Journal of Natural Disaster Science, Volume 27, Number 1, 2005, pp17-23

Characteristics of Precipitation Systems Analyzed from Radar Data over Bangladesh
Md. Nazrul ISLAM1, Taiichi HAYASHI2, Toru TERAO3, Hiroshi UYEDA4 and Katsuhiro KIKUCHI5
1
Department of Physics, Bangladesh University of Engineering & Technology, Bangladesh
2
DPRI, Kyoto University, Japan
3
Faculty of Informatics, Osaka Gakuin University, Japan
4
HyARC, Nagoya University, Japan
5
Department of Biological Environment, Akita Prefectural University, Japan(Received for 19 Jan., 2004 and in revised from 21 Sep., 2005)

ABSTRACTAt present, a low-dense rain-gauge network over Bangladesh is the only tool by which to measure rainfall, so
there is no way of understanding the properties of the precipitation systems occurring there. To obtain precise
rainfall distributions and to clarify the characteristics of precipitation systems, radar data from the Bangladesh
Meteorological Department (BMD) were used in the first investigation in Bangladesh. The BMD radar collects
rain status data in six ranges. The precipitation rate was retrieved from the rain status obtained from BMD radar
data. Correlation between three-days-running averaged rainfall estimated by radar and rain-gauges ranged from
0.63-0.89 over the months of 2000. The radar data was systematically underestimated by about one-fourth therain-gauge value. Small size (〜80 km2) echoes contribute greatly to the total precipitation, but there were few
large size echoes. The distribution of precipitation over Bangladesh obtained from radar data shows heavy precipitation in the northeast and southeast of the country, which is consistent with the rain-gauge rainfall data.
Using the radar data characteristics of 185 analyzed convective systems, the development location, lifetime,
shape, size, propagation speed, and direction were identified. The BMD radar data coverage (600km×600km) is
divided into 3-regions: Northern, Central, and Southern, each being 600km east to west and 200km north to south.
In general, precipitation systems develop in the Northern region and tend to move eastward. Precipitation systemdevelopment in the Central and Southern regions is significant during the peak-monsoon months (June-August).
Analysis of the radar data shows that the average time of maximum precipitation in the Northern region is 00-06
LST, in the Central region 06 LST and 15-18 LST, and in the Southern region 06-09 LST. The average lifetimeof these systems is 〜5.7 hours. Movement speed is 〜5 ms-1.

1. INTRODUCTION
Precise estimation of precipitation is essential because of suchimportant consequences as flooding and drought, which are particularly common natural disasters in Bangladesh. Thunderstorms in
Bangladesh cause death and huge damage due to flash floods every
year. Matsumoto (1998) discussed the flood situation in
Bangladesh and pointed out that heavy rainfall there together withother phenomena is one of the causes of floods. Precipitation is aninvaluable source of renewable fresh water, the importance of
which is vital to a region such as Bangladesh, which has an excessive amount of Arsenic in its ground water. Knowledge of the precise distribution and quantification of precipitation therefore may
prove helpful for disaster prevention and water management.
Agriculture-dependant and flood-affected Bangladesh also needs
to obtain accurate information on the distribution and exact amountof precipitation on a monthly basis. However, there have been few
detailed studies on the distribution of precipitation that have notbeen based on radar data for the entire country. A few investigations have used rain gauge data, Karmaker and Khatun (1995),
Islam and Shafee (1998), and Wahid and Islam (1999), Wahid andIslam (2000). The information obtained from rain-gauges is notalways sufficient for the quantification of the exact amount of precipitation, in particular, precipitation from small clouds that cannot
be collected by the low density network of rain-gauges. A denserain-gauge network is required to produce accurate estimates ofprecipitation, but this is impossible in remote areas and is very
expensive for extensive coverage. Moreover, characteristics ofprecipitation such as growth location, lifetime, shape, size, propagation speed and the direction of precipitation systems are notobtainable from rain-gauge data, but they are very important for
determining the mechanism of a monsoon system as well as model
parameterization. Efforts to estimate surface rain over Bangladesh
using every three-hours satellite data were made by Islam and
Wahid (1999), but due to the low spatial and temporal resolutionsof the data there were very poor correlations (average 0.3) between
the rainfalls calculated from twelve rain-gauges and satellite data.
Radar is more advantageous than rain-gauges for determining
various characteristics of precipitation over the country, which isstill unknown by researchers and is essential for understanding themonsoon mechanism. Obtaining the actual precipitation distribution over Bangladesh and making BMD radar useful is a challeng-

