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Tuesday, January 26, 2010

Winds

Wind Atmospheric motion characterized by direction and speed. The

direction of the wind is the direction from which the wind

approaches the station (Example Northerly wind – Wind

approaching the station from North)

Gales A gale is a very strong wind (34 to 47 knots)

Squall A sudden increase of wind speed by atleast 3 stages on the

Beaufort Scale, the speed rising to force 6 or more, and

lasting for atleast one minute is called a squall

Gust A rapid increase in the strength of the wind relative to the

mean strength at the time


 

Monsoon

"MONSOON" has originated from the Arabic word "MAUSIM" which

means season. It is most often applied to the seasonal reversals of the wind

direction along the shores of the Indian Ocean, especially in the Arabian Sea,

that blow from the southwest during one half of the year and from the

northeast during the other.

Onset & Advance of Monsoon

The guidelines to be followed for declaring the onset of monsoon over Kerala

and its further advance over the country are enlisted below:

Onset over Kerala

a) Rainfall

If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy,

Amini,Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam,

Kochi, Thrissur, Kozhikode, Thalassery,Kannur, Kasargode and

Mangalore report rainfall of 2.5 mm or more for two consecutive days, the

onset over Kerala be declared on the 2nd day, provided the following

criteria are also in concurrence.

b) Wind field

Depth of westerlies should be maintained upto 600 hPa, in the box

equator to Lat. 10ºN and Long. 55ºE to 80ºE. The zonal wind speed over

the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of

15 – 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/

satellite derived winds.

3. Outgoing Longwave Radiation (OLR)

INSAT derived OLR value should be below 200 wm-2 in the box confined

by Lat. 5-10ºN and Long. 70-75ºE.


 

Further Advance of Monsoon over the Country

a) Further advance be declared based on the occurrence of rainfall over

parts/sectors of the sub-divisions and maintaining the spatial continuity of

the northern limit of monsoon, further advance be declared.

The following auxiliary features may also be looked into:

b) Along the west coast, position of maximum cloud zone, as inferred from

the satellite imageries may be taken into account.

c) The satellite water vapour imageries may be monitored to assess the

extent of moisture incursion.

Northern Limit of Monsoon (NLM)

Southwest monsoon normally sets in over Kerala around 1st June. It

advances northwards, usually in surges, and covers the entire country

around 15th July. The NLM is the northern most limit of monsoon upto

which it has advanced on any given day.

Withdrawal of SW Monsoon

a) Withdrawal from extreme north-western parts of the country should not

be attempted before 1st September.

b) After 1st September:

The following major synoptic features should be considered for the first

withdrawal from the western parts of NW India.

i) Cessation of rainfall activity over the area for continuous 5 days.

ii) Establishment of anticyclone in the lower troposphere (850 hPa and

below)

iii) Considerable reduction in moisture content as inferred from satellite

water vapour imageries and tephigrams.

Further Withdrawal from the Country

i) Further withdrawal from the country may be declared, keeping the

spatial continuity, reduction in moisture as seen in the water vapour

imageries and prevalence of dry weather for 5 days.


 

ii) SW monsoon should be withdrawn from the southern peninsula and

hence from the entire country only after 1st October, when the circulation

pattern indicates a change over from the southwesterly wind regime.

Commencement of NE monsoon rains

i) Withdrawal of SW monsoon upto Lat. 15ºN.

ii) Onset of persistent surface easterlies over Tamil Nadu coast.

iii) Depth of easterlies upto 850 hPa over Tamil Nadu coast.

iv) Fairly widespread rainfall over coastal Tamil Nadu, south coastal Andhra

Pradesh and adjoining areas.


 

Criteria for Declaring Onset of Northeast Monsoon

For declaring onset of Northeast Monsoon following criteria may be

considered:

(1) Withdrawal of south west Monsoon upto 15º N.

(2) Onset of persistent surface easterlies over Tamil Nadu coast.

(3) Depth of easterlies upto 850 hpa over Tamil Nadu coast.

(4) Fairly widespread rainfall over the coastal Tamil Nadu and adjoining areas.

(5) Onset is not to be declared before 10th October even, if the conditions

described above exist.

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