Examination of Surface Wind Asymmetry in Tropical Cyclones Over the Northwest Paciﬁc Ocean Using SMAP Observations

remote sensing

Article Examination of Surface Wind Asymmetry in Tropical Cyclones over the Northwest Paciﬁc Ocean Using SMAP Observations

Ziyao Sun 1, Biao Zhang 1,2,* , Jun A. Zhang 3 and William Perrie 4 1 School of Marine Sciences, Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] 2 Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China 3 Hurricane Research Division at NOAA/AMOL and CIMAS at University of Miami, Miami, FL 33149, USA; [email protected] 4 Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, NS B2Y 4A2, Canada; [email protected] * Correspondence: [email protected]; Tel.: +86-25-58695692

Received: 9 October 2019; Accepted: 4 November 2019; Published: 6 November 2019

Abstract: Tropical cyclone (TC) surface wind asymmetry is investigated by using wind data acquired from an L-band passive microwave radiometer onboard the NASA Soil Moisture Active Passive (SMAP) satellite between 2015 and 2017 over the Northwest Pacific (NWP) Ocean. The azimuthal asymmetry degree is defined as the factor by which the maximum surface wind speed is greater than the mean wind speed at the radius of the maximum wind (RMW). We examined storm motion and environmental wind shear effects on the degree of TC surface wind asymmetry under different intensity conditions. Results show that the surface wind asymmetry degree significantly decreases with increasing TC intensity, but increases with increasing TC translation speed, for tropical storm and super typhoon strength TCs; whereas no such relationship is found for typhoon and severe typhoon strength TCs. However, the degree of surface wind asymmetry increases with increasing wind shear magnitude for all TC intensity categories. The relative strength between the storm translation speed and the wind shear magnitude has the potential to affect the location of the maximum wind speed. Moreover, the maximum degree of wind asymmetry is found when the direction of the TC motion is nearly equal to the direction of the wind shear.

Keywords: surface wind asymmetry; tropical cyclone; storm movement; wind shear

1. Introduction Tropical cyclones (TCs) can induce storm surges and intense rainfall during landfall, thereby causing serious damage to society and affecting the safety of residents in coastal areas. Therefore, in operational forecast centers, risk evaluation of potential TC damage is becoming increasingly significant. Obtaining key parameters is vital for TC forecasts, including the maximum wind speed (MWS), radius of maximum wind (RMW), and wind field distribution. Meteorologists usually determine TC intensity by identifying the MWS; thus, understanding the two-dimensional surface wind structure has great importance. Over the past decades, efforts have been made to understand the possible reasons for TC surface wind asymmetry by considering large-scale environmental impacts, such as the beta effect, vertical shear of environmental flow, and uniform environmental flows, on storm intensity. Thus, it is suggested that these environmental effects induce quasi-stationary asymmetries near the TC eyewall and exert inhibitory actions on storm intensity [1]. Moreover, previous studies have shown that other factors

Remote Sens. 2019, 11, 2604; doi:10.3390/rs11222604 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 2604 2 of 18 may affect TC structural asymmetries, including variations in surface friction [2], Rossby waves [3], and blocking actions [4]. Storm motion is one of the major impact factors on an asymmetric structure in the TC surface wind field. The effect of storm movement induces the wind field to be asymmetric on two sides of the storm motion direction, with higher winds in the right forward quadrant than in the left, and the effect of surface friction leads to front-rear asymmetry due to the storm motion [5]. However, TC surface wind fields are axisymmetric when simulated with the traditional parametric hurricane wind profile model [6]. An asymmetric parametric wind profile model was later developed when the impact of storm movement was considered, which maximizes the wind asymmetry to the right side of the forward storm movement [7]. Moreover, early studies found that the effect of storm motion contributes to convective asymmetries, with updrafts being stronger in the right forward quadrant, based on a linear analytical boundary layer flow model in a moving TC [8]. Vertical wind shear also has an important effect on hurricane structural asymmetry [9]. Although the storm motion is traditionally considered to be the primary factor that contributes to the TC structural asymmetry, its effect is significantly weaker than the influence of vertical wind shear on TC convection, based on lightning data in TCs [10]. Recent studies have shown that large environmental vertical wind shear can induce strong asymmetric structures observed in hurricanes and typhoons [11,12]. Moreover, two kinds of rain-rate data have been used to investigate TC rainfall asymmetry; these data show that precipitation asymmetry is much more associated with wind shear than storm motion [13]. Mesoscale analysis data from four typhoon seasons between 2004 and 2007 acquired from the Japan Meteorological Agency (JMA) were used to investigate the azimuthal wavenumber 1 asymmetry [14] of surface winds in the typhoon core region, suggesting that the − phenomenon of asymmetric surface wind is related to both the vertical wind shear and also to the storm motion. Moreover, a previous study has reported that the maximum surface wind speed occurs on the left side of the storm motion when the direction of shear is approximately identical to the storm heading [15]. Airborne stepped-frequency microwave radiometer (S-FMR) hurricane wind observations at the surface and flight level have been used to study the effect of storm movement and vertical wind shear on surface wind asymmetry in hurricanes [16]. At the flight level, the wavenumber 1 wind speed − asymmetry amplitude increases with storm translation speed, while no obvious increasing tendency can be found at the surface. SFMR can only observe along-track wind speeds in TCs, not the complete two-dimensional surface wind field of an individual storm [17]. Satellite scatterometers, such as Ku-band QuikSCAT, have the potential to measure TC surface wind fields [18]. Scientists have used QuikSCAT winds to qualitatively investigate the link between typhoon surface wind asymmetry, as well as both the vertical wind shear and storm motion [19]. The highest wind speeds tend to arise favorably on the left side of the shear and on the right side of the storm movement. However, QuikSCAT cannot accurately provide the maximum winds in the eyewall region of most TCs because of radar signal saturation and attenuation by rain [20]. By conducting a composite analysis of TC wind fields in each TC-inclined region and different TC intensity conditions, based on rain-corrected scatterometer surface winds, a recent investigation has shown that the entire TC surface wind asymmetry is located to the left side of the storm movement, in a motion-relative frame, for all areas and for weaker TCs, when the movement velocity is removed [21]. For the purpose of further study of the effect of wind shear and storm motion on TC surface wind asymmetry, a low-wavenumber analysis method [16] was adopted to quantify the asymmetry index; thus, it was shown that tropical storms exhibit the most obvious surface wind asymmetry characteristics among all TC intensity groups [22]. However, whether the vertical wind shear or the storm motion dominantly affects TC surface wind asymmetry remains an open question. The L-band spaceborne radiometer is an excellent passive microwave sensor for remotely measuring the wind speed in TCs, because it is much less susceptible to rain attenuation than Ku-band scatterometers. Recent investigation has shown that there is a good relation between the Remote Sens. 2019, 11, 2604 3 of 18 wind-induced brightness temperatures from the SMAP L-band radiometer and TC wind speeds, and that brightness temperatures are not saturated under extreme weather conditions [23], thereby providing an opportunity for remote sensing of ocean surface winds during severe storms [24,25]. Compared to Ku-band scatterometers, the L-band radiometer can provide more accurate measurements of high winds in extreme weather conditions. Although aircraft-based platforms, such as SFMR, are useful instruments to measure TC-force wind speeds in the along-track direction, they cannot provide comprehensive wind speed measurements over the entire two-dimensional spatial domain. However, the SMAP L-band radiometer has a wide swath and therefore has the capability of providing complete and accurate wind speed measurements in severe storms. Thus, it is possible to study TC wind asymmetry using SMAP winds. In this paper, our focus was the study of surface wind asymmetry in relation to both factors, namely storm motion and vertical wind shear, for the first time, using SMAP wind measurements of TCs between 2015 and 2017 over the Northwest Pacific Ocean. Our motivation was to probe the roles of storm motion and vertical wind shear in affecting the surface wind speed asymmetry degree under differing TC intensity conditions. The sensitivity of the surface wind asymmetry to the difference between the storm motion direction and wind shear direction was also investigated. The remaining part of this paper is organized as follows. The data and the methodology are described in Sections2 and3. The results and discussion are presented in Sections4 and5. Conclusions are summarized in Section6.

