Taiwan Lemon

The Taiwanese lemon tree is a natural hybrid (pomelo × mandarin), native to the island of Formosa but now primarily cultivated in Japan under the name Natsudaidai.

It produces large, somewhat flattened fruits (250 g, 9 cm) that are yellow when young and turn orange when ripe.
The abundant juice is tart and subtly bitter, with a pronounced floral aroma.
Harvested in winter and into spring.

The tree is vigorous, highly productive, and thorny, with an upright growth 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