Direct LW radiative forcing of Saharan dust aerosols

Vincent Gimbert, H.E. Brindley, J.E. Harries

Imperial CollegeLondon

GIST 25, 24 Oct 2006, UK Met Office

Outline of Presentation

Mineral aerosols – Radiative effects

Case study March 2004: dust storm OLR perturbation

Cloud and Dust detection

GERB / RT Model comparisons over dust storm

Analysis of clearsky OLR

Conclusion and future work

Primary aerosols emitted from desert surfacesLifted in atmosphere by strong surface windsResidence time 1 day ~ 1 weekPresent in the Lower troposphere but can travel 1000s of km

Mineral dust aerosols

Composite image from 3 SEVIRI Visible channels

Composite image from 3 SEVIRI Infra-Red channels

Credit EUMETSAT

Dust LW radiative effects

Dust are large aerosols (~ 1 micron) so also interact with IR thermal radiation.

Absorption of LW radiation and re-emission at level temperature

Scattering of LW radiation

Both Absorption and Scattering decrease the TOA OLR

ΔOLR (dust type, height, AOD, surface/atmosphere)

Strong decrease of OLR

Strong daytime surface temperature anomaly associated with dust event

GERB , 3 March 2004 12:00

Cloud / Dust detection issues

Dust misidentified as clouds by the RMIB GERB cloud product (visible detection)

Similar problem with MPEF cloud mask (Visible/IR detection)

SAFNWC dust detection

Daytime dust detection algorithm developed at Meteo France for SAFNWC

Empirical algorithm based on SEVIRI 10.8 μm and 12μm channels thresholds.

Generated operationally at SEVIRI spatial and temporal resolution

Strategy for estimating Direct Radiative Forcing

Use of ECMWF analyses and assume the data assimilation provides adequate information on state of the atmosphere

Assume that the dust feedback effects via modification of surface temperature and atmospheric profiles are accounted for in analyses (e.g. strong daytime surface temperature drop picked up by analysis on 3rd March 2004)

Compare modelled clearsky OLR to GERB measurement over dust and Estimate direct LW forcing as the difference.

Use clear sky scenes to test the model.

Radiative transfer modelling / GERB measurement

Modelling of radiances using MODTRAN 4 v3r1 from 3.5 μm to ∞• Minor gases, heavy molecules from std profiles• Surf Temp, Temp, Humidity, Ozone profile from ECMWF analysis• 1 Surface type spectral emissivity (Dunesand)• 1 * 1 Degree resolution – 60 vertical levels

Comparison with GERB measurement• GERB L2 ARG product (Not Edition)• Use radiances because of suspected problems with ADM• GERB viewing zenith angle form BARG products• Comparison at 06:00, 12:00, 18:00 (No GERB data at 00:00) for the

Month of March 2004

12:00 GERB - Model difference

GERB/model anomaly coincides with dust front on 3rd March 2004

Anomalies over clear sky as well (cloud identification problem? representation of orography in model?)

GERB and Model 12UTC through March

Cloud/dust free pixels

averages over dust

front region

Over dust front (20040303)

Mean diff = -7W/m2/sr

Over clear sky:

Mean diff = -0.4W/m2/sr

σ = 0.9 W/m2/sr

Strong reduction of OLR diurnal amplitude on 3rd March 2004 OLR decrease smaller at 18 and not significant at 06

Model Warm bias at 06 and 18 (No dust detection)

Need to understand clearsky GERB-Model differences to

understand model deficiencies

GERB/Model Clearsky comparisons

Study the Sahara region over clearsky pixels at 06, 12 and 18 for March 2004

Apply 1 pixel security margin to cloud detection (GERB PSF not

accounted for in Cloud Mask)

• Model warm bias at 06 and 18

• Larger errors at 12 (larger Std Dev than 06 and 18)

Time dependence of clearsky errors

Daytime Conditional Bias

Model does not reproduce the range of GERB OLR at 12

Similar finding as Trigo and Viterbo (2003) with ECMWF model compared to MS7Window channel

We look at possible sources of errors at the surface

Land surface emissivity

Land surface emissivity retrived from ASTER/MODIS (Ogawa et al., 2004)

Varies from 0.86 to 0.99 in the8-13.5μm region

Sensitivity study shows that it could change the OLR by up to 6W/m2/sr for hot and dry scenes.

• Early results show that model warm biais is reduced at 18UTC• Model is offest at 12 as well• In both cases, slight reduction of GERB-model variance• Models need better surface emissivity over Sahara (effect on surface energy budget)

Land SurfaceTemperature

Early study on 2 days of March 2004 at 12 and 18 with SEVIRI 10.8μm channel(‘Hottest’ SEVIRI channel)

Tb(10.8μm) > Tskin (ECMWF) over some regions indicate Pb in model Tsurf

How does that relate to GERB-model differences?

12:00

Temperature differences correlated to GERB/Model errors at 12:00

18:00

• Temperature differences much smaller at 18:00• Some strong OLR errors (e.g. orography) can be explained by a cold biais in model surface temperature• Early comparisons suggest model errors in the diurnal cycle of surface temp

Conclusion and future work

Dust storm was associated with OLR anomaly (wrt clearsky) of

7W/m2/sr at 12:00 with max forcing reaching 12W/m2/sr Strong reduction of OLR diurnal amplitude associated with storm

Clearsky GERB/Model comparisons: Land surface emissivity has significant effect on OLR Evidence for surface temperature problem in ECMWF model, responsible for strong

local model inconsistencies with GERB

,

Future work: Analyse Tb(10.8) for the whole of March 2004 and look for

systematic regional biases over North Africa Model comparisons with SEVIRI WV channels Use ECMWF profile uncertainty data to estimate clearsky OLR error

budget

Many thanks to:

Helen Brindley for providing some of the SEVIRI data

Alessandro Ipe for the RMIB cloud reprocessing

Nicolas Clerbaux for running the SAFNWC software

3rd March 2004 12:00,Srong decrease in SurfaceTemperature

Surface Temperature anomalyfrom the 1st to the 18th of March2004 at12:00 wrt March 2004 12:00 average.ECMWF operational model

Precipitable Water (mm)from the 1st to the 18th of March2004 at12:00ECMWF operational model