Understanding ある as a 連体形

Question

The sentence

人間の背丈ほどもある雑草の中を探検すると蛇に出くわした。

translates to

I ran into a snake when I was exploring through weeds as tall as a person.

Question: What is ある connected to? Is it a 連体形 modifying 雑草, as in

ある雑草

If so, what purpose is it serving? It sounds like it's just saying "existing-weeds", which is pretty much the same thing as "weeds", no?

Answer 1

Yes that ある is in the 連体形, and 人間の背丈ほどもある is a relative clause that modifies 雑草. The "original" sentence is:

(その)雑草は(高さが)人間の背丈ほどもある。

The weeds are as tall as a human.
(literally: As for the weeds, the height is as much as a human's height.)

This ある may still seem tricky, but we use ある this way to express the weight or the size of something. You may temporarily forget the meaning of "to exist".

• この蛇は(長さが)3メートル(も)ある。
  This snake is 3 meters long.
• 彼は(体重が)80kg以上(も)ある。
  He weights more than 80 kg.

(も indicates the number is large. The が-marked subject like 体重 is typically omitted.)

And we can make relativized phrases like so:

• (長さが)3メートルある蛇
  a snake that is 3 meters long
• (体重が)80kg以上ある男
  a man who weighs more than 80 kg

Here's a relevant dictionary definition:

ある

❼《数値を伴って》あるものがそのような物理的な属性をもっている。そのような数量が認められる。「彼は体重が一〇〇キロもある」「塔の高さは五〇メートルある」
_{(Source: 大修館書店 明鏡国語辞典 第三版)}