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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 ensemble control forecast values are available on ecCharts. If users have access to ensemble K-Index values (these are not currently available on ecCharts) they should refer to those alongside the 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 ensemble control forecast values are available on ecCharts.  If users have access to ensemble Total Totals Index values (these are not currently available on ecCharts) they should refer to those alongside the ensemble control values.

Examples

Fig9.6.2-1: 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 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 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 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 ensemble values may be quite different. If possible they should compare with those via local workstation tools.

(FUG Associated with Cy49r1)