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Tropical cyclone structure in the South China Sea based on high-resolution reanalysis data and comparison with that of ‘bogus’ vortices
Dynamics of Atmospheres and Oceans 89 (2020) 101128
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Tropical cyclone structure in the South China Sea based on highresolution reanalysis data and comparison with that of ‘bogus’ vortices
T
Yanyan Huang*, Bin Zheng Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, and Guangdong Provincial Key Laboratory of Regional Numerical Weather Prediction, Guangzhou, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Tropical cyclone South China Sea Structure feature Bogus model Composite analysis
Based on high-resolution reanalysis data of the European Centre for Medium-Range Weather Forecasts, several samples of tropical cyclones (TCs), including tropical storms, severe tropical storms, and typhoons, in the South China Sea (SCS), were selected for composite analysis. The structures of these three types of vortices and their differences with ‘bogus’ vortices were investigated. Results showed that TCs in the SCS have characteristics that are distinctly different from vortices formed by the bogussing scheme used at Guangzhou Institute of Tropical and Marine Meteorology, such as no anticyclone in higher layers, strong convergence concentrated at the bottom of the troposphere, and strong divergence happening in higher layers instead of at 400 hPa. These differences provide clues for constructing a more realistic structure for TCs in the SCS. It was also found that the three types of vortices have some structural features in common. The area with high wind speed is fan-shaped in the north around the TC center, the maximum vorticity appears at 925 hPa, the strongest convergence appears at 1000 hPa, and strong divergence is located from 150 to 100 hPa. On the contrary, significant differences between them were revealed. The warm cores in tropical storms, severe tropical storms, and typhoons are located at 600–400 hPa, 400−300 hPa, and 400−250 hPa, respectively. Among the three types of TCs, the bogus vortex of tropical storms has the largest errors in structure and suffers the largest errors in track forecasts. However, typhoons have the largest errors in the forecast of intensity. This may be related to the great impacts of ocean on TC intensity.
1. Introduction The structure of a tropical storm (TC) has an important impact on its development and movement. Thus, understanding TC structure and its impacts on track and intensity changes is a crucial step towards improving prediction. In Europe and the United States, observations and research on TC structure have mainly focused on TCs in the western North Pacific (WNP) (Zehr, 1992; Gallina and Velden, 2002; Wu et al., 2005) and hurricanes in the North Atlantic (Koteswaram, 1967; Weatherford and William, 1988; Dodge et al., 1999; Chen et al., 2006; Abarca and Corbosiero, 2011). In China, early research (e.g., Yang and Liang, 1988; Wu and Lin, 1989; Lin and Song, 1992) on the structure of TCs in the South China Sea (SCS) was mainly based on low-resolution analysis data or sounding observations for individual cases. Therefore, some crude characteristics were obtained, such as the scale being smaller than ⁎ Corresponding author at: Guangzhou Institute of Tropical Marine and Meteorology, China Meteorological Administration, Guangzhou, 510640, China. E-mail address: [email protected] (Y. Huang).
https://doi.org/10.1016/j.dynatmoce.2019.101128 Received 9 August 2019; Received in revised form 6 December 2019; Accepted 16 December 2019 Available online 19 December 2019 0377-0265/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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TCs in the WNP, the radius of gale force wind being about 300–500 km, and an asymmetry in horizontal structure. In recent years, much work has been published on TCs in the WNP (e.g., Wang and Jiang, 2005; Zhong et al., 2008; Shu et al., 2011; Zhao et al., 2012), but relatively few attempts have been made to systematically study TCs in the SCS. As a TC spends most of its lifetime over the tropical ocean, where conventional observations are sparse, it is difficult to acquire observations that are able to characterize the features of TCs in the SCS. Recent works (e.g., Willoughby, 1990; Reasor et al., 2000; Zhan et al., 2014) have displayed the capability of satellite and radar data in obtaining the inner-core structure of a TC. Wang (2001) demonstrated that a high-resolution model has the ability to simulate many aspects of TCs. The reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF) have a high spatial resolution and, besides soundings, benefit from the assimilation of numerous satellite and radar data. Many previous works have demonstrated this dataset to be observationally efficient in several aspects (Annamalai et al., 1999; Cui et al., 2000; Renfrew and Mooreg, 2002; Bromwich and Fogtr, 2004; Trigo, 2006). Therefore, in this research, the structure of TCs in the SCS was investigated using ECMWF high-resolution reanalysis data. Because of the substantial impact that the initial structure has on TC forecasting, so-called ‘bogus’ vortices are often adopted in the initialization of operational numerical TC models in the absence of observations. Many studies have indicated that a ‘bogussing scheme’ (Iwasaki et al., 1987; Mathur, 1991; Kurihara et al., 1993; Leslie and Holland, 1995; Wang, 1998) or ‘bogus data assimilation’ (BDA) (Zou and Xiao, 2000; Pu and Braun, 2001; Park and Zou, 2004; Huang et al., 2010) can improve TC forecasting and simulation. The bogussing scheme based on the works of Anthes (1974) and Ueno and Ok (1991) was used in the Tropical Regional Atmosphere Model for the South China Sea (TRAMS)—an operational numerical TC prediction model developed by Guangzhou Institute of Tropical and Marine Meteorology (ITMM). In recent years, considerable progress has been made in TC track prediction with TRAMS. However, track prediction for certain typhoons, especially those over the SCS with weak intensity, are still in urgent need of improvement. In the past few years, it has been found in several cases that TCs in the SCS with weak intensity have significantly larger track errors than the mean errors of all TCs (including TCs in the WNP) in the corresponding year. For example, in 2012, the No. 13 Typhoon Kai-Tak, which had an initial sea level pressure (SLP) of 998 hPa in the SCS, also had 24-h and 48-h track errors that exceeded 300 km and 500 km, respectively, when the mean errors in 2012 for the 24-h, 48-h and 72-h forecasts were 96 km, 173 km and 231 km, respectively. As the SCS is small and close to the mainland, TCs in the SCS may be affected by many factors, such as the local terrain, distribution of land and sea, and the activity of the monsoons and the weather systems in the low latitudes of South China, leading to the distinct structure and great difficulty in track prediction. It is likely that the structure of TCs in the SCS is different from that of the bogus vortex used in TRAMS. Therefore, the first objective of this study was to investigate the structure of TCs in the SCS. The second was to identify clues for constructing a more realistic vortex structure that is accordant with TCs in the SCS. To obtain more detailed information in structure of TCs with different intensity, TCs in the SCS are stratified into three groups of tropical storms, severe tropical storms and typhoons. In this paper, we investigate the structures of the three types of composited vortices obtained from the ECMWF reanalysis data, and compare their structures to that of bogus vortices. Following this introduction, the methods and data are described in Section 2. The bogussing scheme is introduced in Section 3. Analysis of the structure of TCs in the SCS, as well as a comparison with bogus vortices, is presented in Section 4. Conclusions are provided in Section 5.
