Indian Journal of Radio & Space PhysicsVol. 25, February 1996, pp. 8-14

Characteristics of layer type radar echoes around DelhiR N Chatterjee, Prern Prakash, Kaushar A1i & R K Kapoor

Indian Institute of Tropical Meteorology, Pune 411 008

Received 9 Ma.y 1995; revised 25 August 1995; accepted 13 October 1995

Some characteristics of the precipitation echoes from layer type clouds in Delhi region have beenstudied using radar observations made during 1960-1962. The study showed that the height of the lay-er echoes first observed varied between 3.0 and 13.0 krn, Mean height of the first observed layerechoes was about 8 km in summer {April-June}, 10 km during monsoon {July-September} and 7 km

.during winter (November-March), The depth of these echoes have been found to be usually in therange 0.5-6.0 Ian and horizontal extensions varied between wide limits ranging from 12 to 200 km. Oc-currence of layer echoes preceded by convective development has been found to be maximum duringsummer while melting band feature occurred more frequently during monsoon. Rainfall rate and liquidwater content in layer clouds as well as the percentage contribution, towards season's rainfall from suchclouds have also been estimated.

1 IntroductionThe study of different aspects of clouds has

been the matter of great interest in different partsof the world since long. With the advent of radar,tremendous progress has been made in this fieldduring the last four decades. In India, most of thestudies+P made so far have been chiefly confinedto the study of different features of convectiveclouds, such as the mechanisms of precipitationformation in them, their size distributions, verticalgrowth and decay rates, durations, fractal dimen-sions etc. However, only a limited number ofstudies'<" have been made in this country in re-spect of layer clouds. They were mainly confinedto different characteristics of melting band (alsocalled bright band) which is, sometimes, observedin radar echoes from layer clouds as a nearly hor-izontal band of high echo intensity. The presentstudy describes different characteristics of precipi-tation echoes from layer clouds observed in Delhiregion in different seasons, viz., heights at whichlayer echoes are first observed, their horizontalextents, rate of their vertical spread, rainfall rateand liquid water content in such clouds andheight at which melting band appears. For thispurpose radar echoes of such clouds observedduring summer (April-June), monsoon (July-Sep-tember) and winter (November-March) seasons ofthe three year period from 1960 to 1962 havebeen examined.

2 Equipment usedEquipment used is a Japanese radar of type

NMD-451A. The Characteristics of the radar set

are given in Table 1. In addition to the conven-tional plan position indicator (PPI), range heightindicator (RIll) and A-scope, the radar isequipped with a special arrangement called rangeelevation indicator (REI) on which depth andrange of the echoes are portrayed in true propor-tions when scanned at elevation angles between-1° and 90°.

3 Analysis and discussionIn analysing the data, we have taken those rad-

ar echoes as layer type when the horizontal exten-sion of an echo was more than five times its verti-cal depth and the structure, as seen on the radarscope, was not cellular or columnar. The growthand decay rates of the echoes were much slowerand their life. as such was much longer as com-pared to the convective type of echoes. Theywere generally associated with slow rate of preci-pitation. Particular care was taken not to be mis-led by the group of convective precipitating cellsappearing extensive from ground as layer typeechoes. Depending on the origin, development

Table 1- Radar characteristics

WavelengthPeak power transmittedPulse lengthMinimum detectable signalPulse repetition frequencyHorizontal and vertical beam width

3.2cm250kW1 ,u s-90dBm300Hz1.2°

CHATTERJEE et al.: LAYER TYPE RADAR ECHOES AROUND DELHI

TYPE - A

TV~E- B

TVPE-C

Fig. 1- REI pictures showing different types of layer echoes [Type-A: Layer without melting bandfeature; Type-B: Layer with melting band feature; Type-C: Convective (Ief~) to layer (rightl]

.'

9

10 INDIAN J RADIO & SPACE PHYS, FEBRUARY 1996

and growth characteristics, these layer echoeshave been classified into three types as follows:Type-A: Those layer type echoes which did notshow any melting band feature and were rrot asso-ciated with any form of convective activity.Type-B: Those layer type echoes in which melt-ing band feature was noticed either from the be-ginning or at later stage, but at no stage convec-tive activity was observed.Type-C: Those layer type echoes that appeared atthe dissipating stage or end phase of convectiveactivity with or without showing any melting bandfeature.

