Reprint 1006

颱風納沙(1117)的個案分析

沈志泰 & 柯銘強

第二十六屆粵港澳氣象科技研討會

澳門，2012 年 1 月 17-19 日

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Case study of Typhoon Nesat (1117)

[Corrigendum]

Shum Chi-tai Or Ming-keung

Hong Kong Observatory

Abstract

Typhoon Nesat (1117) developed over the western North Pacific to the east of the

Philippines on 23 September 2011, and moved west-northwest steadily towards Luzon

in the following few days. As Nesat entered into and traversed the northern part of the

South China Sea during 27 to 29 September, the maximum sustained wind speed near

its centre maintained around 130 km/h. However, satellite images showed there were

signs of expansion of its circulation. NOAA analysis of wind field on the night of 28

September and the early morning of 29 September indicated that the gale radius of

Nesat increased by over 100 kilometres in 6 hours. The expansion of Nesat’s

circulation resulted in a more significant beta effect, causing Nesat to take a more

northwesterly track closer to Hong Kong from the night of 28 September. Owing to

the extensive circulation of Nesat and its closer approach to the south China coast,

gales affected many places in Hong Kong during the morning of 29 September and

the Observatory needed to issue the No.8 Gale or Storm Signal. This paper reviews

the performance of various numerical weather prediction models on the forecast track

of Nesat, in particular during its passage over the northern part of the South China Sea.

Factors favouring the expansion of Nesat’s circulation are also analyzed with an aim

to improving operational forecasting of tropical cyclones.

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1. Introduction

Typhoon Nesat was the only tropical cyclone (TC) necessitating the issuance of

No.8 Signal (Gale or Storm Signal) in Hong Kong in 2011. Although it passed at a

distance as far as 350 km to Hong Kong, gale force winds affected many places in the

territory because of its extensive circulation. This has made Nesat one of the TCs

having the farthest closest approach from the territory necessitating the No.8 Signal

(another case was Typhoon Joe in 1980). Past statistics showed that the chance for

TCs to cause general gales in Hong Kong was very low at such distance. It is

understandable that statistical method for predicting the onset of gales did not work

well for the case of Nesat. In order to let forecasters predict changes in local wind

strength better, more understanding is obviously required in the structural and track

changes of large and expanding TCs similar to Nesat.

While forecasters mainly concern the forecasting of the track and intensity

changes of TCs in weather operations, the size of TC is also a crucial factor

determining local wind changes. According to Weatherford (1988), there is a weak

relationship between the outer-radius wind strength and the inner-core intensity of a

TC. This means that a TC can have its intensity (maximum wind speed near its centre)

remaining unchanged but the gale radius expanding. A larger TC is also associated

with a more significant beta-effect (Chan 1987), which may in turn influence its track.

In this paper, an overview and some highlighting features of Nesat are described in

Section 2. Section 3 analyzes the size changes of Nesat, and discusses factors

favouring its expansion using surface and upper-air observation data, NCAR/NCEP-2

Reanalysis data and satellite imagery data. The performance of numerical weather

prediction models on the forecast track of Nesat and the associated beta-effect are

discussed in Section 4. Section 5 provides hints for forecasters on predicting size and

track evolution of large TCs.

2. Overview of Typhoon Nesat (1117)

Nesat originated as a tropical depression over the western North Pacific to the east

of the Philippines on 23 September 2011. It tracked in the general direction of

west-northwest in the following few days and intensified gradually to typhoon in the

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morning of 26 September (Figure 1a). Just before crossing Luzon, Nesat reached its

peak intensity with an estimated maximum sustained wind of 145 km/h (80 knots)

near its centre on 27 September. It soon entered the South China Sea (SCS) in the

evening on the same day and slightly weakened, but still maintained at typhoon

strength. Nesat traversed the northern part of the SCS in the following two days and

made landfall over Hainan Island in the afternoon of 29 September. It weakened

gradually because of land interaction and finally dissipated over northern Vietnam on

1 October.

Nesat was characterized by its rapid expansion of gale radius and the more

northwesterly track when it passed to the south-southwest of Hong Kong from the

night of 28 September to the early morning of 29 September. From NOAA

multi-platform analysis (Figure 3), the gale radius of Nesat increased rapidly from

12UTC to 18UTC on 28 September. The most distinguishable increase occurred in the

northeastern quadrant where the gale radius increased by more than 100 kilometres.

