Analyzing the Impact of the Three Gorges Reservoir on Local Precipitation with TRMM Satellite Data

After Three Gorges Dam (TGD) was built in China in June 2003, the water level abruptly rose from 66 to 135 m. Its impact on regional precipitation is examined using NASA Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis (TMPA) 3B42 3-hourly data from 1998 to 2010. We hypothesize that the reservoir would reduce precipitation in its vicinity under "synoptically benign" conditions, due to a "lake breeze" effect on local circulation. Our assumption is that days with moderate rainfall in summer are likely to be “synoptically benign”. To test our hypothesis, first we analyzed daily composites of moderate precipitation (between 1 and 10 mm/day, averaged over the whole study area) of each July, both before and after the water level rise. We found that after the water level rise, the ratio of near-reservoir rainfall to whole-area rainfall is significantly reduced. To further support our hypothesis, we continued to investigate the diurnal characters of the moderate rainfall. The results show rainfall suppression near the reservoir mostly occurred in the afternoon, when largest temperature difference between the reservoir and its adjacent land surface is present. These findings suggest the “lake breeze” effect might be the mechanism for the observed change of moderate precipitation pattern. In contrast, no significant changes in the composite of heavy precipitation days (> 10 mm/day) are found, as these days are more likely to be dominated by synoptic-scale processes (e.g., the position and movement of the zonal rain belt). Overall, this analysis presents one of the most thorough and long-term analysis of TGD-hydroclimate relationships to date.

Keywords: Land Use/Cover Change, Climate Impact, TRMM, Three Gorges Reservoir