diagnosis and numerical simulation of western disturbance ... level disturbance with orography over...

International Journal of Earth and Atmospheric Science | October-December, 2015 | Vol 2 | Issue 4 | Pages 126-148 © 2015 Jakraya Publications (P) Ltd

INTERNATIONAL JOURNAL OF EARTH AND ATMOSPHERIC SCIENCE

Journal homepage: www.jakraya.com/journal/ijeas ORIGINAL ARTICLE

Diagnosis and Numerical Simulation of Western Disturbance and Associated Unusual Rainfall over India during the Month of March, 2015 S. I. Laskar*, S. D. Kotal, S. C. Bhan and B.A.M. Kannan India Meteorological Department, New Delhi-110003, India. *Corresponding Author: S.I. Laskar Email: [email protected] Received: 09/08/2015 Revised: 03/10/2015 Accepted: 05/10/2015

Abstract Two cases of intense western disturbances (WD) which caused

widespread rainfall that occurred over northwest and central India and continued for three consecutive days for each case (01-03 March, 2015 and 15-17 March, 2015) have been investigated using the India Meteorological Department’s (IMD) operational numerical weather prediction model Global Forecast System (GFS). The results of the study showed that the cause of these unusual rainfall events were the interaction of western upper level disturbance with orography over the regions and moisture convergence from the Arabian Sea. The presence of divergent wind field aloft along with orographic lifting aided in vertical rising of this unstable warm moist air resulting in severe weather conditions. A jet stream present at 200 hPa caused the development and eastward movement of this baroclinic westerly wave due to strong convergence at lower level and positive divergence at upper level over these regions. The events were also diagnosed using satellite and radar data sets.

Key words: Western Disturbance, Rainfall, Vertical velocity, Vorticity and Moisture flux.

1. Introduction The northern part of India, is a vast land mass

consisting of six meteorological subdivisions, namely, Jammu and Kashmir (J and K), Himachal Pradesh (HP), Haryana, Punjab, Uttarakhand (UK) and West Uttar Pradesh. Precipitation during winter season in North India is very important for Rabi crops, particularly for wheat, as it supplements moisture and maintains low temperature for the development of the crops. Precipitation in the form of snow over the hilly regions of North India also helps in glacier maintenance for the supply of water to rivers throughout the year, hydropower production, transport and logistics, etc. Therefore, precipitation during the winter season is critically important for the agriculture and economy of the country.

The primary weather systems responsible for winter time precipitation in the region are the high level westerly synoptic scale waves known as ‘Western Disturbances (WD)’ noticed as cyclonic circulation/trough in the mid and lower tropospheric levels or as a induced low pressure area on the surface, which occur in middle latitude westerlies and originate over the Mediterranean Sea, Caspian Sea and Black Sea and move eastwards across north India. As pointed out in earlier studies (Kalsi, 1980; Kalsi and Halder, 1992; Pisharoty and Desai, 1996), WDs are

manifestations of the interaction between the tropics and mid-latitude systems and are associated with extensive sheets of mid- and high level clouds and a maxima in the subtropical jet. Kalsi and Halder (1992)

suggest that mobile cloud systems, related to short waves on the subtropical jet, facilitate the interaction between the tropics and mid-latitude systems by amplifying the long-wave troughs, leading to a larger influence of mid-latitude westerlies over subtropical and lower latitudes. The usual sequence of precipitation with western disturbance is first over Jammu and Kashmir and even Himachal Pradesh, Punjab and Haryana and the next day over East Rajasthan and West Uttar Pradesh as well. Many times rainfall may cease at this stage. In other cases, rainfall commences over Madhya Pradesh during the following two days (though not always) spreads to East Uttar Pradesh, Bihar, Orissa, West Bengal and Assam. This is the broad pattern of synoptic development of rainfall belts though wide variations are possible (India Meteorological Department, 1969).

In previous studies, WDs have been discussed by number of authors. Hatwar et al. (2005) have investigated two cases of intense western disturbances which affected the northwest India using the India Meteorological Department’s operational limited area analysis and forecast system. Their analysis shows that

Laskar et al...Diagnosis and Numerical Simulation of Western Disturbance and Associated Unusual Rainfall Over India During the Month of March, 2015


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the 24-hour model forecasts are in good agreement with the observations both in respect of western disturbance’s movement and intensification. Even the numerical model could predict the spatial distribution of precipitation with a high success rate and was found to be very useful in providing numerical guidance in day-to-day operational short range forecasts.