Nesat also took a more northwesterly track at the same time, making its closest

approach to Hong Kong about 50 km closer than that indicated by an earlier forecast

track based on 12UTC 28 September (Figure 1b, Distance of closest approach based

on forecast of 12UTC 28 September was about 400 km, compared with the actual

distance of 350 km). A combination of these two factors has resulted in general gales

affecting Hong Kong on 29 September (Figure 2), and the Observatory issued the

No.8 Signal on the same day.

3. Analysis on the size changes of Nesat

3.1 Size of Nesat

Although there is no universal definition of TC size, the radius of 17.5 ms-1

(gale

radius) is adopted in this study as this parameter is important in operational weather

forecasting. Lee et al. (2010) analyzed 73 TCs over the western North Pacific

attaining typhoon strength during the period 2000-2005. They defined TCs having

radius of 15 ms-1

wind greater than 2.6o latitude (67% percentile in their study) as

large TCs. In the case of Nesat, the radius of gales (17.5 ms-1

) reached a maximum of

3.5o latitude according to NOAA multi-platform analysis. Nesat is thus considered a

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large typhoon from these observations.

Before focusing on the factors leading to the size expansion of Nesat, its

formation mechanism may have already hinted its future evolution into a large size

storm. Lee et al. (2008) identified six synoptic patterns for TC formation by analyzing

the zonal and meridional wind magnitude in the four quadrants of the mesoscale

convective system (MCS) before the TC formation (Table 1). They are easterly wave

(EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow

(NE-SW), southwesterly flow (SW), monsoon confluence (MC) and monsoon shear

(MS). The MS pattern has strong easterlies to its north and westerlies to its south in

the outer core region during the formation stage, which is the most favourable pattern

for formation of large TCs. In the case of Nesat, two relatively maxima of zonal winds

could be identified to the northeast and southwest of the MCS at 12UTC on 21

September (Figure 4), which was about 48 hours before its formation. The 850 hPa

streamlines at the same time (Figure 5) showed organized westerlies associated with

cross-equatorial flow to its south and easterlies associated with subtropical ridge to its

north, satisfying the criteria of MS pattern described by Lee at al. (2008). As such,

there were already hints that Nesat might develop into a large TC during the

formation stage.

3.2 Factors favouring size expansion of Nesat

During the period when Nesat was a typhoon, it underwent two major phases of

size expansion according to NOAA multi-platform analysis. The first period is from

00UTC to 18UTC on 26 September when it was still to the east of the Philippines.

This can be more easily understood as the increase in gale radius was in line with its

intensification (increase in maximum sustained wind speed near its centre from 120

km/h to 145 km/h). The second period is from 12UTC 27 September to 18UTC 28

September when Nesat was over the SCS, with the most pronounced expansion

during the last 6 hours of the period. Such expansion is attributed to three factors: a)

increasing strength of southwesterlies (SW’lies) to the south of Nesat; b) more

significant upper-level divergence in its outer core; c) interaction with the northeast

monsoon.

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a) Strength of southwesterlies

Moisture supply associated with SW’lies in the outer core of a TC is an important

ingredient supporting its development. Visible satellite images from 25 September to

28 September (Figure 6) suggest that the low-level southwesterly jet to the south of

Nesat was intensifying as it traversed from the western North Pacific to the SCS, as

indicated by more deep convection developing on the southern quadrants of Nesat. On

25 September, TC Haitang (1118) still lingered over the SCS and SW’lies were being

drawn into its circulation. As Haitang moved westward and weakened, Nesat became

the only TC drawing the SW’lies on 27 and 28 September. Water vapor images

(Figure 7) during the same period also show that mid-to-upper level convections

triggered by low-level SW’lies intensified significantly after Nesat entering the SCS.

