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Western Disturbances Operational Aspects of Western Disturbances B. P. Yadav Physical features of Himalayas and Neighbourhood Climatological Characteristics of Himalayas Complex Terrain and a variety of climates : Beauty of the Himalayan region. It prevents cold, dry Arctic northerly winds blowing into the subcontinent, which keeps South Asia much warmer than corresponding temperature regions in the other continents. The mountain ranges also influence mid latitude systems ( WD , STJ ), resulting in snow in Western Himalayas and rainfall for parts of northern India. There is a large variation in the annual average precipitation in the Himalayas. The southern slopes of Eastern Himalayas experience some of the highest annual rainfall totals on Earth while other areas receive as low as 50 mm a year. Rainfall decreases from east to west. Common Natural Hazards Western Himalayas (Heavy snowfall, avalanche, flash flood, cloud burst, earthquake…) Central Himalayas (Heavy snowfall, heavy rainfall, Landslide, avalanche, cloud burst, earthquake…. ) Eastern Himalayas (Thunderstorm, heavy rainfall, Landslide, earthquake ) Summer climate Winter climate North Atlantic Oscillation (NAO) NAO The North Atlantic Oscillation (NAO) is a climatic phenomenon in North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia. The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low. The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years. Positive phase of NAO The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track. This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland. The eastern US experiences mild and wet winter conditions Negative Phase of NAO The negative NAO index phase shows a weak subtropical high and a weak Icelandic low. The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west- east pathway. They bring moist air into the Mediterranean and cold air to northern Europe The US east coast experiences more cold air outbreaks and hence snowy weather conditions. Greenland, however, will have milder winter temperatures. Northern Asia experiences colder and drier winters Large scale features VS Winter Rainfall A composite set of El Nino (10 years) and La Nina (8 years) is prepared. Significant correlation indicates increased (decreased) wintertime precipitation over Northwestern India during warm (cold) phase of ENSO. Higher geopotential height, with stronger southwesterly wind flow suggests more moisture flux from the Caspian and Arabian Sea. (Dimri et al.) Western disturbances (WDs) are intensified over northwest India during the warm phase of ENSO. During ENSO years, an upper-level cyclonic circulation anomaly north of India and a low-level anticyclonic anomaly over southern and central India is seen. The cyclonic circulation anomaly intensifies the WDs passing over northwest India and affects inter-annual variability of northwest India winter precipitation (Yadav et al.) There is no significant relation between IOD and NE monsoon and winter rainfall Weather in WHR o In General weather over Western Himalayan region occurs under the influence of WD in winter months. o In monsoon season, it occurs mainly due to shifting of southeasterly monsoon current close to foothills of the Himalayas. o Some time extreme weather occurs over the region by interaction of mid-tropospheric westerlies due to WD and strong southeasterly monsoon current developed due to low pressure area over central & adjoining northwest India. Western Disturbance defined o Western Disturbances are the lower or mid- tropospheric synoptic scale systems that originate from frontal system over Mediterranean & Caspian Sea areas and move eastwards while causing precipitation in form of snow in high reaches of WHR and rainfall in low reaches & plains of northwest India. o In the absence of clouds and rain, it is more usual to refer to them as troughs. o The word “western" implies that the disturbances come up from the west, in compare to most of the rain-giving systems in the principal rainy season approaching India from the east. o Stronger Systems with two closed isobars are usually referred to as Western Depression. Cont….. o The Cyclonic systems that develop over Rajasthan and adjoining Pakistan Area under the influence of WDs moving further north across India or West Pakistan, are called induced cyclonic circulations. o In general, when the trough in mid-latitude westerlies is around Longitude 65° E and north of Latitude 20° N and there is good moisture feeding from Arabian Sea or/and from Bay of Bengal over Western Himalayan Region, especially over Jammu & Kashmir and adjoining plains. Under such scenario, Western Himalayan Region receives intense precipitation activity in form of snowfall or rainfall. o Movement of WDs and induced systems is about five degrees in a day, sometimes much lesser. 