An Analysis of Daily Precipitation over East Asia:Current Status and Future Improvements*Pingping Xie1, Akiyo Yatagai2, Mingyue Chen1,Tadahiro Hayasaka2, Yoshihiro Fukushima2, and Changming Liu3
(1: NOAA/NWS/NCEP, USA; 2: RIHN, Japan; 3: CAS/IG, China)* NOAA/NWS/NCEP Climate Prediction Center,5200 Auth Road, Camp Springs, MD 20746, USA.
E-Mail: Pingping.Xie@noaa.gov
Abstract
A new gauge-based data set of daily precipitation has been created on a 0.5olat/lon grid over the East Asia for a 20-year
period from 1978 to 1997 for meteorological and hydrological applications.
Keyword: precipitation, daily, East Asia, gauge.

1. Introduction
As part of the Yellow River Project, a suite of daily
precipitation analyses products are being constructed forpotential applications in meteorological, hydrological andwater resources research. The product suite, designed tobest take advantage of the available information to meetthe needs for various programs, is composed of three major
components: i.e. 1) a gauge-based analysis on a 0.5olat/lon
grid over the East Asia (65oE-155oE; 5oN-60oN) for anextended period from 1978 to the present (the BaseProduct); 2) a gauge-based analysis on 0.1olat/lon grid overthe Yellow River Basin (the Derived Product); and 3) asatellite-gauge merged analysis on 0.25olat/lon grid overthe East Asia over both land and ocean for a recent period(The Merged Product).
The gauge-based analyses are the mainstay of theproducts, both for their extended temporal coverage and fortheir role in determining the magnitude of the precipitation
fields in the merged analysis. As of October, 2004, thegauge-based analyses have been created for a 20-year
period from 1978 to 1997 and the prototype algorithm for
combining the satellite estimates and the gauge
observations has been developed.

precipitation climatology are then defined by interpolating
the station climatology and by adjusting it against thePRISM (Daily et al. 1994) monthly climatology over Chinaand Mongolia and against the PREC/L (Chen et al. 2002)
elsewhere. The purpose of this adjustment is to better
represent the orographic effects in precipitation that are notaccounted for in the interpolation of the stationclimatology.
The second step of the algorithm involves thedefinition of an analyzed field of ratio of daily precipitation
to daily climatology. This is done by interpolating thecorresponding station values, defined as the ratio of daily
observation at a station to the daily climatology at the gridbox at the gauge location, using the Optimal Interpolation(OI) algorithm.
In the final step, the analysis of total daily
precipitation is calculated by multiplying the analyzed
daily climatology with the ratio.

In this paper, we describe the construction of thegauge-based analyses and their quantitative accuracy.

In creating the products, the analysis is first produced
on a 0.05olat/lon grid over the entire East Asia domain.
The Base Product and the Derived Product are thencomputed by averaging the values at the 0.05olat/lon
resolution. A gauge-based analysis on 0.25olat/lon is also
created for use as input to the Merged Product, ensuring
quantitative consistency among the various products.

2. Methodology

3. Gauge Data

The overall strategy for constructing the gauge-based
daily precipitation analyses is a modification of Chen et al.
(2002), which was originally designed for interpolation ofmonthly precipitation over the global land areas.

Station observations of daily precipitation from three
individual data sets are used to construct the gauge-based
analyses over the East Asia. These are the Global
Telecommunication System (GTS) daily summary files
archived by the NOAA Climate Prediction Center (CPC)for a period from 1977 to the present; a personal collectionof Chinese daily observations (CHN) at over 700 stations
for a period from 1971 to the present; and the daily gaugedata at over ~1000 hydrological stations from the Chinese
Yellow River Commission (YRC) for a period from 1930S
to 1997. Since most of the GTS stations are included in theCHN data set, only daily observations from CHN and YRC
data sets are used inside China, while the GTS gauge data

The creation of the daily precipitation analysis isconducted in three steps. First, a gridded analysis of daily
precipitation climatology is created for each of the 365
calendar days. For this purpose, daily precipitation
climatology are defined for all stations with 90% or higher
reporting rates by accumulating the first 6 harmoniccomponents of the 365-day time series of the 20-year(1978-1997) mean values. Analyzed fields of daily

are used over the regions outside China. Combined,observations of daily precipitation at over 3000 stations areavailable over the target domain. Fig.1 shows gauge
locations from the individual and the combined data sets.
Reasonable gauge coverage is available over most of theland areas of the East Asia domain, while network density
is very high along the Yellow River, making it possible tocreate a high resolution precipitation analysis with reliable
quality over the region.

