Chasing the 'Wind Quarrels' Not Drawn on Weather Maps: A Meteorologist Discusses the Depths of Summer Local Winds on the Tokyo Bay Coast

The 'Local Betrayals' That Plague Forecasters

On summer mornings, we meteorologists face our weather maps. High pressure covers the Japanese archipelago, and the isobars trace gentle curves. 'Today will be stable, clear weather across the entire region'—a few hours after making that judgment, reality mercilessly betrays the forecast. In a corner of the city center, or perhaps in a specific area of Saitama, a fierce thunderstorm suddenly erupts. The AMeDAS rain gauges record figures exceeding 50 millimeters in a short time, and social media is flooded with voices saying, 'The forecast was wrong.'

The surface weather maps we show on television and apps tell the 'big story' of the pressure distribution across all of Japan. However, in the actual atmosphere, especially on the Kanto Plain in summer, another drama is unfolding behind the scenes of the weather map. This is the world of 'local winds.'

In this article, I would like to explain, from a somewhat technical perspective, the 'summer local winds of the Tokyo Bay coast,' which are a headache for meteorologists and at the same time one of the most intellectually stimulating subjects of study. This is not merely a list of meteorological knowledge, but a story of a complex system woven by the city and the atmosphere.

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The Tug-of-War Happening in the 'Gaps' of Weather Maps

The Asymmetry Created by the Temperature Difference Between Sea and Land

The Tokyo Bay coast in summer becomes a massive natural laboratory. As the sun rises, the land, covered in asphalt and concrete, heats up rapidly. Surface temperatures sometimes exceed 50°C, and the air above it expands, becomes lighter, and begins to rise. This creates a relative low-pressure area near the ground.

On the other hand, the sea behaves differently. The specific heat of water is about five times that of concrete. In other words, even when receiving the same solar energy, the temperature rise of the sea surface is more gradual compared to the land. As a result, relatively high-pressure air is maintained over the sea.

This pressure gradient is the driving force of the sea breeze. Air flows from the sea to the land, from high pressure to low pressure. However, what is noteworthy here is the point of 'how far inland it can penetrate.'

The 'Invisible Boundary' Called the Sea Breeze Front

The sea breeze advances inland as a mass of cold, dense air, like a dome crawling along the ground. We call its leading edge the 'sea breeze front.'

The moment this front passes, dramatic changes are recorded in meteorological observation data. The temperature drops sharply by 2 to 5°C, and the wind direction reverses from southerly to northerly. Humidity also often rises sharply. When you animate AMeDAS time-series data, you can visually see the sea breeze front crawling like a living thing from Tokyo Bay toward the city center and further inland.

What excites me most in my research is predicting the behavior of this 'invisible front.' The penetration speed of the sea breeze varies from day to day. The higher the inland temperature and the weaker the surface wind, the deeper the sea breeze penetrates. Conversely, on days when the wind blowing down from inland is strong, the sea breeze is held back at the coast.

Deciphering this delicate balance is the core of local summer forecasting.

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A Microcosm of Kanto Plain Winds—The True Nature of the 'E-S Pattern'

The Opposing Structure Drawn by Two Sea Breezes

When talking about summer winds in Kanto, one cannot avoid the phenomenon called the 'E-S pattern.' While this has become established as a quasi-technical term in the meteorological research community, it is almost unknown to the general public.

The E-S pattern refers to a situation in the Kanto Plain, which spreads in an east-west direction, where two sea breezes blowing from different directions coexist and collide.

- E: North-easterly to easterly winds penetrating from the Kashima-nada and Pacific side. Notable from southern Ibaraki Prefecture to eastern Saitama.
- S: Southerly to south-easterly winds penetrating from Sagami Bay and Tokyo Bay. Moving north from the Kanagawa and Tokyo Bay coasts through the city center.

Because of the vastness of the Kanto Plain, an 'area where a cool wind blows from the east' and an 'area where a hot and humid wind blows from the south' coexist at the same time. And these two wind masses with different properties collide somewhere on the Kanto Plain—often in the 23 wards of Tokyo to southern Saitama.

The 'Invisible Battlefield' Called a Shear Line

In meteorology, this collision zone is called a 'shear line' or a 'convergence line.' It is a boundary where the wind direction changes suddenly, and at the same time, it is a place where the wind 'converges'—that is, where the blown-together air has nowhere to go.

