CHARACTERISTICS OF PRECIPITATION PHYSICS IN THE

CONVECTIVE CELLS IN A HUMID ENVIRONMENT�Mariko Oue, Hiroshi Uyeda, and Yukari Shusse

Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan

1. INTRODUCTIONMeso-� -scale convective cells are com-

ponents of a precipitation system, whichsometimes produces heavy rainfalls. A largenumber of precipitation particles in a con-vective cell produce a strong rainfall whenthey reach the ground. Distributions of pre-cipitation particles in the convective cell varywith formation processes. Therefore, it is im-portant to clarify the distribution of precipita-tion particles in the convective cell for under-standing precipitation physics.

Polarimetric radar is available to char-acterize properties of precipitation particles.Polarimetric variables have information ofvarious properties of precipitation particleswhich predominate within a sensitive volumeof a radar observation. To clarify the distribu-tion of precipitation particles in a convectivecell, a direct observation of raindrop size dis-tribution (DSD) is effective in addition to thepolarimetric radar observation.

Several studies have been made on pre-cipitation particles in severe convective cells(e.g. Zeng et al., 2001). These convectivecells grew above the height of 0 � C level (0 � C-height) in echo-top of 30 dBZ. However, in ahumid environment, as in Okinawa, Japan,during a rainy season, called the “Baiu”, fromJune to July, convective cells have low echo-top heights. Shinoda et al. (2007) repre-sented that convective cells around OkinawaIsland during the Baiu period have low al-titudes of reflectivity peaks (1.5-2.0 km alti-tude) and low echo-top heights of 30 dBZ(around 0 � C-height) by statistical studies.Shusse et al. (2006) analyzed the charac-teristics of polarimetric variables in relativelyshallow convective cells around Okinawa Is-land during the Baiu period using a C-bandpolarimetric radar. They showed that a con-vective cell embedded in stratiform precipita-tion area existed with small raindrops belowthe height of 2 km dominantly.

Convective cells exist not only in a con-vective rain zone but also in a stratiform rainzone associated with a precipitation systemaround Okinawa during the Baiu period. Dis-tributions of precipitation particles in the con-vective cells with low echo-top heights arenot revealed. To understand precipitation

physics of convective cells developing in thehumid environment, it is necessary to clarifydistributions of precipitation particles in theseconvective cells existing in the stratiform rainzone and convective rain zone. We madeobservations at Okinawa Island during theBaiu period in 2006 using a C-band polari-metric radar and a disdrometer which mea-sures DSD on the ground.

2. OBSERVATION AND DATAIn our observation, the C-band polari-

metric radar (COBRA: CRL Okinawa bistaticpolarimetric radar) and a Joss-Waldvogeldisdrometer of Okinawa Subtropical Environ-ment Remote Sensing Center, National In-stitute of Information and CommunicationsTechnology, Japan, were used. COBRA lo-cated at Nago, Okinawa, performed plan po-sition indicator (PPI) scanning of 14 eleva-tions (0.5, 1.1, 1.8, 2.5, 3.3, 4.2, 5.3, 6.5, 8.1,10.0, 12.3, 14.8, 17.4, and 20.5) and rangeheight indicator (RHI) scanning directed tothe disdrometer observation site (Fig. 1).Both scanning were performed every 6 min-

Fig. 1. Locations of each observation site. � and �show the locations of COBRA and the disdrometer, re-spectively. The circle shows the radar range of 100 km.Topography is shown with grayscales from 100 m every200 m.

Fig. 2. The surface weather map at 09 LST, on 10 June2006.

utes. The radar coverage of PPI is 100 km inradius as shown in Fig. 1. Spatial resolutionof RHI data is 75 m in range direction and 0.4degree in elevation direction, and that of PPIdata is 300 m in range direction and 1 degreein azimuth direction.

