Journal of the Meteorological Society of Japan, Vol. 76...

Journal of the Meteorological Society of Japan, Vol. 76, No. 4, pp. 4. 73-496, 1998 473

Mesoscale Structures of Air Flow in a Mei-yu Front Leading Edge Observed by

Aircraft off the East Coast of Taiwan during TAMEX TOP 9

By Tai-Hwa Hor,l Mou-Hsiang Chang

Department of Applied Physics, Chung Cheng Institute of Technology, Taiwan 835, Republic of China

and

Ben Jong-Dao Jou

Department of Atmospheric Sciences, National Taiwan University, Taiwan 106, Republic of China

(Manuscript received, 26 May 1997, in revised form 17 April 1998)

Abstract

In order to investigate the mesoscale structures and appropriate mechanisms for their maintenance in the Mei-yu front leading edge during the TAMEX (Taiwan Area Mesoscale EXperiment) TOP 9 (15 June 1987) off the east coast of Taiwan, we measured the fine-scale in-situ data by different sensors and sensed radar data by two airborne radars mounted on the NOAA P-3 research aircraft traversing the frontal system at six different altitudes. Based upon the sudden increase of the turbulence intensities, the position of the frontal leading edge at each flight level was identified exactly and, after deducting the propagating distance of the frontal system, the composited vertical cross sections of the system in kinematics, dynamics and thermodynamics was feasible.

The significant findings are: the frontal edge was parallel to the isolines of horizontal component of wind and perturbed air density and the frontal edge demonstrated a well-defined density current, the variation of thermodynamic parameters was not obvious, but the pattern of potential temperature revealed a cold core in the lower level behind the frontal edge, which coincided with the position of the heaviest air density. The probable mechanisms for the propagation of the density current and the maintenance of the frontal system were proposed to be the intense horizontal pressure gradient force from rear to front in the cold core region and the moderate convective instability at the head of the system as well as the kinetic energy transport from the mean flow.

1. Introduction

During the late spring and early summer, the weather over southern China, Taiwan and Japan are dominantly influenced by a sort of transition zone called the "Mei-yu" front (or "Baiu" front in Japan), and is frequently oriented from east-north-east to west-south-west in a southeastward slowly moving or quasistationary state. The transition zone forms between the cold, dry northeast monsoon and the warm, moist southwest monsoon flows. The kine-matic migration of the front is greatly controlled by the subtropical warm-core high over the Pacific Ocean and the cold-core high over central China.

The significance of the frontal system is its asso-ciation with MCSs (Mesoscale Convection Systems) and extremely heavy rainfall of at least 100mm/day (Ramage, 1971; Chen, 1983; Tao and Chen, 1987). Previous studiers of the Mei-yu front using conven-tional data sources have emphasized the change of the structure of the Mei-yu front from the synoptic-scale point of view over different areas and time. Chen and Chang (1980) noted that the structure of the eastern section of the Mei-yu front over the ocean showed a typical midlatitude baroclinic fea-ture with a strong horizontal temperature gradient. However, the western part over land featured a weak horizontal temperature gradient and strong horizon-tal wind shear in the lower atmosphere. The onset of the Mei-yu period over the Yangtze River valley was closely related to the changes in the circula-tion features in the monsoon regions, such as: the

1 Corresponding author: Tai-Hwa Hor, Dept. of Applied

Physics, Chung Cheng Institute of Technology, Ta-Hsi, Tao-Yuan, Taiwan 335, Republic of China. E-mail: hor(a)

ap02.ccit.edu.tw (c) 1998, Meteorological Society of Japan

474 Journal of the Meteorological Society of Japan Vol. 76, No. 4

onset of the Indian summer monsoon, the abrupt northward jump of the westerly jet from India or south of the Tibetan Plateau to the northern part of the plateau, the northward shift of the axis of the anticyclone at 100 hPa to 30N, the northward ad-vance of the subtropical high over the West Pacific from 20N to 25N, and the cross-equatorial airflow at about 110E (Yeh et al., 1958; Yoshino, 1971; Wang and Li, 1982). A recent study (Kato, 1989) has indicated that the onset and maintenance of the Mei-yu resulted from the enhancement of the low-level southerly wind toward the Mei-yu front around China, which occurred nearly simultaneously with the onset of the South Asian summer monsoon. Furthermore, Ninomiya (1984) found that the

structure of the Baiu front around the Japan Islands was characterized by a narrow steady precipitation zone of strong gradient of equivalent potential tem-perature, a thick moist neutral layer, and generation of convective instability and significant upward mo-tion, suggesting that the gradients of water vapor and equivalent potential temperature were suitable signs for defining Mei-yu (or Baiu) fronts. Kato (1985) also noted that the formation of the Baiu front was associated with the abrupt decrease of the thermal gradient and increase of the convective in-stability over southern China. By executing several numerical models with and without the convective condensation process, Ninomiya and Tatsumi (1980, 1981) and Ninomiya et al. (1984) investigated the role of condensation heating on the development of meso-c scale disturbances and the maintenance of the Baiu front. The most outstanding feature of the Baiu front over and around the Japan Is-lands was that many scales of motions (planetary-,

large-, meso-a-, meso-B-, and meso-y-scale motions) were interacting with one another (Ninomiya and Akiyama, 1992). Although the Mei-yu front can be several thou-

sand kilometers long, the cross-front dimension is within the mesoscale range. Satellite pictures usually show a long stratiform cloud band along the Mei-yu front with vigorous convection embed-ded within the band (Chen, 1978). The primary mesoscale meteorological phenomena related to the

Mei-yu front include the mesoscale circulation associated with the Mei-yu front, a density current, a low-level jet, pre-frontal squall lines, mountain convection, MCSs, frontal deformation due to topography, terrain-induced mesoscale circulation, and a land-sea breeze. The slowness with which our un- derstanding of these mesoscale weather systems has developed has been caused not by their lack of im- portance but rather by the difficulty involved in ob- taining useful observational data concerning them.