Typical examples of the above three types ofechoes are shown in Fig. 1. It may be mentionedthat the illS picture shown under type-C inFig. 1 is of convective type of precipitation, whichdepicted layer-type situation with melting bandfeature towards its end phase as shown in theRHS picture under type-Co

Table 2 shows the seasonwise distribution ofdifferent types of layer echoes. It is seen fromTable 2 that during summer, maximurrt frequencyof occurrence of layer type echoes was of C-type(as much as 75% of the total). Only in 8 and 17%cases they were of type-A and type-B, respect-ively. During monsoon, 50% of the layer echoesw.ere of B-type, and 17 and 33% were of type-Aand type-C, respectively. In winter, A-type layerechoes occurred more frequently as compared toB- and C-types (their percentage frequency of oc-currence being 46, 27 and 27, respectively). Asalready defined, C-type echoes are those layerechoes that appeared at the dissipating stage orend phase of convective activity with or withoutshowing any melting band feature. In this connec-tion, it may be clarified that melting band featurein C-type of echoes has been observed more fre-quently in winter (in 62% ..cases) as compared tothose in other two seasons. In summer and mon-

Table 2 ....-Percentage frequency distribution of layer echoesamong different types in different seasons

Season Type of layer echoes Totalno. of

A B C cases

Summer 8.0 17.0 75.0 12(Apr.-June]

Monsoon 17.0 .50.0 33.0 36(July-Sep.)

Winter 46.0 27.0 27.0 30(Nov.-Mar.)

For the complete period 27:0 36.0 37.0 78

soon seasons melting band feature in C-typeechoes has been observed in 44 and 42% cases,respectively. Table 3 gives the general summary ofthe different characteristics of layer echoes ob-served during different seasons. Height of the lay-er echoes first observed varied between 5.5 and11.0 km in summer, between 5.5 and 13.0 km inmonsoon and between 3.0 and 9.0 km in winter.Mean height of the first observed layer echoeswere 8.3, 9.6 and 6.7 km during the above threeseasons, respectively. The height of the layer asseen mostly corresponds to medium cloud leveland in a few cases to the high cloud level. Thesehigh level echoes were observed with the layerformation after convective activity suggestingthereby that the cirrostratus formation from Cbclouds contain supercooled water or ice particlesof sufficiently large size to give radar reflection.Maximum height of the layer echoes observed inthe three seasons varied in the same ranges asthose of first observed layer echoes. Mean maxi-mum heights of layer echoes were 8.8, 9.8 and6.7 km during summer, monsoon and winter sea-sons, respectively.

It may also be seen from Table 3 that there is agood parallelism in the general trend of the sea-sonal variation of the height of the layer echoesand height of the freezing level. Height of the lay-er echoes was lowest in winter corresponding tolow freezing level, which slowly increased by sum-mer and. became maximum during monsoon sea-son. Mean heights of the freezing level over Delhiduring the above three seasons are 3.0, 4.3 and5.5 km, respectively. Melting band level (whendeveloped) is also lowest in winter, being 2.7 kmwhich increased through summer to 3.9 km andreached the maximum during the monsoon to 5.1km. Observations also revealed that in most ofthe cases, centre of melting band generally lies be-tween 200 and 500 m below the freezing level.The average value was 300 m. This average valueis very close to the value reported by Austin andBemis 16. They reported an average value of 250m, while Hooper and Kippax'? reported an aver-age value of 100 Ill. The depth of the layer echofirst' observed varied between 0.5 and 5.5 km insummer, between 0.5 and 5.0 km in monsoon,and between 0.5 and 6.0 km in winter season.However, the depth of majority of the first layerechoes has been found to vary between 1.5 and3.0 km. The percentage of such layer echoes insummer, monsoon and winter seasons were 89,64 and 56%, respectively.

Horizontal extents of the layer varied between12 and 100 km in summer, between 20 and 200

.'

CHATTERJEE et aL: LAYER TYPE RADAR ECHOES AROUND DELHI 11

Characteristics of layer echoes

Table 3-Summary of the characteristics of layer echoes in different seasons

Season

Summer Winter

Number of cases

Range of variation of the height of the layer first observed (Ian)

Mean height of the layer first observed (km)

Range of variation of the maximum height reached by the layer (km)

Mean maximum height (km)

Range of variation of the depth of the first observed layer (km)

Mean depth of the first layer echo (km)

Range of variation of the maximum horizontal extent.of the layer (Ian)

Mean horizontal extent (km)

Rate of vertical spread (m s - I)

Mean rate of vertical spread (rn s - I)

-Percentage of cases when precipitation from layer clouds reached ground

Mean height of the melting band (km)

Mean height of the freezing level (Ian)

