cc: Phil Jones date: Wed, 24 Sep 2008 12:33:56 +0100 from: Adrian Simmons subject: Re: Downward trend in relative humidity over land? to: kate.willett@metoffice.gov.uk Thanks for this, Kate. This will need some digging into, as for Europe I see a higher RH anomaly in the first half of the period for ERA than for HadCRUH, when I do the comparison by upscaling and sampling ERA to match HadCRUH. This needs to be understood. The agreement for RH is not as good as for q. What remains a solid result is that RH is notably low in the last few years of the ERA period (which currently ends in July 2006 - two more years to go). First challenge (after finding some spare time) will be to work out exactly how RH has been calculated. I take the archived 2m T and Td for ERA and put it into a library routine to produce RH. That routine was written more than thirty years ago, and I have to find the documentation. Don't hold your breath, but I'll let you know when I've managed to make some progress. Best regards Adrian kate.willett@metoffice.gov.uk wrote: > Over land I found non-significant and very small decadal trends, with > the Northern Hemisphere trend actually being slightly positive. (G = > -0.03, NH = 0.07, T = -0.10 and SH = -0.34 - SH data is very sparse and > likely of low quality). In contrast, the Marine data showed very > significant negative trends but I'm highly suspicious about the pre-1982 > data which has a strong positive bias relative to the rest of the > timeseries. > > The climatology period and possibly the way that anomalies are obtained > differ for HadCRUH (1974 to 2003) although I'm guessing this should have > little effect. The scale on my plots is large and so its perhaps > difficult to see any trends and I agree that the additional 3 years in > the ERA data bring make the negative trend clearer. I have had problems > with different trend fitting techniques giving more/less attention to > end points of the series and so giving very different trends. Ideally it > would be nice if ERA and HadCRUH were in good agreement but I think it > may be realistic that they are not given the variability in RH over land > and the likely very different ways that RH is derived for both. > > I like the idea of a rough sanity check to see if the q and RH changes > in ERA are consistent. For HadCRUH I did a very rough version of > percentage change in q for 1K increase in T using the idea that for > constant RH a ~7% increase in q would be expected. Results suggest > global land q increases are consistent with constant RH (~7%), NH land q > increases suggest a slight increase in RH (~8 %) and Tropics land q > increase suggest a decreasing RH (~5.5 %). > > Not sure if all the above is helpful or just rambling. I think > presenting the plots at Boulder is a good idea though. > > Kate > > Dr Kate Willett Climate Research Scientist > Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB > tel. +44 1392 884288 fax +44 1392 885681 (mark for my attention) > www.metoffice.gov.uk/hadobs -- -------------------------------------------------- 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 --------------------------------------------------