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.
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)
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
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
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?
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 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.
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 Tat
UM Td
Observations are seriously lacking in this region!
*
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.
GERB science meeting 2006
Direct effect of tropospheric aerosols (clear skies for simplicity)
Aerosol Layer
Surface
Scattering solar Absorbing terrestrialAbsorbing solar
Increased planetary albedo: -ve forcing
Decreased planetary albedo: +ve forcing
Incident solar radiation Incident solar radiation
Emitted terrestrial radiation
Re-emitted terrestrial radiation: +ve forcing
COOLING WARMING WARMING
GERB science meeting 2006
Mie scattering calculations suggest that the scattering efficiency is a maximum when x=2r/i.e. when r~
Therefore only large particles significantly influence terrestrial radiation in the atmospheric window (~10m).
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 8-12m
atmospheric window.
ClearDusty
AV
HR
R C
h4
AV
HR
R C
h5
Nadir views from 18,000ft (R6) (above aerosol).
Measured surface temperature (from 100ft) 302.5K
GERB science meeting 2006
GERB science meeting 2006
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.
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
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
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.
GERB science meeting 2006
The inter-annual variability of DOLR
GERB science meeting 2006
The Facility for Airborne Atmospheric Measurements
GERB science meeting 2006
The BAe146 is booked for flying from Niamey to Nouakchott in June 2007 – GERBIL :-
GERB science meeting 2006
GERB/SEVIRI
Flight plan for GERBIL
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.
GERB science meeting 2006
Weather Forecast
http://www-nwp/~obr/weather.html
GERB science meeting 2006
GERB science meeting 2006
GERB science meeting 2006
GERB science meeting 2006
GERB science meeting 2006
GERB science meeting 2006
GERB science meeting 2006