Azadi et al. (2001) simulated an active western disturbance (WD) that affected the Indian region in January 1997 using a non-hydrostatic version of the PSU/NCAR mesoscale model. They investigated the role of the planetary boundary layer (PBL) and convection parameterization schemes in the development of the WD. Analysis and predictions for some fields, including sea level pressure, geopotential height, temperature, horizontal wind and precipitation are also examined by them. Their study found that the performance of the Hong-Pan (as implemented in the NCAR MRF model) and Betts-Miller (or Grell) schemes as PBL and convection parameterization schemes respectively are best compared to the other schemes used in their study.

Dimri and Mohanty (2009) simulated the characteristic features of the Western Disturbances (WDs ) occurring over the Indian region during a winter season by using a non-hydrostatic version of the Penn State University/National Center for Atmospheric Research, US, (PSU/NCAR) mesoscale model by selecting four cases of active WDs. They found that in all the cases the rate of movement of the system is, in general, a little slower in the simulations. Examining the differences between the predicted and analysed zonal component of the wind reveal that the model simulated zonal winds are generally weaker/under-estimated in the location of the upper trough at 500 hPa or aloft and even in the position of the WDs at lower levels. They also found that the model has a systematic easterly bias.

Das (2005) has investigated the predictability of rainfall over different parts of the Himalayas and the Western Ghats Mountains during the passage of a western disturbance and an active monsoon spell respectively by using a Fifth-Generation Mesoscale Model (MM5).

A performance evaluation study has been carried out by Soni et al. (2014) over a semi-arid region, Jaipur, India using Weather Research and Forecasting (WRF) model. In their study the efficiency of the model is determined during winter season over Jaipur by considering the study period from 11th to 14th January 2012, which was the coldest period during the winter season 2011-2012. They found that the model simulations show encouraging and better statistical results with the combination of Quasi-

Normal Scale Elimination (QNSE) surface layer, QNSE planetary boundary scheme along with NOAH land- surface and Kain-Fritsch (new Eta) cumulus parameterization scheme than any other combinations of physical parameterization schemes over study region.

In the present study, the two very strong western disturbances that affected India during the period 28th February–2nd March and 14–16 March, 2015 that causes copious rainfall across a large swathe of the country which can be described as a rare occurrence for this time of the year have been investigated using IMD operational global model (GFST574L64) products.

2. Model Description

The Global Forecast System GFS T574L64 (~ 25 km in horizontal over the tropics), adopted from National Centre for Environmental Prediction (NCEP), was implemented at IMD, New Delhi on IBM based High Performance Computing Systems (HPCS) with Peak Computing Speed 14. 4 TF, 28 Nodes: POWER-6, 4.7 GHz Processors and 128 Giga Bytes Memory per Node.

In horizontal, it resolves 574 waves (≈ 23 Km) in spectral triangular truncation representation (T574), for which the Gaussian grid of 1760 x 880 dimensions are used. The model has 64 vertical levels (hybrid; sigma and pressure). There are 15 levels below 800 hPa (in lower Troposphere), 14 layers in the middle troposphere (800-350 hPa), 8 levels are in jet-stream region (350-150 hPa) and 27 levels above 150 hPa in the stratosphere. Sigma level is considered from surface to 100 hPa (40 levels) and 24 P-level above up to 0.27 hPa. It has time step 2 minutes and Global Data Assimilation System (GDAS) runs four times per day (0000, 0600, 1200, and 1800 UTC). The GFS (T574L64) model runs two times per day (0000, 1200 UTC) and forecast up to 168 hours.

3. Data Used and Methodology

Synoptic features associated with this heavy rainfall event and rainfall data have been collected from India Meteorological Department, New Delhi. IMD GFS (T574L64) dataset has been used to analyze wind at 850 hPa level, vorticity, vertical velocity, moisture flux convergence and rainfall forecast. The observed rainfall of IMD Daily Merged Satellite Guage Rainfall data (0.5 x 0.5 degree) (Mitra et al., 2009) have been used for verification. The Radar observations of Jaipur, New Delhi, Patiala and Srinagar have been used to generate a mosaic and reflectivity cross section.



