Weather conditions of mesoscale system

Sketch of a mature squall line system

The horizontal scale of the squall line is 150 ~ 300 km, and its service life is 4 ~ 10 hour. In the squall line mesoscale system, the pressure gradient can reach (1 ~ 3) hPa/100 km, and the horizontal gradient is obvious, which is 0.5℃/ km. When the squall line passes, the air pressure rises 1 hectopascals /( 1 ~ 2). The favorable environmental conditions for the squall line are: unstable atmospheric stratification and abundant water vapor in the lower layer; There is dry cold air intrusion in the middle layer; There are jet streams in the upper and lower layers, which are properly coupled, and there is some dynamic mechanism in the atmosphere to release unstable energy (such as low-level jet streams and gravity waves). ).

(2) Mesoscale rain cluster

In a large-scale heavy rainfall area, there may be a mesoscale rain belt (commonly known as mesoscale rain belt), and the mesoscale rain belt contains a mesoscale rain mass (commonly known as mesoscale rain mass). In a rainstorm, there may be two or more mesoscale rain belts and multiple mesoscale rain masses, which are important members of rainstorm weather. Mesoscale rain cluster has the following basic characteristics: ① the horizontal scale is small, usually less than 200 kilometers; ② Short service life, generally within 10 hour; ③ Strong convergence at low altitude and horizontal divergence in the lower troposphere; (4) Multiple occurrences, and multiple mesoscale rain clusters can appear in one rainstorm; ⑤ The precipitation intensity is high, and the precipitation of 1 hour can reach more than 50 mm; ⑥ Two kinds of dynamics: mobility and quasi-stationarity. Most mesoscale rain masses are moving.

(3) Mesoscale rain belt

Distribution map of mesoscale rain belt of temperate cyclone

(Note: 1. Warm front rain belt; 2. Rain belts in warm areas; 3. Wide cold front rain belt; 4. Narrow cold front rain belt; 5. Front cold wave; 6. The rain belt is in front and behind)

Mesoscale rain belt is an important part of precipitation in mid-latitude area. In most cases, the mesoscale rain belt consists of thunderstorm cell and small-scale system, and its weather feature is rainstorm or strong convection, and its length is about 100~200 km. To sum up, mesoscale rain has the following basic characteristics:

① Several mesoscale rain belts often coexist, parallel to each other, with roughly the same spacing, about 100 km. The width of each rain belt is about 10 ~ 50km. Sometimes there is only one mesoscale rain belt;

② Mesoscale rain belt is composed of smaller convective cells at different development stages. In many cases, the monomer in the north is mature or aging, and the monomer in the south is new or young;

③ The moving direction of mesoscale rain belt is obviously biased to the right of the average wind direction, and the moving speed can be greater than the average layer wind speed;

(4) Mesoscale rain belts mostly appear in areas with unstable atmospheric potential;

⑤ When the mesoscale rain belt passes through the station, the meteorological elements on the ground will change obviously, such as the sudden increase of air pressure, the decrease of temperature, the increase of humidity, the sudden change of wind direction and speed, and sometimes there will be thunderstorms.

There are many kinds of mesoscale rain belts, including frontal cyclone zone, meiyu zone and typhoon zone, among which the rain belt in frontal cyclone zone is more common and important.