Lemon tree from Taiwan

The Taiwan lemon tree is a natural hybrid (pomelo x mandarin), originating on the island of Formosa but now mainly cultivated in Japan under the name Natsudaidai.

Produces large, rather flattened fruits (250gr, 9cm), yellow then orange when ripe.
The abundant juice is acidic and subtly bitter, with a pronounced floral fragrance.
Harvested in winter and into spring.

The tree is vigorous, highly productive and thorny, with an upright habit and dense foliage.
Extremely cold-hardy, including the fruit.

References:
https://idtools.org/tools/1076/index.cfm?packageID=1179&entityID=8696
https://citrusvariety.ucr.edu/crc2588