Spin around the Sun through a movie of Meteosat Second Generation water vapour imagery.
09 March 2023
09 March 2023
By Martin Setvák (CHMI) and Ivan Smiljanic
Geostationary weather satellites and digital photography cameras supporting time-lapse mode (or 'interval shooting') have one common feature: both capture series of images at certain time intervals, which can next be displayed with certain acceleration. Speeding-up these image sequences enables viewers to much more efficiently follow the evolution and motion of clouds and their systems, or other phenomena in the atmosphere, revealing the dynamics of the atmosphere. There is a saying, that one picture is worth a thousand words. Similarly, one time-lapse movie is worth a thousand images.
The first digital cameras with dedicated time-lapse mode, began to appear about two decades ago, which coincides with the time of launch of the first Meteosat Second Generation satellite, MSG-1 (Meteosat-8), in August 2002. In principle, the MSG satellites can be perceived as giant, very complex digital cameras, with a half-meter diameter lens, taking an image of the entire Earth every 15 minutes — which is the reason why we can consider satellite animations as a special form of time-lapse movie.
The main instrument of MSG satellites, SEVIRI, scans the Earth in 12 different spectral channels (at different wavelengths, ranging from about 0.5 to 14µm). One of these channels is the band centred at 6.2µm, where significant absorption of thermal radiation emitted by the Earth's surface and lower clouds occurs. This means that the satellite sees only the highest clouds — mainly cirrus and cumulonimbus — and the distribution of water vapour in the upper third, or so, of the troposphere (at mid-latitudes, roughly the levels of the atmosphere between 7 and 12km above the Earth's surface).
The pair of images of the Earth in Figure 1, shows a near-real-colour view (VIS-IR, a combination of two visible and one thermal channel) on the left, and a WV6.2 (6.2µm) channel image on the right. While in the VIS-IR image we see the distribution of all clouds and cloud-free land and sea/ocean surfaces roughly as the human eye would see everything, in the WV6.2 image we see only the highest clouds (white) and distribution of water vapour in the upper troposphere — the more water vapour in the atmosphere, the lighter is the shade of grey. So the main advantage of images in the 6.2µm channel is that we can see structure of the atmosphere and its flow, even in cloud-free areas. On top of that, having in mind the fact that the lower the moisture resides in the troposphere the darker (warmer) WV6.2 channels gets, we can see this imagery as a 3D rendering of the upper troposphere.
It is this feature that makes it fascinating to observe the Earth's atmosphere dynamics in this channel in the movie below, covering one whole year — from storm clouds and cirrus in the tropics, driven by the trade winds to the west (in the so-called intertropical convergence zone), followed by meandering subtropical bands of jet streams flowing eastward, to the highly turbulent flow of cyclones and anticyclones at higher latitudes. For ease of orientation, a few days in the VIS-IR product are included at the beginning of the video, followed by the rest of the year in the channel WV6.2 (Figure 2).
Figure 2: Time-lapse movie. Watching in full screen is recommended
This time-lapse movie is dedicated as a tribute to MSG satellites, which have been reliably providing their data to meteorologists for more than two decades by now, and to all the EUMETSAT and ESA personnel developing, constructing, operating and promoting these satellites and their data.
We've extracted a few of more distinguished features from the loop (as these are static images, we also advise looking at the associated time-lapse movie in Figure 2). However, many more stories about the atmosphere can be told from the looping imagery, especially if you enjoy the whole year-around loop, perhaps a couple of times.
Something that one could easily overlook when watching this timelapse (because of its still low pace for this feature) is the north-south oscillation of the Inter-Tropical Convergence Zone (ITCZ). This belt of tropical convergence can be tracked through a line of pulsating convection (on a daily basis) near the equator. In the winter (northern hemisphere) the ITCZ is closer to the south pole, in the summer it moves towards the north (and changes shape). This is obvious when two seasons are compared, with two different snapshots six months apart (Figure 3).
Broad-scale, quasi-persistent subsidence regions, are a consequence of planetary circulation cells, most prominent at the belt where Hadley and Ferrel cells meet up and drive large-scale, descending motions in the troposphere. These are seen as dark (dry) areas in WV imagery (snapshot taken in Figure 4). Other examples can be seen throughout the time-lapse, in areas that are quasi-equidistant from the equator.
