Figure 1: Using GERB and SEVIRI data to evaluate the Met Office NWP model in near-real time; example SINERGEE comparisons for 8th March 2006 at 1200 UTC ( http://www.nerc-essc.ac.uk/~rpa/GERB )

METHODOLOGY• At the Met Office, single time-step runs of the global Numerical Weather Prediction (NWP) model are initiated from 0, 6, 12 and 18 UTC analyses

• Near-real time transfer of model diagnostics + GERB/SEVIRI data to ESSC

• Output comparisons of outgoing longwave radiation (OLR) and top of atmosphere albedo: SINERGEE project

• Archive model-GERB comparisons for further analysis (Allan et al. 2005)

INTRODUCTION• The top of atmosphere radiation balance is (i) an important diagnostic of clouds, aerosol and water vapour (ii) a critical driver of atmospheric circulation

• Here, we use new satellite data from GERB/SEVIRI to (i) evaluate models, (ii) understand radiative processes and (iii) monitor satellite instruments / model

CONCLUSIONS• Near-real time comparisons between GERB data and the Met Office NWP model since 2003

• Exploitation of the synergy between GERB, SEVIRI and an NWP model has contributed to (i) the monitoring of the satellite instruments, (ii) model evaluation, and (iii) improved understanding of radiative processes

• Clear-sky OLR simulated by the model agrees to within about ±5-10 Wm-2 of the GERB fluxes

• The longwave radiative signature of mineral dust aerosol is present in the GERB/SEVIRI data

• Monthly mean model OLR is underestimated by around 50 Wm-2 at 1200 UTC in summer months over west Sahara compared to GERB/Meteosat

• Model deficiencies including simulation of tropical convection and cirrus outflow, low-altitude oceanic cloud and Sahara surface albedo

• The near-real time comparisons have contributed to the process of model improvement through updated parametrizations

REFERENCES Allan, R. P., A. Slingo, S. Milton and I. Culverwell (2004), SINERGEE: simulation and exploitation of data from Meteosat-8 using an NWP model, Proc. EUMETSAT Meteorological Satellites Conf., Prague, Czech Republic, EUMETSAT, 122-130.

Allan, R. P., A. Slingo, S.F. Milton I. Culverwell (2005), Exploitation of Geostationary Earth Radiation Budget data using simulations from a numerical weather prediction model: Methodology and data validation, J. Geophys. Res., 110, D14111, doi: 10.1029/2004JD005698.

Harries, J. E., J. E. Russell, J. A. Hanafin and coauthors (2005), The Geostationary Earth Radiation Budget (GERB) Experiment, Bull. Amer. Meteorol. Soc., 86, 945-960.

Haywood, J. M., R. P. Allan, I. Culverwell, A. Slingo, S. Milton, J. M. Edwards and N. Clerbaux (2005), Can desert dust explain the outgoing longwave radiation anomaly over the Sahara during July 2003? J. Geophys. Res., 110, D05105, doi:10.1029/2004JD005232.

Acknowledgement: This work was funded through the NERC/Met Office Connect-B grant NER/D/S/2002/00412

DATA AND VALIDATION• The Geostationary Earth Radiation Budget (GERB) instrument is the first broad-band radiometer to fly on a geostationary satellite

• Along with the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI), GERB provides unprecedented 15-minute temporal resolution of the radiation budget over the Africa-Atlantic hemisphere

• Validation of the data is ongoing including comparisons with Clouds and the Earth’s Radiant Energy System (CERES) data (left)

• For further details see Harries et al. (2005)

Figure 2: GERB minus CERES longwave (left) and shortwave (right) flux difference, July 2004

CLEAR-SKY RADIATION• Since the Met Office NWP model uses data assimilation we have confidence in the clear-sky longwave radiative fluxes over the ocean

• We compared longwave and shortwave fluxes from the model with GERB data over the ocean only where both the model and satellite data indicated cloud-free conditions (e.g., right)

• Clear-sky OLR agrees to within ~±5-10 Wm-2

• Clear-sky albedo showed more scatter with agreement to within ±0.01 (approximately 10%)

• For further details see Allan et al. (2005)

Figure 3: Time series of model/GERB (a) clear-sky OLR and (b) clear-sky albedo over Mediterranean

Evaluation of clouds and radiation simulated by the Met Office NWP model using GERB dataRichard P. Allan, A. Slingo (ESSC, University of Reading, UK)

M.E. Brooks, S.F. Milton (Met Office, Exeter, UK)Email: rpa@mail.nerc-essc.ac.uk

Figure 6: Evaluation of Namibian marine stratocumulus. Cloud albedo effect for (a) model and (b) GERB; cloud fraction for (c) model and (d) Meteosat; 1200 UTC data

Figure 7: OLR (shading) and 500 hPa vertical motion (contours) for (a) model and (b) GERB, 31/03/2004

EVALUATION OF MODEL CLOUD AND RADIATION• Distribution of cloud across mid-latitudes well simulated by the model due to data assimilation

• At lower latitudes the model has trouble simulating the diurnal cycle of convection, the radiative properties of low-altitude oceanic clouds and the distribution of convectively driven cirrus outflow

• Surface albedo over north Africa is not adequately represented by the model; the magnitude has recently been improved but the spatial distribution is inconsistent with clear-sky albedo from GERB

• The simulated spatial structure of marine stratocumulus shows artefacts; recent changes in boundary layer parametrizations have improved the reflective properties of low-altitude clouds

• The diurnal cycle of tropical clouds over land remains a problem with an early onset of convection and lack of spatial organisation

• For additional details see Allan et al. (2004)

(d) Figure 5: High mineral dust optical depth explains large model error in clear-sky OLR during July 2003: 0.55 μm aerosol optical depth from (a) TOMS and (b) MISR, (c) mean model-GERB clear-sky OLR difference and (d) time-series with estimated direct radiative effect based on radiative calculations

MINERAL DUST AEROSOL• Recent analysis of GERB and SEVIRI data highlight an appreciable longwave radiative signature of dust aerosol (above; see http://www.nerc-essc.ac.uk/~rpa/AMMA )

• The NWP model does not include these dust events

• Analysis of GERB and Meteosat data during July 2003 indicate an enhanced greenhouse effect due to mineral dust aerosol of magnitude ~50 Wm-2 over west Sahara not captured by the model (left; see Haywood et al. 2005)

• Future work seeks to assess the impact of desert dust on (i) model circulation and forecast skill (ii) the atmospheric radiative flux divergence (http://radagast.nerc-essc.ac.uk )

(a)(c) Figure 4: Dust storm, 8th

March 2006: (a) SEVIRI dust product, (b) GERB OLR, (c) model-GERB clear-sky OLR difference + SEVIRI 10.8 minus 12 μm brightness temperatureDust

(b)