Special ClimaTea Lecture


Thursday, May 5, 2016, 3:00pm


HUCE Seminar Room

"Gradient Wind Imbalance at the Outflow of Hurricanes" by Yair Cohen


The gradient wind balance is an integral part of tropical 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 locally reduced to sensible scales so that a quasibalance flow may organize in a scale of a Hurricane. Moreover, 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 axisymmetric models, in which this balanced is imposed via the Sawyer Eliasson equation (e.g. Charley and Eliassen, 1964; Ooyama, 1969; Emanuel, 1986). However, an axisymmetric 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 topcezter is located directly above the Low at its bottom center. When a storm is small enough, the combined gradient and curvature of the isobaric map at its top could yield a scenario in which the sum of the outwards pointing centrifugal and pressure gradient forces cannot be balanced by the inward pointing Coriolis force. In that case the gradient wind balance is violated (gradient imbalance) which enhances the divergence at the hurricanes' outflow.

In this work, some fifty three Hurricanes during 20042015 in the Pacific and Atlantic basins are tracked in a 12km resolution North American Mesoscale model (NAM12). This data set is used in order to examine the imbalance at the top of Hurricanes and the associated divergence mechanism. We find that the storms in the data are almost equally divided into two groups: 1. Small, organized (i.e. equivalent barotropic) storms in which the gradient wind balance in violated at their top and 2. Large, less organized storms, in which the gradient balance holds anywhere in the free atmosphere. It is found that the smaller storms the intensity of the imbalance is well correlated with outflow intensity, minimum 850mb geopotential height and maximum wind speed. Similar correlations in the group large storms is very poor. Moreover for a given maximal temperature perturbation in the warm core, in the smaller (organized) storms the minimum 850mn geopotential height is exactly 60m lower than in the larger storms implying stronger storms for the same temperature perturbation.

These findings motivate a relaxation of the gradient wind balance in an idealized Hurricane model. Towards the end of the talk a novel, convective permitting, layer model will be presented along with its conservation laws.