Dynamics behind extreme rains in Ghat section between Aug 06-09, 2019

-
Very heavy rains are no stranger to Ghat sections of Maharastra, Karnataka, Kerala, and TamilNadu and the districts nestled within the Nilgiris and the Sahyadris.

However, extremely extremely heavy rains ( > 300 mm/day) that are continuous in nature require some local dynamics that help in continuous cloud burst like situation.

Here, I make an attempt to explain the dynamics behind these events.

BAY OF BENGAL SYSTEMS + OFFSHORE TROUGH IN ARABIAN SEA:

It all starts from the "Bay of Bengal", where a low pressure system forms. Under the influence of tropical easterlies, these systems move westward towards land, and dissipate due to frictional effects, land cooling effect, and moisture dissipation (i.e. the system runs out of moisture - the heat engine needed to run the system - and hence doesn't sustain). 

If the system is strong (i.e. depression or deep depression, it will go all the way to the west coast and sustain for a longer time - e.g. the system that formed between Aug 06-Aug 10, 2019).

The basic mechanism is that the low pressure system activates a trough in the Arabian Sea, which allows warm air from the Arabian Sea to interact with the cold air from land. The cold air over land is due to an instability at higher level (500 mb) that is a result of dipping pattern of winds.

OROGRAPHICAL LIFTING NEAR GHATS:

The interaction between the warm and cold air is called a "FRONT", which aids in convection and formation of rain bearing Nimbus clouds. This happens due to the orographical lifting.

In the Ghats sections, due to the topography, the front becomes stationary - and is called a "Stationary Front". A stationary front allows convection in a small region and results in intense cloud formation for many days.



MOUNTAIN LEE WAVES (THE BOOSTER):

Exacerbating the situation is the formation of lee waves on the lee side of the mountain. These are special atmospheric waves that form as a results of stationary front. 

These waves get trapped on the lee side of the mountain. [The lee waves pattern can be seen in atmosphere from satellite images.]

Owing to the dynamics of the lee waves (refer to any meteorological book), these lee waves provide energy vertically upwards, adding additional mode of convection that form very intense clouds due to updraft of moisture. 

Moreover, the vertical energy is such that is moves towards the windward side of the Ghats. This results in very intense formation of clouds in the windward side. 

Due to the trapped lee waves, the leeward side of the mountain also see clouds bands and hence gets heavy rains. However, the Ghat section get extremely heavy rains, resulting in landslides. Moreover, this rain water moves downstream (due to higher elevation of Ghats) to districts which are on the slope of the Ghats, resulting in extensive damage.

Either these waves have to break down or loose energy for the intensification to die down. This happens when the atmospheric instability reduces, which in turn happens when the low pressure system dissipated.

The Lee wave phenomena was the reason behind 2018 Kerala floods, where a stationary front was formed. The strong upper level instability resulted in the formation of lee waves and result is clear from the above dynamics.


You can follow my twitter handle @SriGMFL

Disclaimer: The content given here are all personal views and the author will not be held responsible or held liable for any oversight or misinterpretation.





Comments

Post a Comment

Popular posts from this blog

Onset of Indian summer monsoon

-
Image
The onset of Indian summer monsoon (or southwest monsoon) is the most anticipated event for the Indian population. The first monsoon shower is the most awaited one as many seek respite from the scorching heat, little ones gleefully wanting to play in the cool showers, and the agricultural community get in full swing to prepare the land for farming. Over the past few years, owing to climate change, the onset has been delayed ( https://www.firstpost.com/economy/imd-forecasts-delay-in-onset-of-monsoon-may-hit-kerala-coast-around-5-june-1971211.html ) and the monsoon itself has been erratic, with brief periods of heavy rain (active phase) and periods of no rain (break phase). This is in contrast to a decade or two back, when the onset on monsoon would be well-timed and the nature of monsoon was also steady (with consistent rains through out the season). However, urbanisation, greenhouse pollution, deforestation, and many other anthropogenic activities have led to the late onset and incon...
Read more

Cyclone ASANI

-
Image
Our team has been on the fence with regards to the SEVERE CYCLONIC STORM (SCS) ' ASANI' , since the ridge dynamics were highly tricky. The cone of uncertainty surrounding this cyclone is quite huge, with still some divergence in the weather models as to its exact track, landfall location, as well as intensity. This uncertainty is very uncommon given that the cyclone is just 48 hours within possible landfall. However, as an upcoming (read startup) private player in the field of weather and climate services, it is important to spell out what we feel is likely to happen based on the dynamics that we interpret. The Cyclone has enough fuel to maintain Cyclonic Storm status. The vertical wind shear is the only unfavourable factor, but it is not too high. The ridge action has been the main X-factor. The Indo-Pacific ridge movement has been very unstable for past few days and hence the models were divergent on the track. But now, it looks like the Indo-Pacific ridge has moved closer to...
Read more

Indian Summer Monsoon 2018

-
Image
  A few days back, both Skymet weather services (a private weather forecasting firm) and the Indian Meteorological Department (IMD, a government organization) gave their predictions for the Indian summer monsoon 2018. Overall, they painted a positive picture, where both confirmed that monsoon 2018 is likely to remain normal at 100% (with error margin of +/-5%) of the long period average (LPA), where the nationwide LPA stands at 887 mm for the four-month period from June to September (for more details on this prediction see https://www.skymetweather.com/content/tag/monsoon-2018/ ). To elaborate, >105% of LPA indicates excess rainfall, <95% of LPA indicates below normal rainfall, and <90% of LPA indicates drought. Their prediction is based on the nationwide average and hence does not account for the state- and region-wise variabilities. Hence, although the prediction for 2018 is ‘normal rainfall’, this does not exclude the possibility of excessive rainfall in some regio...
Read more