Reprint 1073

Application of Dvorak Technique during the Weakening Stage of Tropical

C.T. Shum & S.T. Chan

The 6th -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, 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 (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 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 or the prior to entering the 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. Rule C3 for re-development is included in this option as a gap can still exist between the final T-number and CI as re-development starts. Fig. 4 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 only held 0.5 higher than the final T-number at T = 12 h (as the decrease in T-number is only 0.5 from the peak value) and then 1.0 higher than the final T-number from T = 18 h to 30 h. As re-development occurs at T = 36 h, CI is kept unchanged. When the final T-number has plateaued for more than 12 hours at T = 54 h, the CI can then converge with it according to rule C3.

5 5

T 4.5 CI

4 I C , T 3.5

3

Landfall Re-development 2.5 0 6 12 18 24 30 36 42 48 54 hour T-number flattened for 12 hours

Fig. 4. Evolution of the final T-number and CI under Option C.

4. Case studies

The three options described in Section 3 are evaluated based on twenty cases from 2009 and 2010 (Table 1) involving TCs making landfall over China, , Taiwan or the Philippines. The results from the original Dvorak scheme are also included for comparison with those from the three different options.

Table 1. List of TCs included in the study Year TCs 2010 Conson (1002), Typhoon Chanthu (1003), Severe Tropical Storm Mindulle (1005), Severe Tropical Storm LionRock (1006), Severe Tropical Storm Meranti (1010), Severe (1011), Super (1013)

2009 Typhoon Chanhom (0902), Severe (0903), Tropical Storm Nangka (0904), Tropical Storm Soudelor (0905), Tropical Depression, (0906), Severe Tropical Storm Goni (0907), (0908), Tropical Storm Mujigae (0913), (0915), (0916), Super (0917), (0921) a. TCs weakening over the sea

For TCs weakening over the sea without any landfall, there is no difference between the original scheme and the new options before the final T-number flattens. The rule of holding CI the same for 12 hours after initial weakening is still applied as it

6 is not uncommon for TCs to re-develop over the sea. Table 2 illustrates the determination of CI under the original scheme and the three new options in such situation. It should be noted that although weakening stops from Day 2 00Z to Day 2 06Z in this example, the gap between the final T-number and CI will persist. For Option B and Option C, the final T-number and CI will only converge if the final T-number has plateaued for more than 12 hours.

Table 2: Illustration of a TC weakening over the sea in original scheme and the 3 proposed options. Time Final T-number CI Day 1 00Z 5.0 5.0 Day 1 06Z 4.5 5.0 Day 1 12Z 4.0 5.0 Day 1 18Z 3.5 4.5 Day 2 00Z 3.0 4.0 Day 2 06Z 3.0 4.0 b. Case of TC weakening immediately upon landfall – Typhoon Molave (0906)

Typhoon Molave traversed the northeastern part of the South China Sea on 18 July 2009 and made landfall within 50 km to the east of Hong Kong in the early morning of 19 July (Fig. 5). The storm reached its peak intensity just before landfall as indicated by the maximum final T-number of 5.0 (Table 3). A discernible could also be identified in the enhanced IR imagery at 1130Z on 18 July 2009 (Fig. 6a).

Fig. 5. Track of Typhoon Molave (0906).

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(a) (b)

Fig. 6. Enhanced IR imagery of Typhoon Molave at (a) 1130Z on 18 July 2009; (b) 1730Z on 18 July 2009; and (c) 2330Z on 18 July 2009.

(c)

Just prior to making landfall to the east of Hong Kong, the peak intensity of Molave was attained at 12Z on 18 July with a final T-number and CI of 5.0. Immediate weakening could be observed after landfall at 18Z with a progressive drop in the final T-number thereafter. A comparison of CIs and MSW computed from different options against the best-track intensities from HKO, JMA, JTWC (Joint Typhoon Warning Center of the United States) and CMA (China Meteorological Administration) is shown in Table 3. It can be seen that the original scheme gave the highest overestimation when compared with all other options due to the 12-hour holding rule. Option A generated CIs closest to best-track intensities in general as immediate downgrading of CI was allowed upon landfall. Option C gave the same estimates as that of the original scheme most of the time, while Option B lied in between the other two options.