In this study, radar reflectivity (Z ��� )was calculated into a constant altitude PPI(CAPPI). The horizontal and vertical grid in-tervals of the CAPPI were 1 km and 0.25 kmrespectively. Polarimeteric variables of differ-ential reflectivity (Z �� ) and correlation coef-ficient at zero lag (�� � (0)) were utilized. Z ��represents a oblateness of precipitation par-ticles predominating within a sensitive vol-ume of a radar observation. In a rain region,a large value of Z �� represents that rain-drops having large diameters predominatelyexist in the sensitive volume (e.g. Bringiand Chandrasekar, 2001). A large (small)value of ���� (0) represents that size and cat-egory of precipitation particles in a sensitivevolume of a radar observation are homoge-neous (heterogeneous). A value of ���� (0) ina rain region is generally greater than 0.96-0.97 (Doviak and Zrnic, 1993). If snow andraindrops are mixed, �� � (0) is small. A valueof �� � (0) in a melting layer is smaller than0.95.

The disdrometer was located at Ogimi,Okinawa, and at a distance of 14.8 kmfrom COBRA. The disdrometer measured 1-min-integrated DSDs on the ground every 1minute.

3. CHARACTERISTICS OF THE BAIUFRONTAL RAINBAND ON 10 JUNE2006

We selected the Baiu frontal rainbandof 10 June 2006 for analyses becausemany convective cells in the rainband passedover the disdrometer observation site, Ogimi.Sounding data at Ogimi is available on the

Fig. 3. Composite images of horizontal distribution ofrainfall intensity at the 2 km altitude by JMA radar net-work at (a) 0600 LST and (b) 1300 LST.

day. The Baiu front on the surface was an-alyzed at the south of Okinawa Island (Fig.2), and that extended from east-northeastto west-southwest. The rainband, which ex-tended along the Baiu front to the north ofthe Baiu front over East China Sea, movednortheastward and passed over Okinawa Is-land from 6 LST to 18 LST. Figure 3 showscomposite images of horizontal distributionof rainfall intensity by C-band radar networkof Japan Meteorological Agency (JMA). Therainband had a convective rain zone alongthe southern edge of the rainband and astratiform rain zone to the north of the con-vective rain zone. In this study, the convec-tive rain zone is defined as a strong rain-fall area of 80 km in width along the south-ern edge of the rainband. The stratiform rainzone is defined as a broadened weak rainfallarea to the north of the convective rain zone.Several hours after the passage of the strati-form rain zone over Okinawa Island (Fig. 3a),the convective rain zone passed over Oki-nawa Island (Fig. 3b).

Figure 4 shows horizontal radar reflec-tivity at the height of 2 km in CAPPI aroundOgimi. Convective cells (convective echoesexceeding 35 dBZ) in the stratiform rain zoneare embedded in the stratiform precipitationarea (Fig. 4a) and other convective cellsin the convective rain zone are also embed-ded in stratiform precipitation area formedbetween convective cells (Fig. 4b). Bothtypes of convective cells passed over the dis-drometer observation site.

Fig. 4. Horizontal distribution of Z ��� in CAPPI at (a)0630 LST and (b) 1342 LST at the height of 2 km. Cell-A and Cell-B are indicated by arrows in (a) and (b). Thesymbols � and � show the locations of COBRA and thedisdrometer, respectively.

4. DISTRIBUTIONS OF PRECIPI-TATION PARTICLES IN CONVECTIVECELLS

A convective cell in the stratiform rainzone and a convective cell in the convectiverain zone are selected for detailed analysesusing RHI data which have higher spatial res-olution than PPI data. The former passedover the disdrometer observation site, atabout 0630 LST (Fig. 4a) and is named “Cell-A”. The latter passed over the disdrometersite at 1342 LST (Fig. 4b) and is named“Cell-B”. Figure 5 shows vertical distributionsof Z ��� in RHI of Cell-A and Cell-B. Both con-vective cells passed over the disdrometer siteduring their mature stages. The disdrome-ter measured intense rainfall for a short timeduring the passage of these convective cellsover the disdrometer site.

4.1. CONVECTIVE CELL IN STRATI-FORM RAIN ZONE

Around Cell-A, a bright band of Z ��� isclear at the height of 4-5 km (Fig. 5a). The0 � C-height was 4.8 km from sounding dataat Ogimi. The bright band indicates a meltinglayer because the altitude of the bright band

Fig. 5. Vertical distribution of Z ��� in RHI along (a) A-A�

and (b) B-B�

in Fig. 4. Rectangles indicate analysesareas of Z ��� , Z ��� and ����� (0). The symbol � denotesthe location of the disdrometer site.

almost corresponded to the 0 � C-height. Theecho-top height of 30 dBZ was 5.6 km andthe maximum reflectivity was 49.6 dBZ at theheight of 3.2 km in an RHI scanning. Cell-Ahad a low echo-top height less than 6.3 kmduring the mature stage.

