Reprint 1073 Application of Dvorak Technique during the Weakening Stage of Tropical Cyclones C.T. Shum & S.T. Chan The 6th China-Korea Joint Workshop on Tropical Cyclones Shanghai, China 26-28 May 2013 APPLICATION OF DVORAK TECHNIQUE DURING THE WEAKENING STAGE OF TROPICAL CYCLONES C.T. SHUM and S.T. CHAN Hong Kong Observatory, Hong Kong, China Abstract The Dvorak technique has been widely used by operational warning centres in different parts of the world as a major analysis tool to determine the intensity of tropical cyclone (TC). However, there exist noticeable differences in the application of the technique among different warning centres. In particular, the weakening rules in the technique that governs the determination of TC intensity during the TC weakening stage constitute one such difference and are the subject of review in this paper. Three options to modify the weakening rules are introduced and evaluated based on verification against the best-track datasets from various centres. Keywords: Dvorak technique, tropical cyclones, weakening rules. 1. Introduction Based on the infrared imageries from geostationary satellites, the Enhanced Infrared (EIR) Dvorak technique (Dvorak, 1984) has long been used in the Hong Kong Observatory (HKO) as one of the references for estimating the intensity of tropical cyclones (TCs). Once a potential TC is suspected to be forming within the area 0o-36oN, 100o-140oE, Dvorak analysis will be performed at 6-hourly intervals. For TCs within the HKO area of responsibility (i.e. 10o-30oN, 105o-125oE), additional analysis will be performed at 3-hourly intervals. While Dvorak analysis serves as one of the important references in determining the maximum sustained surface wind speed (MSW) of the TC, surface wind reports, Doppler wind observations from weather radars, reconnaissance aircraft reports when available, wind scatterometer data and other objective analysis data, e.g. from Automatic Dvorak Technique, SATCON and the Multi-platform Tropical Cyclone Surface Wind Analysis by NOAA, are also taken into account. 2. HKO practice and local adaptations in other operational centres The Dvorak technique has been widely used by operational warning centres in different parts of the world to determine the MSW of TCs, obtained by converting directly the Current Intensity number (CI) from the Dvorak analysis. The Dvorak technique adopted in HKO essentially follows the original scheme constructed by Dvorak (1984). However, there exist noticeable differences in the application of the technique across different warning centres. The weakening rules (viz. Rule 9 in the Dvorak technique), that governs the determination of TC intensity during the weakening stage, constitute one such difference and are evaluated in this paper. 1 According to Dvorak (1984), CI of weakening TCs is determined by the following rules: a. CI is held constant during the first 12 hours of weakening. b. After 12 hours of holding period, CI is held 1.0 higher than the final T-number as the storm weakens (CI is held 0.5 higher than the final T-number when the final T-number shows a 24-hour decrease of only 0.5). c. When redevelopment occurs, CI is not lowered even if the final T-number is lower than CI. CI is held the same until the final T-number increases to the value of CI. Rules (a) and (b) are sometimes controversial in operation, particularly when applying to landfalling TCs (the applicability on which though not specifically mentioned in the Dvorak technique, a number of centres including HKO have been applying the technique on such cases). The rationale behind rule (a) is to take the time lag between the weakening of TC pattern and the drop in TC intensity into account (see Fig. 1a). However, it is found that the rule has sometimes led to intensity estimates lagging behind the actual weakening rate for landfalling TCs, thereby devaluing its usefulness in intensity determination. Examples will be discussed in Section 4. Weakening rule (b) only gives directions on handling CI when the TC weakens but no explicit guidance is given when the TC stops weakening and the final T-number has flattened for some period of time. A supplementary rule to handle such situation is indeed necessary for completeness. (a) (b) Fig. 1. Relationship between the final T-number (blue line) and CI (pink line) in cases of TCs (a) weakening over the sea; (b) weakening over land; (c) weakening partly over sea and over land as adopted by JMA. The red line along the x-axis indicates the timing of landfall. (Adapted from Velden et al. 2006) (c) Some centres