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Climatology of the Premonsoon Indian Dryline

Nasreen Akter and Kazuhisa Tsuboki

Abstract

A northeast‐ to southwest‐oriented surface dryline with a dew point gradient of 1 °C per 10 km develops along the eastern Indian coast during the entire premonsoon season. Deep, hot, dry air of up to 500 hPa moves from arid regions in Southwest Asia and Western India to the Bay of Bengal (BoB), and warm, shallow, moist air from the BoB penetrates below the dry air, forming an inclined moisture gradient aloft to the east and reaching the surface to the west. The slope of the gradient depends on the vertically increased southwesterly wind over the BoB, which gradually intensifies from March to May. Diurnal low‐level temperature variations over the land cause periodic movements of the dry‐moist air convergence zone at the surface in the zonal direction. During the premonsoon, the average east–west diurnal oscillation of the surface dryline is approximately 100 km, and the eastward movement is almost three times faster than the westward movement.

Variability of Low-Level Moisture Flux and Its Relation With Precipitation

N. Naher and Nasreen Akter

Abstract

This study investigates long-term variation of low-level vertically integrated moisture flux (VIMF) from 1000 hPa to 850 hPa for 37-year from 1979 to 2015 and its contributions to the total rainfall over South Asia (SA), which includes also the Bay of Bengal and the eastern part of Arabian Sea. ERA-Interim 6-hourly reanalysis data generated by European Centre for Medium-Range Weather Forecasts (ECMWF) having horizontal resolution of approximately 80 km are used in this study. The details of spatial and temporal characteristics of low-level moisture flux and its effect on precipitation are analyzed for the sub-regions of South-West (SW), Bay of Bengal (BoB), South-East (SE), North-West (NW), Bangladesh and surroundings (BAN), and North-East (NE). The average value of 37 years VIMF over the study area is found 113.71 kg m-1 s-1, which remains almost constant over the year. About 41% of the total VIMF is found to be advected in the monsoon season, whereas, in the winter season only 17% is transported. The value of VIMF is highest (191.74 kg m-1 s-1) in July over SA which is 33% more than the minimum value in March. For six sub-domains, 27% of the total VIMF is found over the BoB whereas only 6% is found over the NE domain. The SW, SE, NW and BAN regions receive 21%, 19%, 13% and 14% of the total flux, respectively. Southern part of the study area (SW, BoB and SE) acquires 67% of the total VIMF, whereas, 33% is found over the Northern part (NW, BAN and NE). Seasonal variation indicates that the Southern parts of the study area have two times higher VIMF than the Northern parts in the monsoon season. Low level moisture flux contributes large amount of precipitation over the study area. 72% of total precipitation is coming from moisture transport whereas only 28% contributes from local evaporation. Individual domains also show the similar contributions. The coefficient of determination (correlation coefficient) between VIMF and the advective component of precipitation is 0.87 (0.96) and that for evaporation and the precipitation arising from local evaporation (Pm) is 0.456 (0.85).

An Overview of Tropical Cyclone Formation over the Bay of Bengal

Nasreen Akter

Abstract

Tropical Cyclone is characterized by a warm-core low pressure system at the centre called eye. It is often accompanied with strong gusty winds, torrential rains and storm surges which create great damage to life and property. The outer rain bands of the eye are made up of convective cells embedded in straight form rain. Thehorizontal shape of an outer rain band is generally prominent with equiangular spiral geometry but in the Bay of Bengal the eastern side band of the cyclone Sidr (2007) was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone centre. In fact, the Bay of Bengal experiences many differences in forming tropical cyclone. The formation of TC is bimodal and the post-monsoon cyclone frequency is almost double than that of pre-monsoon cyclone. Again, pre-monsoonenvironments are more favourable for cyclogenesis. So, it is very important to improve the understanding of the cyclone characteristics and monitor their intensity in the Bay of Bengal.

