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Identification of the Atmospheric River path every 3 hours, for the event of 02 October 2020, using the SMART technique. The Atmospheric River originated over Gulf of Mexico, crossed the Atlantic and entered the Mediterranean basin, severily impacting on nort-western Italy.
Detecting Atmospheric Rivers from space
Detecting Atmospheric Rivers from space
Satellite investigation of ARs is performed with Meteosat Second Generation (MSG) data in Rapid Scan Service mode (MSG-RSS). The MSG-RSS covers a wide geographic area in the African-Euro-Atlantic domain, spanning from intertropical latitudes to approximately 50°N. Their data present a high spatial resolution (3-5 km), so they are suitable for investigating primarily small-scale phenomena, but also some continuative and long-lived events like ARs, because of their high temporal frequency of observation (5 minutes). The novel technique here proposed for the satellite detection and monitoring of the ARs is called Satellite Method for Atmospheric River Tracking (SMART). This method relies on the extinction processes that the electromagnetic radiation experiences in the AR due to the presence of the large amount of water vapor that generates a significant absorption and reduction of the signal towards the satellite.
The channels used for the implementation of the SMART technique are those centered in the water vapor absorption bands at 6.2 and 7.3 µm, respectively. Their weighting functions peak at 6-8 km and 4-6 km, making possible a better monitoring of the ARs confined in the middle-to-upper atmosphere.
By introducing the following relations to quantify the magnitude of the variation in the signal:
1) WV_VAR062 = |(TB062/TB062_max) x 100 – 100|
2) WV_VAR073 = |(TB073/TB073_max) x 100 – 100|
where WV_VARs are the water vapor variation indexes and the terms TBs are the point brightness temperatures and their maximum values in the domain for the two MSG-RSS absorption channels respectively, it can be evaluated the strength of the ARs water vapor in the extinction process; at middle latitudes, some typical values are in the range 8-15 for both the WV_VARs, while for higher values the signal tends to be masked by the presence of the clouds.
The combination of these information with the concept of optical flow, allows to recognize the areas which are the most affected by the water vapor transport, that are separated by the other scenarios through a cluster analysis. If the clustered region respects some geometric criteria (mostly, length at least 1000 km, length-width ratio at least 2), it is finally classified as an Atmospheric River.
A well-reconstructed case study with the SMART technique is the event of 2-3 October 2020, when an Atmospheric River crossed the Atlantic Ocean and impacted the Central Mediterranean, with some heavy rainfalls on the North-West Italy. The method properly identified the water vapor structure during all its long-lived track, starting from the previous day (1 October) until the evening of the following day (2 October), when the AR reached the Mediterranean Basin starting to be misleaded with other local small-scale water vapor transports.
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