However, since the development of the observational technology in meteorology has made a great progress

within the last thirty years, this trend has been re-

versed, and a rapid advancement of our understand-ing of these important meteorological processes con-tinues. According to the Doppler radar observa-tions and ground-based tower measurements, the moving speed and characteristics of the surface cold front at midlatitudes in thermodynamic and dy-namic structures resemble that of the density cur-rent. It seems to result in the surface frontal system possessing the features of density current (Garrat et al., 1989; Seitter and Muench, 1985; Carbone, 1982; Shapiro, 1984; Shapiro et al., 1985; Young and Johnson, 1984). Moreover, the forced lifting before the frontal system could release latent insta-bility efficiently and generate convection. The hori-zontal wind shear also could initiate the turbulence along the front. Researchers using high-resolution three-dimensional simulations for studying misocy-clone initiation and development found that the ver-tical vortex sheet occurred along the outflow's lead-ing edge in a region of southerly low-level flow (Lee and Wilhelmson, 1997). Mesoscale structures of the Mei-yu front over the Taiwan area were difficult to investigate due to lack of high resolution data that was before the conduc-tion of TAMEX (Taiwan Area Mesoscale EXperi-ment), which was designed to enhance the forecast-ing of heavy precipitation events through better un-derstanding of the mesoscale circulation associated with the Mei-yu front and the effects of orography on the Mei-yu front and on MCSs during the spring of 1987 (Kuo and Chen, 1990). Some scientists (Trier et al., 1990; Chen and Hui, 1990; Chen and Hui, 1992) analyzed the small-scale frontal features based on the TAMEX mesonet observations, NCAR CP-4 Doppler radar and NOAA P-3 instrumented re-search aircraft measurements and showed that the frontal systems were shallow and moderately baro-clinic near the northern tip of Taiwan and implied the existence of density current at the leading edge of the cold air with a warm and moist tongue ahead of it. Then, as they propagated further south over the warm ocean, the frontal air was greatly modified by oceanic heat fluxes. Besides, the frontal system off the east coast of Taiwan featured a vortex cir-culation with rising motion along the leading edge and sinking motion behind. On 15 June 1987, a Mei-yu front passed northern Taiwan and moved south-ward. This cold front was divided into two parts by the mountainous Taiwan island. The eastern part of the front moved faster than the western part, but it was associated with a very weak event. Six pene-trations at different altitudes normal to the eastern front over the ocean were made by the NOAA P-3 aircraft. In this paper, in addition to confirming the shal-

lowness of the Mei-yu front, we employed NOAA P-3 in-situ and airborne radar data to determine

the exact position of the frontal leading edge at

August 1998 T.-H. Hor, M.-H. Chang and B.J.-D. Jou 475

six different flight legs off the east coast of Tai-wan on 15 June, depending on the turbulence in-tensity (LeMone, 1983; Rao and Hor, 1991) as mea-sured by w'2, w'3, and u'w', and composited the mesoscale kinematic, dynamic and thermodynamic structures. Also, the calculations of horizontal di-vergence, relative vorticity, vertical velocity, pertur-bation of air density and kinetic energy transfer be-tween the frontal system and its environment were executed in order to investigate the fine-scale struc-tures and sources of vigor for its maintenance in the frontal system. Especially, the fine-scale vertical cross section of density current in the vicinity of the Mei-yu front over the Taiwan area was demonstrated for the first time.

2. Synoptic situation

During the TAMEX TOP 9 (from 1600 UTC 14 June to 1700 UTC 15 June in 1987), the predomi-nant weather system over central China was a cold-core high (center pressure N 1012 hPa, located about 32.5N, 109.0E at 0000 UTC of 15 June) that was propagating southeastward at speed of 25 km/hr (Cunning, 1988), and a Mei-yu front asso-ciated with a low system (center pressure N 996 hPa, located about 31.5N, 132.9E at 0000 UTC of 15 June) was over the southern China and Taiwan area and moving southward. Some convection occurred over the mountains and along the east coast of Tai-wan after 0400 UTC on 15 June. One isolated con-vective storm developed about 30 km off the central east coast of the island at 1200 UTC on 15 June and persisted through the night with little move-ment. Convection also developed just off the south-ern coast during the night of 15 June. More detailed information is shown on the surface weather maps in Fig. 1 (at 0000 UTC and 1200 UTC on 15 June 1987 made by the Japan Meteorological Agency). The visible GMS (Geostationary Meteorological

Satellite) imageries clearly show the existence of the frontal leading edge following with a long, widely expanded could band. Fig. 2a demonstrates the vis-ible image at 0531 UTC on 15 June. The arrow in-dicates the exact location of the leading edge follow-ing a widely banded cloud system over the southern China area and the open ocean beyond the northern tip of Taiwan. The appearance of the lined leading edge is used to assume that the system possessed a two-dimensional state. However, Fig. 2b shows the infrared one at the same time, indicating the banded frontal system was quite shallow with less organized convections embedded. But, based upon the obser-vations of the tail radar installed in the NOAA P-3 aircraft discussed in Chapter 4, the convection inside the frontal system could develop higher than 10 km. Therefore, the infrared GMS image might give a dif-ferent impression due to the picture being fourtimes lower in resolution than the visible one and much

coarser than the tail radar observation. This frontal system was separated into two parts when it reached the northern tip of Taiwan, and the eastern section of the frontal system was propagating faster than the western one, which was retarded by the topog-raphy of Taiwan (show in Fig. 3). The propagating speed of the eastern frontal sys-

tem was carefully estimated as 6 m/s by comparing the time difference of the surface and aircraft data (Chen and Hui, 1990). Another estimation of the southward moving speed, by checking the position of the frontal leading edge on the visible GMS im-ageries at a 3-hour interval, was about 5.2 m/s. In this paper, the speed of 6 m/s is used to composite the vertical cross sections of the frontal system.

In this study, the standard coordinate system in meteorology is used with the y-axis normal to the frontal system, and the positive northward and op-posite to the direction of system motion.

3. Data used and data analysis

3.1 P-3 f light track Figure 4 shows the NOAA P-3 flight track dur-

ing the TAMEX TOP 9. The aircraft departed the Kadena Air Base in Okinawa, Japan at 0907 UTC 15 June and flew at about 5700 m until reaching a southward-moving Mei-yu front at 1110 UTC. The wind and thermodynamic discontinuities of the front were previously well marked at the lowest levels ob-served in the P-3 aircraft (Jorgensen and LeMone, 1988). The aircraft penetrated the frontal system in the normal direction at six different straight-and-level legs from 154 m to 1517 m between 1127 UTC and 1412 UTC (Pattern 2A2: the aircraft flight pat-tern during the TAMEX that emphasized the fine-scale front structure over the open ocean), and then did a box-sounding descent from 6000 m to 170 m. The inset depicts the beginning and ending times (in UTC) of the race track, with headings at different elevations. The details of the flight legs, including the mean flight altitude, time duration, leg length, and position, as well as the heading for each leg, are shown in Table 1. A developing convectioe system was observed during the flight on the east side of Taiwan, although the system was quite weak. Later on, the aircraft moved to the southwest of the is-land and set up a sawtooth pattern (pattern 2A1.1: the aircraft flight pattern during TAMEX that em-phasized the structure of the low-level-jet related to an undisturbed front over the open ocean) between 1521 UTC and 1537 UTC. Subsequently, after com-pleting a modified topographic pattern close to the western coast of the island, the P-3 headed back to Kadena Air Base at 1748 UTC.