In addition to the weakening of Haitang, another reason causing intensification of

SW’lies was the development and westward extension of a low-level anticyclone to

the southeast of Nesat. 850 hPa analysis at 12UTC of 27 and 28 September (Figure 8)

both indicated existence of such anticyclone. Isotach analysis at 850 hPa (Figure 9)

show that the wind maxima were located between Nesat and this anticyclone from

00UTC on 27 September to 12UTC on 28 September. According to Liu et al. (2002)

and Carr (1997), this 850 hPa anticyclone is called ‘trailing anticyclone’, which was

observed to the southeast of expanding TCs in a number of case studies and numerical

simulations.

In addition to the visual analysis of satellite images, a method used by Lee et al.

(2008) is taken in this paper to quantify the strength of SW’lies associated with Nesat.

Distributions of cloud-top temperatures within a box of 10o longitude x 5

o latitude that

has its northern boundary 5o south of Nesat’s centre has been examined (Figure 10).

This domain is chosen as we are interested in the intensity of convections associated

with SW’lies outside the central core of Nesat. The thresholds of temperatures used

for this study correspond with the color scale used in the D’vorak analysis. A general

increase in the percentage coverage of temperatures <-241K (Dark Gray or colder)

and <-231K (Medium Gray or colder) regions can be identified from 06UTC on 25

September to 12UTC on 27 September (Figure 11a). Gale radius of Nesat was found

to increase steadily during the period except the last 12 hours when it was crossing

Luzon. Gale wind radius of Nesat also increased significantly from 12UTC to 18UTC

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on 28 September when it was located to the south-southwest of Hong Kong. The

percentage coverage of these two thresholds peaked (indicated by the blue arrows in

Figures 11a & b) about 24 hours before the significant expansion of Nesat at the night

of 28 September. In other words, the maximum convective activity in the southern

outer circulation of Nesat preceded its maximum gale radius.

According to Hill (2009), latent heat release in the outer rainbands of TCs can

result in the diabatic lateral expansion of the cyclonic potential vorticity distribution,

favouring expansion in its size. Hence, increase in convective activity outside the TC

core should occur before expansion of the TC circulation. The distribution of

cloud-top temperatures quantifying the intensity of convections, as defined in Figure

11, may become a precursor for increase in gale radius of a TC. Although more case

studies are needed to conclude a reference value of ‘lead time’ for operational use,

forecasters should remain alert if there is a sharp increase in convections associated

with the SW’lies outside the TC circulation.

b) Upper-level divergence

As shown in Figure 3, the expansion of gales mainly occurred in the northern and

eastern quadrants of Nesat at 18UTC on 28 September. Such asymmetric distribution

can be explained by the more significant upper-level divergence associated with a 200

hPa anticyclone to the northeast of Nesat. 200 hPa analysis (Figure 12) show an

anticyclone moved from the east of the Philippines at 12UTC on 27 September to

southeastern China at 12UTC on 28 September. As indicated by the black dotted

circle in Figure 12, more significant divergence was observed to the northeast of

Nesat, matching with the asymmetry of gales indicated in the NOAA multi-platform

analysis.

c) Interaction with northeast monsoon

In the morning of 28 September (Figure 13), surface analysis indicated a ridge

extending from a high cell over northwestern China, but pressure gradient over

southern China coastal areas remained relatively slack. As the northeast monsoon

associated with the high cell spread southward and Nesat edged closer to the coastal

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areas, surface pressure gradient tightened over southern China to the north of Nesat in

the morning of 29 September. To quantify the combined effect between the northeast

monsoon and Nesat, ΔP972, i.e. the pressure difference between Hong Kong and

Chenzhou, China, can be used as a reference index. From Table 2, there was no

increase in ΔP972 from 06UTC to 12UTC on 28 September with local pressure in

Hong Kong remaining almost unchanged. The gale radius of Nesat had not expanded

during that period. However, from 12UTC to 18UTC on 28 September, ΔP972

increased by 4.5 hPa (from 5.1 hPa to 9.6 hPa). After eliminating the effect of local

pressure drop of 2.5 hPa (from 1002.1 hPa to 999.6 hPa), an increase of pressure

gradient of 2 hPa could be attributed to the effect of pressure rise from the northeast

monsoon. This coincided with the period of Nesat’s expansion. Surface pressure

gradient further tightened from 18UTC 28 September to 00UTC 29 September with

ΔP972 increasing to 10.9 hPa while local pressure had ceased dropping. Gales

affected many places in Hong Kong at the time of maximum ΔP972 (from Figure 2).

