Marine-Atmospheric Boundary Layer Characteristics over the South China Sea Durin

时间：2024-09-03

CHENG Xueling,WU Lin,SONG Lili,WANG Binglan, and ZENG Qingcun

1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

2Public Weather Service Center,China Meteorological Administration,Beijing100081

3Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

Marine-Atmospheric Boundary Layer Characteristics over the South China Sea During the Passage of Strong Typhoon Hagupit

CHENG Xueling1,WU Lin1,SONG Lili2,WANG Binglan2, and ZENG Qingcun3∗

1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

2Public Weather Service Center,China Meteorological Administration,Beijing100081

3Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

The structures and characteristics of the marine-atmospheric boundary layer over the South China Sea during the passage of strong Typhoon Hagupit are analyzed in detail in this paper.The typhoon was generated in the western Paci fi c Ocean,and it passed across the South China Sea, fi nally landfalling in the west of Guangdong Province.The shortest distance between the typhoon center and the observation station on Zhizi Island(10 m in height)is 8.5 km.The observation data capture the whole of processes that occurred in the regions of the typhoon eye,two squall regions of the eye wall,and weak wind regions, beforeand afterthe typhoon’s passage.The results show that:(a)during the strong wind(averagevelocity≥10 ms−1)period,in the atmospheric boundary layer below 110 m,u¯ is almost independent of height, and vertical velocityw¯ is greater than 0,increasing withu¯ and reaching 2–4 m s−1in the squall regions; (b)the turbulent fl uctuations(frequency＞1/60 Hz)and gusty disturbances(frequency between 1/600 and 1/60 Hz)are both strong and anisotropic,but the anisotropy of the turbulent fl uctuations is less strong; (c)u¯ can be used as the basic parameter to parameterize all the characteristics of fl uctuations;and(d)the vertical fl ux of horizontal momentum contributed by the average fl ow(u¯·w¯)is one order of magnitude larger than those contributed by fl uctuation fl uxesandimplying that strong wind may have seriously disturbed the sea surface through drag force and downward transport of eddy momentum and generated large breaking waves,leading to formation of a strongly coupled marine-atmospheric boundary layer.This results inw¯＞0 in the atmosphere,and some portion of the momentum in the sea may be fed back again to the atmosphere due tou¯·w¯＞0.

Typhoon Hagupit,marine-atmospheric boundary layer,turbulent fl uctuation,gusty disturbance,air-sea interaction,South China Sea

1.Introduction

Oceans occupy two-thirds of the world’s surface, and the in fl uence of air-sea interaction on weather process and climate change is much more signi fi cant than that of land-atmosphere interaction(Ola et al.,2005). The disastrous weather and climate phenomena in marine areas are more severe than those in land areas. However,due to the limitation of observational conditions,it is difficult to make measurements of the marine-atmospheric boundary layer.Thus,to date, observations and studies of the marine-atmospheric boundary layer remain inadequate,especially for situations involving strong winds.Most existing work concerning the marine-atmospheric boundary layer has involved applying the results obtained in laboratory-based fl uid dynamics experiments and atmospheric boundary layer station observations.The laws governing the characteristics of the marine-atmospheric boundary layer under windy conditions have not yet been clearly revealed.Actually,strong wind occurs much more frequently above the oceanic surface than above the land surface.Driven by the strong wind,the oceanic surface current and breaking waves together with the atmospheric motion form a coupled marineatmospheric boundary layer,constituting a complicated air-sea system.

