Global Instability Index
The Global Instability Index (GII) product is one of the MPEF meteorological products and it describes the instability of the clear atmosphere by a number of airmass parameters. Although of highly empirical nature, the instability indices often indicate the potential for convection a few hours prior to the actual onset of convection.
For the 15 August 2010 case, the unstable airmass is equally well visible in the K-Index.
The Lifted Index is by definition driven by near surface heating and thus shows the expected diurnal cycle, while both the K-Index and the TPW show much less temporal changes.
48-hour loop of the GII K-Index.
48-hour loop of the GII Lifted Index.
48-hour loop of GII Total Precipitable Water.
SEVIRI Physical Retrieval
The PGE13 SPhR algorithm is responsible for the physical retrieval of the atmospheric temperature and humidity profiles. It calculates precipitable water parameters (Total and Layer Precipitable Water) and several thermal instability indices (LI, KI and SHOWALTER) for one clear sky SEVIRI pixel or a box containing M x M pixels. A detailed description on the SEVIRI Physical Retrieval products can be found at EUMeTrain or at NWCSAF.
Humidity Parameters
The most unstable regions are correlated to those with the strong south-easterly flow, which brings very humid and warm airmasses especially to the eastern parts of Europe. This can be seen especially in the water content in the lowest layers (BL). On August 14 and 15, the regions with very high values can be detected from the Ionian Sea to the Adriatic Sea, Hungary, Slovakia and to Poland. As a consequence, high values of MLCAPE are found in these regions and soundings like Budapest 12Z support this (previous chapter).
- TPW: Total Precipitable Water (surface - top) from the retrieved profiles of temperature and humidity in kg m-2.
Precipitable water in three layers from the retrieved profiles of temperature and humidity in kg m-2.
Dry air reaches parts of central Europe in the higher levels of the atmosphere. One can detect this by watching the precipitable water content (brownish colour) in the higher levels (HL) during August 15. This causes a general instability.
Thermal Instability Indices
Very low values of the Lifted Index (LI) indicating instability are found in the eastern parts of Europe. The most unstable airmasses with high values of CAPE are situated in these regions, too. Lower values also occur in the Czech Republic in the afternoon of August 15.
- LI: Lifted Index from the retrieved profiles of temperature and humidity in Kelvin.
- SHW: Showalter Index from the retrieved profiles of temperature and humidity in Kelvin.
- KI: K-Index from the retrieved profiles of temperature and humidity in Kelvin.
Differences
Differences between the parameters obtained with the retrieved profile and the same parameters obtained with the NWP model profile.
- DIFF_TPW: Difference between TPW from the retrieved profile and TPW from NWP profiles in kg m-2
- DIFF_BL, DIFF_ML, DIFF_HL: Difference between LPWs (Layer Precipitable Water) from the retrieved profile and LPWs from NWP profiles in kg m-2
- DIFF_LI, DIFF_KI, DIFF_SHW: Difference between instability indices from the retrieved profile and instability indices from NWP profiles in Kelvin.
