[flexcoders] Changing alpha for modal window?

[candysmate]

I've used 
_global.style.modalTransparency = 50;

to change the alpha of a modal window in Flash in the past. However,
I'm not sure how to do this in Flex 2.

I have created a popUp with this function:

private function growerPopup(): void
    {
      growerPopupDialog = PopUpManager.createPopUp(this,
dialogs.growerSearchDialog, true) as TitleWindow;       
      PopUpManager.centerPopUp(growerPopupDialog);
    }

How do I change the alpha for this popUp please?