Geosciences Dept. Seminar: Gradient imbalance at the outflow layer of Hurricanes: observations in high resolution reanalysis and the outline of a novel, idealized, hurricane model

Yair Cohen, TAU​

21 March 2016, 11:00 
Shenkar Building, Holcblat Hall 007 
Geosciences Dept. Seminar

Abstract:

The gradient balance is an integral part of topical meteorology and of Hurricanes in particular. It is only due to this balance that the otherwise ineffectively-large deformation radius in the tropics can be reduced to sensible scales so that a quasi-balance flow may organize into a Hurricane. To zeros-order the primary circulation in a Hurricane is regarded to be in gradient balance and therefore the basic understanding of Hurricanes was sought in axi-symmetric models, in which this balanced is imposed (e.g. Charney and Eliassen, 1964; Ooyama, 1969; Emanuel, 1986). However, an axi-symmetric Hurricane has to be equivalent barotropic - a state in which the contours of geopotential height and of temperature are parallel - and the geostrophic wind does not veer with height. In such Hurricanes a High at the top-center is located directly above the Low at its bottom-center. When a storm is small enough the curvature of the isobars at its top could yield a centrifugal acceleration which violates the gradient wind balance and enhance the divergence at the hurricanes' outflow.

 

In this work, some fifty three Hurricanes during 2004-2015 in the E-Pacific and W-Atlantic basins are tracked in a 12km-resolution North American Meso-scale reanalysis (NAM12). This data set used in order to examine the imbalance at the top of Hurricanes and the associated divergence mechanism. It is found that a larger percentage of these storms are indeed equivalent barotropic. In these storms the divergence at the outflow layer is controlled by gradient imbalance at intensities which are not found elsewhere in the atmosphere. Moreover, the radius at which gradient imbalance is found at the outflow (height of 16 km) is correlated with the radius of maximum winds (1 km above the sea surface). Thus, a novel explanation for the scale of certain tropical cyclones is proposed.

 

These findings motivate a relaxation of the gradient wind balance in an idealized Hurricane model. At the end of the talk I will describe in details a novel, convective permitting, layer-model that is aimed in order to examine the steady state (and the time evolution towards it) in an idealized tropical cyclone controlled by gradient imbalance.  

 

 

Seminar Organizer: Dr. Ravit Heled

 

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