Gp Capt (Dr) B Nandi


In modern era winds are derived from cloud motion vector, especially over data sparse area. One of the methods is to use Water Vapour (WV) Channel Satellite information, especially for upper troposphere. So, there is no wonder to expect a good correlation between WV satellite image and upper tropospheric wind. Present study intends to demonstrate the utilisation of upper tropospheric wind feature for weather forecasting.

2. WV channel images

Following WV image is from Meteorologix.com for 1200UTC 0f 01May2024.

Yellow colour portion denoted area of subsidence, which has made the region dry and warm. Embedded red colour indicates area of strong subsidence. Black is also part of subsidence but stronger in nature than other two colour. White area denotes cloud. Thin whitish are, almost of uniform texture indicate presence of mid-tropospheric moisture.

3. Upper Tropospheric chart

200hPa chart of predicted time for 1200UTC of 01May2024 on the basis analysis of 0000UTC of 01May2024  is presented below.

Inspection of 200hPa chart reveals following

  • (a) There is an upper tropospheric trough at western parts of Tibet approximately along 800E longitude. Deep NWly winds from 450Nto 300N prevails west of the trough. This a zone upper tropospheric convergence zone (Negative Vorticity Advection zone (NVA)). This convergence leads to subsidence as seen in WV image
  • (b) Subtropical Jet stream core line has been marked with yellow line. North of the Jet stream is of subsidence zone and may be about an one degree south of the jet core. Formation of jet stream owes to the convergence of air of upper branch of Hadley cell due to the latitudinal contraction of atmospheric belt of equatorial air with large diameter reduced to half around 300N. This convergence leads to subsidence and mostly responsible for maintaining high pressure belt at hose latitude (300N).

Two reason of subsidence ((a), (b)) combines over North West India and can be seen as red coloured strong subsidence zone in WV image. This kind of subsidence can lead to strong heat wave on ground.

South of Jet stream (about an one degree from core), still an ascending region and supports cirrus cloud flowing or mid tropospheric moisture as seen in WV image.

  • (c) There is an anticyclone centre over peninsular Indian with east-west ridge almost along 130N. It is well known that SE sector of anticyclone is an area of convergence due NVA. This convergence leads to subsidence as can be seen just south of Chennai. This anticyclones SW sector is of divergence quadrant (PVA- Positive Vorticity Advection). Cloud can be seen over this region.
  • (d) East of the Anticyclone there is a trough in westerly wind. Western side of this trough is a region of subsidence and can be seen as yellow region in WV image over Bay of Bengal.
  • (e) Area of confluence lies over SE Bay of Bengal almost along 90N latitude. Marked in yellow on wind chart. This confluence also leads to subsidence as seen in WV image.

As mentioned in (c), (d), (e) they combine to give wide region of subsidence over entire southern Bay of Bengal.

  • (f) The SWly wind west of 650E is the carrier of moist air through mid-troposphere from convection zone, as seen close to 660E. It is quite visible in WV image

4. Conclusions

(a) Subsidence zone and thereby clear weather zone or may be of heat wave zone can be identified with the upper tropospheric wind chart of 200hPa by identifying

  • (i) West of upper tropospheric trough    
  • (ii) North of STJ
  • (iii) SE quadrant of anticyclone
  • (iv) Confluence region

(b) Carrier of moist air through mid-troposphere can be identified with the SWLy flow at 200hPa.

(c) East of trough is a divergent zone and can facilitate weather formation with suitable lower tropospheric feature ( not discussed here) but can be seen on next days weather formation over Eastern India.

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