3. P-3 in-situ data analysis The P-3 in-situ data covered in this study are u

(x-direction wind), v (y-direction wind), w (vertical


Fig. 1. Synoptic-scale surface charts at (a) 0000 and (b) 12000 UTC 15 June 1987 depicting surface winds, isobars (hPa), frontal features and intensities of highs and lows (Adopted from the Japan Meteorological Agency)

August 1998 T.-H. Hor, M.-H. Chang and B.J.-D. Jon 477

Fig. 2. (a) The visible GMS (Geostationary Meteorological Satellite) image at 0531 UTC on 15 June 1987 shows the Mei-yu front over the north of Taiwan was travelling southward with clear leading edge, indicated by an arrow. (b) The infrared GMS image at the same time shows that the Mei-yu front system was quite shallow.

(a)

(b)


wind), T (Rosemount temperature), Td (dew point), P (static pressure), Ql (liquid water content), Zr (radar altitude) and Zd (pressure altitude) . The sensors were sampled by the airborne data acquisi-tion system at a 40-Hz rate (40 samples per second), and then forty observations were averaged to yield the basic 1-Hz data (1 sample per second). A de-tailed description of the P-3 instrumentation and the data system was given by Merceret and Davis (1981) and Jorgensen and LeMone (1988,1989). Before the aircraft-measured data were used in this study, they had to be analyzed objectively and subjectively in order that the bad samples were flagged or adjusted. During processing, data samples beyond three stan-dard deviations from each flight-leg mean were re-moved. Also, when the time series of data exhibited excessive magnitudes, they were not used for flux computation.

3.3 Airborne radar data processing The NOAA P-3 research aircraft was equipped

with a tail-mounted X-band (3.22 cm in transmit-ter wavelength) Doppler radar. This radar scanned with a rotation rate of 7.5 seconds per sweep in a ver-tical to the plane that was perpendicular to the flight track, similar to the RHI scan from a ground-based radar. Combining the rotation rate with the aircraft cruising speed of 120 m/s, the horizontal resolution was on the order of 900 meters. An incoherent C-band (5 cm) horizontally scanning radar was also installed below the fuselage of the aircraft. Data

from both of the radar could be digitally recorded.

Vertical profiles of radar reflectivity and radial

velocity were directly measured from the vertically

scanning X-band Doppler radar. Radar reflectivity

data were reorganized to obtain reflectivity plots in constant altitude planes and in the g-z planes, re-spectively, based on the procedures of data merging, editing, interpolating and filtering (Jou and Yiou, 1991).

4. Internal structures of the front

4.1 Fine-scale structures from the radar data The fuselage radar gave PPI (plan-position indi-cator) views of the front along the flight track at several altitudes. Figures 5a and 5b show that the frontal leading edge system was propagating south-ward and developing convective cells while the P-3 aircraft heading south was penetrating it at an al-titude of 294 m between 115500 UTC and 120444 UTC (shown in Fig. 5a) and later heading north at an altitude of 453 m between 123539 UTC and 124553 UTC (shown in Fig. 5b). The designators "1202" and "1239" stand for the hour and minute

(in UTC) when the location of the frontal leading edge was defined on the basis of aircraft in-situ data, and the straight red line represents the flight track. The detailed identification of the leading edge at each flight leg will be discussed in the next section. The tail radar captured more detailed information about the frontal system's appearance and charac-ter. Figures 6a and 6b give the reflectivity pattern and radial wind of the RHI (range-height indicator) views of the frontal system, respectively, at 124138 UTC of 15 June with radar altitude of 453 m. The former picture shows that several convective cells in-side the frontal system were tilting to east and could reach higher than 10 km in altitude and had maxi-mum reflectivity of 40 dBZ inside deep convections below the 5-km level. The latter one shows that the intense easterly wind occurred below the 3-km level; above that, the wind gradually shifted to the west. This sort of flow pattern made the convective cells eastward-tilting. The horizontal reflectivity patterns at constant

heights of 0.5 km, 1.5 km, 2.5 km, and 3.5 km (shown in Figs. 7a and 7d) delineate that two moderately de-veloping cells existed along the flight track of 450 m in altitude, which was oriented from south to north along the line of x=10 km between 122713 UTC and 124905 UTC, and the intensity of the cells de-cayed gradually with height. The letter "L" at the point of x=10 km and g=11 km stands for the exact position of the leading edge at the flight leg of 450 m altitude, and after passing through this point the plane flew into the frontal system completely. The southern cell was smaller in area with a max-imum reflectivity of 32 dBZ. The northern cell was larger, with a 44 dBZ maximum at the 0.5-km level. Furthermore, the g-z plane view along x=15 km (see Fig. 8) attests that the southern cell was shal-low (lower than 3.5 km in height), but was well orga-nized, with a maximum reflectivity of 33 dBZ and an

Fig. 3. Isochrones of the leading edge of the Mei-yu front determined from the sur-

face and satellite data on 15 June 1987

(from Chen and Hui, 1990)


Fig. 4. The flight track of the NOAA P-3 aircraft on 15 June 1987 during TAMEX IOP 9. The inset depicts the beginning and ending times (UTC) of the six penetrations at different altitudes.

Table 1. Details of flight legs of the NOAA P-3 research aircraft traversing the Mei-yu front off the east coast of Taiwan on 15 June 1987 during TAMEX IOP 9.


Fig. 5. PPI (plane-position indicator) views of the Mei-yu front from the fuselage radar installed in the NOAA P-3 aircraft (a) which was heading for south (177 deg) at an altitude of 294 m over the eastern ocean of Taiwan during 115500 UTC to 120444 UTC of 15 June 1987, (b) which was heading for north (1.7 deg) at altitude of 453 m over the eastern ocean of Taiwan during 123539 UTC to 124553 UTC of 15 June 1987. The solid red line marks the flight track at each duration, which also represents the time for radar operation. The designator "1202" ("1239") stands for the hour and minute (UTC) of the frontal leading edge at the 294 m (453 m) level determined from the aircraft in-situ data. The heavy echoes on the west side between x=0 and x=60 km are false due to the influence of Taiwan topography. The domain size with the origin of (22.4N, 121.2E) in the figure is 240 km x 240 km. The centered square represents the region of 54 km x 54 km which will be discussed in Fig. 7.

