Meteorologically, what is the formation mechanism of the westerly trough and the leeward trough

Westerly trough

In the middle and high latitudes on the north side of the subtropical high in the Northern Hemisphere, a westerly airflow prevails at an altitude of more than 3 kilometers (700 hpa), which is called the westerly wind belt. Fluctuations are often generated in the westerly air current, forming troughs (low pressure) and ridges (high pressure). The trough line in the westerly wind belt is called the westerly trough.

The westerly trough is mostly northeast-southwest trending, and the east side of the westerly trough (in front of the trough) prevails with a warm and humid southwest airflow, and the weather is mostly cloudy and rainy. A dry and cold northwest downdraft prevails on the west side of the westerly trough (behind the trough), and the corresponding ground is the place where the cold and high pressure is active, and the weather is fine. The westerly troughs that affect the weather in China mainly include the northwest trough and the Qinghai-Tibet trough that affect the northern region from the northwest over the Qinghai-Tibet Plateau, and the southern branch trough (Indo-Burma trough) that moves eastward from India and Myanmar and affects the southern region.

The westerly trough is frequently active, and each activity will bring a certain intensity of cold air invasion, resulting in a wide range of precipitation or rainy weather.

When a trough line appears in the upper altitude of the westerly wind belt, it is a westerly trough. The westerly trough is cold and baroclinic, and there is a strong positive vortex in the trough, and there is often the development of an extratropical cyclone in front of the trough, and an anticyclone is formed behind the trough. The westerly trough will separate the north branch trough and the south branch trough when passing through the Qinghai-Tibet Plateau, and if the south branch trough moves eastward to leave the plateau, it will form a low pressure area near Yunnan, which is the southwest vortex, and the southwest vortex usually extends out of the low pressure trough to bring rainy weather to South China.

The westerly trough also caused the subtropical high to retreat eastward, causing the tropical air to rotate and move northeastwards.

Leeward trough. It is a type of low-pressure trough formed on the leeward slope or downwind side of a mountain range that acts as an almost vertical obstacle to the wind. Also known as the power trough.

The westerly airflow prevails in the upper air, and the east side of the westerly trough (in front of the trough) prevails with a warm and humid southwesterly airflow, and the weather is mostly cloudy and rainy. A dry and cold northwest downdraft prevails on the west side of the westerly trough (behind the trough), and the corresponding ground is the place where the cold and high pressure is active, and the weather is fine.

High mountains, mountains block the formation of troughs of low pressure.

Let me be specific about the leeward trough: the leeward trough occurs when the airflow passes from west to east over the mountain range, and is formed due to the conservation of geopotential vorticity ( +f ) h=constant. Here refers to the vorticity in the vertical direction, f refers to the component of the vorticity caused by the rotation of the earth in the local vertical direction, and h refers to the height of the air column.

For details, please refer to the chapters of the atmospheric dynamics book that deal with geopotential vorticity.

A westerly trough is a phenomenon of air convection.

The westerly trough is generally northeast to southwest, and the formation of the westerly trough is roughly as follows: the warm and humid airflow in the southwest direction moves upward, and the cold and dry airflow in the northeast direction sinks downward, and a pressure difference is formed when it rises and falls, and the side with high air pressure is the ridge and the air pressure is low.

On one side is the slot. The direction of this pressure is to the southwest, so it is called the "westerly trough". Because the temperature in the northeastern part of the westerly trough is low.

High, the temperature in the southwest part is high and the pressure is low, which creates three unique climatic phenomena in the westerly trough, which will be explained in detail below.

1. The northeast part of the westerly trough has sunny weather, and the southwest part has more rainy weatherBecause the southwestern part of the trough is warm and moist, as the trough moves southwestward, the water vapor contained in it will condense into nuclei with the movement, resulting in rain or snowfall. The northeastern part of the westerly trough is cold and dry, so the weather is generally sunny and there is no rain or snowfall.

Take this example as an example, Inner Mongolia, North China, Northeast China.

The snowfall is a phenomenon caused by the condensation of water vapor during the Nasheng movement of the westerly trough.

2. The west wind trough is divided into long wave trough and short wave troughAmong them, the shortwave trough moves faster, and the long wave trough moves slowly, because the short wave trough moves fast, so the weather changes more drastically, and there are often showers or heavy snow, and the climate change of the long wave trough is relatively slow, the long wave trough moves slowly, the weather changes not drastically, and even sometimes there is no rain or snowfall, but the temperature changes a little and the bridge is gone. The westerly trough in this example is a longwave trough.

Tibetan PlateauThere is also this phenomenon, which is called the Northwest Trough:The formation principle of the northwest trough is the same as that of the westerly trough, and the essence is air convection. The northwest trough is different from the westerly trough, mainly because the affected areas are different.

The area affected by the northwest trough is Shaanxi, Gansu and northern Sichuan. The above introduces the westerly trough from all aspects, and also lists the northwest trough as a comparison, the westerly trough is just a normal climatic phenomenon, just take precautions.

The Zephyr Trough is a weather system that swims silver. The tremor is affected by the northwest wind, and the passage of the cold wind will cause snowfall and bend in many places in northern China. The snow is followed by a cooler temperature.

The westerly trough is actually cold air, and under the influence of cold air, snow will begin to fall in most areas that pass through.

The westerly trough refers to a phenomenon when the trough line appears at the high altitude of the westerly wind belt. It is an anticyclone formed by cold and baroclinic pressure.

A westerly trough is a type of weather that comes from the west and carries a cold snap that causes the temperature to drop.