2. Data and methods The ECMWF reanalysis data used in this study covered the period October 2014 to December 2017, with a 0.125° × 0.125° spatial resolution and 17 vertical pressure levels. For simplicity, the domain of the SCS was set as (105 °E–125 °E, 4 °N–25 °N). TCs in this area, including those formed in this region as well as those formed over a different ocean region and then travelled to the SCS, were regarded as TCs in the SCS. According to the observed maximum wind in TC reports at 0000 UTC and 1200 UTC, TCs in the SCS were divided into three categories (tropical storm, severe tropical storm, and typhoon) for composite analysis. The selection of the individual cases was based upon the availability of the ECMWF reanalysis data. Thus, 50 samples of tropical storms, 22 samples of severe tropical storms, and 13 samples of typhoons in the SCS were obtained. Though the ECMWF reanalysis data has a high spatial resolution and has assimilated large amounts of satellite and radar data, it is not “true” observation. Hence, all the samples had been used as the initial condition for TC prediction with TRAMS to pick out “good samples”. Since a better prediction will be achieved when an initial condition that represents the TC structure more realistically is used, samples with “poor prediction” were deleted to remove the data which may not represent the realistic TC structure, samples with ‘good prediction’ were selected to construct the composite vortices, to ensure they are close to the “truth”. Thus, it was acceptable to treat these data as the ‘observation’ to investigate the structure of TCs in the SCS. Here, ‘good prediction’ refers to a track error smaller than or close to the mean error of tracks forecasted by TRAMS in the corresponding year. Thus, 8 samples of tropical storm, 8 samples of severe tropical storm, and 7 samples of typhoon were obtained, as shown in Table 1. Secondly, according to the times given in Table 1, the fields (mass, wind, temperature, etc.) within a radial distance of about 600 km from the TC center were extracted from the ECMWF reanalysis data and the “bogus” data, the latter was obtained from the bogussing scheme. Thirdly, taking the TC center as the center of the composite fields, the average of the extracted fields, including the reanalysis group and the “bogus” group, was calculated as the composite vortex according to the three categories, separately. The composited vortices from the reanalysis data were used to analyze the structure of TCs in the SCS and make comparisons with the bogus vortices. 2
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Table 1 Samples of tropical storms (TS), severe tropical storms (STS), and typhoons (TY) in the South China Sea. Category
Time
No.
Name
Radius of gale force (km)
Central pressure (hPa)
Maximum Wind (m s−1)
TS (8)
2014112800 2014112812 2014121000 2015091400 2017061112 2017082612 2017091300 2017101600 2014112900 2015100212 2015100300 2016101812 2017091400 2017101412 2017110300 2017111100 2015100312 2016101612 2016101700 2017082212 2017091412 2017101500 2017110312
1421 1421 1422 1519 1702 1714 1719 1720 1421 1522 1522 1621 1719 1720 1723 1724 1522 1621 1621 1713 1719 1720 1723
SINLAKU SINLAKU HAGUPIT VAMCO MERBOK PAKHAR DOKSURI KHANUN SINLAKU MUJIGAE MUJIGAE SARIKA DOKSURI KHANUN DAMREY HAIKUI MUJIGAE SARIKA SARIKA HATO DOKSURI KHANUN DAMREY
260 260 280 300 160 200 180 180 260 180 260 148.2 220 180 300 130 260 230 260 260 220 350 450
998 993 990 995 1000 990 995 998 985 985 982 985 985 980 980 985 965 965 965 975 965 965 960
18 20 23 20 18 23 20 18 25 25 28 28.3 25 30 30 25 38 38 38 33 38 38 40
STS (8)
TY (7)
3. Description of the bogussing scheme The bogussing scheme used in TRAMS was developed by He and Wang (1993), following the approach provided by Anthes (1974) and Ueno and Ok (1991). It uses parameters such as the reported TC center, the central SLP, the radius of force-8 wind, and the initial movement, to construct an asymmetrical vortex satisfying the static equilibrium, gradient wind equilibrium, and quasi-thermodynamic equilibrium. The main steps involved in formulating a bogus vortex are as follows: (1) The average SLP, temperature, and humidity are calculated in a ring area around the TC center as parameters of the ambient air. (2) The SLP in the central region is specified using 2
r p (r ) = pE − Δp [1 + ⎛ ⎞ ]−0.5 ⎝ R0 ⎠ ⎜
⎟
(1)
where pE is SLP in the environment, Δp is the difference in pressure between the TC center and the environment, r is the distance from the TC center, and R 0 is a required parameter calculated based on the static equation, gradient wind equation, and the radius of 15 m s−1. (3) The cloud temperature and the level of the cloud top and cloud bottom are calculated according to the adiabatic process, and then anticyclones are constructed near the cloud top. (4) The temperature and tangential and radial wind are specified according to the static equation, gradient wind equation, and momentum equation. The temperature of the TC center is set lower than the atmosphere in cloud and higher than the surrounding environment, to be consistent with the convective parameterization scheme in the model. The moisture in the lower troposphere is empirically set at 95 %. (5) The bogus vortex is implanted into the environment by combining its variables (geopotential height, temperature, humidity, etc.) with the objective analysis fields by weighting coefficients. For a more detailed description, readers are referred to He and Wang (1993); Anthes (1974), and Ueno and Ok (1991).
4. Results The structures of the composite reanalysis vortex in the SCS and the composite bogus vortex are stratified into three groups (i.e., tropical storm, severe tropical storm and typhoon) and compared, in order to obtain structure characteristics of TCs with different intensity. To simplify, the composite reanalysis vortex is referred as “the reanalysis vortex”, and the composite bogus vortex is referred as “the bogus vortex”. 3
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Fig. 1. The (a, b) 250-hPa geopotential height (gpm) and (c,d) 1000-hPa wind for (a, c) the reanalysis vortex of tropical storms in the SCS and (b, d) the bogus vortex. The shaded area indicates high wind speed (m s−1). TC center is located at (0,0) origin. The spatial resolution is 0.125° × 0.125°, and the interval on the x- and y-axis is 1.0°.
4.1. Tropical storm In the middle and lower layers the cylone in the bogus vortex is more intensified, as the geopotential height is lower than that in the reanalysis vortex (figure omitted). Significant differences exist in higher layers above 300 hPa. There is a cyclone in the central region of the reanalysis vortex at 250 hPa (Fig. 1a), and there shows no anticyclone in higher layers. However, a strong anticyclone is found in the bogus vortex at 250 hPa (Fig. 1b) and remains till 70 hPa. At 1000 hPa, the strong-wind zone in both vortices is fanshaped, located in the north of the center, the maximum wind speed in the bogus vortex exceeds 18 m s−1 (Fig. 1d) higher than that in the reanalysis vortex (Fig. 1c)). In the middle layers (i.e., 500 hPa), the wind speed in the north of the center remains higher than that in the south (figure omitted). A cross section of u-component wind speed and relative humidity is shown in Fig. 2. As the quantities were averaged in the longitudinal direction, the point on the left (right) of the origin (i.e., the TC center) on the x-axis refers to the location south (north) of the TC center. In the reanalysis vortex (Fig. 2a), the negative wind speed (namely, the easterly wind) in the north of the center is obviously higher than the positive wind speed (namely, the westerly wind) in the south, with the maximum value exceeding 16 m s−1 between 925 and 700 hPa. Also, negative wind speed prevails in the levels beyond 250 hPa. In the bogus vortex (Fig. 2b), the range of values greater than 8 m s−1 extends to 500 hPa within 3 degree from the center, while that in the reanalysis vortex is much smaller. Beyond 250 hPa, there is negative value in the south of the center and positive value in the north, suggesting anticyclonic circulation. Regarding the differences of relative humidity, the relative humidity in the reanalysis vortex (Fig. 2c) decreases with increasing height from 800 to 500 hPa. The maximum humidity (i.e., > 90 %) is mainly distributed in the lower central region. However, in the central region of the bogus vortex, the humidity between 600 to 200 hPa is higher, suggesting greater pumping of vapour. This is accordant with previous work (Huang et al., 2017) reporting that in the bogussing scheme the cloud top extends to 150 hPa. In Fig. 3a, the contour lines of relative vorticity in the reanalysis vortex are dense and distributed within a narrow region, suggesting a large gradient. The vorticity in lower troposphere (1000−850 hPa) exceeds 75 × 10−5 s−1, and in higher layers there is positive vorticity. The strong convergence in the middle layers (850−700 hPa) and strong divergence in the higher layers (400−150 hPa) are both located in the south of the center. In the bogus vortex (Fig. 3b), the vorticity in the lower layers is about 60 × 10−5 s−1 with a smaller gradient; and above 300 hPa, there is negative vorticity. Moreover, the convergence is mainly 4
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Fig. 2. Pressure–latitude cross section of (a, b) u-component wind speed (m s−1) and (c, d) relative humidity (%) for (a, c) the composited tropical storm in the SCS and (b, d) the bogus vortex, averaged in the longitudinal direction.
