Study of Rainfall with Wind - Simulation of Heavy Rainfall Events of 09-11 June 2007 using MM5

4.2 Simulation of Heavy Rainfall Events of 09-11 June 2007 using MM5 and WRF Model

4.2.2 Study of Rainfall with Wind

During summer monsoon period, one of the main synoptic conditions for occurrence of heavy

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For the present case, westerly wind comes from the Arabian Sea into the Indian region and SW'ly wind comes from the Bay of Bengal and both of these winds enter into the Bangladesh region. Westerly winds carry moisture from the Arabian Sea and SW'ly winds carry moisture from the Bay of Bengal and convergence occurred in the whole Bangladesh especially near the hilly region. There was a cyclonic circulation developed over Bangladesh and West Bengal of India. A well marked low pressure area forms over this region. Due to this heavy rainfall occurred over Bangladesh and Bay of Bengal. The detailed analyses of the system using both the MM5 and WRF models are given below and shown in Figures 4.2.2.1 (a-d), 4.2.2.2 (a-b) and 4.2.2.3 (a-b).

Using MM5 models, the distributions of low level wind flow at 850 hPa and upper level wind flow at 500 hPa and 200 hPa level valid from 00 UTC of 09 June to 21 UTC of 11 June 2007 are presented in Figures 4.2.2.1(a-b), 4.2.2.2a and 4.2.2.3a respectively. The prominent feature is a strong SW'ly flow transporting high magnitude of moisture from the Bay of Bengal into northeast and central Bangladesh during the whole simulation period. Westerly flow also transports significant amount of moisture from the Arabian Sea through India to main land of Bangladesh. From the Figure 4.2.2.1 (a-b), it is seen that the area of convergence (i.e., zone of high convective activity) observed over Bangladesh and neighborhoods. At time 00 UTC on 10 July, 2008, at level 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 of Bangladesh. It is seen that very 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 wind (Figure 4.2.2. 1 (a-b)). The wind speed is almost about 20 m/s from 00 UTC of 10 June to 21 UTC of 11 June 2007 with varied amount of 3-hourly rainfalls from 18 to 65 mm. The SW'ly wind speed of about 20 m/s prevails over North Bay of Bengal and south Bangladesh up to 500 hPa level with cyclonic circulation (Figure 4.2.2.2a). Another anticyclonic circulation is also observed at this level over west side of India. Anticyclonic circulation is also observed at 200 hPa level with wind speed range 40 to 50 m/s (Figure 4.2.2.3a).

Using WRF model, the low level wind flow at 850 hPa and upper level wind now at 500 and 200 hPa level valid from 00 UTC of 09 June to 21 UTC of 11 June 2007 are presented in Figures 4.2.2.1(c-d), 4.2.2.2b and 4.2.2.3b 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 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 or equal to those obtained using MM5 model.

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Figures 4.1.2.4a and 4.1.2.4b are obtained using inner most Domains i.e. Domain 3 of MM5 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 10 June to 21 UTC of 11 June, 2007. It is clear from the Figures 4.2.2.4a and 4.2.2.4b that simulated rainfall obtained from WRF model is more than that obtained from MM5 model.

Dalam dokumen Prediction of High Impact Precipitation Events over Bangladesh and its surrounding areas using NWP Models (Halaman 120-131)