Convective convergence lines are small-scale to meso-scale phenomena, therefore data from a relevant Limited Area Model (LAM) are absolutely necessary to supplement large scale (e.g. ECMWF) forecast data. In the studies made by ZAMG and DHMZ, both models were used: the synoptic situation can be described sufficiently by ECMWF data, whereas the LAM model ALADIN is excellent for the interpretation of divergence, vertical motion and the wind field. The most important level for the detection of convection is 925 hPa. However, due to the fact that this level is frequently found under the ground in the mountainous area, 850 hPa level has also been used. Convergence Lines can be detected in the model up to 800 hPa or in a few cases even higher. Whilst Convergence Lines show differences synoptically, they are similar in terms of small scale key parameters.

Typical synoptic environment

1. Greek convective Convergence Lines

These lines seem to occur in certain synoptic situations:

• There is a low or, at least, an upper level low over Turkey and a High over the central Mediterranean.
• An upper level ridge (sometimes also a surface ridge) is situated over Greece.
• As there is no frontal situation over Greece, the equivalent thickness shows only a small gradient; The TFP sometimes shows a positive value originating from a frontal system passing by to the north.
• The isobars at low levels show a weak gradient but with winds from a northerly direction.

In this image, there is an upper level low over Turkey and a high over the Mediterranean. Greece is situated at the eastern edge of a ridge at 700 hPa.

2. The Carpathian Convergence Lines

In case of the Carpathian Convergence Lines, there is no distinct synoptic pattern such as ridge or trough features or even a frontal system.

• Large scale winds show quite a homogeneous flow with mainly northerly and easterly components.

3. Convergence Lines over the French Mountains

• The typical synoptic situation is characterised by a north-westerly upper level flow, a surface low in the Mediterranean near the Gulf of Genoa and high pressure west of the Massif Central (France).
• Situations with a strong north-westerly flow creating a Mistral situation. However, in the case presented here the wind speed is rather low, since the pressure gradients are relatively weak.

Typical small scale parameters

Wind, convergence and vertical motion data detected in small scale model output show remarkable changes during day. The Greek and the Dinaric Convergence Lines are chosen to show common features. Some additional features are noted for the other Convergence Lines.

Typical distribution of the wind fields in the morning

In the morning (06 UTC) divergence (fields not shown here) prevails over the mountain chains whereas convergence occurs at higher levels. The wind fields show a flow around the mountains rather than over them. Orographic effects vanish at 800 hPa where a more or less uniform flow pattern (north-west wind) can be observed. The example shows northerly winds around the mountain chain, with difluence at the northern edge at 925 hPa. Some confluence can be found east of the mountains. At 700 hPa there is mostly northerly to north-westerly flow.

In the example case of the Dinarides the wind in the lower levels is presented by the 10m wind field:

In the region of the Dinarides there is mostly easterly to south-easterly flow passing over the mountain chain. Upper level flow, represented by the 700 hPa wind is mostly south to south-easterly.

Typical distribution of the wind fields, convergence and vertical motion at noon

• Wind fields are influenced by orography up to about 700 hPa: There is a confluence at lower levels.
• As a consequence of this wind field, small scale convergence can be observed up to 800 hPa. At the 925 hPa level, the maximum of convergence is shifted to the west (the windward side of the mountain ridge).
• Therefore, the maximum of upward motion can also often be found west of the mountain chain (around 1 degree), in a layer which is somewhat thicker than can be observed for the convergence. Above 700 hPa, there appears to be no upward motion which can be related to convergence cloudiness.

Typical distribution of wind-fields at noon

There is general confluence at low levels. At high levels, the wind direction is more or less parallel to the mountain chain, depending on the synoptic situation. The image shows pronounced confluence around the mountain chain generated by easterly and north-easterly winds in the east and westerly winds in the west. At higher levels, winds are parallel to the mountain chain. Over the Dinarides confluence is seen over the mountain chain, in the region where clouds already start to develop. At 700 hPa there is southerly to south-westerly flow in the western part and nort-easterly flow over the mountains in the eastern part of the Dinarides.

Typical distribution of divergence

Low level convergence is occurring at the mountain, divergence alongside. The situation at higher levels is reversed. The clouds of the convective Convergence Line closely follow a narrow zone of convergence (solid lines) at 850 hPa.

At 700 hPa the flow is more divergent over the mountains, specially in the south-eastern part, suggesting that the phenomenon is rather shallow.

Typical distribution of vertical motion at noon

There is low level upward motion occurring at the mountain, downward motion alongside. The situation at higher levels is reversed. Both images show upward motion in the region of the mountains. At 925 hPa, the velocities are much higher. At 850 hPa there is still some upward motion, although at this level there is also some areas of divergence (see above).

Typical distribution of wind-fields in the afternoon

In the afternoon (15.00 UTC) the model parameter are getting similar to those at 06.00 UTC, but they still show some remnants of "daytime features". The convergence zone that developed at noon is starting to decay at this point in time and, consequently, shows a less distinct pattern.
Wind fields are reverting to a typical night-time non-convergent flow. However, at that point there are still convective cells along the Line, sometimes even well developed. The noon confluent pattern is disappearing, but there are still convective cells along the Convergence Line.

Additional features apparent in the Carpathian Convergence Lines

• In general, the wind field can be characterised by north-easterly winds east of the Eastern Carpathians which become south-easterly winds at the Southern Carpathians. Therefore, there are components directed towards the mountain ranges. In Transylvania southerly and easterly winds prevail. In addition, the wind field shows a lot of small vortices within this orographic region.
• As a consequence of the wind field, areas of convergence can be found at lower levels. At 925 hPa it is more or less associated with the lines of cloudiness. The Southern Carpathians and the Eastern Carpathians show two separate areas of convergence with a small area of divergence at the "corner".
• In the afternoon and evening, some small areas of convergence still exist, although they are more likely connected with huge convective cells (MCS) which can survive for longer points of time.

The image shows pronounced orographically induced convergence at 925 hPa. At the transition between the Southern and Eastern Carpathians there is almost no convergence, and even some divergence. The image shows pronounced upward motion at 925 hPa. At the "corner", downward motion is found, the position of this area is more clearly seen with this parameter than with the field of convergence.

Additional features for the Convergence Lines in the French mountains

• The wind fields show northerly directions becoming northwest and west with height but show distinct confluence at several mountain ridges (Massif Central, Jura mountains, West Alps) leading to upward motion from the surface up to levels of 850/800 hPa.