Convective Asymmetries Associated with Tropical Cyclone ...

1560 VOLUME 60J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S

q 2003 American Meteorological Society

Convective Asymmetries Associated with Tropical Cyclone Landfall.Part I: f -Plane Simulations

JOHNNY C. L. CHAN

Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

XUDONG LIANG

Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong, andShanghai Typhoon Institute, Shanghai, China

(Manuscript received 22 March 2002, in final form 6 January 2003)

ABSTRACT

This study investigates the physical processes associated with changes in the convective structure of a tropicalcyclone (TC) during landfall using the fifth-generation Pennsylvania State University–National Center for At-mospheric Research Mesoscale Model, version 3 (MM5). The land surface is moved toward a spunup vortexat a constant zonal speed on an f plane. Four experiments are carried out with the following fluxes modifiedover land: turning off sensible heat flux, turning off moisture flux, setting a higher surface roughness, andcombining the last two processes.

The results suggest that sensible heat flux appears to show no appreciable effect while moisture supply is thedominant factor in modifying the convective structure. Prior to landfall, maximum precipitation is found to thefront and left quadrants of the TC but to the front and right quadrants after landfall when moisture is turnedoff and surface roughness increased.

To understand the physical processes involved, a conceptual experiment is carried out in which moisturesupply only occurs over the ocean and at the lowest level of the atmosphere, and such supply is transportedaround by the averaged circulation of the TC. It is shown that the dry air over land is being advected up andaround so that at some locations the stability of the atmosphere is reduced. Analyses of the data from the morerealistic numerical experiments demonstrate that convective instability is indeed largest just upstream of wherethe maximum rainfall occurs. In other words, the effect of the change in moisture supply on the convectiondistribution during TC landfall is through the modification of the moist static stability of the atmosphere.

1. Introduction

The problem of tropical cyclone (TC) landfall hasreceived more attention in recent years especially afterit has been listed as one of the foci of the U.S. WeatherResearch Program (Marks et al. 1998). The concernsinclude track oscillations, wind and rainfall distribu-tions, storm surges, etc. Most observational research hasbeen based on individual case studies (e.g., Miller 1964;Parrish et al. 1982; Powell 1982, 1987; Powell andHouston 1996, 1998; Geerts et al. 2000; Blackwell2000; Ching et al. 2000; Ching 2002). While these stud-ies point out peculiarities of meteorological parametersassociated with particular TCs, or even common featuresassociated with different TCs, the understanding of theunderlying physical processes is quite difficult. Nu-

Corresponding author address: Johnny Chan, Dept. of Physics andMaterials Science, City University of Hong Kong, 83 Tat Chee Ave.,Kowloon, Hong Kong, China.E-mail: [email protected]

merical modeling studies therefore become a useful toolin this respect.

Two of the earlier modeling efforts by Ooyama (1969)and Rosenthal (1971) investigated the sensitivity of TCsto surface boundary conditions using a simple model.Ooyama (1969) noted that the cyclone-scale circulationdepends on the convection for the release of heat energy,while the continued and organized activity of convectiveclouds in the storm must depend on the large-scale cir-culation for efficient supply of water vapor. It is alsounlikely that convective activity in a column would besupported mainly by the evaporative supply of watervapor from the part of the sea surface directly belowthe column. However, the evaporation from a large areaunder the TC circulation is extremely important for sup-porting the convective activity in the central region ofthe cyclone. Tuleya and Kurihara (1978, hereafterTK78) simulated TC landfall using a three-dimensional,primitive equation model and investigated the effects ofland friction and evaporation through three experiments:friction on land without evaporation, smooth land with-

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out evaporation, and increase in surface roughness withevaporation. Their study suggested that evaporation de-pletion over land is the single most important parameterin the decay sequence during landfall. Increased surfaceroughness tends to augment boundary layer inflow,which enhances kinetic energy generation so that theeffect of surface dissipation is partially offset by thisgeneration. They also found that during TC landfall, theforward-right quadrant of the TC is a prime location ofmaximum precipitation intensity. To have better reso-lution, Tuleya et al. (1984, hereafter TBK84) used amore sophisticated model with triple-nested movabledomains. A moist land with a different surface temper-ature than the ocean, and a roughness parameter of 25cm, were specified in that study. A constant steeringflow and a variable Coriolis parameter were also in-cluded. They found that the evolution of the wind andpressure fields and the rainfall distribution were signif-icantly changed by different land surface conditions.Tuleya (1994) used an improved version of the Geo-physical Fluid Dynamics Laboratory TC model that in-cludes diurnal radiation and a bulk subsurface layer withexplicit prediction of land temperature to test the sen-sitivity of surface boundary conditions to tropical cy-clone development and decay at landfall. He found thatthe thermal property of the subsurface had a dramaticinfluence on TC development.

