Vertical variations of rain intensity in different rainy periods in and around Bangladesh derived...

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 28: 273–279 (2008)Published online 23 July 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1585

Short Communication

Vertical variations of rain intensity in different rainy periodsin and around Bangladesh derived from TRMM observations

Md. Nazrul Islama* and Hiroshi Uyedab

a Department of Physics, Bangladesh University of Engineering & Technology, Dhaka, Bangladeshb Hydrospheric Atmospheric Research Center, Nagoya University, Japan

ABSTRACT: In this paper, differences in rain intensity in and around Bangladesh during various rainy periods aredescribed. The data were obtained from tropical rainfall measuring mission (TRMM) -2A25, -3B42 and -3B43. Theversion 5 (V5) 3B42 daily products overestimated rainfall in pre-monsoon and post-monsoon periods but underestimatedit in the monsoon period compared with rain-gauge data. The version 6 (V6) daily products reduced the over- and under-estimation of rainfall. On the basis of the analysed rainfall characteristics in different rainy periods, the 2A25 data wasanalysed to reveal the vertical variations of rain intensity. Pre-monsoon rainfall was characterized by convective rain withstrong intensity upto high altitudes and high echo tops that usually occurred over land and not over the Bay of Bengal.In contrast, during monsoon, intense rain occurred over the Bay of Bengal and land; however, the existence of strong andhigh echo tops was less significant than that during the pre-monsoon period. The radar echo tops and the height of strongrain intensity measured in the post-monsoon period were lower than those obtained in the monsoon period. Copyright 2007 Royal Meteorological Society

KEY WORDS rainfall; pre-monsoon; monsoon; post-monsoon; intensity variation; echo top; TRMM

Received 21 December 2006; Revised 23 May 2007; Accepted 24 May 2007

1. Introduction

Monsoon rainfall is considered to be an atmosphericheat source that influences the global atmospheric cir-culation (Ose, 1998). It is known that a large amountof rainfall occurs in Bangladesh (88.05° –92.74 °E and20.67° –26.63 °N). In and around Bangladesh, the rainyseason is divided into the following periods: (1) pre-monsoon (March–May), (2) monsoon (June–September)and (3) post-monsoon (October–November) (Islam andUyeda, 2007). Pre-monsoon convections are character-ized by high-intensity rainfall, whereas monsoon rainfallis less intense (Kodama et al., 2005). Terao et al. (2006)analysed data from rain gauges in Bangladesh to show thedifference in rainfall intensity between pre- and mature-monsoon periods. Some preliminary work on the rainfallcharacteristics in Bangladesh has been reported by Islamet al. (2005b). However, the cause of the variations ofrain intensities in different parts of the rainy season isnot clear enough. The existence of the liquid contents atvarious altitudes may be one of the candidates in findingthe possible reasons of the variations of rain intensities indifferent periods. The study of the vertical structure of the

* Correspondence to: Md. Nazrul Islam, Department of Physics,Bangladesh University of Engineering and Technology, Dhaka-1000,Bangladesh. E-mail: [email protected];nazrul [email protected]

liquid contents is not possible using available data fromthe rain-gauge and radar plan position indicator (PPI); thevolume scan radar data is not available yet in Bangladesh.In such a situation of data paucity, the tropical rainfallmeasuring mission (TRMM) data products help studyingthe vertical variations of rain intensity of the precipitationfields in Bangladesh.

Launched on 27 November 1997, the TRMM satelliteis a joint effort of NASA (USA) and JAXA (Japan)to improve the measurement and characterization oftropical rainfall. The TRMM satellite orbits the Earthin a low Earth orbit at about 400 km above the surfaceproviding observations over a region between 38 °Nand 38 °S. Recently, data from the TRMM precipitationradar (PR) have been utilized to investigate rainfallover Hawaii and the surrounding ocean (Kidd andMcGregor, 2007). TRMM datasets have been utilizedto obtain the echo tops of convection developed overBangladesh in the pre-monsoon and monsoon periods(Kodama et al., 2005). As a next step, characteristicsof vertical structure of precipitation fields are expectedto be revealed. Therefore, attempts are made to clarifythe vertical variations of rain intensity using the TRMM2A25 data in order to understand one of the basiccharacteristics of rainfall in and around Bangladesh thatdifferent rain intensities occurred during the differentrainy periods.

