Tropical cyclones and western disturbances

Introduction

Tropical cyclones are among the most powerful and destructive natural phenomena on Earth, originating over warm ocean waters and causing significant damage in coastal regions. They are characterized by high wind speeds, heavy rainfall, and low atmospheric pressure, with their intensity often reaching devastating levels. Unlike temperate cyclones, which are larger but less violent, tropical cyclones are concentrated systems driven by the energy released from the warm ocean. This article will explore the features, structure, formation, life cycle, and monitoring of tropical cyclones, as well as their differences from other weather systems.

Tropical Cyclones

Characteristics of Tropical Cyclones

• Destructive Storms: Tropical cyclones originate in warm oceans and intensify over warm tropical waters. They are highly destructive due to their wind speeds, which can exceed 119 km per hour.
• Warm Core Cyclone: The wind circulates around a low-pressure center that is thermally induced, making it a warm core cyclone.
• Cyclostrophic Systems: They are rotating systems of the tropical regions, unlike temperate cyclones, which are larger and less violent.
• Wind Velocity: Wind speeds in tropical cyclones typically range between 130-150 km/hr but can reach even higher velocities.
• Coastal Impact: Tropical cyclones have a significant impact on coastal locations, differing from thunderstorms and cloudbursts.
• Life Span: These storms last from a few days to up to 3 weeks.
• Formation over Water: They always form over water, as they require the latent heat of condensation to mature and become violent.
• Cold Currents Requirement: Cold currents are necessary to sustain their pressure systems.
• Never at the Equator: They do not form at the equator and are more common around 8-10 degrees north of it.
• Rotation (Ferrel’s Law): According to Ferrel’s Law, tropical cyclones rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Structure of a Tropical Cyclone

• Eye:
  • A roughly circular area located at the center with light winds and fair weather.
  • Little to no precipitation, sometimes with visible blue sky and stars.
  • The warmest region of the cyclone, with temperatures being 10 degrees or more warmer at an altitude of 12 km than the surrounding environment.
  • Varies in size from 8 km to over 200 km across, with an average diameter of 30-60 km.
• Eye Wall:
  • Surrounds the eye with a circular ring of deep convection.
  • Contains the highest surface winds in the cyclone.
  • Fastest sustained winds occur in this region.
  • The air in the eye is slowly sinking, while the eye wall has a net upward flow, resulting in strong updrafts and downdrafts.
• Spiral Bands:
  • Long, narrow rain bands spiraling towards the cyclone’s center, hence called spiral bands.
  • These bands exhibit the most pronounced low-level convergence and upper-level divergence.
• Vertical Structure:
  • Inflow Layer: Extends up to 3 km, responsible for driving the storm.
  • Middle Layer: Extends from 3-7 km, the main area of cyclonic activity.
  • Outflow Layer: Above 7 km, with maximum outflow at around 12 km, where the flow is anticyclonic.

Conditions for the Formation of Tropical Cyclones

• Sea Surface Temperature: The water temperature should be above 27 degrees Celsius.
• Coriolis Force: The presence of Coriolis force is essential for the cyclone’s rotation.
• Vertical Wind Speed Variation: A small variation in vertical wind speed is required.
• Low-Pressure Area: A pre-existing weak low-pressure area aids in cyclone formation.
• Upper-Level Divergence: There should be upper divergence above sea areas or in the Inter-Tropical Convergence Zone (ITCZ).

Life Cycle of a Tropical Cyclone

1. Origin: Multiple thunderstorms merge over warm ocean areas, creating an intense low-pressure system. The warm, moist air rises, cools, and condenses, releasing latent heat and initiating the cyclone’s development.
2. Early Stage: Condensation releases latent heat, making the air warmer and lighter, leading to further uplift. This cycle continues, intensifying the storm as more moisture is drawn in.
3. Mature Stage: The cyclone reaches its strongest point with a well-developed eye and eye wall. Spiraling winds create multiple convective cells, leading to heavy rainfall under cumulonimbus clouds.
4. Dissipation: The cyclone weakens and dies off when it hits land due to the lack of moisture supply.

Weather Transition Across a Tropical Cyclone

• Light winds gradually increase, and scattered clouds with light showers give way to heavy rains and high wind speeds.
• As the eye passes over, there is a brief calm with clear skies.
• The reverse sequence occurs as the cyclone moves away, with winds and rainfall gradually ebbing.

Four Stages of Tropical Cyclone Development

1. Tropical Disturbance: Warm ocean water evaporates, rises, and condenses, releasing heat and initiating cloud formation.
2. Tropical Depression: Large columns of clouds form, and divergence occurs due to low atmospheric pressure at the surface and high pressure aloft.
3. Tropical Storm: The formation of an eye and rotation of winds around it begins, with wind speeds reaching around 39 miles per hour.
4. Tropical Cyclone: The fully developed cyclone has wind speeds reaching their maximum.

