Tuesday, May 01, 2012

Monsoon Watch - 4 (Part 1).

Forecasting Methods adopted for the Indian Monsoons:

Gowariker(1989) developed parametric and multiple power regression (MPR) models with 15 predictors for LRF of AISMR, which were later modified in 1991 to include 16 predictor parameters. The parametric model is qualitative and indicates the likelihood of the monsoon rainfall to be excess or deficient.

Thapliyal (1990) evaluated the relative performance of multiple regression, and Navone and Ceccatto (1994) have used 'feed-forward' neural network technique for the prediction of Indian monsoon rainfall with two predictors (500 hPa ridge location and Darwin SLP tendency from January to April). 

The results of a recent work by Krishna Kumar (1997) indicate that a single component accounts for about half of the total variance in the predictors

Most of the studies on LRF of Indian monsoon rainfall were based on empirical or statistical techniques till 2010. 
IITM, Pune has recently implemented the state-of-the-art coupled climate model, the Coupled Forecasting System (CFS) developed by the National Centers for Environmental Prediction (NCEP), USA. These statistical techniques range from simple correlation analysis to advanced procedures.

I still have faith, and believe in the "Gowariker Method" with 16 parameters. The results of this old but proven method has been successful. For Vagaries' forecast and analysis made in the "Arrival and quantum Monsoon Watch" series, I combine a few of them alongwith Dynamical models for a personal analysis and estimate. 

15 initial parameters from the months March, April and May are used to analysis and forecast the simulation of Indian monsoon rainfall during June to October. These initial conditions were carefully selected by experts to span the evolution of both the atmosphere and ocean.

Calculating the amount of rainfall, region wise, is of utmost importance, as SWM contributesto almost 80% of the annual total rainfall over India.
The performance of Vagaries' Monsoon Watch Quantum analysis is evaluated for the amount of summer monsoon rainfall over Indian monsoon region during June to SWM withdrawal.

The 500 hpa parameter is of importance in April. The mean latitudinal location of the 500 hPa ridge along 75°E in April over India, first identified by Banerjee (1978), is considered to be one of the most important predictors. The mid-tropospheric anticyclone over southern India migrates from 11.5°N in January to its northernmost position of 28.5°N during July. From October, the ridge starts shifting back southward. 
A more Northward location indicates better performance of the monsoon and vice versa. It also showed that the negative correlation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of Northern India.

ENSO factor in arriving at the rainfall amount is also of importance. The interface is the sea surface: that is where the transfers of water (evaporation/precipitation) and momentum occur. An accurate coupling of the fast atmosphere to the slow ocean is essential to simulate the ENSO, which in turn can simulate the interannual variability of Indian monsoon. Unfortunately, getting an accurate, or near to accurate forecast of the ENSO for the next 2 months is difficult, with no guarantee of accuracy or performance surety.

We see neutral conditions now. But, there is a possibility of a mild El-Nino event occuring as early as June/July. Now, this would result in "severe" break Monsoon condition in July. That is the last thing we want. But I would not endorse the event as yet. I would give it a 50-50 chance.

Limited pulses from the Eastern Pacific area, that is S.China sea, would mean lesser depressions and systems originating from the Bay. 
'In Situ" systems and systems from the Far east would contribute to the rainfall to the East Coast thru Central India and into Gujarat and even into Sindh (Pakistan).
El-Nino, even a mild one, would sort of restrict these pulses, and in fact encourage WDs into coming down South, more South than normal. 
But, for this, we would have to hope and pray for the Neutral ENSO conditions (presently on) to prevail and hang on till September. Again, a 50-50 chance for Neutral conditions.

Regarding the IOD parameter, recent forecasts from a few  coupled models  suggest  the possibility of development of a weak negative Indian Ocean Dipole event after July 2012.

1. Seasonal Low: Indicator: -ve

The heat is about to pick up in the Rajasthan/Sindh regions from the 3rd/4th of May, so good chance for the seasonal low to form and establish itself, at least by the 10th.Today the pressure at MSL is 1000 mb, with the core at 1000 mb. 
The sub Continent hot spots have just about touched the 44c mark (it is already Late. Refer MW-1) and should head for the 50c mark by around 16th May.

2. ITCZ: Indicator: Normal
As it is by the 10th, that we should have the ITCZ nearing the Equator. Only then would the Bay branch of the SWM could commence in the Southern Bay by the 15th of May.

3. Jet Streams: Indicator:-ve

These are streaming away in a Westerly direction above the 25N line. They are just about getting Easterly around 5N. Needs a rapid change now.

4. Cross Equatorial Winds: Indicator: Normal in Bay Branch. -ve in Arabian Sea Branch

Well established in the Bay region. SW winds, with sufficient speed hitting the S.Myanmar coasts, covering the Bay Islands with 20-30 knts, and could hearald the arrival of the SWM on time there.

Arabian Sea winds are now picking up, and rubbing the Somali coast at sufficient speed in getting the "Somali Current" established. The SST off the Somali Coast is 22c, While the required SST in First week June is 17/18c. (Required to form masses of Monsoon clouds in the warm central Arabian Sea).

The Initial Forecast for the SWM 2012, Region wise and Month wise, could go in for a revision if ENSO and Upper Winds change drastically. Possible could  put up another Revised Forecast by May End if needed.

Second Part of MW-4 with Map of Monsoon Rain Estimated Forecast region wise will be put up on 2nd May at 1 pm IST.

All my personal views and estimates. Not binding on anyone and do not use for commercial purposes.

See Current Weather Page for Today's Weather.


Rajan Alexander said...

Hi Rajesh. Trust that you are enjoying your vacation (??) in the US.

Europe to face record cold wave:

Just wanted to pick your brains whether this could affect the Indian Monsoon, and if so, how

Rajesh said...

Rajan: Thanks, Rajan, yes, I'm on a holiday with my son in Cal.
Discussed with a few metmen here to about forecasting thru the computer method, and other ways of looking at the weather, the sub continent weather.
Actually, as per the method i rely on, the Eurasian snow in april,the far western region, is a parameter, that plays a important role in the quantum for our NW and N areas.There is a low predicted by Mark that would bring precipitation to the Central/Eastern Europe regions.The Easterly jet streams are still prevelent in our region, and with rain bearing lows around them over Eastern and SE Europe, the WDs are encouraged to move more Southwards. As a result, they "block" our required Monsoon lows from coming towards the Indian Peninsula. So, the lows, if any, move towards Banglaedash and Myanmar. I will be putting up this rainfall aspect in my seasonal forecast map today, showing our NE as excessive rain regions.

Rajan Alexander said...

Thanx Rajesh. Cal? California - must be beautiful.

So what I gather from your clarification, is the "block" has adverse impact on the monsoon, though NE (which since recently were rain deficient) gets floods!

A regional rain distribution forecast would be really useful from the perspective of disaster preparedness and agriculture. So look foward to read it.

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