As a result, interaction with the northeast monsoon could be another reason

explaining the asymmetry of gales of Nesat.

3.3 Hints from numerical models

The expansion of gales associated with Nesat is generally not evident from

analysis of major numerical models. In fact, the performance of forecast surface wind

field inside the circulation of a TC is dependent on the performance of TC intensity

forecast, of which models still cannot handle very well at present. According to

Rogers et al. (2006), the slow improvement in intensity forecasting is the result of

deficiencies in collecting inner-core data of the TC, inadequate specification of the

vortex in the initial conditions of models and gaps in our understanding of the physics

of TCs. Lajoie et. al. (2010) pointed out that surface wind profiles of TCs from

numerical models were not performing better than other derived techniques. In the

case of Nesat, guidance from 2011092612UTC run by ECMWF, JMA and HKO

meso-model (Table 3) generally showed that Nesat would intensify during its passage

over the SCS on 28 and 29 September. Such predictions did not materialize as the

intensity of Nesat flattened at 130 km/h during the period. Hence, performance of

intensity forecast of the TC should be taken into account when assessing the

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reliability of forecast surface wind profile.

Despite the limitations mentioned above, some favourable synoptic patterns

leading to increasing size can still be identified. Both ECMWF (Figure 14) and JMA

(not shown) indicated the development of a 850 hPa anticyclone to the southeast of

Nesat from T+24 hours to T+72 hours in the 12UTC run on 26 September. Such

development of an anticyclone, as mentioned in Section 3.2, is a mechanism for

enhancing the SW’lies associated with Nesat. A 200 hPa anticyclone was also forecast

to move to the northeast of Nesat at 12UTC on 28 and 29 September (Figure 15), and

enhanced divergence between this anticyclone and Nesat may be hinted. Tightened

surface pressure gradient from interaction with northeast monsoon was also predicted

for 29 September by the same model run (Examples of ECMWF runs are shown in

Figure 16). As a result, even though numerical models may not directly predict a

larger gale radius for Nesat, forecasters can identify some hints in the prognostic

charts to assess the possibility of gale radius expansion.

4. Performance of numerical models on track prediction of Nesat and the

associated beta-effect

4.1 Performance of numerical models on track prediction of Nesat

When Nesat was still located to the east of the Philippines, numerical models

generally forecast correctly its west-northwestward track as it moved across the

northern part of the SCS. Model runs at 12UTC on 26 September (Figure 17) show

that ensemble track consisting of four models (ECMWF, NCEP, JMA and EGRR)

and EGRR were the best performers in terms of forecasting the direction of

movement before 06UTC on 28 September, with ECMWF and NCEP showing

poleward bias and JMA showing equatorward bias. However, all models have

underestimated the speed of Nesat during its passage over SCS, even for the ensemble

track. The forecast positions on 29 September by the 2011092612UTC ensemble track

generally lagged behind the actual positions by nearly 100 kilometres although initial

position analysis was quite good.

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4.2 Evidence of beta-effect and hints from numerical models

As Nesat traversed the northern part of the SCS, the most significant change is

its more northwesterly track from the night of 28 September to the morning of 29

September (Figure 18). This coincided with the timing associated with expanding gale

radius of Nesat. According to Carr (1997), an expanding TC can gain poleward

component in its movement by modifying the environmental anticyclone to its east to

become more north-south oriented. Such change in synoptic environment is an

evidence for occurrence of beta-effect. In the case of Nesat, 500 hPa analysis (Figure

19) show a ridge could be observed to its east at 12UTC on 28 September, which was

more north-south oriented compared with the analysis at 12UTC on 27 September.

