date: Wed Feb 25 13:09:59 2004 from: Phil Jones subject: Re: Trends in DTR from ERA-40 to: Adrian.Simmons@ecmwf.int Adrian, A couple of things have come up that need rapid attention, so I should get to your draft on Friday and the weekend. If I go through and put my comments on my copy and then email this back - will that be OK ? I say this as I've got into the habit of adding comments to a word version. You might be using a different system as you've sent a pdf file. I think hard copy comments should work. Cheers Phil At 09:26 25/02/2004 +0000, you wrote: Phil The SSTs in the simulation were the same analyses as used in ERA-40. So the simulation can be regarded as an ERA-40 analysis without atmospheric, land-surface and ocean-wave observations, but still, effectively, using observations of SST. Obviously that provides quite a control. The analyses were from the Hadley Centre for the period up to November 1981 and from NCEP thereafter. The switch was made because NCEP provided higher time frequency. There is a report on this in an advanced stage of preparation. Best regards Adrian Phil Jones wrote: Adrian, I've been away, but I've printed off the draft paper and the recent set of figures. I will go through these when I've cleared a few pressing things in the next couple of days. A quick look at the figures seems to indicate that ERA-40 is much better than NCEP and a paper would be a useful addition to the literature. I will contact the people at NCDC about the max/min series and possible updates to nearer the present as well as a gridded dataset. I have a couple of quick questions: - the specified SST run, is this the same as ERA-40 but with SSTs just having the same annual cycle year on year? The proper ERA-40 run used SSTs as they were measured - a dataset developed at the HC. Cheers Phil At 13:08 24/02/2004 +0000, Adrian Simmons wrote: Phil I've now computed a set of monthly means of daily max and min temperatures from ERA-40. They are not based directly on any observations of Tmax and Tmin. Instead they are derived from maximum and minimum values computed during each of the six-hour background forecasts of the data assimilation system. The first attachment shows trends in the daily temperature range for the periods 1958-2001 and 1979-2001. This shows a reduction over time for North America (1958-2001 at least) and for eastern Europe and much of Asia. Values in the tropics must be treated with most caution as we know we have spurious trends in some aspects of the hydrological cycle which could well impact the daily temperature range. The trend over the US may be biased a bit by a too-dry troposphere from 58-63 caused by wrongly encoded humidity observations in one of the radiosonde datasets supplied to us - we picked this up too late to allow correction in ERA-40 production. The other plots show the trends in maximum and minimum temperatures for 1958-2001 and 1979-2001. Overall the short-range forecast trends are quite consistent with the trends for the daily mean analyses (Fig 7 in the document attached to yesterday's mail). The increase in daily temperature range over northern latitudes is seen to be associated with a larger rate of increase of minimum than maxiumum temperatures - at least we seem to have got the most basic feature right. The Tmax and Tmin trends for 1958 to 2001 also show that the almost certainly erroneous cooling trends seen in the daily mean analyses for Australia and much of tropical South America arise much more from a decrease over time in maximum temperatures than from a decrease in minimum temperatures. This suggests that the ERA-40 background forecasts (and "analyses") of daytime (rather than nightime) temperatures are the ones that are biased warm in the earlier years in which we assimilated little or no SYNOP data over these areas. Western Europe seems to show a slightly larger trend in maxiumum than minimum temperatures. Hotter summers and drier soils? It will be interesting to make more quantitative comparisons with the new data from NCDC. Best regards Adrian -- -------------------------------------------------- Adrian Simmons Head of Data Division 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 ---------------------------------------------------------------------------- -- -------------------------------------------------- Adrian Simmons Head of Data Division 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 ----------------------------------------------------------------------------