(a)

(b)


intense gradient of reflectivity at the frontal leading edge designated by letter "L" which was the same as the location of leading edge along x=10 km under the assumption that the lined leading edge system was two dimensional. The determination of the lead-ing edge will be discussed in the next section. The determination of the leading edge will be discussed in the next section. The northern one seemed to grow higher (above 9.5 km) with a maximum inten-sity of 35 dBZ. Both of the cells were tilting to the south. The existence of the well-organized south-ern cell hints that the confluent region between the northerly and southerly flows below 3.5 km was in an unstable state and capable of initiating convec-tive clouds. This phenomenon will be illustrated with more detail in the following section.

.4.2 Fine-scale structures from the in-situ data The lengths of the legs at six elevations (154 m,

297 m, 450 m, 602 m, 908 m and 1517 m) around this front varied with a mean 153 km (refer to Ta-ble 1). In order to examine the characteristics of the Mei-yu front from the mesoscale point of view, it is necessary to isolate the mechanisms and de-termine the scales of motions involved. Following LeMone and Zipser (1980), the updraft and down-draft cores are responsible for considerable trans-port of mass, moisture and momentum in convec-tive events from small entities. We define an up-draft core (a downdraft core) as an area where up-ward (downward) vertical velocity is continuously greater than an absolute value of 1 m/s for hori-zontal length of 500 m. The diameter of an event

Fig. 6. (a) Reflectivity pattern (dBZ), and (b) radial wind (m/s) of the RI-II (range-height indicator) view of the Mei-yu front at 124138 UTC from the tail radar installed in the NOAA P-3 aircraft, which was heading north (1.7 deg) at an altitude of 453 m ("O" stands for the position of the aircraft) off the east coast of Taiwan on 15 June 1987 during TAMEX IOP 9. The scale interval in the abscissa

(x-axis) is 2 km and the interval in the ordinate (z-axis) is 1 km. The indication for radical wind measured by the airborne Doppler radar is different from that observed by the ground-based Doppler

radar. In this figure, the inward flow is positive and the outgoing flow is negative.


Fig. 7. The reflectivity patterns (dBZ) of the frontal leading edge system at altitudes of 0.5 km, 1.5 km, 2.5 km, 3.5 km, respectively, between 1238 and 1244 UTC on 15 June 1987 observed by the tail radar

mounted on the P-3 aircraft. The origin is 23.2N, 122.4E and the domain, which is equivalent to the size of the square in the center of Fig. 5, is 54 km x 54 km. The cross-hatched areas stand for the

regions where the reflectivity was larger than 30 dBZ. In Fig. 7a, the arrow stands for the heading of the aircraft and the capital letter "L" at the point of x =10 km and y =11 km is the position of the frontal leading edge, which was identified on the basis of the aircraft in-situ data at z=450 m level. The y-z cross-section of the radar echoes at x=15 km (marked by "*" ) will be shown in the Fig. 8.


is defined as the product of the duration of the event in seconds and the true airspeed in m/s. The updraft core coverages associated with convective

events within this system ranged from 0.85 km to

4.32 km and were about 2.4 km on an average. This

means that significant amounts of kinetic energy

were contained in the meso-'y scales of motion and

attests that it was possible to form a composited

frontal leading edge in the 2.4 km scale by applying

a 10-sec running mean to the P-3 in-situ data. This

Fig. 7. (Continued)


is based on the assumption that the frontal system was in a quasi-steady state along the y-direction dur-ing the entire flight time (about 2 hours 45 minutes), while traversing the frontal zone at six different ele-vations, with aircraft cruising speed 120 m/s relative to the ground.

In order to explain the observed u- and v-momentum fluxes in the mesoscale frontal zone, it is necessary to separate this motion into a mean (of an averaged flight-leg-scale of 153 km, designated by an overbar) and fluctuation (all scales less than 153 km and represented by a prime) such that u=u+u', etc. Then, the leg-averaged vertical fluxes of u-momentum and v-momentum are calculated from the product of the deviations u', v' and w', such as

u'w'-uiw2, (1) 2=1

and

u'w'=1/n√ni=1v'iw'i, (2)

where n is the number of good points in the data for

each flight leg. Results of the calculations will be dis-

cussed in Chapter 6. An important factor related to

the accuracy of momentum fluxes was the sensitiv-

ity of the calculated u- and v-momentum fluxes with

the variety of the flight-leg length at each run. The-

oretically, the flight-leg length for each run should

be as long as possible in order that the flight-leg av-

eraged i, v, w, etc. can represent the u, v, w, etc.

in the undisturbed environment (personal communi-cation with LeMone, 1988). The typical leg length selected in this study was 153 km, which is longer than the standard flight-leg length (140 km) for the GATE cloud bands (LeMone, 1983). After the nec-essary data analysis and rejection of unsuitable data discussed in Chapter 3, this sensitivity error factor at each run was carefully removed. The identification of the leading edge at each

flight leg was quite significant for completing the composite of the frontal system. The sudden in-crease in intensity of turbulence wi2, w'3 and prod-uct of u' and w' for more than 2-3 km in horizontal scale was used to identify the exact position of the leading edge at each flight level (LeMone, 1983; Rao and Hor, 1991). The abrupt changes of horizon-tal wind direction and speed were also good indi-cations to support the determination. Figures 9a-9e show the time series of the turbulence intensity, w'2 and wi3, as well as the product of u' and w' and the horizontal wind along the flight leg heading north (2.7 deg) at 908 m between 132812 UTC and 134817 UTC. The location of the leading edge at this leg was well defined at 133732 UTC with sharp changes of wi2, wi3, u'w', wind direction and wind speed. After this, the aircraft flew into the frontal system. Using the above two schemes (the turbu-lence intensity scheme and the wind change scheme) might lead to problems. For example, the locations of leading edges determined from the wind direc-tion at the 297 m level (between 115233 UTC and

Fig. 8. The y-z cross section of reflectivity (dBZ) within the frontal leading edge system at x=15 km in Fig. 7a (marked by "*" ). The capital letter "L" at y=11 km represents the position of the frontal leading edge determined on the basis of the aircraft in-situ data at z=450 m level. The cross-hatched areas stand for the regions where the reflectivity was larger than 30 dBZ.


