Study of Rainfall with Wind - Simulation of Heavy Rainfall Events of 1-3 July 2008 using WRF an

SIMULATION OF HEAVY RAINFALL EVENTS USING MM5 AND WRF MODELS

4.1 Simulation of Heavy Rainfall Events of 1-3 July 2008 using WRF and MM5 To analyze the convective system of 1-3 July 2008 with their vertical structure, the models

4.1.2 Study of Rainfall with Wind

During summer monsoon period, one of the main synoptic conditions for occurrence of heavy rainfall over Bangladesh and neighborhood is the southwesterly flow streaming from the head Bay of Bengal into Bangladesh [110]. In this case, westerly wind comes from the Arabian Sea into the Indian region and south westerly wind comes from the Bay of Bengal enter into the Bangladesh region. This southwesterly wind carry moisture from the Bay of Bengal and convergence occurred in the southeastern part (hilly region) of Bangladesh. A well marked

Bangladesh and the Bay of Bengal. The detailed analyses of the system are given below using both the MM5 and WRF models and shown in Figures 4.1.2.1 (a-d), 4.1.2.2 (a-b) and 4.1.2.3 (a-b).

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Using MM5 model, the distributions of low level wind at 850 hPa and upper level wind at 500 and 200 hPa levels valid from 00 UTC of 01 July to 00 UTC of 04 July 2008 are presented in Figures 4.1 .2.1 (a-b), 4.1 .2.2a and 4.1 .2.3a respectively. The prominent feature is a strong southwesterly (SW'ly) flow transporting high magnitude of moisture from the Bay of Bengal into southeast and central Bangladesh during the whole simulation period. From the Figure 4.1.2. l(a-b), it is seen that the area of convergence (i.e., zone of high convective activity) observed over Bangladesh and neighborhood especially in the northeastern sector of the low pressure region. At time 00 UTC on 02 July, 2008, at level 850 hPa, the amount of moisture is very low. Due to convergence, small cells merge with others cells 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 of Bangladesh. It is seen that vary developed MCS form near the foot hills of Himalayan Mountain and hence heavy rainfall occurred in the north-eastern part of Bangladesh. The cyclonic circulation is observed at 850 hPa level through Out the simulation (Figure 4.1.2.1(a-b)). Figures make us clear that rainfall happens because of the combined effect of SW'ly and southerly wind which carrying moisture from Arabian Sea and Bay of Bengal respectively. The southwesterly is prevailed over North Bay of Bengal and southern part of Bangladesh up to 500 hPa level with cyclonic circulation (Figure 4.1.2.2a). Anti-cyclonic circulation is observed at 200 hPa level (Figure 4.1 .2.3a). The maximum wind speed for the levels 850, 500 and 200 are 20, 30 and 50 m/s respectively.

Using WRF model, the distributions of 850 hPa, 500 hPa and 200 hPa level wind valid from 00 UTC of 01 July to 00 UTC of 04 July 2008 are presented in Figures 4.1.2.1(e-h), 4.1.2.2b and 4.1.2.3b respectively. Similar features are observed using both the models with different amount of moisture contained and wind speed. The simulation of MCS using WRF model is more than that using the MM5 model. The maximum wind speed at the levels 850, 500 and 200 are 20, 10 and 30 m/s respectively. This maximum wind speed is smaller than those obtained using MM5 model. From the above figures it is clear that the rainfall occurred due to the combined effect of southwesterly and southerly wind which carrying moisture from Arabian Sea and Bay of Bengal respectively.

Figures 4.1.2.4a and 4.1.2.4b are obtained using ilmer most Domains i.e. Domain 3 of MM5 and WRF models respectively. It is noted that resolution of Domain 3 is 10 km for both the models. Figures show the development of cloud (accumulated rainfall in every 3 hours) with the advancement of time from 00 UTC of 02 July to 21 UTC of 03 July, 2008. It is clear from the Figures 4.1.2.4a and 4.1.2.4b that simulated rainfall obtained from WRF model is more than that obtained from MM5 model.

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Figure 4.1.2.4a: MM5 Model simulated precipitation field valid for 00 UTC on 02 July to 21 UTC of 03 July 2008 (sequence is top left to right).

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Dalam dokumen Prediction of High Impact Precipitation Events over Bangladesh and its surrounding areas using NWP Models (Halaman 90-101)