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SUMMARY:Identifying Rossby wave packets using local finite amplitude wave 
 activity - Paolo Ghinassi\, Institut für Physik der Atmosphäre\, Univers
 ität Mainz
DTSTART:20170928T110000Z
DTEND:20170928T120000Z
UID:TALK83481@talks.cam.ac.uk
CONTACT:9780
DESCRIPTION:Upper tropospheric Rossby wave packets (RWPs) are often associ
 ated with weather systems and can thus have a strong influence on surface 
 weather. They sometimes act as precursors to blocking or intense extratrop
 ical cyclones and are\, in this sense\, connected with\nsevere weather epi
 sodes. Therefore\, understanding the dynamics of RWPs is of fundamental\ni
 mportance in the context of predictability.\nThis contribution presents lo
 cal finite amplitude wave activity (LWA) as a novel diagnostic for RWPs. L
 WA\, which is an extension of the finite amplitude wave activity of Nakamu
 ra\nand Solomon\, is proportional to the local meridional displacement of 
 contours of potential\nvorticity (PV) from zonal symmetry. The advantage o
 f using LWA consists in the fact that\nits formulation does not make any s
 mall amplitude assumption\; it is able to faithfully identify nonlinear ph
 enomena such as Rossby wave breaking\, blocking\, PV streamers\, or cutoff
 s.\nFurthermore\, LWA has an exact conservation relation which allows one 
 to formulate a budget equation for its evolution and to quantify the impac
 t of non-conservative processes as a\nresiduum from the LWA budget.\nFollo
 wing the main idea of upscale error propagation\, this diagnostic is appli
 ed to data\nfrom the global forecast model ICON to quantify the error grow
 th at the planetary scale.\nAt the same time\, the LWA budget equation is 
 used to estimate the magnitude of the\nnon conservative term. A challenge 
 in this context is the cascade of enstrophy to smaller\nscales\, which res
 ults in an unavoidable sink term in the LWA budget due to diusion of\nvor
 ticity. Preliminary results show that non conservative processes play a no
 n-negligible role\nin the propagation of RWPs\, suggesting that their misr
 epresentation in the models can lead\nto poor forecasts.
LOCATION:MR14\, Centre for Mathematical Sciences
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