Evaluation of upper tropospheric moisture in the GEOS5CCM and MERRA reanalyses and implications for contrail formation Richard Damoah1, H. B. Selkirk1, A. R. Douglass2, L. Oman2, L. Ott2, S. Pawson3, and M. Manyin3 1Universities Space Research Association (GESTAR/NASA), Greenbelt MD 2NASA Goddard Space Flight Center, Greenbelt, MD 3System Science and Applications, Inc. Lanham, MD INTRODUCTION The relative humidity of the upper troposphere plays a primary role in the formation of contrails and cirrus induced from contrails. However, the Inter-governmental Panel on Climate Change (IPCC) has indicated that there is a low level of scientific understanding of the radiative effect of cirrus-induced from contrails. This in large part is due to the challenges in modeling upper tropospheric water vapor. In order to reduce uncertainties in the prediction of the climate impact of contrails and contrail-induced cirrus, we have examined probability density functions (pdfs) of relative humidity with respect to ice (RHI) in two critical regions of the atmosphere: the tropics (30S – 30N, 180W – 180 E) and the northern mid-latitude (40N – 60N, 0 – 75W), using data from MOZAIC, MLS, MERRA and GEOS-5. APPROACH In our analysis we compare fraction of RHI from MOZAIC (in the northern mid-latitude at 250 hPa) and MLS (tropics at 215 hPa) exceeding ice supersaturation (>100 %) to the fraction of GEOS5CCM grid boxes meeting Schmidt-Appleman criteria for contrail formation. Since RHI is not an end product of MOZAIC, MERRA and GEOS5CCM, we computed the RHI from water vapor and temperature fields using Goff-Gratch function. The fraction of GEOS5CCM grid boxes (2° latitude by 2.5° longitude) for contrails formation was computed according to Burkhardt et al. (2008). DATA RHI pdfs are derived from NASA’s GEOS-5 atmospheric GCM, the foundation of the GEOSCCM chemistry-climate model, from observations and from re-analyses. For observational data sets we used 5 years (2005-2009) of measurement data from the Aura’s Microwave Limb Sounder (MLS) and 14 years (1995-2008) of aircraft data from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) project. We used the same spans of reanalysis data from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). Likewise, we use the same number of years of output from a multi-year integration of GEOSCCM. Midlats: MOZAIC [40-60N, 0-75 W] 250 hPa layer Tropics: MLS [30S-30N] 215 hPa In this distribution of individual MOZAICobservations from the North Atlantic region, descending dry air peaking at 15 % RHI is clearly distinguished from a long tail of moist air that flattens out from the mean (59%) to the ice supersaturation mark (100 %). The number at the lower right corner represents the counts equal or greater than the ice supersaturation. MLS PDF of RHI at 215 hPa for 5 years (2005-2009) measurement data (black) and 2 years (red) data in the tropics (30S-30N, 180W-180E). Similar to MOZAIC in the mid-latitudes, the MLS distribution also shows a peak around 15 % RHI but a tropical RHI mean of 46 %. Midlats: MERRA [40-60N, 0-75 W] 250 hPa layer The bimodality of the MOZAIC data is not resolved on the 2°x2.5° grid of the MERRA reanalysis data here. Sampling only gridboxes with MERRA data, THE MERRA mean RHI is some 9% dryer than MOZAIC. GEOS-5 (red) and MERRA (black) PDFs subsampled at MLS grid points in the tropics. MERRA and model distributions on average are moister than MLS by 10 % and 14 %, respectively. Contrary to the MERRA, the CCM shows two modes, a dry mode which coincides with the single mode of the MERRA distribution and a moist mode peaks at 80 % RHI. Tropics: GEOS-5 and MERRA [30S-30N] 215 hPa Midlats: GEOS-5 [40-60N, 0-75 W] 250 hPa layer RHI from 14 years of the 26-year GEOSCCM control run using all grid points on the 2° x 2.5° CCM grid (black) in the North Atlantic region and points subsampled at MOZAIC points (red). The distribution is similar to the MERRA reanalysis however, the 60 % mean is 9 % more moist than MERRA but comparable to the MOZAIC mean. CONCLUSION: In the northern mid-latitudes at 250 hPa, the CCM shows approximately similar RHI mean as MOZAIC but 9 % more moist than MERRA. 20% of MOZAIC data points exceeded ice supersaturation compared to 15% of the CCM grid points that meet the criteria for contrail formation. At 215 hPa in the tropics, the CCM and MERRA were 14 % and 9 % wetter than MLS, respectively. MLS had 26% of its measurement data exceeding ice supersaturation compared to 20% of CCM grid points that fall within the condition for contrail formation. Acknowledgments: We are grateful to NASA and FAA for their financial support through ACMAP and ACCRI (DTRT57-09-20030) projects, respectively.