Sunday, January 03, 2010

Ensemble Forecasting ??? ..

Ensemble Forecasting ???

Initially using weather proverbs and today using super computers.

However, modelling the atmosphere and the interconnected oceans is so complicated that we still have a long way to go.

I remember many years ago when I was in Exeter someone said to me "I never watch the weather forecasts because they are always wrong".

I found difficulty seeing how if he never watched how he knew we were always wrong! I had even less chance of persuading him that if we were always wrong, if he always believed the opposite was the correct forecast, he could always be right!

Traditionally, meteorologists collect weather data twenty-four hours a day all around the world, which includes observations from land, sea and in the air. This information is fed very quickly into the super computers, which, by solving a series of mathematical equations, give the meteorologist a variety of weather maps up to a week or so ahead.

The supercomputers provide only one solution to the evolution of the weather patterns.
The supercomputers provide only one solution to the evolution of the weather patterns. This is called a deterministic forecast and gives little idea of the probability of certain events occurring. Scientists, therefore, tried to find a way of giving the meteorologist the probability of the deterministic forecast being correct; they called this an ensemble forecast.

In the atmosphere Chaos Theory reins supreme. It states that if a butterfly flaps its wings in one area of the world, it can have an effect on our weather in the UK. It is probably easier to understand, from the weather point of view, with a different example.

If a raindrop falling onto the top of the Andes in South America encounters a light easterly breeze, it will blow the rain onto the western side of the mountain. It will eventually reach the Pacific Ocean and interact with weather systems there.

If, however, there is a gentle westerly wind the raindrop will fall onto the eastern side and eventually become part of the Atlantic Ocean weather systems.

To cater for this very small change in the initial position of the raindrop above, ensemble forecasts run the deterministic forecast many times using slightly different starting points ending up with many different forecasts for a few days ahead.

While this may seem like a time consuming process, it allows the meteorologist to determine the probability of certain weather events happening.

If the results for most of the forecasts are very similar... the meteorologist has a high degree of confidence in the forecast.
If the results for most of the forecasts are very similar for a certain point in time, then the meteorologist has a high degree of confidence in the forecast. If, on the other hand, the ensemble forecast has a wide scattering of results, then the confidence in a particular forecast is much lower.

In this way the meteorologist can give the listener or viewer a probability of the forecast being correct.

Taken from BBC
BBC also forecast Showers for Central & North Tamilnadu coast from Friday, 8-Jan-10...
Final days (8,9-Jan) of ATP Chennai - 2010 may be in for Heavy showers.. for regular weather updates visit
Chennai - Is expected to receive showers on 7,8 and 9 of Jan-2010.
Chennai - for the Past 2 years has received showers from 5th to 10th of January.
From our records:: "Surprise Day in January! " .. 7, 8-Jan-09 .. ....
From our records:: Previous history of January RAINs for Chennai .. "3-Jan-08.. Unusual day!" ..
Coming weekend from 8-Jan-10 for Chennai & central,North Tamilnadu coast is going to be wet ..
Heavy shower activity can also be seen over south-west Srilanka .. .. Dead high clouds are seen moving north into Bay and some are over Chennai too.
5:30pm IST, Heavy convective clouds over south-east Bay just along 10th parallel .. .. SST along Tamilnadu coast upto south-Andhra coast is on higher side. .. Cyclone potential is ZERO above 10th parallel into Bay, so if the system tries to move north-west.. it will die. ..this present potential system is expected to give Heavy showers for TN coast from 7-Jan..
Cold day conditions would continue in Punjab, Haryana, Uttar Pradesh and Uttar Pradesh during next 1-2 days ... A feeble western disturbance lies over Jammu & Kashmir and neighbourhood. .. The trough in the easterlies now extends from Lakshadweep to south Gujarat coast.
Still this system .. .. is not even MARKED as LOW. But soon it'll be.
GFS predicts that the present whirl .. may not become as heavy as it was predicted a week back.
Present Whirl is at the same place as that of Cyclone "ward"..
A good whirl can be seen over south & south-west Bay ..
Chennai - Why, minimum temperature is high?? this is the reason ..
RT @IL_world: Heavy snow hits Beijing, stranding thousands - Times of India
All of sudden from Yesterday afternoon south Bay has become very active, and now this is the picture ..
Chennai - Today Low cloud formation will be heavy and prolonged.
Chennai - A sudden rise in minimum temperature.. 24.1°C (6:48am) .. why??