While these numerical studies have shown the effectsof land conditions on the TC structure, the underlyingphysical processes are still not clearly understood. Inparticular, changes in one condition often affect otherconditions. For example, the primary effect of an in-crease in roughness is to reduce the wind speed. How-ever, such a reduction leads to a concomitant decreasein heat and moisture fluxes so that it is difficult to isolatethe physical processes involved.

This study therefore represents a numerical modelingeffort to investigate the physical processes that lead tochanges in the convective distribution in a TC prior to,during, and after landfall under idealized conditions.The mesoscale model used is briefly described in section2. Results of a control run are also presented, not onlyto demonstrate the validity of the model, but also toserve as a basis for comparison with other experiments.The basic effects of sensible heat, moisture (latent heat),and momentum fluxes on modifying the convective dis-tributions are described in section 3. While the resultsappear to be reasonable and consistent with a numberof observations, they also deviate from the conclusionsof some modeling studies. To investigate this apparentdiscrepancy, a conceptual experiment is introduced insection 4 to understand the fundamental physical pro-cesses involved. Based on the result from this experi-ment, the numerical data generated in section 3 are an-alyzed to demonstrate that these processes can be usedto explain the present simulation results. Section 5 givesa summary of the study together with future investi-gation directions.

2. The model and control run

Version 3 of the fifth-generation Pennsylvania StateUniversity–Mesoscale National Center for AtmosphericResearch Model 5 (MM5; Dudhia 1993; Grell et al.1994) is employed. There are 301 3 301 grid points inthe x and y directions with a grid spacing of 15 km.The vertical coordinate consists of 16 s (5p/p*) levels.1.0, 0.99, 0.97, 0.94, 0.90, 0.85, 0.79, 0.71, 0.61, 0.49,0.37, 0.27, 0.18, 0.10, 0.04, and 0.00. The value of theCoriolis parameter f is fixed at 208N. The physics op-tions include the Betts–Miller cumulus parameterization(Betts and Miller 1993; Janjic 1994), simple ice scheme(Dudhia 1989), and high-resolution Blackadar planetaryboundary layer (PBL) scheme (Zhang and Anthes 1982;Blackadar 1979).

The initial conditions of all experiments are obtainedby integrating for 36 h a prespecified vortex in a qui-escent atmospheric environment, in the same way asdescribed in Chan et al. (2001). The prespecified vortexhas an initial minimum sea level pressure (MSLP) of980 hPa and a radius of 15 m s21 winds of 250 km.The vertical temperature structure of the environmentis obtained from the reanalysis data of the EuropeanCentre for Medium-Range Weather Forecasts for a par-ticular day over the western North Pacific and the hu-midity values are specified. For further details, the read-er is referred to Chan et al. (2001).

Since the TC cannot move on an f plane, the ‘‘coast-line’’ is moved in the model domain, similar to thestrategy used in TK78. The land and sea parts of thedomain (separated by the coastline) are characterizedby different values of various PBL parameters depend-ing on the experiment to be performed. The coastlineis oriented north–south with a movement of 5 m s21

(i.e., towards the TC from west to east).1 As pointedout by TK78, moving the coastline toward the vortexavoids any complication involving a steering current.Peng et al. (1999) and Chan et al. (2001) indeed foundthat a uniform steering flow could change the vortexintensity in their respective models. However, by elim-inating the steering current, the effect of asymmetriesarising from the superposition of a steering current ontoa vortex, which is present in the real atmosphere, cannotbe simulated. Nevertheless, it is felt that since the ob-jective here is to examine the effects of the changes inthe various fluxes over land on the convective structureof the vortex, the lack of such asymmetries should stillbe acceptable, and in fact desirable. Inclusion of theseasymmetries will be left to future studies.

The control experiment (to be labeled as experiment1 for future reference—see Table 1) is run with oceanPBL conditions throughout the domain. Chan et al.(2001) have demonstrated that the model is capable of

1 It should be pointed out that directions such as east and west arefor reference only and do not have much physical meaning since allexperiments are performed on an f plane.



TABLE 1. Location of the TC center in the model domain in ex-periments 2, 3, and 4. The coordinates are from the bottom-left cornerof the domain.

