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NWP model OLR errors over the Sahara - detection, causes, and possible solutions

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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,JohnEdwards, 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.

nwp model olr errors over the sahara detection causes and possible solutions
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)
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


Data from 12Z, July 2003

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

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)

  • Explanation?
  • Surface temperature?
  • Emissivity?
  • Atmospheric transmission?

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 dTs.

300band version of the code in LW using observed atmospheric profiles from radiosondes.

Ts of ~308K (35C) near the centre of the anomaly would be necessary.


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 Tat



Observations are seriously lacking in this region!


The emissivity of the surface does not appear to fix the difference between the UM and GERB:-

For a Ts of 330K, OLRe=1-OLRe=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 e=0.95.


Direct effect of tropospheric aerosols (clear skies for simplicity)

Incident solar radiation

Incident solar radiation

Re-emitted terrestrial radiation: +ve forcing

Decreased planetary albedo: +ve forcing

Increased planetary albedo: -ve forcing

Aerosol Layer

Emitted terrestrial radiation





Scattering solar

Absorbing solar

Absorbing terrestrial


Mie scattering calculations suggest that the scattering efficiency is a maximum when x=2pr/l~6 i.e. when r~l.

Therefore only large particles significantly influence terrestrial radiation in the atmospheric window (~10mm).


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 8-12mm atmospheric window.





Nadir views from 18,000ft (R6) (above aerosol).

Measured surface temperature (from 100ft) 302.5K


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.


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


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 ataer0.5of 1.0, and Ts of 330K, dOLR~30Wm-2


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.



  • The utility of NWP model comparisons against GERB under the SINERGEE project are clearly demonstrated.
  • 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.
  • Other factors such as errors in the surface reflectance, surface temperature (Malcolm Brooks), surface emissivity may also be important.
  • Further work is necessary ……………….. GERBIL should prove a more definitive test-bed.
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