200 hPa of 11/3/2014 300 hPa of 11/3/2014 400 hPa of 11/3/2014 500 hPa of 11/3/2014 700 hPa of 11/3/2014 850 hPa of 11/3/2014 925 hPa of 11/3/2014 MSL Chart of 11/3/2014 Cont….. o Life period of a simple sequence of a WD is two to four days, induced systems tending to fill up within that time. o Longer periods of disturbed weather occur over northern India, even upto ten days, if a fresh western disturbance from west comes over India while the induced system of the previous disturbance has not decayed. In such situations trough line complex may develop extending to even south of 20°N and cause extensive rainfall over the country. o Western disturbances causing rainfall over Punjab or Uttar Pradesh seem to come in succession at intervals of 2 to 5 days while they last. At other times, it is usual for periods of one to two weeks to be continuously free from such western disturbances in any month. Cont….. o Roughly on one half of the occasions, the activity of western disturbances is confined to Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Punjab and Haryana, while during the remaining spells the weather extends to the south also. o On an average there are six to seven disturbances per month (including the induced systems) moving across India during the winter. Cont….. Strong lower level convergence and higher level divergence over Western Himalayan Region & adjoining plains are seen before commencement of widespread precipitation along with heavy falls. Cont….. Position of the westerly trough in mid-tropospheric level may also be analysed by using the vorticity field at 500 hPa level. In Northern Hemisphere, vorticity is positive then the wind turns counter-clockwise i.e. having cyclonic rotation and negative vorticity shows anti-cyclonic rotation. Therefore, trough or cyclonic circulation in mid-tropospheric level can be identified with the help of vorticity. Cont….. location of induced cyclonic circulation can also be identified by using vorticity field at 850 hPa level. Observed precipitation rd 500 hPa 0000 UTC of 03-02-2011 0300 UTC of 3 52º E/ north of 25º N 850 hPa 0600 UTC of 3rd Sct. Ppt. over WHR. 500 hPa 0000 UTC of 06-02-11 0300 UTC of 6th 65º E/ north of 20º N th 850 hPa 0600 UTC of 6 WS ppt with scattered very heavy fall over WHR. 0300 UTC of 7th 0000 UTC of 7th 500 hPa 70º E/north 25º N 850 hPa 0600 UTC of 7th FWS ppt over WHR. Sct ppt over Pb, HR, DL & W. Raj 500 hPa 0000 UTC of 11th 0300 UTC of 11th 52º E/ north of 23º N 850 hPa 0600 UTC of 11tth FWS ppt over WHR. 500 hPa 0000 UTC of 13th 0300 UTC of 13th 63º E/ north of 20º N 850 hPa 0600 UTC of 13th WS ppt with Scattered very heavy and isolated extremely heavy fall over WHR. Sct ppt over plain of NW India. 0000 UTC of 14th 0300 UTC of 14th 500 hPa 72º E/ north of 25º N 850 hPa 0600 UTC of 14tth WS ppt over WHR. FWS ppt over plain of NW India. Typical synoptic features associated with extreme precipitation over J&K during winter Typical features associated with extreme precipitation during winter Naresh and Yadav (Atmosphera) Easterly-Westerly Interaction 04-09-14 16 June 2013 Synoptic Charts M.S.L. Pressure 925 hPa winds 700 hPa winds 500 hPa winds Rainfall reported at 0300 UTC of 17-06-2013 23 to 25 September 1988 (max ppt 31.6 cm) LPA 23 to 25 September 1988 (max ppt 31.6 cm) SOME STUDIES ABOUT CLIMATIC TRENDS OVER WESTERN HIMALAYAS Winter ( Dec – Mar) Trends over HP (1977-2006) WDs frequency Winter precipitation Winter heavy precipitation (>64.5 mm) Naresh and Yadav (Atmosphera) Monthly Precipitation over WHR J & K HP UK 44 Stations 1901 – 2010 Tyagi, Naresh, Yadav (2012) Precipitation variation with longitude in WHR Region I Region II Region III Region I Region II Region III 35° N Zone III 33° N Zone II 31° N Zone I 29° N 73° N 75° N 77° N Precipitation variation with latitude in WHR Zone I Zone II Zone III Region I Region II Region III 35° N Zone III 33° N Zone II 31° N Zone I 29° N 73° N 75° N 77° N Precipitation variation with altitude in WHR Annual ppt (mm) Winter monthly ppt (mm) Monsoon monthly ppt (mm) Seasonal precipitation variation with altitude Pre-monsoon Post-monsoon Precipitation climatology of WHR Extreme PPT.
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