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4. Synoptic Features 4.1 Case I (28th February to 2nd March, 2015) This is an unusual western disturbance, which gave precipitation over the Western Himalayas, Jammu and Kashmir, Himachal Pradesh, Uttarakhand and also over central parts and Gangetic plains. KALPANA-I satellite imagery with cloud top temperature (CTT) indicating the presence of western disturbance and strong convection during the period 28th February to 2nd

March, 2015 as shown in the Fig 1(a–b). The convective cloud mass of CTT –500C or less was observed over Uttar Pradesh and neighbourhood on 28th February 2015. The convection zone extended up to Jammu and Kashmir on 2nd March. During the period of study (28th February to 2nd March) the lowest value (–600C) of CTT was observed over Uttarakhand on 1st March, 2015.

Radar echoes were visible from 0500UTC of 28th February 2015 gradually intensifying with time.

(a)

(b)

Fig 1: KALPANA-I satellite imagery with cloud top temperature: (a), 1650 UTC 28 February, (b) 0545 UTC 02 March, 2015.



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Fig 2: RADAR images showing (a) reflectivity composite of Srinagar, Patiala, New Delhi Radars on 28-02-2015;

(b) X-Z cross section reflectivity on 01-03-2015 (at location marked with a black line in image a); (c) Reflectivity image of Delhi radar on 02-03-2015 (d) Cross section imagery of Delhi Radar on 02.03-2015 (at location marked with a black line in image c)

These clouds were widely distributed and mostly stratiform in nature with few embedded convective cells. The overall movement of the cloud mass was from West South West to North North East. A composite of Srinagar, Patiala and Delhi Radars is affixed in Fig 2(a), the cross section of the weather phenomenon is shown in Fig 2(b).

The radar reflectivity factor is proportional to the complex index of refraction (approx.0.93 for liquid particle and 0.1 for ice particles) apart from the sixth power dependence of scatterer diameter. The strati-form wide spread rain signature in radar is formed from stably stratified clouds, where normally an enhanced reflectivity just below zero degree isotherm is perceived because of the aloft ice particle while slipping down gets an outer skin of water and virtue of its bigger size gives an enhanced reflectivity termed as melting layer marked with a rectangle in Fig 3. In contrast the strong up/down drafts in convective cells disrupt the melting band signature, such propping In present study over Aaravalli mountains between Jaipur and Delhi where the orographic lifting caused enhanced reflectivity even above melting band indicative of strong updraft in the system (marked with arrows in the Fig 3). The melting layer is clearly

pronounced at 3 km with few clouds propping up from convective enhancements (orographic lifting).

On an average the cloud tops were 5-6 kms. This lasted up to 1052 UTC of 1st March, till the cloud mass reached the foothills. A subsequent swell of development started from 1900 UTC of the same day which were mostly convective in nature and were aligning into a (squall) line echo (Fig 2c-d). The echo intensities were higher reaching 54.2dBZ and cloud heights reaching about 7-8 km. The movement of this was purely westerly.

The synoptic features during the period 28th February to 2nd March, 2015 have been shown in the Fig 4(a-c). On 28th February a Western Disturbance as an upper air cyclonic circulation lies over Afghanistan and neighbourhood extends up to 3.1 km above mean sea level with a trough aloft roughly along Longitude 62.0°E and North of Latitude of 15 °N and an induced upper air cyclonic circulation over south Pakistan and adjoining Kutch area and extends up to 1.5 km above mean sea level. The system moved east wards and lies over north Pakistan and neighbourhood with trough aloft roughly along Longitude 68.0°E and north of Latitude 15.0°N and induced upper air cyclonic circulation over south Rajasthan and -

(a)

(b)

(c)

(d)



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Fig 3: RADAR image of 00:42:55 UTC of 1 March,2015 showing melting layer (highlighted with a rectangle) a

signature of stratiform precipitation with few embedded convective cells formed due to orographic lift (marked with arrows).

adjoining Gujarat extends up to 3.1 km above mean sea of the westerly trough into central Arabian Sea, wind confluence continued over central India and neighbor- hood. Wind analysis at 200 hPa (Fig 5a-c) display the day-to-day evolution of the jet stream core during 28th February to 2nd March, 2015. The figure shows a gradual deepening of a westerly trough over the North Indian region, which then starts to move away to the east on 1st March, 2015. Due to deepening of trough on 1st March, the core of the jet stream weakened and concentrated only over a smaller area as compared to 28th February and 2nd March.

