SatManu

Appearance in satellite images

Figure 1: Occlusion front

The schematic shows and describes features typical for occlusion cloud bands in the basic satellite channels. These are to be compared with an actual case:

Synoptic scale spiral structure
Bright in IR, VIS, WV channels
Black stripe in WV on the rear side
Lumpy structure from embedded Cbs

Meteorological-physical background

Figure 2a & 2b

For the CM "occlusion" the meteorological physical background has to describe the development. A well-developed occlusion cloud band is often the end stage of the classical polar front theory.

This can and should be checked with a sequence of images showing each stage of development from wave to occlusion.

Key parameters

For the CM "Occlusion" six key parameters have been derived which have a special relation to the cloud band and show special configurations:

Height contours
Thickness advection
Temperature advection
Isotachs
Vorticity
Vorticity advection

An example for height contours:

14 July 2016 / 18 UTC
Developed WCB Occlusion

Upper image:
Height contours 1000 hPa

Lower image:
Height contours 500 hPa

Fully developed low centre from surface to upper levels

Figures 3a-3c: Height contours

Another schematic indicates a jet axis spiralling around the cloud spiral, a jet streak on the rear side of the cold front cloud band and a maximum of cyclonic vorticity advection in the left exit region of the jet streak, which is the center of the cloud spiral.

14 July 2016 / 18 UTC
Developed WCB Occlusion

Upper image:
Relative Vorticity 300 hPa
Orange/black: zero line - jet axis at 300 hPa
Red: cyclonic vorticity
Blue: anticyclonic vorticity

Lower image:
Isotachs 300 hPa; jet streak

Jet streaks are along the rear edge of the CF and the Occlusion cloud band; jet axis extends from the inner cloud edge of the occlusion to the outer one

Figure 4: Jet stream

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Chapter 4: Appearance in Vertical Cross Sections

For the CM "Occlusion" there are seven key parameters which are indicative and show special features in vertical cross sections:

(Moist) isentropes and humidity
Divergence and vertical motion
Temperature advection
Vorticity advection
Potential vorticity

An example for isentropes:

Figure 5: Vertical cross section of relative humidity

The schematic shows a trough over the spiral cloud band, indicating the presence of warm air in middle and upper levels.

Another schematic indicates a large area of warm air advection on the leading frontal side and within the isentropic trough and a thick layer with cold air advection to the rear of the isentropic trough.

Figure 6: Vertical cross section of temperature advection

Weather Events

Parameter	Description
Precipitation	Broad precipitation lines (5-50 km wide) are found within and ahead of the surface front. Maximum precipitation close to the Occlusion point(in contrast to Occlusion: Cold Conveyor Belt Type) At the rear part of the cloud spiral showers and thunder, even hail.
Temperature	Seasonal dependance; rising temperature in wintertime, cool conditions in the summer.
Wind (incl. gusts)	Moderate to strong winds. The most instense wind on the outer edge of the cloud spiral.
Other relevant information	Generally, Occlusions are connected with multi-level cloudiness. Therefore the weather activities are highly variable.

Figure 7: Weather types in Warm Conveyor Belt Occlusion

Weather and radar observation are basic parameters for cloud and precipitation conditions.

Figure 8a & 8b: Synoptic observations (left), synoptic observations + Opera Radar (right)

For more detailed cloud information, satellite evaluations such as from the Nowcasting SAF are supplementary information sources.