This example illustrates that for TCs undergoing rapid weakening upon landfall, the original Dvorak technique is particularly undesirable because of the significant overestimation of TC intensity. Option A performed the best in this case while Option B came the second.

8 Table 3. Dvorak estimates given by the original scheme and the three options. The bolded figures refer to analysis of TC over land. Highest intensity estimates at specific times are marked in red, while the lowest estimates are marked in blue. (MSW are expressed as 10-minute mean winds in knots and converted directly from CI based on a conversion factor of 0.93 for converting 1-minute mean winds to 10-minute means. The best-track intensities from JTWC have been converted to 10-minute means using the same conversion factor, while the best-track intensities from CMA denote 2-minute means without any conversion applied.) Time Final Original A B C HKO JMA JTWC CMA T-number CI/MSW CI/MSW CI/MSW CI/MSW Intensity Intensity Intensity Intensity 12Z, 18 Jul 5.0 5.0 / 5.0 / 5.0 / 5.0 / 84 84 84 84 75 65 60 74 18Z, 18 Jul 4.5 5.0 / 4.5 / 5.0 / 5.0 / 84 72 84 84 65 60 51 64 00Z, 19 Jul 3.5 5.0 / 3.5 / 4.0 / 4.5 / 84 51 60 72 45 40 33 39 06Z, 19 Jul 3.0 4.0 / 3.0 / 3.5 / 4.0 / 60 42 51 60 30 -- -- 35 12Z, 19 Jul 2.5 3.5 / 2.5 / 3.0 / 3.5 / 51 33 42 51 25 -- -- 25 c. Case of TC weakening partly over sea and land – Super Typhoon Megi (1013)

Super Typhoon Megi developed over the western Pacific and reached its peak intensity over the sea to the east of the Philippines in the early morning of 18 October 2010. It weakened as it traversed across . It then took a northward turn and subsequently made landfall over the southeastern coast of China on 23 October 2010 (Fig. 7).

Fig. 7. Track of Super Typhoon Megi (1013).

9 (a) (b)

(c) (d) Fig. 8. Enhanced IR imagery of Super Typhoon Megi at (a) 1730Z on 17 October 2010; (b) 2330Z on 17 October 2010; (c) 0530Z on 18 October 2010; and (d) 1130Z on 18 October 2010.

The Dvorak analysis for Megi at 18Z on 17 October still gave a final T-number and CI of 7.5, with no sign of weakening observed yet. Slight weakening in the storm structure could be observed in the IR imagery at 00Z on 18 October (Fig. 8b), giving a final T-number of 7.0. As the storm was still over the sea, the original scheme and all three options would hold CI the same (7.5) at this moment. Megi then made landfall between 00Z to 06Z on 18 October. The original scheme continued to hold the same CI but all the new options would give lower estimates without the holding constraint. The estimates by Options A and B were the same, and were closer to the best-track intensities from the operational centres. The original scheme again gave the largest overestimation among all schemes at 06Z on 18 October (first analysis after landfall).

Table 4. Same as Table 3, except for Super Typhoon Megi. Time Final Original A B C HKO JMA JTWC CMA T-number CI/MSW CI/MSW CI/MSW CI/MSW Intensity Intensity Intensity Intensity 18Z, 17 Oct 7.5 7.5 / 7.5 / 7.5 / 7.5 / 144 144 144 144 145 125 144 140 00Z, 18 Oct 7.0 7.5 / 7.5 / 7.5 / 7.5 / 144 144 144 144 140 125 140 136 06Z, 18 Oct 6.0 7.5 / 6.5 / 6.5 / 7.0 / 144 118 118 130 115 105 112 117 12Z, 18 Oct 5.5 6.5 / 6.0 / 6.0 / 6.5 / 118 107 107 118 95 90 84 97 18Z, 18 Oct 5.0 6.0 / 5.5 / 5.5 / 6.0 / 107 95 95 107 90 85 88 97 00Z, 19 Oct 5.0 6.0 / 5.5 / 5.5 / 6.0 / 107 95 95 107 90 80 88 87

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Although the three options mainly concern about weakening of TCs over land, the effect of the rules can extend to the period after the TCs re-emerge over the sea. As Megi re-emerged over the sea at 18Z on 18 October, both Option A and Option B maintained lower estimates than Option C and the original scheme as the weakening trend of Megi continued. The intensity estimates given by Option A and Option B matched closer to the best-track intensities of the operational centres. d. Case of possible under-estimation under the new options – Severe Typhoon Fanapi (1011)

Severe Typhoon Fanapi (1011) crossed Taiwan on 19 September 2010 and then traversed the Taiwan Strait, finally making landfall over the southeastern coast of China on 20 September. Fanapi then tracked westward across and dissipated near Guangzhou to the north of Hong Kong.