The vorticity analysis at 850 hPa level based on 0000UTC of 28th February to 2nd March 2015 respectively have displayed in Fig 6(a-c). From the figure it is seen that high positive vorticity of value more than 10x10-4/sec over northwestern parts of Punjab on 2nd March.

The vertical velocity analysis at 850 hPa level based on 0000UTC of 28th February to 2nd March 2015 along with 24, 48 and 72 hours forecasts respectively have been displayed in Fig 7(a-d) to Fig 9(a-d). From the Fig 7 to Fig 9 the highest magnitude of rising motion over Indian region during the period have been observed mostly over Jammu and Kashmir and Himachal Pradesh. From the figures it has also been observed that the model has well predicted the vertical velocity in its 24, 48 and 72 hours forecasts.

The accumulation of moisture occurred due to persistent currents from Arabian Sea in the lower troposphere. From the figures it has been observed that the moisture flux convergence over the northwest India gradually increased from 28th February to 2nd March. During the period, the highest value of moisture flux convergence is found to be of the order of 10x10-

4g/kg/sec or more over Jammu and Kashmir on 2nd March.

The observed (analysis) rainfall for 1st March and the corresponding 24 hour forecast based on 28th February valid for 1st March, 48 hour forecast based on 27th February valid for 1st March and 72 hours forecasts based on 26th February valid for 1st March by the GFS model have been shown respectively in the Fig10 (a-d).

Fig11 (a-d) represents respectively the observed (analysis) rainfall for 2nd March and the corresponding 24 hour forecast based on 1st March valid for 2nd March, 48 hour forecast based on 28th February valid for 2nd March and 72 hours forecasts based on 27th February valid for 2nd March.

Similarly, Fig12(a-d) represents respectively the observed (analysis) rainfall for 3rd March and the corresponding 24 hour forecast based on 2nd March valid for 3rd March, 48 hour forecast based on 1st March valid for 3rd March and 72 hours forecasts based on 28th February valid for 3rd March. On 1st March, observed rainfall was well captured by the GFS model



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(a)

(b)

(c)

Fig 4: Wind (knot) analysis at 850hPa level based on 0000UTC of (a) 28-02-2015, (b) 01- 03- 2015 and (c) 02-03

2015



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(a)

(b)

(c)

Fig 5: Wind (knot) analysis at 200hPa level based on 0000UTC of (a) 28-02-2015, (b) 01-03-2015, (c) 02-03-2015.



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(a) (b)

(c )

Fig 6: Vorticity (10-5/sec) analysis at 850hPa level based on 0000UTC of (a) 28-02-2015, (b) 01-03-2015, (c) 02-03-2015



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(a) (b)

(c ) (d) Fig 7: Vertical velocity (10-2 hPa/sec) at 850hPa level (a) analysis based on 0000UTC of 28-02- 2015 (b) 24

hours forecast (c ) 48hours forecast and (d) 72 hours forecast in its 24, 48 and 72 hours forecasts but it underestimates the heavy rainfall which occurred over the Vidharbha subdivision of India. On 2nd March, spatial distribution of the observed rainfall has been well captured in 24, 48 and 72 hours forecasts and also captured the intensity of the rainfall well in its 24 and 48 hours forecast but it underestimated the rainfall in the 72 hours forecast. The model over estimated the rainfall forecast in terms of amount and spatial distribution for 3rd March. 4.2 Case-II (14-16 March, 2015)

Due to this western disturbance unusual precipitation occurred over the Western Himalayas, Jammu and Kashmir, Himachal Pradesh, Uttarakhand and also over central parts and Gangetic plains of India during 15-17 March 2015.

Fig 13(a) and Fig 13(b) show the KALPANA-I satellite imagery with cloud top temperature (CTT) indicating the presence of western disturbance and strong convection. The intense convection zone of CTT -50° was observed over West UP, East Rajasthan, north eastern parts of West Rajasthan, Haryana, Chandigarh, Delhi and some parts of Punjab. The convection zone –



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(a) (b)

(c ) (d)

Fig 8: Vertical velocity (10-2 hPa/sec) at 850hPa level (a) analysis based on 0000UTC of 01-03- 2015 (b) 24 hours forecast (c ) 48hours forecast and (d) 72 hours forecast

moved northeastward and was concentrated over UP and adjoining Uttarakhand on 15th March.