5. Quantitative Accuracy of the Analyses
Cross-validation tests are conducted to examine thequantitative accuracy of the gauge-based analyses. For
this purpose, daily precipitation observations at 10%
randomly selected stations are withdrawn and the daily
data over the remaining 90% stations are used to create thegauge-based analyses as described in section 2. This
process is repeated for 10 times so that each station iswithdrawn once. The withdrawn gauge observations arethen compared with analyzed daily precipitation at the0.05olat/lon grid box where the gauge is located. Table 1
presents the comparison results for a 365-day period of
1997.
The biases are very small for all stations,indicating that the daily analysis has very good overallmagnitude agreements with the observations.
Thecorrelation between the analyzed daily precipitation andthe withdrawn (independent) gauge observations is 0.504
for GTS stations which are sparsely distributed, while itreaches 0.860 for YRC stations that are very denselydistributed.
Overall, the correlation for all stations
combined is 0.594, suggesting good performance of theanalysis in representing variations of daily precipitation ona spatial scale of 0.05olat/lon.
Table 1: Cross-Validation Results for 1997

Fig.1:oNumber of gauges in each 0.5 lat/lon grid box
from the individual and combined data sets.

4. Gauge-Based Analyses
As of October, 2004, the gauge-based analyses of daily
precipitation have been generated for a 20-year period
from 1978 to 1997. Presented in fig.2 is an animation of
the daily precipitation maps for a 31-day period from July
1 – 31, 1997. Day-to-Day variations of precipitation are
well captured in our gauge-based products.

Fig.2:An animation of daily precipitation maps
(mm/day) for a 31-day period from July 1 – 31,
1997. This animation is based on the Base
Product on 0.5olat/lon resolution.

Stations

Mean(mm)Bias(mm)

RMSE
(mm)

Corr.

ALL
GTS
CHN
YRC

1.972
2.590
2.211
0.913

0.036
0.086
-0.0090.005

7.194
10.122
5.801
2.131

0.594
0.504
0.716
0.860

Dependence of the performance of the gauge-based
analyses on gauge-network density can be further exploredby comparing the statistics for grid boxes with various
network densities.
Fig.3 presents the scatter plots
between the analysis – gauge observation correlation andthe distance to the closest gauge location. It is very clearthat the correlation, or the quality of the analysis, improvesas the gauge network becomes denser.
One of the areas with relatively poor gauge coverage is
the South East Asia. To further investigate how the current
version of our gauge-based analysis may be improved byincluding more station observations, a variation of theanalysis is constructed for the 365 days of 1998 using GTS,
CHN and gauge observations at ~1000 additional stations
collected by the GAME project. Fig.4 shows the maps ofdaily precipitation and gauge locations for products withand without the GAME gauge data. Derived from gauge
observations of GTS and CHN stations, the current versionof the analysis (right side) missed some important featuresof precipitation over Nepal, Myanmar and Thailand where
the GTS gauge network is too sparse to represent thespatial distribution patterns of daily precipitation.
Including additional gauge over the region greatly
improves the quality of the analysis over the region.

The analysis is capable of representing precipitation
variations with good quality over most of the regions andits quality depends primarily on the gauge-network density.
A variation of the analysis is created for 1998 using
additional gauge data collected by the GAME project andthe results showed that including more gauges in thedata-sparse areas such as the Southeast Asia will greatly
improve the quality of our gauge-based analysis.
The Base Product of daily precipitation described in thispaper is available to all interested scientists. We welcome
collaborations of any form in improving this analysis byrefining the algorithm and including more gauge data.

References

Fig 3:

Scatter plots between the analysis – gauge
observation correlation and the distance to theclosest gauge.

Fig.4:

Maps of daily precipitation (top, mm/day) andnumber of gauges in a 0.5olat/lon grid box(bottom) for the analyses with and without
additional GAME gauge data.

6.

Summary

An analysis of gauge-based analysis of daily
precipitation has been constructed on a 0.5olat/lon grid fora 20-year period from 1978 to 1997 by interpolating
observations from over 3000 stations collected in three
individual data sets.

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