2. Data

2.1. SMAP Radiometer Winds In this study, SMAP radiometer winds in storms were used to explore the degree of surface wind asymmetry in TCs. The NASA SMAP mission was successfully launched in January 2015 and started to provide data to the science community in April 2015. SMAP was originally designed for the purpose of measuring soil moisture and the freeze-thaw state [26], by using synergistic observations from an onboard passive radiometer and active scatterometer. The spatial resolution and swath of the SMAP L-band radiometer are about 40 and 1000 km, respectively. Moreover, it has been demonstrated that the SMAP radiometer is an excellent microwave instrument to observe ocean surface winds in storms [23–25]. SMAP wind data with a spatial resolution of 0.25 0.25 were publicly obtained ◦ × ◦ through the remote sensing systems website (www.remss.com/missions/smap/). The SMAP radiometer has the ability to provide accurate estimates of storm intensity (~70 m/s) and hurricane-force wind radii, even in heavy rainfall conditions [23]. In order to investigate surface wind asymmetry in TCs, a total of 125 samples of SMAP-measured surface wind ﬁelds for 43 TCs were acquired over the Northwest Paciﬁc Ocean between April 2015 and December 2017. Table1 summarizes the TC names and corresponding numbers of SMAP observations for each TC.

Table 1. Names of TCs and corresponding numbers of SMAP wind ﬁeld snapshots. The year in which each storm occurred is also shown in the table.

Year Storm (No. of Wind Fields) Atsani (6), Chan-hom (2), Champi (4), Dolphin (3), Dujuan (5), Goni 2015 (3), Haishen (1), In-fa (5), Koppu (2), Krovanh (4), Linfa (1), Maysak (2), Melor (2), Nangka (7), Noul (3), Soudelor (3), Choi-wan (1) Haima (2), Lionrock (7), Megi (1), Meranti (2), Nepartak (4), Sarika 2016 (1), Songda (4), Ghaba (3), Malakas (4), Meari (3), Mindulle (1), Namtheun (1), Omais (1), Conson (1) Khanun (1), Kulap (1), Lan (4), Nalgae (1), Nanmadol (1), Nesat (2), 2017 Noru (10), Sanvu (4), Saola (3), Talim (3), Banyan (5), Kai-tak (1) Total 43 (125) Remote Sens. 2019, 11, 2604 4 of 18

Remote Sens. 2019 4 of 19 2.2. TC Track Data and Environmental Wind Data The directiondirection and and magnitude magnitude of of the the TC TC translational translational motion motion were were determined determined from from the Best the Track Best dataTrack from thedata Joint Typhoonfrom Warningthe CenterJoint (JTWC)Typhoon (https: //www.metoc.navy.milWarning Center/jtwc /jtwc.html(JTWC)). It(https://www.metoc.navy.mil/jtwc/jtwc. is difficult to accurately determine thehtml). TC center It is from difficult SMAP to wind accurately data due determine to its sparse the resolution.TC center Thus,from SMAP Best Track wind datadata weredue to used its sparse to derive resolution. the TC Thus, center Best positions Track data from were SMAP used winds to derive via the linear TC interpolation.center positions The from temporal SMAP resolution winds via is 6linear h for interp the JTWColation. Best The Track temporal data. The resolution environmental is 6 h verticalfor the windJTWC shear Best wasTrack estimated data. The by environmental calculating the vertical change wind in winds shear at was 850 andestimated 200 hPa by levelscalculating averaged the withinchange ain circle winds with at 850 a radiusand 200 of hPa 300 levels km about averaged the stormwithin center,a circle from with thea radius NCEP of Climate300 km about Forecast the Systemstorm Reanalysiscenter, from (CFSR) the VersionNCEP 2 (Climateapdrc.soest.hawaii.edu Forecast System/dods /Reanalysispublic_data /(CFSR)CFSv2). TheVersion spatial 2 (apdrc.soest.hawaii.edu/dods/public_data/CFSv2). The spatial and temporal resolutions of the CFSR and temporal resolutions of the CFSR data are 0.5◦ 0.5◦ and 1 h, respectively. data are 0.5° × 0.5° and 1 h, respectively. × 3. Methodology 3. Methodology 3.1. Asymmetry Parameter 3.1. Asymmetry Parameter The SMAP wind speeds were used to estimate the degree of TC wind speed asymmetry by using an asymmetryThe SMAP parameter wind speeds (ε). were Following used to [4 ],estimate the dimensionless the degree of asymmetry TC wind speed parameter asymmetry was defined by using as an asymmetry parameter (ε). Following [4], the dimensionless asymmetry parameter was defined as Vmax VRMW ε = 𝑉−−𝑉 (1) 𝜀= VRMW (1) 𝑉 wherewhere V𝑉max isis thethe maximummaximum windwind speed,speed, andand V 𝑉RMW is the average azimuthal wind speed at thethe locationlocation ofof thethe radius radius of of the the maximum maximum wind wind (RMW). (RMW). In thisIn this study, study,Vmax 𝑉and VandRMW 𝑉were were derived derived from thefrom SMAP the SMAP surface surface wind data.wind Largerdata. Larger values values of the asymmetryof the asymmetry parameter parameter correspond correspond to a larger to a degree larger ofdegree asymmetry of asymmetry for the TCfor windthe TC field. wind field. The asymmetry parameter for each TC case was estimatedestimated followingfollowing EquationEquation (1).(1). All TCs were divided intointo twotwo groups groups according according to to the the asymmetry asymmetry degree degree parameter. parameter. Strong Strong or weak or weak asymmetry asymmetry was denotedwas denoted by the by asymmetry the asymmetry parameter, parameter, corresponding correspo tonding larger orto smallerlarger or values smaller than values 0.2, respectively. than 0.2, Figurerespectively.1 shows Figure the geographical 1 shows the locations geographical of these locati two groupsons of ofthese TCs, two indicating groups noof apparentTCs, indicating di fference no inapparent spatial difference distributions in spatial between dist theributions two groups. between The the linear two groups. effect of The storm linear motion effect was of storm removed motion by subtractingwas removed half by ofsubtracting the TC translation half of the speed TC translation [27] from speed the SMAP [27] from surface the SMAP wind speed surface data, wind and speed the asymmetrydata, and the parameter asymmetry was parameter recalculated. was recalculated.

Figure 1. Geographical locations of 125 TC cases in the Northwest Pacific Paciﬁc Ocean Ocean observed observed by by SMAP. SMAP. Blue starsstars andand black black circles circles denote denote that that the correspondingthe corresponding asymmetry asymmetry parameter parameter (ε) is larger(ε) is orlarger smaller or thansmaller 0.2, than for a 0.2, particular for a pa TC,rticular respectively. TC, respectively.

3.2. Composite Wind Field Analysis A composite wind field analysis method was used to investigate TC surface wind asymmetry. Compared to an individual case analysis, composite wind field analysis of a group of TCs can directly

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3.2. Composite Wind Field Analysis A composite wind field analysis method was used to investigate TC surface wind asymmetry. Compared to an individual case analysis, composite wind field analysis of a group of TCs can directly reveal the overall characteristics of the surface wind field, including the average intensity and asymmetric structure. Since the composite field is derived from the average of the individual fields, the noise in the SMAP wind data is eliminated, to a certain extent. Accordingly, TC asymmetric structure can be elucidated, based on composite analysis, which thus enabled us to explore the effect of various factors on TC surface wind asymmetry. The basic steps for the composite analysis were as follows: (1) Select the SMAP wind field within a radius of 250 km from the TC center to ensure that the composite wind fields can exhibit the essential asymmetric structural features; (2) rotate the TC wind fields so that the forward direction of the composite field coincides with the direction of the wind shear or storm motion; and (3) bin average the TC wind fields as a function of the distance from the TC center and normalize, by using the radius of maximum wind speed (RMW) and the maximum wind speed (MWS).