121323 UTC) and the wind speed at the 154 m level (between 112707 UTC and 114913 UTC) are differ-ent from those defined by the turbulence intensity. In those exceptional cases, the turbulence intensity scheme is adopted to define the location of the lead-ing edge. Due to that the leading edge of the front at each leg was distinct, it served as a distinguishing

Fig. 9. The time series of (a) the turbulence intensity wr2, (b) the turbulence intensity wi3, (c) the product of u' and w', (d) the horizontal wind direction, and (e) the horizontal wind speed along the flight leg heading for north (2.7 deg) at height of 908 m between 132812 UTC and 134817 UTC on 15 June 1987. The location of the frontal leading edge at this leg was defined at 133732 UTC (about 560 seconds after the beginning

of this flight leg) and represented by "L."

Fig. 9. (Continued)


mark enabling the construction of the vertical cross section of this frontal leading edge.

1) um, vm, wm, pm and QL fields Based on these discussions, the composited frontal

leading edge system on a mesoscale, denoted by the subscript m, was isolated by first applying a 10-sec running mean to the 1-sec data and then averaging the data in corresponding positions relative to the leading edge at each flight leg. Figure l0a shows the um component (m/s). The strong negative (east-erly) winds (larger than 10 m/s in magnitude at y=10 km) below 600 m within the system repre-sented a low-level easterly jet. It decreased steadily above 600 m. The isotach of 4 m/s matched the frontal leading edge pretty well, and a very strong u-component wind shear along the y-direction (about 6 x 10_4 s-1) was observed just behind the leading edge at the level of 300 m, implying that an intense relative vorticity occurred in the region. Figure lOb shows the vm front-relative field by

deducting the southward-moving speed (6 m/s) of the frontal system. The relative positive (southerly) and negative (northerly) winds (more than 4 m/s in magnitude) just merging at y=7 km immediately behind the leading edge below 400 m represented a confluence taking place inside the system. Above 400 m, the system was dominated by the southerly flow with a maximum speed of 10 m/s in the vicinity of the edge. Figure lOc demonstrates the composited w-field

based upon the aircraft in-situ measurement. No mean value was substracted from the data. It shows at least three updrafts existing with a maximum magnitude of 0.4 m/s along the leading edge lower than 750 m, which is a crude response to the hor-izontal convergence of the v-component wind (see Fig. 10b) and the intense gradient of reflectivity of the smaller convective cell (see Fig. 8) . A relatively significant ascent or descent was present behind the edge as well as ahead of the edge. Both ascend-ing and descending motions at higher levels above z=1200 m (between y=22 km and y=38 km in Fig. 10c) coincided with intense reflectivity higher than 30 dBZ within the larger cell observed by the tail radar (between y=25 km and y=41 km shown in Fig. 8). Due to the smallness of ascent and de-scent magnitude in this study, the difference in re-flectivity between ascending and descending motions could not be recognized. The mesoscale field of deviation pressure p'm was

determined from the differences between radar alti-tudes Zr and pressure altitudes Zd measured by the P-3 aircraft using the equation

p m=pDg, (3)

where the D value equals to the difference of radar

altitude Zr and pressure altitude Zd at each ob-

servation point and was averaged at 2.4 km inter-val. Figure lOd stands for the series of three low-and three high-pressures in p'm fields. The binary low-pressure area with a minimum value of -30 Pa just behind the edge underneath the height of 750 m accompanying two mesohighs attests that updrafts were distinct along the lower part of the system at y=-1 km, 8 km, 17 km, 28 km and 33 km (see Fig. 10c). There were at least two mesohighs behind the mesolows within the system, suggesting that the downdrafts with higher density were prevailing in the region. The existence of a moderate mesohigh with 10 Pa just above the head of the frontal system (centered at y=3 km and z=600 m) was probably an indication to inhibit the directly vertical devel-opment of the smaller cell just behind the frontal edge (see Fig. 8). The distribution of the pressure field resembled the GATE (GARP Atlantic Tropi-cal Experiment) and SMONEX (Summer MONsoon EXperiment) cloud bands in the development of a low pressure system immediately to the rear of the leading edge (LeMone, 1983; Rao and Hor, 1991). The measurements of liquid water content Qi were

obtained by using the Johnson-Williams' hot wire with resolution of 0.1 g/m3. Occasionally, the in-strument gave negative liquid water. Therefore, a downward adjustment was necessary to make the value zero. The mesoscale liquid water content in the vicinity of the system was quite small (shown in Fig. 10e). The maximum intensity was larger than 0.6 g/m3 located between y=20 km and 30 km aloft, which is consistent with the radar echo above 30 dBZ in reflectivity inside the large convective cell measured by the P-3 tail radar (see Fig. 8). The lack of liquid water content near the lower portion of the leading edge (referred to Fig. 10e) is probably due to the new cells continuously growing along the edge with weak reflectivity by the suppression of the high pressure aloft.

2) Divergence and vorticity fields Figure ha shows the vertical cross section of hor-

izontal divergence field (3v/0y) in the y-z plane cal-culated from the in-situ data along the flight tracks neglecting the u-component wind shear in the x-direction due to the assumption of the two dimen-sionality of the frontal system. The frontal zone did not vary along the x axis. It shows that strong convergence occurred along and immediately behind the leading edge. A maximum value of 10_4 s-1 was located at y=8 km and z=300 m where the con-tinuous vertical motions were obvious (Fig. lOc). The calculated relative vorticity (-13u/5y) in the

z-direction, based on the same assumption of the two dimensionality, shows positive values in the frontal edge and delineates the strong instability there (shown in Fig. lib).


3) Thermodynamic fields The vertical cross section (y-z plane) of the poten-

tial temperature constructed from six flight runs at different altitudes in use of Poisson's equation shows that a distinct cold core with the lowest temperature

of 303 K almost covered the entire lower portion of the frontal zone (z<600 m) and relative warmer air was ahead of the system. Also, the air possessed discontinuity along the frontal edge and weak unsta-ble feature at low level from y=0 to 23 km (shown

Fig. 10. The y-z cross section of the composite mesoscale (a) u-wind field (m/s) with a cross-hatched area where the easterly wind was larger than 10 m/s, (b) v-wind field (m/s) with a cross-hatched area where the northerly wind was larger than 4 m/s, (c) w-wind field (m/s) with cross-hatched areas for upward motion, (d) pressure deviation field (Pa) with cross-hatched areas where the deviated pressure was less than -20 Pa, and (e) liquid water content (g/m3) with cross-hatched areas where the liquid water content was bigger than 0.6 g/m3, measured by the P-3 aircraft in the Mei-yu front off the east coast of Taiwan between 1127 ti 1412 UTC on 15 June 1987 during TAMEX TOP 9. The dashed line stands for the frontal leading edge.


