Study of Rainfall with Wind

ISO ''4

4.3 Simulation of Heavy Rainfall Event of 1-3 May 2009 using WRF and MM5 To analyze the convective system of 01-03 May, 2009 with their vertical structure, MM5

4.3.2 Study of Rainfall with Wind

One of the main synoptic conditions for occurrence of heavy rainfall over Bangladesh and neighborhood is the SW'ly flow streaming from the head of Bay of Bengal into Bangladesh [1]. For this present study case, westerly wind comes from the Arabian Sea into the Indian region and SW'ly wind comes from the Bay of Bengal and both these winds enter the Bangladesh region. Westerly winds carry moisture from Arabian Sea and SW'ly winds carry moisture from the Bay of Bengal and convergence occurred in Bangladesh especially near the hilly part. Heat flow from the Indian land mass. So, warm and moist environment helps to develop the convective system over Bangladesh. The detailed analyses of the system are given below using both the MM5 and WRF models and shown in Figures 4.3.2.1 (a-d), 4.3.2.2 (a-b) and 4.3.2.3 (a-b).

Using MM5 model, the distributions of low level wind flow at 850 hPa and upper level wind at 500 hPa and 200 hPa levels valid for 00 UTC of 01 May to 21 UTC of 03 UTC of 03 May 2009 are presented in Figures 4.3.2.1(a-b), 4.3.2.2a and 4.3.2.3a respectively. The prominent feature is a SW'ly flow transporting moisture from the Bay of Bengal into Bangladesh and heat flow from the India land mass during the whole simulation period. NW'ly flow also transports moisture from Arabian Sea through India to main land of Bangladesh. The area of convergence (i.e., zone of high convective activity) is observed over Bangladesh and neighborhoods. At time 00 UTC on 02 May, 2009, at 850 hPa, the amount of moisture is very low. Due to convergence, small cells merge with others cells and make clusters. With the advancement of time other cells make another cluster. Clusters merge to form mesoscale convective system (MCS) and rainfall occurs in and outside Bangladesh. It is seen that a MCS forms near the foot hills of Himalayan Mountains and hence heavy rainfall occurred in the north-eastern part of Bangladesh. The cyclonic circulation is observed at 850 hPa (Figure

10 4.3.2. 1 (a-b)). The wind with varied speed is from 10 to 20 m/s during the 00 UTC of 01 May to 21 UTC of 03 May 2009 with varied amount of 3 hourly rainfalls. Since SW'ly is weak, cyclonic circulation is not observed at 500 hPa. Anticyclonic circulation is observed over north side of the India with wind speed ranges 20 m/s for the whole simulation time (Figure 4.3.2.2a)). Anticyclonic circulation is observed at 200 hPa level wind with speed range 40 to 50 m/s (Figure 4.3.2.3a).

Using WRF model, the distributions of 850 hPa, 500 hPa and 200 hPa level wind valid from 00 UTC of 01 May to 00 UTC of 04 May 2009 are presented in Figures 4.3.2. 1 (c-d), 4.3.2.2b and 4.3.2.3b respectively. Similar features are observed using both the models with different amount of moisture content and wind speed. The simulation of MCS using WRF model is less than that using the MM5 model. The maximum wind speed at the levels 850, 500 and 200 are 10, 10 and 40 m/s respectively. This maximum wind speed is smaller than those obtained by MM5 model. From figures it is clear that the rainfall occurred due to the

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- 1026 - 1023 1028 1017 1014 1012

1009 1006 1004 1 002 1000

Figure 4.3.1.1a: MM5 Model simulated MSLP (hPa) valid from 00 UTC of 01 May to 00 UTC of 04 May 2009.

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combined effect of southv'estcr1v and southerly wind. carrvi ne moisture loni Arabian Sea and Bay of Bengal respectively.

Fiures 4...2.4a and 4.3.2.4b are obtained usine inner most Domains i.e. Domain 3 of M \15 and WRF models. It is noted that resolution of Domain 3 is 10 km for both the models.

Figures show the development of cloud i.e. MCS in every 3 hours with the advancement of time from 00 UTC of 02 May to 21 UTC of 03 May, 2009. It is clear from the Figures 4.3.2.4a and 4.3.2.4b that simulated rainfall obtained from WRF model is more than that obtained from MM5 model.

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Figure 4.3.1.1b: WRF Model simulated MSLP (hPa) valid from 00 UTC of 01 May to 00 UTC of 04 May 2009.

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Figure 4.3.2.1a: MM5 Model simulated wind flow (m/s) at 850 hPa valid 00 to 21 UTC of 02 May 2009.

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Figure 4.3.2.1b: MM5 Model simulated wind flow (m/s) at 850 hPa valid 00 to 21 UTC of 03 May 2009.

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Figure 4.3.2.1c: WRF Model simulated wind flow (m/s) at 850 hPa valid for 00 to21 UTC of 02 May 2009.

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Figure 4.3.2.1d: WRF Model simulated wind flow (m/s) at 850 hPa valid for 00 to 21 UTC of 03 May 2009.

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Figure 4.3.2.2a: MM5 Model simulated wind flow (m/s) at 500 hPa valid for 00 UTC of 02 May to 21 UTC of 03 May, 2009.

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Figure 4.3.2.3a: MM5 Model simulated wind flow (m/s) at 200 hPa valid for 00 UTC of 02 May to 21 UTC of 03 May, 2009.

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Figure 4.3.2.3b: MM5 Model simulated wind flow (m/s) at 200 hPa valid for 12 to 21 UTC of 03 May 2009

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0 10 20 40 80 80 100 120 140 160 10 200 220

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Figure 4.2.2.4a: MM5 Model simulated precipitation field valid from 00 UTC of 02 May to 21 UTC of 03 May 2009. Sequence is top left to right.

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Figure 4.2.2.4b: WRF Model simulated precipitation field valid from 00 UTC of 02 May to 21 UTC of 03 May 2009. Sequence is top left to right.

Dalam dokumen Prediction of High Impact Precipitation Events over Bangladesh and its surrounding areas using NWP Models (Halaman 154-165)

ISO ''4

4.3 Simulation of Heavy Rainfall Event of 1-3 May 2009 using WRF and MM5 To analyze the convective system of 01-03 May, 2009 with their vertical structure, MM5

4.3.2 Study of Rainfall with Wind

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