Experiment 36 h 72 h

Expt 2Expt 3Expt 4

(151, 151)(151, 151)(151, 151)

(151, 150)(151, 148)(151, 149)

FIG. 1. Total hourly rainfall within 300 km at each azimuth from 1 to 36 h after the vortex has been spun upfor 36 h (i.e., 1 h means 37 h of integrating time from the initial conditions) for the control run (experiment 1).

developing and sustaining the vortex. An importantanalysis in the control run is the temporal variation ofrainfall distribution near the TC center (Fig. 1), whichis obtained by summing the hourly rainfall within each15-km, 18 azimuth box (centered on the TC center) outto 300 km. The radius of 300 km is chosen becausealmost all of the rainfall occurs within this radius (the10-cm isohyet after 36 h of integration in most of theexperiments has roughly a radius of this value). It isobvious from Fig. 1 that the maximum rainfall areasrotate cyclonically around the TC center.

3. Basic experiments on the effects of PBL fluxesThree properties can be exchanged between the at-

mosphere and its underlying surface through their re-spective fluxes: sensible heat, moisture (latent heat), andmomentum. Therefore, to examine the changes in thecharacteristics of the TC that is about to make landfall,the following experiments are performed (see also Table1): experiment 2—turning off the sensible heat flux overland; experiment 3—turning off the moisture flux overland; experiment 4—setting the roughness length to be0.25 m over land; and experiment 5—combining theeffects in experiments 3 and 4. As in TK78, landfall isdefined as the instant when the TC center encountersthe coastline.

In the high-resolution Blackadar PBL scheme (Grellet al. 1994), the friction velocity u* is computed from

kV u* 5 max , u* , (3.1) 0z a ln 2 cmz 0



where k is the von Karman constant, cm a nondimen-sional stability parameter that is a function of the bulkRichardson number, u*0 a background value (0.1 m s21

over land and zero over water), and V is given by

2 2 1/2V 5 (V 1 V ) ,a c (3.2)

with Va being the wind speed at the lowest model layer,and Vc a convective velocity defined under unstable andneutral conditions as

1/2V 5 2(u 2 u ) ,c g a (3.3)

which is equal to zero under stable conditions, ug thepotential temperature at the lowest model layer and ua

that at the ground, za the height of the lowest s level,and z0 the roughness parameter. Over land, z0 is specifiedas a function of land-use category. Over water, z0 iscalculated as a function of friction velocity such that

2z 5 0.032u /g 1 z ,0 0c* (3.4)

where z0c is a background value of 1024 m and g theacceleration due to gravity.

In all the experiments, land use at each grid point isset to be water body, and the surface temperature isinvariable with time during the integration. Therefore,the surface temperature will not be affected by thechanges in the heat flux in the PBL. In experiment 2(turning off the sensible heat flux over land) only thesensible heat flux is set to zero over ‘‘land’’ and nothingelse is changed. Similarly, only the latent heat flux isset to zero in experiment 3 (turning off the moistureflux over land). The land in each experiment is thereforenot real land but only an area over which one or moreparameters is/are changed, because the land use is stillthe water body. In other words, when the sensible heatflux is set to zero in experiment 2, the roughness lengthz0 is also calculated according to (3.4). In experiment4 (friction experiment), when calculating the momentumflux, the roughness length z0 over the land is set to 0.25m [that over the sea still calculated according to (3.4)];however, when calculating the sensible and latent heatfluxes, z0 is calculated according to (3.4) for the entiredomain (i.e., both over land and sea). This way, theeffect of friction on the momentum transfer can be iso-lated from the effect due to heat exchanges.

a. Effect of sensible heat flux

In experiment 2, the sensible heat flux is turned offover land while that over the sea surface is the same asin the control run. The largest departure of total rainfallafter 36 h in this experiment from the control one isonly 40 mm (not shown), which is very small relativeto the total rainfall of .1000 mm in the inner area. Theeffect of sensible heat flux change due to landfall istherefore apparently not evident.

b. Effect of moisture (latent heat) flux

In a TC, although most of the latent heat is releasedin the areas near the eyewall and the moisture can betransported into the TC through inflow in the lowertroposphere, the bulk of the water vapor comes fromthe PBL through moisture flux. Therefore, cutting offthe moisture flux over land should have a profound ef-fect on the TC characteristics.

Significant differences between this experiment (ex-periment 3) and the control run begin to occur at ;12h (216 km) before landfall when the maximum rainfalloccurs to the west (i.e., ahead) of the TC (Fig. 2). Then,another maximum to the north of the TC begins to rotatewestward and by 6 h (108 km) before landfall, the max-imum rainfall is located in the west to southwest quad-rant (i.e., in the forward and left quadrants relative tothe TC motion). This maximum remains stationary inthis quadrant up to 9 h after landfall, with the highestvalue occurring due west at around the landfall time.

