Parameter description of strong convective potential prediction system

Parameter description of strong convective potential prediction system

(1) Charcot index SI

An index reflecting atmospheric stability. It is defined as the difference between the air mass temperature Ts850 on the isobaric surface of 850 hectopascals and the ambient temperature T500 on the isobaric surface of 500 hectopascals when the air mass rises along the dry adiabatic line and reaches the condensation height, and then rises to 500 hectopascals along the wet adiabatic line. When si

SI= T500- Ts850

According to foreign data, SI has the following relationship with convective weather:

When Si >-3°C, there is little or no possibility of thunderstorm.

0℃ & ltSI & lt3℃ may have showers;

-3c & lt； Si-LT0℃ may have thunderstorm;

-6℃ & ltSI & lt-3℃ has the possibility of strong thunderstorm;

SI & lt-6℃ is in danger of strong convective weather (such as tornado);

(2) uplift index li

When the air mass rises from the height of 900 meters in the lower layer along the dry insulation line to reach the condensation height, and then rises to 500 hectopascals along the wet insulation line, the difference between the temperature Ts on the isobaric surface of 500 hectopascals and the ambient temperature T500. Dangli

LI=T500-Ts

(3) favorable uplift index BLI

The atmosphere below 700 hectopascals is layered at intervals of 500 hectopascals, and each point in the middle height of each layer is lifted to its condensation height according to the dry heat insulation line, and then to 500 hectopascals according to the wet heat insulation line, so that different lifting indexes of each point are obtained, and the most favorable lifting index is the one with the largest negative value. BLI<0, atmospheric stratification is unstable, and the greater the negative value, the greater the degree of instability.

(4)** K index * *

The k index is defined as:

K=(T850-T500)+Td850-(T-Td)700

Where t and Td represent temperature and dew point temperature, respectively; The following tables 500, 700 and 850 represent 500, 700 and 850 hectopascals respectively.

The first term in the K index formula indicates the direct cooling rate, the second term indicates the low-level water vapor condition, and the third term indicates the middle-level saturation. Therefore, the K index can reflect the stratification stability of the atmosphere. The greater the k index, the more unstable the stratification is. The statistical results show that there is no thunderstorm when K < 20. Isolated thunderstorm with 20 < k < 25; 25 < k < 30 sporadic thunderstorm; 30 < k < 35 scattered thunderstorm; K > 35 thunderstorms.

(5) Modified K index MK

MK = 0.5(T0+T850)+0.5(Td0+Td 850)-T500-(T-Td)700

Refers to the improved K index considering the geothermal conditions. Here, T0 represents the ground temperature. The greater the mK value, the warmer and wetter the lower air mass, and the smaller the stability, so it is more conducive to convection.

(6)TT total index

Defined as TT = T850+TD 850-2T500.

Subscripts 850 and 500 represent 850 hectopascals and 500 hectopascals respectively. The bigger TT, the more prone to convective weather.

(7) Severe Weather Threat Index Sweat

Sweat =12td850+20 (TT-49)+2f850+f500+125 (s+0.2)

Td850 represents the dew point temperature (C) of 850hPa; If Td850 is negative, this item is 0;

TT = T850+TD 850-2T500, that is, the total index. If TT is less than 49, item 20 (TT-49) is 0; F850 is the wind speed of 850hPa (nautical miles per hour), and the wind speed in meters per second should be multiplied by 2; F850 is the wind speed of 500hPa (nautical miles per hour), and the wind speed in meters per second should be multiplied by 2; , representing 500 hectopascal wind direction and 850 hectopascal wind direction respectively; The last term 125(S+0.2) is zero when any of the following four conditions are not met: the wind direction at 850hPa is between130 and 250; 500 hPa wind direction is between 2 10 and 3 10; 500 HPA wind direction minus 850 HPA wind direction is positive; Wind speeds of 850 hectopascals and 500 hectopascals are at least equal to 15 knots (7.5 m/s).

It is often used to predict tornadoes. According to the analysis of tornadoes and severe thunderstorms in the United States, the relationship between sweat index and weather is that the critical value of sweat is 400 when tornadoes occur and 300 when severe thunderstorms occur. Severe thunderstorms mainly refer to thunderstorm weather accompanied by strong winds with a wind speed of at least 25 meters per second or hail with a diameter of 1.9 cm.

(8) Deep convection index DCI

Diagnostic deep convection index: Deep convection refers to a convection system whose extension height is equal to or greater than the uniform atmospheric height H0 (closer to the isobaric surface height of 400hPa). The deep convection index calculated by using the brightness temperature of the black body at the cloud top can be used as an indicator that the cloud top is equal to or higher than 400 hectopascals.

Deep convection index DCI for prediction

DCI = T850+TD 850- Li

Plum uplift index. Almost all strong local storm events are related to deep convection. The deep convection index combines the temperature of 850hPa layer with the buoyancy characteristics from the ground to 500hPa, and estimates the occurrence of deep convection potential. Where the index is very high, if there is also a trigger mechanism to lift the air mass, strong convective weather events may occur.