KEY WORDS: Radar, Precipitation, Lifetime, Propagation speed and direction, Cloud development

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MD. N. ISLAM, T.HAYASHI, T. TERAO, H. UYEDA AND K. KIKUCHI

Fig. 1 Regional map showing BMD radar coverage (solid bold line, left panel). Rain-gauge locations (plusmark, right panel) throughout Bangladesh with the station names. The star at the center ofBangladesh shows the location of the BMD radar. The northern, central, and southern regions in theright panel are used for Fig. 3. The arrows are used for Fig. 6.

ing task. At present, there is no reliable way to retrieve precipitation from the Bangladesh Meteorological Department (BMD) radar
data. There are two problems in using BMD radar data. One isthat the data provide only a six-range rain status, the other is that a
precise Z (Reflectivity) - R (Rain rate) relationship is needed forthis region. In this work, attempts have been made to obtain the
precipitation rate from the rain status of BMD radar data. Of
course, rain-gauge data are also invaluable in calibrating the results
obtained from radar data.
In addition, detailed examinations of precipitation systems bymeans of high spatial and temporal resolution data is essential toclarify the characteristics of the various types of mesoscale cloudsystems that develop. Fortunately, high spatial and temporal resolution data for Year 2000 from radar over Bangladesh is available
to analyze.
2. DATA USED AND ANALYSIS METHODS
Radar PPI (Plain Position Indicator) scans and rain-gauge
rainfall data from the Bangladesh Meteorological Department(BMD) were used. The BMD collects 3-hour rain-gauge rainfall at
33 ground stations (plus marks) throughout the country (rightpanel, Fig. 1). It has installed an S-band weather radar (wavelength 〜10cm, beam width 1.7°
, elevation angle 0°at the center ofthe country (star in the right panel), which gives data coverage fora 600km×600km rectangular area (Fig. 1). The BMD radar isdesigned to cover a radius of 400km, at present the effective radius
being about 250km. The radar collects 2-3 minute-interval PPI
scan data (reflectivity in dBZ) continuously for 1-hour with a 2hour interval in operation. Radar data (pixel size 2.5km×2.5km)
are available for the analysis of the period April 16 to August 30,
2000.

The BMD radar collects reflectivity (dBZ) data and automatically converts it to the precipitation rate (mm/h) and stores data insix statuses; 1 (1-4mm/h), 2 (5-16mm/h), 3 (17-32mm/h), 4 (3364mm/h), 5 (65-128mm/h), and 6 (>128mm/h). A procedure for
retrieving the precipitation rate from the precipitation status is proposed. Radar data are sampled in a 10km grid box; each grid containing 16 pixels with a pixel resolution of 2.5km. The precipitation rate is retrieved from the precipitation status using the following equations.
The hourly precipitation rate HPR is defined as
I =N

HPR = (1 / N ) Â RI
I =1…………………………………………(1)where RI is the instantaneous precipitation rate per unit area in a
10km grid box, and N is the total number of scans per hour withr =6

RI = (1 / A) Â Sr AR , r
r =1…………………………………………(2)where A is the grid area (100km2 in this analysis), Sr the status precipitation rate averaged from the possible values in a status, and AR
the rainy area corresponding to each status in the 10km grid box.
For r =1, 2, 3, 4, 5, and 6 the respective possible status values are
1-4, 5-16, 17-32, 33-64, 65-128, and 129mm/h. The respective Srvalues obtained are 2.5, 10.5, 24.5, 48.5, 96.5, and 129 for r =1, 2,
3, 4, 5, and 6.
As stated, BMD radar usually stores data continuously for one
hour with a two hour interval in operation. Sometimes the radar isoperated for several hours without a break. There are about 20 PPI
data scans available during each hour of operation. Radar data aresampled in each PPI scan, and all the available PPI scans per hour
are used to obtain the hourly value. Hourly values are used toCHARACTERISTICS OF PRECIPITATION SYSTEMS ANALYZED FROM RADAR DATA OVER BANGLADESH

