MAY 2016 S H I M A D A E T A L . 1823 Evaluation of the Accuracy and Utility of Tropical Cyclone Intensity Estimation Using Single Ground-Based Doppler Radar Observations UDAI SHIMADA AND MASAHIRO SAWADA Meteorological Research Institute, Tsukuba, Ibaraki, Japan HIROYUKI YAMADA University of the Ryukyus, Nishihara, Okinawa, Japan (Manuscript received 18 July 2015, in final form 21 December 2015) ABSTRACT Intensities (central pressures) of 28 cases of 22 tropical cyclones (TCs) that approached Japan were estimated by using single ground-based Doppler radar observations, and the accuracy and utility of the estimation method were evaluated. The method uses the ground-based velocity track display (GBVTD) technique, which retrieves tangential winds, and the gradient wind balance equation. Before application of the method to the 28 cases, a preliminary experiment was performed with pseudo-Doppler velocities obtained by numerical simulation to confirm that the method could reasonably estimate central pressures. Compared with best track data from the Regional Specialized Meteorological Center (RSMC) Tokyo, the estimated intensities of the 28 cases had a root- mean-square error of 8.37 hPa and showed a bias of 1.51 hPa. This level of accuracy is comparable to or better than the accuracies of Dvorak and satellite microwave-derived estimates. Two distance metrics are defined: 1) the distance between the TC center and the radar location and 2) the distance between the TC center and the weather station whose sea level pressure was used as an anchor for pressure measurement. In general, the accuracy of the Doppler radar estimates was higher when the distance metrics were shorter, as well as when wind retrieval accuracy was better and radar coverage was denser. For TCs with a radius of maximum wind of 20– 70 km, the estimated central pressures had a root-mean-square error of 5.55 hPa. These results confirm that Doppler radar intensity estimates have sufficient accuracy and utility for operational use. 1. Introduction and Nakazawa 2007; Sakuragi et al. 2014). The Dvorak technique is an empirical method based on past re- It is of great importance to analyze tropical cyclone connaissance observations and the subjective classifica- (TC) intensity (i.e., central pressure and maximum sus- tion of cloud patterns, although the advanced Dvorak tained wind) with high accuracy, not only from the per- technique is a fully automated method (Olander and spective of improving scientific understanding of various Velden 2007). Methods using satellite microwave phenomena associated with TCs, but also for disaster sounding and imager data are entirely objective, but in prevention and mitigation. TC intensity estimates are the western North Pacific, satellite methods refer to the generally made using infrared satellite data, for example, best track data as truth. Because these data are based by the Dvorak technique (e.g., Dvorak 1975, 1984; Koba mainly on the Dvorak technique, the uncertainty of the et al. 1990; Velden et al. 1998; Olander et al. 2004; satellite methods includes the estimation error associated Olander and Velden 2007), satellite microwave sounding with the Dvorak technique. In addition, these conven- data (Brueske and Velden 2003; Herndon and Velden tional methods estimate TC intensity from relevant 2004; Demuth et al. 2004; Oyama 2014), or satellite mi- physical values such as cloud patterns and upper-level crowave imager data (Bankert and Tag 2002; Hoshino warm-core anomalies, rather than by using a straight- forward physical equation that describes pressure distri- butions. Therefore, conventional estimates can have a Corresponding author address: Udai Shimada, Typhoon Research Department, Meteorological Research Institute, 1-1 Nagamine, large margin of error. Although it is not realistically Tsukuba, Ibaraki 305-0052, Japan. possible at present to obtain dropsonde observations of E-mail: [email protected] minimum sea level pressures (MSLPs) of TCs in the DOI: 10.1175/MWR-D-15-0254.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/01/21 03:37 AM UTC 1824 MONTHLY WEATHER REVIEW VOLUME 144 western North Pacific Ocean, it is still necessary to esti- GBVTD retrievals. The Japan Meteorological Agency mate TC intensities with as much accuracy as possible, (JMA) upgraded all of the radars in its observation particularly for TCs that approach populated areas. network to Doppler radars between 2006 and 2013, and One method of addressing this problem is to estimate the new Doppler radars have collected data on many central pressures by using a physical equation, namely, TCs since their installation. the gradient wind equation. Lee et al. (2000) developed a The purpose of this study was to estimate TC in- method that uses data from a single ground-based tensities by using the accumulated Doppler radar data Doppler radar (DR; hereafter the DR method). In