Multi-Factor Intensity Estimation Method of Tropical Cyclone in the Western North Pacific with the Deviation Angle Variance Technique

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  • In this paper, the standardized infrared cloud images from Fengyun series geostationary satellites and Best Track Data from China Meteorological Administration (CMA-BST) within 2015–2017 are used to investigate the effects of two multi-factor models, generalized linear model (GLM) and long short-term memory (LSTM) model, for the tropical cyclone (TC) intensity estimation. The typical single-factor Sigmoid function model (SFM) with the map minimum value of deviation angle variance (DAV) is also reproduced for comparison. Through applying the sensitivity experiments to the DAV calculation radius and different training data groups, the estimation precision and optimum calculation radius for DAV in the western North Pacific (WNP) are analyzed. The results show that the root mean square error (RMSE) of the single-factor SFM is between 8.79 and 13.91 m s−1 by using the individual years as test sets and the remaining two years as training sets with the optimum calculation radius of 550 km. However, after selecting and using the high-correlation factors by GLM, the RMSE of GLM and LSTM models decreases to 5.93–8.68 and 4.99–7.00 m s−1 respectively with their own optimum calculation radius of 350 and 400 km. All sensitivity experiments indicate that the estimation results of SFM can be significantly influenced by the DAV calculation radius and characteristics of the training set data, while the results of multi-factor models appear more stable. Furthermore, the multi-factor models reduce the optimum radius within the process of DAV calculation and improve the precision of TC intensity estimation in the WNP, which can be an effective way for TC intensity estimation in marine area.
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  • Fig. 1.  The schematic of the deviation angle calculation process with the (a) cloud image (shaded; K) of Meranti (1614) and (b) corresponding deviation angle variance (DAV) map (shaded; deg2) at 0600 UTC 13 September 2016. Or is the reference point marked by the blue tringle. The blue circle denotes the calculation area within a given radius, taking 350 km as an example here. Two blue lines indicate the radial and Block-body Brightness Temperature (TBB) gradient direction of the given point A, and $ \theta $ is their deviation angle. The blue square and red dot denote the location of the map minimum value (MMV) and recorded tropical cyclone (TC) center of Meranti. (c) The main region infrared cloud image (shaded; K) and (d) distribution of DAV (shaded; deg2) at the same time. The blue circles are centered on the position of each recording TC from the Best Track Data from China Meteorological Administration (CMA-BST) at that time with the radius of 350 km. The TC name and its intensity level are marked in red above each blue circle.

    Fig. 2.  (a) The Sigmoid-based fitting curve of the FMMV (deg2) obtained from different calculation radii in the G-all test with the maximum wind speed ($ {V}_{\max} $; m s−1) in the CMA-BST dataset. The scattered points refer to the median of FMMV values corresponding to different $ {V}_{\max} $; the solid line is the fitting curve based on the Sigmoid function; and the seven colors represent the results with seven different radii for DAV calculation varying from 250 to 550 km at an interval of 50 km, respectively. (b) The distribution of the RMSE (labeled with the straight line; m s−1) of the test results calculated at different radii in the four groups of experiments. Different colors refer to different experiments; G-2015, G-2016, and G-2017 represent the RMSE of the test groups, respectively; and G-all represents the fitting RMSE of the training set.

    Fig. 3.  (a) Average variance contribution rates of different radii in the all-factor model with G-all as the validation data. The dark bar represents the variance contribution of the single factor, and the light bar represents the cumulative variance contribution of all pairwise interactions involving this factor. (b) The RMSE change rate of the all-factor model after the elimination of certain factor. (c) The RMSE change with the increasing number of factors in the model at different calculation radii. Bars and lines with different colors represent the results with different calculation radii.

    Fig. 4.  The schematic diagram of the structure of (a) LSTM cell and (b) 2-D LSTM chains. $ {f}_{t} $, $ {i}_{t} $, and $ {o}_{t} $ denotes the forget, input, and output gates, respectively. $ \sigma $ and tanh are activate functions, in which $ \sigma $ represents the Sigmoid function while tanh represents the hyperbolic tangent function. $ {x}_{t} $ is the input value at the current time. $ {C}_{t} $ and $ {h}_{t} $ are the state of the cell and hidden layer at the current moment, respectively.

    Fig. 5.  Boxplots for biases of the estimation results in the (a) single-factor SFM, (b) multi-factor GLM, and (c) LSTM model, from the CMA-BST dataset at different stages of the TC. “A”, “B”, “C”, and “D” in the four boxplots of different colors at each stage of the TC correspond to the G-2015, G-2016, G-2107, and G-all tests, respectively.

    Fig. 6.  Comparison of the TC intensity among CMA-BST records (black heavy line), SFM estimations (green heavy line), GLM estimations (blue heavy line), LSTM estimations (purple heavy line), and trends of four high-correlation factors after normalization (light lines with different colors as shown in the figure legend). Three cases [Dujuan (1521), Meranti (1614), and Talim (1718)], are taken as examples, whose life cycles are divided by red dashed lines.

    Table 1.  Groups of different test and training sets

    GroupTest setSize (h)Training setSize (h)
    A: G-2015201540222016, 20174992
    B: G-2016201629412015, 20176073
    C: G-2017201720512015, 20166963
    D: G-all2015, 2016, 201790142015, 2016, 20179014
    Download: Download as CSV

    Table 2.  Pre-selected factors for the TC intensity estimation

    NameDescription
    FMMVFiltered MMV
    MDAVAveraged DAV around the MMV location with the radius of 200 km
    RDThe distance between the MMV location and circulation left
    TBBmMinimum Block-body Brightness Temperature within the radius of 200 km.
    S-20The cloud area lower than −20° within the radius of 50–200 km.
    TBBstdThe standard deviation within the radius of 100–300 km.
    LatLatitude of circulation left*
    LonLongitude of circulation left*
    * The circulation left is defined by the local vorticity maximum at 850 hPa calculated with ECMWF reanalysis (ERA)-Interim.
    Download: Download as CSV

    Table 3.  RMSEs of the multi-factor GLM and LSTM model estimation results with different calculation radii

    Radius (km)G-2015 RMSE (m s−1)G-2016 RMSE (m s−1)G-2017 RMSE (m s−1)G-all RMSE (m s−1)
    GLMLSTM GLMLSTM GLMLSTM GLMLSTM
    2508.687.587.447.566.835.947.356.61
    3008.497.157.417.406.335.547.246.41
    3508.327.017.467.375.985.127.216.30
    4008.277.007.577.505. 934.997.236.22
    4508.267.257.747.495.974.697.276.33
    5008.307.307.917.755.974.807.356.38
    5508.417.288.177.935.965.157.486.60
    Download: Download as CSV

