Example: 22 October 2012, 12:00 UTC

Figure 6: WV 6.2 Ám image from 22 October 2012 at 12:00 UTC. The yellow isolines depict the isotachs [m/s] of the jet at 300 hPa.

The WV image from 22 October 2012, 12:00 UTC (figure 6) shows a branch of the polar jet stream over the Atlantic heading towards the southeast and a branch of the subtropical jet west of Morocco and Portugal heading towards the northeast. A dark stripe in the WV 6.2 µm image is clearly visible on the cold side of the jet axis for both jet streaks. The moisture gradient zone corresponds to the axis of the jet.

a) b)
c) d)

Figure 7a-7d: WV 6.2 Ám image from 22 October 2012, 12:00 UTC
a) The WV 6.2 Ám satellite image shows scallops at the back of a frontal cloud band (green arrow) and transverse cloud bands inside it (blue arrow).
b) WV 6.2 Ám satellite image with shear vorticity isolines.
c) Isolines of the relative humidity [%] at 300 hPa.
d) Isotachs [m/s] at 300 hPa.

The WV satellite image of 22 October 2012 shows scallops near the axis of the subtropical jet and transverse cloud bands at the center and eastern parts of the frontal cloud band (figure 7a). The shear vorticity shows high values in this area at the 300 hPa level. A maximum of cyclonic shear vorticity, which favors the formation of scallops, can be distinguished along the dark WV stripe. In the middle of the cloud band there is a maximum of anticyclonic shear vorticity (negative sign), where transverse cloud bands can be seen.

A vertical cross section (figure 8a-8d) through the cloud band west of Morocco shows that the frontal zone is located above 600 hPa. This upper level cold front marks the transition from a dry stratospheric air mass to a warmer and more humid one. ECMWF cloud parameterization shows the presence of high level cloudiness above 500 hPa.

a) b)
c) d)

Figure 8a-8d: Vertical cross section through the upper level cold front with ECMWF model data. Date is the same as in figure 7.
a) Position of the vertical cross section (red line)
b) Equivalent potential temperature (black) and relative humidity (green/brown)
c) Equivalent potential temperature (black) and cloud cover (green)
d) Equivalent potential temperature (black) and isotachs (brown)

The isolines of the equivalent potential temperature in figure 8b-8c show the position of the upper level front between 16W/34N and 14W/36N. The cloud cover visible in satellite imagery is reflected in the model fields as well as the high values of relative humidity as a precondition for condensation. The isotachs suggest strong vertical wind shear.

This environment is favorable for Kelvin-Helmholtz waves to become visible in IR images, and in WV images they are even clearer. While descending air masses in the wave trough tend to warm up and evaporate cloud droplets/ice crystals, the moisture in the air condenses at wave crests. High values of relative humidity are necessary for the wave pattern to show up in satellite imagery.