Future projection of precipitation extremes with 20km-mesh Atmospheric General Circulation Model

Future change in precipitation extremes were analyzed by the global warming projection experiments conducted with a 20km horizontal grid atmospheric general circulation model (MRI-AGCM). This high-resolution model enabled to reproduce precipitation extremes which are difficult to be simulated by a low resolution model and to project those future changes in small spatial scale. Two simulations, control run and global warming run, were conducted. The latter is realized by the "time-slice" method forced by the observed climatological sea surface temperature (SST) plus anomalies around the year 2090, obtained from the atmosphere-ocean coupled model simulations with MRI-CGCM2.3 (Yukimoto et al., 2005).The CO2 concentration in the model is based on the IPCC Special Report on Emission Scenario (SRES) A1B. High-resolution model is expected to excel in ability to reproduce precipitation extremes. Because such a heavy precipitation is local phenomenon, therefore they could be weaken its magnitude or disappear in a coarse resolution model. Compared with the observation (TRMM 3B42) and control run, the model reproduces very well the PDF (probably density function) of daily precipitation and extremes indices (explained later) in the global. Precipitation extremes are diagnosed by the "extremes indices" proposed by Frich et al (2002). Five extremes indices are calculated with daily precipitation; CDD (The annual maximum number of consecutive dry days), R10 (the number of days in a year with precipitation greater than 10mm/day), R5d (the annual maximum consecutive 5-day precipitation total), SDII (simple daily intensity index) and R95t (fraction of annual total precipitation due to extreme wetdays exceeding 95th percentile of AJ-run). CDD is used as a "dry index", and the other are used as "heavy precipitation indices". In the United States, dry index increases remarkably in the West Coast and around the Rocky Mountain; Oregon, Washington, California, Idaho, Utah and Nevada. Heavy precipitation indices increase in large in those areas between the Appalachian Mountain and the Great Lakes, where big rivers, such as Mississippi, Missouri and Tennessee run through. In a lot of places, changes in extremes indices are resemble to that of annual precipitation; dry index decreases and heavy precipitation indices increase in the area where annual precipitation increase. However, in such states as Virginia or North Carolina, changes in extremes indices are opposite to that of annual precipitation. These geographically detailed information have never been derived by general circulation model, though validity of these results should be further studied. Meteorological reasons of changes in extremes are being analyzed now and they will be shown on the day of presentation.