    Table 4.  Comparison of the model performance with the exiting satellite-based TC intensity estimation approaches

    ModelApproachCovered durationData sourceInputFitting test* RMSE (m s−1)
    Ritchie et al. (2014)SFM2005–2011GOES-seriesIR (10.7 μm)7.36
    Combinido et al. (2018)CNN1996–2014GMS-5,
    GOES-9,
    MTSAT-1R,
    MTSAT-2,
    Himawari-8
    IR (11.0 μm)6.80
    Zhang et al. (2016)RVM2005–2014FY-2CIR (10.3 μm)6.58
    Lee et al. (2020)CNN2011–2016COMSVIS (0.67 μm),
    VIS (3.7 μm),
    VIS (6.7 μm),
    IR (10.8 μm),
    IR (12.0 μm)
    4.27
    In This paperSFM
    (500 km)
    2015–2017FY-2F/2GIR (10.7 μm)12.00
    GLM
    (350 km)
    7.21
    LSTM
    (400 km)
    6.22
    * The unit of fitting test RMSE from the referenced literature has been uniformed as m s−1. Geostationary Operational Environmental Satellite—GOES; Geostationary Meteorological Satellite—GMS; Multi-functional Transport Satellite—MTSAT; Communication, Ocean, and Meteorological SatelliteCOMS; infrared radiation—IR; visible band—VIS.
    Download: Download as CSV
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    [5] Meng YUAN, Wei ZHONG. Detecting Intensity Evolution of the Western North Pacific Super Typhoons in 2016 Using the Deviation Angle Variance Technique with FY DataJournal of Meteorological Research,  doi: 10.1007/s13351-019-8064-7.
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Multi-Factor Intensity Estimation Method of Tropical Cyclone in the Western North Pacific with the Deviation Angle Variance Technique

    Corresponding author: Meng YUAN, yuanm2012@foxmail.com
  • 1. College of Meteorology and Oceanology, National University of Defense Technology, Nanjing 211101
  • 2. Submarine College of Navy, Qingdao 266199
Funds: Supported by the National Key Research and Development Project of China (2018YFC1507402) and National Natural Science Foundation of China (42075011)

Abstract: In this paper, the standardized infrared cloud images from Fengyun series geostationary satellites and Best Track Data from China Meteorological Administration (CMA-BST) within 2015–2017 are used to investigate the effects of two multi-factor models, generalized linear model (GLM) and long short-term memory (LSTM) model, for the tropical cyclone (TC) intensity estimation. The typical single-factor Sigmoid function model (SFM) with the map minimum value of deviation angle variance (DAV) is also reproduced for comparison. Through applying the sensitivity experiments to the DAV calculation radius and different training data groups, the estimation precision and optimum calculation radius for DAV in the western North Pacific (WNP) are analyzed. The results show that the root mean square error (RMSE) of the single-factor SFM is between 8.79 and 13.91 m s−1 by using the individual years as test sets and the remaining two years as training sets with the optimum calculation radius of 550 km. However, after selecting and using the high-correlation factors by GLM, the RMSE of GLM and LSTM models decreases to 5.93–8.68 and 4.99–7.00 m s−1 respectively with their own optimum calculation radius of 350 and 400 km. All sensitivity experiments indicate that the estimation results of SFM can be significantly influenced by the DAV calculation radius and characteristics of the training set data, while the results of multi-factor models appear more stable. Furthermore, the multi-factor models reduce the optimum radius within the process of DAV calculation and improve the precision of TC intensity estimation in the WNP, which can be an effective way for TC intensity estimation in marine area.

1.   Introduction
  • Tropical cyclone (TC) is one of the most catastrophic weather systems all over the world (Zhang and Guo, 2008). As we know, the majority of meteorological observations are on the land whereas the genesis and development of TCs mostly occur over the ocean, which means that we are unable to directly acquire the information regarding the TC structure, intensity, trajectory, and associated variation due to the shortage of observations and limits of data extraction techniques. Therefore, the detection and forecast of TCs have always been the most difficult priorities in predicting and warning hazardous weather. On the basis of current methods to acquire as much as accurate data about TCs, to strength the ability of monitoring TCs is the most essential way to improve TC forecasts.

    Over the vast ocean, the most effective method to track and analyze TC life cycles relies on the visible and infrared cloud images (Piñeros et al., 2010). At present, the most widely utilized way to analyze TCs with satellite cloud images is the Dvorak technique proposed by Dvorak (1975) in the 1970s. In comparison with the TC eye and its surrounded clouds with a series of standard models of TCs, Dvorak technique can evaluate the TC intensity by a comprehensive analysis of cloud circulations and distribution of brightness temperature. On this basis, the Advanced Objective Dvorak Technique (AODT) was introduced by Olander and Velden (2007) that integrated subjective analysis statistics and regression equations to predict the TC intensity and also achieved self-located TC centers. Dvorak technique has been the prevalent analysis method of TC study because of good performance in practical application. Although AODT lays more emphasis on the objectivity in the analysis process, its cloud classification still relies on the subjective analysis so that users are required to consecutively analyze cloud images. Thus, the prediction results are seriously dependent on users’ experience (Piñeros et al., 2010).

    Based on the infrared radiation (IR) cloud images to accomplish real-time TC monitoring, Piñeros et al. (2008) raised an objective analysis method—Deviation Angle Variance Technique (DAV-T)—that can differentiate the TC intensity through the morphology and dynamic characters of clouds. As TCs with different intensities present significant differences in their organization processes, DAV-T will first quantify the TC axisymmetry and then establish the relationship of the obtained characteristic values and TC intensity via the Sigmoid function model (SFM) to implement the real-time monitoring of the TC structure and intensity. Note that the calculation radius in DAV-T that needs to acclimatize TC scales have obvious impacts on the distributions of DAV values in clouds. Considering that the large-scale environment and TC formation vary in different genesis regions, it is necessary to conduct sensitivity experiments on the calculation radius in corresponding regions (Briegel and Frank, 1997; Ritchie and Holland, 1999; Hill and Lackmann, 2009). Then Piñeros et al. (2011) applied DAV-T to monitor TCs over the North Atlantic areas during 2004 and 2009 and employed the map minimum value (MMV) to represent the TC axisymmetric degree. Results indicated that: (1) the root-mean-square error (RMSE) of the TC intensity from predictions and national hurricane center (NHC) records was 24.8 kt with 2009 best tracks as test sets; (2) when the radius was calculated every 50 km within the range of 150–500 km to select the optimum parameter for the North Atlantic, the error of DAV-T was relatively small with the calculation radius between 300 and 400 km. Ritchie et al. (2014) took the DAV of the TC circulation center (E-DAV) as the proper value to represent the TC axisymmetric degree, and found that: (1) for the northwestern Pacific region from 2007 to 2011, RMSEs of the TC intensity from predictions and Join Typhoon Warning Center (JTWC) records were between 12.9 and 15.1 kt with any one year as the test set and the rest as the training set; (2) for the northeastern Pacific region from 2007 to 2011, RMSEs of the TC intensity from predictions and JTWC records were between 9.4 and 16.9 kt with any one year as the test set and the rest as the training set; (3) the error of DAV-T was the minimum with a calculation radius of 300 km and the error reduced further if the radii of 250 and 500 km were taken.