obtain the daily and monthly values for different parameters. In
the radar analysis, the Echo Embedded Area (EEA) is the pixel
coverage for the rain rate 1mm/h. The Frequency of Occurrence(FO) is the number of echoes or echo-systems determined from all
the PPI scans used. The echo length, lifetime, propagation speed,
and direction of the echo systems are obtain subjectively from 2-3
minutes interval PPI scan data. The pattern-matching technique(Islam et al., 1997) is used to detect echoes during their lifetime.
In this analysis, we have tried to use available continuous PPI
scans, but when radar data are missing for a continuous two hours,
Japanese Geo-stationary Meteorological Satellite (GMS-5) data
were used as supporting information to detect the system in thenext available PPI scan. Note that 30-minute interval GMS-5 data
were available during the analysis period.

ed in Fig. 2 together with the amount of rainfall averaged for 33
rain-gauge stations located throughout the country. With few
exceptions, the EEA and rainfall provide very similar patterns for
all the days. Some days, the EEAs were not as large as the highamount of rainfall; e.g., on May 23 and 24 the amounts of rainfall
were high but the EEA was not very large. This situation continued for many days in July. The reasons for the differences include
the fact that the low-density rain-gauge network (25-140km)
throughout the country is insufficient for catching the rainfall fromtiny clouds. Sometimes when the precipitation rate is very high, asmall echo area provides a large amount of rainfall. Other reasons

3. RESULTS AND DISCUSSION
3.1 Disasters in Bangladesh in 2000
In 2000, Bangladesh suffered from severe flooding and rivererosion for the third consecutive year. Heavy monsoon rains
began six weeks early that year with no sign of letting up. In 2000,
monsoon-related disasters left some 4 million people homeless(Report 6). The nine worst-affected western districts wereSatkhira, Jessore, Jenaidah, Chuadanga, Meherpur, Kustia,
Magura, and Rajshahi. By August 11, 2000, approximately 1.26
million people in 41 sub-districts had been affected by flooding(ADRC Disaster Report, 2000/09). The usually dry southwest partof Bangladesh was affected in the last week of September 2000.
Flooding killed more than 100 people, damaged 625,000 acres(250,000 hectares) of rice and other crops, and washed away nearly 675 miles (1,080km) of roads.
3.2 Precipitation in Bangladesh in 2000
Data coverage by the BMD radar is shown on the regionalmap (solid bold line, left panel, Fig. 1) together with the raingauge network over Bangladesh (right panel, Fig. 1). Plus marks
represent the location of rain-gauge stations. The entire coverage(87.44°-93.33°E, 21.08°-26.45°N, right panel, Fig. 1) represents therectangular coverage of the BMD radar data, whereas the radar
center is positioned at the center of the country (star mark).
Time sequences for EEA, determined by the BMD radar thatcovers the whole of Bangladesh and surrounding areas are present-

19

Fig. 3(a) An instantaneous precipitation (mm/h) system at
20:04 LST on 25August. (b) An event from 10:40-11:59
LST on 28 April detected by the BMD radar in 2000.

Fig. 2 Time sequences of daily rain-gauge rainfall (mm) showing echo-embedded area (EEA) detected by the BMD radarfrom 16 April to 30 August 2000.

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MD. N. ISLAM, T.HAYASHI, T. TERAO, H. UYEDA AND K. KIKUCHI

Table 1.

Characteristics of precipitation systems in Bangladesh as observed by BMD radarare given in Section 3.4.
3.3 Characteristics of precipitation in Bangladesh
For the five months (April-August) analyzed, 20846 PPI scans
were available for analysis. On inspection of all the PPI scans, 185cases were selected to determine such characteristics of the cloudclusters as location of growth, movement, shape, size, and lifetime;
all of which are discussed here.
To understand the properties of the precipitation areas thatdeveloped over Bangladesh, an example of an instantaneous precipitation system at 20:04 LST on August 25(upper panel) and an
event from 10:40 to 11:59 LST on April 28(lower panel), detectedby the BMD radar are shown in Fig. 3. Rain-gauge data cannot
recognize the fact that strong precipitation cores are surrounded bylight precipitation regions. The strength of precipitation in Fig. 3
is reduced because the PPI scans were processed in a 10km×10km
grid box. This analysis however, facilitates our understanding of
the shapes, sizes, dimensions, patterns, movements, and lifetimes
of precipitation systems.
In Bangladesh, pre-monsoon cloud systems usually develop in
the northern part of the country, whereas monsoon cloud systems
develop throughout the country and are associated with southwestern monsoon flow. Radar data coverage (600km×600km) is
divided into three regions: Northern (northern boundary to 200km
south), Central (radar center to 100km north and 100km south) andSouthern (southern boundary to 200km north), each measuring

