Explanation of various parameters of the severe convection potential forecast system
Description of various parameters of the severe convection potential forecast system
(1) Sabouraud index SI
An index that reflects the stability of the atmosphere. It is defined as the difference between the air mass temperature Ts850 when the moist air mass on the 850hPa isobaric surface rises along the dry adiabatic line, reaches the condensation height and then rises along the moist adiabatic line to 500hPa, and the ambient temperature T500 on the 500hPa isobaric surface. . When SI<0, the atmospheric stratification is unstable, and the larger the negative value, the greater the degree of instability. On the contrary, it means that the gas layer is stable.
SI= T500- Ts850
According to foreign data, SI has the following relationship with convective weather:
SI> Possibility of thunderstorms at -3°C Little or no chance of showers at 0°C< SI<3°C;
Chance of thunderstorm at -3°C< SI<0°C
-6°C< SI<-3°C, there is the possibility of severe thunderstorms;
SI<-6°C, there is the possibility of severe convective weather (such as tornadoes) The danger;
(2) Lift index LI
The air parcel has the characteristics when it rises along the dry adiabatic line from a height of 900m at the lower level, reaches the condensation height, and then rises along the wet adiabatic line to 500hPa The difference between the temperature Ts and the ambient temperature T500 on the 500hPa isobaric surface. When LI<0, the atmospheric stratification is unstable, and the larger the negative value, the greater the degree of instability. On the contrary, it means that the gas layer is stable.
LI=T500-Ts
(3) Beneficial lifting index BLI
The atmosphere below 700hPa is layered at 50hPa intervals, and the middle height of each layer is Each point at the point is raised to its own condensation height according to the dry adiabatic line, and then raised to 500hPa according to the wet adiabatic line, and different lifting indexes are obtained at each point. The one with the largest negative value is the most favorable lifting index. When BLI<0, the atmospheric stratification is unstable, and the greater the negative value, the greater the degree of instability.
(4) ** K index **
The definition of K index is:
K=（T850-T500）+Td850-（T-Td ) 700
Among them, T and Td represent the temperature and dew point temperature respectively; 500, 700 and 850 in the table below represent 500, 700 and 850hPa respectively.
The first term in the K index calculation formula represents the temperature lapse rate, the second term represents the low-level water vapor conditions, and the third term represents the saturation degree of the middle layer. Therefore, the K index can reflect the stratification stability of the atmosphere. The larger the K index, the more unstable the stratification. The statistical results are: K＜20, no thunderstorm; 20＜K＜25, isolated thunderstorm; 25＜K＜30, sporadic thunderstorm; 30＜K＜35, dispersed thunderstorm; K＞35, widespread thunderstorm. .
(5) Modified K index MK
Mk=0.5 (TT850)+0.5 (TdTd850)-T500-(T-Td)700
Refers to the modified K-index that takes into account ground temperature conditions. Here T0 represents the ground temperature. The larger the mK value, the warmer and wetter the lower air mass, and the smaller the stability, which is more conducive to the generation of convection.
(6) The total index TT
is defined as: TT= T85Td850-2T500
The subscripts 850 and 500 represent 850hPa and 500hPa respectively. The larger the TT, the easier it is for convective weather to occur.
(7) Severe weather threat index SWEAT
SWEAT=12Td8520 (TT-49) +2f85f50125 (S+0.2)
Td850 represents the 850hPa dew point temperature (°C). If Td850 is a negative number, this item is 0;
TT= T85Td850-2T500, which is the total index. If TT is less than 49, then 20 (TT-49) items is 0; f850 is the wind speed of 850hPa (nautical miles/hour), the wind speed in m/s should be multiplied by 2; f850 is the wind speed of 500hPa (nautical miles/hour), the wind speed in m/s should be multiplied by 2; , and represent the 500hPa wind direction and the 850hPa wind direction respectively; the last term 125 (S+0.2) is zero when any of the following four conditions is not met: the 850hPa wind direction is between 130° and 250°; the 500hPa wind direction is between 210° and Between 310°; the 500hPa wind direction minus the 850hPa wind direction is positive; the 850hPa and 500hPa wind speeds are at least equal to 15 knots/hour (7.5m/s).
Commonly used in tornado forecasting. According to the analysis of tornadoes and severe thunderstorms in the United States, the relationship between the SWEAT indicator value and the weather is: the critical value of SWEAT when a tornado occurs is 400, and the critical value of SWEAT when a severe thunderstorm occurs. for 300. Severe thunderstorms mainly refer to thunderstorm weather accompanied by strong winds with a wind speed of at least 25 m·s or more, or hail with a diameter of more than 1.9cm.
(8) Deep convection index DCI
Diagnostic deep convection index: Deep convection refers to an extension height equal to or greater than the homogeneous atmospheric height H0 (closer to the 400hPa isobaric surface height ) convection system. The deep convection index calculated using the equivalent blackbody brightness temperature of the cloud top can be used as an index indicating that the cloud top is equal to or higher than 400hPa deep convective clouds.
