1. GERB science meeting, Oct 2006 The bias in OLR over west Africa in the Met Office Unified Model: detection, attribution, and future plans Jonathan Taylor & Jim Haywood (OBR, Met Office, UK) Richard Allan, Tony Slingo (ESSC, Reading) Nicolas Bellouin, Ian Culverwell, Sean Milton, John Edwards, Malcolm Brooks (Met Office) Haywood, J.M, Allan, R.P., Culverwell I., Slingo, A., Milton, S., Edwards. J.M., and Clerbaux, N., Can desert dust explain the outgoing longwave radiation anomaly over the Sahara during July 2003? J. Geophys. Res., 110, D05105, doi:10.1029/2004JD005232, 2005.

2. GERB science meeting 2006 NWP model OLR errors over the Sahara - detection, causes, and possible solutions Detection: Observations of OLR from GERB vs model Causes: –Surface temperature errors? –Surface emissivity errors? –Observations/radiative effect of mineral dust »Aircraft »Sun-photometers »Satellite Possible solutions: –The effect on dynamics in the NWP model (preliminary)

3. GERB science meeting 2006 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 The +ve anomaly over desert is ~ -ve anolmaly over ITCZ clouds

4. GERB science meeting 2006 Cloud screened data The Geostationary Earth Radiation Budget instrument (GERB) shows significantly less OLR over regions of the desert during July 2003 by up to ~50Wm -2 in the monthly mean. Data from 12Z, July 2003

5. GERB science meeting 2006 Cloud screened data The GERB dOLR shows a similar spatial distribution and magnitude to the dOLR calculated from Meteosat-7 data (although the potential error in the Meteosat-7 data is larger owing to the narrowband- broadband conversion) Data from 12Z, July 2003 Explanation? Surface temperature? Emissivity? Atmospheric transmission?

6. GERB science meeting 2006 What would be the surface temperature required to explain the UM-GERB OLR? Perform Radiative transfer calculations using the Edwards and Slingo (1996) radiative transfer code to determine dT s. 300band version of the code in LW using observed atmospheric profiles from radiosondes. T s of ~308K (35C) near the centre of the anomaly would be necessary.

7. GERB science meeting 2006 Tephigrams from the area show T of lowest layers to be ~42C. Pringent et al. (2001) suggest that Tat is less than Ts by >15K. Therefore maximum values of Ts should be >57C (~330K) which is close to the model Ts. Therefore Ts of 35C is not reasonable suggesting that errors in Ts cannot explain the discrepancy between the model and GERB. UM T at UM T d Observations are seriously lacking in this region! *

8. GERB science meeting 2006 The emissivity of the surface does not appear to fix the difference between the UM and GERB:- For a Ts of 330K, OLR  =1 -OLR  =0.9 is ~ 15Wm -2. This is well short of the 50Wm -2 discrepancy. Use of Snyder wavelength dependent emissivity for bare soil gives results very similar to  =0.95.

9. GERB science meeting 2006 Direct effect of tropospheric aerosols (clear skies for simplicity) Aerosol Layer Surface Scattering solarAbsorbing terrestrialAbsorbing solar Increased planetary albedo: -ve forcing Decreased planetary albedo: +ve forcing Incident solar radiation Emitted terrestrial radiation Re-emitted terrestrial radiation: +ve forcing COOLINGWARMING

10. GERB science meeting 2006 Mie scattering calculations suggest that the scattering efficiency is a maximum when x=2  r/  i.e. when r~  Therefore only large particles significantly influence terrestrial radiation in the atmospheric window (~10  m).

11. GERB science meeting 2006 The terrestrial radiative effect is also significant. C-130 measurements using ARIES (cm -1 resolution interferometer) clearly show the effect of Saharan dust in the 8- 12  m atmospheric window. Clear Dusty AVHRR Ch4 AVHRR Ch5 Nadir views from 18,000ft (R6) (above aerosol). Measured surface temperature (from 100ft) 302.5K

12. GERB science meeting 2006

13. The monthly mean TOMS AI can be converted to a monthly mean AOD using empirical relationships based on AERONET observations. The results agree with the (v. much more) sophisticated MISR instrument.

14. GERB science meeting 2006 The July 2003 monthly mean aerosol size distribution from the nearby Dahkla AERONET site can be used with suitable refractive indices (Volz) to estimate the optical parameters associated with mineral dust. Specific extinction coefficient Single scattering albedo Asymmetry Factor

15. GERB science meeting 2006 A look-up table may be produced whereby the dOLRc caused by mineral dust may be calculated as a function of aerosol optical depth and of Ts. For a  aer0.5 of 1.0, and Ts of 330K, dOLR~30Wm -2

16. GERB science meeting 2006 If we account for the effect of the aerosol on the SW at the surface which reduces the surface temperature and hence reduces the OLR as well, we end up with this. Which is in good(ish) agreement with the dOLR between GERB and the UM.

17. GERB science meeting 2006 The inter-annual variability of DOLR

18. GERB science meeting 2006 The Facility for Airborne Atmospheric Measurements

19. GERB science meeting 2006 The BAe146 is booked for flying from Niamey to Nouakchott in June 2007 – GERBIL :-

20. GERB science meeting 2006 GERB/ SEVIRI Flight plan for GERBIL

21. GERB science meeting 2006 Conclusions 1.The utility of NWP model comparisons against GERB under the SINERGEE project are clearly demonstrated. 2.The terrestrial signal for mineral dust may reach 50Wm -2 locally over warm desert regions, which may explain much of the discrepancy between the OLR from the NWP Unified Model of the Met Office, and the new GERB instrument on Meteosat-8. 3.Other factors such as errors in the surface reflectance, surface temperature (Malcolm Brooks), surface emissivity may also be important. 4.Further work is necessary ……………….. GERBIL should prove a more definitive test-bed.

22. GERB science meeting 2006 Weather Forecast http://www-nwp/~obr/weather.html

23. GERB science meeting 2006