Pre-convective environment

From satellite data one can retrieve information on instability and moisture content in cloud free pixels. These parameters are useful for spotting future convective potential.

EUMETSAT provides two product groups on moisture and instability info in cloud-free pixels:

  • the Global Instability Indices (GII) derived by the Central Application Facility (CAF) and
  • the SEVIRI Physical Retrieval products (SPhR) derived by NWCSAF.

GII consists of (among others) total precipitable water, K and lifted indices. SPhR includes (among others) total precipitable water, K, lifted and Showalter indices. Note that the definitions of GII lifted index and of SPhR lifted index are not the same. The GII lifted index is calculated by 'lifting' a virtual air parcel from the surface to 500 hPa and comparing its temperature to that of the environment. The SPhR lifted index is calculated likewise, except that the parcel's initial temperature is an average from a layer just above the surface. The thickness of this layer can be 100 hPa or 1000 m.

Table 1 shows typical values of satellite-retrieved total precipitable water and instability indices for the studied area or close to it between 05 and 17 UTC. In the afternoon these values indicated a buoyant atmosphere, with significant moisture.

Table 1. Typical values of the moisture and instability parameters between 05 and 17 UTC.

Fig. 7.1 presents total precipitable water and K index at 08:55 UTC retrieved by both methods, while Fig. 7.2 presents GII and SPhR lifted indices and Showalter index. GII lifted index has a higher diurnal cycle than the SPhR lifted and Showalter indices.

Fig. 7.1. GII and SPhR total precipitable water (left panels) and K index (right panels) for 08:55 UTC). The upper panels show the SPhR products, the lower panels show the GII products (colors in clear air pixels). Cloudy pixels are black.

Fig. 7.2. GII and SPhR instability indices for 08:55, 12:10, 15:55 UTC in the upper, middle and bottom rows, respectively. GII lifted index (left column), SPhR lifted index (middle column), SPhR Showalter index (right column). The colors represent the instability values in cloud-free pixels.