In the past two decades,due to increased interest in tropical storms,hurricanes,and their formation and evolution,as well as the signi fi cant impact of strong winds on marine engineering and operation, atmospheric and oceanographic scientists,along with structural engineers,have made some important observations and analyses of strong marine winds,especially those associated with hurricanes and typhoons. Although such observations have largely been made on a case-by-case basis,they are still extremely valuable.Observations of this kind are mostly performed by dropsonde,aircraft,Doppler radar,and so on(e.g., Franklin et al.,2002;Powell et al.,2003;Schroeder and Douglas,2003;Knupp et al.,2005;Aberson et al., 2006;French et al.,2007;Kudryavtsev and Makin, 2007;Sanford et al.,2007;Zhang et al.,2008;Zedler et al.,2009).Some valuable directly observed data have also been obtained during typhoon landfalls and when tropical cyclones passing over islands or marine observational platforms(Sparks,2001;Xu and Zhan, 2001;Song et al.,2005,2010;Harper,2008;Cao et al., 2009;Li et al.,2010;Chen et al.,2011;Liu et al.,2011; Peng et al.,2012;Xiao et al.,2012).In particular,the works of Song et al.(2005,2010)were based on strong typhoons such as Damrey,Nuri,Chunchi,Prapiroon, Hagupit,and so on.They focused on studying the engineering problems faced by structures such as buildings,bridges,and wind power installations.

Using the data of Song et al.(2005,2010),in this paper we focus on the marine-atmospheric boundary layer characteristics,such as those concerning turbulence and wind gusts,and their parameterization. We will employ our own specially developed method, which has already been successfully applied to the previous studies of the boundary layer during strong winds related to sand storms and cold surges(Zeng, 2006;Cheng et al.,2007;Zeng et al.,2010;Cheng et al.,2011,2012a,b).We divide the fl uctuations(or turbulences)into two parts,one being high-frequency turbulent fl uctuation(frequency higher than 1/60 Hz), and the other being gusty wind disturbance(frequency between 1/600 and 1/60 Hz).The two parts have different structures and play di ff erent roles,such as in the transport of heavy aerosol particles(Zeng,2006; Zeng et al.,2010;Cheng et al.,2012b).Meanwhile, by comparing with strong cold-surge cases(Cheng et al.,2014),we show that there are many common features in the marine-atmospheric boundary layer under strong winds.

2.Data and instruments

In China,there are many marine meteorological observational stations in the coastal areas of Guangdong Province and over the islands of the South China Sea.Certain stations play host to periods of denser and one-o ffobservational campaigns during the typhoon season to monitor the landfalling of particular typhoons of interest.The station on Zhizi Island is one such station.The data used in this study were mainly obtained from Zhizi(additional data from some other coastal meteorological stations were also used).For a detailed description of this station and its instruments, readers can refer to Liu et al.(2011)and Xiao et al. (2012).However,for convenience,a brief overview is provided as follows.

Zhizi Island is located o ffthe coast of Bohe, Maoming City.The shortest o ff shore distance is 4.6 km,and the exposed part of the island is about 90 m long and 40 m wide(Fig.1).The observation tower on Zhizi is 100 m high atand the base altitude is 10 m above sea level.The surrounding water depth is 6–10 m.There are 6 sets of cup anemometers(NRG-Symphonie type)installed at heights of 10,20,40,60,80,and 100 m on the tower, and 3 sets of wind direction observation instruments at heights of 10,60,and 100 m.In the typhoon seasonfrom August 2008 to August 2010,there was also one ultrasonic anemometer at 60-m height(Gill-Windmaster Pro.;sampling frequency 10 Hz).

Fig.1.Location of Zhizi Island and trajectory of Typhoon Hagupit(after Liu et al.,2011).

3.Typhoon Hagupit and its structure and development

Typhoon Hagupit formed on 19 September 2008 in the western Paci fi c Ocean near the Philippines. It moved westward and strengthened into a strong typhoon,and landed at 0645 BT 24 September near Chen Village,Bohe,Maoming City,Guangdong Province(Liu et al.,2011;Xiao et al.,2012).The trajectory of Hagupit is shown in Fig.1.The shortest distance between the typhoon center and Zhizi station is 8.5 km.During the typhoon’s landing,the 10-min averaged wind speed reached force 15(48.5 m s−1)at Bohe meteorological station near Zhizi,and the 3-s instant maximum gust reached 63.9 m s−1,as observed at 60-m level of the Zhizi tower.The lowest surface pressure was 956 hPa,which lasted for 8 min(Song et al.,2010;Liu et al.,2011;Xiao et al.,2012).

