Changes between Version 3 and Version 4 of DevelopmentActivities/CMIP6/DevelopmentsCMIP6/zo_evaporation

10/20/15 17:22:38 (5 years ago)



  • DevelopmentActivities/CMIP6/DevelopmentsCMIP6/zo_evaporation

    v3 v4  
    113113  (5) We also agreed to make a review of how z0s are calculated in both ORCHIDEE and LMDZ, highlighting the inconsistencies between the two models, the zones that remain unclear (like around topographic drag), and the order of magnitudes of the involved quantities (z0 mostly), since the goal is not only to change the calculations, but also to improve them, and for good reasons. [[BR]] 
     116== Additional results by Jan Polcher == 
     118Within the Earth2Observe project a paper on the Tier 1 simulations has been submitted by Schellekens et al.. it include the following table : 
     120[[Image(Table3.png, 80%)]] 
     122For this dataset it was the Trunk of July 2014 which was run but with the new driver. This meant a more accurate calculation of ETP from the WFDEI_CRU forcing. One immediately notes that evaporation is too high in ORCHIDEE and thus sets us apart from the other models. In order to test how much this is the result of years of accepting too high turbulence in order to compensate for too low ETPs produced the old driver, I re-ran the same code but dividing z0 by 10. The simulation is not yet finished so only the 90s are analysed. 
     124The change in z0 is pictured below : 
     126[[Image(Z0_Diff.png, 80%)]] 
     128This modification leads to a weak but significant reduction of evaporation as pictures below : 
     130[[Image(Evap_Diff.png, 80%)]] 
     132Globally this translate into a reduction of evaporation of about 4%. 
     134If we consider that the 90's are a representative sample in the 34 WFDEI_CRU forcing, then we can apply this reduction to the mean ORCHIDEE number of 609 kg/m²/y and obtain 585kg/m²/y. With this reduction, ORCHIDEE is still in the higher evaporating models but very close to HTESSEL and SURFEX.  
     136This suggests to me that indeed we have been using Z0h=Z0m for years as a compensation for errors in the forcing. This is corroborated by analysis of the water cycle of the Mediterranean basin where ORCHIDEE systematically shows too much water in the rivers and thus too little evaporation. Choosing lower Z0h values in ORCHIDEE will allow to differentiate Z0m and Z0h in LMDZ, lowers evaporation and probably prepare us for a more complete parametrisation for estimating Z0h, like the one proposed by Su et al.