cc: d.viner@uea.ac.uk, aidan.burton@ncl.ac.uk, buishand@knmi.nl date: Mon, 18 Sep 2000 11:47:50 +0100 from: "Chris Kilsby" subject: relative humidity bias to: jmmurphy@meto.gov.uk, phillip.reid@uea.ac.uk, p.jones@uea.ac.uk Dear James and Wrincle colleagues, we are rather concerned about the RH bias in the HadCM2/3 output. My concern is that it may be caused by a feature of the parameterizations, and we are not performing a fair comparison wiuth the NCEP data. Adri's comments below lend weight to this. Any comments on how we should proceed ? Chris ------- Forwarded Message Follows ------- From: "Adri Buishand " To: "Chris Kilsby" Date sent: Mon, 18 Sep 2000 12:21:25 +0000 Subject: relative humidity bias Priority: normal Dear Chris, I had a short discussion with Erik van Meijgaard about the relative humidity bias in the HADCM simulations. The HADCM2 simulation shows a cold bias at 700 hPa. A temperature bias of 2 degrees has already an effect of 15% on the saturated vapour pressure and may thus substantially contribute to the bias in relative humidity. There is, however, little temperature bias in the winter season. Erik told me that GCMs work with the relative humidity over ice if the temperature is below zero. Have these relative humidities been converted to relative humidities over water? The difference between the saturated vapour pressure over water and ice is about 10% if the temperature is - 10 degrees (average winter temperature at 700 hPa for the grid-point considered). Wrong parametrizations were also suggested as causes for the relative humidity bias. In the old low-resolution UKMO GCM, the zonally averaged 700 hPa relative humidity at 50 N is about 70% (Fig. 1b in Mitchell and Ingram, J. Climate, 5(1992), 5-21), which is comparable with that in the HADCM2 simulation. We do not have the relative humidity in the ECHAM/OPYC3 simulations. Over the Netherlands and Northern Germany there is a bias in the specific humidity of about 10%, whereas the bias in 700hPa temperature is less than 1 degree. We have the impression that it looks somewhat better than the HADCM simulations. Best regards, Adri Chris Kilsby Lecturer in Hydrology Water Resource Systems Research Laboratory Dept of Civil Engineering, University of Newcastle upon Tyne NE1 7RU United Kingdom Tel + 191-222-5614 Fax + 191-222-6669 http://www.staff.ncl.ac.uk/c.g.kilsby