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Although simply constructed, the indices do have a part to play in assessment of any potential convection, particularly when used in conjunction with other meteorological information (e.g. low-altitude convergence, bulk shear, atmospheric cooling at higher altitudes etc.)  Users may have become accustomed to using an appropriate preferred instability index for their region of interest, but for a more detailed and comprehensive assessment users are encouraged to use these alongside forecast values of CAPE (Convective Available Potential Energy) and CAPE-shear, and CIN (Convective Inhibition). All such parameters are available in ecCharts.

Whilst CAPE may be the preferred variable for many, note that using CAPE alone does not always bring the best results.

K-index (K)

The formula is:

K-Index (K) = (T₈₅₀ - T₅₀₀) + Td₈₅₀ - (T₇₀₀ - Td₇₀₀)

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Charts of K-Index derived from HRES ensemble control forecast values are available on ecCharts. If users have access to ENS ensemble K-Index values (these are not currently available on ecCharts) they should refer to those alongside the HRES ensemble control values.

Total Totals Index (TT)

The formula is:

TT = (T₈₅₀ – T₅₀₀) + (Td₈₅₀ – T₅₀₀)  =   T₈₅₀ + Td₈₅₀ – 2(T₅₀₀)

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Charts of Total Totals Index derived from HRES ensemble control forecast values are available on ecCharts.  If users have access to ENS ensemble Total Totals Index values (these are not currently available on ecCharts) they should refer to those alongside the HRES ensemble control values.

Examples

Fig9.6.72-1: Composite diagram showing ecChart presentation of the vertical profile product, and of CIN, CAPE, K-index, and Total totals index from ensemble control/HRES, all valid for T+60hr valid 12UTC 28 July 2018, forecast base time 00UTC 26 July 2018.

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The values of the parameters and indices at Freiburg (location denoted by the green pin) are shown in the probe box and the vertical profile product for the same location is shown on the left.  Both the K-Index and the Total Totals Index attain high values in Freiburg. Meanwhile the vertical profiles show that ensemble control/HRES Lifting Condensation Level (LCL) and Level of Free Convection (LFC) are at fairly low altitude and close together, potentially facilitating energetic convection to occur (reflected by the low CIN and high CAPE values shown on the charts). However many ENS ensemble members suggest a LCL at a greater altitude with release of less energetic convection. This is highlighted by the CAPE chart on the vertical profile plot (lower right panel), where 8 ENS ensemble members suggest moderate energy is required to overcome CIN but these only have moderate CAPE for subsequent convection. Meanwhile 23 ENS ensemble members (and the Control member) suggest a good deal of energy is needed to overcome CIN, and 18 ENS ensemble members suggest no propensity for convection (CAPE is zero). Only ensemble control/HRES and one ENS ensemble member suggest deep and active convection can be released. Instability indices on ecCharts are derived from ensemble control/HRES forecast values and the user must always bear in mind that ENS ensemble values may be quite different. If possible they should compare with those via local workstation tools.

Fig9.6.82-2: Composite diagram showing ecChart presentations of the vertical profile product, and CIN, CAPE, K-index and Total totals index from ensemble control/HRES, all valid for T+60hr valid 12UTC 28 July 2018, forecast base time 00UTC 26 July 2018. 

• CIN values >1000J/kg are uncoloured; <10J/kg shown as red.
• CAPE values <100J/kg are uncoloured; >3000J/kg shown as blue.
• K-index values <10°C are uncoloured; >35°C shown as magenta.
• Total totals index values <40°C are uncoloured; >50°C shown as purple.

Updated/Amended 02/07/20 - Removed link to issue with CIN. Not applicable with 47R1The values of the parameters and indices at Paris (location shown by the pin) are shown in the probe box and the vertical profile product for the same location is shown on the left. The vertical profile shows very dry air at 700hPa with a low dewpoint (about -35°C) which has a direct influence on evaluation of the K-index (ensemble control/HRES suggests -4.3°C!).  Some ENS ensemble members, however, show air at 700hPa being moister (dewpoint as high as -5°C) and would therefore produce a higher K-index. Therefore the ensemble control/HRES value may well be extreme. A glance at the CAPE diagram suggests about a third of the ENS ensemble members could permit instability release, albeit weak. In contrast to the ensemble control/HRES K-index, the ensemble control/HRES Total totals index (43.3°C, just within the yellow area) suggests that the thunderstorm risk is borderline. Instability indices on ecCharts are derived from ensemble control/HRES forecast values and the user must always bear in mind that ENS ensemble values may be quite different. If possible they should compare with those via local workstation tools.

_{Updated/Amended 30/12/19 - Link to issue with CIN}

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