Dust and vapour cloud the view Richard Allan Environmental Systems Science Centre, University of...

Dust and vapour cloud the view

Richard Allan

Environmental Systems Science Centre, University of Reading, UK

Thanks to Tony Slingo, Ruth Comer, Sean Milton, Malcolm Brooks, and the GERB International Science Team

Introduction

• Climate and NWP models

• Model evaluation: – Top down/bottom up approach

• Diagnosing variability and feedbacks

• Nuts and bolts: model parametrizations and physical processes– fast feedbacks

• Limitations of the observing systems

Intro 3

• Clouds and climate

Bony and Dufresne (2005)

Clouds and Climate

Objectives

• Validation of new datasets (GERB/SEVIRI)

• Timely Model Evaluation• Understanding of physical processes

GERB July 2006 OLR Animation Model

Sinergee project: www.nerc-essc.ac.uk/~rpa/GERB/gerb.html

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Mean model bias: 2006

All-sky Clear-sky

All-sky Clear-sky

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Convective cloud

Surface albedo

Mineral dust aerosol

Marine stratocumulus

Cirrus outflow

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Mineral dust aerosol

Dust

- Major dust source for Amazon

- Large component from March 2004 dust storms

March 2004: an interesting month

Loeb et al. (2007) J. Climate, 20, p.582

March 2006 was interesting too…

1200GMT, 6 March 2006

In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.

On 6 March, unusually strong northerly winds bring cold air at low levels over the desert, creating a broad front of dust as the air moves south.

The location of Niamey is marked by a cross.

RADAGAST project: http://radagast.nerc-essc.ac.uk

1200GMT, 7 March 2006

The shallow layer of cold air cannot rise over the mountains of the central Sahara (light blue in colour), so it is forced to follow the valleys. Streaks appear where it accelerates through gaps in the topography.

The dust reached Niamey at 0930 on 7 March.

In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.

RADAGAST project: http://radagast.nerc-essc.ac.uk

1200GMT, 8 March 2006

By 8 March, dust covers the whole of West Africa and is moving out over the Atlantic.

In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.

Animation available: http://radagast.nerc-essc.ac.uk

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Radiative transfer models underestimate the solar absorption in the atmosphere during March 2006 dust storm

Slingo et al. (2006) GRL, 33, L24817

Dust impact on longwave radiation

• Large perturbation to Met Office model OLR during summer over west Sahara

• Correlates with high mineral dust aerosol optical depth

Model minus GERB OLR: July 2006, 12-18 UTC

Consistent with calculations of dust longwave radiative effect

Clear-sky OLR bias (Wm-2) in 2003

Calculations:

Direct radiative effect

Direct plus shortwave feedback effect

Haywood et al. (2005) JGR 110, D05105

All-sky Clear-sky

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Marine stratocumulus

Convective cloud

Cirrus outflow Radiative biases in the Met Office global model

Marine Stratocumulus

• Curious banding structure– Transition across

model levels

• Cloud reflectivity bias

Changes in albedo bias (ocean)

• Model upgrade (March 2006) reduced but did not remove albedo bias– Compensating errors: ITCZ/stratocumulus

Stratocumulus composites

Cloud liquid water path

Bias: model minus GERB; SSM/I; SEVIRI

Albedo Liquid Water Path Cloud

Reduction in model bias from June to July 2006 - relates to cloud liquid water

LWPWentz: overestimate for low cloud fraction?

TMIWentz/MODIS LWPOvercast boundary layer clouds: good agreement

Horváth and Davies (2007) JGR 112, D01202

Convective cloud

5th June 2006

Model evaluation: near-real time

• Change in model minus GERB flux differences

• Relate to change in model physics implementation

13th March | 14th March

Model SW albedo

2005 2006

Convective Decay Time-scale

• Unrealistically low levels of convective cloud

• On-off; common problem in models

• Simple fix…

Improved shortwave reflectivity

• Increased convective cloud cover

• But is the physics any better?

• Future work: Comparisons with CloudSat

Gulf of Guinea Model CloudSat

5th July 2006

19th July 2006

Clouds and water vapor

• Combine GERB/SEVIRI

• Diurnal changes in cloud and humidity

• Radiatively driven subsidence

• work by Ruth Comer

2-3 hr lag between tropical convection and

upper tropospheric water vapor

(Soden 2000, 2004)

above: central/South Americaright: Lagrangian tracking

Tracking over

Africa difficult?

Complex picture locally

due to propagating disturbances

Clouds and Water Vapor: Africa

~3-hour lag

Work by Ruth Comer

Clear-sky radiative cooling and the

atmospheric hydrological cycle

• Clear-sky radiative cooling: – radiative convective balance– atmospheric circulation

• Earth’s radiation budget– Understand clear-sky budget to

understand cloud radiative effect• Datasets:

– Reanalyses – observing system– Satellites – calibration and sampling– Models – wrong by definition

Links to precipitation

Tropical Oceans

1980 1985 1990 1995 2000 2005

Ts

CWV

LWc

SFC

ERA40

NCEP

SRB

HadISST

SMMR, SSM/I

Derived:SMMR, SSM/I, Prata)

Allan (2006) JGR 111, D22105

Surface LWc and water vapourdLWc/dCWV ~ 1.5 Wkg-1

ERA40 NCEP

dCWV/dTs ~ 3 kgm-2K-1

Allan (2006) JGR 111, D22105

Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB

Calibration or sampling?

Tropical Oceans

Surface Net LWc

Clear-sky OLR

Clear-sky Atmos LW cooling

QLWc

ERBS, ScaRaB, CERES

Derived

ERA40

NCEP

SRB

HadISST

Allan (2006) JGR 111, D22105

Linear least squares fit

• Tropical ocean: descending regime

• Dataset dQLWc/dTs Slope

• ERA-40 3.7±0.5 Wm-2K-1

• NCEP 4.2±0.3 Wm-2K-1

• SRB 3.6±0.5 Wm-2K-1

• OBS 4.6±0.5 Wm-2K-1

ERA40 NCEP

Implications for tropical precipitation (GPCP)?

ERA40 QLWc

GPCP P

OBS QLWc

Pinatubo?

IPCC AR4 models: tropical oceans

• CWV

• Net LWc

• OLRc

• Q_LWc

IPCC AR4 models: tropical oceans

• QLWc

• Precip

Ongoing work…

Ongoing work…CMIP3 models

Also considering coupled model experiments including greenhouse gas and natural forcings

Conclusions• Top down-bottom up approach

– Good for feedback to modelers• Mineral dust aerosol

– Shortwave absorption; longwave radiative effect– Large effect of single events

• Marine stratocumulus– Reflectivity and seasonal variability: issues

• Deep convection– Intermittent in models; issues with detrainment

• Clear-sky radiative cooling– Links to atmospheric hydrological cycle– Need to understand before can understand changes in

cloudiness• Observing systems: capturing decadal variability

problematic

Spurious variability in ERA40

• Improved performance in water vapour and clear-sky radiation using 24 hour forecasts

• Reduced set of reliable observations as input to future reanalyses?

Clear-sky vs resolution

Sensitivity study

• Based on GERB- SEVIRI OLR and cloud products over ocean:

• dOLRc/dRes ~0.2 Wm-2km-0.5

• Suggest CERES should be biased low

by ~0.5 Wm-2 relative to ERBS

Links to precipitation