Figure 6 shows scatter diagrams of Z ���versus Z �� and �� � (0) of Cell-A within rect-angle regions in Fig. 5a. At the height of 4-5km (box-1) corresponding to the 0 � C-height,the maximum Z �� is greater than 2.0 dB(Fig. 6a). In the same region, the minimum�� � (0) reaches a small value to 0.92 (Fig.6d). A large Z ��� represents that flat parti-cles are dominant in a sensitive volume of aradar observation. At the 0 � C-height, �� � (0)smaller than 0.98 indicates mixed phase ofice and liquid particles. Therefore, it is con-sidered that Cell-A had a large number of wetsnowflakes around the height of 4-5 km. Be-low the height of 3.5 km (box-2 and box-3),Z �� are smaller than 1.5 dB (Figs. 6b andc), and �� � (0) are greater than 0.98 (Figs. 6eand f). The small Z ��� and large �� � (0) repre-sent that small raindrops were predominantat the low altitudes of Cell-A. The distributionas noted above is similar to that of the con-

Fig. 6. Scatter diagrams of polarimetric parameters inRHI for Cell-A. The diagrams of the left row is Z ��� ver-sus Z ��� within the box-1 (a), the box-2 (b) and the box-3 (c) in Fig. 5a. The diagrams of the right row is Z ���versus ����� (0) within the box-1 (d), the box-2 (e) and thebox-3 (f) in Fig. 5a.

vective cell embedded in stratiform precipita-tion area of Shusse et al. (2006).

Vertical profiles of averaged Z �� and���� (0) of Cell-A are represented in Figs. 7aand b. Areas of the average are indicated inFig. 7c. The peak height of averaged Z ��almost corresponds to the height of the mini-mum of averaged �� � (0) (4-4.5 km altitude).Below the height of 3.5 km, averaged Z ��and �� � (0) are constant values of 1 dB andgreater than 0.99, respectively. The constantvalues of small Z �� and large ���� (0) rep-resent that coalescence processes of rain-drops were less in Cell-A.

While Cell-A was passing over the dis-drometer, an average rainfall intensity was18.5 mm h �

�and the maximum rainfall in-

tensity was 43.0 mm h ��. DSD during the

passage of Cell-A is shown in Fig. 8. Thenumber concentration of raindrops from 1 to2 mm in diameter is large and few raindropsexceed 3 mm in diameter. Raindrops from 1to 2 mm in diameter account for 63.3 % ofthe average rainfall intensity. From the re-

Fig. 7. Vertical profiles of averarged (a) Z ��� and (b)����� (0) of Cell-A. Averaging areas are indicated by rect-angles in (c). The symbol � in (c) denotes the locationof the disdrometer.

Fig. 8. DSDs during the passage of Cell-A (from 0627LST to 0636 LST). Dotted markes show DSDs every1 minute. The solid line shows the DSD fitted to agamma distribution (Kozu and Nakamura, 1991). Vari-ables right above show averaged rainfall intensity (R)and parameters of gamma distribution (� , � and N ).

Fig. 9. As Fig. 6 but for Cell-B.

sults of polarimetric variables and DSD onthe ground, it is considered that small rain-drops having 1-2 mm in diameter predomi-nated near the ground.

4.2. CONVECTIVE CELL IN CONVEC-TIVE RAIN ZONE

Around Cell-B, a melting layer is alsoclear such as a bright band of Z �!� at theheight of 4-5 km (Fig. 5b). The echo-topheight was 5.6 km and the maximum reflec-tivity was 54.2 dBZ at the height of 1.2 kmin an RHI scanning. Cell-B also had a lowecho-top height less than 5.8 km during themature stage.