distributed beyond 600 hPa, equivalent on both sides of the center. Strong convergence and divergence are distributed below 850 hPa and from 500 to 250 hPa, respectively. For further investigation, the differences in specific humidity and temperature between the TC center and the environment were calculated in different layers. Similarly, obviously positive anomalies of temperature and specific humidity exist in the central region from 850 to 200 hPa (Fig. 3c and d). However, there are significant differences in thermal structure and humidity field. In the reanalysis vortex (Fig. 3c), the maximum positive anomaly of temperature (warm core) and specific humidity (wet core) is less than 3 K located between 700 and 400 hPa, and about 2 × 10−3 kg kg−1 mainly distributed below 500 hPa, respectively. The warm core and wet core in the bogus vortex is much stronger with the maximum anomaly exceeding 5 K and 4 × 10−3 kg kg−1, respectively. Obvious negative temperature anomalies appear at 100 hPa. The region with obvious positive anomaly of humidity extends to 300 hPa, which confirms the analysis of relative humidity in Fig. 2d. Clearly, the location of the warm core is higher than that in the reanalysis vortex. The middle and upper tropospheric warm core is more effective than a lowerlevel one to cause a great surface pressure fall from the hydrostatic equation (Chen and Gopalakrishnan, 2015; Chen and Zhang, 2013; Zhang and Chen, 2012). It is easy to construct an overestimated vortex for tropical storms. Fig. 4 shows the vertical profiles of wind speed, humidity, vorticity, and divergence of both vortices. The values of v-component wind speed, humidity, vorticity, and divergence in each layer were obtained by calculating the average within the distance of 1 ° from the TC center. Beyond 200 hPa the u-component wind speed in the reanalysis vortex is significantly higher than that in the bogus vortex (figure omitted). This can also be deduced from Fig. 2a and b. As the v-component is concerned, in the reanalysis vortex there are southerly winds from 925 to 200 hPa with obvious vertical shear and these convert to northerly winds at 100 hPa. From 1000 to 5
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Fig. 3. Pressure–latitude cross section of (a, b) vorticity (contoured; 10−5 s−1) and divergence (shaded; 10−5 s−1) and (c, d) differences in specific humidity (contoured; 10−3 kg kg−1) and temperature (shaded; K) between the atmosphere in the central region and the environment for (a, c) the composited tropical storm in the SCS and (b, d) the bogus vortex.
10 hPa the speed is below 3 m s−1. It can be infered that the easterly winds prevail in the higher layers. Low wind speeds and small vertical shear are found in the bogus vortex (Fig. 4a, solid circle). Below 300 hPa, the specific humidity of the bogus vortex is larger than that of the reanalysis vortex (Fig. 4b). The vorticity of the bogus vortex at 1000 hPa is much larger than that of the reanalysis vortex. The maximum vorticity of both vortices appears at 925 hPa (Fig. 4c). Negative vorticity is found in the bogus vortex beyond 300 hPa, while in the reanalysis vortex vorticity remains positive until 200 hPa. Moreover, in the reanalysis vortex convergence mainly occurs in the lower troposphere (i.e., below 925 hPa) and divergence mainly happens in the upper layers from 150 to 100 hPa. In a deep layer from 850 to 250 hPa, the value is close to zero, suggesting near non-divergence. However, in the bogus vortex, strong convergence exists till 700 hPa and maximum divergence happens at 400 hPa (Fig. 4d). In summary, the structure of the composite tropical strom has distinct characteristics, such as no anticyclone in higher layers, a warm core located in the middle layers, convergence concentrated in the lower troposphere, and divergence located in upper layers of the troposphere. The vortex constructed by the bogus scheme is unable to represent tropical storms in the SCS. 4.2. Severe tropical storm There also exists a false anticyclone in the upper layers of the bogus vortex for severe tropical storms (figure omitted). Vertical distribution of the u-component in both vortices (figure omitted) are similar to that of tropical storms, but the eastly winds in the north-central region exceeds 24 m s−1 and in the south-central region the zero-velocity line appears at higher layers in both the reanalysis and bogus vortex, indicating that the cyclone extends to higher levels. Relative humidity in both vortices has similar distribution in vertical section (figure omitted). 6
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Fig. 4. Vertical profiles of (a) v-component wind speed (m s−1), (b) specific humidity (10−3 kg kg−1), (c) vorticity (10−5 s−1), and (d) divergence (10−5 s−1), where the open and solid circles represent the composited tropical storm in the SCS and the bogus vortex, respectively.
In both vortices, the maximum vorticity is located in the lower layers. In the bogus vortex the region of value greater than 100 × 10−5 s−1 extends higher to 600 hPa (Fig. 5b) than that in the reanalysis vortex (Fig. 5a). In the central area the zero-vorticity curves appear beyond 100 hPa in the reanalysis vortex and near 200 hPa in the bogus vortex. Strong convergence and divergence are found below 925 hPa and near 150 hPa in the reanalysis vortex (Fig. 5a), respectively. In the bogus vortex (Fig. 5b), the convergence in the lower and middle layers is too strong and beyond 600 hPa there is apparent divergence over a wide range. The differences in temperature and specific humidity between the atmosphere in the central region and the surrounding environment are provided in Figs. 5c and 5d. In the reanalysis vortex (Fig. 5c), the maximum positive anomaly of temperature exceeds 3 K located at 700 hPa and between 400 and 300 hPa. The maximum positive anomalies of temperature and specific humidity in the bogus vortex exceed 5 K and 4 × 10−3 kg kg−1, respectively. The warm core is located between 300 and 200 hPa and the wet core is at 600 hPa. Also, there is a considerable negative anomaly of temperature (cold core) at 100 hPa (Fig. 5d). Clearly, the warm core and wet core in the bogusvortex is too strong and distributed at higher levels. Vertical profiles of v-component wind speed, humidity, vorticity and divergence are given in Fig. 6. There exists more apparent vertical shear in the reanalysis vortex (Fig. 6a). Profiles of u-component wind speed (figure omitted) and humidity (Fig. 6b) are similar to those of the tropical storm, but the differences in the middle and lower layers between the reanalysis and bogus vortex become smaller. The maximum vorticity appears at 925 hPa (Fig. 6c). At 150 hPa, there is negative vorticity in the bogus vortex and positive vorticity in the reanalysis vortex respectively, indicating different circulation in the two vortices. Regarding divergence (Fig. 6d), differences between the two vortices are similar to that of tropical storms. 4.3. Typhoon In the reanalysis vortex, the maximum negative and positive speed of u-component (Fig. 7a) exceed −30 m s−1 and 20 m s−1, respectively. The range of high speeds (e.g., −20 m s−1 or 10 m s−1) exceeds the distance of 350 km from the center. In the bogus 7
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Fig. 5. As in Fig. 3, but for severe tropical storms.
vortex, high speeds (e.g., −20 m s−1 or 10 m s−1) are distributed within a distance of 200 km from the center, and the zero-velocity curve appears at a lower level (i.e., 200 hPa) (Fig. 7b). The distribution of relative humidity in both vortices is similar (figure omitted), in the central region from 1000 to 150 hPa the relative humidity exceeds 90 %. In the reanalysis vortex, the maximum vorticity and the gradient of vorticity is larger than that in the bogus vortex, and the zerovorticity curves appear at 100 hPa (Fig. 8a), higher than that in the bogus vortex (Fig. 10b). Furthermore, in the reanalysis vortex, strong convergence mainly occurs below 925 hPa and the maximum divergence is mainly distributed between 200 and 100 hPa (Fig. 8a, shaded). However, strong convergence in the bogus vortex extends to 850 hPa and the maximum divergence is located at 400 hPa (Fig. 8b, shaded). The atmosphere in the central region of both vortices is significantly warmer and wetter than the ambient atmosphere, as obvious positive anomalies of temperature and humidity are found in the vertical cross sections (Fig. 8c and d). Nevertheless, the maximum anomaly of temperature in the bogus vortex exceeds 7 K, while that of the reanalysis vortex is below 6 K. The warm core in the bogus vortex is much stronger and it is located a bit higher than that in the reanalysis vortex. In the middle layers (i.e., from 700 to 500 hPa) the anomaly of specific humidity in the bogus vortex is larger than that in the reanalysis vortex. It is obviously that the warm core constructed by the bogus scheme in higher layers is too strong while the vorticity and the range of high speeds are small. The vertical changes of speed, humidity, vorticity and divergence in both vortices (figure omitted) are similar to that of tropical storms, but the vorticity of the reanalysis vortex is considerably larger than that in the bogus vortex, and the v-component speed in the middle layers becomes larger. Making a comparison between the three types of TCs (reanalysis vortices) in the SCS, there are some similarities as well as significant differences in their structures. The area with maximum wind speed exists in the north of the center, and is fan-shaped around the center. Easterly winds prevail beyond 100 hPa. The strongest convergence appears at 1000 hPa, strong divergence is located from 150 to 100 hPa, and the maximum vorticity appears at 925 hPa. Obvious warm core and wet core are distributed in the 8
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Fig. 6. As in Figs. 4, but for severe tropical storms.
