Kids naturally do a lot of screaming, crying, and other annoying behaviors...unless you teach them better and more effective strategies for expressing themselves. I don't think there's anything wrong with a little time alone if she's totally out of control and needs some chill time to regroup, but for most things, the key is in prevention, not in how you react after the fact. Behavior is a major form of communication--so once you figure out what she's trying to accomplish, and teach her a better way of doing that, you'll get less of the screaming and crying.
A few typical causes of whiny/screamy behavior (in no particular order):
1. Child needs help with something she can't do for herself. (screams out of frustration)
2. Child is trying to communicate something and doesn't feel understood. (screams out of frustration)
3. Transitions (child is doing one thing but it's time to do something else) (screams out of frustration, anger)
4. Child wants something but can't have it.
Some ways to head off the screaming meltdowns:
1. Do you need some help (with that)?
1. Suggest another alternative
1. Do you need help or do you want to do it yourself?
1. When I'm finished with...I'll help you do that.
2. I'm sorry, I didn't understand that.
2. Are you asking me...?
2. Can you say that again? (Use your words)
3. Prepare child ahead of time (we're leaving the house in 5 minutes, it's time to get ready to go home..., reminders of what happens next...always add some extra time into major transitions so the stakes are lower if it takes longer than expected. Countdowns and timers work great for difficult transitions)
4. I'm sorry, but that's not yours... you can have this.
4. I'm sorry, but that's not safe. Let's do this instead...
4. I know you're mad, but you can't...
* All purpose: "I'll listen to you when you aren't screaming", "I can't understand you when you're screaming like that." If it gets a bit much, just walk away, that often stops the screaming real fast (no audience takes the fun out of it)
Basically the goal is to replace the screaming/crying with more socially acceptable methods of communication. So the more you encourage appropriate and effective means of communication, the less screaming and crying there will be.
It's always good to follow up with behavioral information when the child is calm:
I like it when you... (ask nicely, ask for help, use nice words, whatever the child did that was not screaming/crying)
When you...(ask nicely, ask for help, use nice words, whatever the child did that was not screaming/crying) it makes me happy.
Next time when you want to...try (asking nicely, asking for help, using nice words)
When you're debriefing after an incident, it's better to emphasize the behavior you DO want, rather than the behavior you DON'T want.
Oh, and don't reward the screaming/crying by giving in. Ever. That gives her NO incentive to change.