[MIS01-P08] Quantitative Evaluation of Winter Daily Precipitation over Northern Japan
Keywords:APHRO_JP, winter precipitation, water resource
First, we validated and adjusted the interpolation bias. We created daily precipitation gridded data based on the distribution of the Automated Meteorological Data Acquisition System (AMeDAS) using hourly precipitation from the NHRCM (spatial distribution 2 km) and we compared it with precipitation from the NHRCM. We found that precipitation from the Mesh precipitation climatology which is used in APHRO_JP underestimated precipitation over mountain areas where rain-gauges were not set up. Because of this, APHRO_JP underestimated winter precipitation over mountain areas by approximately 3 mm/day. This daily precipitation bias over mountain areas was halved when we used climatology based on precipitation from the NHRCM instead of Mesh precipitation climatology.
Even with this adjustment, precipitation over mountain areas was still underestimated. This stemmed from the interpolation method used to predict rainfall, because the climatology indicate that the ratio of precipitation increases seasonally with elevation over mountain areas. This underestimation was controlled by an adjustment to the interpolation bias from the precipitation pattern detected by an Empirical Orthogonal Function (EOF) analysis.
On the basis of these validation results, we created daily precipitation gridded data based on the AMeDAS rain-gauge. We then applied the three adjustments from the climatology, precipitation pattern and precipitation undercatch and evaluated their contribution. We found that the annual precipitation over Japan was 2054 mm/year, which was an 18% increase on APHRO_JP. Furthermore, at the four dam catchments in Northern Japan, this adjusted precipitation explained 99% of the precipitation estimated by inflow from the dam catchments and evapotranspiration.
Finally, we measured the contribution of the systematic bias in winter precipitation in Northern Japan (138.5–141°E, 36.8–38.8°N). Precipitation was increased 11% by changing the climatology and increased 12% by adjusting for precipitation undercatch. The impact of the precipitation pattern adjustment was small. For extreme events, the adjustment effect was different. On average, in the four dam catchments, precipitation was increased 18% by the change of climatology 7% by adjustment for precipitation undercatch. However, these ratios were different at each dam, and in dams where the catchments were in high mountain regions, there was a much greater effect from a change in the climatology. Because there are few rain-gauges in high mountain regions, it important to change climatology used in APHRO_JP. Furthermore, our results indicate uncertainty in precipitation from internal variability when the precipitation pattern adjustment was not used. Our application of these adjustments to APHRO_JP reduced systematic bias and improved precipitation measurements.