Copyright 2007 Royal Meteorological Society

274 MD. N. ISLAM AND H. UYEDA

2. Data and methods of analysis

TRMM is NASA’s first mission dedicated to observingand understanding tropical rainfall and how it affectsglobal climate (Simpson et al., 1988). TRMM includeda number of precipitation-related instruments such asthe visible and infrared sensor (VIRS), the SSM/I-likeTRMM microwave imager (TMI) and the PR (Kum-merow et al., 2000). It is the first platform from whichspace-borne radar observations were made with a PR. Thelevel 2 standard algorithm that retrieves the instantaneousvertical profiles of rainfall rate and attenuation-correctedradar reflectivity factor from the TRMM PR data is des-ignated as 2A25 in the TRMM project, details of theTRMM 2A25 is described in Iguchi et al. (2000a,b). TheTRMM -3B42 and -3B43 data products are explained inHuffman et al. (2004).

TRMM V5 3B42, V5 3B43, V6 3B42 and V6 3B43data for 1998–2002 were analysed to obtain daily andmonthly rainfall amounts in Bangladesh. Bangladeshground-based rain-gauge (BG) data collected at 33 sta-tions (Figure 1, triangles) by the Bangladesh Meteorolog-ical Department (BMD) are utilized for the calibration ofTRMM products. The availability of reliable 3-hourly BGdata for the years 1998–2002 is considered to compareTRMM calculated rainfall with rain-gauge data. The dailydata from TRMM V5 3B42 products estimated griddedrainfall of 1° × 1° resolution. The V5 3B43 product isproduced by merging the monthly accumulation of dailyV5 3B42 with the monthly accumulated climate assess-ment and monitoring system (CAMS) or Global Precipi-tation Climatology Centre (GPCC) rain gauge analysis(3A45) (Huffman et al., 1995). The V6 3B42 is a 3-hourly 0.25° product based on multi-satellite precipita-tion analysis (Huffman et al., 2004). The multi-satelliteand gauge analysis are merged (Huffman et al., 1997) tocreate a post-real-time satellite-gauge monthly product,which is the TRMM product V6 3B43. The 2A25 data,with a nominal 4.3 km spatial resolution and 250 m ver-tical sampling from the surface to 20 km altitude (Kiddand McGregor, 2007), is used in this analysis. The periodof PR data is different from 3B data and BG data, but theeffect of different integration time is small. Iguchi et al.(2000b) describes the TRMM 2A25 algorithm that esti-mates the vertical profiles of attenuation-corrected radarreflectivity factor and rainfall rate. The PR instrumentobserves a swath of 220 km with 49 beams every 0.6 s.Each beam width is about 0.7° with the pulse width of1.67 µs. TRMM 2A25 data are analysed in the domainof 87–94 °E and 18–28 °N (Figure 1) to obtain the ver-tical variations of rain intensities and spatial distributionof location of maximum rain rates in different rainy peri-ods from 2000 through 2003. The rain rates derived fromthe radar reflectivity of TRMM PR using the algorithmsof TRMM science data and information system (TSDIS)are considered as rain intensities. Some good cases, 34 inApril, 82 in July and 36 in October, which includes rain inand around Bangladesh, are manually selected from 2A25overpass. The number of selected cases is few due to only

a single overpass of the TRMM PR during a 24-h period.If there is a TRMM pass in the analysis domain with noconsiderable convection, the overpass is excluded fromthe analysis. Among the selected overpasses, the maxi-mum rain rate and its position are extracted from eachoverpass. The maximum rain rate is defined as the mostintense rate in an overpass. The 2A25 has 80 layers, eachof which is 250 m thick. Using these layers, the verticalextension of the precipitation field upto 20 km from thesurface is obtained along with the maximum precipitationarea. The echo top is defined as the top of the verticalextension of the precipitation field for a specific thresh-old rain rate. In this analysis, echo tops for different rainthresholds 5, 10, 15, . . ., 50 and 60 mm/h are determinedmanually. First, the vertical extension of the precipitationfield is created and determined by the echo top height foreach specified threshold. The months of April, July andOctober are selected from each year for detailed anal-ysis as the peak months of the pre-monsoon, monsoonand post-monsoon periods, respectively. Performance ofa TRMM product is defined by the difference betweenthe TRMM and BG values divided by the BG value, andis expressed in percentage.

3. Results and discussion3.1. Wind conditions for different rainy periods in andaround Bangladesh

Figure 2 shows the spatial distribution of the precipitationfield calculated from V5 3B42 datasets superimposed

Figure 1. Analysis domain and location of the rain gauges installed byBangladesh Meteorological Department throughout Bangladesh.