This result appears to be consistent with time-lapseanimations of radar reflectivities associated with Hur-ricane Andrew (1992) that show a convective cycle withcells forming on the north side of the eyewall, maturingon the west and south sides, and decaying on the eastside (Dodge and Burpee 1993; Willoughby and Black1996). A recent study of radar reflectivities associatedwith four typhoons making landfall near Hong Kongalso showed a maximum in convection to the west (left)of the typhoons at and after landfall (Ching 2002).

It is interesting to note that even when the moistureflux is turned off over land, the rainfall in some areasis still increased (not shown). TK78’s experiment alsosuggested a small increase in the average precipitationrate in the model storm near landfall.

c. Effect of surface friction

In this experiment (experiment 4), the roughnesslength over land is set at 25 cm (same as that in TK78)while the value over the sea is calculated according to(3.4) for the rest of the domain. The temporal variationsof rainfall indicate that when the TC reaches about 108km (6 h) from the coast, the rainfall in the sector fromthe northwest to west and south is enhanced (Fig. 3).After landfall, the maximum rainfall is located in thewest and southwest (i.e., again, forward and left) quad-rants. The difference in the total rainfall between thisexperiment and the control run shows a steady increasefrom the time when the TC is still nearly 300 km fromthe coast, reaching a maximum about 3 h after landfall(Fig. 4). The difference then begins to decrease althoughthe rainfall in experiment 4 is still larger than that inthe control run even at 8 h (144 km) after landfall. Thedistributions of surface wind speeds (decrease overland), divergence and vorticity [both parameters beingpositive (negative) in the offshore (onshore) sector] havesimilar patterns as those in TK78 (and hence not shown).



FIG. 2. As in Fig. 1 except for experiment 3 (moisture flux turned off over land). The shaded area is the amount ofincrease in rainfall (values indicated in the shaded bar at the top of the figure; unit: cm) compared with the control run.The horizontal line at 27 h indicates the time when the coastline meets the vortex center. Only contours of $6 cm areplotted.

d. Changes in track and intensity

Since the experiments are all on an f plane, any dis-placement of the vortex must be due to the specificchanges in the various fluxes. However, it is observedthat the track changes are minimal (Table 1), with ex-periment 3 giving the largest southward displacementof 45 km in 36 h. This suggests that in the absence ofother factors, changes in the track due to landfall aregenerally not significant, which is consistent with theresult of TBK84.

On the other hand, since the amount of latent heatrelease varies among the experiments, the MSLP also

varies (Table 2). While the vortex in the control exper-iment (experiment 1) continues to deepen slightly, thatin experiment 2 maintains its intensity. If the moistureis cut off (experiment 3) or the roughness length in-creased (experiment 4), the vortex weakens by ;10 hPain 36 h. The result from experiment 3 agrees with thatof TK78, but that in experiment 4 does not. One pos-sibility for this discrepancy is in TK78’s statement that‘‘evaporation was not suppressed’’ (section 5b inTK78’s paper). This is interpreted to mean that evenwhen the surface roughness was increased, the moistureflux was maintained. As a result, an increase in surface



FIG. 3. As in Fig. 2 except for experiment 4 (land surface friction). Only contours of $10 cm are plotted.

friction would enhance the amount of surface conver-gence through Ekman pumping so that more moisturebecame available, which led to an increase in the in-tensity of the modeled TC. In experiment 4 of the currentstudy, the moisture flux is reduced because of the de-crease in surface wind, which leads to a decrease inintensity.

e. Combining experiments 3 and 4

In TK78’s basic experiment, the total rainfall beforeTC landfall tends to increase to the right of the stormtrack where winds are onshore. However, a careful ex-amination of the locations of maximum rainfall (theirFig. 10) suggests that these locations actually tend to

occur more often to the left of the TC. During and afterlandfall, the location of maximum precipitation is biasedtoward the forward-right quadrant of the storm. In thecurrent experiments 3 and 4, the maximum rainfall gen-erally exists ahead and in the forward-left quadrant ofthe TC near, during, and after landfall. In other words,an apparent discrepancy between the current study andTK78 exists for the period near and after landfall, whichmay be due to the fact that in experiments 3 or 4, onlyone of the fluxes is specified to be different over land.

To explore this further, an experiment (experiment 5)similar to the basic landfall experiment of TK78 is de-signed, with the moisture flux being cut off and theroughness length set to 0.25 m over land. Prior to land-fall, the location of maximum precipitation still occurs



FIG. 4. Difference in the cumulative total rainfall within 300 km at each hour betweenexperiments 4 and 1. Positive numbers indicate the rainfall in experiment 4 is larger. Unit: cm.

TABLE 2. Minimum MSLP at the TC center in experiments 1, 2, 3,and 4.