3.3. Grouping Approach and Coordinate Systems The relationship between the asymmetry parameter (ε) and the dominant factors that influence it were examined, such as the environmental vertical wind shear, storm movement, and intensity. First, TC samples were divided into four groups according to the TC intensity, namely, the tropical storm group (17.2 m/s < Vmax < 32.6 m/s), typhoon group (32.7 m/s < Vmax < 41.4 m/s), severe typhoon group (41.5 m/s < Vmax < 50.9 m/s), and super typhoon group (Vmax > 51.0 m/s). TC samples were also divided into four groups according to the angle difference (∆θ = θ θstorm), between the shear − storm motion direction, θstorm, and wind shear direction, θshear. For the downshear (DSHR) group, wind shear and storm motion directions were equal; the angle difference was |∆θ| 22.5◦ . For the ≤ upshear (USHR) group, the wind shear and storm motion were in the opposite directions; to be specific, the angle difference was |∆θ| 157.5◦ . When the wind shear was on the right side of the storm motion ≥ direction (RSHR) or to the left side of the storm motion direction (LSHR), they were categorized as 22.5◦ < ∆θ < 157.5◦ and 157.5◦ < ∆θ < 22.5◦ , respectively. − − Several different coordinate systems were used to evaluate the wind asymmetry. These coordinate systems include (1) earth-relative coordinates, in which cardinal directions are designated as North, East, South, and West; (2) motion-relative coordinates, in which cardinal directions are defined as to the front of the storm motion, to the right side of the storm motion, to the rear of the storm motion, and to the left side of the storm motion; and (3) wind shear-relative coordinates, in which the cardinal directions are defined as ‘upshear’,‘downshear’, left side of shear, and right side of shear. The frequency distributions of some key parameters for the TC samples are shown in Figure2. TC intensity (Vmax) was directly derived from the SMAP wind observations. Figure2a shows that the TC samples used in this study cover a wide range of TC intensities, from ~20 to 75 m/s. The average TC translation speed (Vstorm) and direction (θstorm) over all samples are 5.8 m/s and 231◦, respectively. The average environmental vertical wind shear magnitude (Vshear) and direction (θshear) of the TC samples are 6.15 m/s and 196◦, respectively. The range of the angle differences between the wind shear directions and storm motion directions (θ θstorm) is shown in Figure2f. shear − Remote Sens. 2019, 11, 2604 6 of 18 Remote Sens. 2019 6 of 19

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Figure 2. HistogramsHistograms of ofpara parametersmeters used used for: for:(a) TC (a )intensity TC intensity (𝑉); ( V(bmax) TC); translation (b) TC translation speed (𝑉 speed);

((Vc)storm environmental); (c) environmental vertical verticalwind shear wind magnitude shear magnitude (𝑉 ();V (sheard) );TC (d motion) TC motion direction direction (𝜃 (θstorm); (e);) Figure 2. Histograms of parameters used for: (a) TC intensity (𝑉 ); (b) TC translation speed (𝑉 ); (environmentale) environmental vertical vertical wind wind shear shear direction direction (𝜃 (θshear); );and and (f ()f )angle angle difference difference between the shear (c) environmental vertical wind shear magnitude (𝑉 ); (d) TC motion direction (𝜃 ); (e) direction andand thethe TCTC motionmotion directiondirection (θ(𝜃shear −𝜃θstorm). Note). Note that that 0◦ 0°in in (d )(d and) and (e) ( representse) represents North; North; 0◦ environmental vertical wind shear direction− (𝜃 ); and (f) angle difference between the shear in0° (inf) indicates(f) indicates that that the windthe wind shear shear and and storm storm motion motion directions directions are equal. are equal. RS indicates RS indicates that the that wind the direction and the TC motion direction (𝜃 −𝜃 ). Note that 0° in (d) and (e) represents North; shearwind shear is located is located on the on right the right side ofside the of TC the motion TC motion and and LS indicates LS indicates that that the windthe wind shear shear is located is located on 0° in (f) indicates that the wind shear and storm motion directions are equal. RS indicates that the theon the left left side side of theof the TC TC motion. motion. wind shear is located on the right side of the TC motion and LS indicates that the wind shear is located on the left side of the TC motion. 4. Results 4. Results 4.1. Intensity Effect on the Degree of Wind Asymmetry 4.1. Intensity Effect on the Degree of Wind Asymmetry To4.1. study Intensity the Effect TC intensityon the Degree eff ectof Wind on surface Asymmetry wind asymmetry, the TC samples were divided into To study the TC intensity effect on surface wind asymmetry, the TC samples were divided into four groups as mentioned in Section 3.3 and the distribution of the asymmetry parameters was four groupsTo studyas mentioned the TC intensity in Section effect on3.3 surface and thewind distribution asymmetry, ofthe theTC samplesasymmetry were parametersdivided into was examinedfour groups in each as group. mentioned As shownin Section in Figure3.3 and3 ,the on distribution average, tropical of the asymmetry storms have parameters stronger was surface examined in each group. As shown in Figure 3, on average, tropical storms have stronger surface windexamined asymmetry in each than group. typhoons, As shown severe in typhoons, Figure 3, andon average, super typhoons tropical storms as indicated have stronger by the surface asymmetry wind asymmetry than typhoons, severe typhoons, and super typhoons as indicated by the parameter.wind asymmetry We also computed than typhoons, the linear severe regressions typhoons, to and estimate super thetyphoons dependence as indicated of the asymmetryby the asymmetry parameter. We also computed the linear regressions to estimate the dependence of the parameterasymmetry on the parameter. TC intensity. We also Figure computed4 clearly the linear shows regressions that the asymmetryto estimate the parameter dependence significantly of the asymmetry parameter on the TC intensity. Figure 4 clearly shows that the asymmetry parameter decreasesasymmetry with increasingparameter on TC the intensity. TC intensity. This Figure finding 4 suggestsclearly shows that that strong the surfaceasymmetry wind parameter asymmetry significantlysignificantly decreases decreases with with increasing increasing TC TC intensity. intensity. ThisThis finding finding suggests suggests that that strong strong surface surface wind wind (ε > 0.2) often exists in weak storms, which is in line with the results shown in Figure3. The values of asymmetryasymmetry (ε > (0.2)ε > 0.2) often often exists exists in in weak weak storms, storms, whichwhich isis in in line line with with the the results results shown shown in Figure in Figure 3. 3. the linear regression coefficients in Figure4 are given in Table2. The valuesThe values of the of thelinear linear regression regression coeffici coefficientsents inin Figure 44 are are given given in in Table Table 2. 2.

𝜀 FigureFigure 3. Histogram 3. Histogram of theof the asymmetry asymmetry parameter parameter (ε)) for for ( (aa) )a a tropical tropical storm; storm; (b) (typhoon;b) typhoon; (c) severe (c ) severe typhoon;typhoon; and and (d) super(d) super typhoon. typhoon. The The number number ofof cases in in each each group group and and the the mean mean value value of 𝜀 offorε eachfor each Figure 3. Histogram of the asymmetry parameter (𝜀) for (a) a tropical storm; (b) typhoon; (c) severe groupgroup is marked. is marked. typhoon; and (d) super typhoon. The number of cases in each group and the mean value of 𝜀 for each group is marked.

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Figure 4. Asymmetry parameter (𝜀) versus TC intensity (𝑉 ). Black solid and dashed lines represent Figure 4. Asymmetry parameter (ε) versus TC intensity (Vmax). Black solid and dashed lines represent linearlinear regressions regressions and and 95% 95% conﬁdence confidence intervals intervals for for the the ﬁts, fits, respectively. respectively.

Table 2. V Table 2.Linear Linear regressionregression coecoefficientsﬃcients forfor thetheasymmetry asymmetry parameterparameter ((ε𝜀)) versusversus TCTC intensityintensity ((𝑉max)) [ε = a + bV ] (Figure4). [𝜀=𝑎+𝑏𝑉max] (Figure 4).