have modified the weakening rules to cope with the above two issues. Regarding the first issue, the Bureau of Meteorology, Australia (BoM) and the Tropical 2 Prediction Centre in Miami have relaxed the holding rule to 6 hours only (Velden et al. 2006). The Regional Specialized Meteorological Centre (RSMC) at La Reunion has also applied the same modification for small systems (Burton & Velden, 2011). The Japan Meteorological Agency (JMA) has adopted the following modifications specifically for TCs making landfall (Koba et al. 1989): J1. If the final T-number is steady or increasing before landfall, but decreases immediately after landfall, the 12-hour holding rule to determine CI is not applied. CI is determined to be equal to the final T-number over land. (Fig. 1b). J2. If the final T-number is decreasing prior to landfall, and continues the way after landfall, then CI is decreased by the same amount as the final T-number (Fig. 1c). J3. The above relationships are maintained even if the TC re-emerges over the sea until there are apparent signs of re-development. For weakening TCs over the sea, the original rule of holding CI for 12 hours is still applied in the JMA scheme considering higher chance of re-development in such cases (Fig. 1a). Regarding the second issue discussed above, there is no mechanism in the modified Dvorak technique by JMA to eliminate the gap between the final T-number and CI even when the TC stops weakening. 3. Options of modifications to the original scheme This paper specifically reviews the weakening rules in the Dvorak technique for landfalling TCs as a TC affecting Hong Kong will very often cross Taiwan or the Philippines prior to entering the South China Sea and then make landfall and dissipate over China. To cope with the two issues discussed in Section 2, three different options for modification of the rules have been tested and evaluated. Option A: Adopting the JMA rules In this option, the JMA-modified Dvorak technique with rules J1 to J3 listed in Section 2 are followed. Fig. 2 shows the flowchart of this option. 3 Final T-number decreases in current analysis TC over sea or over land? Land Sea Did the TC re-emerge from landmass after some weakening over land?* Yes No Decrease CI by the same Hold CI the same within amount as the drop in final 12 hours from initial T-number weakening, then hold CI 1.0 higher as storm weakens. (Hold CI 0.5 higher if final T-number only shows 24-hour decrease of 0.5.) * This question is to cater for TCs crossing landmass and then re-emerging over the sea. If weakening occurs over land, the rule of ‘decreasing CI by the same amount as the drop in final T-number’ will be followed until re-development. If weakening occurs over the sea, the original weakening rules in Dvorak technique will apply. Fig. 2 Flowchart of option A. Option B: Holding CI 0.5 higher than the final T-number immediately upon landfall In this scheme, the weakening rules are modified as follows: B1. As the TC makes landfall and weakens, CI is immediately held 0.5 higher than the final T-number. B2. When the final T-number has already plateaued for more than 12 hours, CI is held the same as the final T-number. This applies to TCs over land or TCs returning to sea after landfall. B3. When redevelopment occurs over land, CI is held the same as the final T-number. Fig. 3 illustrates the evolution of the final T-number and CI under this option. Landfall is made between T = 6 h and T = 12 h for this TC. CI is held 0.5 higher than 4 the final T-number from T = 12 h after landfall. As the final T-number has plateaued for more than 12 hours at T = 42 h, CI can then converge with it according to rule B2. 5 T 4.5 CI 4 I C , T 3.5 3 Landfall 2.5 0 6 12 18 24 30 36 42 T-number flattened for 12 hours hour Fig. 3. Evolution of the final T-number and CI under Option B. Option C: Holding CI 1.0 higher than the final T-number immediately upon landfall In this scheme, the weakening rules are modified as follows: C1. As the TC makes landfall and weakens, CI is held 1.0 higher than the final T-number (CI is held 0.5 higher than the final T-number if the decrease in final T-number is only 0.5 from the peak value). C2. If the final T-number has already plateaued for more than 12 hours, CI is held the same as the final T-number. This applies to TCs over land or TCs returning to sea after landfall. C3. When redevelopment occurs, CI is held the same until the final T-number increases above the value of CI. Option C is the same as Option B except that CI is held 1.0 higher than the final T-number instead of 0.5.