Objective Estimation of the Radius of the Outermost Closed Isobar in Tropical Cyclones

ABSTRACT: Increased understanding of the importance of TC structure in dynamical, climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of outermost closed isobar (roci) has been developed. The new method uses storm position and global analyses of mean sea level pressure to compute a mean (axisymmetric) roci. This radius can be used, together with the central pressure, for the construction of a synthetic vortex that is initialized in a numerical prediction model. The method also has important applications in dynamical and climatological studies of TC intensity, size and structure. The algorithm is robust and capable of estimating roci, even in the case of a weak system that may not have a closed isobar in the global analysis. The values produced by the new method are shown to be more consistent than the corresponding operational estimates which are subjective and produced under strong time constraints. Statistical comparison between subjective and objective estimates gives a mean absolute difference of 110 km, which given the difficulty in making a subjective estimate, is satisfactory. In addition, even though limitations exist with the estimates of vortex parameters like the radius to gales (r34), comparison with estimates from an extended best track data set provides independent evaluation of the scheme. Mean absolute difference for r34 for around 3200 cases is near 80 km, even though the best track estimates are subjective and the objective r34 is estimated only from storm central pressure and the objective roci. This validation suggests that the algorithm can be used to obtain useful size estimates of TCs. Keywords: tropical cyclone, size, estimation