AsymmetryAsymmetry Parameter Parameter 𝜺=𝒂+𝒃𝑽ε=a+bVmax𝒎𝒂𝒙 a a b b 𝜀=𝑎+𝑏𝑉ε= a + bVmax 0.31120.3112 0.0033−0.0033 −

4.2.4.2. CompositeComposite AnalysisAnalysis forfor DiDifferentfferent IntensityIntensity GroupsGroups CompositeComposite analysisanalysis ofof thethe TCTC surfacesurface windwind fieldsfields waswas conductedconducted forfor didifferentfferent intensityintensity groupsgroups toto examineexamine thethe asymmetryasymmetry ofof thethe windwind distributiondistribution relativerelative toto factorsfactors ofof bothboth thethe environmentalenvironmental verticalvertical wind wind shear shear andand stormstorm movement.movement. FigureFigure5 5 shows shows the the shear-relative shear-relative surface surface wind wind composites composites forfor fourfour intensityintensity groups.groups. FigureFigure5 5a,ba,b show show that that the the maximum maximum wind wind speed speed (denoted (denoted by by black black stars) stars) approximatelyapproximately occurs occurs on on the the left left side side of of the the wind wind shear shear (LS) (LS) in the in the composites composites of the of tropicalthe tropical storm storm and typhoonand typhoon intensity intensity categories. categories. However, However, with with regard rega tord the to severethe severe typhoon typhoon category, category, the the position position of theof the maximum maximum wind wind speed speed is closeis close to to ‘downshear ‘downshear’, as’, as shown shown in in Figure Figure5c. 5c. On On the the other other hand, hand, in in the the compositecomposite analysis, analysis, Figure Figure5d shows5d shows that forthat the for super the typhoonsuper typhoon category, category, the location the of location the maximum of the windmaximum speed wind is within speed the is left within quadrant the left of quadrant ‘upshear’. of ‘upshear’. Figure6 shows the surface wind composites in a motion-relative coordinate framework for all four intensity groups. As expected, the location of the maximum wind speed mainly occurs to the right side of the TC movement for all groups. For the typhoon and super typhoon groups, the maximum wind speed occurs in the right-rear quadrant. Yet, for the tropical storm and severe typhoon groups, the maximum wind speed arises just to the right side of the storm motion. The composite fields in Figures5 and6 all pass the student’s t-test with 95% confidence.

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Figure 5. Composites of normalized two-dimensional SMAP wind speed fields are illustrated for (a) a tropical storm, (b) typhoon, (c) severe typhoon, and (d) super typhoon, as shown in Figure 3. Isoline intervals are 0.025 normalized units, and the maximum wind speed is equal to a value of 1. The red arrow and the black star indicate the direction of the wind shear and the location of the maximum wind speed in each group, respectively. The average maximum wind speed in each of the different groups is marked at the top of each panel. Black circles with radii 2 × RMW and 4 × RMW are also Figureshown, 5. 5.for Composites easy comparison ofof normalizednormalized among two-dimensionalthetwo-dimensional different groups. SMAP SMAP wind wind speed speed fields fields are are illustrated illustrated for for (a ()a a) tropicala tropical storm, storm, (b ()b typhoon,) typhoon, (c ()c severe) severe typhoon, typhoon, and and ( d(d)) super super typhoon, typhoon, as as shown shown in in FigureFigure3 3.. Isoline Isoline Figure 6 shows the surface wind composites in a motion-relative coordinate framework for all intervalsintervals areare 0.0250.025 normalizednormalized units,units, andand thethe maximummaximum wind speed is equal to aa valuevalue ofof 1.1. The red four arrowintensity andand groups. thethe blackblack As starstar expected, indicateindicate the thethe directiondirection location ofof of thethe the windwind maximum shearshear andand wind thethe locationlocationspeed mainly ofof thethe maximummaximum occurs to the rightwindwind side speedspeedof the inin TC eacheach movement group,group, respectively.respec fortively. all groups The average. For maximumthe typhoon wind and speed super in each typhoon of the didifferentgroups,fferent the maximumgroups wind is markedmarked speed atat thetheoccurs toptop of ofin eacheach the panel. panel.right-rear BlackBlack circlesqucirclesadrant. withwith radiiYet,radii 2for2 × RMWRMWthe tropical andand 44 × stormRMW areand alsoalso severe × × typhoonshown,shown, groups, forfor easyeasy the comparisoncomparison maximum amongamong wind thethe speed didifferentfferent arises groups.groups. just to the right side of the storm motion. The composite fields in Figures 5 and 6 all pass the student’s t-test with 95% confidence. Figure 6 shows the surface wind composites in a motion-relative coordinate framework for all four intensity groups. As expected, the location of the maximum wind speed mainly occurs to the right side of the TC movement for all groups. For the typhoon and super typhoon groups, the maximum wind speed occurs in the right-rear quadrant. Yet, for the tropical storm and severe typhoon groups, the maximum wind speed arises just to the right side of the storm motion. The composite fields in Figures 5 and 6 all pass the student’s t-test with 95% confidence.

Figure 6. Similar to Figure5 but in a motion-relative coordinate framework. The yellow arrow indicates the typhoon motion direction. Other details are the same.

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Figure 6. Similar to Figure 5 but in a motion-relative coordinate framework. The yellow arrow Remoteindicates Sens. 2019 ,the11, typhoon 2604 motion direction. Other details are the same. 9 of 18

4.3. Dependence of Wind Asymmetry on Storm Motion and Shear 4.3. Dependence of Wind Asymmetry on Storm Motion and Shear Figure 7 shows the asymmetry parameter versus storm translation speed for various intensity Figure7 shows the asymmetry parameter versus storm translation speed for various intensity groups. Figure 7a,d show that the asymmetry parameter (ε) significantly increases with increasing groups. Figure7a,d show that the asymmetry parameter ( ε) signiﬁcantly increases with increasing TC TC translation speed (𝑉) for tropical storms and super typhoons. However, for typhoons and translation speed (V ) for tropical storms and super typhoons. However, for typhoons and severe severe typhoons, thestorm asymmetry is weakly dependent on the storm motion speed, as shown in Figure typhoons, the asymmetry is weakly dependent on the storm motion speed, as shown in Figure7b,c. 7b,c.

FigureFigure 7.7. Asymmetry parameter (ε𝜀) versus typhoon translation speed (V𝑉storm)) forfor ((a)) aa tropical storm,storm, ((bb)) typhoon,typhoon, ((cc)) severesevere typhoon,typhoon, andand ((dd)) supersuper typhoon,typhoon, asas shownshown inin FigureFigure3 .3. The The number number of of storms storms forfor eacheach groupgroup isis marked.marked. The black solidsolid andand dasheddashed lineslines representrepresent linearlinear regressionsregressions andand 95%95% conﬁdenceconfidence intervalsintervals forfor thethe ﬁts,fits, respectively.respectively.

FigureFigure8 illustrates8 illustrates the the relation relation between between the asymmetry the asymmetry parameter parameter and the and environmental the environmental vertical windvertical shear wind magnitude. shear magnitude. For all For intensity all intensity groups, groups, the asymmetry the asymmetry parameter parameter (ε) increases (ε) increases with with the increasingthe increasing vertical vertical wind wind shear shear (Vshear )( 𝑉 magnitude. ) magnitude. Table3 showsTable the3 shows values the of thevalues linear of regression the linear coeregressionﬃcients coefficients in Figures7 inand Figures8. 7 and 8.

Table 3. Linear regression coeﬃcients for the asymmetry parameter (ε) versus the typhoon translation speed (Vstorm)[ε = a + bVstorm] (Figure7), and the asymmetry parameter ( ε) versus the environmental vertical wind shear magnitude (Vshear)[ε = c + dVshear] (Figure8) for di ﬀerent intensity groups.