Copyright 2007 Royal Meteorological Society Int. J. Climatol. 28: 273–279 (2008)DOI: 10.1002/joc

VERTICAL VARIATIONS IN RAIN INTENSITY IN BANGLADESH DERIVED FROM TRMM 275

with the wind circulation at 850 hPa derived from NCEPreanalysis data for 1998–2002. In Bangladesh, a low-level westerly blows in April usually with thunderstorms.With the advancement of the month, the wind directionturns from the northwest to the southwest, starting themonsoon. In October (post-monsoon), the wind directionagain changes from the southwest to the southeast. About62% of the annual rainfall in Bangladesh occurs duringthe monsoon period (Islam and Uyeda, 2005a). Pre-monsoon rain is usually observed in Bangladesh and tothe north, whereas monsoon rain occurs in and around thecountry; rain is especially heavy over the Bay of Bengal.In Figure 2, a smaller amount of rainfall is observed in

April due to the brief duration of rainfall and fewer rainydays than in July.

3.2. Comparison of TRMM and BG rainfall inBangladesh

Figure 3(a) shows the comparison of seasonal rainfallobtained from TRMM V5 3B42-, V5 3B43-, V6 3B42-,V6 3B43 and BG. Rainfall in each season is averagedfrom 1998 to 2002. The V5 3B42 overestimated rain-fall in the pre-monsoon and post-monsoon periods andunderestimated it in the monsoon period. The V5 3B43underestimated rainfall in all periods. The V6 3B42- and

Figure 2. Spatial distribution of rainfall obtained from V5 3B42 for (a) April, (b) July and (c) October. The wind field is calculated from NCEPreanalysis data for the same months of rainfall. Wind and rainfall are averages for 1998–2002.



V6 3B43-estimated rainfall amounts are very close to theBG-estimated one.

The V5 3B42, V5 3B43, V6 3B42 and V6 3B43performances obtained for 1998–2002 are about −2.8,−10.1, −6.8 and −4.2%, respectively. But the factis that V5 3B42 substantially overestimates the gaugemeasurement in the pre-monsoon and underestimates itduring the monsoon, whereas V6 measure almost closerto the observed value in both periods (Figure 3(a)).Hence, the performance of V5 3B42 is the best. AllTRMM products underestimated the monsoon rainfall.Figure 3(b) shows the scatter plots of monthly rainfallmeasured by V5 3B42, V5 3B43, V6 3B42 and V6 3B43with BG during 1998–2002. The V5 3B42 overestimatedlow rainfall and underestimated high rainfall totals. Thedegree of underestimation is much higher for V5 3B43.Overall, V6 3B43 is better in all measurements. TheTRMM has some false detection for low rainfall totalsand limitations in detecting very high rainfall. The V5has large false overdetection whereas V6 has small falseunderdetection (Islam and Uyeda, 2007). The fact is thatthe low bias of V5 3B43 may be due to the non-inclusion

of some rain events in the operational gauge analysesthat are used in the production of V5 3B43 (Chiuet al., 2006). The reduction of rainfall overestimationin both pre-monsoon and post-monsoon periods, andunderestimation in monsoon periods in V6 (compared toV5) results in better agreement between BG estimatesand TRMM V6 products. The correlations between BGand TRMM rainfall estimations are 0.95, 0.96, 0.97 and0.98 for V5 3B42, V5 3B43, V6 3B42 and V6 3B43,respectively. These correlations are better than 0.65, 0.81,0.84 and 0.86 for V5 3B42, V5 3B43, V6 3B42 andV6 3B43, respectively, obtained by Chokngamwong andChiu (2006) for Thailand who analysed V5 data for1998–2004 (March) and V6 data for about 1998–2005.This occurs due to the different rainfall climatology inThailand and Bangladesh and the integration time of data.

The scatter plots between daily rainfall measured byV5 3B42 and V6 3B42 with BG for 1998–2002 aredisplayed in Figure 3(c). The correlations between BGand 3B42 daily rainfalls are 0.91 and 0.89 for V5 andV6, respectively. Hence, the correlation is reduced only4–8% for daily rainfall compared to the monthly data,

Figure 3. (a) Seasonal rainfall obtained from TRMM datasets and BG, (b) Scatter plots of TRMM products versus BG rainfall and (c) Scatterplots of V5 3B42 and V6 3B42 versus BG rainfall.



TRMM estimates are therefore useful for understandingthe monthly rainfall in Bangladesh and informative fordaily rain estimations.

