1. ENSEMBLES/RT3 worksession Land atmosphere coupling in the RACMO West-Africa simulations Bart van den Hurk Erik van Meijgaard Presented by Geert Lenderink

2. ENSEMBLES/RT3 worksession Status runs Model used: RACMO cy31 (≈ EC-Earth configuration,  European RT2b/RT3 simulations) –old (shallower) soil scheme (TESSEL) –different convection ERA40/Interim run –1986 – 1988: ERA40 –1989 – 2005: ERA-Interim –2006 – 2007: ERA-Interim currently running (until?) ECHAM5r3 –(almost) finished (at time of leave 2047)

3. ENSEMBLES/RT3 worksession Science questions Does RACMO reproduce precipitation climate well? Which areas expose strong land influence on precipitation variability on monthly timescales? Is this interaction explained by local atmospheric processes?

4. ENSEMBLES/RT3 worksession Seasonal patterns of precipitation 1986 – 2005

5. ENSEMBLES/RT3 worksession Reproduction of precipitation CRU TS3 monthly precipitation Mean land area 15S-25N, 15E-30E ERA40 > ERA-Interim discard 1986 – 1988

6. ENSEMBLES/RT3 worksession Anomaly correlation modelled-observed precipitation 1989 – 2005 Poor reconstruction local processes in soil/atmosphere of importance

7. ENSEMBLES/RT3 worksession Where is land effective in changing precipitation? Criteria (After Dirmeyer et al, 2008): 1.Surface perspective: Evaporation sensitive to soil moisture significant +corr. E and soil moisture 2.Atmospheric perspective: moisture recycling depends on local moisture supply significant +corr. recycling ratio and soil moisture * Recycling ratio: “Lagrangian” algorithm that accumulates evaporation in upstream grid points, and measures E/q within L km radius (100 < L < 1000 km)

8. ENSEMBLES/RT3 worksession Recycling ratio L = 200km

9. ENSEMBLES/RT3 worksession Recycling ratio L = 500km Length scale unimportant for spatial structure

10. ENSEMBLES/RT3 worksession Area of significant correlation Evap – soil moisture anomaly E controlled by soil moisture except in convective rainfall areas (where E is energy limited)

11. ENSEMBLES/RT3 worksession Area of significant correlation recycling – soil moisture + correlation: recycling senses local moisture supply In the landward part of the monsoon area In the retreat area of the wet season (wet  dry transition)

12. ENSEMBLES/RT3 worksession Soil memory* in areas with both positive correlations Interesting areas where land-atm interaction may affect precipitation on indicated time scale *autocorrelation time scale

13. ENSEMBLES/RT3 worksession Science questions – Conclusions Does RACMO reproduce precipitation climate well? –mean climate yes, but free interior leaves many degrees of freedom for local processes Which areas expose strong land influence on precipitation variability on monthly timescales? –at wet  dry transitions Is this interaction explained by local atmospheric processes? –work in progress –hurkvd@knmi.nl

14. ENSEMBLES/RT3 worksession Some processing Soil moisture memory: –e-folding time scale of soil moisture = time lag where autocorr. < 1/e Statistical significance –t-test 5% one-sided –degrees of freedom = nr of simulation days / soil moisture memory Recycling ratio –“Lagrangian” algorithm that accumulates evaporation in upstream grid points, and measures E/P within L km radius (100 < L < 1000km)

15. ENSEMBLES/RT3 worksession Mechanisms of coupling strong d LCL/d sm: LCL can reach LFC in dry conditions (neg feedback) strong d CTP/d sm: moisture supply strong d Hilow /d sm: break-up of convective inhibition

16. ENSEMBLES/RT3 worksession Experimental set-up RACMO cy31 (≈ EC-Earth configuration,  European RT2b/RT3 simulations) –old (shallower) soil scheme (TESSEL) –different convection 1986 – 1988: ERA40 1989 – 2005: ERA-Interim –2006 – 2007: ERA-Interim currently running

17. ENSEMBLES/RT3 worksession Soil moisture memory Long memory in dry seasons… …and in core of wet season In wet  dry transition some memory is maintained

18. ENSEMBLES/RT3 worksession Correlation E – soil memory + correlation: simultaneous occurrence of high memory and high E (transition from wet to dry)