Asymmetry Parameter ε=a+bVstorm ε=c+dVshear a b c d tropical storm 0.1736 0.0105 0.1908 0.0129 typhoon 0.2315 0.0082 0.0977 0.0186 − severe typhoon 0.1185 0.0029 0.0778 0.0122 super typhoon 0.0589 0.0263 0.0447 0.0354 − −

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Figure 8. Asymmetry parameter (ε) versus the environmental vertical wind shear magnitude (𝑉 ) Figure 8. Asymmetry parameter (ε) versus the environmental vertical wind shear magnitude (Vshear) for (afor) a ( tropicala) a tropical storm, storm, (b )(b typhoon,) typhoon, ( c(c)) severe severe typhoon, and and (d ()d super) super typhoon, typhoon, as shown as shown in Figure in Figure 3. 3. The number of storms for each group is marked. The black solid and dashed lines represent the linear The number of storms for each group is marked. The black solid and dashed lines represent the linear regressions and 95% confidence intervals for the fits, respectively. regressions and 95% conﬁdence intervals for the ﬁts, respectively.

4.4. ImpactTable of Motion 3. Linear and regression Shear oncoefficients the Maximum for the asymmetry Wind Speed parameter Position (𝜀) versus the typhoon translation speed ( 𝑉 ) [ 𝜀=𝑎+𝑏𝑉 ] (Figure 7), and the asymmetry parameter (ε) versus the Theenvironmental location of thevertical maximum wind shear wind magnitude speed (𝑉 azimuthal ) [𝜀=𝑐+𝑑𝑉 is another ] (Figure important 8) for different property for an investigationintensity of the groups. TC asymmetric wind structure, aside from the degree of asymmetry as estimated by the asymmetricAsymmetry parameter.Parameter Figure9 illustrates𝜺=𝒂+𝒃𝑽 the𝒔𝒕𝒐𝒓𝒎 azimuthal dependence𝜺=𝒄+𝒅𝑽 of the maximum𝒔𝒉𝒆𝒂𝒓 wind on the storm translation speed for differenta intensity groups.b The maximumc wind speed is mostlyd located to the right sidetropical of storm the storm motion 0.1736 direction for all 0.0105 intensity groups. 0.1908 The azimuth 0.0129 location of the maximum windtyphoon speed has a tendency0.2315 to move from the−0.0082 right-front quadrant0.0977 to the right0.0186 side and then severe typhoon 0.1185 0.0029 0.0778 0.0122 to the right-rearsuper quadrant,typhoon as the TC− translation0.0589 speed0.0263 increases for all−0.0447 intensity groups.0.0354 Similarly, the relationship between the environmental vertical wind shear and maximum wind speed4.4. position Impact of is Motion illustrated and Shear in Figure on the Maximum10. The maximum Wind Speed wind Position speed mainly occurs in the downshear and downshear-leftThe location quadrantsof the maximum for all wind intensity speed groups. azimuthal The is another azimuthal important location property of the for maximum an windinvestigation speed has aof tendency the TC asymmetric to shift from wind thestructure, downshear aside from to the the left degree side of of asymmetry the wind as shear estimated when the shearby magnitude the asymmetric increases parameter. for di Figurefferent 9 illustrate intensitys the groups. azimuthal These dependence results are of consistentthe maximum with wind [16 ] for typhoons,on the andstorm severe translation and super speed typhoons. for different It shouldintensity be groups. noted The that maximum the relationship wind speed for theis mostly location of the maximumlocated to the surface right windside of and the thestorm wind motion shear dire magnitudection for all also intensity holds groups. for tropical The azimuth storm-strength location TCs. The linearof the maximum regression wind coeffi speedcients has of a Figures tendency9 and to move 10 are from presented the right-front in Table quadrant4. to the right side and then to the right-rear quadrant, as the TC translation speed increases for all intensity groups.

Table 4. Linear regression coeﬃcients for the maximum wind speed position (θmax) with respect to the typhoon translation speed (Vstorm)[θmax = a + bVstorm] (Figure9), and also with respect to the environmental vertical wind shear magnitude (Vshear)[θmax = c + dVshear] (Figure 10) for diﬀerent storm intensity groups.

Maximum Wind Speed Position θmax=a+bVstorm θmax=c+dVshear a b c d tropical storm 9.2108 7.0214 –33.802 –2.2985 − typhoon 43.619 1.4731 –25.739 –3.8484

severe typhoon 3.574 6.7655 26.554 –6.3546 super typhoon 30.355 2.4134 –5.9658 –3.2414 Remote Sens. 2019 11 of 19

Remote Sens. 2019, 11, 2604 11 of 18 Remote Sens. 2019 11 of 19

Figure 9. Maximum wind speed position (𝜃) versus the typhoon translation speed (𝑉) for (a) a tropical storm, (b) typhoon, (c) severe typhoon, and (d) super typhoon, as shown in Figure 3. The black solid and dashed lines represent linear regressions and 95% confidence intervals for the fits,

respectively. 𝜃 (°) is in the clockwise azimuthal direction relative to the typhoon movement direction. RM indicates that 𝜃 is located on the right side of the TC motion, LM indicates that

𝜃 is located on the left side of the TC motion, FM indicates that 𝜃 is located to the forward of

the TC motion, and BM indicates that 𝜃 is located behind the TC motion.

Similarly, the relationship between the environmental vertical wind shear and maximum wind Figure 9. Maximum wind speed position (𝜃 ) versus the typhoon translation speed (𝑉 ) for (a) speedFigure position 9. Maximum is illustrated wind in speed Figure position 10. The (θmax maximum) versus the wind typhoon speed translation mainly occurs speed (inV stormthe )downshear for (a) aa tropicaltropical storm,storm, ((bb)) typhoon,typhoon, ((cc)) severesevere typhoon,typhoon, andand ((dd)) supersuper typhoon,typhoon, asas shownshown inin FigureFigure3 3.. The The and downshear-left quadrants for all intensity groups. The azimuthal location of the maximum wind blackblack solidsolid andand dasheddashed lines represent linear regressionsregressions and 95% conﬁdenceconfidence intervals for the ﬁts,fits, speed has a tendency to shift from the downshear to the left side of the wind shear when the shear respectively.respectively.θ 𝜃max( )(°) is inis thein clockwisethe clockwise azimuthal azimuthal direction direction relative relative to the typhoon to the typhoon movement movement direction. magnitude increases◦ for different intensity groups. These results are consistent with [16] for RMdirection. indicates RM that indicatesθmax is that located 𝜃 on is the located right sideon the of theright TC side motion, of the LM TC indicates motion, that LMθ indicatesmax is located that typhoons, and severe and super typhoons. It should be noted that the relationship for the location of on𝜃 the is left located side of on the the TC left motion, side of FM the indicates TC motion, that FMθmax indicatesis located that to the𝜃 forward is located of the to TC the motion, forward and of the maximum surface wind and the wind shear magnitude also holds for tropical storm-strength TCs. BMthe TC indicates motion, that andθmax BMis indicates located behind that 𝜃 the TC is located motion. behind the TC motion. The linear regression coefficients of Figures 9 and 10 are presented in Table 4. Similarly, the relationship between the environmental vertical wind shear and maximum wind speed position is illustrated in Figure 10. The maximum wind speed mainly occurs in the downshear and downshear-left quadrants for all intensity groups. The azimuthal location of the maximum wind speed has a tendency to shift from the downshear to the left side of the wind shear when the shear magnitude increases for different intensity groups. These results are consistent with [16] for typhoons, and severe and super typhoons. It should be noted that the relationship for the location of the maximum surface wind and the wind shear magnitude also holds for tropical storm-strength TCs. The linear regression coefficients of Figures 9 and 10 are presented in Table 4.

Figure 10. Maximum wind speed position (θmax) versus the environmental vertical wind shear magnitude (Vshear) for (a) a tropical storm, (b) typhoon, (c) severe typhoon, and (d) super typhoon, as shown in Figure3. The black solid and dashed lines represent linear regressions and 95% conﬁdence

intervals for the ﬁts, respectively. θmax(◦) is clockwise azimuth relative to the wind shear direction. RS indicates that θmax is located on the right side of the wind shear, LS indicates that θmax is located on the left side of the wind shear, DS indicates that θmax is ‘downshear’, and US indicates that θmax is ‘upshear’.