Water vapor images (Figure 7) also indicate the subsidence to the east of Nesat

becoming more prominent from 27 to 29 September. Further in-depth study of

meso-scale analysis may also be required to explain quantitatively the degree of such

poleward turn.

Although model runs at 12UTC on 26 September (Figure 18) generally cannot

capture the slight northwestward turn of Nesat, ECMWF run at 12UTC on 27

September did hint occurrence of such turn (Figure 18). Forecast timing for the turn

was however about 12 hours earlier than the actual timing. Further analysis of the 500

hPa geo-potential height field forecast by ECMWF (Figure 20) shows that the line of

5880 m became obviously more north-south oriented from 12UTC on 26 September

to 12UTC on 29 September, matching with the actual analysis of the 500 hPa level.

Such change in orientation provides clues for Nesat to take a more northwesterly track

at some stage as it traversed the SCS.

5. Conclusion and hints

In conclusion, Nesat was characterized by its expanding gale radius and the more

northwesterly track as it traversed the SCS from the night of 28 September to 29

September. Expanding size of Nesat was mainly due to a) increasing strength of

SW’lies to the south of its circulation; b) more significant upper-level divergence in

its outer core; c) interaction with the northeast monsoon. As the onset of gales is a

major concern in operational weather forecasting, the following hints are suggested

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for the monitoring of TC development:

1. To analyze the formation mechanism of the TC, in particular patterns favourable

for expansion into large size, such as the MS pattern.

2. To analyze possible signs of intensifying SW’lies by a) visual analysis of satellite

images; b) analysis of the percentage coverage of cloud-top temperatures <-241K

and <-231K in the southern outer circulation of the TC.

3. To identify formation of a ‘trailing’ anticyclone at 850 hPa to the southeast of the

TC from actual analysis or numerical models.

4. To identify signs of enhanced 200 hPa divergence in particular quadrants of the

TC from actual analysis or numerical models.

5. To assess possible tightening of surface pressure gradient from interaction

between the northeast monsoon and the TC over the SCS.

In case indications for size expansion exist, forecasters should remain alert if a

poleward turn will be followed because of more significant beta-effect. This can be

hinted by the more north-south orientation of the ridge at the steering layer, or

increased subsidence associated with such ridge as shown in the satellite images. In

reality, such as the case of Nesat, there may not be enough lead time for an early alert.

This should remain to be a challenge for the forecaster in the near future.

Further studies on the role of low-level SW’lies in TC development may be

carried out to enhance our understanding on the size evolutions of TCs. The feasibility

of incorporating relevant findings into numerical models may also be explored.

.

Acknowledgement:

The authors would like to thank Mr C.K. So for his assistance in providing the

cloud-top temperature data for analysis, Mr K.C. Tsui and Dr. C.M. Cheng for their

comments on the paper.

References:

Carr, L.E., III, and R.L. Elsberry, 1997: Models of tropical cyclone wind distribution

and beta-effect propagation for application to tropical cyclone track forecasting. Mon.

Wea. Rev., 125, 3190-3209.

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Chan, J. C.-L., and R.T. Williams, 1987: Analytical and numerical studies of the

beta-effect in tropical cyclone motion. Part I: Zero Mean flow. J. Atmos. Sci., 44,

1257-1265.

Hill, K.A., and G.M. Lackmann, 2009: Influence of Environmental Humidity on

Tropical Cyclone Size. Mon. Wea. Rev., 137, 3294-3315.

Lajoie, F., and K. Walsh, 2010: A Diagnostic Study of the Intensity of Three Tropical

Cyclones in the Australian Region. Part I: A Synopsis of Observed Features of

Tropical Cyclone Kathy (1984). Mon. Wea. Rev., 138, 3-21.

Lee, C.-S., K.K.W. Cheung, J.S.N. Hui, and R.L. Elsberry, 2008: Mesoscale features

associated with tropical cyclone formation in the western North Pacific. Mon. Wea.

Rev., 136, 2006-2022.