Experiment 36 h (hPa) 72 h (hPa)

Expt 1Expt 2Expt 3Expt 4

942942942942

939941952954

in the forward and forward-left quadrants (Fig. 5), sim-ilar to the results of experiments 3 and 4. However, afterlandfall, the location of maximum rainfall shifted to theforward-right quadrant of the TC. Thus, to a large ex-tent, the current result is consistent with that of TK78.In other words, a cutoff in moisture together with anincrease in roughness tends to shift the maximum pre-cipitation at landfall to the forward-right quadrant. Thephysical processes involved are to be discussed in thenext section.

4. Effect of moisture supply

Two main results from the last section are: 1) themaximum rainfall tends to rotate cyclonically, whichsuggests that the rainfall variations should depend onthe three-dimensional wind and moisture distributions

of the TC; and 2) prior to and during landfall, the max-imum rainfall primarily occurs in the forward and for-ward-left quadrants. The results from experiment 3 (nomoisture flux over land) further suggest that the con-vergence (divergence) in the onshore (offshore) areadoes not appear to play a key role in determining theprecipitation distribution, since no change in the windfield exists in this experiment and only the moisture fluxis cut off over land. Furthermore, the rainfall distribu-tions in experiments 3, 4, and 5 before landfall are verysimilar, which suggests that the moisture flux is veryimportant in determining the rainfall pattern before andduring TC landfall since in all three experiments, thecommon element that is modified is the moisture flux.In a case study of Hurricane Donna of 1960, Miller(1964) also concluded that the removal of oceanic heatsource was the primary reason for the weakening ofDonna.

However, the effect of a change in moisture flux inthe convective structure is obviously not local since therainfall pattern is not uniform over land. In other words,some other physical processes must be operating to pro-duce the simulated rainfall distribution when the mois-ture flux is cut off or decreased. To address this issue,a conceptual experiment is performed to understand the



effect of moisture supply on the convective structureassociated with a landfalling TC.

a. Conceptual experimentsTo study the effect of moisture flux, a conceptual

experiment is designed such that the variation of watervapor (represented by the specific humidity q) at eachgrid point is determined by the three-dimensional ad-vection and surface moisture flux:

]q dq5 2V · =q 1 , (4.1))]t dt

s51

where V is the three-dimensional wind vector (u, y, w)and (dq/dt) | s51 is the surface moisture flux.

In this experiment, moisture flux is assumed to existonly at the lowest level (s 5 1), and is controlled ateach time step by

24 2 24.67 3 10 (i 2 ic) 1 ( j 2 jc) . 900(4.2) 0.003dq

2 2(i 2 ic) 1 ( j 2 jc) # 900,5 2 2dt i 2 ic j 2 jc

exp 1 1 2 1 2[ ]22 22

where (ic, jc) is the grid point of the TC center. Sincedq/dt exists only at the lowest level (s 5 1), the qualifier| s51 for dq/dt will be omitted for simplicity. The valueof dq/dt calculated by (4.2) is very close to the moistureflux in experiment 1 at the initial time (Fig. 6). Notealso from (4.2) that the value of dq/dt is independentof time.

The discrete form of (4.1) is

Dq Dq Dq dqq 5 q 2 u 2 y 2 w 1 Dt,t t21 1 2 )[ ]Dx Dy Dz dt t21t21

(4.3)

where Dt is the length of the time step. The domainused to solve (4.3) is similar to that in experiment 1(control run) except that the domain size is smaller. Thehorizontal grid spacing is 15 km with 113 3 113 gridpoints, and 15 vertical levels. The TC center is fixed atthe middle of the domain (ic 5 57, jc 5 57).

The initial value of q at each grid point and each levelis equal to the averaged specific humidity between 1and 36 h in experiment 1. The time step is 90 s, and ateach time step the ‘‘moisture flux’’ at the lowest levelis calculated from (4.2). Because no prognostic equationis available to predict the temporal evolution of the windfields, the wind fields of the control run averaged be-tween 1 and 36 h are used to calculate the advectivequantities. In other words, in solving (4.3), the three-dimensional wind fields do not change with time.

The solution of (4.3) for the control run (experiment1) after 36 h (to be labeled as experiment 6) gives theexpected result that q is advected from the outer areato the eyewall of the TC in the lower levels and sub-sequently moved up to higher levels (Fig. 7). To sim-ulate the effect of moisture flux change due to the en-counter with land, another experiment (experiment 7)is run with dq/dt set to zero over land and the coastline

moved from west to east at a speed of 5 m s21 (i.e.,similar to experiment 3). Because the coastline movestowards the TC, dq/dt is also changed when the seasurface becomes the land surface. Therefore, q at thelowest level should be reduced over land. Subtractingthe value of q in experiment 7 from that in experiment6 will then give a negative value, which will be termednegative anomaly. These negative anomalies will be ad-vected around by the three-dimensional flow.