The terror of a shear line lies in its invisibility. If there are no clouds, nothing can be seen from the ground. However, intense mechanical processes are underway inside the atmosphere.

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The 'Nest of Thunderclouds' Drawn by a Shear Line

Forced Ascent and the Explosion of Convection

When the wind from the east and the wind from the south collide head-on, the air that can no longer move horizontally is pushed vertically—that is, upward. This is 'forced ascent.'

This is where it gets terrifying for a forecaster. If the lapse rate (the rate at which temperature decreases with altitude) in the upper atmosphere is unstable, the air that has been lifted once will continue to rise on its own. This is called 'convective instability.'

The shear line of the E-S pattern functions extremely powerfully as a trigger for this convection. Even if there is no strong cold air in the upper atmosphere, if the convergence near the ground is strong enough, explosive cumulonimbus clouds will develop starting from that point. This is the true nature of the 'guerrilla rainstorms' that suddenly occur on clear afternoons.

The Difficulty of Forecasting—An Error of a Few Kilometers Can Be Fatal

'It will be hot in Tokyo today because the south wind is strong'—this is not correct as a weather forecast. To be precise, one should say, 'It will be hot and humid in areas where the south wind prevails, and relatively cool in areas where the east wind enters. And in the zone where the two collide, intense convection can occur.'

The problem is that the position of this shear line can change the result significantly with an error of just a few kilometers. If the shear line forms in southern Saitama, the city center is safe. However, if it forms over the city center, it will be hit by a sudden downpour.

Although the horizontal resolution of current numerical weather prediction models has improved to the kilometer scale, it is still difficult to perfectly predict the collision position of local winds. This is one of the biggest factors lowering the accuracy of forecasts on summer afternoons.

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Interaction with the 'Hot Wall' of the City

Heat Islands Distort the Wind

The discussion so far has been about local winds in a 'natural state,' so to speak. However, the metropolis of Tokyo makes this landscape even more complex.

The city affects the wind in two ways. First, high-rise building complexes act as physical barriers that block and slow down the wind. Second, artificial waste heat released from buildings and air conditioner outdoor units injects a huge amount of thermal energy into the atmosphere.

This is the heat island phenomenon. The temperature in the city center is 2 to 3°C, and in some cases more than 5°C, higher than in the surrounding suburbs. This 'island of hot air' functions as an invisible wall that blocks the intrusion of sea breezes.

'Wind Paths' Revealed by Latest Research

With recent high-resolution simulations and observational research, the interaction between urban structure and local winds is gradually being elucidated.

For example, it has been pointed out that the tower apartment complexes standing in the coastal area may delay the inland penetration of sea breezes. On the other hand, specific streets and areas along rivers function as 'wind corridors,' serving as routes that efficiently carry cool air inland.

The riverbeds of the Sumida River and Arakawa River are prime examples. Because the surface of the river is water, the temperature is lower than the surroundings, making it easier for sea breezes to penetrate. As a result, residents along the rivers feel that 'the wind blows through.'

This is the front line where meteorology and urban planning intersect. 'Urban design that is conscious of wind paths' may be the key to future sustainable cities.

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Increasing the Resolution of Looking at the Sky

The Landscape That Knowledge Changes

'Is it a south wind or a north wind today?'—many people perceive the wind in those two choices.

However, if you happen to look up at the sky on a summer evening next time, I would like you to imagine just a little bit. Above your head right now, the east wind blowing from Kashima-nada and the south wind crossing Tokyo Bay might be colliding fiercely at an invisible boundary line. Directly above that collision, moist air might be rising rapidly and eventually growing into a huge cumulonimbus cloud.

The 'atmospheric structure' and 'boundary lines' that are not drawn on weather maps but certainly exist. Once you develop the habit of looking at the sky while imagining them, even a just hot summer day will start to look like a showcase of magnificent physical phenomena.

When a forecast is wrong, it is not necessarily the forecaster's negligence. It is proof that the complex system called the atmosphere has slipped through the meshes of our predictions. Maintaining that humility, while at the same time continuing to challenge this complexity, is the essence of the job of a meteorologist.

The wind is invisible.
But it can be understood.
And for those who understand, the sky will never look the same again.

References and for those who want to learn more

- Japan Meteorological Agency: 'To Protect Yourself from Localized Heavy Rain'
- Meteorological Society of Japan: 'Urban Climate and Heat Island' Special Issue
- Tokyo Metropolitan Research Institute for Environmental Protection: 'Tokyo Wind Path Survey' Report