Fig. 9. Track of Severe Typhoon Fanapi (1011).

11

(a) (b)

(c) (d)

(e) (f) Fig. 10. Enhanced IR imagery of Severe Typhoon Fanapi (1011) at (a) 2330Z on 18 September 2010; (b) 0530Z on 19 September 2010; (c) 1130Z on 19 September 2010; (d) 1730Z on 19 September 2010; (e) 2330Z on 19 September 2010; and (f) 0530Z on 20 September 2010.

12 The enhanced IR imageries showed that Fanapi still displayed a clear banding feature and an eye could be identified prior to its landfall over Taiwan (Fig.10a). Dvorak analysis using the eye pattern gave a final T-number and CI of 5.5 at that moment. As it made landfall over Taiwan, the eye disappeared and the centre also became exposed with the major convection shifting to its southeast. As a result, the Dvorak estimates indicated rapid weakening with the final T-number. dropping to 4.0 as it crossed the Taiwan Strait. Owing to the 12-hour holding rule, the original scheme would still give a CI of 5.5 at 12Z 19 Sep, but CI of 4.0 under Option A. The difference in intensity estimates between these two schemes is 33 knots. Comparing with the HKO and CMA best-track intensities, Option A indicates an underestimation as high as 10 knots when Fanapi was located over Taiwan and the Taiwan Strait. The best-track intensity from JMA is closer to that given by Option A, probably because Option A is simply a JMA-modified version of the Dvorak technique. Option C essentially results in CI very similar to the original scheme while Option B gives estimates in between Option A and Option C.

Table 5. Same as Table 3, except for Severe Typhoon Fanapi. Time Final Original A B C HKO JMA JTWC CMA T-number CI/MSW CI/MSW CI/MSW CI/MSW Intensity Intensity Intensity Intensity 00Z, 19 Sep 5.5 5.5 / 5.5 / 5.5 / 5.5 / 95 95 95 95 90 80 98 87 06Z, 19 Sep 4.5 5.5 / 4.5 / 5.0 / 5.5 / 95 72 84 95 80 65 70 78 12Z, 19 Sep 4.0 5.5 / 4.0 / 4.5 / 5.0 / 95 60 72 84 70 60 65 68 18Z, 19 Sep 4.0 5.0 / 4.0 / 4.5 / 5.0 / 84 60 72 84 65 60 65 68 00Z, 20 Sep 4.0 5.0 / 4.0 / 4.5 / 5.0 / 84 60 72 84 65 50 60 64 06Z, 20 Sep 3.5 4.5 / 3.5 / 4.0 / 4.5 / 72 51 60 72 55 45 47 49 12Z, 20 Sep 3.0 4.0 / 3.0 / 3.5 / 4.0 / 60 42 51 60 40 40 37 35 18Z, 20 Sep 2.5 3.5 / 2.5 / 3.0 / 3.5 / 51 33 42 51 35 -- 28 31

After Fanapi made landfall over southeastern China, it continued to edge closer to Hong Kong as it took a westerly track across Guangdong. Dvorak analysis suggested a progressive decrease in the final T-number while Fanapi was traversing southern China but some intense convection in the active southwesterlies could still be identified to the south of Fanapi’s centre (Fig. 11a). CMA indicated a rather rapid weakening from a warning intensity of 25 m/s (49 knots) at 06Z on 20 September to 18 m/s (35 knots) at 12Z on 20 September, while the best-track intensities of HKO and JMA still indicated an intensity of 40 knots at 12Z on 20 September. The last warning of Fanapi by CMA was 18Z on 20 September, while HKO only downgraded Fanapi to an area of low pressure about 5 hours later than CMA. According to reports, torrential rain triggered by Fanapi has incurred huge damage and economic losses over inland Guangdong (HKO, 2011).

