Dense fog over southern Finland

By Robert Mäkitie (FMI)

On 11 September a large low pressure area occurred over Fennoscandia, bringing warm air over southern Finland (Figure 1).

As it moved south warm air that picked up a lot of moisture from the Baltic Sea. The low pressure area and its frontal system slowly travelled northwards, but its rapid movement was blocked by a high pressure area located over northern Finland. This area of high pressure kept northern Fennoscandia much colder than the southern parts. The warm and moist air that the fronts brought led to fog development and formation of low clouds over southern Finland and the Gulf of Finland, while it travelled north over cooler area.

How it was forecast

The synoptical scale that occurred over northern Europe was well predicted by all the numerical weather models used at the Finnish Meteorological Institute (FMI) — the ECMWF HRES (global model), FMI HIRLAM, and MetCoOp (a cooperated weather model between Finland, Sweden and Norway). Every model predicted fog and low clouds over southern parts, but when comparing the model data to surface observations, it can be seen that the models weren't quite as precise about the areas to be affected. This means that forecasters have to use both model data and surface observations as the base for nowcasting, along with forecaster knowledge and understanding of how fog forms.

All three weather models predicted that the low cloud base over southern Finland would rise to VFR (Visual Flight Rule) conditions during the day. and in the evening the fog over the Gulf of Finland would slowly advect over southern Finland, reducing visibility to 400m. During the morning the cloud base started to rise, but did not rise as high and rapidly as predicted by the weather models. Later in the afternoon the cloud base broke for a short moment into VRF condition. The breaking of the cloud occurred in a completely different place and time compared to all three weather models.

The situation was difficult to follow from a satellite point of view, because of high cirrus clouds that occurred along the front over southern Finland. The high cirrus clouds appeared to be thick clouds in most of the RGB satellite images on SEVIRI, which prevented FMI forecasters from seeing the fog clouds below (example Figure 3).

The only RGB satellite image that captured the true situation of the cirrus clouds was the HRV Cloud product (Figure 2).

From the HRV cloud product, forcasters could see that the cirrus cloud over the Gulf of Finland was thin and transparent, and some low clouds were detectable through it. It was hard to know if the cloud below the cirrus cloud was fog, because middle and low clouds have similar colours in the HRV cloud product.

Fortunately, during that same period, the polar satellite Suomi-NPP passed over Fennoscandia, which included Finland in its field of view. From its satellite images forecasters could see the fog that occurred over the Gulf of Finland. Due to the high spatial resolution of the polar satellite images the transparent cirrus cloud was more detectable. The fog cloud over the Gulf of Finland could be observed much more clearly using the Natural Colour RGB products (Figure 4 for VIIRS, Figure 5 for SEVIRI), due to its different channels and colour shades. The Natural Colour RGB is a good product both in polar and geostationary world.

In the product, low water clouds are shown in greyer colour shade, and middle to high clouds are shown with white and cyan colour shades, due to ice crystals in the cloud. In the polar satellite image, the breaking in the low cloud was also much more detectable. Seeing the breaking in the cloud in the polar satellite images, forecasters could determine how different the model predictions were from what actually occurred. Knowing that the weather models differ from the observations, and seeing the fog that will advect later further inland, helped the forecaster predict that the fog could advect much earlier than estimated over the Helsinki-Vantaa airport, the main airport in Finland, located only 17km from the south coast.

Every model predicted that after 18:00 UTC the fog would slowly start to advect over Helsinki-Vantaa airport and around 21:00 UTC the denser fog would arrive. These time estimations were predicted in the Helsinki-Vantaa airport 18:00 UTC TAF (Terminal Aerodrome Forecast). The other forecast products that were issued to Helsinki-Vantaa airport contained the information of an earlier fog risk.

About half an hour after the 18:00 UTC TAF was issued the dense fog had already advected over Kumpula (12km south of Helsinki-Vantaa), where FMI headquarters is located. This meant that the dense fog had started to advect inland earlier than the models predicted. FMI quickly called the airport and alerted the air traffic controllers that the fog would reach the airport in approximately 90 minutes — exact time frame the airport needs to prepare the LVP-lights (Low Visibility Procedure) into a standby mode.

In similar weather situations, forecasters would usually follow the surface observations stations south of Kumpula to see when the fog starts to move inland. However, in this situation all the surface observations south of Kumpula were under low clouds, and bad visibility conditions prevailed the whole day, which made it difficult to to see the changes in the visibility and cloud base at the time when the fog started to advance. The dense fog did arrive over Helsinki-Vantaa 90 minutes after the phone call, giving the airport the time they needed to prepare.

The next generation of satellite images

The big question after reviewing this case is how this fog case would have looked in the coming MTG FCI (Flexible Combined Imager) or the Metop-SG METimage instrument. To get a better understanding of this FMI has created proxy data that simulates how SEVIRI images from this fog case would look through the lens of the FCI.

Figures 6-8 show three RGB satellite images where of two of the images are the Natural Colour RGBs and Day Microphysics.

Comparing these images to SEVIRI’s images (Figures 2 and 5) one clear improvement is easily noticeable in the simulated FCI images — the enhanced spatial resolution. Cloud cover and cloud edges can be seen much more clearly in the FCI than in the SEVIRI. This shows that the increased spatial resolution in the FCI instrument will improve a lot the old RGB satellite products that are in use at FMI.

Note: It is important to remember that this proxy data does not take into account other factors that have influence on the image sharpness (see more details in High latitude simulator for MTG/FCI instrument under the section Limitations of the method).

The FCI also includes four new channels, including NIR1.38, which is made for better detection of thin cirrus clouds. This channel is used in one of the newest RGB satellite images called Cloud Type RGB, that will arrive with the new FCI and the METimage instrument.

Image comparison

Cloud type HRV clouds

 

Figure 9 (left) is a proxy image of the new Cloud Type from the same time frame. In the new RGB product the thin cirrus that is located over the Gulf of Finland can much better observed (red colour) due to the new channel. It is also possible to see through the thin cirrus cloud, which is a big improvement compared to SEVIRI images. Also, the METimage will be a big improvement compared to the old AVHRR images. The new METimage will contain much better spatial resolution and more channels, so it will be able to produce the same RGB products as the FCI, including the new NIR1.38 for thin cirrus clouds. Seeing through thin cirrus clouds has been a problem for many years but hopefully these new instruments will bring a change to that.