Development of storm Valtteri

By Petteri Pyykkö, Robert Mäkitie, Kaisa Solin

Approximately 20-30cm of snow accumulated over southern Finland and winds speeds over 96 km/h were recorded during the storm. The strongest wind speed occurred after the low pressure centre passed over the sea south of the Åland Islands. In Finland, the storm cut the power for up to 30,000 households, and several traffic accidents took place due to visibility and slippery roads.

The storm itself isn’t that interesting over Finland from a satellite point of view, because a lot of thick high clouds occurred with the low-pressure centre, blocking the view. The aim of this case study is to look at the development of the storm Valtteri (also known as Malik) by using the next generation RGB satellite products to come from the Meteosat Third Generation Flexible Combined Imager (FCI) and Metop-SG METimage instruments.

Development of the storm

On the evening of 28 January, two shortwaved troughs occurred over the northern Atlantic, one over Iceland and the other southwest of Iceland (Figure 2). The trough over Iceland occurred in the exit region of a 75kt jet flow, which had already stated to strengthen the trough. The trough southwest of Iceland moved much faster towards the east than the one ahead of it. On 29 January, the faster moving trough eventually caught up with the trough located over Iceland. The two troughs merged, creating an upper low over the Norwegian Sea (Figure 3). The upper low was located behind a surface low pressure centre, which started to strengthen rapidly while moving over Scandinavia. The upper low was vertically aligned with the surface low on 30 January, when the surface low had reached southwestern Finland, and the storm started to weaken. The maximum surface pressure drop of the storm was approximately 15hPa in 24 hours.

The new generation of satellite images

In Figure 3 the vortex east of Iceland doesn't suffer from the limb-cooling effect as in Figure 2b where the vortex appears a bit darker/purple. Here MODIS Airmass RGB imagery is a good proxy of what we can except from future METimage imagery. This kind of higher resolution Airmass RGB imagery (Figure 2) in comparison to imagery from SEVIRI (Figure 3) might facilitate improvements in the timeliness of spotting PV-anomalies and to better observe their strengthening in high latitudes.

Figures 4 and 5 show the development at a later stage when the disturbances had reached the same longitude. On both MODIS images the lower cloudiness is more visible. Looking closely some smaller-scale vortices near the center of the low pressure in Figure 4 can be seen, whereas the resolution of SEVIRI imagery is not sufficient for making such observations.

On Figure 4 the southern edge of the most intense red colouration matches quite nicely the location of the PV anomaly region from the ECMWF model, where it starts increasing northwards towards the centre of the upper level circulation.

As low pressure systems usually evolve and propagate quite quickly, high temporal resolution plays an important role in a forecaster's capability to detect whether the model has a good understanding of the situation i.e. whether the satellite observations match the development in a weather model. By comparing Figures 8 and 9, it shows that the ECMWF model forecast had a very good idea of the storm development with a 12 hour lead time; the dry (dark) and bright (moist) regions in both images match well. Also the dry region matches well with where the PV anomaly region from the ECMWF model starts increasing northwards towards the centre of upper level circulation.

In this case simulated imagery (Figure 9) gives an unrealistic expectation of future FCI water images because of the limb-cooling that is not taken account in simulation (see High-latitude simulator for MTG/FCI instrument, in the sections Limitations of the method and Limb cooling ). However, from Figure 9 it can be noted that imagery that we can expect from FCI in the future will be more detailed than the imagery currently available from Meteosat-11. At high latitudes new generation polar satellites will give more a detailed perspective of water vapour structures e.g. moisture gradients are more pronounced and smaller scale vortices near the centre of circulation are also possible to observe here.

Typically in-situ ground observation are located near the coastline or on the continent, and observations over sea area are seldom available, if at all. With scatterometer wind measurements (Figure 10) it is possible to get valuable (almost) real time information from the coastal area in nowcasting. The wind field (Figure 10) is measured in approximate 2.5 minutes (satellite flight velocity ~7.43km/s). In the case of polar low development, with ASCAT wind measurement it is easy to see the location of the strongest winds and, also, the scale of the strongest winds. While comparing these measurements with model output on wind and sea surface forecasts, it is possible to analyse the reliability of numerical prediction. The upcoming Metop-SG has the Scatterometer (SCA) instrument onboard, which supports measurements at higher wind speeds than the current ASCAT, for the benefit of observing extra-tropical storms.