Bow Echoes and Related Severe Storms

Delhi experienced an intense storm marked by strong winds and unusual radar imagery on May 25, 2025. The Indian Meteorological Department (IMD) noted that the storm displayed a crescent or archer’s bow-like shape on its radar. This is an indication of a meteorological event known as a ‘bow echo’. The storm’s winds reached up to 100 kmph causing significant impact across the region. It also brought record rainfall, making May 2025 Delhi’s wettest on record, with over 186 mm total precipitation and more than 80 mm in a single day, disrupting transport and city life.

Understanding Bow Echo

A bow echo is a specific radar signature of a mesoscale convective system (MCS) associated with strong, wind-driven storm fronts. On Doppler radar imagery, it resembles a bowed or arched line of thunderstorms. These echoes are named for their characteristic shape, where the centre of the storm line is pushed outward due to strong winds behind the system, forming a shape similar to an archer’s bow.

The term ‘bow echo’ was first coined in the 1970s by a Japanese American meteorologist, Ted Fujita. He is also well-known for developing Fujita scale (F-scale) to classify tornadoes. A bow echo is essentially a line of storms, also called a squall line, which can sometimes be embedded in larger squall line. A bow echo can extend from 20 to 100 kilometres in length and typically last between three to six hours. They are often precursors to serve weather phenomena, including damaging straight-line winds, downbursts and in some cases, transient tornadoes.

How a Bow Echo Forms

Bow echoes form due to complex interaction between warm moist surface air and cooler rain-cooled air masses descending from a thunderstorm. It begins when rain-cooled air descends (downdraft) and spreads horizontally upon reaching the ground. This cool and dense air forms a boundary that separates it from the warmer and more buoyant air at the surface known as gust front.

This gust front forces the warm-moist surface air upward into the atmosphere, which triggers the development of new thunderstorm cells. These new cells generate more rain, reinforcing the downdraft and maintaining the gust front’s intensity. As this cycle continues, a rear inflow of the air develops behind the storm, pushing the centre of the storm line forward and creating the characteristic ‘bow’ shape. Bookend vortices, or rotating eddies (vortices) on the end of the bow, can also develop and sometimes spawn brief tornadoes.

Over time, the thunderstorm complex could reach a semi-steady state, where new storm cells continue to develop at the front of the bow while older cells dissipate at the rear. The constant regeneration and reinforcement help sustain strong winds, which often lead to widespread surface damage.

Types of Bow Echo System

Bow echo systems are broadly classified into two patterns based on their structure and behaviour:

(i) Progressive Pattern In this configuration, the bow echo is relatively short and moves rapidly, typically perpendicular to the environmental wind direction. They are common during summer and associated with high instability. They usually result in rapid and intense downbursts along a narrow path.

(ii) Serial Pattern Serial bow echoes are extensive and aligned nearly parallel to the environmental wind. These often occur in the cool season and may include multiple embedded bowing segments and line echo wave patterns (LEWP).

They are associated with sustained damaging winds and possible transient tornedoes, especially when high precipitation supercell characteristics (mesovortices) are present.

An LEWP is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south or equatorward (towards the equator) of a mesoscale low-pressure area with a rotating ‘head’.

Downbursts: The Companion Phenomenon

A downburst is a powerful and localised wind event caused by a descending column of cooled air from a thunderstorm or rain cloud. These events originate when a cumulus-type cloud could no longer support the heavy mass of water suspended within it. Once the weight becomes too heavy, the water rapidly falls towards the ground, accelerating as it descends.

As this mass of precipitation plunges, it displaces the air beneath, creating a strong downdraft. When this air hits the ground, it spreads outwards in all directions, resulting in damaging straight-line winds at the surface. In some cases, the presence of dry air below the storm enhances this process. The falling rain partially or fully evaporates into the dry air, cooling it further and increasing its density. This denser air accelerates downwards, intensifying the downburst.

Downbursts are commonly observed within bow echoes and other convective systems, and they could be responsible for extensive wind damage similar to that caused by tornadoes—though their wind pattern is linear rather than rotational.

Bow Echoes and their Connection to Derechos

While all derechos are bow echoes, not all bow echoes become derechos. (A derecho is a specific type of long-lived, widespread windstorm that must meet particular criteria to earn its name.) For a storm to qualify as a derecho, it must produce:

• A swath of wind damage extending at least 400 miles;
• A width of at least 60 miles;
• Wind gusts of at least 58 miles per hour along most of its length; and
• Several gusts exceeding 75 miles per hour.