Figure 9 shows scatter diagrams of Z ���versus Z �� and �� � (0) of Cell-B within rectan-gle regions in Fig. 5b. At the height of 4-5 km,Z �� are mostly smaller than 1.5 dB for Z ���greater than 43 dBZ (Fig. 9a). In the sameregion, �� � (0) are greater than 0.97 (Fig. 9d).This represents that there were a large num-ber of small raindrops at the height of 4-5km in Cell-B. Around the height of 2.5-3.3 km(box-2), Z �� reaches the maximum of 2.5 dBand ���� (0) reaches the minimum of 0.95. Be-low the height of 2 km (box-3), the maximumZ �� is greater than 4 dB and the minimum���� (0) is smaller than 0.90. The small �� � (0)

Fig. 10. As Fig. 7 but for Cell-B.

Fig. 11. As Fig. 8 but for Cell-B (from 1331 LST to1345 LST).

below the 0 � C-height represents oscillationsof large raindrops and broadened DSD fromsmall size to large size. It is considered thatlarge raindrops existed below the 0 � C-heightin Cell-B.

Figure 10 shows vertical profiles as wellas that of Cell-A (Fig. 7). Below the heightof 4 km, the averaged Z �� increases withdecreasing altitude (Fig. 10a). The aver-aged �� � (0) decreases with decreasing alti-tude (Fig. 10b). The results represent co-alescence processes of raindrops were pre-dominant in Cell-B.

While Cell-B was passing over the dis-drometer, the average and maximum rainfallintensity were 17.6 mm h �

�and 56.4 mm

h ��, respectively. DSD during the passage of

Cell-B is shown in Fig. 11. The DSD broadento large diameter. The number concentrationof raindrops from 1 to 2 mm in diameter issmaller than that of Cell-A and a large num-ber of raindrops exceed 3 mm in diameter.Raindrops exceeding 3 mm in diameter ac-count for 19.1 % of the average rainfall in-tensity. This percentage is grater than thatof Cell-A (2.3 %). It is considered that largeraindrops in Cell-B had diameters exceeding3 mm near the ground.

6. SUMMARY AND CONCLUSIONSIn order to clarify distributions of precip-

itation particles in the convective cells, weanalyzed polarimetric variables and DSD intwo convective cells associated with the Baiufrontal rainband on 10 June 2006. One con-vective cell existed in a stratiform rain zoneassociated with the rainband, the other ex-isted in a convective rain zone associatedwith the rainband.

The former had a low echo-top height (of30 dBZ) of about 5-6 km and strong rainfallintensity of about 18 mm h �

�. Around the

convective cell, the melting layer was clear.Below the height of 3.5 km, Z �� was lessthan 1.5 dB and ���� (0) was greater than 0.98.In the DSD on the ground, the number con-centration of raindrops of 1-2 mm in diameterwas large and few raindrops exceeded 3 mmin diameter. The results represent a largenumber of small raindrops of 1-2 mm in di-ameter existed in the convective cell in thestratiform rain zone.

The latter also had a low echo-top height(of 30 dBZ) of about 5-6 km and strong rain-fall intensity of about 18 mm h �

�. Around the

convective cell, the melting layer was clear.Below the height of 3.5 km, the Z �� indi-cated greater than 1.5 dB and the �� � (0) in-dicated smaller than 0.98. In this case, thenumber concentration of 1-2 mm in diame-ter was smaller than that of the former caseand a large number of raindrops exceeded 3mm in diameter. The results represent large

raindrops exceeding 3 mm in diameter werepredominant in the convective cell in the con-vective rain zone.

We show that a convective cell embed-ded in stratiform precipitation area of the Baiufrontal rainband existed in a convective rainzone associated with the Baiu frontal rain-band, as well as in a stratiform rain zone.Distributions of preciptaition particles in theconvective cells embedded in stratiform pre-cipitation area in the stratiform rain zone andin the convective rain zone of the Baiu frontalrainband were clarified: small raindrops inthe convective cell of the stratiform rain zoneand large raindrops in the convective cell ofthe south convective rain zone of the Baiufrontal rainband. Although these convectivecells had common characteristics of the lowecho-top height and providing strong rain,distributions of precipitation particles weredifferent. The results show a clue for under-standing precipitation physics of convectivecells developing in the humid environment.

Acknowledgments: This observation wasconducted in collaboration with National In-stitute of Information and CommunicationsTechnology and Hydrospheric AtmosphericResearch Center, Nagoya University.

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