The Variation of Sea Salt Aerosol over North Indian Ocean Including Bangladesh and Its Impact

M. A. Amin and Nasreen Akter

Abstract

Aerosols are the severe threats to human life as well as ecosystem. Highest amount of natural and anthropogenic aerosols are observed over the South Asian region. Among the natural aerosols, sea salt aerosols (SSAs) have significant concentrations which are analyzed using the Monitoring Atmospheric Composition & Climate (MACC) reanalysis data for 10 years from 2003 to 2012. Average SSA is found to be about 17% of the total aerosols over the North Indian Ocean (NIO). On the other hand, Bangladesh receives 9% SSAs which are mostly advected by the southerly wind from the Ocean. The annual variations of SSA are found to be significantly positive with an approximate value of 19% in the NIO and 28% in Bangladesh. Seasonal distributions of SSA provide a highest concentration of 0.13 during the monsoon season (June-September) and lowest value of 0.03 in winter (January-February). The average wind velocity over the NIO has displayed strong positive correlation with SSA (r = ~0.93), which plays the major role for the seasonal variability of SSA distribution. The monthly average precipitation and relative humidity at surface are positively correlated with SSA by the values of 0.83 and 0.70, respectively for the NIO.

The Effect of Atmospheric Variables on Soil Moisture over Bangladesh

M. Paul and Nasreen Akter

Abstract

The moisture for four different depths of soil is analyzed using 36-years reanalysis data. The spatial variation shows that moisture increases with the soil depth. This characteristic is prominent in the dry season (November-May), whereas in the wet season (June-October) moisture reaches the saturated conditions for all layers. The value of soil moisture provides an annual periodicity with maximum value of 0.28 m3m-3 in July and minimum value of 0.17 m3m-3 in March. Atmospheric variables of evaporation minus precipitation and relative humidity are compared to soil moisture in the surface layer which indicates the strong positive relation with the correlation coefficient value of -0.87 and 0.88, respectively. Monthly temperature variation also affects on the amount of soil moisture. The changing of surface temperature changes soil moisture after two months later with an approximate value of 0.01 m3m-3.The long-term annual variability of soil moisture provides a periodic trend for all layers which indicates about the effect of climate change.

Mesoscale Convection and Bimodal Cyclogenesis over the Bay of Bengal

Nasreen Akter

Abstract

Mesoscale convective systems (MCSs) are an essential component of cyclogenesis, and their structure and characteristics determine the intensity and severity of associated cyclones. Case studies were performed by simulating tropical cyclones that formed during the pre- and postmonsoon periods in 2007 and 2010 over the Bay of Bengal (BoB). The pre- (post) monsoon environment was characterized by the coupling of northwesterly (southwesterly) wind to the early advance southwesterly (northeasterly) monsoonal wind in the BoB. The surges of low-level warm southwesterlies with clockwise-rotating vertical shear in the premonsoon period and moderately cool northeasterlies with anticlockwise-rotating vertical shear in the postmonsoon period transported moisture and triggered MCSs within preexisting disturbances near the monsoon trough over the BoB. Mature MCSs associated with bimodal cyclone formations were quasi linear, and they featured leading-edge deep convection and a trailing stratiform precipitation region, which was very narrow in the postmonsoon cases.

In the premonsoon cases, the MCSs became severe bow echoes when intense and moist southwesterlies were imposed along the dryline convergence zone in the northern and northwestern BoB. However, the development formed a nonsevere and nonorganized linear system when the convergence zone was farther south of the dryline. In the postmonsoon cases, cyclogenesis was favored by squall-line MCSs with a north–south orientation over the BoB. All convective systems moved quickly, persisted for a long time, and contained suitable environments for developing low-level cyclonic mesovortices at their leading edges, which played an additional role in forming mesoscale convective vortices during cyclogenesis in the BoB.

Role of Synoptic-Scale Forcing in Cyclogenesis over the Bay of Bengal

Nasreen Akter and Kazuhisa Tsuboki

Abstract

The cyclone frequency distribution over the Bay of Bengal during 1990–2009 was distinctly bimodal, with a primary post-monsoon peak and a secondary pre-monsoon peak, despite the very high convective available potential energy (CAPE) during the pre-monsoon. The location of the monsoon trough over the bay is a primary factor in tropical cyclogenesis. Because the trough was in the northernmost bay during the pre-monsoon season, cyclogenesis was inactive in the southern bay, where a strong southwesterly wind shear was found. In this season, moreover, a hot, dry air mass extending vertically from 950 to 600 hPa was advected from northwestern India toward the bay. Moist, warm southwesterly winds penetrating below the deep, dry air mass caused a prominent dryline to form aloft on the northwestern side of the bay. The synoptic-scale hot, dry air forcing to the bay suppressed the active convection necessary for cyclogenesis. The strength of the stable environmental layer, represented by convective inhibition (CIN), was extremely large, and acted as a cap over the northern and northwestern bay. Conversely, during the post-monsoon, there were no horizontal temperature or moisture gradients, and CAPE and CIN were fairly modest. The entire bay was covered by a very deep, moist layer from the surface to 700 hPa transported from the east. The monsoon trough position and the environmental CIN in combination can explain the lower frequency of cyclogenesis during the pre-monsoon compared with the post-monsoon season.

