How to jump to top of browser page

You could do it without javascript and simply use anchor tags? Then it would be accessible to those js free.

although as you are using modals, I assume you don't care about being js free. ;)

Without animation, you can use plain JS:

scroll(0,0)

With animation, check Nick's answer.

If you're using jQuery UI dialog, you could just style the modal to appear with the position fixed in the window so it doesn't pop-up out of view, negating the need to scroll. Otherwise,

var scrollTop = function() {
    window.scrollTo(0, 0);
};

should do the trick.

You can set the scrollTop, like this:

$('html,body').scrollTop(0);

Or if you want a little animation instead of a snap to the top:

$('html, body').animate({ scrollTop: 0 }, 'fast');