Fig. 7. As in Figs. 2a and 2b, but for typhoons.
9
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Fig. 8. As in Fig. 3, but for typhoons.
central region, but the location and the intensity are different. The warm core in tropical storms, severe tropical storms, and typhoons is located from 600 to 400 hPa, 400 to 300 hPa, and 400 to 250 hPa, respectively. As the TC intensity increases, the intensity of the warm core and wet core increases and the range becomes wider.
4.4. Analysis on structure errors and forecast errors of tropical storms, severe tropical storms and typhoons In this subsection, structure errors and forecasting errors of the three types of TCs in the SCS are analyzed to investigate whether the larger structure errors lead to larger forecasting errors? From the comparison above, it is clear that the major errors of the bogus vortex are the false anticyclone in the higher layers, the too intense and higher-located warm core and the inaccurate vertical distribution of divergence. As shown in Table 2, the anticyclone in the bogus vorex of tropical storms, sever tropical storms and typhoons appears at 250, 200 and 150 hPa respectively, and it maintains till 70 hPa. In the bogus vortex of tropical storms, the size of the anticyclone is the largest both in horizontal (figure omitted) and vertical direction, and the intensity is the strongest. In the bogus vortices of severe tropical storms and typhoons, the size and the intensity of the anticyclone are comparable. Significant differences of temperature deviation from the environment between the reanalysis and bogus vortex are distributed in the middle and higher layers (i.e., 600−100 hPa, Fig. 9). The differences of temperature deviation in higher troposphere (i.e., 10
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Table 2 The geopotential height in the reanalysis and bogus vortex of tropical storms (TS), severe tropical storms (STS), and typhoons (TY). The characters “H” and “L” represent anticyclone and cyclone system, respectively. Level(hPa)
300 250 200 150 100 70 50
TS
STS
TY
Reanalysis
Bogus
Reanalysis
Bogus
Reanalysis
Bogus
L L – – – – –
– H H H H H –
L L L – – – –
L L H H H H –
L L L L – – –
L L – H H H –
Fig. 9. Differences of temperature between the TC center and the environment in the vortices of (a) tropical storms, (b) severe tropical storms and (c) typhoons, where the open and solid circles represent the reanalysis vortex and the bogus vortex, respectively.
300−200 hPa) in tropical storms are the largest (Fig. 9a) and there exist big errors in 100 hPa. The errors between 300 and 200 hPa of severe tropical storms and typhoons are comparable, while in 100 hPa the errors of severe tropical storms are larger than that of typhoons (Fig. 9b and c). In the reanalysis vortices, the location of the warm core in tropical storms is lower than that of severe tropical storms and typhoons (as shown in Figs. 3c, 5 c 8 c and 9), which is consistent with previous studies on WNP TCs (Zhang et al., 2007; Durden, 2013; Gao et al., 2017). Since the warm cores in the bogus vortices are all located between 300 and 200 hPa, the bogus vortex of tropical storms also has the largest deviation in the warm core location. The differences of the divergence between the reanalysis and bogus vortex in tropical storms (Fig. 3a and b) are more significant than those in severe tropical storms (Fig. 5a and b) and typhoons (Fig. 8a and b), as the convergence in the bogus vortex of tropical storms below 925 hPa is much stronger. In summary, the vortex of tropical storms constructed by the bogus scheme has the largest errors in structure. Forecasting tracks errors of tropical storms, severe tropical storms and typhoons using the bogus scheme are shown in Fig. 10a. The track errors of tropical storms are generally the largest, corresponding to the largest structure errors. And the track errors of typhoons are the smallest. It can be deduced that the deviation in structure of TC vortex has great impact on track forecasts. Fig. 10b shows the forecasting intensity errors of the three types of TCs. It is clear that within 48-h forecast, typhoons have the largest intensity errors, most errors result from underestimate. Since sea surface temperature used in TRAMS is climate average, it probably has more difficulty in energy supporting for intensification of a stronger TC. Ocean basin may have a great impact on intensity changes, so that the large errors in the initial structure of tropical storms do not lead to large errors in intensity forecast. 4.5. How to adjust the bogus scheme Based on the analysis above, the bogus scheme will be adjusted according to the errors in the structure of TC vortex. The intensity of the warm core needs to be weakened. The false anticyclone in the higher layers needs to be removed. Profiles of temperature deviation from the environment can be introduced to the scheme to make the location and the intensity of the warm core more 11
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Fig. 10. Forecasting (a) track errors and (b) intensity errors of tropical storms (blue and solid circle), severe tropical storms (red and open circle) and typhoons (green and open square).
realistic. Since there is nearly non-divergence between 850 and 250 hPa (Figs. 4d and 6 d), the convergence can be restricted below 850 hPa and the divergence can be restricted beyond 200 hPa.
5. Conclusion In this study, samples of tropical storms, severe tropical storms, and typhoons in the SCS were selected based on ECMWF highresolution reanalysis data for composite analysis. Their structures were investigated and compared with the bogus vortices. Structure errors and forecasting errors of the three types of TCs are compared and analyzed. And the issue of how to adjust the bogus scheme is discussed. The following conclusions can be drawn: (1) The vortices constructed by the bogussing scheme are not consistent with the ‘observed’ structures. For instance, they show a strong anticyclone in higher layers and strong divergence in middle layers (i.e., 400 hPa). The warm core is stronger and locates higher. (2) Tropical storms, severe tropical storms, and typhoons in the SCS share some structural features in common. They have obvious warm core structures. The area with high wind speeds is fan-shaped in the north around the TC center, the strongest convergence appears at 1000 hPa, strong divergence locates from 150 to 100 hPa, and the maximum vorticity appears at 925 hPa. (3) Nevertheless, there are significant differences in thermodynamic structures between them. For instance, the warm core in tropical storms, severe tropical storms, and typhoons is located from 600 to 400 hPa, 400 to 300 hPa, and 400 to 250 hPa, respectively. As TC intensity increases, the warm air in the central region becomes wider and wetter. (4) The bogus vortex of tropical storms has the largest errors in structure among the three types of TCs. Correspondingly, it suffers the largest forecasting track errors. However, typhoons have relatively larger errors in the forecast of intensity. Ocean may play a more important role in the development of intensity than TCs’ initial structure. In future, the bogussing scheme will be adjusted based on this study. The composited vortices of tropical storms, severe tropical storms, and typhoons obtained can also be used to construct the initial vortex. Numerical experiments will be designed and demonstrated based on the two schemes provided above to verify the validity of the schemes in improving forecasts of TCs in the SCS.
