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

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)


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


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)

Data from 12Z, July 2003

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

UM Td

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, 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.


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


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).


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


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 a aer0.5 of 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 inter-annual variability of DOLR


The Facility for Airborne Atmospheric Measurements


The BAe146 is booked for flying from Niamey to Nouakchott in June 2007 – GERBIL :-


GERB/SEVIRI

Flight plan for GERBIL


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.


Weather Forecast

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

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