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(a) (b) Fig. 11. Enhanced IR imagery of weakening Severe Typhoon Fanapi (1011) on (a) 1130Z on 20 September 2010; and (b) 1730Z on 20 September 2010.

The widespread damage in Mainland China by Fanapi suggests that the potential issue of intensity underestimation should be handled with care when reviewing the weakening rules of the Dvorak technique. The underestimation involved in Option A may indicate some of its possible drawbacks. Other insights from this case are:

1) As some of the patterns in Dvorak analysis (e.g. shear pattern, eye pattern and embedded eye pattern) strongly depend on the satellite fix position, possible formation of a secondary centre (replacing the primary centre later) as a TC traverses the mountains over Taiwan/Luzon may result in a seemingly lower final T-number. Immediate reduction of CI to the final T-number in Option A may not be desirable in such cases, as re-organization over the sea may wrap the convection around the TC centre to form a better structure again.

2) When TCs re-emerge over the Taiwan Strait after crossing Taiwan and then track westward across the south China coastal areas, extra caution should be taken if deep convection could still be identified in one of its quadrants (especially on the quadrant of active southwesterlies from traditional wisdom). Intense rainstorms and gusty conditions could occur on that side even though Dvorak may give a seemingly lower final T-number.

5. Verification a. Data samples

In the 20 TC cases included in this study, a total of 86 Dvorak analyses involve weakening of TCs over land. The intensity estimates given by the original Dvorak scheme and the three options introduced in Section 3 are verified against the best-track intensities from different operational centres, namely, HKO, JMA, JTWC and CMA.

The 10-minute mean wind data of HKO and JMA are used for the verification. For JTWC data, the 1-minute mean wind speed data are first converted to 10-minute

14 means using a conversion factor of 0.93 as recommended by Harper et al. (2010). The simple ensemble mean of HKO, JMA and converted JTWC intensities is also deployed for the verification. Note that no conversion for CMA data is made and they represent 2-minute mean values. b. Root mean-square error (RMSE) of the Dvorak estimates

The RMSE of the Dvorak estimates from various schemes are as shown in Fig. 12. The original Dvorak scheme has resulted in the largest RMSE irrespective of the operational centre chosen as the basis for verification. Among the three options discussed, Option A gives the lowest RMSE (less than 10 knots apart from verifying against JTWC data) while Option C gives the largest. The RMSE of Option B lies somewhere in between. In fact, the RMSE of Option C is very close to that of the original scheme, which means that there will be almost no appreciable benefits by adopting Option C as the new weakening rule.

It is also found that the best-track intensities from JTWC are the ones having the largest difference from the Dvorak estimates in general. This can be due to the fact that JTWC stops performing Dvorak analysis over land and relies more on other analysis data.

30 HKO

JMA 25 JTWC

CMA

) 20 Ensemble of HKO, JMA & JTWC ts o n k ( 15 E S M R 10

5

0 ABCOriginal Option

Fig. 12. The RMSE of Dvorak estimates from Options A to C and the original scheme, against the best track intensities from HKO, JMA, JTWC, CMA and the simple ensemble mean of HKO, JMA and JTWC.

15 c. Distribution of overestimation and underestimation cases

HKO JMA 100% 100% 90% 90% 80% e 80% e g g 70% a 70% ta t n 60% n 60% e e c 50% rc 50% r 40% e 40% e P P 30% 30% 20% 20% 10% 10% 0% 0% A B C Original A B C Original (a) (b)

JTWC CMA 100% 100% 90% 90% 80% 80% e g 70% e 70% a g t 60% ta 60% n n e 50% e 50% c c r 40% r 40% e e P 30% P 30% 20% 20% 10% 10% 0% 0% ABCOriginal A B C Original (c) (d)

Ensemble of HKO, JMA & JTWC 100% 90% 80% e 70% g ta 60% n e 50% rc 40% e P30% 20% 10% 0% A B C Original (e) Fig. 13. Percentage of overestimation > 7.5 knots, underestimation > 7.5 knots and errors within 7.5 knots of the Dvorak estimates from Options A to C and the original scheme, against the best-track intensities from (a) HKO; (b) JMA; (c) JTWC; (d) CMA; and (e) the simple ensemble mean of HKO, JMA and JTWC.

