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Fitxer:Thunderstorms over Estonia on a Copernicus Sentinel-1 synthetic aperture radar image.jpg

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English: Synthetic Aperture Radar (SAR) is said to be weather independent. It is true in a majority of the cases - the microwaves do not interact with condensed water of clouds and light rain and the image represents the underlying land or sea. But sometimes in case of heavy rain or hail, like during the thunderstorms the size and density of water particles is enough to interact with the microwaves of a SAR. The image presents Estonia around the lakes of Võrtsjärv and Peipsi in the evening of August 12th, 2015 at 18:56 local time. During that time heavy thunderstorms were passing over Estonia, bringing car damaging apple sized hail particles with them. According to Estonian Weather Service ground-based meteorological radar the vertical profiles of the thunderstorm centres were exceeding 10 km of height.

Notice the white and purple areas in the photo over land and lake Peipsi at left. Notice also the dark areas over land. The bright-white and purple areas correspond to backscattering from the atmosphere in the heavy rainfall areas. Dark areas is land backscattering, which is attenuated, because the SAR signal travelled through the rainfall cloud. Due to the side-looking geometry of the SAR system and tall profile of the thunderstorm clouds the bright backscattering and dark attenuation areas of the same cloud are misplaced. The bright backscattering is located in the left of the dark attenuation area as satellite beam was looking from left side. Using ground based weather radar and spaceborne SAR data it is possible to re-construct the thunderstorm clouds in 3D.

Satellite look direction left (ascending orbit), incidence angle range 34-45 degrees, false colour RGB, R=VV-, G=VH- and B=VV+VH-polarisation backscatter.
Eesti: Tehisava-radareid loetakse ilmast sõltumatuteks. Enamikel juhtudest on see tõsi - mikrolained läbistavad pilvi ja nõrka vihmasadu minimaalse vastastikmõjuga ning mõõdetud signaal vastab maapinnale või merele. Vahel harva tugevate äikesetormide ajal võib ilma mõju siiski radaripildil näha olla. Äikesetormides on veepiisad ja pooleldi jääs rahe osakesed piisavalt suured ning nende tihedus pilves piisav, et radari mikrolaineid märgatavalt summutada ja hajutada.

Sentinel-1 radaripilt on tehtud Eestis Võrtsjärve ja Peipsi ümbruses 12. augustil 2015 kell 18:56 kohaliku aja järgi, kui Eesti kohal arenesid ja liikusid tugevad äikesetormid tuues endaga kaasa kuni õunasuurusi autosid lõhkuvaid raheterasid. Eesti Ilmateenistuse maapeale ilmaradari mõõtmiste järgi ulatus nende sambakujuliste äikesepilvede kõrgus üle 10 km. Sentinel-1 radaripildil paistavad mitmes kohased helevalged ja lillakad laigud nii maa kui ka Peipsi kohal. Osade alade kohal on ka tumedaid laike. Heledad ja lillakad laigud vastavad äikesetormi sajualast vee- ja rahepiiskadelt tagasihajunud signaalile ning tumedad alad maapinnalt tagasihajunud signaalile, mis läbis oma teekonnal äikesepilve ja sumbus selles osaliselt. Kuna tehisava-radari kiirgusvihk on alati küljele suunatud on äikesepilve asukoht pildil nihutatud - tegelik pilve asukoht on heleda ja tumeda laigu vahepeal. Sama pilve pealt tagasihajunud signaal on pilve tegelikust asukohast vasakul ja vastavalt pilves sumbunud maapinnalt hajunud signaal pilvest parema, sest satelliit vaatas uuritavat ala vasakult (lääne poolt). Sentinel-1 pildi ja maapealse ilmaradari mõõtmiste kombineerimise abil on võimalik arvutada pilve 3D mudel antud ajahetkel.

Satelliit vaatas vasakult (tõusev orbiit), langemisnurkade vahemikus 34-44 kraadi, valevärvipilt, punane=VV-, roheline=VH- ning sinine=VV+VH-polarisatsiooni tagasihajumine.
Data
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English: * Satellite data: European Space Agency, 2015
Eesti: * Satelliidi toorandmed: Euroopa Kosmoseagentuur, 2015
Posició de la càmera58° 30′ 00″ N, 26° 00′ 00″ E Kartographer map based on OpenStreetMap.Aquesta i altres imatges properes a: OpenStreetMapinfo

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© This image contains data from a satellite in the Copernicus Programme, such as Sentinel-1, Sentinel-2 or Sentinel-3. Attribution is required when using this image.
Reconeixement: Contains modified Copernicus Sentinel data 2015

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nom curt de l'autor: Kaupo Voormansik
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