After 36 h, the moisture anomalies show that moistureflux is reduced in the sector of the TC over land (Fig.8b). Further, these anomalies are advected from the outerinto the inner area through convergence at the lowerlevels and then vertical advection (Fig. 8a). Note thatonly negative anomalies exist since no mechanism existsto increase the moisture.

The time variations of q suggest that prior to landfall,negative anomalies of q due to the cutoff in moistureflux over land occur in the southwest at the surface (9h; Fig. 9) and in the southeast at the lower levels (9 h,s 5 3; Fig. 10). The maximum negative anomalies arefound to the east and northeast at the midtroposphere(s 5 5 and 7; Fig. 10). In other words, the maximumnegative anomaly is rotated cyclonically upward fromsouth to north when the TC moves from sea to land.These anomaly distributions have a profound effect onthe stability of the atmosphere. For example, to the northand west, water vapor is reduced in the midtropospherebut remains at about the same value as in experiment6 at the low levels (because of moisture advection fromthe ocean). As a result, the atmosphere in the northwestto southwestern quadrants becomes more unstable,which then leads to stronger convection, and hence morerainfall. After landfall, these areas should still remainunstable but since the reduction of water vapor now alsooccurs at the lower levels in the north and east, therainfall should be reduced compared to the situation



FIG. 5. As in Fig. 2 except for experiment 5. Only contours of $10 cm are plotted.

before landfall. When the TC moves farther inland, therainfall should become less and less, and the TC shouldweaken.

Because of horizontal and vertical advection, thethree-dimensional anomalies shown in Figs. 9 and 10rotate cyclonically as the TC moves from sea to land.However, this is not exactly the case in experiment 3(see section 4b below). The reason for this differenceis because in experiment 7, the negative anomalies, oncegenerated, will always be present (since no mechanismexists to cause their removal or destruction) and areadvected around by the cyclonic flow and vertical mo-tion of the TC. As a result, these anomalies are accu-mulated and rotated cyclonically. In the more realisticcase (experiment 3), the water vapor anomalies wouldbe vertically advected by upward motion, and removed

by rainfall and other mechanisms such as diffusion etc.Nevertheless, the results in these conceptual experi-ments can be used to provide an understanding of therainfall distributions associated with TC landfall, as willbe seen in the next section.

b. Analyses of the data in the numerical experiments

In interpreting the diagnoses of the data from thenumerical experiments, the following conceptual frame-work needs to be established. First, it is assumed thata reduction of moisture flux from the land surface willlikely decrease the amount of water vapor in the air, andthis drier air is subsequently advected into the TC center.Second, the drier air may not always decrease the pre-cipitation, since it can be advected to higher levels,



FIG. 6. Values of the moisture flux (solid; unit: 1024 kg kg21) along the line across the TC center inthe zonal direction defined by Eq. (4.2). The dashed line is the value (unit: kg m22) in the initialconditions of the control run (experiment 1). The abscissa is the grid number starting from the westside of the domain.

which then increases the instability of the whole columnof air, and enhances the upward motion of the moist airadvected from the sea surface. Finally, rain occurs onlywhen the air is raised up to a certain height (way abovethe lifting condensation level) and not at the very lowlevels.

The potential pseudoequivalent temperature use is giv-en by (Bolton 1980):

230.2857(120.28310 r)1000u 5 Tse 1 2p

3.3763 exp 2 0.002541 2[ TL

233 r(1 1 0.81 3 10 r) , (4.4)]where p is the pressure (hPa), r the mixing ratio (g kg21),T the temperature (K), and TL the temperature at liftingcondensation level computed from

2840T 5 1 55, (4.5)L 3.5 lnT 2 lne 2 4.805

where e is the water vapor pressure (hPa) given by

pre 5 . (4.6)

622 1 r

Since the configuration in the conceptual experimentin section 4a is closest to that in experiment 3 (no mois-ture flux over land), the values of ]use /]z (used as in-dicator of instability) are first calculated at 1-h intervalsusing the data from experiment 3. The asymmetric hor-

izontal distribution of ]u se /]z and q (specific humidity)are then computed by subtracting the wavenumber 0(i.e., the axially symmetric component) from the totalvalue. Since the heaviest rainfall and the strongest ver-tical motion occur within ;100 km from the TC center(not shown), the values of these two quantities within150 km are examined.

