The Atmospheric Sciences Friday colloquium yesterday was delivered as
a Ph.D. defense by Angeline Prendergast, titled “The Atmospheric
Energy Constraint on Precipitation Change”. I’ve never interacted
with her directly, but at some remove perceive her as active and
bright. She has published two or more papers as a grad student, and
will shortly be submitting her third.
Yesterday’s talk reports studies of a family of climate models’
[CMIPS5] predictions of global, temperature-affected changes in
rainfall amounts, rates, and their probability distributions, and
supporting observations in the El Nino bands across the equatorial
Pacific. [Quit here, if you’re bored.]
First, some background:
The vapor pressure [and water vapor content] in the air above the
oceans and moist continents increases with temperatures by about 7% /
degK. The averaged rainfall, however, increases by only 2% / degK.
What causes the difference?
Rainfall is stimulated by rising air, which cools, condensing water
vapor to form clouds and rain. Rising air is driven by complex
effects of converging lower level winds, and radiation balance
affected by absorption and re-emission by water vapor and clouds.
This is the core stuff of atmospheric dynamics and the bread and
butter of atmospheric sciences. Climate models deal with all of this.
A word of caution, however. Both the horizontal and vertical scales
of much of that rising air are less than the models can resolve.
Various ‘parameterizations’ [that is, fudge factors .. not all of them
unreasonable] are inserted into those models to bring their outputs
closer to what is observed. Closer, but not all that close, when
compared with observations. Depending upon taste, the models do
pretty well with precipitation, or not so well.
OK. Back to the presentation.
My ‘carry away’ from Angie’s talk is that the models [some of them]
predict increased frequency and amount of heavy rain and diminished
frequency of moderate rain. She did not discuss changes in the
lateral distributions of rainfall, perhaps the most interesting effect
expected from global warming. She did not discuss effects of micro-
physics, that is, cloud condensation nucleii and ice-forming nucleii.
During question time after her talk, I asked whether the models
include microphysics, and understood her to say ‘yes’. I’m doubtful
of this, however.
One splendid tid-bit: the highest recorded one-day’s rain is 1.75
meters [!!!], in New Calidonia. Wow.
Cheers,
Halstead