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CRediT authorship contribution statement Yanyan Huang: Conceptualization, Methodology, Writing - original draft. Bin Zheng: Investigation, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Key Projects of China (Grant 2018YFC1506902), the National Natural Science Foundation of China (Grants 41705089 and 41675099), and the Science and Technology Program of Guangdong Province (Grants 2017B030314140 and 2017A020219005). The authors thank Prof. Jishan Xue and Dr. Yerong Feng for offering insightful assistance during the early stage of this research. References Abarca, S.F., Corbosiero, R.L., 2011. The world wide lightning location network and convective activity in tropical cyclones. Mon. Weather Rev. 139, 175–191. Annamalai, H., Slingoj, M., Sperberk, et al., 1999. 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Tropical cyclone structure in the South China Sea based on highresolution reanalysis data and comparison with that of ‘bogus’ vortices
T
Yanyan Huang*, Bin Zheng Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, and Guangdong Provincial Key Laboratory of Regional Numerical Weather Prediction, Guangzhou, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Tropical cyclone South China Sea Structure feature Bogus model Composite analysis
Based on high-resolution reanalysis data of the European Centre for Medium-Range Weather Forecasts, several samples of tropical cyclones (TCs), including tropical storms, severe tropical storms, and typhoons, in the South China Sea (SCS), were selected for composite analysis. The structures of these three types of vortices and their differences with ‘bogus’ vortices were investigated. Results showed that TCs in the SCS have characteristics that are distinctly different from vortices formed by the bogussing scheme used at Guangzhou Institute of Tropical and Marine Meteorology, such as no anticyclone in higher layers, strong convergence concentrated at the bottom of the troposphere, and strong divergence happening in higher layers instead of at 400 hPa. These differences provide clues for constructing a more realistic structure for TCs in the SCS. It was also found that the three types of vortices have some structural features in common. The area with high wind speed is fan-shaped in the north around the TC center, the maximum vorticity appears at 925 hPa, the strongest convergence appears at 1000 hPa, and strong divergence is located from 150 to 100 hPa. On the contrary, significant differences between them were revealed. The warm cores in tropical storms, severe tropical storms, and typhoons are located at 600–400 hPa, 400−300 hPa, and 400−250 hPa, respectively. Among the three types of TCs, the bogus vortex of tropical storms has the largest errors in structure and suffers the largest errors in track forecasts. However, typhoons have the largest errors in the forecast of intensity. This may be related to the great impacts of ocean on TC intensity.
1. Introduction The structure of a tropical storm (TC) has an important impact on its development and movement. Thus, understanding TC structure and its impacts on track and intensity changes is a crucial step towards improving prediction. In Europe and the United States, observations and research on TC structure have mainly focused on TCs in the western North Pacific (WNP) (Zehr, 1992; Gallina and Velden, 2002; Wu et al., 2005) and hurricanes in the North Atlantic (Koteswaram, 1967; Weatherford and William, 1988; Dodge et al., 1999; Chen et al., 2006; Abarca and Corbosiero, 2011). In China, early research (e.g., Yang and Liang, 1988; Wu and Lin, 1989; Lin and Song, 1992) on the structure of TCs in the South China Sea (SCS) was mainly based on low-resolution analysis data or sounding observations for individual cases. Therefore, some crude characteristics were obtained, such as the scale being smaller than ⁎ Corresponding author at: Guangzhou Institute of Tropical Marine and Meteorology, China Meteorological Administration, Guangzhou, 510640, China. E-mail address: [email protected] (Y. Huang).
https://doi.org/10.1016/j.dynatmoce.2019.101128 Received 9 August 2019; Received in revised form 6 December 2019; Accepted 16 December 2019 Available online 19 December 2019 0377-0265/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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TCs in the WNP, the radius of gale force wind being about 300–500 km, and an asymmetry in horizontal structure. In recent years, much work has been published on TCs in the WNP (e.g., Wang and Jiang, 2005; Zhong et al., 2008; Shu et al., 2011; Zhao et al., 2012), but relatively few attempts have been made to systematically study TCs in the SCS. As a TC spends most of its lifetime over the tropical ocean, where conventional observations are sparse, it is difficult to acquire observations that are able to characterize the features of TCs in the SCS. Recent works (e.g., Willoughby, 1990; Reasor et al., 2000; Zhan et al., 2014) have displayed the capability of satellite and radar data in obtaining the inner-core structure of a TC. Wang (2001) demonstrated that a high-resolution model has the ability to simulate many aspects of TCs. The reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF) have a high spatial resolution and, besides soundings, benefit from the assimilation of numerous satellite and radar data. Many previous works have demonstrated this dataset to be observationally efficient in several aspects (Annamalai et al., 1999; Cui et al., 2000; Renfrew and Mooreg, 2002; Bromwich and Fogtr, 2004; Trigo, 2006). Therefore, in this research, the structure of TCs in the SCS was investigated using ECMWF high-resolution reanalysis data. Because of the substantial impact that the initial structure has on TC forecasting, so-called ‘bogus’ vortices are often adopted in the initialization of operational numerical TC models in the absence of observations. Many studies have indicated that a ‘bogussing scheme’ (Iwasaki et al., 1987; Mathur, 1991; Kurihara et al., 1993; Leslie and Holland, 1995; Wang, 1998) or ‘bogus data assimilation’ (BDA) (Zou and Xiao, 2000; Pu and Braun, 2001; Park and Zou, 2004; Huang et al., 2010) can improve TC forecasting and simulation. The bogussing scheme based on the works of Anthes (1974) and Ueno and Ok (1991) was used in the Tropical Regional Atmosphere Model for the South China Sea (TRAMS)—an operational numerical TC prediction model developed by Guangzhou Institute of Tropical and Marine Meteorology (ITMM). In recent years, considerable progress has been made in TC track prediction with TRAMS. However, track prediction for certain typhoons, especially those over the SCS with weak intensity, are still in urgent need of improvement. In the past few years, it has been found in several cases that TCs in the SCS with weak intensity have significantly larger track errors than the mean errors of all TCs (including TCs in the WNP) in the corresponding year. For example, in 2012, the No. 13 Typhoon Kai-Tak, which had an initial sea level pressure (SLP) of 998 hPa in the SCS, also had 24-h and 48-h track errors that exceeded 300 km and 500 km, respectively, when the mean errors in 2012 for the 24-h, 48-h and 72-h forecasts were 96 km, 173 km and 231 km, respectively. As the SCS is small and close to the mainland, TCs in the SCS may be affected by many factors, such as the local terrain, distribution of land and sea, and the activity of the monsoons and the weather systems in the low latitudes of South China, leading to the distinct structure and great difficulty in track prediction. It is likely that the structure of TCs in the SCS is different from that of the bogus vortex used in TRAMS. Therefore, the first objective of this study was to investigate the structure of TCs in the SCS. The second was to identify clues for constructing a more realistic vortex structure that is accordant with TCs in the SCS. To obtain more detailed information in structure of TCs with different intensity, TCs in the SCS are stratified into three groups of tropical storms, severe tropical storms and typhoons. In this paper, we investigate the structures of the three types of composited vortices obtained from the ECMWF reanalysis data, and compare their structures to that of bogus vortices. Following this introduction, the methods and data are described in Section 2. The bogussing scheme is introduced in Section 3. Analysis of the structure of TCs in the SCS, as well as a comparison with bogus vortices, is presented in Section 4. Conclusions are provided in Section 5.
2. Data and methods The ECMWF reanalysis data used in this study covered the period October 2014 to December 2017, with a 0.125° × 0.125° spatial resolution and 17 vertical pressure levels. For simplicity, the domain of the SCS was set as (105 °E–125 °E, 4 °N–25 °N). TCs in this area, including those formed in this region as well as those formed over a different ocean region and then travelled to the SCS, were regarded as TCs in the SCS. According to the observed maximum wind in TC reports at 0000 UTC and 1200 UTC, TCs in the SCS were divided into three categories (tropical storm, severe tropical storm, and typhoon) for composite analysis. The selection of the individual cases was based upon the availability of the ECMWF reanalysis data. Thus, 50 samples of tropical storms, 22 samples of severe tropical storms, and 13 samples of typhoons in the SCS were obtained. Though the ECMWF reanalysis data has a high spatial resolution and has assimilated large amounts of satellite and radar data, it is not “true” observation. Hence, all the samples had been used as the initial condition for TC prediction with TRAMS to pick out “good samples”. Since a better prediction will be achieved when an initial condition that represents the TC structure more realistically is used, samples with “poor prediction” were deleted to remove the data which may not represent the realistic TC structure, samples with ‘good prediction’ were selected to construct the composite vortices, to ensure they are close to the “truth”. Thus, it was acceptable to treat these data as the ‘observation’ to investigate the structure of TCs in the SCS. Here, ‘good prediction’ refers to a track error smaller than or close to the mean error of tracks forecasted by TRAMS in the corresponding year. Thus, 8 samples of tropical storm, 8 samples of severe tropical storm, and 7 samples of typhoon were obtained, as shown in Table 1. Secondly, according to the times given in Table 1, the fields (mass, wind, temperature, etc.) within a radial distance of about 600 km from the TC center were extracted from the ECMWF reanalysis data and the “bogus” data, the latter was obtained from the bogussing scheme. Thirdly, taking the TC center as the center of the composite fields, the average of the extracted fields, including the reanalysis group and the “bogus” group, was calculated as the composite vortex according to the three categories, separately. The composited vortices from the reanalysis data were used to analyze the structure of TCs in the SCS and make comparisons with the bogus vortices. 2
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Table 1 Samples of tropical storms (TS), severe tropical storms (STS), and typhoons (TY) in the South China Sea. Category
Time
No.
