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SYSTEMATIC BIASES 3-hourly comparisons of top of atmosphere radiation from GERB and the Met Office global forecast model Systematic biases in model fluxes.

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Presentation on theme: "SYSTEMATIC BIASES 3-hourly comparisons of top of atmosphere radiation from GERB and the Met Office global forecast model Systematic biases in model fluxes."— Presentation transcript:

1 SYSTEMATIC BIASES 3-hourly comparisons of top of atmosphere radiation from GERB and the Met Office global forecast model Systematic biases in model fluxes (Wm -2 ), April-September 2006 (Fig. 1): - poor representation of the spatial structure of Saharan surface albedo - underestimate in reflectivity of tropical convective cloud over Africa - stratocumulus decks are too bright - northern Africa: clear-sky outgoing longwave radiation is overestimated Changes in model parametrizations strongly affect mean radiation budget time-series, especially for land (Fig. 2) INTRODUCTION The top of atmosphere radiation balance is (i) an important diagnostic of surface properties, clouds, aerosol and water vapour and (ii) a critical driver of the atmospheric circulation and global water cycle We use new satellite data from GERB/SEVIRI to (i) evaluate models, (ii) understand radiative processes and (iii) monitor satellite instruments (Allan et al. 2005) DECADAL CHANGES IN WATER VAPOUR AND CLEAR-SKY RADIATION IPCC AR4 models generally reproduce observed variations in column integrated water vapour (CWV) & derived clear-sky surface net longwave flux (SNLc) Reanalyses and satellite observations have difficulty representing interannual variability in clear-sky radiation (Fig. 7) All datasets indicate robust increase in longwave radiative cooling of clear-sky atmosphere (Q_ LWc ) with surface temperatures (Fig. 8) of ~3-5 Wm -2 K -1 CONCLUSIONS Instantaneous satellite estimates of radiative fluxes help to identify and reduce systematic model biases Errors in radiative properties of the surface, marine stratocumulus clouds and tropical convection over land and the lack of representation of mineral dust aerosol lead to the largest systematic model biases Water vapour greenhouse effect well simulated REFERENCES Allan, R. P. (2006) Variability in clear-sky longwave radiative cooling of the atmosphere J. Geophys. Res., 111, D22105, doi: /2006JD Allan, R. P., A. Slingo, S.F. Milton, and 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: /2004JD 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: /2004JD Slingo, A., T. P. Ackerman, R. P. Allan, E. I. Kassianov, S. A. McFarlane, G. J. Robinson, J. C. Barnard, M. A. Miller, J. E. Harries, J. E. Russell, and S. Dewitte (2006), Observations of the impact of a major Saharan dust storm on the Earth's radiation balance, Geophys. Res. Lett., 33, L24817, doi: /2006GL This work was funded through the NERC grant NE/C51785X/1 EVALUATION OF MODEL CLOUD AND RADIATION Model marine stratocumulus reflects too much shortwave radiation compared to GERB (Fig. 3) Angolan stratocumulus: bias diminishes from June-July 2006; coincides with reduced positive bias in model cloud liquid water verses SSM/I Convection across tropical Africa is overly intermittant in the model (Fig. 4) The reflectivity of tropical convective cloud over Africa is low compared to GERB Implementation of a decay time-scale param- etrization for convection improved comparisons Figure 5 (above): A large discrepancy in model OLR during July 2006 (a) is also present for cloud-screened data (b) and correlated with high mineral dust optical depth from MISR (c). Similar results were found for other years (Haywood et al. 2005). Figure 6: (a) Dust storm, 8 th March 2006 from the SEVIRI dust product.and (b) surface and (c) the atmosphere component of solar absoprtion from ARM/GERB data and in radiation models Dust Evaluation of the radiative energy balance in atmospheric models using satellite data, reanalyses and ground-based observations Richard P. Allan, A. Slingo (ESSC, University of Reading, UK) S.F. Milton, M.E. Brooks, (Met Office, Exeter, UK) Figure 1 (right): Mean model minus GERB systematic biases in the (left) all- sky and (right) clear-sky radiation budget for April-September 2006, 12 UTC data. Top: outgoing longwave radiation (OLR). Bottom: reflected shortwave radiation. Figure 2 (above): Time series of mean model minus GERB systematic biases in (a) outgoing longwave radiation and (b) shortwave albedo for land regions using 12 UTC data. Figure 4: Shortwave albedo at 12 UTC for model (left) and GERB (right). Top: 5 th June Bottom: 5 th December 2006 after model upgrade. Figure 7 (right): de-seasonalised low-latitude ocean anomalies of water vapour and clear-sky radiation for re-analyses, observations and IPCC AR4 models Models reproduce the increases in water vapour with surface warming (Fig. 8a) There is a strong increase in the net cooling of the atmosphere to the surface as the surface warms (Fig. 8b) Satellite estimates of changes in top of atmosphere radiation suffer from sampling and calibration issues (Fig. 8c) Figure 8 (left): scatter plots of de-seasonalised anomalies in clear-sky radiation, water vapour and surface temperature (Ts) for NCEP, satellite observations and a climate model over oceans a Figure 3: Angolan marine stratocumulus. Model minus GERB cloud albedo effect (left) and model minus SSM/I cloud liquid water (right) for June 2006 (top) and July 2006 (bottom). MINERAL DUST AEROSOL Recent analysis of GERB and MISR data (Fig. 5) indicate an appreciable longwave radiative signature of desert dust aerosol; the NWP model does not at present represent mineral dust aerosol events Work is currently underway to assess the impact of desert dust on (i) model circulation and forecast skill, and (ii) the atmospheric radiative flux divergence: A case study for the large dust storm in March 2006 (Fig. 6) indicates that radiation models underestimate atmospheric absoprtion of solar radiation during this event (Slingo et al. 2006) ab cd


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