NCEP数据资料获取 地面抬升指数

先放上数据地址：https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.surface.html

美国国家环境预报中心（NCEP）和国家大气研究中心（NCAR）联合执行的全球大气40年资料再分析计划通过CDC（Climate Dianogistic Center）利用磁带的形式向外发行。

地面抬升指数

抬升指数是一种表示大气对流性不稳定的指数，指一个气块从地面出发，上升到500百帕（海拔5500米左右高度）处所具有的温度被该处实际大气温度所减得到的差值。

The lifted index (LI) is the temperature difference between the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa (mb). The temperature is measured in Celsius. When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.

LI can be computed using computer algorithms but can also be determined graphically. To do this, generally, the parcel is lifted from the portion of the planetary boundary layer (PBL) that lies below the morning inversion. The air here should be about 60 to 65% RH, which is then lifted along the dry adiabat (see also adiabatic process) to the lifting condensation level (LCL), which is the intersection of that curve with the average mixing ratio in the boundary layer. Once the LCL is found, the parcel is lifted along the moist adiabat to 500 mb. It is then that one finds LI = Te(p) - Tp(p).

LI is generally scaled as follows:

• LI 6 or Greater, Very Stable Conditions
• LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
• LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, ...)
• LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
• LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism

The lifted index can be used in thunderstorm forecasting, however, convective available potential energy (CAPE) is considered by most as a superior measurement of instability and is preferred by many meteorologists for convection forecasting.^[1] However, LI is easier and faster to determine without using a computer, as determining CAPE requires integration from one level to another.

Reanalysis output includes two lifted indices. the surface lifted index (LFTX) and the best (4-layer) lifted index (4LFTX).

The surface lifted index is computed by finding the temperature of an air parcel lifted from the lowest layer to 500 mb and subtracting
the ambient 500 mb temperature.

The best lifted index is computed by lifting the parcel from the layer with the warmest equivalent potential temperature to 500 mb
and subtracting the ambient 500 mb temperature.
最暖等效位温

For computation, the vertical profile was converted to six layers of depth of 30 mb starting from the surface.

Stability of the Atmosphere

Lifted Index (LI)

The LI is a commonly utilized measure of stability which measures the difference between a lifted parcel's temperature at 500 mb and the environmental temperature at 500 mb. It incorporates moisture and lapse rate (static stability) into one number, which is less vulnerable to observations at individual pressure levels. However, LI values do depend on the level from which a parcel is lifted, and rally cannot account for details in th environmental temperature curve above the LCL and below 500 mb. LI was originally intended to utilize average moisture and temperature properties within the planetary boundary layer.

     LI  = T(500 mb envir) - T(500 mb parcel)

in degrees C, where T (500 mb envir) represents the 500 mb environmental temperature and T (500 mb parcel) is the rising air parcel's 500 mb temperature.

LI over 0:	Stable but weak convection possible for LI = 1-3 if strong lifting is present.
LI = 0 to -3:	Marginally unstable.
LI = -3 to -6:	Moderately unstable.
LI = -6 to -9:	Very unstable.
LI below -9:	Extremely unstable.

These LI values are based on lifted parcels using the average lowest 50 to 100 mb moisture and temperature values (i.e., the boundary layer). Variations exist on how LI values are calculated, as discussed below.

Surfaced-based LI: Surface-based LIs can be calculated hourly, and assume a parcel is lifted from the surface using surface-based moisture and temperature values, as well as assigned environmental temperatures at 500 mb. This method is valid for a well-mixed nearly dry adiabatic afternoon boundary layer where surface characteristics are similar to those in the lowest 50 to 100 mb layer. However, these values would not be representative of the ambient elevated instability if a nocturnal inversion or shallow cool air to the north of a frontal boundary is present. In these cases, more instability resides above the surface, and parcels may be lifted to form thunderstorms from the top of the inversion.

Best LI: The Best LI represents the lowest (most unstable) LI computed from a series of levels from the surface to about 850 mb. This index is most useful during cases when shallow cool air exists north of a frontal boundary resulting in surface conditions and boundary layer-based LI values that are relatively stable. However, the airmass at the top of the inversion, from which lifting may occur, is potentially unstable. An example of this would be elevated ("overrunning") convection (possibly a nocturnal MCS).