Interpolation Methods of China Daily Precipitation Data

Precipitation is one of the most important meteorological factors. Its spatial and temporal distribution is critical in climate diagnostics, numerical models verifications and hydrological models initiation. Substantial progress has been made in the last two decades in quantitatively documenting global precipitation. The development and operation of precipitation analysis products over China are relatively lagging behind other countries. Through the cooperation with the Climate Prediction Center (CPC) affiliated to National Oceanic and Atmospheric Administration (NOAA), the real time operational system of daily precipitation analysis over China is established in National Meteorological Information Center (NMIC). Based on the daily precipitation observations of over 2419 gauges, a system with the resolution of 0.5°×0.5° is established. It is consisted of four modules: Data obtaining, quality controlling, data processing and products serving. The kernel interpolation algorithm of this system is introduced from the CPC known as the climatological optimal interpolation (OI) method which can reduce the analysis error substantially arising from the large spatial discontinuity of precipitation. Daily climatology is defined for each station as the summation of the first 6 harmonics for the 365 calendar day time series of the mean daily precipitation, and then climatology fields are created by interpolating the truncated station climatology through the algorithm of Shepard. These fields are then adjusted by the PRISM (Parameter elevation Regressions on Independent Slopes Model) monthly precipitation climatology data, and the temporal variation patterns in the original daily climatology time series which also reflect orographic effects reasonably are retained. Analyzed fields of the ratio between daily precipitation and climatology are derived by interpolating the corresponding station values through the OI technique. Analyses of total daily precipitation are finally calculated by multiplying the daily climatology with the daily ratio. This system runs stably with reasonable products and users can get the products in three formats after 09:20 everyday on CDC website. It is an integral part of China real time and fine resolution precipitation analyses system under construction. Further improvements of the gauge based analysis will be implemented to correct the bias of wind effects. Collaborations in improving this system by refining the algorithm and merging with the satellite based precipitation products are welcome.