3.3. Maximum rainfall in and around Bangladesh

The occurrence of maximum rainfall is obtained from2A25 datasets for the domain of 87–94 °E and 18–28 °N.The maximum rain intensity in April (dark closed circle,Figure 4) occurs over land and is not significant overthe Bay of Bengal. Maximum rain events are observedin the northeastern parts of Bangladesh and the northof the country. In contrast, during July (medium darkclosed circle, Figure 4) maximum rain intensity eventsare distributed over the Bay of Bengal and land. Thedistribution of locations of maximum rain intensity inOctober (light dark closed circle, Figure 4) is somehowsimilar to that of July, but the number of events isfewer, with 36 in October compared with 82 in July.The strength of the rain intensity of maximum rain rateevents in April is higher than that in the July and Octoberevents (from the size of circle in Figure 4). The reasonfor the variations in strength in different rainy months isdiscussed in the next sub-section.

3.4. Vertical variations of rain intensity in differentrainy periods in and around Bangladesh

As explained in the previous section, the rain intensityof maximum rain rate events in April is higher compared

Figure 4. Distribution of the locations of maximum rain rates in April(dark closed circle), July (medium dark closed circle), and October(light dark closed circle). The open circle at the lower left of the panel

represents the size of rain rate.

to that in July and October. To understand the distinctionof intensity in different periods, maximum rain rates andecho top heights are calculated for 34 cases in April, 82cases in July and 36 cases in October using 2A25 datasetsduring 2000–2003. Averages from the above mentionedcases, show that rain rates are high in April compared toother months, and low in October (Figure 5). Echo topheight is also higher in April than in other months, andlow in October.

Figure 6 shows the echo top heights for differentthresholds calculated from the same number of casesused in Figure 5. In April, a pre-monsoon month, highecho tops are found for large threshold rain rates(Figure 6(a)). On the other hand, low echo tops are foundfor large threshold rain rates in July, a monsoon month(Figure 6(b)), and in October, a post-monsoon month(Figure 6(c)). Above 5 km, a small number of echo topsare observed in July and October beyond the thresholdlimit of 20 mm/h. For April, many echo tops are foundabove 5 km beyond the same threshold limit. It is clearthat heavy rainfall has higher echo top height correspond-ing to strong rain rate in the pre-monsoon than in otherrainy periods. Thus, this explains the high rain intensityof maximum rain rate events in pre-monsoon convec-tive systems. Results from the manual detection of echotop may be useful in developing an automatic echo topdetection program.

The rain intensity of maximum rain rate events issomewhat low in July, a monsoon month, and very lowin October, a post-monsoon month, relative to that inApril, a pre-monsoon month (Figures 4 and 5). Accordingto Figure 3(a), daily rainfall in July is higher comparedto other months because in this month the number ofrainy days is many. So, monthly averaged daily rainfallis higher in this month that does not represent thehigh intensity of rain rates. Moreover, daily rain rateduring pre-monsoon is not large but rain intensity ishigh. It can be concluded that the existence of highrain intensities at high altitudes, that is deep systems(Figure 6), produces high rain rates during pre-monsoonconvection (Figure 5).

Figure 5. Maximum rain rates and echo tops are calculated by using2A25 data in April, July and October.



Figure 6. Echo tops calculated at different threshold rain rates by using 2A25 data in (a) April, (b) July and (c) October. The horizontal andvertical lines represent 5 km and 20 mm/h, respectively.

4. Conclusions

A comparison of rainfall amounts determined by TRMMand BG shows that the performance of V6 3B43 is usefulfor understanding rainfall climatology (monthly rainfall)in Bangladesh, and V6 3B42 is useful for estimatingshort-term (daily) rainfall. In addition, pre-monsoon con-vection activities are observed over land, whereas mon-soon and post-monsoon activities are distributed overthe Bay of Bengal and land. One of the basic char-acteristics of rainfall in and around Bangladesh is thatpre-monsoon rainfall is more intense than that duringother rainy periods. The analysis of TRMM 2A25 prod-ucts reveals that intensified echo regions extend uptorelatively higher altitudes for pre-monsoon echoes than

for those in other periods. On average, the echo topheight is high in pre-monsoon events. These results canexplain the vertical variations of rain intensities in dif-ferent rainy periods. This availability of analyses of rainintensity manually judged the TRMM 2A25 data encour-ages our future plan to analyse TRMM 2A25 data inpreparing a routine to detect three-dimensional echo cellsautomatically.

Acknowledgments

The authors would like to thank BMD for providingrain-gauge data and Dr A. K. M. Saiful Islam ofIWFM, BUET for helping with TRMM data processing.



This work is supported by the TRMM-RA4 project ofJAXA. In addition, the authors acknowledge the workof the reviewers in providing helpful suggestions andcomments.

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