1. IACETH Institute for Atmospheric and Climate Sciences Indirect aerosol effects in EC earth Trude Storelvmo and Ulrike Lohmann, ETH-Zurich

2. IACETH Institute for Atmospheric and Climate Sciences Outline  Motivation – the treatment of the aerosol indirect effect in the IPCC AR4 transient simulations  Experimental setup  Reults  Ongoing work / Plans for the (near) future

3. IACETH Institute for Atmospheric and Climate Sciences Aerosol indirect effects 1)1st aerosol indirect effect/radius effect = an increase in cloud albedo due to the decrease in cloud droplet sizes that follows when cloud droplet number concentration increases. 2)2nd indirect effect/cloud lifetime effect = an increase in cloud thickness, cloud cover and/or lifetime as a consequence of 1). IPCC, 2007

4. IACETH Institute for Atmospheric and Climate Sciences Aerosol indirect effects, Ch.10 IPCC AR4 (2007) Out of 23 models/model versions in Chapter 10 of IPCC AR4, 9 included aerosol indirect effects. 3 models included an online sulfur cycle All except one used empirical relationships to calculate cloud droplet number concentrations (CDNCs) LW SW

5. IACETH Institute for Atmospheric and Climate Sciences Experimental setup: ● Question: What governs the spread in shortwave forcings in the transient IPCC AR4 models? How much of this spread is due to the different methods of calculating CDNC and the first aerosol indirect effect alone? ● Experimental setup: 2 simulations are carried out for each CDNC scheme: One with present- day aerosol concentrations and one with preindustrial aerosol concentrations - In total 8 5-year simulations - All with climatological SSTs - Resolution: T95 and 40 vertical levels.

6. IACETH Institute for Atmospheric and Climate Sciences MODEL IMPLEMENTATIONS. Monthly mean aerosol fields (sulfate, OC, BC, sea salt and dust) are implemented. The aerosol fields are the average fields from the models contributing to the 3rd IPCC report (2001), and there are different fields for present-day and preindustrial runs. The first aerosol indirect effect (i.e. the radius effect) has been calculated, based on 4 different empirical relationship between cloud droplet number concentration (CDNC) and aerosol mass

7. IACETH Institute for Atmospheric and Climate Sciences 4 CDNC schemes: 1) 2) 3) 4) Boucher & Lohmann (1995): CDNC function of SO 4 only Jones et al. (2001): CDNC function of SO 4 and seasalt. Menon et al. (2002): CDNC function of SO4, POM and seasalt Dufresne et al. (2005): CDNC function of SO4 only, relationship fitted to POLDER data

8. IACETH Institute for Atmospheric and Climate Sciences

9. IACETH Institute for Atmospheric and Climate Sciences Cloud droplet number concentration (cm -3 ) in EC earth based on 4 different methods: Based on monthly mean aerosol fields from IPCC TAR

10. IACETH Institute for Atmospheric and Climate Sciences CDNC schemeAerosol indirect effect Natural CDNC at 950 hPa ΔCDNC at 925 hPa (cm) Boucher & Lohmann (1995) -0.91111.422.3 Jones et al. (2001) -0.97107.839.8 Menon et al. (2002) -1.94158.7123.1 Dufrese et al. (2005) -0.6241.910.6

11. IACETH Institute for Atmospheric and Climate Sciences Resulting aerosol indirect effect The uncertainty in SW radiative forcing due to using different methods for predicting cloud droplets (all else being equal) is as large (~2 W/m 2 ) as the uncertainty range in SW forcing from the coupled simulations for IPCC AR4. !

12. IACETH Institute for Atmospheric and Climate Sciences Cloud droplet number and albedo Hobbs, 1993: Academic Press

13. IACETH Institute for Atmospheric and Climate Sciences Quick validation of cloud fields CC (%)IWP (g/m 2 ) LWP (g/m 2 ) IFS 62 - 62.2 28.7- 29.0 58.4 – 59.9 OBS 62-6729.450-84 The IFS cloud fields compare very well with satellite & surface observations!

14. IACETH Institute for Atmospheric and Climate Sciences Conclusions & plans for the near future: ● A comparison of 4 different methods to predict CDNC has been carried out in EC earth, leading to a wide spread in the aerosol indirect effect. ● Plans for the near future: 1) Present results at ICCP conference in Mexico, July 2008 2) Submit paper on CDNC scheme comparison, Summer 2008. 3) Prepare IFS microphysics for merging with TM5/M7 aerosol treatment, ~ August 2008.