Upload
theresa-webster
View
223
Download
0
Tags:
Embed Size (px)
Citation preview
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