Fig. 4

Regional development precipitation systems analyzed for
185 cases in 2000.

600km east to west and 200km north to south as shown in the rightpanel of Fig. 1. Characteristics of the 185 analyzed precipitation
systems were examined in the three predefined regions, some ofwhich are given in Table 1 as examples. Details of type and shape
are described in Islam et al. (2004). The types are solitary andmerged the shapes Oval, Line, and Arc. The preferred locations
for the development of precipitation systems are presented in Fig.

CHARACTERISTICS OF PRECIPITATION SYSTEMS ANALYZED FROM RADAR DATA OVER BANGLADESH

4. In the Northern region, development is 90% in April, gradually
decreasing to 50% in June, continuing to decrease until August.
Precipitation systems tend to develop in the Southern region during
the peak-monsoon months (JJA). In the Central region they tend
to develop before and after the peak-monsoon month, July. On
average, the preferred locations of the development of precipitation
systems are Northern 60%, Central 21%, and Southern 19%.
Fig. 5 shows the size distribution and frequency of occurrence(FO) during the analysis period. Small size echoes contributemarkedly below the cluster size of 〜80km2. Alternatively, convective activity becomes less organized for producing large clusters. Large clusters, however, contribute to the total precipitation
due to their huge coverage.
Fig. 6 shows the propagation speed and direction of the 185
cases analyzed. Propagation speed varies from 0.4 to 19.8ms-1, but
on average is 〜5ms-1. The propagation direction, with few exceptions is almost east-northeast or east-southeast. The lifetime aver-Fig. 5Frequency of occurrence and cluster size determined by
the BMD radar for 185 cases in 2000.

Fig. 721

aged of all 185 cases is 〜5.7 hours (not shown) for the five month
period from April to August (AMJJA).
3.4. Spatial distribution and times of maximum precipitationin Bangladesh
Fig. 7 presents the spatial distribution of the averaged daily
rainfall (mm) as estimated from BMD radar data (left panel) andthat estimated by the rain-gauge system (right panel) during theentire analysis period. The radar-estimated precipitation is calculated for a 100km2 grid box with 2.5km pixel resolution at eachrain-gauge site. Precipitation in the northeast and southeast partsof Bangladesh is significantly high. In the southeast corner, precipitation is low due to the long distance from the radar center, andone station is beyond radar coverage. There are a number of stations located outside the effective radar radius of 250km (circle,
Fig. 7). Precipitation varies greatly with location in the country,
and this distribution pattern coincides with rain-gauge results,

Fig. 6

Propagation speed and direction of the 185 cases analyzed in 2000.

Distribution of precipitation (mm/day) obtained from (a) BMD radar and (b) Rain-gauge data. Average for 16
April to 30 August 2000. Large circle represents the effective radius of BMD radar.

