cc: Eric Wood , Kevin Trenberth , Michael Roderick , Phil Jones date: Tue, 18 Mar 2008 11:16:03 -0600 from: Aiguo Dai subject: Your JC paper on drought to: justin@princeton.edu Dear Dr. Sheffield and Eric, I enjoyed reading your recent J Clim paper (Feb. 2008) uisng VIC model-simulated soil moiture to study trends and variability in global drought during 1950-2000. I noticed that your soil moisture trend map (Fig.1 in your paper) and precip trend map (Fig. 3 from CRU TS2.0) are very similar. In other words, the soild moisture trends are largely determined by the precip forcing data from CRU TS2.0. As reported in IPCC AR4 (chapter 3) and shown in the attached plots, there are substantial differences in the several precip data sets over land currently available. The large differences come from Canada, Brazil, and central Africa, where raingauges are sparse (see atthached 3rd file--the station count is the number of stations within a gridding radius of ~400km? CRU TS2.10 is an updated version of CRU TS2.0) and thus the differences in gridding/analysis methods play a bigger role. The increasing precip and soil moisture (which appears to be affected little by any changes in evaporation in your study) over northern Canada shown by your Fig.3 appears to be at odds with reported decreases in Canadian streamflow, also shown in the 4th attached Word file (taken from our new manuscript on global discharge trends. Note the decreasing Canadian runoff and precip in this plot, and the negative but insignificant streamflow trend for Amazon). The final attached plot shows that the trend in the CLM3-simulated soil moisture generally follows that of PDSI. Note that in our CLM3 forcing data (Qian et al. 2006, JC), solar radiation changes were estiamted using cloud cover data. In summary, I felt that uncertainties in current precip and other atmospheric forcing data are large. Combined with differences in models, they make current assessments of historical drought quite uncertain. This has prevented us from writing up the CLM3-simualted soil moisture changes for publication. Best regards, Aiguo Dai -- Aiguo Dai, Scientist Email: adai@ucar.edu Climate & Global Dynamics Division Phone: 303-497-1357 National Center for Atmospheric Research Fax : 303-497-1333 P.O. Box 3000, Boulder, CO 80307, USA www.cgd.ucar.edu/cas/adai/ Street Address: 1850 Table Mesa Drive, Boulder, CO 80305, USA Attachment Converted: "c:\eudora\attach\precip.trendmap.1951-2000.p1.gif" Attachment Converted: "c:\eudora\attach\precip.trendmap.1951-2000.p2.gif" Attachment Converted: "c:\eudora\attach\pre-stn-1950-60-70-75-80-90-95-00-Jan.gif" Attachment Converted: "c:\eudora\attach\Fig9_from_Dai_etal.doc" Attachment Converted: "c:\eudora\attach\pdsi.vs.soilMoisture-trend-fromDaiAMSTalk2005.ppt"