As might be expected from the results of the con-ceptual experiment, prior to landfall, dry air from theland is advected into the inner area of the TC in thesouth and east at the low levels and then raised up inthe east and north (18 h; Fig. 11). At the same time,moist air from the sea is advected into the inner areasin the north and west at the low levels and raised up inthe west and south. As the distance between the TC andthe coastline decreases (9 and 18 h; Fig. 11), the neg-ative asymmetries (dry air) at the low levels extendcounterclockwise from south to east and north. How-ever, the negative asymmetries in the midtropospherereach farther downstream due to upward motion andhorizontal advection. For example, at 18 h, the negativeasymmetries cover from the south to east and north ats 5 3, while they are over the east and north at s 55, and even reach to the west at s 5 7. At around landfalltime (27 h; Fig. 11), moisture asymmetries at the lowlevels are positive (negative) in the north and west(south and east) but are nearly opposite in the midtro-posphere. After landfall (36 h; Fig. 11), the distributionsof moisture asymmetries remain generally the same asat 27 h except in the inner areas (within 50 km) wherethe moist (dry) air reaches the southern (northern) partof the TC at the low levels, but dry (moist) air is stillpresent in the northwest (southeast) at s 5 7.



FIG. 7. Distribution of q in experiment 6. (a) Vertical section acrossthe TC center from west to east and (b) horizontal distribution onthe lowest level (s 5 1). Unit: kg kg21. The large values are due tothe fact that moisture cannot be removed in this experiment. (Alldistances are in km.) FIG. 8. Distribution of negative anomalies of q in experiment 7.

(a) Vertical section across the TC center from west to east and (b)horizontal distribution on the lowest level (s 5 1). The long dashedline in (b) is the coastline at 36 h. (All distances are in km.)The vertical distributions of dry and moist air will

decrease or increase the instability, which can be eval-uated by examining the asymmetric distributions of ]use /]z. The asymmetric distributions of q in Fig. 11 suggestthat the distributions of instability in each level will bechanged. In experiment 3, before landfall (18 h; Fig.12), the atmosphere is more unstable to the east at s 53. However, the area of negative asymmetric ]u se /]zcovers from east to north and west at s 5 5 and northto west and south at s 5 7. It is obvious that this ‘‘bandof instability’’ is rotated from the outer area at the lowlevels to the inner areas in the midtroposphere through

horizontal and vertical advection. At around landfall (27h; Fig. 12), instability is mostly confined to the outerareas to the north, east, and south at s 5 3, but coversmost of the northeast to northwest at s 5 5 and almostthe entire western half at s 5 7. The patterns are gen-erally similar after landfall (36 h; Fig. 12). The increased(decreased) instability will lead to increase (decrease)of upward motion, so that enhanced upward motion be-gins to the east and north of TC at the lower levels and



FIG. 9. Distribution of negative anomaly of q (unit: kg kg21) in experiment 7 on the lowest s level at (a) 9,(b) 18, (c) 27, and (d) 36 h. The abscissa and ordinate are the zonal and meridional distances (km) relative tothe TC center, which is represented by the typhoon symbol. The straight long-dashed line indicates the coastline.

migrates to the west and south in the midtroposphere.Since rainfall occurs only after the air is raised to certainheights and not at the very low levels, enhanced rainfallis found to the northwest and southwest of the TC.

An examination of the corresponding distributions ofmoisture and instability for experiments 4 and 5 givesessentially the same patterns. The major difference inexperiment 4 (increased friction over land) is that be-cause the moisture flux over land is only reduced ratherthan cut off, the moisture and instability asymmetriestend to be smaller in magnitude (not shown). In exper-iment 5 (moisture cut off over land plus increased fric-tion), the coupling of these two effects created verysimilar asymmetric distributions to those in experiment3 (cf. Figs. 12 and 13) except for some details. At aroundand after landfall, the maximum negative asymmetriesin experiment 3 (27 and 36 h; Fig. 12) at s 5 7 extendmore southwestward than those in experiment 5 (27 and36 h; Fig. 13). As a result, after landfall, the maximumrainfall shifts towards the northwest in experiment 5

(see Fig. 5) compared with that in experiment 3 (seeFig. 2). These results also suggest that, before and dur-ing TC landfall, the coastal convergence, which existsin experiments 4 and 5 to the north of the TC, is notvery important in determining the rainfall distributions.Of course, after landfall, friction over land has moredistinct effects because a large part of the TC circulationis over land and affected through stronger friction. Sucha decay process after landfall is more complicated andwill be considered in future studies.