    Using the above DAV-T to determine the TC intensity actually presented obvious flaws since predictive factors only considered the single parameter of cloud DAV. When E-DAV was adopted as the predictor for the TC intensity, the performance was much improved, but the location of the TC eye exerted a great influence on the model and its employment. E-DAV alone failed to reveal the whole TC structure because this value only represented the axisymmetric degree around the TC eye. However, MMV can indicate the axisymmetric degree of deep convective clouds within the whole TC system and will not be affected by the TC location (Yuan and Zhong, 2019).

    The objective methods of TC intensity estimation have made a lot of progress in recent years. Firstly, the factors were expanded to describe the TC intensity characteristics more completely. Zhang et al. (2016, 2019) extracted the deviation angle gradient co-occurrence matrix (DAGCOM) with the IR image. Then the Relevance Vector Machine (RVM) was used for building the estimation model that performs much better than the traditional linear regression model. In addition, they also investigate the optimal calculation radius of factors and optimal numbers of factors to the model. On the other hand, the intelligent estimation models such as machine learning has significantly improved the accuracy of results. Combinido et al. (2018) used 11.0 μm-channel infrared satellite data as the input by constructing a 2-D Convolutional Neural Network (CNN) to estimate the TC intensity in the western North Pacific (WNP). Moreover, the research by Lee et al. (2020) proved that expanding the input data to the multi-channel infrared is more effective than the single-channel method. Until now, most of the TC intensity estimation methods using deep learning are based on CNN model, which can capture the spatial features from an image and be helpful to identify the object accurately, as well as its relation with other objects in an image. However, although the value of intensity is closely related to the spatial feature of the TC structure from the same time, each TC case still has its own time evolution influenced by the environmental atmosphere, external forces, internal dynamic process, and so on. So the long short-term memory (LSTM) model is introduced in this paper to trace the time evolution of the TC intensity. LSTM model is a modified version of Recurrent Neural Network (RNN), which makes up for problems of the gradient disappearance and gradient explosion of RNN, insufficient long-term memory capacity, and so on, and makes the RNN be used effectively in the long-range timing information (Graves, 2012).

    Based on the DAV-T and FY series satellite data, this paper discusses the TC intensity estimation techniques over the WNP Ocean. Section 2 presents the data and methodology. Multi-factor models, generalized linear model (GLM), and LSTM model, are introduced and compared with SFM in Section 3, which also includes the sensitivity experiments on the calculation radius. Section 4 compares the performance of the single-factor and multi-factor models with the optimum calculation radius. The final section is the conclusion and prospect for future work.

2.   Data and methods
  • The satellite data we utilized in this paper originates from the long-wave (10.7 μm) IR image that are generated by Fengyun (FY) series geostationary satellites of China from 2015 to 2017. Influenced by satellite debugging or regional encryption observation, the single geostationary satellite may not supply the information in some periods. In addition, in order to cover the most area of the WNP Ocean, the observations from satellite FY-2F are mainly used here, while images from satellite FY-2G are supplemented to expand the available satellite data. Considering the different coverages of FY-2F and FY-2G, the spatial area of available satellite data is defined to be 0°–40°N, 100°–160°E. The spatial resolution is 0.05° per pixel (approximately 5 km per pixel), and the time resolution is 1 h. Piñeros et al. (2011) find out that reducing the spatial resolution of the research data will not have a significant impact on the calculation results, but reduce the calculating time. Therefore, all available satellite data used here are reshaped with the spatial resolution of 0.1° per pixel (approximately 10 km per pixel) to improve the computing efficiency.

    The Best Track Data from China Meteorological Administration (CMA-BST: http://tcdata.typhoon.org.cn/zjljsjj_zlhq.html) with the spatial resolution of 6 h is used in this paper to analyze the position, intensity, and division of different life history stages of typhoons (TYs). According to the National Standard for TC Class (GB/T 19201-2006), TC can be divided into eight classes: Weaker than Tropical Depression (TD), TD, Tropical Storm (TS), Severe Tropical Storm (STS), TY, Severe TY (STY), Super TY (SuperTY), and Extratropical Cyclone (ET). In order to make full use of the satellite data with the high-time resolution, the cubic spline function is used to interpolate CMA-BST data to coordinate the satellite data in timescales. Furthermore, it should be noted that, affected by the noise and quick-timescale disturbance, physical variables calculated from the satellite data will appear high-frequency fluctuation, especially in the complex formation process of TCs (Piñeros et al., 2008). Therefore, Chebyshev Filter with the 0.1π-cutoff frequency (low pass) is used to smooth the calculating variables from satellite data as in Piñeros et al. (2011) in order to get a better match with CMA-BST.

  • The organization degree of TC cloud structures varies significantly at different development stages. Observational studies based on satellite data have shown that the TC genesis with the concentration and closure of scattered deep convective cloud clusters develop with the axisymmetric process of the organized annular deep convection belts around the eye, and weaken with the dissipation of the organized convective cloud belts over the land or cold ocean surface (Hubert and Timchalk, 1969; Dvorak, 1975). As a result, the symmetry of convective clouds relative to the center of circulation can be regarded as a significant parameter to estimate the TC intensity. So, an ideal axisymmetric TC system can be supposed as an annulus deep convection belt encircling a cloud-free eye, which means that the corresponding brightness temperature in the satellite image drops off with the radius from the eyewall to periphery. In other words, the direction of all brightness temperature gradient vectors is pointing toward or away from the eye. Hence, the symmetry degree of any vortex system can be quantified through its deviation to the ideal TC system, which is the basic theory of DAV-T. The specific process of DAV-T can be found in Piñeros et al. (2008).