Remote Sens. 2019 12 of 19

Figure 10. Maximum wind speed position (𝜃 ) versus the environmental vertical wind shear magnitude (𝑉) for (a) a tropical storm, (b) typhoon, (c) severe typhoon, and (d) super typhoon, as shown in Figure 3. The black solid and dashed lines represent linear regressions and 95% confidence

intervals for the fits, respectively. 𝜃(°) is clockwise azimuth relative to the wind shear direction. RS indicates that 𝜃 is located on the right side of the wind shear, LS indicates that 𝜃 is located

on the left side of the wind shear, DS indicates that 𝜃 is ‘downshear’, and US indicates that 𝜃 is ‘upshear’.

Table 4. Linear regression coefficients for the maximum wind speed position (𝜃) with respect to

the typhoon translation speed (𝑉) [𝜃 =𝑎+𝑏𝑉] (Figure 9), and also with respect to the

environmental vertical wind shear magnitude (𝑉) [𝜃 =𝑐+𝑑𝑉] (Figure 10) for different storm intensity groups.

Maximum Wind Speed Position 𝜽𝒎𝒂𝒙 =𝒂+𝒃𝑽𝒔𝒕𝒐𝒓𝒎 𝜽𝒎𝒂𝒙 =𝒄+𝒅𝑽𝒔𝒉𝒆𝒂𝒓 a b c d tropical storm −9.2108 7.0214 –33.802 –2.2985 typhoon 43.619 1.4731 –25.739 –3.8484 severe typhoon 3.574 6.7655 26.554 –6.3546 Remote Sens. 2019, 11, 2604 12 of 18 super typhoon 30.355 2.4134 –5.9658 –3.2414

To simultaneously simultaneously investigate investigate the the impacts impacts of ofstorm storm motion motion and andwind wind shear shear on TC on surface TC surface wind asymmetry,wind asymmetry, the TC the samples TC samples were divided were divided into two into categories two categories based basedon the onmagnitudes the magnitudes of the storm of the translationstorm translation speed speedand vertical and vertical wind windshear, shear,for each for eachintensity intensity group. group. As shown As shown in Figure in Figure 11, the 11, maximumthe maximum wind wind speed speed occurs occurs favorably favorably to the to left the side left sideof the of wind the wind shear shear and andto the to right the right side sideof the of stormthe storm motion motion for forall alltyphoon typhoon intensity intensity groups, groups, which which is isconsistent consistent with with [19]. [19]. The The relative relative strength strength between the storm translation speed and wind shear magnitude magnitude has has the the potential potential to to affect aﬀect the wind maximum location. As As shown shown in in Figure Figure 11,11, the the maxi maximummum wind speed mostly occurs to the right side of the storm motion when the storm translation spee speedd is larger than the magnitude of the wind wind shear shear (blue diamonds in Figure 11).11). By By contrast, contrast, when when th thee storm storm translation translation speed is smaller than the wind shear magnitude (red(red crossescrosses inin FigureFigure 11 11),), the the maximum maximum wind wind speed speed mostly mostly occurs occurs to to the the left left side side of ofthe the wind wind shear shear direction. direction.

Figure 11. Azimuth angle of the wind maximum to the storm movement versus the azimuth angle of the wind maximum to vertical wind shear for (a) a tropical storm, (b) typhoon, (c) severe typhoon, and (d) super typhoon as shown in Figure3. The blue diamond indicates that Vstorm is larger than Vshear, and the red cross indicates that Vstorm is smaller than Vshear.

4.5. Influence of the Angle Difference between the Storm Motion and Wind Shear on Wind Asymmetry The 125 TC samples were divided into four groups to explore the influence of the angle difference between the vertical wind shear and storm motion on the surface wind asymmetry. The composite wind speed of each group is shown in Figure 12. Similar to Figure5, Figure 12 shows the normalized wind speed versus the normalized radius in the wind shear-relative framework. The maximum wind speed (black star) rotates clockwise from LSHR, USHR, and RSHR, to DSHR, as defined in Section 3.3. The maximum wind speed mainly occurs in the left-front and left-rear quadrants for the RSHR, USHR, and LSHR groups, which agrees with the results shown in [16]. However, the maximum wind speed in the DSHR group occurs to the right side of the wind shear. Remote Sens. 2019 13 of 19

Figure 11. Azimuth angle of the wind maximum to the storm movement versus the azimuth angle of the wind maximum to vertical wind shear for (a) a tropical storm, (b) typhoon, (c) severe typhoon,

and (d) super typhoon as shown in Figure 3. The blue diamond indicates that 𝑉 is larger than

𝑉, and the red cross indicates that 𝑉 is smaller than 𝑉.

4.5. Influence of the Angle Difference between the Storm Motion and Wind Shear on Wind Asymmetry The 125 TC samples were divided into four groups to explore the influence of the angle difference between the vertical wind shear and storm motion on the surface wind asymmetry. The composite wind speed of each group is shown in Figure 12. Similar to Figure 5, Figure 12 shows the normalized wind speed versus the normalized radius in the wind shear-relative framework. The maximum wind speed (black star) rotates clockwise from LSHR, USHR, and RSHR, to DSHR, as defined in Section 3.3. The maximum wind speed mainly occurs in the left-front and left-rear quadrants for the RSHR, USHR, and LSHR groups, which agrees with the results shown in [16]. However, the maximum wind speed in the DSHR group occurs to the right side of the wind shear. The distribution of the asymmetry parameter for each group was also evaluated for the storm motion and wind shear directions. As shown in Figure 13, the DSHR and LSHR groups have a larger degree of surface wind asymmetry than the USHR and RSHR groups. On average, when the direction of the wind shear is nearly the same as the direction of the storm motion, the wind asymmetry degree is the largest; and as the wind shear direction is opposite to the storm motion direction, the wind Remoteasymmetry Sens. 2019 degree, 11, 2604 is the smallest. These results are consistent with those shown by [22]. 13 of 18

FigureFigure 12.12. CompositesComposites ofof normalizednormalized two-dimensionaltwo-dimensional SMAP wind speed ﬁeldsfields for:for: (a) WindWind shearshear directiondirection andand stormstorm motionmotion directiondirection areare equalequal (DSHR);(DSHR); ((bb)) windwind shearshear isis onon thethe leftleft sideside ofof thethe stormstorm motionmotion (LSHR);(LSHR); ((cc)) windwind shearshear andand stormstorm motionmotion areare inin thethe oppositeopposite directiondirection (USHR);(USHR); andand ((dd)) windwind shearshear isis onon thethe rightright sideside ofof thethe stormstorm motionmotion (RSHR).(RSHR). OtherOther detailsdetails areare thethe samesame asas inin FigureFigure5 .5.

The distribution of the asymmetry parameter for each group was also evaluated for the storm motion and wind shear directions. As shown in Figure 13, the DSHR and LSHR groups have a larger degree of surface wind asymmetry than the USHR and RSHR groups. On average, when the direction of the wind shear is nearly the same as the direction of the storm motion, the wind asymmetry degree is the largest; and as the wind shear direction is opposite to the storm motion direction, the wind asymmetryRemote Sens. 2019 degree is the smallest. These results are consistent with those shown by [22]. 14 of 19

Figure 13. Histogram of the asymmetry parameter (ε) for: ( (a)) Wind Wind shear and storm motion directions are equalequal (DSHR);(DSHR); (b)) windwind shearshear isis onon thethe leftleft sideside ofof thethe motionmotion (LSHR);(LSHR); ((cc)) windwind shearshear andand stormstorm motion areare in in the the opposite opposite direction direction (USHR); (USHR); and and (d) wind(d) wind shear shear is on is the on right the sideright of side the stormof the motion storm (RSHR).motion (RSHR). The number The ofnumber storms of in storms each group in each and group the mean and valuethe mean of ε forvalue each of group ε for areeach indicated. group are indicated.