Derechos typically arise from one or more bow echoes, often within larger quasi-linear convective systems (QLCS). QLCS is a broad thunderstorm structure that can evolve from squall lines or other bowing line segments, and they are closely monitored for sudden severe wind hazards.

Major Severe Wind Phenomena Associated with Thunderstorms

In addition to rain and lightning, thunderstorms are capable of producing powerful and hazardous wind phenomena that differ markedly in scale, duration, and intensity.

The table below provides a concise summary of the principal severe wind types associated with convective storms:

Bow Echo Structure and Radar Features

The radar reflectivity patterns associated with bow echoes are essential for meteorologists in predicting potential damage. Early in the development of the storm, a strong downburst may initiate the bulging of the reflective pattern, leading to the arched shape.

A pronounced reflectivity gradient appears along the leading edge of the storm, highlighting zones of strong convection and vertical updrafts.

Behind the leading edge, radar often reveals rear inflow notches (RINs) or weak echo channels (WECs). These are indicators of strong inflow of air into the storm system. These are frequently associated with a rear-inflow jet (RIJ). RIJ is a stream of fast-moving mid-tropospheric air, descending into the back of the bow echo, and contributing to high surface wind speeds.

Additionally, mid-altitude radial convergence (MARC) signature may be evident in storm-relative velocity map (SRM) data at an altitude of about 3–7 kilometres. MARC involves strong wind velocity differences at mid-levels, signalling potential downbursts and leading-edge wind damage.

Seasonal and Environmental Conditions

Bow echo development is influenced by the atmospheric conditions preceding the storm. These pre-storm environments differ significantly between the warm and cool seasons.

Warm or Summer Conditions During summer months, bow echoes often form under relatively weak synoptic-scale forcing but high instability:

• Surface patterns feature east-west frontal boundaries with high dewpoints pooled near the front.
• Strong surface convergence and high equivalent potential temperature (Theta-e) values near the genesis region.
• Moderate to strong warm advection (the horizontal movement of a mass fluid, such as air or an ocean current) at mid-levels (850 and 700 mb) near genesis area.
• High moisture content near the surface through drier air may exist at higher altitudes, (700 and 500 mb) which can intensify damaging winds.

Thermodynamically, these systems often form in highly unstable air masses with convective available potential energy (CAPE), a measure of the capacity of the atmosphere to support upward air movement that can lead to cloud formation and storms, values ranging from 2400 to over 6000 J/kg. Wind shear in the lower atmosphere, up to 2.5 km altitude, is generally strong in speed but limited in direction.

Cool or Winter/Spring Conditions In contrast, bow echoes in the cooler months are driven more by strong dynamic forcing:

• Progressive low-pressure systems with associated warm and cold fronts dominate.
• Moderate to strong wind fields prevail throughout the atmosphere.
• Wind momentum from aloft could transfer downwards, especially in the absence of a low-level inversion.
• Instability is lower (CAPE values between 500 and 2000 J/kg), but strong wind shear compensates.

Cool season bow echoes often form along or ahead of cold fronts and could be embedded within longer squall lines, typically resulting in more organised and widespread wind damage.

In India, bow echoes are more likely during pre-monsoon months (April–June), when high surface heating and moisture contrasts create ideal conditions for severe thunderstorms.

Tornedoes within Bow Echoes

Although bow echoes are primarily known for their straight-line winds, embedded mesovortices along the leading edge can spawn short-lived tornedoes, particularly

near the apex or northern side of the bow.

The mesovortices originate were enhanced low-level convergence and cyclonic shear exist. Rapid vertical stretching in updraft regions intensifies these vortices which may become strong enough to produce EF0 to EF2 tornadoes.

BOW Occurrences in India

Although bow echoes are more commonly studied in the United States and Europe, they are not unfamiliar to India. For instance:

• On May 31, 2022, a bow echo formed over Delhi and Noida, producing winds up to 100 kilometres per hour. However, the event was short-lived, for about an hour, but caused notable damage.
• In May 2025, Delhi experienced another bow echo-driven storm with similar wind speeds accompanied with heavier rain and greater urban disruption.
• Similar patterns were observed during thunderstorm activity in Odisha in May 2025, highlighting the potential recurrence of such events in India’s monsoon and pre-monsoon climate.

According to IMD scientists, these bow echo events often coincide with intense thunderstorm activity during seasonal transitions and could have significant weather implications.

Conclusion

Bow echoes are powerful and visually distinctive radar phenomena that signify the presence of severe thunderstorms capable of causing widespread wind damage representing most striking displays of organised convection. Although commonly observed in temperate climates, recent events over Delhi and other Indian regions underscore their growing relevance in Indian meteorology.

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