(9) Convective effective potential angle

or

Where ZLFC is the height of free convection, that is, the height of (TVP-TVE) from negative to positive; ZEL is the equilibrium height, that is, the height at which (TVP-TVE) changes from a positive value to a negative value.

Its physical meaning means that when the gravity and buoyancy of a gas block are not equal and the buoyancy is greater than gravity, part of potential energy can be released, because this part of energy has a positive effect on atmospheric convection and can be converted into atmospheric kinetic energy, which is called convective effective potential energy. It represents the energy that an air block can obtain by doing work with positive buoyancy above the free convection height. CAPE usually calculated corresponds to the energy corresponding to the positive area on Emma diagram.

(10) optimal convective effective potential BCAPE

At the lowest layer of 200 hectopascals, find the highest value of false equivalent potential temperature, lift the gas block there and calculate CAPE.

(1 1) times the convective effective potential energy DCAPE.

Among them, subscripts e and p indicate density temperature, surrounding environment and air mass, Pi indicates the air pressure when the air mass begins to sink, Pn indicates the air pressure when the air mass reaches the neutral buoyancy layer or the ground, and r indicates the water vapor mixing ratio.

Physical significance: In a storm, when liquid water evaporates in unsaturated air or solid water melts below the frozen layer, it will produce the effective potential energy of sinking convection.

(12) storm intensity index SSI

SSI= 100 [2+(0.276 inch (SHR))+(2.0110-4 Cape)]

It is composed of average wind shear and buoyancy energy from 0 to 3600 m, which reflects the comprehensive effect of vertical wind shear and convective effective potential energy. In Australia, SSI>= 120 was judged as a severe thunderstorm.

(13) rough Richardson number BRN

In practical calculation, U and V are often taken as two components of the wind difference (or wind speed difference) between the density-weighted wind of 0 ~ 6 km and the average wind of 0 ~ 500 m near the surface. Namely:

Strong convective weather can occur in an environment where weak vertical wind shear is combined with strong potential instability, or vice versa. The index consists of convective effective potential energy and vertical wind shear in the middle and lower troposphere, which can reflect the balance relationship between vertical wind shear and potential instability when strong convection occurs. Some analysts believe that medium-intensity supercells often occur when the number is 5≤BRN≤50, and multi-cell storms usually occur at BRN-GT35.

(14)** Relative helicity RSH**

Within a few kilometers of the lower troposphere, the wind direction relative to the storm rotates with height, which is the key factor for the development of storm rotation. Relative helicity is introduced to quantitatively estimate the horizontal vorticity along the inflow direction of the storm and the comprehensive influence of inflow intensity on the rotation of the storm. The experimental results show that for weak tornadoes, moderate tornadoes and strong tornadoes, the helicity is 150 ~ 299, 300 ~ 499 and greater than 450 respectively. When h> 150, there is a great possibility of strong convection.

(15)EHI energy spiral index

EHI=(Hs-r*CAPE)/ 160000

CAPE represents the effective potential energy of convection, and Hs-r represents the relative helicity of 0 ~ 2 km low-altitude storm.

Severe convective weather can occur in low helicity (HS-R: 2500 Jkg- 1) or in the opposite environment (HS-R >；; 300 square meters of s-2 combined with CAPE & gt 1000Jkg- 1). The energy helicity index is composed of convective effective potential energy and helicity, which reflects the balance between convective effective potential energy and helicity when severe convective weather occurs. Research shows that when EHI>；; The 2 o'clock direction indicates that there is a great possibility of strong convection. The greater the EHI value, the greater the potential intensity of strong convective weather.

(16) convection inhibition index CIN

Where Tb is the average temperature of the layer, Te and Tp represent the temperatures of the environment and the gas block respectively, Tv represents the virtual temperature, Tve and Tvp represent the virtual temperatures of the environment and the gas block respectively, and Zi (or Pi) represents the initial lifting height (or air pressure) of the gas block. Convection suppression index refers to the work done by the gas block in the uniform boundary layer from the stable layer to the free convection height, which is proportional to the area (negative area) surrounded by the state curve from the initial position of the gas block to the free convection height and the stratification curve. For the case of strong convection, CIN often has a suitable value: too large, which inhibits convection to a great extent and makes convection difficult to occur; Too small, energy is not easy to accumulate in the lower layer, and convection adjustment is easy to occur, so that convection cannot develop to a strong degree.

(17) 0℃ * * * height ZHT

The height of 0℃ temperature is closely related to the occurrence of hail. It has been pointed out that when the height of 0℃ layer is above the ground 1524 ~ 3658 m, 90% of hail falls. When the height of 0℃ layer is 2 134 ~ 3353 m above the ground, large hailstones are most likely to appear.

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