12

5.5-11.Q.8.3

5.5-11.0

8.8

0.5-5.5

2.3

12-100

48.0

0.7-2.01.3

58.0

3.94.3

Monsoon

36

5.5-13.0

9.6

5.5-13.0

9.8

0.5-5.0

2.2

20-200

50.0

0.5-2.11.3

78.0

5.1

5.5

30

3.0-9.0

6.7

3.0-9.0

6.7

0.5-6.0

2.8

15-180

64.00.."-1.7

0.6

57.0

2.7

3.0

Table 4-Contribution of rainfall from layer type cloud days towards seasons total rainfall

Season Totaino. Amount of Amount of Percentageof case rainfall rainfall contributionstudies recorded per recorded per of rainfall

station on station in by layer typelayer type the season cloud dayscloud days towards seasons'

total rainfall(A) (B) (NB)x 100

Summer 12 42.0 98.0 42.9(Apr.-June)

Monsoon 36 562.0 1436.0 39.1(July-Sep.)

Winter 30 33.0 172.0 19.2(Nov.-Mar.)

For the complete 78 637.0 1706.0 37.2period

Ian in monsoon and between 15 and 180 Ian inwinter. Mean horizontal extents of the layerechoes in the above three seasons were 48.0, 50.0and 64.0 Ian, respectively.

Rate of vertical spread of the layer echoes vari-ed between 0.7 and 2.0 m S-1 in summer, be-tween 0.5 and 2.1 m S-I in monsoon and 0.5 and1.7 m s - 1 in winter season, respectively. Themean rates of vertical spread in the above threeseasons were, respectively, 1.3, 1.3 and 0.6 m s - I.

Earlier, Chatterjee and Prem Prakash:' studied thevertical growth rates of radar echoes from con-vective clouds in this region and found that the

vertical growth rates of such clouds varied be-.tween 1.0 and 25.0 m ~- 1 in summer and be-tween 0.9 and 12.8 ni s -1 in monsoon season re-spectively. The average growth rates in the aboveseasons were 4.3 and 4: 1 m s - I, respectively. It is,therefore, clear that the growth rJites of layerechoes are much smaller as compared to thegrowth rates of convective echoes.

Number of cases, when precipitation from layerclouds reached ground, have been found to bemaximum during monsoon (in 78% cases). In theother two seasons such cases are comparativelylow (in about 58% cases).

12 INDIAN J RADIO & SPACE PHYS, FEBRUARY 1996

Table 4 shows the percentage contributions tothe total rainfall by layer type clouds in Delhi re-gion in different seasons as well as for the com-plete period. For this purpose, rainfall data ob-tained from a raingauge network consisting of 25raingauge stations within 25 km around Delhihave been considered and the total' of mean rain-fall per raingauge station on layer type cloud dayshas been compared with the total amount ofmean rainfall per station for the entire season. Itmay be seen from Table 4 that about 43, 39 and19% of the seasons total rainfall are contributedby layer type clouds during summer, monsoonand winter seasons, respectively. For the entireperiod, i.e., summer, monsoon and winter seasonstaken together, about 37% of the total rainfall iscontributed by layer type clouds in this region.

Attempts have also been made to measure rain-fall rate and liquid water contents of the layersfrom the radar reflectivity measurements. Theradar reflectivity was determined by the pre-cali-brated radar receiver sensitivity which waschecked from time to time. For computing therainfall rate and liquid water content, the follow-ing empirical relations given by Kutty and Shirvai-kar" have been used.Z= 345.8 R 1.618

M= 67.8 RO.85

where, the rainfall rate, R, is expressed in mmh - I, liquid water content, M; in mg m- 3 and rad-ar reflectivity, Z, in mm" m-3. Table 5 shows themean values of rainfall rate and liquid water con-tent in layer echoes for the days when radar ref-lectivities were measured for layer echoes in dif-ferent seasons. Although the data presented arevery small, nevertheless it would give an idea re-garding the range of variations of rainfall ratesand liquid water contents in precipitation fromlayer clouds. It is seen from Table 5 that themean value of the rainfall rate of layer clouds ob-served on different days varied from day to daybetween 8.4 and 12.1 mm h-I in summer, be-tween 0.9 and 23.9 mm h -I in monsoon and be-tween 0.6 and 13.1 mm h-1 in winter season.When mean rainfall rate of individual seasons isconsidered, it has been found that rainfall rate oflayer clouds was maximum during monsoon (11.3mm h - I) and minimum in winter (5.4 mm h -I)~Insummer, it was 10.3 mm h -I which is comparableto that obtained in monsoon season. Similar tothe rainfall rate, there was marked day-to-day var-iations in mean liquid water contents of the layerclouds also. It varied between 413.8 and 564.4mg m-3 in summer, between 62.0 and 1006.6 mg

Table 5-Rainfall rate and liquid water content of layerclouds in different seasons

