MONSOON PREDICTION MODELS

Subject: Geography

Context : Monsoon to be ‘normal’ in 2021; East and Northeast may get less rain: IMD

Concept :

The India Meteorological Department (IMD) on Friday said the 2021 southwest monsoon starting in June is expected to be normal at 98 per cent of the Long Period Average (LPA).
A good monsoon will mean another year of bumper farm production and cascading positive impact for the economy battling Covid-19 infections. If the forecast comes true, this will be the third consecutive year of normal or above normal monsoon.
Earlier this week, private weather forecasting agency Skymet had made a similar prediction that the southwest Monsoon would be normal at 103 per cent of the LPA. Rainfall between 96 and 104 per cent of the LPA is considered normal.
Both IMD and Skymet predictions come with a model error of plus and minus 5 per cent.

Dynamical Model:

It is also called the Monsoon Mission Coupled Forecast System. It relies on the supercomputers, mathematically simulating the physics of the ocean and the atmosphere.
This model is better at forecasting the state of the weather a week or two in advance and is not yet considered reliable by meteorologists in forecasting the monsoon.

Statistical Model:

Traditional statistical model equates relationships of physical parameters, such as for instance sea surface temperatures, snowfall, the temperature of landmass etc, with the actual observed rainfall in the past.
For example , it takes into consideration the global weather models pointing to negligible chances of El Nino, a warming of the central equatorial Pacific which is associated with the drying up of monsoon rain. The IMD relies on this model.
In any given year, there is a 33% chance of a normal monsoon that’s why there is high confidence that the monsoon in 2020 would be normal.

Proposed Models

The three models under consideration are:
12 global circulation models (dynamical) whose outputs would be combined into a single one.
Model that gauges rainfall based on the sea surface temperature in the tropics .
Statistical model based on climate variables observed during the pre-monsoon.
All of them are ‘ensembles’ meaning smaller models are combined to arrive at an average value.