Lee, C.-S., K.K.W. Cheung, W.-T. Fang and R.L. Elsberry, 2010: Initial Maintenance

of Tropical Cyclone Size in the Western North Pacific. Mon. Wea. Rev., 138,

3207-3223

Liu, K.S., and J.C.L. Chan, 2002: Synoptic Flow Patterns Associated with Small and

Large Tropical Cyclones over the Western North Pacific. Mon. Wea. Rev., 130,

2134-2142.

Rogers, R., S. Aberson, M. Black, P. Black, J. Cione, P. Dodge, J. Gamache, J.

Kaplan, M. Powell, J. Dunion, E. Uhlhorn, N. Shay and N. Surgi, 2006: The Intensity

Forecasting Experiment: A NOAA Multiyear Field Program for Improving Tropical

Cyclone Intensity Forecasts. Bulletin of American Met. Soc., 87, 1523-1537.

Weatherford, C.L., and W.M. Gray, 1988: Typhoon structure revealed by aircraft

reconnaissance. Part II: Structural variability. Mon. Wea. Rev., 116, 1044-1056.

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Synoptic

Pattern

NE quadrant NW quadrant SW quadrant SE quadrant

U V U V U V U V

EW S W S W

NE S S S

NE-SW S S

SW S W S S

MC S S

MS S S

Table 1: Classification of six synoptic patterns for formation of TCs as defined by Lee

et al. (2008). ‘S’ refers to strong zonal(U)/meridional(V) wind components with

average speed greater than 5 ms-1

in the respective 5o x 5

o quadrant from the TC centre.

‘W’ refers to weak zonal(U)/meridional(V) wind components with average speed

smaller than 2 ms-1

in the respective 5o x 5

o quadrant from the TC centre.

Date/Time (UTC) ΔP972 (hPa) Hong Kong MSLP (hPa)

28 Sep 06 5.7 1002.0

12 5.1 1002.1

18 9.6 999.6

29 Sep 00 10.9 1000.0

06 8.5 1001.9

Table 2: Summary of ΔP972 and mean sea level pressure (MSLP) of Hong Kong

from 06UTC 28 September to 06UTC 29 September. ΔP972 is calculated by

subtracting the MSLP at Hong Kong (45007) from that of Chenzhou, China (57972).

Table 3: Summary of intensity forecasts of Nesat by numerical models from the

2011092612UTC run.

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Figure 1 (a): Track of Nesat from 23 September to 1 October.

(6-hourly positions are marked according to UTC.)

Figure 1 (b): Actual track of Nesat (in black solid line) and forecast track of Nesat at

2011092812UTC (in black dotted line).

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Figure 2: Wind distribution over Hong Kong at 08HKT on 29 September 2011 when

No.8 Gale or Storm Signal was in force and Nesat was centred about 350 km from

Hong Kong.

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Figure 3: Evolution of wind radii of Nesat as indicated by NOAA multi-platform

analysis from 2011092806UTC to 2011092900UTC at 6-hourly intervals. Green,

yellow and red color roughly corresponds to strong, gale force and storm force winds

respectively.

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Figure 4: Isotachs (in ms-1

) of zonal flows at 850hPa (left) and 925 hPa (right) of

MCS of Nesat at 2011092112UTC from NCEP/NCAR-2 Reanalysis data. The centre

of MCS of Nesat was around 9oN 139

oE, indicated by the red dot.

Figure 5: 850 hPa streamlines at 2011092112UTC associated with the MCS of Nesat

(centre indicated by red dot.)

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(a) 201109250201UTC

(b) 201109260201UTC

(c) 201109270201UTC

(d) 201109280201UTC

Figure 6: Visible imageries of Nesat and Haitang from 201109250201UTC to

201109280201UTC at 24-hour intervals, showing the enhanced low-level SW’lies in

its southern outer circulation as it traversed the SCS.

