Development of Global Statistical-Dynamical Tropical Cyclone Wind Radii and MSLP Guidance

Global tropical cyclone (TC) warning centers routinely estimate the maximum radial extent of significant wind speed thresholds (e.g., the radial extent of 34-knot wind speed) as part of their TC advisory and warning process. These estimates are typically referred to collectively as wind radii. The production of quality wind radii is important to operations for a number of reasons beyond the obvious specification of TC vortex structure. The primary purpose of wind radii is to provide quantitative estimates of the TC wind structure for the production of effective warnings of on-station and on-ship gale-force (34-kt), damaging (50-kt), and destructive winds (64-kt) winds. Wind radii also provide initial conditions for a number of applications such as wind speed probabilities, TC Conditions of Readiness, and wave forecasting. Operational wind radii forecast guidance currently comes from purely statistical models and numerical models. To complement these forecast tools, this study develops a statistical-dynamical wind radii forecast capability for all of the global tropical cyclone basins.

Since historical wind radii are generally of poor quality and temporally and spatially inhomogeneous, this study uses a satellite-based tropical cyclone size estimates that are both temporally and spatially homogeneous as a basis for prediction. The satellite-based TC sizes that are scaled by climatology as a function of intensity (FR5) are used as the independent variable (predicant) in the statistical-dynamical scheme and predictors are derived from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) large-scale environmental diagnostics. Using the predicted FR5 and the predicted intensity (e.g., from SHIPS), forecast of TC size estimates is obtained. Azimuthally averaged wind radii can then be estimated from statistical relationships between TC size and azimuthally averaged Atlantic and East Pacific best tracked wind radii. Asymmetries are applied using relationships from the wind radii Climatology and Persistence (CLIPER) statistical model, which are a function of storm motion and latitude, to provide wind radii in all quadrants. Finally, given storm motion, intensity, and wind radii, estimates of MSLP are then also estimated using the Knaff/Zehr/Courtney methodology.

Specifics about the data sets, the statistical model formulation, the statistical TC size and wind radii relationships, as well as vortex model development and assumptions will be presented in this talk. In addition, details of the environmental and climatological factors important for TC size and thus for wind radii variations will also be discussed, highlighting differences between the global tropical cyclone basins. Finally, some verification statistics will be presented for the North Atlantic and East Pacific basins, where best tracked wind radii are available.

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. This research was sponsored by the GOES-R project office and the Joint Hurricane Testbed.