Another convective zone of CTT -50°C was observed over Punjab and HP on 15th March and moved in a northward direction and lies over Jammu and Kashmir on 16th March.

In radar echoes the clouds of 14th March were mainly convective in nature extending above 8 kms, which at 0800 UTC had severe development over Rajasthan (Fig 14c). Similar severe thunderstorm with hail signature event was observed around 0700 UTC of 15th March (Fig 14d) peaking at 1000 UTC and dissipated at 2100 UTC.

The synoptic situations during the period 14th February to 16th March, 2015 are shown in the Fig 15(a-c). On 14th March the western disturbance as an

upper air cyclonic circulation lies over Afghanistan and adjoining Pakistan and extends up to 7.6 km above mean sea level. The induced upper air cyclonic circulation lies over South Pakistan and adjoining southwest Rajasthan and extends up to 3.1 km above mean sea level.

On 15th March the western disturbance as an upper air cyclonic circulation over Pakistan and neighbourhood extending up to 7.6 km above mean sea level persists.

The induced low pressure area over West Rajasthan and neighbourhood with associated upper air cyclonic circulation extends up to 2.1 km above mean sea level.

On 16th March the western disturbance as an upper air cyclonic circulation over North Pakistan and



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(a) (b)

(c ) (d) Fig 9: Vertical velocity (10-2 hPa/sec) at 850hPa level (a) analysis based on 0000UTC of 02-03 2015 (b) 24 hours

forecast (c ) 48hours forecast and (d) 72 hours forecast adjoining Jammu and Kashmir persists and extends up to 3.1 km above mean sea level with a trough aloft roughly along Longitude 72.0°E and north of Latitude 30.0°N.

The induced low pressure area over West Rajasthan and neighbourhood seen as an upper air cyclonic circulation over East Rajasthan and neighbourhood and extends upto 1.5 km above mean sea level.

Wind analysis at 200 hPa (Fig 16a-c) displays the day-to-day evolution of the jet stream core during 14th to 16th March, 2015. Figure shows the jet maxima over eastern and north eastern parts in the Indian region.

Fig 17 (a-c) shows the vorticity analysis at 850 hPa level based on 0000UTC of 14th to 16th March 2015 respectively. On 15th March high positive vorticity has been observed over north western parts of west UP and on 16th it shifted to the north western parts of Punjab. During the period rising motion of maximum intensity has been observed mostly over Himachal Pradesh and Jammu and Kashmir which can be seen in the Fig 18(a-c).

The accumulation of moisture over central India increased from 14th to 15th March and again decreased on 16th March whereas it gradually increased over North West India from 14th to 16th March with maximum value 10x10-4g/kg/sec or more over Punjab and adjoining Jammu and Kashmir on 16th March.



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(a) (b)

(c) (d) Fig 10: Rainfall(mm) on 01 March 2015 (a) observation (analysis), (b) 24 hours forecast based on 28 February 2015,

(c ) 48 hour forecast based on 27 February 2015, and (d) 72 hours forecast based on 26 February 2015.

The observed (analysis) rainfall for 15th March and the corresponding 24 hour forecast based on 14th March, 48 hour forecast based on 13th March and 72 hours forecasts based on 12th March by the GFS model have been shown respectively in the Fig 19(a-d).

Fig 20(a-d) represents respectively the observed (analysis) rainfall for 16th March and the corresponding 24 hour forecast based on 15th March, 48 hour forecast based on 14th and 72 hours forecasts based on 13th March. For the rainfall of 15th March the model

forecasts for 24, 48 and 72 hours are in agreement with the observation in terms of spatial distribution but it fails to predict heavy rainfall occurred over parts of West MP and East Rajasthan. For the rainfall of 16th March also the model forecasts for 24, 48 and 72 hours are in agreement with the observation in terms of spatial distribution but it failed to capture heavy rainfall occurred over Jammu and Kashmir. The model forecasts for 24, 48 and 72 hours also underestimated the 17th March rainfall.



