Is anyone interested in starting a separate thread on water vapor and cloud feedbacks? While it is of some relevance to the hurricane/global warming topic, the relevance is indirect and this topic certainly has enough scientific meat for its own thread (provided people are sufficiently interested.
Comment by Judith Curry “¢’¬? 17 September 2006 @ 9:39 am | Edit This
Your wish is my command.
There are a number of topics in this area that intrigue me. In the mid-1990s, Ramanathan and others wrote about “anomalous atmospheric absorption” in which they argued that atmospheric absorption of near infrared solar radiation was up to 25 wm-2 greater than modeled (or in Trenberth’s standard figures). IPCC TAR used TRenberth, mentioning Ramanathan’s concerns only in passing without analyzing or explaining them. IPCC 4AR makes no reference to the issue at all as far as I could tell (I requested an explanation but probably won’t get one.)
A number of recent articles by specialists in water vapor have proposed significantly higher NIR absorption by water vapor than in climate models. Belmiloud and Tennison have written scathingly about this. There was an interesting clerical error in NIR absorption levels in HITRAN-96 used in all IPCC TAR models – the error in wm-2 was greater than 2x CO2. I’ve seen some discussion that observed NIR absorption can be (sort of) explained if cloud droplet sizes are assumed to be three times larger than generally believed. (I’ll check this reference).
I get the impression that recent work mostly attributes “anomalous” absorption to aerosols and, to some extent, I get the feeling that aerosols and higher parameterizations for NIR absorption by water vapor/liquid/clouds are competing mechanisms. Intuitively it seems to me that this would result in quite different feedback consequences – higher NIR absorption from water cycle would be a negative feedback whereas aerosols are not.
The recent discussion at realclimate [link] was also interesting – I was very struck by Held’s surprise that cloud feedbacks were all uniformly strongly positive – Held said that he expected the feedbacks to be evenly distributed between positive and negative. It’s a bit disquieting that such a leading expert should be surprised by this.