Fig. 13 shows the distribution of cases of underestimation and overestimation cases when verified against the best-track intensities from different centres. As anticipated, the percentage of overestimation > 7.5 knots is highest for the original scheme. The corresponding figure for Option C is almost the same as that of the original scheme. In general, Option A is able to provide the highest percentage of ‘good’ estimates within 7.5 knots while Option B is the second best. One interesting finding is that the percentages of overestimation when verified against JTWC data are indeed the highest among all cases, implying that the best-track intensities from JTWC are generally lower

16 (as can be observed in the case of Typhoon Molave or Severe Typhoon Fanapi discussed in Section 4). This seems to contradict with the impression that JTWC gives higher intensities for intense storms. Such contradiction occurs as the verification in this section only involves Dvorak estimates over land. It is found that JTWC sometimes downgraded TCs earlier than other centres. One probable reason is that JTWC does not conduct Dvorak analysis for TCs over land and therefore there is no need for it to consider the weakening constraints upon landfall of TCs.

While Option A seems to provide the lowest percentage of overestimation and highest percentage of ‘good’ estimates, the percentage of underestimation is also highest. A further analysis of the underestimation cases is shown in Fig. 14. 12%

ts HKO o n JMA k 10% .5 7 JTWC > n o 8% CMA ti a Ensemble of HKO, JMA & JTWC m ti s 6% re e d n u f 4% o e g ta n 2% e rc e P 0% A B C Original Option

Fig. 14. Percentage of underestimation cases (>7.5 knots) when verified against HKO, JMA, JTWC, CMA and the ensemble intensity.

Fig. 14 shows that the percentage of underestimation cases (>7.5 knots) using Option A is close to 10% when verified against HKO or CMA intensities. This can create concerns as exemplified by the case of Fanapi that indicates a fast downgrade during landfall can be risky. Option B results in a lower percentage of underestimation in general (6% or below).

6. Conclusion

In conclusion, all the three modified Dvorak schemes give generally better estimates of TC intensity for cases over land with reduction of RMSE and lower percentage of overestimation. The benefit given by Option C is, however, comparatively minimal.

In terms of RMSE, percentage of overestimation or ‘good’ estimates, Option A outperforms the other two options tested. However, Option A can result in the highest percentage of underestimation cases and this can increase the risk of downgrading a TC

17 too early over land. Option B appears to alleviate this problem by reducing such percentage to 6% or below. When compared with the original scheme, it can still reduce the percentage of significant overestimation >7.5 knots by more than 20% in general when verified against the best-track intensities from different centres. After balancing all factors, Option B is considered more suitable for operation and has been adopted for trial operation in HKO in the TC season of 2013.

One should also note that intensity estimate from Dvorak analysis is only one of the references for determining the final TC intensity and other available information should be taken into account. The TC may also lose most of its banding or identifiable features in some cases over land and credibility or quality of the analysis should be handled with care.

References:

Burton, A. and C. Velden, 2011: International Satellite Analysis of Tropical Cyclones: Final Report. World Meteorological Organization.

Dvorak, V. F., 1984: Tropical cyclone intensity analysis using satellite data. NOAA Tech. Rep. 11, 45 pp.

Harper, B.A., J.D. Kepert and J.D. Ginger, 2010: Guidelines for converting between various wind averaging periods in tropical cyclone conditions. World Meteorological Organization, WMO/TD-No. 1555.

Hong Kong Observatory, 2011: Tropical Cyclones in 2010. Hong Kong Observatory. (Available online at http://www.weather.gov.hk/informtc/fanapi/fanapi.htm).

Koba, H., S. Osano, T. Hagiwara, S. Akashi, and T. Kikuchi, 1989: Determination of intensity of passing through the Philippine Islands (in Japanese). J. Meteor. Res., 41, 157-162.

Velden, C., B. Harper, F. Wells, J.L. Beven II, R. Zher, T. Olander, M. Mayfield, C. Guard, M. Lander, R. Edson, L. Avila, A. Burton, M. Turk, A. Kikuchi, A. Christian, P. Caroff and P. McCrone, 2006: Supplement To: The D’vorak Tropical Cyclone Intensity Estimation Technique: A Satellite-Based Method that Has Endured for over 30 Years. BAMS, 87, S6-S9.

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