    Specifically, the process of DAV-T is as follows. Firstly, calculating the gradient direction of brightness temperature at each pixel on the IR-image. Secondly, choosing any pixel at the IR image as the reference point Or, and obtaining the deviation angle θi between the gradient and radial directions of all pixels within the calculating radius Rv. Then, the variance of all the above deviation angles is defined to be the DAV value of this reference point (as shown in Fig. 1a). Finally, after taking all pixels in the satellite image as the reference point in turn, the DAV values at every pixel constitute the DAV map (Fig. 1b).

    Figure 1.  The schematic of the deviation angle calculation process with the (a) cloud image (shaded; K) of Meranti (1614) and (b) corresponding deviation angle variance (DAV) map (shaded; deg2) at 0600 UTC 13 September 2016. Or is the reference point marked by the blue tringle. The blue circle denotes the calculation area within a given radius, taking 350 km as an example here. Two blue lines indicate the radial and Block-body Brightness Temperature (TBB) gradient direction of the given point A, and $ \theta $ is their deviation angle. The blue square and red dot denote the location of the map minimum value (MMV) and recorded tropical cyclone (TC) center of Meranti. (c) The main region infrared cloud image (shaded; K) and (d) distribution of DAV (shaded; deg2) at the same time. The blue circles are centered on the position of each recording TC from the Best Track Data from China Meteorological Administration (CMA-BST) at that time with the radius of 350 km. The TC name and its intensity level are marked in red above each blue circle.

    Following the above procedures, the IR image at 0600 UTC 13 September 2016 and the corresponding DAV map can be seen in Figs. 1c, d. There are three organized deep convective cloud clusters with distinct vortex properties respectively in the northern Indochina Peninsula, Taiwan Island southeast ocean, and west of Marshall Islands, respectively. These cloud clusters correspond to the three TCs existing in the WNP Ocean at that time: the landed TY Rai (1615) at the stage of TS, SuperTY Meranti (1614) moving to the southeast coast of China, and TY Malakas (1616) that had just reached TS. In addition, there are also large area of exuberant deep convections in the South China Sea, Philippine peninsula, and central Pacific. Convections in the central Pacific (0°–16°N, 150°–165°E) appears to concentrate as cloud clusters, while the structure of deep convections in other places is relatively loose.

    According to the relationship between the DAV and axisymmetric deep convective cloud cluster, the higher the organization degree of the cloud cluster, the better the symmetry, which means the relatively lower DAV value; on the contrary, the more scattered the cloud, the higher the DAV value (Piñeros et al., 2008). Wood et al. (2015) used 2000 deg2 as the maximum value of the organized cloud clusters based on on Geostationary Operational Environmental Satellite (GOES) series geostationary satellite. However, considering the characteristics of observation information from FY series satellites, we set the threshold value to be 2200 deg2, which can capture all TC records in CMA-BST even when they are at the stage of weaker than TD (Yuan and Zhong, 2019.

    The calculation results of DAV map (Fig. 1d) show that there are obvious DAV lows within the blue circles, which are centered by positions of the three recorded TCs from CMA-BST with a radius of 350 km. The DAV low relating to the SuperTY stage of Meranti (1614) appears to be a circular shape, and its MMV is located at the TC center with the minimum value of 1231 deg2. However, when comes to the DAV distribution of the TS stage of TY Rai (1615) and Malakas (1616) at the same time, although the shape of the two DAV lows still presents a relatively complete circular structure, the positions of MMV deviate to the TC center, which depends on the location of axisymmetric deep convections in the TC system. Furthermore, the MMV value of Malakas (1616) and Rai (1615) at the TS stage is 1598 and 1783 deg2, respectively.

    Meanwhile, the results of DAV map also indicate that the deep convections in the South China Sea and Philippines Peninsula belong to the unorganized convections. However, there are sporadic DAV lows within the large aggregation cloud clusters in the central Pacific, which means that the clusters here may have the possibility to spin up. Hence, through the DAV-T method, we can detect TC by the IR image and quantify its axisymmetric degree, which can provide a valid basis for TC identification and intensity estimation at sea.

    In this paper, 83 TC cases that occurred in the WNP from 2015 to 2017 are studied to obtain the TC intensity estimation model, including 9014 hourly satellite IR image samples. Four groups (A to D; Table 1) are designed to check the model efficiency. Groups A, B, and C are designed to test the intensity estimation with CMA-BST records in an individual year from 2015 to 2017 and to train the model with the remaining 2-yr samples. Group D is used to obtain the fitting results with all the 3-yr samples.

    GroupTest setSize (h)Training setSize (h)
    A: G-2015201540222016, 20174992
    B: G-2016201629412015, 20176073
    C: G-2017201720512015, 20166963
    D: G-all2015, 2016, 201790142015, 2016, 20179014

    Table 1.  Groups of different test and training sets

3.   Models and results
  • Earlier studies have shown that the corresponding cloud cluster becomes more symmetric when the TC intensity is stronger. Therefore, both the DAV value at the TC center and MMV in the regional system correspond significantly well to the maximum wind speed ($ {V}_{\max} $) of the TC (Piñeros et al., 2011). Considering the uncertainties in TC positioning in the operational detection, in this paper, the MMV is used to construct the statistical estimation model between the DAV and $ {V}_{\max} $ of the TC.

    As the Sigmoid function not only coincides with the variation characteristics of the MMV sequence, but also converges at the upper and lower boundaries of $ {V}_{\max} $, the estimation deviating much from the actual intensity can be avoided, which makes it a general model for TC intensity estimation (Piñeros et al., 2011; Ritchie et al., 2014). The SFM is defined as below.

    where $ {\tilde V}_{\max} $ is the fitted value of $ {V}_{\max} $ obtained from the model; $ {I}_{\max} $ and $ {I}_{\min} $ are corresponding to the upper and lower limits of $ {V}_{\max} $ in the best track dataset, respectively. To make the fitting results of the model cover all TC intensities, $ {I}_{\max} $ and $ {I}_{\min} $ are taken as 80 and 5 m s−1, respectively. $ x $ is the value of MMV; $ \alpha $ and $ \beta $ are two model parameters obtained by fitting samples. Since the time resolution of satellite IR image data is higher than that of the CMA-BST dataset, the high-frequency filtered MMV (FMMV) sequence, obtained by filtering the MMV sequence with Chebyshev Filter referred in Section 2.1, is used here, and $ {V}_{\max} $ of the TC system in the CMA-BST dataset is interpolated. Yet the $ {V}_{\max} $ and DAV values are not in the one-to-one correspondence, so it is difficult to build a stable model for intensity estimation. Therefore, to solve this problem, the median of MMV values corresponding to each $ {V}_{\max} $ in the CMA-BST dataset is taken as the fitting data, which could avoid the problem of low intensity during the fitting process caused by different sizes of samples with various intensity levels (Piñeros et al., 2011).

