The Effect of Runoff Scheme Modification on the Summer
Rainfall Simulation in East Asia
SHI Xueli1 , LIU Yiming1 , YANG Hongwei2 , XIE Zhenghui2（1: National Climate Center, China Meteorological Center, Beijing, China,
2: LASG, Institute of Atmospheric Physics, Chinese Academy of Science)e-mail: shixl@cma.gov.cn
Abstract
The runoff generation mechanism in the Biosphere-Atmosphere Transfer Scheme (BATS)
has been modified in the regional climate model RegCM_NCC. The difference between themodified and original runoff scheme in BATS is that the former accounts for the simultaneous
happening of the two surface runoff generation mechanisms.
The abnormal heavy rainfall process over the Yangtze River valley in summer 1998 has
simulated with the RegCM_NCC by using the modified and original runoff scheme. Primary
results have shown that by using the modified runoff scheme, the fraction of runoff in thesimulated water cycle partitioning has been effectively modified. The runoff is increased,correspondingly the evaporation and soil moisture are decreased, the surface temperature andsensible heat are increased. The precipitation is differently modified in different regions, but ismore consistent with the observation. The land-surface change also has an impact on thecirculation over middle-lower troposphere layer (the highest at 700hPa).
Key Words: Runoff scheme, regional climate model, precipitation, land-surface hydrology

1. Introduction
According to the hydrology, there are two important surface runoff generation
mechanisms, i.e., the infiltration and saturation excess runoff processes, which are also calledas Philip and Horton runoff respectively. As an important part of the hydrological cycle in themodel, the unreasonable simulation of the runoff will affect the portioning of the precipitation
into the other variables (such as the evaporation and soil moisture), and then affect the energyand mass exchange. So it’s necessary to couple the hydrological with the climate model.
The importance of the runoff for the land-surface model and atmospheric model has also
been highlighted (e.g., Koster and Milly, 1997). But the results of the PILPS (Project ofIntercomparison of the Land Process Scheme) have shown that runoff is one of the simplest
treatment parts in the land surface model (Liang et al., 1998). And the difference of thehydrological partitioning between various land-surface models might be even larger than theactuary runoff and drainage (runoff+ drainage, PILPS 2(c), Anderson et al., 2003). There aresome gaps between the hydrological model and atmospheric models because of the scalemismatch and coupling, this deficiency will significantly impact the hydrological cycle, andeven the mass and energy exchange in the land-atmosphere interactive system.
Some efforts have been made on the coupling of the hydrology and climate model, or theimprovement of the hydrological cycle in the climate model during these years (e.g., Kim etal., 2001; Liang and Xie , 2001; Xie et al., 2003, and Zeng et al., 2003). In this context, westudy the effect of the improvement of the runoff scheme in the land-surface scheme (BATS)
in the regional climate model RegCM_NCC on the model performance.

2．Model Description
The regional climate model RegCM_NCC used in this study has been developed basedon the RegCM2 during 1996-2000, by modifying the physical process parameterization
schemes, with the support of the “95” National Key project “Studies on short-term climate
prediction system for China”. The detailed description of the model is not listed here, which
can be referred to as Ding et al. (2000).
The runoff generation mechanism has adopted part of the New Runoff Model (NRM) of
Liang and Xie (2003), which is characterized by incorporating two excess runoff generation
mechanisms (Horton and Dunne) at the same time. The heterogeneity of the sub-scale grid is
not considered currently in this study.

3．Model experiments and results analysis
The case study is the abnormally heavy rainfall over the Yangtze River valley during thesummer 1998. Two simulations have been made, that is, the control experiment (CTL) withthe original runoff scheme in BATS, and the modified experiment (RUN1) with the modified
runoff scheme in BATS. The observed precipitation is from the observations over the 160
stations in China.
Figure 1 shows the differences between the RUN1 and CTL during June 1998. We cansee that by using the modified runoff scheme, the simulated runoff (Fig.1(a))has been
increased in most of the model regions, especially in the rainfall center along the mid-low
reach of the Yangtze River Valley (YRV). Correspondingly, the simulated ground temperature(Fig.1(b)) is increased in most of the regions. As for the precipitation, the difference between
the RUN1 and CTL is not so uniform(Fig.1(c)). The precipitation in RUN1 is decreased inareas both the north of the YRV and the south of the YRV. Since the precipitation isoverestimated in most regions in the CTL simulation (figures omitted), so by modifying therunoff scheme, the RUN1 simulation seems more consistent with the observations in both thelocation of the rain-belt and the magnitude, except that the rainfall amount is underestimatedin the regions north to the SYR center(figures omitted).

(a)

(b)

(c)
Fig.1

The simulation differences between RUN1 and CTL in June 1998((a) Runoff, (b) Ts, and (c) precipitation)
The land-surface change of the heat and moisture will also have an impact on theatmosphere over higher levels. Comparison of the RUN1 and CTL experiment results, wefound that the distinguished difference is temperature of the mid-lower troposphere. Thisdifference can reach the height of 700hPa (Figures omitted).

4．Summary and discussionThe effect of the modified runoff generation mechanism of the land-surface scheme
in the RegCM_NCC on the model performance has been studied. The results have shown
that:（1）By accounting for the two runoff generation schemes simultaneously, the simulatedrunoff has increased, accordingly, the partitioning of the precipitation into the evaporationand soil moisture has decreased, so the ground temperature is increased. The simulated
precipitation has also changed because of the feedback processes, with the rainfall pattern
being more consistent with the observation.
（2）The changes of the land-surface hydrology also have certain affect on the variables
over the mid-lower troposphere. As for the temperature, the impact can reach as high as
700hPa.
In this study, only the runoff generation scheme is modified, such other importantfactors, such as the heterogeneity of the sub-scale grid is not accounted, which will be ournext research work.

5. References
Anderson Christopher J., Arritt Raymond W., et al., J. Hydrometeor., 4, 584-598, 2003
Ding, Yihui, et al., Development of Operational Short-term climate Model Series2, 217-231, 2000
Koster, R.D., and Milly, P.C..D., JC, 10, 1758-1591, 1997
Liang X., Wood E.F., Lettenmaier D.P., et al, Glob. Planet. Change, 19, 137-159, 1998
Liang Xu, Xie Zhenghui, Adv. Water Resources, 24, 1173-1193, 2001
Xie Zhenghui, Su Fengge and Liang Xu, Adv. Atm. Sci., 20, 165-172, 2003
Zeng Xinmin, Zhao ming, Su Bingkai, et al., Adv. Atm. Sci., 20, 227-236, 2003