(Pictures taken from MTSAT of JMA)

Haitang Nesat Haitang Nesat

Nesat Nesat

Haitang

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(a) 201109250032UTC

(b) 201109260032UTC

(c) 201109270032UTC

(d) 201109280032UTC

(e) 201109290032UTC

Figure 7: Water vapor imageries of Nesat and Haitang from 201109250032UTC to

201109290032UTC at 24-hour intervals, showing the enhanced convection associated

with the SW’lies in its southern outer circulation as it traversed the SCS (from a-d),

and more prominent subsidence from 27 to 29 September in black box (from c-e).

(Pictures taken from MTSAT of JMA)

Haitang Nesat

Haitang Nesat

Haitang

Nesat Nesat

Nesat

19

Figure 8: Streamline analysis at 850 hPa at 2011092712UTC (top) and

2011092812UTC (bottom). The anticyclone to the southeast of Nesat is marked in the

red box.

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Figure 9: Isotachs (in ms-1

) and wind barbs at 850 hPa associated with Nesat from

2011092700UTC to 2011092812UTC at 12-hour intervals, showing the isotach

maxima to the southeast of Nesat.

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Figure 10: Dimensions of the box with 10o longitude x 5

o latitude that has its northern

boundary 5o south of Nesat’s centre.

Figure 11: Time series of percentage coverage of cloud-top temperatures lower than

various thresholds of color scheme in the D’vorak channel computed in (a) the box

defined in Figure 10; (b) the right half of the box defined in Figure 10. Red arrow

indicates the period when Nesat underwent rapid increase in gale radius from 12UTC

to 18UTC on 28 September. Blue arrows indicate the peaks in percentage coverage of

cloud-top temperatures <-241 K and <-231 K before Nesat’s expansion.

5o

5o

10o

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Figure 12: Streamline and divergence analysis at 200 hPa at 2011092712UTC (top)

and 2011092812UTC (bottom). An anticyclone can be analyzed to the northeast of

Nesat at 2011092812UTC. Black dotted circle indicates the northeastern quadrant of

Nesat with enhanced divergence from 2011092712UTC to 2011092812UTC.

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(a) 00UTC on 28 September 2011

(b) 00UTC on 29 September 2011

Figure 13: Surface analysis at (a) 00UTC on 28 September 2011 (b) 00UTC on 29

September 2011. The tightened pressure gradient because of interaction with northeast

monsoon is marked in the red box to the north of Nesat.

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Figure 14: ECMWF 850 hPa prognostic charts of the 2011092612UTC run from T+0

hour to T+72 hours at 24-hour intervals. Note the presence of an anticyclone to the

southeast of Nesat (marked in black box), especially for 2011092712UTC and

20011092912UTC forecasts.

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Figure 15: ECMWF 200 hPa prognostic charts of the 2011092612UTC run from T+0

hour to T+72 hours at 24-hour intervals. Note the presence of an anticyclone to the

northeast of Nesat (marked in black box) for 2011092812UTC and 20011092912UTC

forecasts.

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Figure 16: ECMWF surface prognostic charts of the 2011092612UTC run from T+0

hour to T+72 hours at 24-hour intervals. Note the tightened pressure gradient (marked

in black box) spreading from inland China for 2011092812UTC and 2011092912UTC

forecasts.

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Figure 17: Forecast tracks of Nesat from various models (Red-JMA; Purple-ECMWF;

Blue-EGRR; Light blue-NCEP; Green-Ensemble) of the 2011092612UTC runs with

the best track analysis in black.

Figure 18: Forecast tracks of Nesat from various models (Red-JMA; Purple-ECMWF;

Blue-EGRR; Light blue-NCEP; Green-Ensemble) of the 2011092712UTC runs with

the best track analysis in black. The northwest turn by ECMWF is marked.

NW turn by ECMWF

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Figure 19: Streamline analysis at 500 hPa at 2011092712UTC (top) and

2011092812UTC (bottom). The ridge to the east of Nesat (marked in red box) was

found to be more north-south oriented at 2011092812UTC compared with

2011092712UTC.

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Figure 20: ECMWF 500 hPa prognostic charts of the 2011092612UTC run from T+0

hour to T+72 hours at 24-hour intervals. The contour of 5880 m line is marked in red.