    Studies show that, the calculation results of DAV and application of DAV-T in the objective estimation of the TC intensity are greatly affected by the radius of calculation. For instance, during most of the years in the North Atlantic, the optimum calculation radius for DAV-T is within 200–250 km, while it is within 350–400 km in the WNP (Ritchie et al., 2014). This is attributed to the significant differences in TC scales caused by different oceanic environmental fields and development characteristics of TCs at different stages. Thus, if the selected radius differs greatly from the actual TC scale, it would lead to deviations in the magnitude and value of the DAV map, further affecting the accuracy of intensity estimation models.

    To compare the influence of different calculation radii on the model construction, the DAV is calculated with the each-time IR image data corresponding to all cases in three years. The calculation is conducted for seven different radii varying from 250 to 550 km at the interval of 50 km. The Sigmoid fitting curve of the TC $ {V}_{\max} $ obtained from the FMMV based on different radii for calculation is provided in Fig. 2a. Overall, the Sigmoid function can well reflect the correspondence of the TC $ {V}_{\max} $ in the effective intensity range to the FMMV. When the TC intensity is weaker than TD or stronger than STY, the smaller (absolute) slope of the Sigmoid function curve means that within the same TC intensity variation interval, the corresponding DAV difference is more obvious. In other words, this model is significantly better than other periods in the case of very weak (weaker than TD) or very strong (stronger than STY) TCs.

    Figure 2.  (a) The Sigmoid-based fitting curve of the FMMV (deg2) obtained from different calculation radii in the G-all test with the maximum wind speed ($ {V}_{\max} $; m s−1) in the CMA-BST dataset. The scattered points refer to the median of FMMV values corresponding to different $ {V}_{\max} $; the solid line is the fitting curve based on the Sigmoid function; and the seven colors represent the results with seven different radii for DAV calculation varying from 250 to 550 km at an interval of 50 km, respectively. (b) The distribution of the RMSE (labeled with the straight line; m s−1) of the test results calculated at different radii in the four groups of experiments. Different colors refer to different experiments; G-2015, G-2016, and G-2017 represent the RMSE of the test groups, respectively; and G-all represents the fitting RMSE of the training set.

    It can be seen from Fig. 2a that the FMMV corresponding to the equivalent $ {V}_{\max} $ tends to increase as the radius increases, indicating that the axisymmetry of the whole system reduces with the increasing calculation radius. Meanwhile, as the radius increases, the gradient of the fitting curve increases obviously when $ {V}_{\max} $ is below 60 m s−1, while it weakens significantly when the function becomes saturated. Such a distribution characteristic of the function can lead to the following results. When the TC intensity is relatively weak, the larger the calculation radius is, the larger the gradient of the fitting function will be, thereby the corresponding relation between the $ {V}_{\max} $ and FMMV becomes more accurate. While for the high-intensity TC, the larger the calculation radius is, the weaker the gradient of the function will be, and it could easily result in the obvious fitting deviation during the model training.

    To further test the application effect of the single-factor SFM on the TC intensity estimation, the test results of the model calculated at different radii are obtained according to the settings of four experiments as mentioned above (Fig. 3). G-2015, G-2016, and G-2017, represent the test data composing of the TC cases in 2015, 2016, and 2017 respectively, while TCs in the other two years are used as the training data, and the results are represented by the RMSE between $ {\tilde V}_{\max} $ from the results of the model and $ {V}_{\max} $ from the CMA-BST dataset in the test year. The result of G-all test is represented by the RMSE between the hindcast $ {\tilde V}_{\max} $ from the model based on all samples during the period of 2015–2017 and the $ {V}_{\max} $ from the CMA-BST dataset. From the test results, it can be seen that the RMSE of the single-factor SFM is within the range of 8.79–13.77 m s−1, the average effect of G-2017 performs slightly better than that of G-2016, and that of G-2015 performs the worst, and its RMSE at each radius all exceeds 13 m s−1. According to the aforementioned case information, 2015 is the high-occurrence year of SuperTYs, and 15 out of 29 cases reach the level of SuperTY throughout the year; whereas there are only 12 cases of SuperTY in total during 2016 and 2017. Thus, it can be seen that, the accuracy of the high-intensity TC estimation based on the Sigmoid function is affected by both the disappearance of the gradient at boundaries of the function and the limitation in the number of cases, which can lead to considerable errors in estimation.

    Figure 3.  (a) Average variance contribution rates of different radii in the all-factor model with G-all as the validation data. The dark bar represents the variance contribution of the single factor, and the light bar represents the cumulative variance contribution of all pairwise interactions involving this factor. (b) The RMSE change rate of the all-factor model after the elimination of certain factor. (c) The RMSE change with the increasing number of factors in the model at different calculation radii. Bars and lines with different colors represent the results with different calculation radii.

    Considering the test results obtained under different radii, the optimum radius of 450–500 km is found in G-all, G-2015, and G-2016. This optimum radius is larger than that from the tests of similar models in the Atlantic Ocean and mid-East Pacific Ocean, which is accordant with the conclusion given by Knaff et al. (2010) that the TC size in the WNP Ocean is larger than those in other sea areas, statistically. And it is interesting that, unlike the other tests, the RMSE in the G-2015 test does not decrease monotonically under different radii varying from 250 to 350 km. Its RMSE under the radius of 250 km is smaller than those under the radii of 300 and 350 km. However, the RMSE in the G-2017 test decreases with the increasing calculation radius, while the variation amplitude reduces significantly when the calculation radius is greater than 500 km. The statistical analysis on TC samples shows that in 2015, there are more SuperTYs but fewer low-intensity TC cases (only six cases) whose intensities are below the TS. However, in 2017 there are only four high-intensity cases selected for testing and the number of TC cases with the intensity below the TS reaches 10. In fact, statistical studies show that there is a certain correspondence between the TC size and intensity. Especially the size of SuperTY is the minimum compared with its size at the other stages, and the size of low-intensity TCs (below the TS intensity) is relatively large. Therefore, in the G-2015 test, the calculation radius of 250 km can better depict the scale of SuperTY. While in the G-2017 test, the larger the calculation radius is, the closer it will be to the size of the low-intensity TC, which also coincides with that the gradient of the Sigmoid function increases with the increasing calculation radius, thus it is favorable for improving the resolving accuracy of the corresponding relations between the $ {V}_{max} $ and FMMV. Thus, it indicates that the setting of the calculation radius suitable for the size of TC system facilitates the improvement in the accuracy of TC intensity estimation. Besides, the test results reveal that the errors among different calculation radii exhibit small differences while those among different test groups are relatively large, indicating that the stability of the single-factor SFM is poor and that the differences among the models are significant as the sample data change.