Remote Sens. 2019, 11, 2604 14 of 18

5. Discussion Previous studies have shown that both storm motion and environmental vertical wind shear are the primary factors that account for TC surface wind asymmetries [15,16,19,21,22]. However, it is still not clear which factor has greater influence on the TC asymmetries. In this study, the relationships among TC surface wind asymmetry, storm motion, and vertical wind shear were further explored using SMAP-measured winds. An analysis of composite wind fields of the intensities for tropical storm and typhoon groups show that the location of the maximum wind speed approximately occurs on the left side of the wind shear (Figure5a,b), which further corroborates the previous results reported in [21,22]. Note that the average maximum wind speeds of the composite wind fields from the SMAP wind data were significantly larger than those derived from scatterometer winds [21,22], as shown in Figure5c,d, especially for severe typhoons and super typhoons. Moreover, for these two categories, the positions of the maximum wind speed were found to be closer to left of ‘downshear’ and ‘upshear’, which are different from those shown in Figure5a,b in this study, Figure4 in [ 21], and Figure5 in [ 22]. TC structures tend to be more symmetrical for the range from the center to about 4 RMW, with × increasing storm intensity, especially for super typhoons, whether in shear-relative or in motion-relative coordinate frameworks. The dependence of TC asymmetries on storm motion and environmental vertical wind shear is probably based on the estimated asymmetric parameter. It was shown that the asymmetry parameter increases with increasing TC translation speed and vertical wind shear magnitude for the tropical storm and super typhoon categories (Figure7a,d and Figure8a,d); however, this tendency was not given by [16]. The influence of the angle difference between the storm motion and vertical wind shear on TC asymmetries was also examined by using the composite wind field analysis. The maximum wind speed position rotates on the basis of the angle difference between the storm motion and the wind shear; however, the order of rotation is different from previous studies [16,22]. This difference may be due to the different TC intensity ranges between our study and other studies [16,22], because [16] only included hurricane-strength TCs, and in [22], the maximum wind speeds of TC samples were not as large as those in our study. In order to further study the relationship between TC asymmetries and intensity, storm motion, and environmental vertical wind shear, SMAP multi-temporal wind observations of three typical typhoons (Atsani, Nangka, and Noru) were selected from 125 samples to conduct individual case study analyses. Figure 14 shows the asymmetry parameter versus intensity, translation speed, and environmental vertical wind shear magnitude for each typhoon. As shown in Figure 14b,e,h, as the translation speed increases, the asymmetry parameter has an obvious increasing tendency for all three typhoons. However, an exception is typhoon Atsani, where the intensity increases, but the asymmetry parameter shows a significant decreasing trend. Additionally, for this case, the asymmetry parameter also increases with the wind shear magnitude, whereas no similar features are found for typhoons Noru and Nangka. The explanation may be that the range of magnitudes for wind shear for these two typhoons (~1.7–7.6 m/s for Noru and ~3.4–8.8 m/s for Nangka) is significantly smaller than that of Atsani (~0.8–13.5 m/s). A time series analysis of key parameters of typhoon Nangka was carried out to further investigate the variations of TC surface wind structure. The results are shown in Figure 15. In the early stage of development (5 July), Nangka was classified as a tropical storm (~24 m/s). It moved westward with a relatively fast translation speed (~6–7 m/s) and a small vertical wind shear (~3.4 m/s); its structure appears to be asymmetric. On 6 July, Nangka was upgraded to a typhoon with a maximum wind speed of about 40.8 m/s. The structure of the surface wind field gradually became symmetrical. On 10 July, Nangka reached the super typhoon level (~51.3 m/s). Subsequently, Nangka began to slowly weaken due to its large vertical wind shear (~8.8 m/s) and the eye became cloud filled. In the evening of 12 July, Nangka commenced to have eyewall replacement and its intensity continued to weaken. As a result, the asymmetric structure again emerged for typhoon Nangka. On 13 July, Nangka started to move Remote Sens. 2019 15 of 19 translation speed increases, the asymmetry parameter has an obvious increasing tendency for all three typhoons. However, an exception is typhoon Atsani, where the intensity increases, but the Remoteasymmetry Sens. 2019 parameter, 11, 2604 shows a significant decreasing trend. Additionally, for this case,15 ofthe 18 asymmetryRemote Sens. parameter 2019 also increases with the wind shear magnitude, whereas no similar 16features of 19 are found for typhoons Noru and Nangka. The explanation may be that the range of magnitudes for northwardwindtranslation shear with for speed a these slow is translationlargertwo thantyphoons the speed magnitude (~1.7–7.6 (~4 m/s). of th Thereafter,m/e swind for shear.Noru the In eyewalland the first~3.4–8.8 replacement observation m/s for wason 10Nangka) completed July, is andsignificantly Nangkathe maximum underwentsmaller wind than speed re-intensification that position of Atsani is located (~0.8–13.5 between on the 13m/s). left and side 15 of July. the storm motion and the wind shear (Figure 15d,e) because the translation speed (~3 m/s) is relatively smaller than the magnitude of the vertical wind shear (~6.3 m/s). In addition, the TC motion direction and the direction of the wind shear are similar (Figure 15c), which is in accordance with the conclusion in [15] that the position of the maximum wind speed occurs on the left side of the storm motion when the angle difference between the storm motion and wind shear is small. The angle difference between the direction of the storm motion and the direction of the wind shear, and relative strength between the storm translation speed and the wind shear magnitude both have an important influence on the position of the maximum wind speed. The existing TC parametric models [4,27,28] only take into account the impact of the storm motion on the structure of wind field while ignoring the effect of the environmental vertical wind shear. Studies have shown that significant wave heights simulated by numerical ocean wave models using asymmetric wind fields are closer to buoy measurements than those resulting from symmetric wind fields [29]. Therefore, it is necessary to refine parametric wind field models by incorporating the effect of wind shear on TC asymmetries, thereby providing the potential to improve the accuracy of TC simulations, storm warnings, and risk assessments. In this study, we used spaceborne radiometer wind data to investigate the effect of storm motion and wind shear on TC wind asymmetry, and moreover, we suggest that the relative strength between the storm translation speed and the wind shear magnitude can affect the location of the maximum wind speed. Compared to spaceborne Ku-band scatterometer winds, L-band SMAP radiometer winds have the advantage that for investigation of TC wind field asymmetry, the radiometer- measured brightness temperatures are largely unaffected by rain and have excellent sensitivity to Figure 14. Asymmetry parameter (𝜀) versus TC intensity (𝑉 ), translation speed (𝑉 ), and Figurewind speed 14. Asymmetrywhen winds parameterare very high. (ε) versus However, TC intensitydue to its (V maxcoarse), translation resolution, speed SMAP (V windsstorm), andare environmental vertical wind shear magnitude (𝑉) for Noru (a–c), Nangka (d–f), and Atsani (g–i). environmentallimited in their vertical ability wind and shear cannot magnitude accurately (Vshear resolve) for Noru the detailed (a–c), Nangka structure (d–f ),of and TC Atsani storms, (g– isuch). Black as solidaccurateBlack and solid RMW, dashed and or dashed lines provide represent lines reliable represent linear radial regressions linear profiles. regre and ssions 95% confidence and 95% confidence intervals for intervals the fits, respectively. for the fits, respectively.