There have been many detailed analyses of the 10-min averaged velocity characteristics and turbulent statistics of Hagupit(Song et al.,2010;Liu et al.,2011; Xiao et al.,2012),but the goals of these studies were to examine the typhoon’s e ff ects on engineering structures.Based on these works,we reanalyzed the original data with added quality control tests.The 10-min averaged wind velocities(u¯,w¯)that we reanalyzed this time were the same as those in the previous studies. However,we now employ a di ff erent method in turbulence decomposition to analyze the characteristics of fl uctuations,and hope to obtain some new results.

Figure 2 shows the 10-min averaged horizontal wind speedu¯ recorded by the cup anemometers at 6 levels and the ultrasonic anemometer at 1 level on the tower.Figure 2 captures the whole process of Hagupit, as observed by Zhizi station from 0000 BT 22 to 2400 BT 27 September,including the region of the typhooneye,two regions of eye wall squall,and the outside fi eld of the typhoon before(24 September)and after (25 September)its landing.Figure 3 showsu¯ andw¯ observed by the ultrasonic anemometer,but only for 24 September 2008.

Fig.2.Time series of 10-min averaged horizontal velocityat 6 levels by cup anemometers and 1 level by ultrasonic anemometer on the Zhizi tower during 22–24 September 2008.

These two fi gures indicate that the results of the cup anemometers and the ultrasonic anemometer are consistent.There are some clear characteristics in the average fl ow(¯u, ¯w).Firstly,in the 20–110-m layer (above sea surface), ¯uis almost independent of height (note that by adding the data of the highest velocity from Bohe station during the typhoon’s landing,we could conclude that ¯ufrom sea level to 110 m is the same).Only for a very short time during the period of strongest wind(＞35 m s−1)is there an exception—the di ff erence between ¯uat the highest two levels is about 10 m s−1.Secondly,at the 60-m level(70 m above the sea surface),there is upward motion( ¯w＞0);speci fi cally, ¯wreaches 2–4 m s−1at the squall wall, and 0.5–1 m s−1in the typhoon eye region.The variable ¯wincreases with ¯u.Only for a very short time after the typhoon’s passage is ¯wreduced to nearly 0 m s−1.This is an important point,and why ¯wis＞0 m s−1is to be discussed in the last section of this paper.It should be noted that the observed ¯wis correct,because before and after the typhoon’s passage the di ff erence in wind direction is about 180°,but the observed ¯wdoes not change sign,and it is unlikely that the inclinometer has produced any error.

Figure 4 shows the pairs(¯u, ¯w)and their regression.It can be seen that the accuracy of the regres-

Fig. 3. (a)and(b)obtained by the ultrasonic anemometer at 60-m level of the Zhizi tower on 24 September 2008.

Fig.4.Pair pointsand their regression.

sion is high.Note that here ¯uis considered as the wind velocity at 10 m above sea surface,as suggested in the literature.

4.Characteristics of the turbulent fl uctuations and gusty wind disturbances

The methods by Zeng et al.(2010)and Cheng et al.(2011,2012a,2014)are employed here to analyze the characteristics of fl uctuations(u′,v′,w′)of the ultrasonic anemometer data,whereu′=u-¯u;v′=v,=0;w′=w-¯w,anduis along the downwind direction.The averaging interval is 10 min.We further divide the fl uctuations into two types:turbulent fl uctuations(ut,vt,wt)and gusty disturbances (ug,vg,wg).The former consists of components with frequency＞1/60 Hz,and the latter with frequency between 1/600 and 1/60 Hz.The variableui(i=t,g) is along the 10-min averaged wind direction.The data are analyzed for the whole period,i.e.,22–27 September 2008,but with special attention paid to the strong wind period(≥ 10 m s−1).

Fig.5.Gusty disturbancesugand turbulent fl uctuationsutduring 0400–0500 BT 24 September 2008.