Fig. 12a). The cross section of virtual potential tem-perature based upon the empirical formula devel-oped for navigational and meteorological variables measured by the NOAA P-3 aircraft (Merceret and David, 1981) shows that discontinuity and unstable features at lower level were more obvious (shown in Fig. 12b). It suggests that the vertical transport of heat, water vapor and momentum was supported by the ocean. Furthermore, the equivalent potential temperature field (refer to Fig. 12c) calculated from the Merceret and David's formula (1981) shows that the warm, moist air was dominant in front of the leading edge below 900 m in altitude. In addition, a region with high equivalent potential temperature was located along the leading edge from z=900 m to z=1500 m, which coincided with intense re-flectivity (higher than 30 dBZ between y=25 km and y=35 km shown in Fig. 8) and downdrafts (with magnitude larger than 0.4 m/s shown in Fig. lOc). The similar feature was also found by Chen and Hui (1990), but the location of the phenomenon in their study was much further behind the leading edge. The reason why the high equivalent poten-tial temperature region was accompanied by intense reflectivity and downdrafts is not clear. Generally

speaking, the thermodynamic characteristics of the system at the lower level (z<600 m) was unstable and cold-core centered with discontinuity along the leading edge, although the upper portion between 600 m and 1500 m in altitude did not have much distinction.

4) Deviated density field The Mei-yu front is the intermediate zone of cold

air propagating from the north and warm air coming from the south, generally appearing as the geometry of density current. Other examples of atmospheric density flows are thunderstorm outflows and squall lines. Droegemeier and Wilhelmson (1987) inves-tigated the dynamics of thunderstorm outflows by using a two-dimensional numerical model and found that the internal outflow head circulation was gov-erned primarily by the outflow's vertical tempera-ture distribution and it played a key role in deter-mining the gust front propagation speed and outflow head depth. Moreover, Fovell and Ogura (1988) sim-ulated the observed structure of a midlatitude squall line and showed that the storm survived through the production of new cells at the leading edge of the storm, over the system's gust front, and the forward boundary of the cold pool. According to the corn-

Fig. 10. (Continued)


posited aircraft in-situ data on 15 June 1987 and the execution of the ideal gas low, the vertical cross sec-tion of deviated density was completed. The deriva-tion of deviated density is shown as

p'=p-, (4)

where

p=Ps, (5) RdTv

and

P= n (6) In Eq. 5, ps stands for the static pressure measured by aircraft, Rd the gas constant for dry air, and Tv the virtual temperature calculated by using the equation defined by Merceret and Davis (1981). The averaged density p shown in Eq. 6 is estimated based upon the entire-flight-leg data and n, the number of good points of data for each flight leg. Figure 13 shows that the intrusion of a denser flow beneath a lighter flow featured the Mei-yu front with a density

current, with 450 m for the height of the current head. The two heaviest densities were at the ele-vation of 300 m, one at y=7 km, the other one at y=22 km, corresponding to the small and large convective cells, respectively, which were determined by the P-3 tail radar (refer to Fig. 8). The isoline of zero deviation density was coincident with the lead-ing edge, especially in the lower level (z<600 m). It attests that the schemes in this study to identify the exact position of the frontal leading edge for each flight leg were very reasonable. As a matter of fact, this figure confirms the existence of density current in the vicinity of the Mei-yu front over the Taiwan area. The estimated speed for steadily propagating density current at great depths of submergence was equivalent to

c2=2gH(P1P2) (7)

(Karman, 1940; Benjamin, 1968) where c stands for the propagation speed, H is the asymptotic height of the interface above the bottom and P1, and P2 represents the heavier and lighter density.

Fig. 11. The y-z cross section of the composite mesoscale (a) divergence field 10-5 s1), and (b) vertical vorticity field (10_5 s-1), computed on the basis of the P-3 aircraft in-situ data in the Mei-yu front off the east coast of Taiwan on 15 June 1987 during TAMEX TOP 9. The dashed line stands for the frontal leading edge.


In this case, H=450 m, p2=1.12 kg/m3, and pl-p2=0.0035 kg/m3. The estimated propagat-ing speed was 5.25 m/s, quite close to the realistic moving speed of the frontal system.

5. Vertical momentum transfer

Figure 14a shows the flight-leg-averaged profile of the u and v component winds (also refer to Table 2). The magnitude of u component wind differed slightly, but it didn't have any change in direction. The easterly flow was dominant. In contrast, the v

component wind in magnitude changed a lot, with a maximum speed of 6.7 m/s at the elevation of 297 m, and the wind direction shifted from north to south between 602 m and 908 m. The momentum flux profiles for 15 June are shown in Fig. 14b. Note that the vertical flux of u-momentum u'w' was more intense than v'w', despite the considerably larger vertical shear in v component wind at the levels lower than 908 m. Both u'w' and dii/dz were of the same sign between the surface and 300 m, 450 m and 600 m, 750 m and 1200 m. The momentum

Fig. 12. The y-z cross section of the mesoscale (a) potential temperature field (K), (b) virtual potential temperature field (K), and (c) equivalent potential temperature (K), computed on the basis of the P-3 aircraft in-situ data in the Mei-yu front off the east coast of Taiwan on 15 June 1987 during TAMEX TOP 9. The dashed line stands for the frontal leading edge.


fluxes at these levels were of the opposite sign from those predicted by the mixing length theory. Actu-ally, this property is called the countergradient mo-mentum transport. Many case studies suggest that two-dimensional convection in the atmosphere can transport momentum normal to the system up the vertical shear gradient, for example countergradient transfer (Newton, 1950; Starr, 1966; Shapiro and Stevens, 1980; LeMone, 1983; Rao and Hor, 1991). Also, the and dv/dz had the same sign at alti-tudes higher than 450 m. It shows the countergra-dient transport again. Since the v component wind was perpendicular to the frontal system, this finding agrees with LeMone (1983) for a GATE cloud band and Rao and Hor (1991) for a SMONEX cloud band.

