“Uncertainty  in  climate  sensitivity:  Cloud  adjustment  and  feedback”  	  Youichi	  KAMAE	  Assistant  Professor,  Faculty  of  Life  and  Environmental  Sciences  University  of  Tsukuba    During	  the	  recent	  decades,	  uncertainty	  range	  of	  climate	  sensitivity	  (CS),	  determined	  as	   global-­‐mean	   surface	   temperature	   increase	   in	   response	   to	   doubling	   of	   CO2	  concentration,	  has	  not	  been	  reduced	  efficiently	  despite	  improving	  the	  quality	  of	  clim	  ate	   model	   projections.	   Literatures	   revealed	   that	   it	   is	   essential	   how	   to	   determine	  radiative	  forcing	  and	  climate	  feedback	  for	  decomposing	  sources	  of	  uncertainty	  in	  CS.	  
It	   is	   also	   commonly	   known	   that	   an	   instantaneous	   CO2	   forcing	   can	   lead	   to	   fast	  responses	   in	   stratosphere	   and	   troposphere	   (e.g.	   vertical	   profiles	   of	   temperature,	  humidity,	   cloud	   and	   circulation)	   which	   results	   in	   a	   rapid	   perturbation	   in	   radiative	  balance	   at	   the	   top	   of	   the	   atmosphere.	   Atmospheric	   processes	   associated	   with	   the	  tropospheric	  adjustment	  including	  slowdown	  of	  hydrological	  cycle	  were	  suggested	  to	  	  be	   important	   for	   projections	   of	   global	   and	   regional	   climate	   changes	   including	  monsoons.	  	  In	   addition,	   cloud	   feedback	   that	   is	   essential	   for	   uncertainty	   in	   CS	   shows	   an	   apparent	  land-­‐sea	  contrast.	  Physically-­‐robust	  positive	  cloud	  feedback	  found	  over	  tropical	  land	  contributes	   to	   anomalous	   land	   warming	   even	   if	   the	   imposed	   radiative	   forcing	   is	  spatially	   uniform.	   As	   for	   the	   oceanic	   clouds,	   tropical	   low-­‐cloud	   and	   middle-­‐cloud	   are	  suggested	   to	   be	   essential	   for	   CS	   spread	   among	   climate	   models.	   Here	   a	   systematic	  modeling	   framework	   was	   applied	   to	   decompose	   structural	   and	   parametric	  uncertainties	   in	   CS.	   Model-­‐parameter	   sensitivity	   found	   in	   perturbed	   parameter	  ensembles	   reveals	   that	   the	   lower	   tropospheric	   mixing	   intensity	   can	   explain	   drying	   of	  the	   boundary	   layer	   in	   a	   warming	   climate.	   As	   a	   result,	   extremely-­‐high	   CS	   (e.g.	   6-­‐10	   K)	  can	  be	  constrained	  by	  the	  observed	  intensity	  in	  lower	  tropospheric	  mixing.	  However,	  a	  large	  spread	  is	  also	  found	  in	  middle	  cloud	  feedback,	  resulting	  in	  a	  large	  uncertainty	  in	  CS.	  A	  combination	  of	  different	  observational	  metrics	  is	  needed	  for	  more	  effective	  constraint	  of	  CS	  uncertainty.	  	  In	  this	  talk,	  the	  recent	  progresses	  in	  understanding	  of	  sources	  of	  CS	  uncertainty	  will	  be	   introduced.	   The	   effectiveness	   and	   limitation	   of	   a	   linear	   forcing-­‐adjustment-­‐
feedback	  concept	  are	  also	  discussed.	  The	  systematic	  frameworks	  including	  structural	  and	  parametric	  uncertainties	  will	  contribute	  to	  dispel	  the	  clouds	  of	  uncertainty	  in	  CS.