Tracking & Movement of Tropical Cyclones

• Tropical cyclones generally move from east to west under the influence of trade winds.
• They are monitored using radars, aircraft, and satellites.
• Monitoring is crucial for issuing warnings and taking preventive measures like evacuation and suspension of activities.

Naming of Cyclones

• Cyclones are named for easier identification and to avoid confusion.
• Names are given by Regional Specialized Meteorological Centers (RSMCs) and Tropical Cyclone Warning Centers (TCWCs).
• In the Indian Ocean region, the World Meteorological Organization/ESCAP Panel on Tropical Cyclones decides the names.

What Are Western Disturbances?

Definition and Origin

• The term “Western” refers to the direction from which they originate relative to India.
• The word “disturbance” is used because the air within these low-pressure systems tends to be unstable and disturbed.
• Extra-Tropical Storms: Western disturbances are extra-tropical storms that derive moisture from upper atmospheric winds, particularly jet streams, unlike tropical cyclones that get moisture from lower atmospheric winds.
• They are also referred to as western cyclonic disturbances.

Characteristics

• Low-Pressure Systems: These disturbances are embedded in western winds (westerlies) that flow from west to east.
• Western disturbances develop in the mid-latitude region north of the Tropic of Cancer and not in the tropical region.
• Origin: The primary origin of western disturbances is the Mediterranean Sea, where evaporated moisture condenses to form clouds, leading to the development of low-pressure systems.
• Movement: As they move eastward toward India, they gather additional moisture from the Black Sea and the Caspian Sea.

Impact of Western Disturbances on India

• Western disturbances contribute 5-10% of India’s total annual rainfall, with an average of 4-5 disturbances forming during the winter season.
• The rainfall distribution and amount vary with each disturbance.

Impact on Weather

• Night Temperatures: Before the arrival of a western disturbance, night temperatures increase.
• Day Temperatures: Day temperatures decrease during the impact of these disturbances.
• Winter Rainfall: They cause moderate to heavy rainfall, especially in the northwestern part of India, from December to March, affecting regions like Punjab, Haryana, northwestern Uttar Pradesh, and the western Himalayas.
• Snowfall: Western disturbances bring heavy snowfall to the Himalayan region, causing a cold wave in the north Indian plains.

Impact on Agriculture

• Rabi Crops: Moderate rainfall from western disturbances is beneficial for rabi crops such as wheat, barley, mustard, gram, and lentils.
• Heavy Rainfall: However, heavy rainfall can lead to floods, resulting in crop damage in the plains.
• Weak Disturbances: Can cause crop failure and water scarcity, affecting agriculture adversely.

Casualties and Hazards

• Although western disturbances are not high-intensity weather systems, they have become more disastrous in recent years.
• Cloudbursts: An example is the cloudburst in Leh in 2010, which resulted in flash floods and significant damage to life and property.
• Landslides and Avalanches: Heavy snowfall or rainfall from western disturbances can trigger landslides and avalanches in the western Himalayas.

Weather Associated with Western Disturbances

• The arrival of these disturbances is associated with remnants of temperate cyclones, leading to clear weather after they pass away.
• They can occasionally cause dense clouding and heavy precipitation, even during the monsoon season.

Cloud Burst

• A cloudburst is an intense, short-duration torrential rainfall brought by a thunderstorm, often leading to flash floods.
• It specifically occurs when an air mass with high humidity is struck at a place due to various reasons.
• Cloudbursts can cause substantial damage to life and property.

Origin and Development of Western Disturbances

• The origin of western disturbances lies in the Mediterranean Sea, where evaporated moisture condenses to form clouds.
• A low-pressure system typically forms over the Mediterranean Sea due to the subtropical westerly jet stream in the upper atmosphere.
• The Himalayas act as a barrier, trapping moisture and causing rainfall in the northwestern part of India.
• As these disturbances travel toward India, they gather additional moisture from the Black Sea and Caspian Sea.

Weather Transition Across a Western Disturbance

• As a western disturbance approaches, the temperature and weather patterns change:
  • Night temperatures increase, and clouds start forming.
  • Rainfall begins, leading to colder day temperatures.
  • Heavy snowfall occurs in the Himalayan regions.
  • After the disturbance passes, clear weather follows.

Conclusion

Western disturbances play a vital role in shaping the winter weather of India, providing necessary moisture for agriculture and maintaining water resources. While they bring beneficial rainfall for crops, they can also cause hazards like landslides, avalanches, and cloudbursts. Understanding these disturbances helps in better managing their impacts on life, agriculture, and infrastructure.

1. Explain the structure of a tropical cyclone, focusing on the role of the eye, eye wall, and spiral bands. (250 words)
2. Discuss the role of the Himalayas in influencing the weather patterns associated with western disturbances in India. (250 words)
3. How do western disturbances differ from tropical cyclones in terms of origin, movement, and impact on the Indian subcontinent? (250 words)