A Novel Multiscale Intensity Metric for Evaluation of Tropical Cyclone Intensity Forecasts

Abstract

In this study, a new multiscale intensity (MSI) metric for evaluating tropical cyclone (TC) intensity forecasts is presented. The metric consists of the resolvable and observable, low-wavenumber intensity represented by the sum of amplitudes of azimuthal wavenumbers 0 and 1 for wind speed within the TC vortex at the radius of maximum wind and a stochastic residual, all determined at 10-m elevation. The residual wind speed is defined as the difference between an estimate of maximum speed and the low-wavenumber intensity. The MSI metric is compared to the standard metric that includes only the maximum speed. Using stepped-frequency microwave radiometer wind speed observations from TC aircraft reconnaissance to estimate the lowwavenumber intensity and the National Hurricane Center's best-track (BT) intensity for the maximum wind speed estimate, it is shown that the residual intensity is well represented as a stochastic quantity with small mean, standard deviation, and absolute norm values that are within the expected uncertainty of the BT estimates. The result strongly suggests that the practical predictability of TC intensity is determined by the observable and resolvable low-wavenumber intensity within the vortex. Verification of a set of highresolution numerical forecasts using the MSI metric demonstrates that this metric provides more informative and more realistic estimates of the intensity forecast errors. It is also shown that the maximum speed metric allows for error compensation between the low-wavenumber and residual intensities, which could lead to forecast skill overestimation and inaccurate assessment of the impact of forecast system change on the skill.

Assimilation of High-Resolution Tropical Cyclone Observations with an Ensemble Kalman Filter Using HEDAS: Evaluation of 2008–11 HWRF Forecasts

“…Forecast errors remain about constant through 108 h and increase by 120 h, probably due to the small sample size (Fig. The runs with no data assimilated have a substantial high intensity bias after 36 h. The low intensity biases in the short range for all three sets of runs may be due to the vortex spindown issue discussed in Vukicevic et al (2013), and suggest that this issue is also important in the vortex initialization scheme. The difference between the forecast intensity errors is expected to be larger than for track errors, since intensity is at least partially controlled by processes within the TC core.…”

“…Aksoy et al (2013) demonstrated the value of assimilating airborne Doppler radar radial wind data with HEDAS on 83 cases in 20 TCs during the 2008-11 seasons. Vukicevic et al (2013) found that these errors were caused by a short-term spindown of the entire vortex circulation in each data assimilation cycle, and that the magnitude was correlated with the TC intensity. They showed direct positive impact on the vortex wind structure, but also indirect positive impact on the thermodynamic structure.…”

Evaluation and Predictability of Observation-based Surface Wind Asymmetric Structure in Tropical Cyclones

“…One remaining question is can these analyses be used in some predictive or verification capacity? Vukicevic et al (2014) compared the low wavenumber maximum from a TC model against those determined from SFMR data and proposed their use as an alternative method of forecast verification. A next step here is to reproduce a similar comparison but with the scatterometer data.…”

“…To determine and quantify motion and shear-dependent low wavenumber asymmetric surface wind structure using global scatterometer data, the present study expands on previous work from Vukicevic et al [(2014), herein referred to as V14] and U14 that examined surface wind asymmetry from SFMR and model data. In V14, a…”

“…Understanding the physical processes associated with changes in TC surface wind structure can in turn be used to evaluate their appearance in numerically simulated TCs. Similarly derived quantities from model and observational sources can also be used as an alternative method for TC intensity verification purposes (i.e., Vukicevic et al 2014). Note that some of these ideas have been investigated with aircraft data in a general sense ), but because of some of the aircraft constraints mentioned above, it is worthwhile to investigate the usefulness of scatterometer that more regularly observe TCs compared to reconnaissance aircraft.…”