Numerical Simulation of Cyclone Sidr Using Cloud Resolving Model: Characteristics and Formation Process of an Outer Rainband

Nasreen Akter and Kazuhisa Tsuboki

Abstract

Cyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation.

The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band consisted of convection with stratiform rain, whereas the upwind and middle portions contained active convective cells without stratiform rain.

The band was located between the synoptic-scale flows of a weakly sheared, gradient-balanced westerly and a strongly sheared, nongradient-balanced prevailing southerly caused by the complex terrain of the Bay of Bengal’s southeast region. Low-level convergence along the band was dominated by cross-band flow from both sides of the band and was confined below 3 km. As the cyclone moved northward, the convergence zone resulted in the extension of band length up to ~800 km. The southerly at the eastern side of the center gradually accelerated and was directed toward the center by a strong pressure gradient force. The flow accumulated a substantial amount of water vapor from the sea in addition to the increased moisture in the lower troposphere, resulting in further intensification of the convective cells.

Characteristics of Supercells in the Rainband of Numerically Simulated Cyclone Sidr

Nasreen Akter and Kazuhisa Tsuboki
Journal Paper SOLA, 6A, 25-28

Abstract

Cyclone Sidr had an intense rainband east of the cyclone center. The rainband exhibited two strong convective lines (band axes) composed of convective cells. To study the characteristics of the convective cells, a simulation was performed at 1-km resolu-tion by using a cloud-resolving model. In both band axes, some cells showed the characteristic structure of a supercell. Supercells within the outer axis had stronger updraft, more intense precipita-tion, and a longer lifetime than those of the inner axis. The values of the CAPE and helicity are different in the strong vertical shear environment of the inner and outer axes. A large moisture flux was present throughout the troposphere on the east side of the outer axis. On the other hand, the upper level west of the inner axis was relatively dry. These are additional factors affecting the differ-ences in cell characteristics in the two band axes.

Employment of MM5 in Simulating MCSs Developed in and around Bangladesh

Nasreen Akter and Islam M. Nazrul

Abstract

The mesoscale convective systems (MCSs) produce numerous weather hazards with their variety of form. The formation mechanism of MCSs is thus important to know for Bangladesh and its surroundings, because this region is one of the heaviest rainfall areas in the tropical zone. The meteorologists are studying and analysing the formation mechanism of different types of MCSs using radar and satellite observations data. There are limitations to acquire the continuous data from weather radar and rain-gauge network in Bangladesh. Observations are limited to real time, but for planning purposes projected data over a certain period obtained from a climate model is the requirement. Consequently the motivation of this paper is to obtain the evolution and life cycle of MCSs developed in and around Bangladesh in pre-monsoon period on 20 May 2002 using the simulation data from numerical mesoscale model called MM5. Before going to use the model data in application purposes the calibration of MM5 model for cumulus parameterization has been performed during the pre-monsoon period for this region. In the present study two domains with mesh resolutions 45 × 45 km and 15 × 15 km are prepared. Five MM5 runs using different cumulus parameterizations are carried out for sensitivity test. For that purpose the precipitations data of the model are compared structurally and numerically with that of tropical rainfall measuring mission (TRMM) data products, available data from radar scan and observed rain-gauges in Bangladesh. Important features like lifetime, maintenance mechanism, traversed path, propagation speed and direction of MCSs developed in pre-monsoon period in and around Bangladesh could have been pointed out.

Use of MM5 model for wheather forecasting over Bangladesh region

Nasreen Akter and Islam M. Nazrul

Abstract

The severity of weather appears almost every year in Bangladesh. It causes damage of property and takes a very high death toll due to absence of timely and proper forecasting facility. Thus the focus of this paper is to demonstrate the usefulness of weather model for forecasting and estimating convective system. A high resolution mesoscale model MM5 has been used in this regard to observe the accurate rainfall estimation over Bangladesh. The model is run at two nested domains at 45 km and 15 km resolutions for two durations i.e. 31 March to 05 April 2002 and 20 to 25 May 2002. In both the cases, results indicate that the MM5 model has a good capability to estimate rainfall over Bangladesh.