6. Kinetic energy transfer

The products of momentum fluxes on the mesoscale and the mean vertical shears on the large-scale govern the kinetic energy exchange processes between the two scales. The possible conversion of the mean kinetic energy K into the perturbation ki-netic energy K' (motion scales less than 153 km in length) for each layer was found using the following equation (Sun, 1978):

KK'z2dz-z2-dz []=-pu'w'dzPv'w', (8) z1z1dz

where p represents the mean air density at each level and z1 and z2 are the aircraft-flight altitudes adja-cent to the calculated level. Theoretically, the mean vertical shears of the environment should be ob-tained from the sounding observation. But the spa-tial resolution of the sounding data during the flight mission couldn't reach the requirement, and the leg length for each flight level ranging from 145 km to 159 km sounded long enough to represent the u, v,

and w in the environment. Therefore, the flight-leg-averaged vertical shears were used for the com-putation. The objective of such an estimate was to examine the possible direction of energy flow be-tween the frontal system and the environment. The results (shown in Table 2 and Fig. 15) are quite crude and just demonstrate the kinetic energy trans-fer qualitatively. It appears that the kinetic en-ergy conversion [K, K']u changed sign twice and the integrated kinetic energy conversion based on the u-wind was -0.04 joules/m2s. It represents the frontal system energy transfer to the mean flow in the x-direction and more or less supported the low-level easterly jet. Conversely, the integrated con-version [K, K']v contributed by the v component wind was 0.12 joules/m2s, implying the mean flow parallel to the moving direction of the system de-creased in intensity as it converted its kinetic en-ergy to the frontal system partially. As a result, the net conversion in kinetic energy [K, K']t, across the frontal system was 0.08 joules/m2s. It suggests that on the whole the large-scale flow transferred its ki-netic energy partially to the Mei-yu font system and enhanced its development. Actually, the frontal sys-tem was maintained in good shape and kept moving at least five hours off the east coast of Taiwan until it reached to the southern tip of the island. Basically, the SMONEX case (Rao and Hor, 1991) shows the same result, with more intense kinetic energy trans-

fer ([K, K']t=0.14 joules/m2s).

7. Discussions and conclusions

The Mei-yu front system propagating southward off the east coast of Taiwan in TAMEX IOP 9 (15 June 1987) was penetrated by NOAA P-3 instru-mented research aircraft at six different altitudes. High-resolution, good-quality data measured by dif-

Fig. 13. The y-z cross section of the perturbed density (in units of 10-2 kg/m3) measured by the P-3 aircraft in the Mei-yu front off the east coast of Taiwan on 15 June 1987. The dashed line stands for the frontal leading edge.


ferent sensors and Doppler radar mounted on the

aircraft were obtained. In the paper, these data were

used to investigate the fine-scale structures of the

frontal system in kinematics, dynamics and thermo-

dynamics over the Taiwan area.

Based upon the observations of the fuselage and

tail radars, several developing convective cells tilt-

ing to the east existed inside the frontal system and

could reach the level higher than 10 km with maxi-

mum reflectivity of 44 dB Z at the lower levels. Also,

the intense easterly wind occurred below the 3-km

level and gradually shifted to westerlies above that

level. After more careful management of the tail

radar data, two convective cells within the frontal

Table 2. Values of the mean winds (u, v, w), momentum fluxes (u'w', v'w'), air density (p) and kinetic energy conversions (Ku, Kv, Kt) at six different flight levels computed on the basis of the P-3 aircraft in-situ data in the Mei-yu front off the east coast of Taiwan on 15 June 1987 during TAMEX TOP 9. Ku and Kv stand for the conversions by the u and v winds, respectively, and Kt is the total of these two conversions.

Fig. 14. (a) Vertical profiles of u and v component winds, averaged along flight leg at each level across the composited

Mei-yu front off the east coast of Tai- wan on 15 June 1987 during TAMEX IOP 9. The u component wind (solid line) is positive eastward parallel to the front and the v component (dashed line)

positive northward opposite to the direction of front motion. (b) Same as in Fig. 14a, but for the momentum fluxes u'w' (solid line) and v'iv' (dashed line).

Fig. 15. Profile of the conversion of the mean kinetic energy to the perturbation kinetic energy in the frontal leading edge off the east coast of Taiwan on 15 June 1987. K (dashed line) and K (heavy dashed line) represent the conversions by the u and v wind, respectively, and

Kt (solid line) is the grand total of these two conversions.


leading edge system were measured along the flight

track. The southern one was more shallow, but pos-

sessed an intense gradient of reflectivity facing the

head of the front, implying that the new convec-

tive cells kept growing there. The northern one was

deeper, with 35 dBZ (in Fig. 8, 44 dBZ in Fig. 7a) in maximum intensity, and did not change its pattern much with respect to the fast southward motion.

The identification of the leading edge at each flight leg was essential for completing the composite of the mesoscale frontal system from the aircraft in-situ data. The sudden increase in intensity of turbulence w'2, w'3 and u'w' for more than 2-3 km was used to identify the exact position of the leading edge at each flight level. This scheme has been used several times previously (LeMone, 1983; Rao and Hor, 1991), and it appears a good way to define the location of the discontinuity in a fine-scale motion. The abrupt changes of horizontal wind direction and speed were also good indications to support the de-termination. But some disagreements might occur when using these two schemes (the turbulence in-tensity scheme and the wind change scheme). For example, the locations of leading edges determined from the wind direction at the 297 m level and the wind speed at the 154 m level were different from those defined by the turbulence intensity. In those exceptional cases, the turbulence intensity scheme was adopted to define the location of the leading edge. Due to that, the leading edge of the front at each leg was distinct; it served as a distinguishing mark enabling the composite. The composited frontal system on a mesoscale,

denoted by the subscript m, was isolated by apply-ing a 10-sec running mean to the 1-sec data in order to determine the scale of motions larger than 2.4 km due to the existence of updraft cores in intensity of 2.4 km on the average and assuming the the frontal system was in a quasi-steady state while the aircraft was traversing the front at different altitudes. The composite wind fields show that the strong negative prevailing winds (larger than 10 m/s) below 450 m within the system represented a low-level northeast-erly jet. The wind shifted to southeasterlies steadily above that altitude. In front of the edge it was domi-nated by southeasterly flow almost everywhere. The strong u-component wind shear and v-component confluence in the horizontal fine-scale located just behind the leading edge at the lower level (shown in Figs. l0a and lob), as well as the slight and discrete positive (upward) motions along the edge, show that a moderate instability occurred at the head of the frontal system (referred to Fig. lOc and Fig. 12a), and the sharp shifting in wind direction and speed occurred in a very narrow zone (less than 300 m, see Figs. 9d and 9e). The locations of the discrete as-cents behind the leading edge were well coincident with those of the two convective cells observed by

the tail radar shown in Fig. 8. Furthermore, the surface um=-4 m/s with strong discontinuity inside the front was coincident with the frontal leading edge of air motion, supporting the feasibility of turbulence and momentum flux approaches again. The

moderate convergence and positive relative vorticity (about 10-4 s-1) were also proposed as the instability at the head of the frontal system.

