Publication: An investigation into gradient balance of flight-level tropical cyclone windfields

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Title An investigation into gradient balance of flight-level tropical cyclone windfields
Authors/Editors* M Gibbons, C Miller
Where published* 29th Conference on Hurricanes and Tropical Meteorology
How published* Proceedings
Year* 2010
Publisher American Meteorological Society
Keywords gradient balance, flight level, wind speed, tropical cyclone
The assumption that gradient balance exists outside of the boundary layer within a tropical cyclone is an important one for many tropical cyclone modelling applications. In this study we consider the relationship between geopotential height and velocity and whether gradient balance exists using observed 700 hPa flight-level data obtained in North Atlantic hurricanes. Using the method of Kepert (2005) storm tracks are first determined for individual storms for the period 2001-2008 by compositing flight-level centre fixes obtained over all the flights associated with a particular storm. The resulting storm tracks are then used to convert the observed flight-level geopotential height and velocity data from an Earth relative coordinate system to a storm centre relative system. Two-dimensional (2-D) geopotential height fields are then calculated using the storm centre relative geopotential height observations composited over an appropriate time period using the objective analysis method described by Mueller et al. (2006). Following the calculation of the 2-D geopotential height fields, the corresponding velocity fields were calculated using one of two methods. The first is based upon an assumption of axisymmetric flow about the centre of the storm, while the second allows for asymmetries in both the radial and tangential directions about the storm center. The computed velocity values are then compared to the actual observed values along the radial legs flown by the aircraft by calculating the error between the two sets of values. The resulting errors are then classified by such characteristics as the velocity calculation method, the quadrant of the storm within which the observations fall, storm size, storm intensity and change in storm intensity (strengthening or weakening).
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