How cold tolerant are most of the pummelos at present time?
Not cold tolerant at all. Pomelos are the most cold vulnerable out of all the citrus, more cold vulnerable than orange and more cold vulnerable than grapefruit. That's probably one reason you don't see pomelo very often in the U.S.
It's not really surprising. View oranges and grapefruits as being on a continuum between mandarins and pomelo. Mandarins are the more cold hardy of the two, so in general the more mandarin it has in its ancestry, the more cold hardy the citrus type will be.
That's part of what makes this challenging. It would be quite an accomplishment to have a pomelo variety that could grow in zone 9 (let alone even thinking about it surviving in zone 8 ). And I am talking specifically about
pomelo, not grapefruit, because although the two are very similar there is a fundamental difference in flavor between these two groups. Grapefruits have more of a pungent, slightly putrid smell, and carry a small degree of tangy orange flavor. The aroma of pomelo is a little bit more floral and clean, and subtle with nuances, and usually have less bitterness than grapefruit.
Chandler is probably somewhat more cold hardy than other pomelos, but it's still a little more vulnerable to cold than the average grapefruit. What I mean specifically, most Thai pomelo varieties could be grown in zone 10 but they wouldn't grow that well.
People in the U.S. just don't know that much about pomelos. They are a big thing in Southeast Asia. And they were traditionally grown in Japan, although now grapefruits are much more prevalent in Japan than native pomelo varieties because of cheaper grapefruit imports from the U.S. and fruit being very expensive to grow in Japan because of the limited land and high cost of living and labor costs. Another problem is the language barrier. In Vietnamese the word used for grapefruits is the same as for pomelo, so this leads to much confusion. When Vietnamese talk in English they most often think the fruit they ate in Vietnam is called grapefruit in America, and Vietnamese are the main Southeast Asian group that has immigrated to America. Pomelos are also popular New Year festival fruits in the Canton province of China, where they are usually translated into English as "Chinese grapefruit". The early introduction of grapefruits into the American market place sort of superseded the awareness of pomelo, and so it has become the grapefruit that stuck and became the popular one. There are some interesting old stories from the 1930s about Americans encountering grapefruit for the first time and not knowing what to do with them. One housewife, when given a grapefruit, tried boiling it for two hours in water thinking it was like a cabbage.
So basically the existence of grapefruit has displaced pomelo from gaining popularity in the U.S., even though the two are not exactly the same. And all the exports of American grown grapefruit to other countries are displacing the sales and awareness of pomelo even there. Grapefruit is more practical for the U.S. citrus industry to grow than pomelo, because grapefruits have been bred to be more cold hardy than when they first appeared in Jamaica.
Another issue, most of the Asian pomelo varieties would require a lot of heat to fully ripen and, as hot as Southern California is, all the area within zone 10 is in proximity to the coast and still does not get quite enough heat throughout the year to become sweet enough. This has no doubt hindered the introduction of pomelo into California and explains why there is such little variety. All three of the pomelo varieties (2 of them actually being grapefruit-pomelo hybrids) commercially available in the state resulted from hybrids of 'Siamese Sweet', a very sweet but mostly flavorless pomelo variety, and were bred by the University of California research station in Riverside. Pomelos ripen well around Miami (Florida) though, and probably would in Brownsville (Texas) too, I would imagine.
>> One correction to add: I have heard of Chandler pomelos being grown in zone 9b but these are places that have a lot of heat (Texas, Florida)