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(a) (b)

(c) (d) Fig 11: Rainfall(mm) on 02 March 2015 (a) observation (analysis), (b) 24 hours forecast based on 01 March 2015, (c

) 48 hour forecast based on 28 February 2015, and (d) 72 hours forecast based on 27 February 2015.



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(a) (b)

(c) (d) Fig 12: Rainfall(mm) on 03 March 2015 (a) observation (analysis), (b) 24 hours forecast based on 02 March 2015, (c

) 48 hour forecast based on 01 March 2015, and (d) 72 hours forecast based on 28 February 2015.



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5. Conclusions

In this study, unusual rainfall events during 01-03 March 2015 and 15-17 March 2015 over the northwest part of India associated with western disturbances (WD) have been investigated using the India Meteorological Department’s operational numerical weather prediction model Global Forecast System (GFS). The analyses of synoptic situation show that deepening of the westerly trough into central

Arabian Sea pumped significant amount of moisture hail signature event was observed on 15th March that over the northwest and central India. Subsequently moisture flux convergence over the northwest India gradually increased for both the cases. The analysis of vorticity and vertical velocity shows that high positive vorticity and associated high vertical velocity developed over northwestern parts.

Analyses of model forecasts indicate that the global model has a good capability to predict rainfall

(a )

(b) Fig 13: KALPANA-I satellite imagery with cloud top temperature: (a) 0845 UTC 14 March and (b)

1145 UTC 16 March 2015.



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over the mountains for at least 48–72 hours in advance at high model resolutions. However, the model generally under-predicts the heavy precipitation amounts observed over the region. Analysis of dynamical features based on model analysis clearly indicated that the relative positions of the cyclonic circulation and the position of the westerly jet stream appear significant factors in pumping of huge amounts of moisture into the Western Himalayas.

Radar analysis of clouds shows that, on an average the cloud tops were 5-6 kms and cloud mass reached the foothills by 01st March. A subsequent swell of development also started which were mostly convective in nature and were aligning into a (squall) line echo. In the second case, radar echoes the clouds of 14th March were mainly convective in nature extending above 8 kms and severe thunderstorm with

(a) (b)

(c) (d)

Fig 14: RADAR images showing (a) reflectivity composite of Srinagar, Patiala, New Delhi Radars on 14-03-2015, (b) cross-section on 14-03.2015( at location marked with a black line in image (a)); (c) Reflectivity composite on 14-03-2015 and (d) on 15-03-2015.



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(a)

(b)

(c )

Fig 15: Wind (knot) analysis at 850 hPa level based on 0000UTC of (a) 14-03-2015,(b) 15-03-2015 and (c) 16-03-2015



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(a)

(b)

(c)

Fig 16: Wind (knot) analysis at 200hPa level based on 0000UTC of (a) 14-03-2015, (b) 15-03-2015 and (c) 16-03-2015



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(a) (b)

(c)

Fig 17: Vorticity (10-5/sec) analysis at 850hPa level based on 0000UTC of (a) 14-03-2015,(b) 15-03- 2015, (c) 16-03-2015.



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(a) (b)

(c)

Fig 18: Vertical velocity (10-2 hPa/sec) analysis at 850hPa level based on 0000UTC of (a) 14-03-2015,(b) 15-03- 2015, (c) 16-03-2015.



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(a) (b)

(c) (d) Fig 19: Rainfall(mm) on 15 March 2015 (a) observation (analysis), (b) 24 hours forecast based on 14 March

2015, (c ) 48 hour forecast based on 13 March 2015, and (d) 72 hours forecast based on 12 March 2015.



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(a) (b)

(c) (d) Fig 20: Rainfall(mm) on 16 March 2015 (a) observation (analysis), (b) 24 hours forecast based on 15 March

2015, (c ) 48 hour forecast based on 14 March 2015, and (d) 72 hours forecast based on 13 March 2015, triggered the heavy to extremely heavy rainfall event. This would help in development of early warning and prediction systems to mitigate hazards associated with WDs in the form of extreme precipitation, natural hazards and prevent possible

damages to life, property and crops. However, there is a need for further studies in this direction with better modeling techniques to predict rainfall more accurately.



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Acknowledgement: Authors are grateful to the Director General of Meteorology, India Meteorological Department for his kind support and encouragement to

carry out this work. Authors like to thank the anonymous reviewers for their valuable suggestions and comments to improve the quality of the paper.

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