  • Studies have shown that besides the value of MMV, the relative distance (RD) between the location of MMV and the circulation center of the TC, as well as the distribution characteristics of the DAV value, are good indicators for the TC intensity (Yuan and Zhong, 2019). Based on the DAV-T and referring to the building process of the statistical regression model for the TC intensity (Demaria et al., 2005), the accuracy of the TC intensity determination could be improved by screening and introducing information of the infrared cloud images, TC location, and integration of the DAV and cloud image. So, eight preselected factors (Table 2) have been introduced here. These factors can be classified into three categories: (a) FMMV, MDAV, and RD are symmetrization factors from DAV map, which include the structure of organized deep clouds and their relationship with the circulation center. (b) Minimum Block-body Brightness Temperature (TBBm), S-20, and TBBstd are convection factors from IR images, which reflect the influence of deep convections as in Demaria et al. (2005). (c) Lat and Lon are location factors (Table 2), which indicate the effect of location on the TC intensity.

    NameDescription
    FMMVFiltered MMV
    MDAVAveraged DAV around the MMV location with the radius of 200 km
    RDThe distance between the MMV location and circulation left
    TBBmMinimum Block-body Brightness Temperature within the radius of 200 km.
    S-20The cloud area lower than −20° within the radius of 50–200 km.
    TBBstdThe standard deviation within the radius of 100–300 km.
    LatLatitude of circulation left*
    LonLongitude of circulation left*
    * The circulation left is defined by the local vorticity maximum at 850 hPa calculated with ECMWF reanalysis (ERA)-Interim.

    Table 2.  Pre-selected factors for the TC intensity estimation

  • GLM is an extended form of the general linear model. The feature of GLM is that it is able to establish the relationship between the mathematical expectation of response variables and predictive variables in the form of linear combination through the link function. This method is capable of processing the data without changing their natural metrics, whether they are with nonlinear or non-constant variance. At the same time, while taking into account the single-factor effect, the regression model can also examine the nonlinear interactions between any two predictors (McCullagh, 1984; Sun et al., 2013). Therefore, the GLM regression model can be used to test the factor effects and can be adopted to establish the relationship between the TC intensity and various predictors (Guo et al., 2014), which is defined as below.

    where ${\hat V_{\max }}$ is the fitted value of $ {V}_{max} $ in the TC system obtained from the model; $ {\mathop { X}\limits^ \rightharpoonup}$ = xi (i = 1, 2, ···, n) represents each predictor for determining the TC intensity; and bi (i = 1, 2, ···, n) is the coefficient of each factor term. Accordingly, $\displaystyle\sum\limits_{i = 1}^n {{b_i}{x_i}} $ in the formula represents the single-factor effect of each predictor; $\displaystyle\sum\limits_{i = 1}^n {\displaystyle\sum\limits_{j = i + 1}^n {{b_{i,j}}{x_i}{x_j}} } $ represents the interactions between every two predictors; and ε is the residual.

    Before the construction of the multi-factor estimation model, contributions of the preselected factors to the intensity estimation should be examined to improve the estimation accuracy. In the multi-factor model, for the i-th predictor, the variance contribution is defined as ${{{V}}_i} = b_i^2/{C_{ii}}$, where $ {C}_{ii} $ is the element at line i and column i of the matrix $ {\mathop { C} \limits^ \rightharpoonup}={\left({{\mathop { X}\limits^ \rightharpoonup}}'{\mathop { X}\limits^ \rightharpoonup}\right)}^{{\text{−}}1} $, and $ {\mathop { X}\limits^ \rightharpoonup} $ is the matrix of the preselected factors. Thus, the relative variance contribution rate for the i-th factor can be defined as the ratio of the variance contribution of the i-th factor to the sum of the variance contribution of all factors, that is, ${{P}_i} = {V_i}/\sum\nolimits_{i = 1}^n {{V_i}} $.

    First, all the pre-selected factors are introduced into the GLM. The distribution of factors’ average variance contribution rates to the TC intensity under different radii is analyzed (Fig. 3a). The dark color represents the variance contribution rate of single factor, and the light color represents the total variance contribution rate of all pairwise interactions involving this factor. As can be seen from the figure, in general, the factors with a variance contribution rate of over 10% include the FMMV, S-20, RD, Lon, and TBBstd, while the corresponding rates of the Lat, TBBm, and MDAV are relatively low. Among all the factors, the FMMV rank the highest in the variance contribution, reaching over 70%, which demonstrates the high correlation between the axisymmetry and intensity of the TC system. Meanwhile, the distributions of S-20 and TBBstd represent the region size and uniform distribution of the deep convection near the circulation center of the system, and both the two factors exhibit high correlations with the system intensity, especially S-20. The RD factor can reflect the deviation of the large-area organized convection from the circulation center of the system. It makes up for the deficiency of the MMV’s deviation from the circulation center, which is caused by the influence of large-area organized convections under the severe asymmetry of the system and the increasing baroclinicity at the late development stage of TCs. Hence, it facilitates the improvement of the model accuracy (Yuan and Zhong, 2019). In addition, it should be noted that contribution rates of the average TBB and MMV are quite low, indicating that the TC intensity has a poor correlation to the regional-mean distribution of the convection. This may be attributed to the fact that both the two factors are highly correlated with the key factor FMMV, thus the introduction of the two factors has little effect on the model.

