for slopes.

SLIDE 14

Jan-09-2006

Case study 1: Positive correlation, ρrCRE,Ai = 0.75

Strong positive correlation between rCRE, τc and LWP.

Ai ; LWP

ρrCRE,Ai + ρLWP,Ai +

R = 0.50

Time UTC (hour) Aerosol Index

Negative slopes, as expected.
Large variance for slopes.

Aerosol Index

After ~16h UTC:

SLIDE 15

Apr-26-2006

Case study 2: Negative correlation, ρrCRE,Ai = -0.65

Strong positive correlation between rCRE, τc and LWP.

Time UTC (hour)

SLIDE 16

Apr-26-2006

Case study 2: Negative correlation, ρrCRE,Ai = -0.65

Strong positive correlation between rCRE, τc and LWP. Positive slopes, contrary to expectation.

Time UTC (hour) Aerosol Index

SLIDE 17

Apr-26-2006

Case study 2: Negative correlation, ρrCRE,Ai = -0.65

Strong positive correlation between rCRE, τc and LWP.

Ai ; LWP

Positive slopes, contrary to expectation.

ρrCRE,Ai - ρLWP,Ai -

R = -0.44

After ~14h UTC:

Time UTC (hour) Aerosol Index Aerosol Index

SLIDE 18

Correlation between correlations

Usually, if the aerosol index and LWP are positively correlated, the correlation between rCRE and aerosol index is positive (and vice-versa). N = 323 days

Are we actually seeing the LWP signal instead of the aerosol signal in Cloud Radiative Effect (rCRE)? R = 0.54

Daily correlation of LWP and Ai

SLIDE 19

Summary

1) For SGP, the influence of aerosol on cloud RF is weak; macroscopic cloud properties and dynamics play a much larger role in cloud RF compared to microphysical effects. 2) Microphysical metrics to estimate aerosol-cloud interaction are very uncertain. 3) We propose looking at aerosol indirect effects using higher-order properties that more significantly affect RF. 4) We are using the same approach to study sites under different cloud regimes (Amazônia).

Reference:

Sena, E. T., McComiskey, A., and Feingold, G.: A long-term study of aerosol–cloud interactions and their radiative effect at a mid latitude continental site using ground-based measurements,

Atmos. Chem. Phys. Discuss., 2016.