The distribution of the pressure deviation field resembled the GATE and SMONEX cloud bands in the development of a low pressure system im-

mediately to the rear of the leading edge (LeMone, 1983; Rao and Hor, 1991). The binary low-pressure

area with minimum value of -30 Pa just behind the edge underneath the height of 750 m surrounded

by several mesohighs attests that updrafts less than 0.6 m/s were along the lower part of the system.

There were at least two mesohighs with a maximum intensity of 30 Pa behind the mesolows within the

system. Consistent with the findings of the v-wind profile relative to the front, it appeared that the rear-to-front flow (the northerly wind) prevailed in the region. The existence of a moderate mesohigh system with 10 Pa just above the head of the frontal system probably inhibited the directly vertical de-velopment of the smaller cell just behind the frontal edge, even though the environment was quite unsta-ble. However, the deeper convective cell just after the smaller one shown in Fig. 8 could sustain its development because of the horizontal pressure gra-dient forces above. The mesoscale liquid water content in the vicin-

ity of the system was quite small. The maximum intensity was about 0.8 g/m3 located at y=30 km aloft, which was consistent with the radar echo of the mature convective cell measured by the P-3 tail radar. The lack of liquid water near the lower por-tion of the leading edge was due to the new cells continuously growing along the edge. The thermodynamic fields reveal that the frontal

system was quite flat in the upper levels (above 500 m in altitude) and possessed a strong cold core in the lower levels with an obvious temperature gra-dient and slight convective instability in the vicinity of the leading edge. The features were quite consis-tent with the studies of some scientists (Trier et al., 1990; Chen and Hui, 1992). The most noteworthy result in this study is the

exact demonstration of the vertical cross section of the density current in the fast moving Mei-yu sys-tem over the Taiwan area. It shows the intrusion of a denser flow with head of 450 m beneath a lighter flow. Actually, there were two peak values at the elevation of 300 m inside the system, clearly cor-responding to the small and large convective cells, respectively, which were determined by the P-3 tail radar. The isoline of zero deviation density was sim-ilar to the leading edge, especially in the lower lev-


els (z<600 m). The estimated propagating speed based upon the Karman's empirical formula (1940) was 5.25 m/s, quite close to the moving speed (about 6 m/s) of the frontal system computed by the sur-face and aircraft data. The momentum flux transfer of u-wind parallel

to the frontal system was more intense than that of v-component normal to the system, despite the more considerable vertical shear in the v-wind. Both of the u- and v-wind possessed the countergradient feature in the vertical flux transport. Many case studies suggest that two-dimensional moist convec-tive systems have this countergradient characteris-tic in the normal direction. Moreover, the system must tilt the shear normal to its frontal axis and the mesoscale pressure field accelerates the convective-scale updrafts in the active region front-to-rear, and the downdrafts rear-to-front. Basically, this weak and fast moving front satisfied these requirements as well. The probable mechanisms for the propagation

of the density current and the maintenance of the frontal system were proposed to be the intense hori-zontal pressure gradient force in the cold core region and the slight convective instability at the head of the system, as well as the kinetic energy transport from the mean flow.

In conclusion, this Mei-yu front possessed the characteristics of shallowness, a weak disturbance, an obscure temperature gradient, a cold core and fast movement with the appearance of density cur-rent. These characteristics emphasize that the frontal system was in a transition state between the midlatitude baroclinicity and the tropical instabil-ity. The fact that the frontal edge, defined by abrupt increases in turbulence intensity as well as consider-able changes in horizontal wind direction and speed, was rather parallel to the isoline of perturbed den-sity field, which had quite strong gradient within the frontal region and came to nought outside the front, demonstrates that these approaches were reasonably applied to determine the position of the frontal lead-ing edge at each flight level. The system was ac-companied by at least two convective cells, and the mechanism for lifting the prefrontal air was essential to the development of the convective cells due to the fast propagation of the cold front and the strong in-teraction of two different air masses. Moreover, this investigation attests that the kinetic energy trans-fer from the environment to the front was a partial source for the maintenance of the system.

Acknowledgments

We would like to thank the P-3 Data Research Lab. in the Dept. of Atmospheric Sciences at Na-tional Taiwan University for the full support of air-borne Doppler data analysis. Thanks also go to Dr.

Yi-Leng Chen at the University of Hawaii for dis-cussing the calculation errors in the equivalent po-tential temperature by using the NOAA P-3 aircraft in-situ data. We are also grateful to anonymous re-viewers and Dr. Masataka Murakami, the editor of the Journal of the Meteorological Society of Japan, for their comments and suggestions. This research was sponsored by the National Science Council of ROC under Grants NSC80-0202-M014-01, NSC82-0115-C-002-0033, and NSC87-2111-M-002-008-AP6.

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TAMEX IOP9期間中に台湾の東岸沖で航空機により観測された

梅雨前線先端部内の気流のメソスケ-ル構造について

Tai-Hwa Hor・Mou-Hsiang Chang

(中正理工学院)

Ben Jong-Dao Jou

(国立台湾大学)

TAMEX IOP期間中 (1987年6月15日) に台湾の東岸沖で研究用航空機 (NOAAP-3) を用いて梅雨前線

の鉛直断面 (6高度) 観測を実施した。航空機による直接観測データと2種類の航空機搭載レーダーのデー


タを用いて、梅雨前線南端付近のメソスケール構造と、前線の維持機構を調べた。各高度における水平飛

行中の乱流強度の急増から前線の先端の位置を決定し、前線の移動速度を考慮に入れて、前線を横切る鉛

直断面内の運動学、力学及び熱力学変数の分布を合成した。これらの分布から、前線の先端部は風速の水

平成分と空気密度の偏差の等値線に平行で、密度流的な構造を示すことが明らかとなった。熱力学変数の

変化はざほど顕著ではないが、相当温位の分布は前線後面の下層の寒気コアの存在を示した。寒気コアは

最も空気密度の大きな部分に対応していた。

密度流の進行及び前線系の維持のメカニズムとして、寒気コア内における後方から前方に向かう水平気

圧傾度力、前線の先端部付近における中程度の対流不安定、平均流からの運動エネルギーの変換が考えら

れる。

Date post:	27-Jun-2020
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Journal of the Meteorological Society of Japan, Vol. 76...

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