Date

16 Apr. 6022 June60Mean

14 July 6025 July 6026 Aug:6015 July 6121 July6122 July 6125 July 616 Aug. 618 Aug. 61Mean

6 Jan. 6012 Jan. 6020 Jan. 604 Mar. 6017 Mar. 6030 Dec. 6017Jan.6125 Jan. 6128 Jan. 6131 Jan. 6117 Mar. 6123 Mar. 611 Feb. 623 Feb. 6215 Feb. 621 Mar. 62Mean

Mean heightof layer

echokm

5.67.56.6

6.010.57.59.58.0

11.012.510.07.39.1

6.06.04.94.25.57.05.4

6.35.3

5.57.85.04.07.55.05.25.7

Rainfallrate

mmh-'

Summer

8.4

12.110.3

Monsoon

2.50.96.53.6

18.49.1

13.023.923.611.3

Winter

4.03.8

11.710.77.93.4

5.213.12.22.21.33.4

11.5

0.61.9

2.75.4

I,Liquidwater

contentmg m '?

413.8564.4489.1

147.762.0

333.0201.4806.0442.2600.0

1006.6995.9510.5

220.3210.9548.5508.4392.8191.9275.3603.8132.5132.583.3

187.0540.544.4

117.0158.0271.7

m-3 in monsoon, and between 44.4 and 603.8 mgm - 3 in winter seasons; Similarly mean liquid wa-ter content obtained for different seasons hasbeen, found to be maximum during monsoon sea-son and minimum during winter season. Themean values in different seasons were 489.1,510.5 and 271.7 mg m ? in summer, monsoonand winter, respectively.

4 Special characteristics in different seasonsIn winter, on days of extensive layer situation, a

definite slope was generally observed depending

"

CHATTERJEE et aL: LAYER TYPE RADAR ECHOES AROUND DELHI 13

Fig. 2-REI pictures showing slope in layer echoes

Fig. 3-REI pictures showing multiple layer features

on the movement of the front. These types ofdays were generally associated with western dis-.turbances. Radar REI pictures in Fig ..2 show ex-amples of such situation.

On a number of days, multiple layer features asshown in Fig. 3 have been observed. Table 6shows the percentage frequency of occurrence oflayer echoes displaying different number of signi-ficant layers in different seasons. It is seen fromTable 6 that multiple layer feature is more pre-dominant in monsoon season although the same isnot very rare in winter season 'but are very few insummer.

Lastly, it may also be. mentioned that therewere occasions when convective activities wereseen through the layer (Fig. 4). In winter, this typeof feature was generally noticed in isolated cases,whereas in monsoon it 'appeared on a number of'occasions on days of widespread activity.

Table 6-Frequency distribution (%) of layer echoes showingindicated number of significant layers

Season

Summer(Apr.-June)

Monsoon(July-Sep.)

Winter(Nov.-Mar.)

Combined

'No. of significant layers Totalno. of

2 3 cases

83.4 8.3 8.3 12

66.7 20.0 13.3 30

80.6 16.6 2.8 36

75.6 16.7 7.7 78

5 SummaryRadar data of precipitation echoes from layer

type clouds, within 300 km around Delhi collect-ed during the three year period from 1960 to1962 have been analysed in order to study some

..

14 INDIAN J RADIO & SPACE PHYS, FEBRUARY 1996

Fig. 4- REI pictures showing convective activity through layer echoes

features of this type of clouds forming in this re:"gion during the summer (April-June), monsoon(July-September) and winter (November-March).Following broad features have been noticed:

(i) Layer echoes in this region commonly occurat a height of 8, 10 and 7 km during sum-mer, mO~9n and winter seasons, respect-ively.

(ii) Thickness of the initial layer normally variesbetween 1.5 and 3.0 krn.

(iii) Horizontal extent of the layer varies be-tween wide limits ranging from 12 to 200km. Average horizontal extent of the layerecho observed was 50 km.

(iv) During summer, about 75% of the layerechoes appear towards the end phase ofconvective activity.

(v) Melting band feature in layer echoesap-pears more frequently in monsoon as com-pared to the occurrence of such feature inthe other two seasons. Percentages of layerechoes which show melting band feature indifferent seasons are 50% in monsoon, 27%in winter and only 17% in summer.

(vi) In monsoon, precipitations from layer reachthe ground in about 80% cases. In the othertwo. seasons, precipitations from such cloudsreach the ground in about 60% cases.

(vii) Rate of vertical spread of layer clouds ismuch smaller than that of convective clouds.

(viii) Multiple "layer feature also occurs some-times .. Occurrence of such feature is maxi-mum in monsoon and minimum in summer.

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j