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MD. N. ISLAM, T.HAYASHI, T. TERAO, H. UYEDA AND K. KIKUCHI

however, the rain-gauge provides a large amount of rainfall compared to radar. The discrepancy between the rainfall estimations
based on the BMD radar and rain-gauge systems is due to the following: (i) radar rainfall is estimated from the areal average of100km2 grid boxes, whereas rain-gauges indicate point values; (ii)there is inconsistency in the temporal and spatial averaging of theradar and rain-gauge data; (iii) rain status is calculated based on asingle Z-R relationship, details of spatial variation in the precipitation in Bangladesh are discussed in Islam et al. (2005). The northern border of Bangladesh, close to the Shillong Hills of India, isthe region with the highest rainfall, the second highest rainfall
occurring along the eastern border.
Fig. 8 gives the areal averaged precipitation rates determinedby BMD radar in the three predefined regions. The rate has the
maximum value at 00-06 LST in the Northern, at 06 and 15-18
LST in the Central, and at 06 LST in the Southern regions. These
times are consistent with the results reported by Ohsawa et al.
(2001) in an analysis of rainfall in Bangladesh but differ from thegeneral characteristics reported for inland rainfall (e. g., Gray andJacobson, 1977; Riehl, 1978; Meisner and Arkin, 1987; Al-brightet al., 1985; Short and Wallace, 1980; Mapes et al., 2003; Warner
et al., 2003). In reality, the precipitation rates in the Northern andSouthern regions tend to be much higher because these regions arebeyond the BMD radar effective radius (250km). At any rate, thetime of maximum rate is very important for determining exact
diurnal variations in the tropical convective activity that developedin those regions. Details of diurnal cloud activity variation in
Bangladesh are discussed in Islam et al. (2004). The nature of thediurnal precipitation cycle in Bangladesh is a morning peak at
0600 LST and a minimum at noon.
Rain-gauge rainfall versus radar rainfall scatter plot (leftpanel, Fig. 9) shows the relationships between them for the entire
BMD radar coverage in different months (April-August) of 2000.
The right panel in Fig. 9 shows the relationships between rainfall
calculated from the rain-gauge and radar data averaged from AprilAugust 2000 inside the effective BMD radar coverage (<250km).
Clearly, the radar data underestimates rainfall by about one-fourththat of the rain-gauge value. The correlation coefficients for thedifferent months (0.63-0.89) are tabulated in Table 2. For all fivemonths (April-August) the correlation coefficient is 0.64. The
coefficient increases in magnitude (0.70) inside the effective radiusof the BMD radar. If a new best-fit Z-R relationship is obtainedfor BMD radar that considers the same temporal and spatial aver-Table 2. Relationship between rainfall obtained by radar and rain-gauge

Fig. 8Precipitation rates (mm/h) determined by the BMD radarat three regions in Bangladesh. Averages for 16 April to
30 August 2000 (AMJJA 2000).

a)Fig. 9b)

Relationships between rainfall calculated by radar and rain-gauge data. a) in different months of 2000
and b) averaged for all months (March-August) in 2000 inside radar effective radius. Symbols in a)
represent for different months (M4 = April, M5 = May, M6 = June, M7 = July, and M8 = August).

CHARACTERISTICS OF PRECIPITATION SYSTEMS ANALYZED FROM RADAR DATA OVER BANGLADESH

aging then the correlation further increases. The discrepancy in therainfall values estimated by radar and rain-gauge, as shown in Fig.
9, will be reduced. Research in this topic on going.
4. CONCLUSIONS
The rain rate could be retrieved successfully from the BMD
radar rain status and the correlation coefficients obtained between
the rainfall as calculated by radar and rain-gauge data ranging from
0.63-0.89 for the different months, but there was substantial systematic underestimation. Analysis of 185 cases over five months(AMJJA) of 2000, revealed the characteristics of precipitation systems-development locations, sizes, shapes, dimensions, lifetimes,
propagation speeds, and directions for the first time in Bangladesh.
Most of the precipitation systems were found to develop in the
Northern region then move east-northeast or east-southeast. The
average lifetime of these systems is 〜5.7 hours and the speed of
movement 〜5 m/s. Midnight to morning (00-06 LST) is the optimum period for maximum precipitation in the Northern region of
Bangladesh whereas it is 06 LST in the Southern region. The optimum times in the Central region are 06 LST and 15-18 LST. The
above information on precipitation systems in Bangladesh isimportant for model parameterization, which should prove usefulfor forecasting that can be used for water management and for theprevention of natural disasters. An up-grade of the BMD radarsystem has been proposed, and in the near future it will be possible
to obtain quantitative amounts of rainfall from BMD radar data.
ACKNOWLEDGEMENTS
We thank the BMD for providing the radar and rain-gauge
data collected under the JICA Japan Bangladesh Joint Study
Project-Phase II , grant no 11691151. One of the authors Nazrul
Islam was fully supported during this work by Islamic
Development Bank (IDB), Saudi Arabia, under an IDB merit
scholarship program. Dr. Jun Matsumoto, the University of Tokyo
is thanked for his assistance in the copying of the radar data. We
are also grateful to Takeshi Maesaka of Nagoya University for his
invaluable help in the data processing. Disaster information on
Bangladesh can be obtained at http://www.ifrc.org.
REFERENCES
ADRC Disaster Report, Asian Disaster Reduction Centre Disaster Report
FL-2000-0555-BGD, No 2000/09.
Albright, M. D., E. E. Recker, R. J. Reed and R. Dang, 1985. The diurnal
variation of deep convection and inferred precipitation in the central
tropical Pacific during January-February 1979. Mon. Wea. Rev., 113,
1663-1680.