Selection of parameterization in MM5 for the estimation of rainfall in Bangladesh

Nasreen Akter, Islam M. Nazrul, T. Terao and T. Hayashi

Abstract

Bangladesh is one of the regions of heaviest rainfall in the world. But Bangladesh Meteorological
Department (BMD) is having only s-band weather radar to collect data for one hour with two hours pause to
save the cost of magnetron. Such type of break of data collection is great hindrance to study the convective
systems developed here. Also the surface rain-gauges are very limited in number. In this circumstance, nonhydrostatic
mesoscale model MM5 would be a good tool for the estimation of precipitation in Bangladesh
area once its sensitivity is known to the concerned. With a view to estimate the rainfall in a better manner, a
comparison of rain-gauge data and TRMM 3B42RT products with the outputs of ten MM5 model options has
been conducted for the duration of 138 hours from 20 to 25 May 2002. By this way, the suitable
parameterization scheme of MM5 for the simulation of rainfall in Bangladesh is investigated as a case study.
The simulated rainfall pattern from MM5 nearly agrees with the ground-based observational data. However,
different MM5 options demonstrate some variations in different observation methods as well as domains. For
coarse resolution (45 × 45 km mesh) Anthes-Kuo (Ku) cumulus scheme with Blackadar planetary boundary
layer (PBL) and for nested domain mesh resolutions 15 × 15 km2 and 5 × 5 km2 Ku and Grell cumulus
schemes with MRF PBL respectively are found better with respect to TRMM value.

Retrieval of rain rate from the rain status of BMD radar

Nasreen Akter, Islam M. Nazrul and M. Rafiuddin

Abstract

The climate of Bangladesh is characterised by high temperature, heavy rainfall, often excessive humidity, and fairly marked seasonal variations. One of the conventional methods of monitoring the meteorological variable rainfall in Bangladesh is radar installed by the Bangladesh Metrological Department (BMD).The BMD S-band radar measures the reflectivity to provide an estimation of precipitation intensity and displays in six rain statuses. This paper presents a method for retrieving the unknown rain rate out of the six rain statuses from the radar plan position indicator (PPI) scan. The technique has been developed to sample the PPI radar scan into 10 km x 10 km grid boxes having 16 pixels with 2.5 km mesh and assemble the data for each status by rain rate retrieval algorithm. A statistical method has also been proposed to retrieve the rain rate at any particular location within radar coverage. Two cases of year 2003 have been analyzed in two distinct periods i.e. pre-monsoon and monsoon for retrieving the rain rates from the rain status. The result from the retrieval method has been tested through a comparison made between the retrieval rain rate measured from BMD radar and the same obtained from the Tropical Rainfall Measuring Mission (TRMM) PR (precipitation radar).

Interfacing Arrangement and Software Development for Acquisition and Analysis of Respiratory Data

Nasreen Akter and K S Rabbani

Abstract

Lung diseases are still major killers in Bangladesh due to absence of proper diagnostic equipment primarily because of high cost. Indigenous development of transducer, hardware interface and application software can bring the benefit of modern technology at the doorstep of the people. In this respect, computerisation of a mechanical spirometer is an attempt to make the dream of poor people into reality. A wedge bellow type spirometer as transducer, modified ISA card circuit with low cost arrangement for hardware interface and window based easy operating software have been developed to reach to the target.

Education & Training

  • Ph. D 2010

    Meteorology

    Nagoya University, Japan, Title of the dissertation: Numerical study of an outer rainband and its convective cells associated with Cyclone Sidr

  • Master of Philosophy 2007, A grade (GPA 3.92)

    Atmospheric Physics

    Bangladesh University of Engineering & Technology (BUET), Bangladesh, Title of the dissertation: Sensitivity test of different cumulus parameterizations in MM5 modelling for Bangledesh

  • Master of Science in Physics 1999 (held in 2001), 1st Class (1st Position)

    Physics

    University of Dhaka, Bangladesh, Title of the dissertation: Development of user-friendly software for acquisition of Spirometric data

  • Bachelor of Science in Physics 1995 (held in 1997), 1st Class (2nd Position)

    Physics

    University of Dhaka, Bangladesh

Honors, Awards and Grants

  • 2014
    UGC Award
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     Research paper titled "Role of Synoptic-Scale Forcing in Cyclogenesis over the Bay of Bengal"

  • 2007-2010
    Monbukagakusho Scholarship
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    Ph. D program

  • 2001
    Dhaka University Merit Award
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     being 1st position in Master of Science

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