Monday, August 15, 2011

Can nasty dewpoints be useful?

Have you ever sat around when its 95 over 82, with 6000 J/kg of CAPE laughing at you, frustrated why you can't buy a cumulus cloud, much less a thunderstorm? Well, I have. And since we had quite a few of those days this summer, I did a bit of thinking. Two questions:

1. Why do you never see temperatures well above 100 with dewpoints of ~78F+? Note that all the extremely hot days in the southern plains usual see 100F+ temperatures with dewpoints 70F or lower.
2. In the summer, why is it that you can get very high dewpoints without even a cloud, while other areas with lower dewpoints see storms?

To answer both of these, I'll use a simple equation that explains dewpoint tendency within the boundary layer:

Now let's butcher this equation...

The first term is the dewpoint tendency, because we're talking about those nasty days where the dewpoint stayed at 81 or whatever all day, term (1) is 0. Also, many of those days were fairly calm, so the horizontal advection of dewpoint, term 2, is negligible. In fact, if we're trying to explain 80+ dewpoints, then this term can only be detrimental to us, because dewpoints all around are likely equal or lower than ours, so this can only be a sink term. Thus, all this simplifies down to 2 things, upward motion and plant breathing. Plants consistently add moisture to the air, so term (3) turns out to be the most efficient way to lower the dewpoint on a calm summer day...

Finally, this brings me to the conclusion: the reason why we don't see thunderstorms with 80+ dewpoints is because of the dewpoints themselves! They are telling us that the cap is incredibly strong so there's hardly any mixing going on with the free atmosphere (even though the boundary layer itself is growing in height). This may also explain why it's hard to get 100+ temperatures with 80+ dewpoints. In order to get 100+, there almost has to be mixing down of the high-theta air that resides within the cap...

Anyways, that's two cents for the day.