Figure 5 shows the gusty disturbancesugand turbulent fl uctuationsutfor the eye wall squall period (0400–0500 BT 24 September 2008),as denoted by the two vertical lines in Fig.3.It can be seen that the gusty disturbances and turbulent fl uctuations are all quite strong.By usingas the abscissa,the turbulence kinetic energy(Et)and the gusty amplitude of the horizontal component(Agh)are given in Figs. 6 and 7,respectively.Three direction components of turbulence energy(Etu,Etv,Etw)and three components of gusty amplitude(Agu,Agv,Agw)are also given in Figs.6 and 7,whereEt=Etu+Etv+Etw,and the kinetic energy of gustsEgi=(i=u,v,w).It can be seen that during the strong wind period,the anisotropy of gusty disturbances(Agu＞Agv＞Agw)is signi fi cant,and the turbulent fl uctuations are also anisotropic to some extent,but weaker than that of gusts.This is a common characteristic of windy atmospheric layers,no matter if they are above the land surface(Zeng et al., 2010;Cheng et al.,2011)or above the oceanic surface (Cheng et al.,2014);however,during strong typhoons, this feature is the strongest.

The equivalent period of gusts(Tg,another useful index in practical applications)and its regression are given in Fig.8.The frequency of the gusty wind disturbance is between 1/600 and 1/60 Hz.There are di ff erent periods of gusts in the atmospheric boundary layer.The equivalent period of gusts refers to the main period of gusts by analyzing their power spectra. Note that the points in the region with ¯u＜10 m s−1are scattered due to the randomness and spontaneity of gust occurrences during weak wind cases,but this does not lead to a serious problem in the practical application of the regression because gusts are weak and present only a small in fl uence on heat and mass transport in such situations.

5.Friction velocity and vertical transport of momentum

Fig.6.Kinetic energy of turbulent fl uctuation(Et)and its three components(Etu,Etv,Etw).The regressions are also given in solid lines.

Figure 9 is the superposition ofugandwgduring 0400–0500 BT 24 September 2008.It shows very strong coherence at most moments:ug＞0 is accompanied bywg＜0,andug＜0 bywg＞0.Therefore, the downward vertical fl ux of momentum,contributed by gusty disturbances,is large.The vertical fl ux of momentum contributed by turbulent fl uctuations is also downward.The absolute values of both parts of the momentum fl uxes are equal toandrespectively.Figures 10a and 10b show respectively the gusty friction velocity(ug∗)and turbulent friction velocity(ut∗).Figures 10c and 10d also showu∗(the friction velocity in the conventional sense)andum∗, where (Cheng et al.,2007),um∗is called the average fl ow friction velocity,and the related momentum fl ux is ¯u·¯w,i.e.,In the case of Hagupit, fl uctuations cause downward fl uxes of momentum,but the average fl ow causes upward fl ux.The upward momentum fl ux by the average fl ow is at least one order of magnitude larger than the downward momentum fl ux by fl uctuations,although the latter is very much larger than that under weak wind conditions.It seems that the 10-min average motion is related to the transport of momentum from ocean to the atmosphere during the typhoon period. Note that in strong wind situations,ug∗,ut∗,u∗,and evenum∗can be parameterized by ¯u,as shown by the high accuracy of the regressions in Fig.10.

The sensible heat fl ux and latent heat fl ux are not given in this paper because the heat and water vapor exchanges between the atmosphere and the sea spray and spume generated by breaking waves and entrained into the atmospheric boundary layer are special subjects of research,and will thus be discussed in future work.

6.Summary and discussion

Fig.7.Horizontal amplitude(Agh)and its three component amplitudes(Agu,Agv,Agw)of the gusty disturbances and associated regressions.

Fig.8.The equivalent period of gusty disturbances(Tg) and the associated regression.

Fig.9.Superposition ofug(black line)andwg(red line) during 0400–0500 BT 24 September 2008.

Fig.10.Parameterized friction velocity.(a)Gusty friction velocity,(b)turbulent friction velocity,(c)friction velocity in the conventional sense,and(d)average fl ow friction velocity.

Analyses of the marine-atmospheric boundary layer during strong Typhoon Hagupit in September 2008 and the windy atmospheric boundary layer related to a cold surge in March 2012 over the South China Sea(Cheng et al.,2014)indicate that in the marine-atmospheric boundary layer during strong winds,the characteristics of both the average fl ow (u¯,w¯)and fl uctuations(turbulences and gusts)possess many similarities but also some di ff erences with those during the strong wind cases over the land surface(Zeng et al.,2010;Cheng et al.,2011).The major fi ndings of the study can be summarized as follows.