A time series analysis of key parameters of typhoon Nangka was carried out to further investigate the variations of TC surface wind structure. The results are shown in Figure 15. In the early stage of development (5 July), Nangka was classified as a tropical storm (~24 m/s). It moved westward with a relatively fast translation speed (~6–7 m/s) and a small vertical wind shear (~3.4 m/s); its structure appears to be asymmetric. On 6 July, Nangka was upgraded to a typhoon with a maximum wind speed of about 40.8 m/s. The structure of the surface wind field gradually became symmetrical. On 10 July, Nangka reached the super typhoon level (~51.3 m/s). Subsequently, Nangka began to slowly weaken due to its large vertical wind shear (~8.8 m/s) and the eye became cloud filled. In the evening of 12 July, Nangka commenced to have eyewall replacement and its intensity continued to weaken. As a result, the asymmetric structure again emerged for typhoon Nangka. On 13 July, Nangka started to move northward with a slow translation speed (~4 m/s). Thereafter, the eyewall replacement was completed and Nangka underwent re-intensification between 13 and 15 July. In general, the largest wind speeds are found in the eyewall on the right side of the typhoons in the northern hemisphere. The typhoon’s motion (forward speed) contributes to the counterclockwise wind speed. In our study, and also in previous investigations [16,19,21,22], the maximum wind speeds are found to occur mainly on the left side of the wind shear. However, it is difficult to interpret Figure 15. Time series of key parameters of typhoon Nangka: (a) Asymmetry parameter (𝜀) and TC this Figurephenomenon 15. Time based series ofonly key on parameters statistical of typhoonanalysis. Nangka: Based (ona) Asymmetry SMAP wind parameter observations (ε) and TCon five intensity (𝑉 ); (b) translation speed (𝑉 ) and environmental vertical wind shear magnitude differentintensity days (V maxof typhoon); (b) translation Nangka’s speed lifecycle, (Vstorm) and the environmental positions of verticalthe maximum wind shear wind magnitude speed (wereVshear );found (𝑉); (c) TC motion direction (𝜃) and environmental vertical wind shear direction (𝜃); (d) (c) TC motion direction (θstorm) and environmental vertical wind shear direction (θshear); (d) Azimuth angle to occur fourAzimuth times angle on of the wind right maximum side of (𝜃 the )storm relative motion to storm (Figuremovement; 15d). (e) Azimuth Three of angle these of thefour wind occurrences of wind maximum (θmax) relative to storm movement; (e) Azimuth angle of the wind maximum (θmax) corroboratemaximum our conclusion (𝜃) relative that to vertical the relative wind shear. stre RMngth indicates between that the𝜃 storm is located translation on the right speed side and the relative to vertical wind shear. RM indicates that θ is located on the right side of the TC motion, LM wind shearof the magnitude TC motion, affects LM indicates the position that 𝜃 of is thelocated maxmaximum on the left wind side of speed. the TC Namely,motion, FM the indicates position of the indicates that θ is located on the left side of the TC motion, FM indicates that θ is located forward maximumthat wind 𝜃 ismaxspeed located mostly forward appearsof the TC motion,on the and righ BMt indicates side of that the 𝜃 storm is located motionmax behind when the TC the storm of the TC motion, and BM indicates that θ is located behind the TC motion. RS indicates that θ is max max located on the right side of the wind shear, LS indicates that θmax is located on the left side of the wind shear, DS indicates that θmax is ‘downshear’, and US indicates that θmax is ‘upshear’. Remote Sens. 2019, 11, 2604 16 of 18

In general, the largest wind speeds are found in the eyewall on the right side of the typhoons in the northern hemisphere. The typhoon’s motion (forward speed) contributes to the counterclockwise wind speed. In our study, and also in previous investigations [16,19,21,22], the maximum wind speeds are found to occur mainly on the left side of the wind shear. However, it is difficult to interpret this phenomenon based only on statistical analysis. Based on SMAP wind observations on five different days of typhoon Nangka’s lifecycle, the positions of the maximum wind speed were found to occur four times on the right side of the storm motion (Figure 15d). Three of these four occurrences corroborate our conclusion that the relative strength between the storm translation speed and the wind shear magnitude affects the position of the maximum wind speed. Namely, the position of the maximum wind speed mostly appears on the right side of the storm motion when the storm translation speed is larger than the magnitude of the wind shear. In the first observation on 10 July, the maximum wind speed position is located on the left side of the storm motion and the wind shear (Figure 15d,e) because the translation speed (~3 m/s) is relatively smaller than the magnitude of the vertical wind shear (~6.3 m/s). In addition, the TC motion direction and the direction of the wind shear are similar (Figure 15c), which is in accordance with the conclusion in [15] that the position of the maximum wind speed occurs on the left side of the storm motion when the angle difference between the storm motion and wind shear is small. The angle difference between the direction of the storm motion and the direction of the wind shear, and relative strength between the storm translation speed and the wind shear magnitude both have an important influence on the position of the maximum wind speed. The existing TC parametric models [4,27,28] only take into account the impact of the storm motion on the structure of wind field while ignoring the effect of the environmental vertical wind shear. Studies have shown that significant wave heights simulated by numerical ocean wave models using asymmetric wind fields are closer to buoy measurements than those resulting from symmetric wind fields [29]. Therefore, it is necessary to refine parametric wind field models by incorporating the effect of wind shear on TC asymmetries, thereby providing the potential to improve the accuracy of TC simulations, storm warnings, and risk assessments. In this study, we used spaceborne radiometer wind data to investigate the effect of storm motion and wind shear on TC wind asymmetry, and moreover, we suggest that the relative strength between the storm translation speed and the wind shear magnitude can affect the location of the maximum wind speed. Compared to spaceborne Ku-band scatterometer winds, L-band SMAP radiometer winds have the advantage that for investigation of TC wind field asymmetry, the radiometer-measured brightness temperatures are largely unaffected by rain and have excellent sensitivity to wind speed when winds are very high. However, due to its coarse resolution, SMAP winds are limited in their ability and cannot accurately resolve the detailed structure of TC storms, such as accurate RMW, or provide reliable radial profiles.

6. Conclusions The effects of the storm motion and the environmental vertical wind shear on surface wind asymmetry in TCs were investigated by using SMAP observations in the Northwest Pacific Ocean from 2015 to 2017. The great advantage of the SMAP data is that it can provide accurate TC wind speed measurements up to 70 m/s, thereby allowing studies of the wind asymmetry in TCs with a large range in intensity. The asymmetry parameter was estimated by the ratio of the difference between the maximum wind speed and the azimuthal average of the wind speed, relative to the azimuthal average wind speed at the location of the RMW. The results showed that this asymmetry parameter significantly decreases as the TC intensity increases. The results also showed that the degree of surface wind asymmetry increases significantly with increasing TC translation speed for tropical storms and super typhoons; whereas it has little dependence on the storm motion for typhoons and severe typhoons. However, the asymmetry parameter increases with increasing magnitude of the wind shear for all TC intensity groups. Remote Sens. 2019, 11, 2604 17 of 18

Moreover, the wind maximum tended to occur to the left side of the wind shear direction and to the right side of the storm motion direction in all TC intensity groups. The relative strength between the storm translation speed and the wind shear magnitude also has the potential to aﬀect the location of the maximum wind speed. The position of the maximum wind speed mostly appears to the right side of the storm motion when the storm translation speed is larger than the magnitude of the wind shear. On the contrary, when the storm translation speed is smaller than the wind shear magnitude, the maximum wind speed mostly occurs to the left side of the wind shear direction. The composite analyses show that the wind maximum rotates, in storms where the wind shear is on the left side of the storm motion direction (LSHR), to those in the upshear (USHR) group, to those where wind shear is on the right side of the storm motion direction (RSHR), and to those in the downshear (DSHR) group. A maximum asymmetry exists when the direction of the TC motion is nearly equal to the direction of the wind shear, whereas a minimum asymmetry is found when the directions of motion and wind shear are opposite to each other.

Author Contributions: B.Z. and J.A.Z. conceived the original idea of the study, suggested for the topic, and contributed to the interpretation of the results; Z.S. carried out the satellite and best track data analysis and prepared all of the figures; W.P. assisted in manuscript preparation and revision. Z.S. and B.Z. wrote the manuscript. Funding: This research received no external funding other than what is acknowledging below. Acknowledgments: Z.S. and B.Z. acknowledge support of the National Science Foundation of China for Outstanding Young Scientist under Grant 41622604, the National Key Research and Development Program of China under Grant 2016YFC1401004, the National Program on Global Change and Air-Sea Interaction under Grant GASI-IPOVAI-04. SMAP sea surface wind data are produced by Remote Sensing Systems and sponsored by NASA Earth Science funding. Data are available at www.remss.com/missions/SMAP/winds. The views, opinions, and findings contained in the paper are those of the authors and should not be construed as an official NOAA or U.S. Government position, policy, or decision. Conflicts of Interest: The authors declare no conflict of interest.

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