Prof. Yair Cohen joins us from Tel Aviv University to give an informal seminar on the Gradient Imbalance in the Outflow of Tropical Cyclones from NAM12 reanalysis and the Basics of a New Model
Tropical cyclones (TCs) are some of the most destructive natural disasters on earth (Emanuel, 2005). TCs are known for their intense primary (horizontal) circulation which is sustained by their secondary (in-up- out-down) circulation. The basic understanding of the relation between these two circulation 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 equation). 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 located directly above the Low at the bottom-center of the TC. If the storm is small 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 pressure gradient forces cannot be balanced by the Coriolis force), thus enhancing the divergence at the top of the TC.
In this work the 12km-resolution 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 equivalent barotrpic structure, gradient imbalance at the top, wind divergence at that level and the storm intensity is examined at 6 hours intervals 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 storms the upper level winds are in gradient imbalance and this imbalance correlates to upper level divergence and maximum angular velocity at the 850mb. On the other hand, in the years 2004-2005 the storms are larger, more axi-asymmetric and thus in gradient balance throughout the troposphere. An explanation for causes for the change in structure of these storms needs 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 developed in TAU will be presented.