Forecasting uses the deep convection index DCI
DCI=T85Td850-LI
LI lift index. Almost all severe local storm events are related to deep convection related. The deep convection index combines the temperature at the 850hPa layer with the buoyancy characteristics from the surface to 500hPa to estimate the potential for deep convection to occur. In places where the index is very high, if there is also a triggering mechanism for lifting air parcels, severe convective weather events are likely to occur.
(9) Convection effective potential energy CAPE
or
where ZLFC is the free convection height, which is the conversion of (TVP-TVE) from negative to positive. Height; ZEL is the equilibrium height, which is the height at which (TVP-TVE) changes from positive to negative.
Its physical meaning means: when the gravity and buoyancy of the air mass are not equal and the buoyancy is greater than gravity, part of the potential energy can be released, because this part of the energy has a positive effect on atmospheric convection and can be converted into atmospheric Kinetic energy is called the convective effective potential energy. Represents the energy that an air parcel can obtain from work performed by positive buoyancy above the height of free convection. The usually calculated CAPE corresponds to the energy corresponding to the positive area on the Emma diagram.
(10) Best convective effective potential energy BCAPE
In the bottom 200hPa level, find the highest value of the false equivalent potential temperature and calculate it by lifting the air parcel there CAPE.
(11) Lower multiplied convection effective potential energy DCAPE
Among them, it represents the density temperature, the subscripts e and p are the surrounding environment and the air block, and Pi represents the air pressure at the initial sinking point of the air block. , Pn represents the air pressure when the air parcel reaches the neutral buoyancy layer or the ground, and r represents the water vapor mixing ratio.
Physical meaning: In the storm body, when liquid water in the unsaturated air evaporates or solid water melts below the freezing layer, the effective potential energy of sinking convection will be generated.
(12) Storm Severity Index SSI
SSI=100 [2+(0.276 In(Shr))+(2.011 10-4 CAPE)]
It is composed of the average wind shear and buoyancy energy from 0 to 3600m, reflecting the comprehensive effect of vertical wind shear and convective effective potential energy. In Australia, SSI >= 120 is determined to be a severe thunderstorm.
(13) Rough Richardson number BRN
In actual calculations, U and V are often taken as the density-weighted wind of 0~6KM and the average of the near-surface layer of 0~500M The two components of the wind difference (or wind speed difference) value between the winds. That is:
Strong convective weather can occur in environments where weak vertical wind shear is combined with strong geopotential instability or the opposite. This index is a combination of convective effective potential energy and vertical wind shear in the middle and lower troposphere, which can reflect the balance between vertical wind shear and geopotential instability when strong convection occurs. Some analysts believe that medium-intensity supercells often occur when 5 ≤ BRN ≤ 50, and multi-cell storms generally occur when BRN > 35.
(14) ** Relative helicity RSH **
Within a few kilometers of the lower troposphere, the wind direction relative to the storm rotates counterclockwise with height, which is a key factor in the development of storm rotation. . Relative helicity is introduced to quantitatively estimate the combined effect of the horizontal vorticity along the storm inflow direction and the intensity of the inflow on the storm rotation. The test results show that for weak tornadoes, medium-strength tornadoes and strong tornadoes, the helicity sizes are 150-299, 300-499 and greater than 450 respectively. When h>150, strong convection is highly likely to occur.
(15) Energy helicity index EHI
EHI=(Hs-r*CAPE)/160000
Among them, CAPE represents the convective effective potential energy, Hs -r represents the relative helicity of the storm at low altitude 0 to 2km.
Severe convective weather can occur in an environment with low helicity (Hs-r<150m2 s-2) combined with high convective effective potential energy (CAPE>2500 Jkg-1), or in the opposite environment environment (Hs-r >300 m2 s-2 combined with CAPE>1000Jkg-1).
The convective effective potential energy and helicity are combined to form the energy helicity index, which reflects the mutual balance characteristics between the convective effective potential energy and helicity when strong convective weather occurs. Research shows that when EHI>2, it indicates that strong convection is highly likely to occur. The larger the EHI value, the greater the potential intensity of severe 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 air parcel respectively, Tv represents the virtual temperature, Tve and Tvp represent the virtual temperature of the environment and the air block respectively, and Zi (or Pi) represents the initial lifting height (or air pressure) of the air block. The convection suppression index refers to the work done by a uniform boundary layer air parcel from the stable layer to the free convection height during its ascent. The size of the work is bounded by the state curve and the stratification curve from the starting position of the air parcel to the free convection height. is proportional to the area (negative area). For situations where strong convection occurs, CIN often has a more appropriate value: if it is too large, the degree of convection suppression will be great and convection will not easily occur; if it is too small, energy will not easily accumulate at low levels and convection adjustment will easily occur, thus preventing convection from developing to a higher level. Strong degree.
(17) 0℃****layer height ZHT
The height of 0℃ temperature is closely related to the occurrence of hail. Studies have pointed out that 90% of hail occurs at 0℃ When the layer height is 1524~3658m from the ground; when the 0℃ layer height is 2134~3353m from the ground, large hailstones are most likely to occur.
https://www.weather.gov/source/zhu/ZHU_Training_Page/convective_parameters/skewt/skewtinfo.html