Richard Allan and Tony Slingo (ESSC, University of Reading) The impact of Saharan dust on the OLR: detection, causes, and introduction of mineral dust into the Met Office NWP model Jim Haywood, Nicolas Bellouin, Ian Culverwell, Sean Milton, John Edwards and Malcolm Brooks (Met Office, Exeter) and Richard Allan and Tony Slingo (ESSC, University of Reading) November 2004 © Crown copyright 2004

Data from SINERGEE project using 6Z, 12Z, 18Z, 24Z, July 2003 How does the NWP model OLR compare with new observations by the Geostationary Earth Radiation Budget instrument (GERB)? Data from SINERGEE project using 6Z, 12Z, 18Z, 24Z, July 2003 Positive anomaly over desert Negative anomaly over ITCZ clouds © Crown copyright 2004

So, the difference is not caused by clouds Data from 12Z, July 2003 Cloud screened data The Geostationary Earth Radiation Budget instrument (GERB) shows significantly lower OLR for clear-regions over the desert during July 2003, by up to ~50Wm-2 in the monthly mean. So, the difference is not caused by clouds © Crown copyright 2004

What is the cause of the clear-sky OLR differences over the Sahara? Surface temperature errors? Surface emissivity errors? Wind-blown Saharan dust? © Crown copyright 2004

Saharan dust is an obvious candidate; the TOMS aerosol index shows large amounts of aerosol over this region in July 2003 TOMS Aerosol Index – July 2003 © Crown copyright 2004

© Crown copyright 2004

Aerosols are the most likely explanation Sensitivity studies were performed to assess the potential contributions from; surface temperature errors in the model surface emissivity errors in the model Neither contribution can explain the large magnitude of the OLR differences, although the contributions are by no means negligible Aerosols are the most likely explanation © Crown copyright 2004

Which is in good agreement with the dOLR between GERB and the UM. If we account for the effect of the aerosol on the SW at the surface which reduces the surface temperature and hence the OLR, we end up with this. Which is in good agreement with the dOLR between GERB and the UM. © Crown copyright 2004

Experiments have recently been performed at the Met Office to assess the impact of aerosol on model forecasts daily aerosol optical depths from TOMS with ‘sensible’ radiative properties (derived from aircraft and AERONET measurements) were imposed in the radiative transfer code an ensemble of ten 5-day forecasts with and without aerosol were performed to improve the signal/noise and remove the local synoptic variations in the analysis. © Crown copyright 2004

Radiative forcing after 24 hours © Crown copyright 2004

Differences in surface temperature (T0+24) T0+24h T0+48h T0+72h -8 -6 -3 0 3 6 9 T0+96h T0+120h Area averages: Total -0.5 K, SW effect -0.8 K, LW effect +0.3 K © Crown copyright 2004

Differences in temperature profiles Over land surface Over ocean SW and LW effects included SW effect only LW effect only © Crown copyright 2004

Differences in u-component of winds Analysis Control Run (T0+48h) Aerosol Run © Crown copyright 2004

We saw this effect early on in the SINERGEE project and reported it at GIST meetings We wrote a draft paper, based on the July 2003 comparisons, but at GIST 21 in Boulder it was agreed that this contravened the proposed GERB data policy, since it amounted to exploitation not validation and also because it used early data from GERB (Meteosat 7 imager, geolocation errors, etc) So, we repeated the study using the OLR estimated from the Meteosat 7 imager data by RMIB. The results are very similar to those from GERB We re-wrote the paper to use the estimated OLR and this has now been accepted for publication; Haywood, J.M., R. P. Allan, I. Culverwell, A. Slingo, S. Milton, J. M. Edwards and N. Clerbaux, 2004. Can desert dust explain the outgoing longwave radiation anomaly over the Sahara during July 2003? J. Geophys. Res., accepted for publication © Crown copyright 2004

Data from 12Z, July 2003 Cloud screened data The ΔOLR calculated from Meteosat-7 is very similar to that found from the GERB data (although the potential error in the OLR derived from Meteosat-7 data is larger, owing to the narrow-broadband conversion) © Crown copyright 2004

Was July 2003 unusual? © Crown copyright 2004

Much the same signal recurred in July 2004 When these data are released, we would like to submit a second paper confirming the earlier results, using the GERB data for July 2004. This would get round the problem of trying to use the early data from July 2003, which are subject to much larger errors. The GIST is invited to comment on this proposal © Crown copyright 2004