    Considering that the interactions between factors occupy the absolute predominance in the variance contribution rate of the model, in order to further investigate the effects of the pre-selected factors and ensure the stability of the model, each predictor is removed from the model in turn, and the RMSE of the model without a certain predictor is compared with that of the model containing all the predictors. The increment is defined as the percentage increase in the RMSE before and after the predictor is removed. The larger the increment is, the more important the predictor is for the TC intensity determination, otherwise, the less important the predictor will be. As shown in Fig. 3b, the effect is the most significant when the factor FMMV is removed, with the increase in the RMSE all exceeding 25% under different radii. TBBstd and S-20 are the next. Particularly, the model is more sensitive to the two factors under smaller radii. The influence of RD is equivalent to those of TBBstd and S-20, but it increases with the increasing calculation radius and tends to be stable when the radius is > 400 km. When the two predictors (MDAV and TBBm) are removed respectively, deviations of the RMSE from that of the model with all factors are so small that they are almost negligible, which also corresponds with their variance contribution rates. Besides, the position of the circulation center has a certain influence on the system intensity, and it is interesting that the effect of the longitude is stronger than that of the latitude. For the latitude, it has a greater impact on the modeling under smaller calculation radius, and the impact is quite weak when the calculation radius is greater than 400 km.

    According to the contribution rate of each factor introduced above, we add them to the GLM in the sequence of the MMV, S-20, RD, Lon, TBBstd, Lat, TBBm, and MDAV one by one to compare the estimation results. With the data of all the three years as the validation set (G-all), it can be seen from Fig. 3c that the RMSE is gradually decreasing with the increase of the introduced factor number, especially for the first six factors. However, when the number of factors further increases, the accuracy of the GLM no longer improves significantly. Under the condition that the DAV calculation radius is 350 km, the RMSE is reduced by only 0.21 m s−1 compared with the model with the introduction of six factors. On the other hand, too many factors may weaken the robustness of the model due to collinearity. Therefore, it is necessary to minimize the number of factors to be introduced on the basis of ensuring the estimation accuracy. With the full consideration of both the variance contribution and influence of the factor removal on the model, the six parameters of the FMMV, RD, S-20, TBBstd, Lat, and Lon are introduced to the GLM.

  • In recent years, the models of machine learning have been gradually applied to the research and prediction of time series data. A deep learning model is a deep neural network model with multiple non-linear mapping levels, which can extract features layer by layer from the input data, and dig deeper connections (LeCun and Bengio, 2015). In this section, we use the LSTM model, as well as GLM, to establish the multi-factor model of TC intensity estimation, and compare the results with the single-factor SFM.

    The structure of LSTM model is shown in Fig. 4. Each LSTM cell includes an input, output, and forget gates (Fig. 4a). The input gate determines the new information that can be added to the cell state. The output gate determines what will be regarded as the output value in the current state. The forget gate determines the information that will be discarded from the cell state. LSTM model avoids the problem of long-term dependence by setting these three gates. The hidden layer of LSTM model consists of chain repeating cells. In this study, we used a two-dimensional LSTM (2-D LSTM) model to estimate the TC intensity. We set the number of nodes in the first layer network to 20 and the number of nodes in the second layer network to 5 (Fig. 4b).

    Figure 4.  The schematic diagram of the structure of (a) LSTM cell and (b) 2-D LSTM chains. $ {f}_{t} $, $ {i}_{t} $, and $ {o}_{t} $ denotes the forget, input, and output gates, respectively. $ \sigma $ and tanh are activate functions, in which $ \sigma $ represents the Sigmoid function while tanh represents the hyperbolic tangent function. $ {x}_{t} $ is the input value at the current time. $ {C}_{t} $ and $ {h}_{t} $ are the state of the cell and hidden layer at the current moment, respectively.

    The test results of GLM and LSTM intensity estimation models with optimum factors (Table 3) reveal that the relative effect of the test results is similar to that of the single-factor SFM among different groups, and the test result of G-2017 with relatively fewer high-intensity TC cases is significantly better than those of the other groups, while the group of G-2015 with the most high-intensity TC cases exhibits relatively poor performance. However, the RMSE range of the multi-factor models with GLM and LSTM models are 5.93–8.68 and 4.69–7.93 m s−1, indicating that there is a significant improvement on the overall performance compared with the single-factor SFM. Meanwhile, an optimum calculation radius exists in all the four groups, which mainly ranges from 300 to 450 km. Between the two multi-factor models, LSTM model has lower RMSE in every test group even with different radii, and the optimum radius also narrows to 350–400 km, which means that the estimation results can be further improved with the introduction of the machine learning technology.

    Radius (km)G-2015 RMSE (m s−1)G-2016 RMSE (m s−1)G-2017 RMSE (m s−1)G-all RMSE (m s−1)
    GLMLSTM GLMLSTM GLMLSTM GLMLSTM
    2508.687.587.447.566.835.947.356.61
    3008.497.157.417.406.335.547.246.41
    3508.327.017.467.375.985.127.216.30
    4008.277.007.577.505. 934.997.236.22
    4508.267.257.747.495.974.697.276.33
    5008.307.307.917.755.974.807.356.38
    5508.417.288.177.935.965.157.486.60

    Table 3.  RMSEs of the multi-factor GLM and LSTM model estimation results with different calculation radii

    Through the comprehensive comparison, it can be seen that, physically, the multi-factor models not only consider the symmetrical characteristic of TCs but also can effectively reduce the error of intensity estimation by considering the deep convection, environment information, and their interactions. As the calculation efficiency of DAV is inversely proportional to the square of the radius, it is obvious that the determination of an optimum radius by the multi-factor models can effectively reduce the computational burden.

4.   Comparison of the fitting and estimation results to the TC intensity
  • Although the optimum radii corresponding to different validation sets are different, all of them basically fluctuate around the optimum radii corresponding to the fitting data set. Thus, the optimum calculation radius in the fitting test is adopted for comparison between the results of the above two models. Specifically, the optimum calculation radii are 550 km for the single-factor SFM, 350 km for the multi-factor GLM, and 400 km for the multi-factor LSTM model. Biases of the results with the optimum calculation radii from the CMA-BST dataset at different stages of TCs are compared in Fig. 5. Generally, for the single-factor SFM, the median of intensity biases below the STS intensity is close to the zero line; at the stages of TY and STY, the obvious positive bias exists; and the significant negative bias is observed at the stage of STY except for the G-2017 test. However, large differences still exist among different test groups. For instance, the median of the intensity bias in the G-2016 test is slightly negative at the stages of STS and TY, while that in the G-2017 test is significantly positive at the stage of STY. Moreover, for intensities above TS, the distance between the upper and lower quartiles is larger and the tail is longer. Meanwhile, there are also many outliners and a large number of anomalies exceeding ± 20 m s−1 with a maximum value of over 50 m s−1. Generally, the estimation results from the single-factor SFM over-estimate the TC intensity at the low-intensity stage and under-estimate at the high-intensity stage, meanwhile the test results are very sensitive to the distribution of TC cases, revealing the poor stability of the model.