this and to evaluate the accuracy and utility of the DR method, the ground-based velocity track display method. Before the estimation of TC intensities, we (GBVTD) technique (Lee et al. 1999) is used to retrieve performed a preliminary experiment with pseudo VD axisymmetric tangential wind velocities, Vt,ofaTCfrom obtained by numerical simulation to confirm that the the Doppler radial velocities, VD, under the assumption method could reasonably estimate central pressures. that there is one primary circular vortex around the TC This paper consists of six sections. We describe the center and that the asymmetric radial wind is much data and TC cases used and the study methodology in smaller than the corresponding tangential wind. Then, an section 2. We describe the preliminary experiment and MSLP is estimated by using an axisymmetric pressure present its results in section 3.Insection 4, we present the deficit deduced by applying the retrieved Vt to the gra- DR estimation results for real TC cases and their statis- dient wind equation and using a sea level pressure (SLP) tical verification. In section 5, we discuss the results, in- observation around the TC as an anchor for pressure cluding their utility, reliability, and limitations for measurement. Because the DR method calculates operational use, and present additional statistical verifi- MSLPs by using a physical equation related to the TC cations. Section 6 includes a summary and conclusions. wind field, the estimated MSLPs are expected to have higher accuracy than those derived from cloud patterns 2. Data, cases, and methodology and upper-level warm-core anomalies, although the DR a. Data method is applicable only to TCs that approach radar locations. To apply the DR method, it is essential to re- We used VD data observed by JMA C-band opera- trieve Vt from VD with high accuracy. Because the tional Doppler radars. Figure 1 shows the radar loca- GBVTD technique has been successfully used to retrieve tions used and the observational range of 2-km, Vt for many TCs (Lee et al. 2000; Harasti et al. 2004; constant altitude plan position indicator (CAPPI) data Lee and Bell 2007; Zhao et al. 2008; Zhao et al. 2012), (;200 km) for each location. The Doppler observation the DR method is promising. parameters of the Okinawa radar, which is representa- The DR method has already been implemented by the tive of the JMA radars, are listed in Table 1. We used National Hurricane Center (NHC; Harasti et al. 2007), both high and low dual-pulse repetition frequency (dual- where it is called Vortex Objective Radar Tracking PRF) data for the lowest-elevation plan position in- and Circulation (VORTRAC), but the accuracy with dicator (PPI) in order to see out to ;200-km range at which the DR method estimates MSLPs and the oper- 2-km altitude (see section 2c). Before 2009, the radars ational utility of the DR method have not yet been observed VD at 10-min intervals, and since 2009 the clarified comprehensively, probably owing to the lack of observation interval has been 5 min. In this study, sufficient TC examples. It is of great importance that dealiasing of VD by the Hybrid Multi-PRI (HMP) the disaster prevention information have high reliability method (Yamauchi et al. 2006) and by a newly de- and consistency for the forecasts to be seen as credible veloped correction method (not shown) that assumes and to avoid confusion. Thus, it is essential to examine there is a wavenumber-1 VD pattern around a radar lo- the accuracy of the estimated MSLPs and to determine cation influenced by TC circulation was performed au- their suitability for operational use. In addition, analysts tomatically. Additionally, noise caused by dealiasing adopting the DR method for TC intensity analysis need failure and sea clutter was removed from the data au- to have a good understanding of the DR method’s ac- tomatically by various quality control (QC) procedures. curacy and reliability, as well as of its strengths, weak- Although problems associated with the dealiasing cor- nesses, and limitations. rection of VD can greatly affect the accuracy of TC in- Many strong TCs approach and make landfall in Ja- tensity estimates, describing how to cope with these pan. In particular, many TCs approach southwestern problems is beyond the scope of this study. Therefore, Japan, including the Okinawa Islands, where there is we examine the accuracy of the estimated MSLPs based no topography effect on the TC structure. Therefore, on the premise that the Doppler velocity data have been TCs approaching this region are ideally suited for dealiased and that most noise has been removed. Unauthenticated | Downloaded 10/01/21 03:37 AM UTC MAY 2016 S H I M A D A E T A L . 1825 range (;200 km) of a Doppler radar and we had to be able to subjectively identify the TC center from the VD pattern (an indication that, in general, radar coverage was sufficient).