Adapting the Knaff and Zehr wind-pressure relationship for operational use in Tropical Cyclone Warning Centres

Abstract

A methodology to implement a new operational mean sea-level pressure versus maximum mean near-surface wind relationship (hereafter wind-pressure relationship) based on a modified version of the approach recently developed by Knaff and Zehr is presented. This method incorporates latitude, size, storm motion and environmental pressure. In this formulation tropical cyclone size is expressed in terms of the radius of gales, environmental pressure is expressed in terms of the pressure of the outer closed isobar – parameters that are already routinely available in the operational setting. The resulting wind speed is also converted from a one-minute mean to a ten-minute mean. In addition, a correction is introduced for a weakness in the original Knaff and Zehr formula for storms located at low latitudes. The resulting algorithm is able to be integrated into the operational software to assist in the efficient calculation of the central pressure. Other Tropical Cyclone Warning Centres are encouraged to use this derivation. The new approach also presents the potential to reanalyse past tropical cyclone events to create a more homogeneous data-set. Some historical variations in intensity derivation in the Australian region are also presented. Differences between Atlantic basin best-track reconnaissance data and Dvorak-based data relied upon in other tropical cyclone basins result in some biases associated with the determination of the maximum wind. Dvorak-derived maximum wind corresponds well with the reconnaissance-derived maximum wind for non-weakening systems with intensities associated with Dvorak current intensity (CI) numbers of 3.5 and higher (i.e. greater than ~50 knots). At weaker intensities the Dvorak intensity underestimates the maximum wind. As a result the calculated pressure via a wind-pressure relationship, which is strongly influenced by reconnaissance-based intensity estimates, may result in higher pressures than is actually the case.