Name
Radius of gale force (km)
Central pressure (hPa)
Maximum Wind (m s−1)
TS (8)
2014112800 2014112812 2014121000 2015091400 2017061112 2017082612 2017091300 2017101600 2014112900 2015100212 2015100300 2016101812 2017091400 2017101412 2017110300 2017111100 2015100312 2016101612 2016101700 2017082212 2017091412 2017101500 2017110312
1421 1421 1422 1519 1702 1714 1719 1720 1421 1522 1522 1621 1719 1720 1723 1724 1522 1621 1621 1713 1719 1720 1723
SINLAKU SINLAKU HAGUPIT VAMCO MERBOK PAKHAR DOKSURI KHANUN SINLAKU MUJIGAE MUJIGAE SARIKA DOKSURI KHANUN DAMREY HAIKUI MUJIGAE SARIKA SARIKA HATO DOKSURI KHANUN DAMREY
260 260 280 300 160 200 180 180 260 180 260 148.2 220 180 300 130 260 230 260 260 220 350 450
998 993 990 995 1000 990 995 998 985 985 982 985 985 980 980 985 965 965 965 975 965 965 960
18 20 23 20 18 23 20 18 25 25 28 28.3 25 30 30 25 38 38 38 33 38 38 40
STS (8)
TY (7)
3. Description of the bogussing scheme The bogussing scheme used in TRAMS was developed by He and Wang (1993), following the approach provided by Anthes (1974) and Ueno and Ok (1991). It uses parameters such as the reported TC center, the central SLP, the radius of force-8 wind, and the initial movement, to construct an asymmetrical vortex satisfying the static equilibrium, gradient wind equilibrium, and quasi-thermodynamic equilibrium. The main steps involved in formulating a bogus vortex are as follows: (1) The average SLP, temperature, and humidity are calculated in a ring area around the TC center as parameters of the ambient air. (2) The SLP in the central region is specified using 2
r p (r ) = pE − Δp [1 + ⎛ ⎞ ]−0.5 ⎝ R0 ⎠ ⎜
⎟
(1)
where pE is SLP in the environment, Δp is the difference in pressure between the TC center and the environment, r is the distance from the TC center, and R 0 is a required parameter calculated based on the static equation, gradient wind equation, and the radius of 15 m s−1. (3) The cloud temperature and the level of the cloud top and cloud bottom are calculated according to the adiabatic process, and then anticyclones are constructed near the cloud top. (4) The temperature and tangential and radial wind are specified according to the static equation, gradient wind equation, and momentum equation. The temperature of the TC center is set lower than the atmosphere in cloud and higher than the surrounding environment, to be consistent with the convective parameterization scheme in the model. The moisture in the lower troposphere is empirically set at 95 %. (5) The bogus vortex is implanted into the environment by combining its variables (geopotential height, temperature, humidity, etc.) with the objective analysis fields by weighting coefficients. For a more detailed description, readers are referred to He and Wang (1993); Anthes (1974), and Ueno and Ok (1991).
4. Results The structures of the composite reanalysis vortex in the SCS and the composite bogus vortex are stratified into three groups (i.e., tropical storm, severe tropical storm and typhoon) and compared, in order to obtain structure characteristics of TCs with different intensity. To simplify, the composite reanalysis vortex is referred as “the reanalysis vortex”, and the composite bogus vortex is referred as “the bogus vortex”. 3
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Fig. 1. The (a, b) 250-hPa geopotential height (gpm) and (c,d) 1000-hPa wind for (a, c) the reanalysis vortex of tropical storms in the SCS and (b, d) the bogus vortex. The shaded area indicates high wind speed (m s−1). TC center is located at (0,0) origin. The spatial resolution is 0.125° × 0.125°, and the interval on the x- and y-axis is 1.0°.
4.1. Tropical storm In the middle and lower layers the cylone in the bogus vortex is more intensified, as the geopotential height is lower than that in the reanalysis vortex (figure omitted). Significant differences exist in higher layers above 300 hPa. There is a cyclone in the central region of the reanalysis vortex at 250 hPa (Fig. 1a), and there shows no anticyclone in higher layers. However, a strong anticyclone is found in the bogus vortex at 250 hPa (Fig. 1b) and remains till 70 hPa. At 1000 hPa, the strong-wind zone in both vortices is fanshaped, located in the north of the center, the maximum wind speed in the bogus vortex exceeds 18 m s−1 (Fig. 1d) higher than that in the reanalysis vortex (Fig. 1c)). In the middle layers (i.e., 500 hPa), the wind speed in the north of the center remains higher than that in the south (figure omitted). A cross section of u-component wind speed and relative humidity is shown in Fig. 2. As the quantities were averaged in the longitudinal direction, the point on the left (right) of the origin (i.e., the TC center) on the x-axis refers to the location south (north) of the TC center. In the reanalysis vortex (Fig. 2a), the negative wind speed (namely, the easterly wind) in the north of the center is obviously higher than the positive wind speed (namely, the westerly wind) in the south, with the maximum value exceeding 16 m s−1 between 925 and 700 hPa. Also, negative wind speed prevails in the levels beyond 250 hPa. In the bogus vortex (Fig. 2b), the range of values greater than 8 m s−1 extends to 500 hPa within 3 degree from the center, while that in the reanalysis vortex is much smaller. Beyond 250 hPa, there is negative value in the south of the center and positive value in the north, suggesting anticyclonic circulation. Regarding the differences of relative humidity, the relative humidity in the reanalysis vortex (Fig. 2c) decreases with increasing height from 800 to 500 hPa. The maximum humidity (i.e., > 90 %) is mainly distributed in the lower central region. However, in the central region of the bogus vortex, the humidity between 600 to 200 hPa is higher, suggesting greater pumping of vapour. This is accordant with previous work (Huang et al., 2017) reporting that in the bogussing scheme the cloud top extends to 150 hPa. In Fig. 3a, the contour lines of relative vorticity in the reanalysis vortex are dense and distributed within a narrow region, suggesting a large gradient. The vorticity in lower troposphere (1000−850 hPa) exceeds 75 × 10−5 s−1, and in higher layers there is positive vorticity. The strong convergence in the middle layers (850−700 hPa) and strong divergence in the higher layers (400−150 hPa) are both located in the south of the center. In the bogus vortex (Fig. 3b), the vorticity in the lower layers is about 60 × 10−5 s−1 with a smaller gradient; and above 300 hPa, there is negative vorticity. Moreover, the convergence is mainly 4
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Fig. 2. Pressure–latitude cross section of (a, b) u-component wind speed (m s−1) and (c, d) relative humidity (%) for (a, c) the composited tropical storm in the SCS and (b, d) the bogus vortex, averaged in the longitudinal direction.