23

Gray, W. M. and R. W. Jr. Jacobson, 1977. Diurnal variation of deep
cumulus convection. Mon. Wea. Rev., 105, 1171-1188.
Islam, M. N. and S. Shafee, 1998. Prediction models for drought and different meteorological variables in Bangladesh. Dhaka Univ. J. Sci.,
46(2), 327-334.
Islam, M. N. and C. M. Wahid, 1999. On the development of a technique toestimate surface rain using satellite data over Bangladesh. Bang. J.
Sci. Res., 17(2), 181-188.
Islam, M. N., T. Terao, H. Uyeda, T. Hayashi and K. Kikuchi, 2005.
Spatial and Temporal Variations of Precipitation in and around
Bangladesh. J. Meteor. Soc. Japan, 83(1), 21-39.
Islam, M. N., T. Hayashi, H. Uyeda, T. Terao and K. Kikuchi, 2004.
Diurnal variations of cloud activity in Bangladesh and north of the
Bay of Bengal in 2000. Remote Sensing of Environment, 93(3), 378388.
Islam, M. N., Uyeda, H. and Kikuchi, K., 1997. Characteristics of clouds
and cloud clusters obtained by radar and satellite data during the
TOGA-COARE IOP. J. Fac. Sci. Hokkaido Univ. Japan, Ser. Vll(Geophysics), 10(2), 189-223.
Karmaker, S. and A. Khatun, 1995. Variability and probabilistic estimatesof rainfall extremes in Bangladesh during the southwest monsoon season. MAUSAM, 46, 47-56.
Mapes, B. E., T. T. Warner, M. Xu, and A. J. Negri, 2003. Diurnal; patterns of rainfall in Northwestern South America. Part I: Observations
and context. Mon. Wea. Rev., 131, 799-812.
Matsumoto, J., 1998. Synoptic features of heavy monsoon rainfall in 1987
related to the severe flood in Bangladesh. Bulletin of the Dept.
Geography, Univ. Tokyo, Japan, 20, 43-56.
Meisner, B. N and P. A. Arkin, 1987. Spatial and annual variations in thediurnal cycle of large-scale tropical convective cloudiness and precipitation. Mon. Wea. Rev., 115, 2009-2032.
Riehl, H. and A. L. Miller, 1978. Differences between morning andevening temperatures of cloud tops over tropical continents andoceans. Quart. J. Roy. Meteor. Soc., 104, 757-764.
Ohsawa, T., H. Ueda, T. Hayashi, A. Watanabe and J. Matsumoto, 2001.
Diurnal variations of convective activity and rainfall in tropical Asia.
J. Meteor. Soc. Japan, 79, 333-352.
Report 6, International Federation of Red Cross and Red CrescentSocieties, Situation report no. 6, Appeal no. 20/2000.
Short, D. A. and J. M. Wallace, 1980. Satellite-inferred morning-toevening cloudiness changes. Mon. Wea. Rev., 108, 1160-1169.
Wahid, C. M. and M. N. Islam, 2000. Use of satellite data to estimate rainfall over Bangladesh. Bang. J. Sci. & Tech., 2(1), 141-145.
Wahid, C. M. and M. N. Islam, 1999. Patterns of rainfall in the northernpart of Bangladesh. Bang. J. Sci. Res., 17(1), 115-120.
Warner, T. T., B. E. Mapes, M. Xu, 2003. Diurnal patterns of rainfall in
Northwestern South America. Part II: Model Simulations. Mon. Wea.

Rev., 131, 813-829.