date: Mon Sep 22 14:12:56 2008 from: Phil Jones subject: Re: Downward trend in relative humidity over land? to: Adrian.Simmons@ecmwf.int, Kate Willett Adrian, Thanks for the plots. Kate didn't see much happening with RH, but the 1-1.5% reduction is probably too small to be that clear. It may also not be statistically significant either. Kate's series finished in 2003, so the last 3 years may be a factor as you say. The increase in q wasn't quite as large as expected from temperature, but the thought at the time was that this was due to the air over land not being saturated as it would be over the ocean. Thinking aloud, we could do a simple back of the envelope calculation and see if the slight reduction in RH explains the difference between the expected from T changes in q and what Kate sees with the real q obs. These are probably worth showing, to see if anybody next week has any more thoughts. I guess you can't produce similar series for q? Kate has received the proofs. I'll attach these. The password you need to open this is JCLI2274. JCLI and then 2274. Kate is also putting together another paper. Glad to hear you'll have some time later in the year. Cheers Phil At 11:58 22/09/2008, Adrian Simmons wrote: Phil, Kate I'm afraid I've been hopelessly busy with my day job and standard GCOS matters of late, so the surface humidity stuff was put to one side until yesterday evening. I'm in Boulder next week, and will be giving a talk on reanalysis at NOAA/ESRL on the Friday. I thought I would include the surface humidity comparisons between HADCRUH and ERA, and decided last night to set off the data retrieval needed to plot relative humidity time series from ERA (-40 and -Interim versions). I've plotted the results this morning (for all land points, not just for the grid boxes where there are HADCRUH data) and attach them for selected areas. I hope the plots are self-explanatory. Time series are adjusted to give zero mean value for the overlap period between ERA-40 and ERA-Interim (1989-2001). These plots give a clear impression of a downward trend in relative humidity (cf AR4, where it is stated "trends are uncertain, but suggest that it [relative humidity] has remained the same overall" - perhaps you wrote this Phil). This may not be that inconsistent with earlier evidence - Kate's thesis summary does indicate a trend in specific humidity that is slightly less than that expected for constant relative humidity, although you state that trends in RH are statistically indistinguishable from zero, and the indication of a downward trend in the ERA results is strengthened by the low values analysed for recent years. What do you make of this? I've used a quite standard set of programs to do the calculations, so it's unlikely (but certainly not impossible) that I've made a mistake. I would be wary about the quality of the reanalysis for the tropics, but reasonably confident about it for much of the extratropics. I'll not have much time to work on this for the next five or six weeks, but things may get easier in Nov/Dec, and by then ERA-Interim will be close to the present day. Best regards Adrian -- -------------------------------------------------- Adrian Simmons European Centre for Medium-Range Weather Forecasts Shinfield Park, Reading, RG2 9AX, UK Phone: +44 118 949 9700 Fax: +44 118 986 9450 -------------------------------------------------- Prof. Phil Jones Climatic Research Unit Telephone +44 (0) 1603 592090 School of Environmental Sciences Fax +44 (0) 1603 507784 University of East Anglia Norwich Email p.jones@uea.ac.uk NR4 7TJ UK ----------------------------------------------------------------------------