distributed beyond 600 hPa, equivalent on both sides of the center. Strong convergence and divergence are distributed below 850 hPa and from 500 to 250 hPa, respectively. For further investigation, the differences in specific humidity and temperature between the TC center and the environment were calculated in different layers. Similarly, obviously positive anomalies of temperature and specific humidity exist in the central region from 850 to 200 hPa (Fig. 3c and d). However, there are significant differences in thermal structure and humidity field. In the reanalysis vortex (Fig. 3c), the maximum positive anomaly of temperature (warm core) and specific humidity (wet core) is less than 3 K located between 700 and 400 hPa, and about 2 × 10−3 kg kg−1 mainly distributed below 500 hPa, respectively. The warm core and wet core in the bogus vortex is much stronger with the maximum anomaly exceeding 5 K and 4 × 10−3 kg kg−1, respectively. Obvious negative temperature anomalies appear at 100 hPa. The region with obvious positive anomaly of humidity extends to 300 hPa, which confirms the analysis of relative humidity in Fig. 2d. Clearly, the location of the warm core is higher than that in the reanalysis vortex. The middle and upper tropospheric warm core is more effective than a lowerlevel one to cause a great surface pressure fall from the hydrostatic equation (Chen and Gopalakrishnan, 2015; Chen and Zhang, 2013; Zhang and Chen, 2012). It is easy to construct an overestimated vortex for tropical storms. Fig. 4 shows the vertical profiles of wind speed, humidity, vorticity, and divergence of both vortices. The values of v-component wind speed, humidity, vorticity, and divergence in each layer were obtained by calculating the average within the distance of 1 ° from the TC center. Beyond 200 hPa the u-component wind speed in the reanalysis vortex is significantly higher than that in the bogus vortex (figure omitted). This can also be deduced from Fig. 2a and b. As the v-component is concerned, in the reanalysis vortex there are southerly winds from 925 to 200 hPa with obvious vertical shear and these convert to northerly winds at 100 hPa. From 1000 to 5
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Fig. 3. Pressure–latitude cross section of (a, b) vorticity (contoured; 10−5 s−1) and divergence (shaded; 10−5 s−1) and (c, d) differences in specific humidity (contoured; 10−3 kg kg−1) and temperature (shaded; K) between the atmosphere in the central region and the environment for (a, c) the composited tropical storm in the SCS and (b, d) the bogus vortex.
10 hPa the speed is below 3 m s−1. It can be infered that the easterly winds prevail in the higher layers. Low wind speeds and small vertical shear are found in the bogus vortex (Fig. 4a, solid circle). Below 300 hPa, the specific humidity of the bogus vortex is larger than that of the reanalysis vortex (Fig. 4b). The vorticity of the bogus vortex at 1000 hPa is much larger than that of the reanalysis vortex. The maximum vorticity of both vortices appears at 925 hPa (Fig. 4c). Negative vorticity is found in the bogus vortex beyond 300 hPa, while in the reanalysis vortex vorticity remains positive until 200 hPa. Moreover, in the reanalysis vortex convergence mainly occurs in the lower troposphere (i.e., below 925 hPa) and divergence mainly happens in the upper layers from 150 to 100 hPa. In a deep layer from 850 to 250 hPa, the value is close to zero, suggesting near non-divergence. However, in the bogus vortex, strong convergence exists till 700 hPa and maximum divergence happens at 400 hPa (Fig. 4d). In summary, the structure of the composite tropical strom has distinct characteristics, such as no anticyclone in higher layers, a warm core located in the middle layers, convergence concentrated in the lower troposphere, and divergence located in upper layers of the troposphere. The vortex constructed by the bogus scheme is unable to represent tropical storms in the SCS. 4.2. Severe tropical storm There also exists a false anticyclone in the upper layers of the bogus vortex for severe tropical storms (figure omitted). Vertical distribution of the u-component in both vortices (figure omitted) are similar to that of tropical storms, but the eastly winds in the north-central region exceeds 24 m s−1 and in the south-central region the zero-velocity line appears at higher layers in both the reanalysis and bogus vortex, indicating that the cyclone extends to higher levels. Relative humidity in both vortices has similar distribution in vertical section (figure omitted). 6
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Fig. 4. Vertical profiles of (a) v-component wind speed (m s−1), (b) specific humidity (10−3 kg kg−1), (c) vorticity (10−5 s−1), and (d) divergence (10−5 s−1), where the open and solid circles represent the composited tropical storm in the SCS and the bogus vortex, respectively.
In both vortices, the maximum vorticity is located in the lower layers. In the bogus vortex the region of value greater than 100 × 10−5 s−1 extends higher to 600 hPa (Fig. 5b) than that in the reanalysis vortex (Fig. 5a). In the central area the zero-vorticity curves appear beyond 100 hPa in the reanalysis vortex and near 200 hPa in the bogus vortex. Strong convergence and divergence are found below 925 hPa and near 150 hPa in the reanalysis vortex (Fig. 5a), respectively. In the bogus vortex (Fig. 5b), the convergence in the lower and middle layers is too strong and beyond 600 hPa there is apparent divergence over a wide range. The differences in temperature and specific humidity between the atmosphere in the central region and the surrounding environment are provided in Figs. 5c and 5d. In the reanalysis vortex (Fig. 5c), the maximum positive anomaly of temperature exceeds 3 K located at 700 hPa and between 400 and 300 hPa. The maximum positive anomalies of temperature and specific humidity in the bogus vortex exceed 5 K and 4 × 10−3 kg kg−1, respectively. The warm core is located between 300 and 200 hPa and the wet core is at 600 hPa. Also, there is a considerable negative anomaly of temperature (cold core) at 100 hPa (Fig. 5d). Clearly, the warm core and wet core in the bogusvortex is too strong and distributed at higher levels. Vertical profiles of v-component wind speed, humidity, vorticity and divergence are given in Fig. 6. There exists more apparent vertical shear in the reanalysis vortex (Fig. 6a). Profiles of u-component wind speed (figure omitted) and humidity (Fig. 6b) are similar to those of the tropical storm, but the differences in the middle and lower layers between the reanalysis and bogus vortex become smaller. The maximum vorticity appears at 925 hPa (Fig. 6c). At 150 hPa, there is negative vorticity in the bogus vortex and positive vorticity in the reanalysis vortex respectively, indicating different circulation in the two vortices. Regarding divergence (Fig. 6d), differences between the two vortices are similar to that of tropical storms. 4.3. Typhoon In the reanalysis vortex, the maximum negative and positive speed of u-component (Fig. 7a) exceed −30 m s−1 and 20 m s−1, respectively. The range of high speeds (e.g., −20 m s−1 or 10 m s−1) exceeds the distance of 350 km from the center. In the bogus 7
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Fig. 5. As in Fig. 3, but for severe tropical storms.
vortex, high speeds (e.g., −20 m s−1 or 10 m s−1) are distributed within a distance of 200 km from the center, and the zero-velocity curve appears at a lower level (i.e., 200 hPa) (Fig. 7b). The distribution of relative humidity in both vortices is similar (figure omitted), in the central region from 1000 to 150 hPa the relative humidity exceeds 90 %. In the reanalysis vortex, the maximum vorticity and the gradient of vorticity is larger than that in the bogus vortex, and the zerovorticity curves appear at 100 hPa (Fig. 8a), higher than that in the bogus vortex (Fig. 10b). Furthermore, in the reanalysis vortex, strong convergence mainly occurs below 925 hPa and the maximum divergence is mainly distributed between 200 and 100 hPa (Fig. 8a, shaded). However, strong convergence in the bogus vortex extends to 850 hPa and the maximum divergence is located at 400 hPa (Fig. 8b, shaded). The atmosphere in the central region of both vortices is significantly warmer and wetter than the ambient atmosphere, as obvious positive anomalies of temperature and humidity are found in the vertical cross sections (Fig. 8c and d). Nevertheless, the maximum anomaly of temperature in the bogus vortex exceeds 7 K, while that of the reanalysis vortex is below 6 K. The warm core in the bogus vortex is much stronger and it is located a bit higher than that in the reanalysis vortex. In the middle layers (i.e., from 700 to 500 hPa) the anomaly of specific humidity in the bogus vortex is larger than that in the reanalysis vortex. It is obviously that the warm core constructed by the bogus scheme in higher layers is too strong while the vorticity and the range of high speeds are small. The vertical changes of speed, humidity, vorticity and divergence in both vortices (figure omitted) are similar to that of tropical storms, but the vorticity of the reanalysis vortex is considerably larger than that in the bogus vortex, and the v-component speed in the middle layers becomes larger. Making a comparison between the three types of TCs (reanalysis vortices) in the SCS, there are some similarities as well as significant differences in their structures. The area with maximum wind speed exists in the north of the center, and is fan-shaped around the center. Easterly winds prevail beyond 100 hPa. The strongest convergence appears at 1000 hPa, strong divergence is located from 150 to 100 hPa, and the maximum vorticity appears at 925 hPa. Obvious warm core and wet core are distributed in the 8
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Fig. 6. As in Figs. 4, but for severe tropical storms.
