Improving Tropical Cyclone Guidance Tools by Accounting for Variations in Size

Tropical cyclones (TCs) come in a variety of sizes. The areal extent of largest TC can be 50 times larger than the smallest TC. In terms of radial extent of circulation, differences are generally less than an order magnitude, but are nonetheless important. As a result, the core region of a large TC can nearly encompass the entire circulation of a very small TC. These size variations have been documented by many past studies and are often reflected in subjective and method-dependent operational measures of TC size, namely the Radius of Outermost Closed Isobar (ROCI) and radial extent of gale/34-kt winds (R34). The R34 and ROCI estimates however have been shown to be inhomogeneous both in time and from TC basin to TC basin and are considered to be of relatively low accuracy. As a result, size variations have generally been poorly accounted in TC guidance applications, especially for those whose development require a large number of TC cases. This results in less than ideal statistics and guidance tools that generally perform best for average-sized TCs. To overcome some of the shortcomings of operational ROCI and R34 estimates an infrared (IR, i.e., ~11µm wavelength) satellite algorithm was developed to provide an objective TC size estimate (Knaff et al. 2013). The metric, R5, is in the simplest terms defined as the radius of where the TC wind field is indistinguishable from the background flow in a climatological environment. The R5 algorithm requires only position information and a single IR image; making its applicability global with the current constellation of geostationary satellites. Furthermore the estimates of this technique have been shown to be fairly well related to R34 with the added benefits, like ROCI, that it can be applied to TCs that have maximum winds less than 34 knots. Since R5 is independent of operational intensity estimates, applicable to all TC intensity ranges, and can be estimated from readily available data and information, R5 and related global statistics provide a relatively simple way to account for the variations of TC size while developing TC guidance. This presentation will describe how R5 is calculated, how it can be used to spatially scale TC development applications and test the premise of this scaling on a number of operational and pre-operational TC guidance and diagnostic applications. The guidance applications include the Statistical Hurricane Intensity Prediction Scheme (SHIPS, DeMaria et al. 2005), the Logistic Growth Equation Model (DeMaria, 2009) and experimental version of the Rapid Intensity Index (Kaplan et al. 2010). Specifically we will experiment with scaling digital IR and lightning density information to better define various regions of TC such as the inner core/eyewall region versus the outer rainband region. Furthermore we will also investigate the biases of a few operational objective intensity estimation techniques as a function of the R5 metric to determine its utility for refining subjective operational intensity estimates.

DISCLAIMER: The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or U.S. Government position, policy, or decision.