5. Summary and discussion

a. Summary

This study investigates the physical processes asso-ciated with changes in the convective structure of atropical cyclone (TC) during landfall using the fifth-generation Pennsylvania State University–NationalCenter for Atmospheric Research Mesoscale Model,



FIG. 10. Negative anomalies of q (unit: kg kg21) at s levels 3 (;920 hPa), 5 (;850 hPa), and 7 (;750 hPa)in experiment 7 at 9, 18, 27, and 36 h. The abscissa and ordinate are the zonal and meridional distance (km)relative to the TC center, which is represented by the typhoon symbol. The straight long-dashed line indicatesthe coastline.

version 3 (MM5). The land surface with properties dif-ferent from the ocean is moved toward a spunup vortexat a constant zonal speed of 5 m s21 on an f plane. Fourexperiments are carried out with the following modi-fications over land: turning off sensible heat flux, turn-ing off moisture flux, setting a higher surface roughness,and combining the last two processes.

The results suggest that sensible heat flux appears toshow no appreciable effect while moisture supply is the

dominant factor in modifying the convective structure.In particular, prior to landfall, maximum precipitationis found to the front and left quadrants of the TC in allthe experiments. If only the moisture is cut off or thesurface roughness is increased, the location of such anasymmetry is generally maintained during and afterlandfall. If both of these processes are included, themaximum rainfall shifts toward the front and right quad-rants, in agreement with the studies of TK78 and



FIG. 11. As in Fig. 10 except for asymmetric distributions of specific humidity in experiment 3. Unit: 0.1 gkg21; contour interval: 0.2 g kg21.

TBK84. Such asymmetries in the rainfall distributionsuggest that the effect of moisture supply is not local.Furthermore, the convergence (divergence) along thecoastline is not very important for producing the rainfallanomalies in the inner area of TC because the maximumprecipitation does not necessarily occur on the con-verging (onshore) side before landfall, and of the factthat in the moisture flux experiment (experiment 3), noextra convergence exists along the onshore side.

To understand the physical processes involved, a con-ceptual experiment is carried out in which moisture sup-ply only occurs over the ocean and at the lowest levelof the atmosphere, and such supply is transported aroundby the averaged circulation of the TC. It is shown thatthe dry air over land (where moisture supply is set tozero) is being advected up and around so that at somelocations, the stability of the atmosphere is reduced (dryair overlying moist air).



FIG. 12. As in Fig. 10 except for the values of ]use/]z in experiment 3. Unit: K km21;contour interval: 1.5 K km21.

Based on the result of this experiment, data from themore realistic numerical experiments are examined. Us-ing the three-dimensional distributions of moisture, thevertical variations of saturated equivalent potential tem-perature are calculated. The results show that instabilityis indeed the largest around and just upstream of wherethe maximum rainfall occurs. In other words, the changein moisture supply modifies the moist static stability ofthe atmosphere and hence the convection distributionduring TC landfall.

To summarize, as a TC makes landfall, the convectionassociated with the TC is modified by the changes inthe fluxes over land. The change in sensible heat fluxappears to have very little effect. The most importanteffect is from a reduction in moisture flux, which occursnot only through a decrease in moisture supply from thesurface, but also through a decrease in wind speed dueto an increase in surface roughness. However, the con-vergence associated with the decrease in surface windspeed on the onshore side of the TC apparently does



FIG. 13. As in Fig. 10 except for the values of ]use/]z in experiment 5. Unit: K km21; contour interval: 1.5 Kkm21.

not produce an increase in rainfall there. Rather, thechanges in the rainfall distribution results from thethree-dimensional advection of the dry air around theTC, which then changes the moist static stability of theatmosphere. Areas where dry air is present over moistair will likely have enhanced convection, and hencemaximum rainfall occurs at and downstream of thoselocations.

b. Discussion

Although the results from this study are from ideal-ized situations, they are largely consistent with thosefrom previous observational and modeling studies, andthe convective asymmetries can be explained physically.Of course, these situations are far from realistic. Crudeassumptions include the use of an f plane and the coast-



line is moved toward the TC. The former (latter) ignoresthe beta (steering) effect, both of which can contributetoward wind asymmetries, and even convective asym-metries, in the TC. How these asymmetries will modifythe present results needs to be determined through ad-ditional modeling work. Further, an implication fromthis result is that the distribution of rainfall associatedwith TC landfall is largely independent of the directionof approach of the TC. Although Ching (2002) did showthat for four typhoons that hit Hong Kong in 1999 fromdifferent directions, the distributions of convection werequite similar, the extent to which this statement is gen-erally true needs to be ascertained through both mod-eling work with more realistic situations and furtherobservational studies. Finally, the effect of topographyis ignored in the present study. The effects of mountains,water bodies, and even urban environments on the mod-ifications of the convective distributions also need to beinvestigated.

Acknowledgments. The authors would like to thankMr. Simon Ching for helpful discussions. This researchis sponsored by the City University of Hong Kong Stra-tegic Research Grant 7001042 and the National NaturalScience Foundation of China Grant 49975014.

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