Another large-scale feature of the upper troposphere is a jet stream — an integral feature of a planetary Rossby wave. One such example is annotated in Figure 5, with more intense period between 9 and 14 February (see the time-lapse). This jet stream may be traced through as a streaming belts of higher level moisture, accompanied by typical high-level clouds, such as transverse cirrus bands. However, note, the most intense core of the jet is actually on the right flank of this belt, seen a dark (dry) line.
In principle, polar jets are more intense than subtropical, but both are equally important for large scale mixing of air masses. A co-existing duo is captured in Figure 6.
Large areas of low-pressure systems, with embedded smaller low-pressure areas, are colloquially called 'mother lows'. These occasionally form across both north and south mid-latitudes, one example was captured in Figure 7.
A similar, but more persistent 'mother low' system, is shown in Figure 8. This one was crossing the Europe during the whole first half of September (watch it's journey in the time-lapse).
Unlike low-pressure systems, anticyclones very seldom undertake a regular rotation pattern, but are shaped in arcs that bend clockwise (in the northern hemisphere). However, on 13 August one high pressure system, over the north-east Atlantic, took a very regular, round shape (Figure 9), and demonstrated atmospheric stability that is associated with anticyclonic systems (note there is very limited vertical cloud development, in contrast to cyclonic system to the west).
A feature that connects low and high pressure systems, and separates two different synoptic or mesoscale cloud systems, is called a Deformation Zone. This elongated, usually s-curved band, is well depicted in Figure 10, with a dark stripe (dry air) on one side, pushed against a very bright, white cloud system on the other. In this case, the cloud system is exceptionally bright white and is know by the acronym DIBS (dust-infused baroclinic storm clouds). As the name suggest, these clouds form when the dust is picked up and ingested into the existing low-pressure system, usually through a warm conveyor belt. In this case (Figure 10), a very heavy dust load was infused from North Africa, changing the microphysical properties of the clouds to a high degree, making them appear as very cold, and lasting exceptionally long in the higher troposphere.
Warm conveyor belt
Warm conveyor belts (WCB) don't only bring dust, they are often responsible for moisture advection. This advection of moisture, and associated air mass streaming (in particular over higher terrains), is responsible for different cloud formations. Figure 11 captures the passage of a WCB (see the time-lapse) over the high terrains of the Pyrenees, Alps and Apennines, which was responsible for the formation of quasi-stationary lee wave clouds behind the mountains, and convective clouds around their slopes.
A number of back-building storms can be observed from 2 to 5 September in the south of Europe (Figure 11). These were embedded in the subtropical jet across Mediterranean Sea, and they appear almost as it they are surfing in the westerly stream.
Pulsating convective clouds in the tropics are usually triggered by the diurnal heating of the tropical belt (more pronounced over land masses, hence, land masses are often outlined by the developing convection). However, bigger mesoscale convective cloud systems (Figure 13) can last for longer periods of time, traversing long distances (usually over land, from east to west).
Intense convective systems are accompanied by strong vertical motions (updrafts and downdrafts), responsible for the generation of gravity waves and outflow boundaries. These emanate from the convective systems in the form of waves, and are responsible for further cloud formations outside the system (observe the arc cloud formation in Figure 14).
A number of tropical low-pressure systems can be observed in this time-lapse, in both hemispheres. During the first few weeks of 2022, four consecutive tropical cyclones hit Madagascar, In Figure 15 the last one, tropical cyclone Emnati, was captured around the time of landfall.
At times the atmosphere becomes 'tired' and loses momentum, so there are no pronounced jet streams to 'drive the weather' in the extra-tropical regions. It starts to appear more homogeneous, with multiple (small and weak) low and high pressure systems, blurred deformation zones, containing well-mixed air masses (Figure 16).
As shown above, this simple time-lapse movie perfectly highlights the adverse behaviours and features of Earth's atmosphere — and if you step back, with a bit of imagination, you can even catch Earth sleeping, yawning occasionally. (Figure 17).
The first author of this case study, Martin Setvák, has been 'time-lapsing' since 2003, roughly the same period as the MSG data availability. In fact, first MSG loops were one of the main 'drivers' which brought him to time-lapse movies of clouds and other phenomena in the sky. On many occasions you can find his time-lapse movies being accompanied by weather satellite images or loops, bringing together the view from ground and space. All of his time-lapse movies are freely available for educational purposes.