(1) In the low-latitude marine-atmospheric boundary layer,during strong wind periods associated with tropical storms,hurricanes,or cold surges, the horizontal average velocityu¯ is almost independent of height below about 100 m,and the vertical velocityw¯ is greater than 0 m s−1.This is di ff erent from land surface cases related to cold air mass outbreaks,whereu¯ increases rapidly with height,andw¯ may be negative(descending cold air).

(2)During the strong wind periods,in the lower part of the atmospheric layer,whether over the ocean or land,gusty wind disturbances are anisotropic and coherent,and turbulent fl uctuations are also anisotropic to some extent(with vertical kinetic energy signi fi cantly less than the horizontal one)but with weak coherency.

(3)During strong wind periods,the energies of gusts(Egi)and turbulences(Eti)(i=u,v,w),as well as the corresponding friction velocities(ug∗andut∗), are all much larger than those in weak-wind situations.The vertical fl uxes of momentum contributed by gusts and turbulences are all downward.They can be parameterized by using ¯uas the controlling factor in marine boundary layer cases because of the independence of ¯uto height.However,the top height of our observations is not sufficient for obtaining their vertical pro fi les.

(4)According to our analysis of observationaldata,strong wind is accompanied by upward vertical velocity in the marine-atmospheric boundary layer, and ¯wis ≥ 0.25 m s−1when ¯uis ≥ 10 m s−1,and ¯wis ≥ 1.0 m s−1when ¯u≥ 30 m s−1.This means that the upward transport of horizontal momentum, ¯u·¯w, is ≥ 2.5 and ≥ 30.0 m2s−2,respectively.These values are at least one order of magnitude larger than the downward fl uxes due to the fl uctuations(=0.04 and 0.6 m2s−2,respectively).This fact is important and should be further studied because strong wind occurs frequently above the marine surface.Generally speaking,we can imagine that strong wind(average atmospheric fl ow)and the superimposed fl uctuations drive oceanic surface currents and generate large breaking waves.Thus,a marine-atmosphere coupled boundary layer is formed,and on the atmospheric side,the boundary layer is di ff erent from that over the land, and some portion of the sea momentum can be fed back to the atmosphere from the ocean.This may be the reason why ¯uis independent of height.Furthermore,the sea spray and spume droplets make special exchanges of heat and mass transport between the ocean and atmosphere.

From the reanalysis data of the ECMWF,NCEP, and GFDL(Geophysical Fluid Dynamics Laboratory), we can see that in the middle and high latitudes ¯w≥0(i.e.,ω≡dP/dt≤ 0)at 10 m over a broad area of the oceanic surface when there is a mature cyclone passing over,and ¯w≤ 0(i.e.,ω＞0)occurs only in the cold frontal region near the center of the cyclone. If this can be con fi rmed by observations,it might be true that ¯w≥ 0 at 10 m occurs very often and covers most of the oceanic surface areas during strong winds.

Another concern is about the 10-m wind.According to the Beaufort scale,when the 10-m wind is force 8(17.2–20.7 m s−1),force 10(24.5–28.4 m s−1), and force 12(hurricane;32.7–36.9 m s−1),the significant wave heights are 5.5(usual)to 7.5 m(highest), 9.0(usual)to 12.5 m(highest),and 14.0(usual)to＞16.0 m(highest),respectively.Therefore,the socalled 10-m wind above the oceanic surface is actually meaningless.Fortunately, ¯uis independent of height in the strong wind situation.

Acknowledgments.We are very grateful to the Guangdong Meteorological Bureau for providing some data.

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(Received December 10,2013;in fi nal form January 8,2014)

Supported by the National Natural Science Foundation of China(40830103 and 91215302),National(Key)Basic Research and Development(973)Program of China(2010CB951804),China Meteorological Administration Special Public Welfare Research Fund(GYHY201306057),and Strategy Guide for the Speci fi c Task of the Chinese Academy of Sciences(XDA10010403).

∗Corresponding author:zqc@mail.iap.ac.cn.