BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//132.216.98.100//NONSGML kigkonsult.se iCalcreator 2.20.4//
BEGIN:VEVENT
UID:20250921T034746EDT-15509vVwVT@132.216.98.100
DTSTAMP:20250921T074746Z
DESCRIPTION:Prof. Yair Cohen joins us from Tel Aviv University to give an i
 nformal seminar on the Gradient Imbalance in the Outflow of Tropical Cyclo
 nes from NAM12 reanalysis and the Basics of a New Model\n\nAbstract\nTropi
 cal cyclones (TCs) are some of the most destructive natural disasters on e
 arth (Emanuel\, 2005). TCs are known for their intense primary (horizontal
 ) circulation which is sustained by their secondary (in-up- out-down) circ
 ulation. The basic understanding of the relation between these two circula
 tion in TCs has been advanced using axi-symmetric TC models (e.g. Charney 
 and Eliassen\, 1964\; Ooyama\, 1969\; Emanuel\, 1986). All of these models
  assume that the winds in the free atmosphere are in gradient wind balance
  – what is known as the balanced vortex assumption (i.e. Sawyer-Eliassen e
 quation). However\, an axi-symmetric TC is also equivalent barotropic - a 
 state in which the contours of geopotential height and of temperature are 
 parallel - and thus geostrophic wind does not veer from one pressure level
  to the other. In this case\, a High at the top-center of the TC is locate
 d directly above the Low at the bottom-center of the TC. If the storm is s
 mall enough then the curvature of the isobars at the top of the storm may 
 violate the gradient wind balance (i.e. the combined centrifugal and press
 ure gradient forces cannot be balanced by the Coriolis force)\, thus enhan
 cing the divergence at the top of the TC.\nIn this work the 12km-resolutio
 n North American Meso-scale reanalysis (NAM12) is used in order to examine
  the imbalance at the top of TCs and the associated divergence mechanism. 
 Following a definition of several indexes\, the correlation between the eq
 uivalent barotrpic structure\, gradient imbalance at the top\, wind diverg
 ence at that level and the storm intensity is examined at 6 hours interval
 s in 14 TCs in the years 2004-2009. In the years 2007-2009 the storms are 
 smaller\, axi-symmetric and surprisingly equivalent barotropic. In these s
 torms the upper level winds are in gradient imbalance and this imbalance c
 orrelates to upper level divergence and maximum angular velocity at the 85
 0mb. On the other hand\, in the years 2004-2005 the storms are larger\, mo
 re axi-asymmetric and thus in gradient balance throughout the troposphere.
  An explanation for causes for the change in structure of these storms nee
 ds to further examined. Moreover\, these findings imply that the balanced 
 vortex assumption should be relaxed in axi-symmteric TC-models. At the end
  of the talk\, the outlines of a new\, axi-asymmetric TC- model which is d
 eveloped in TAU will be presented.\n
DTSTART:20150629T190000Z
DTEND:20150629T210000Z
LOCATION:Room 934\, Burnside Hall\, CA\, QC\, Montreal\, H3A 0B9\, 805 rue 
 Sherbrooke Ouest
SUMMARY:Seminar: Prof. Yair Cohen
URL:/meteo/channels/event/seminar-prof-yair-cohen-2537
 26
END:VEVENT
END:VCALENDAR