Fig. 7. As in Figs. 2a and 2b, but for typhoons.
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Fig. 8. As in Fig. 3, but for typhoons.
central region, but the location and the intensity are different. The warm core in tropical storms, severe tropical storms, and typhoons is located from 600 to 400 hPa, 400 to 300 hPa, and 400 to 250 hPa, respectively. As the TC intensity increases, the intensity of the warm core and wet core increases and the range becomes wider.
4.4. Analysis on structure errors and forecast errors of tropical storms, severe tropical storms and typhoons In this subsection, structure errors and forecasting errors of the three types of TCs in the SCS are analyzed to investigate whether the larger structure errors lead to larger forecasting errors? From the comparison above, it is clear that the major errors of the bogus vortex are the false anticyclone in the higher layers, the too intense and higher-located warm core and the inaccurate vertical distribution of divergence. As shown in Table 2, the anticyclone in the bogus vorex of tropical storms, sever tropical storms and typhoons appears at 250, 200 and 150 hPa respectively, and it maintains till 70 hPa. In the bogus vortex of tropical storms, the size of the anticyclone is the largest both in horizontal (figure omitted) and vertical direction, and the intensity is the strongest. In the bogus vortices of severe tropical storms and typhoons, the size and the intensity of the anticyclone are comparable. Significant differences of temperature deviation from the environment between the reanalysis and bogus vortex are distributed in the middle and higher layers (i.e., 600−100 hPa, Fig. 9). The differences of temperature deviation in higher troposphere (i.e., 10
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Table 2 The geopotential height in the reanalysis and bogus vortex of tropical storms (TS), severe tropical storms (STS), and typhoons (TY). The characters “H” and “L” represent anticyclone and cyclone system, respectively. Level(hPa)
300 250 200 150 100 70 50
TS
STS
TY
Reanalysis
Bogus
Reanalysis
Bogus
Reanalysis
Bogus
L L – – – – –
– H H H H H –
L L L – – – –
L L H H H H –
L L L L – – –
L L – H H H –
Fig. 9. Differences of temperature between the TC center and the environment in the vortices of (a) tropical storms, (b) severe tropical storms and (c) typhoons, where the open and solid circles represent the reanalysis vortex and the bogus vortex, respectively.
300−200 hPa) in tropical storms are the largest (Fig. 9a) and there exist big errors in 100 hPa. The errors between 300 and 200 hPa of severe tropical storms and typhoons are comparable, while in 100 hPa the errors of severe tropical storms are larger than that of typhoons (Fig. 9b and c). In the reanalysis vortices, the location of the warm core in tropical storms is lower than that of severe tropical storms and typhoons (as shown in Figs. 3c, 5 c 8 c and 9), which is consistent with previous studies on WNP TCs (Zhang et al., 2007; Durden, 2013; Gao et al., 2017). Since the warm cores in the bogus vortices are all located between 300 and 200 hPa, the bogus vortex of tropical storms also has the largest deviation in the warm core location. The differences of the divergence between the reanalysis and bogus vortex in tropical storms (Fig. 3a and b) are more significant than those in severe tropical storms (Fig. 5a and b) and typhoons (Fig. 8a and b), as the convergence in the bogus vortex of tropical storms below 925 hPa is much stronger. In summary, the vortex of tropical storms constructed by the bogus scheme has the largest errors in structure. Forecasting tracks errors of tropical storms, severe tropical storms and typhoons using the bogus scheme are shown in Fig. 10a. The track errors of tropical storms are generally the largest, corresponding to the largest structure errors. And the track errors of typhoons are the smallest. It can be deduced that the deviation in structure of TC vortex has great impact on track forecasts. Fig. 10b shows the forecasting intensity errors of the three types of TCs. It is clear that within 48-h forecast, typhoons have the largest intensity errors, most errors result from underestimate. Since sea surface temperature used in TRAMS is climate average, it probably has more difficulty in energy supporting for intensification of a stronger TC. Ocean basin may have a great impact on intensity changes, so that the large errors in the initial structure of tropical storms do not lead to large errors in intensity forecast. 4.5. How to adjust the bogus scheme Based on the analysis above, the bogus scheme will be adjusted according to the errors in the structure of TC vortex. The intensity of the warm core needs to be weakened. The false anticyclone in the higher layers needs to be removed. Profiles of temperature deviation from the environment can be introduced to the scheme to make the location and the intensity of the warm core more 11
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Fig. 10. Forecasting (a) track errors and (b) intensity errors of tropical storms (blue and solid circle), severe tropical storms (red and open circle) and typhoons (green and open square).
realistic. Since there is nearly non-divergence between 850 and 250 hPa (Figs. 4d and 6 d), the convergence can be restricted below 850 hPa and the divergence can be restricted beyond 200 hPa.
5. Conclusion In this study, samples of tropical storms, severe tropical storms, and typhoons in the SCS were selected based on ECMWF highresolution reanalysis data for composite analysis. Their structures were investigated and compared with the bogus vortices. Structure errors and forecasting errors of the three types of TCs are compared and analyzed. And the issue of how to adjust the bogus scheme is discussed. The following conclusions can be drawn: (1) The vortices constructed by the bogussing scheme are not consistent with the ‘observed’ structures. For instance, they show a strong anticyclone in higher layers and strong divergence in middle layers (i.e., 400 hPa). The warm core is stronger and locates higher. (2) Tropical storms, severe tropical storms, and typhoons in the SCS share some structural features in common. They have obvious warm core structures. The area with high wind speeds is fan-shaped in the north around the TC center, the strongest convergence appears at 1000 hPa, strong divergence locates from 150 to 100 hPa, and the maximum vorticity appears at 925 hPa. (3) Nevertheless, there are significant differences in thermodynamic structures between them. For instance, the warm core in tropical storms, severe tropical storms, and typhoons is located from 600 to 400 hPa, 400 to 300 hPa, and 400 to 250 hPa, respectively. As TC intensity increases, the warm air in the central region becomes wider and wetter. (4) The bogus vortex of tropical storms has the largest errors in structure among the three types of TCs. Correspondingly, it suffers the largest forecasting track errors. However, typhoons have relatively larger errors in the forecast of intensity. Ocean may play a more important role in the development of intensity than TCs’ initial structure. In future, the bogussing scheme will be adjusted based on this study. The composited vortices of tropical storms, severe tropical storms, and typhoons obtained can also be used to construct the initial vortex. Numerical experiments will be designed and demonstrated based on the two schemes provided above to verify the validity of the schemes in improving forecasts of TCs in the SCS.
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CRediT authorship contribution statement Yanyan Huang: Conceptualization, Methodology, Writing - original draft. Bin Zheng: Investigation, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Key Projects of China (Grant 2018YFC1506902), the National Natural Science Foundation of China (Grants 41705089 and 41675099), and the Science and Technology Program of Guangdong Province (Grants 2017B030314140 and 2017A020219005). The authors thank Prof. Jishan Xue and Dr. Yerong Feng for offering insightful assistance during the early stage of this research. References Abarca, S.F., Corbosiero, R.L., 2011. The world wide lightning location network and convective activity in tropical cyclones. Mon. Weather Rev. 139, 175–191. Annamalai, H., Slingoj, M., Sperberk, et al., 1999. 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