Left-exit cyclogenesis & gravity waves over Poland

By Jakub Walawender, Tomasz Knopik and Piotr Struzik (IMGW, Polish Weather Service)

On this day, there was an anticyclonic system over the Baltic Sea with ridge extending towards western Poland, and a low over Svalbard with a very long frontal system which waved over Ukraine forming another depression, see synoptic maps: 00:00 UTC and 12:00 UTC (Credit: DWD).

An even deeper 'Ukrainian Low' was very well seen on the maps at levels 850hPa, 700hPa and 500hPa but in those higher levels of troposphere it was located southwards from the surface low. This synoptic situation generated warm air advection from north-eastern directions towards Poland.

A cyclonically curved jet-streak aligned parallel in front of the warm front cloud edge can be well-recognised in the corresponding Meteosat-9 Airmass RGB image (Figure 1). The strong north-eastern wind on the level of 200–300hPa can also be identified in the aerologic diagram from Legionowo (12:00 UTC).

This was a typical left-exit cyclogenesis: a zone of cloud-free air formed in the left-entrance region of the jet-streak, whereas an extensive cloud band appeared in the left-exit region and the low occurred underneath. The low was not reported on the German synoptic map.

The gravity wave occurred over eastern Poland as a result of the vertical wind and temperature distribution. The first maximum of the wind speed was recorded in the layer between 1500 and 2000m AMSL. This maximum was connected with subsidence inversion which is clear between 1500 and 2000m in the aerologic diagram (12:00 UTC). A second maximum of the wind speed was around 4000m AMSL.

A much smaller inversion layer can be observed around 3000m AMSL. The layer below the lower inversion (from surface up to about 1500m AMSL) is characterised by much lower wind speed and a more north-western direction. This layer is also very well mixed and the vertical temperature distribution is dry-adiabatic as opposed to the layer between the inversion and level around 3000m AMSL (second inversion level) which has rather moist-adiabatic temperature distribution. The layer between 3000m AMSL and 5000m AMSL is much drier than the layer below.

The wind shear (mainly velocity but also direction difference) across the interfaces between two fluids observed in the aerologic diagram, due to Kelvin-Helmholtz instability rules, caused the waves in the air. The inversions were the interfaces between the layers of the troposphere which had different profiles of temperature, wind and humidity. The typical wave clouds (Altocumulus lenticularis) observed over Poland were the proof of this phenomenon. The lenticular clouds aligned perpendicular to the wind direction both on the western edge of the frontal system and in the mountains were very well seen in the NOAA-19 AVHRR RGB composites (Figures 2 and 3), as well as in the MSG high resolution visible (HRV) images (Figure 4).

Also lower stratocumulus clouds located between 1500–2000m AMSL inside the frontal system were waved what can be clearly seen, once the contrast of the AVHRR RGB composite image was enhanced. The radiometric values of 10-bit (1024 level for each primary colour) AVHRR non-calibrated data were stretched as follows: for band 1 (VIS0.6) histogram values in between 500 and 800 were taken into account, for band 2 (VIS0.8) values between 500 and 750 and for band 4 (IR11.0) values between 560 and 900. The RGB composite was created which made it possible to distinguish between clouds located in different levels in the atmosphere.

Additional content

Met-9 HRV Animation (00:00–24:00 UTC)
Meteosat-9 Airmass RGB image (11:30 UTC)
Lenticular clouds above Mazovia on the edge of the frontal system
(photo taken by Jakub Walawender from the plane landing in Warsaw-Okęcie on 7 May 2011 at 11:18 UTC)
Lenticular clouds above Mazovia on the edge of the frontal system
(photo taken by Jakub Walawender from the plane landing in Warsaw-Okęcie on 7 May 2011 at 11:19 UTC)