    Figure 5.  Boxplots for biases of the estimation results in the (a) single-factor SFM, (b) multi-factor GLM, and (c) LSTM model, from the CMA-BST dataset at different stages of the TC. “A”, “B”, “C”, and “D” in the four boxplots of different colors at each stage of the TC correspond to the G-2015, G-2016, G-2107, and G-all tests, respectively.

    Comparatively, for the multi-factor models, distributions of the bias medians in different test groups are consistent, except for the positive bias at the STY stage in the G-2017 test, which is different from the others. Overall, it is shown that the medians in all test groups exhibit positive biases at the stages with the intensity below TD and get close to the zero line at the stages of TS, STS, and TY, yet negative biases occur at the stages above the STY intensity. In addition, it can be seen that the distance between the upper and lower quartiles as well as the tail length all decrease significantly, and the number and magnitude of the outliers are within a reasonable interval, also no bias of over 40 m s−1 appears (Figs. 5b, c). It indicates that the estimation results of the multi-factor model are more accordant with records in the CMA-BST dataset. By investigating differences between the two multi-factor models, it shows that the number and magnitude of the outliers can be further reduced in LSTM model.

    Taking three SuperTYs [Dujuan (1521), Meranti (1614), and Talim (1718)] as examples, comparison of the TC intensities among the CMA-BST records, models’ estimations, and trends of four high-correlation factors after normalization, has been shown in Fig. 6. It can be found more clearly that the multi-factor models reduce the estimation deviation at the peak-intensity stage and dissipation stage. Furthermore, both the multi-factor models eliminate the large false increase at the early stage of these three cases. The reason for the greatly improved estimation accuracy of the multi-factor models is more due to the complementary effects among different type factors. On the contrary, since the estimation results of the single-factor SFM only depend on the inverse correlation between the $ {V}_{\max} $ and FMMV, the sharp drop of FMMV in the early stage of the TC led to a large deviation in the intensity estimation. As discussed in Yuan and Zhong (2019), FMMV can indicate the axisymmetry of deep cloud clusters around its location. The closer between the FMMV location and TC center at the same time, the better the estimation result is. However, when there exist the large-scale non-closed deep convective cloud clusters at the TC early stage, the FMMV decreases while the TC axisymmetry is poor. Fortunately, the other three factors (RD, S-20, and TBBstd), exert the positive effects on reducing this false increase by respectively showing the increasing distance between the FMMV location and TC center, limited area of deep convective cloud-clusters, and large standard deviation around the circulation center. When comparing GLM and LSTM models, the consideration of the non-linear interaction among different factors with machine learning avoid the high-frequency oscillation of the estimation results and reduce the deviation.

    Figure 6.  Comparison of the TC intensity among CMA-BST records (black heavy line), SFM estimations (green heavy line), GLM estimations (blue heavy line), LSTM estimations (purple heavy line), and trends of four high-correlation factors after normalization (light lines with different colors as shown in the figure legend). Three cases [Dujuan (1521), Meranti (1614), and Talim (1718)], are taken as examples, whose life cycles are divided by red dashed lines.

    Since the different satellite data, area, and testing methods were used, it is not possible to directly compare the results among recent studies on the TC intensity estimation. But it is still meaningful to understand the effect of the fitting test from the advanced models in literature. As shown in Table 4, compared with the SFM (Ritchie et al., 2014) with seven years TC cases, GLM still slightly reduce the RMSE with no more than half samples. Then the LSTM model also appears better results in complicated models with the single-channel data, even the sample number is much smaller. The results also show that the introduction of multi-channel information could significantly reduce the fitting RMSE (Lee et al., 2020), we will further test the effects of multi-factors with longer samples’ duration and more channel data in the future.

    ModelApproachCovered durationData sourceInputFitting test* RMSE (m s−1)
    Ritchie et al. (2014)SFM2005–2011GOES-seriesIR (10.7 μm)7.36
    Combinido et al. (2018)CNN1996–2014GMS-5,
    GOES-9,
    MTSAT-1R,
    MTSAT-2,
    Himawari-8
    IR (11.0 μm)6.80
    Zhang et al. (2016)RVM2005–2014FY-2CIR (10.3 μm)6.58
    Lee et al. (2020)CNN2011–2016COMSVIS (0.67 μm),
    VIS (3.7 μm),
    VIS (6.7 μm),
    IR (10.8 μm),
    IR (12.0 μm)
    4.27
    In This paperSFM
    (500 km)
    2015–2017FY-2F/2GIR (10.7 μm)12.00
    GLM
    (350 km)
    7.21
    LSTM
    (400 km)
    6.22
    * The unit of fitting test RMSE from the referenced literature has been uniformed as m s−1. Geostationary Operational Environmental Satellite—GOES; Geostationary Meteorological Satellite—GMS; Multi-functional Transport Satellite—MTSAT; Communication, Ocean, and Meteorological SatelliteCOMS; infrared radiation—IR; visible band—VIS.

    Table 4.  Comparison of the model performance with the exiting satellite-based TC intensity estimation approaches

5.   Conclusion
  • In this paper, we have investigated the intensity determination process of the TC in the WNP during 2015–2017 based on the deviation angle variance (DAV) technique and applied the cross validation method with different training sets and test sets for different models. It is found that the SFM has relatively poor stability and accuracy in the intensity determination process. In addition, the calculation radius for the DAV technique is large, resulting in low computing efficiency, which cannot reach the ideal effect of the intensity determination. Then the two multi-factor models (GLM and LSTM) are set up and significantly decrease the RMSE of intensity estimation in all the test groups by introducing the information of deep convection structures. Besides, since the optimum radius for the multi-factor model ranges from 350 to 400 km, the load of computing decreases effectively. The result shows that the multi-factor model has better fitting degree with CMA-BST data, together with the intensive deviation distribution and better stability.

    After comparing with recent studies on the TC intensity estimation, future work includes the following aspects. On one hand, since the size of cases and multi-channel information intensity significantly affect the intensity estimation result besides effects of the multi-factor, expanding the input information and high-correlation factors to improve the accuracy of TC intensity estimation is our main task. On the other hand, the application of machine learning is helpful to reflect the non-linear complicated relation between the physical factors and TC intensity, and adjusting and improving LSTM model with multi-factors will be also important in the future work.

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