East China Summer Rainfall during ENSO Decaying Years Simulated by a Regional Climate Model

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1 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2011, VOL. 4, NO. 2, East China Summer Rainfall during ENSO Decaying Years Simulated by a Regional Climate Model ZENG Xian-Feng 1, 2, LI Bo 1, 2, FENG Lei 1, 2, LIU Xiao-Juan 1, 2, and ZHOU Tian-Jun 1 1 State Key Laboratory of Numerical modeling for Atmospheric Science and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China 2 Graduate University of Chinese Academy of Sciences, Beijing , China Received 8 December 2010; revised 4 January 2011; accepted 5 January 2011; published 16 March 2011 Abstract The performance of the Climate version of the Regional Eta-coordinate Model (CREM), a regional climate model developed by State Key Laboratory of Numerical modeling for Atmospheric Science and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP), in simulating rainfall anomalies during the ENSO decaying summers from 1982 to 2002 was evaluated. The added value of rainfall simulation relative to reanalysis data and the sources of model bias were studied. Results showed that the model simulated rainfall anomalies moderately well. The model did well at capturing the above-normal rainfall along the Yangtze River valley (YRV) during El Niño decaying summers and the below and above-normal rainfall centers along the YRV and the Huaihe River valley (HRV), respectively, during La Niña decaying summers. These features were not evident in rainfall products derived from the reanalysis, indicating that rainfall simulation did add value. The main limitations of the model were that the simulated rainfall anomalies along the YRV were far stronger and weaker in magnitude than the observations during El Niño decaying summers and La Niña decaying summers, respectively. The stronger magnitude above-normal rainfall during El Niño decaying summers was due to a stronger northward transport of water vapor in the lower troposphere, mostly from moisture advection. An artificial, above-normal rainfall center was seen in the region north to 35 N, which was associated with stronger northward water vapor transport. Both lower tropospheric circulation bias and a wetter model atmosphere contributed to the bias caused by water vapor transport. There was a stronger southward water vapor transport from the southern boundary of the model during La Niña decaying summers; less remaining water vapor caused anomalously weaker rainfall in the model as compared to observations. Keywords: East China rainfall, ENSO decaying summers, regional climate model, water vapor Citation: Zeng, X.-F., B. Li, L. Feng, et al., 2011: East China summer rainfall during ENSO decaying years simulated by a regional climate model, Atmos. Oceanic Sci. Lett., 4, Introduction East China is dominated by a typical monsoon climate Corresponding author: ZHOU Tian-Jun, zhoutj@lasg.iap.ac.cn that exhibits multi-time scale variability (Zhou et al., 2009a). On an inter-annual scale, the El Niño-Southern Oscillation (ENSO) affected the anomalous monsoon activity. The impact of ENSO on East China summer rainfall is the most significant during ENSO decaying years. The ENSO mainly affects the East Asia Summer Monsoon (EASM) by modulating the locations and strengths of the western North Pacific (WNP) monsoon trough and the WNP subtropical high (WNPSH; Wang et al., 2009; Wu et al., 2009, 2010). Before the late 1970s, the abovenormal (below-normal) sea surface temperature (SST) anomalies over Niño-3 or Niño-4 regions during the preceding winter often meant that more (less) rainfall would appear in North China and south to the Yangtze River valley (YRV), less (more) rainfall would appear over the YRV, and a later (earlier) Chinese Meiyu would appear during the following summer. After the late 1970s, due to a westward expansion of the WNPSH on inter-decadal scale (Chang et al., 2000; Zhou et al., 2009d), summer precipitation increased in the middle and lower branches of the YRV and decreased in northern and southern China (Chang et al., 2000; Wang et al., 2009). The relationship between rainfall in East China and the ENSO became weaker after the 1980s (Gao et al., 2006a). Climate models are useful tools in monsoon research. However, when driven by observed SST, many Atmospheric General Circulation Models (AGCMs) generally failed to simulate rainfall anomalies in East China (Wang et al., 2009). For instance, all 11 AGCMs that participated in the Atmospheric Model Inter-comparison Project (AMIP) poorly simulated anomalous rainfall patterns in Southeast Asia and the western North Pacific (Wang et al., 2004). The AMIP models also failed to simulate the principle modes of Asian-Australian monsoon rainfall anomalies over the extra-tropics (Zhou et al., 2009c). This may be due to the fact that the current state-of-art AGCMs generally employ low horizontal resolution and are unable to represent the relatively small-scale monsoon front (Wang et al., 2009; Zhou et al., 2008, 2009a, b, d; Li et al., 2010a, b; Chen et al., 2010). Regional climate models (RCMs) are useful tools for dynamically downscaling climate at the regional scale. They have been widely used in regional climate simulation, sensitivity experiments, and future scenario projections (Gao et al., 2001, 2002, 2006b; Zou et al., 2010). In recent years, State Key Laboratory of Numerical Modeling for Atmospherics Sciences and Geophysical Fluid

2 92 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 Dynamics/Institute of Atmospheric Physics (LASG/IAP) developed a RCM named the Climate version of the Regional Eta-coordinate Model (CREM). The model reproduced the spatial distributions of summer climatological mean precipitation and circulation in East China reasonably well (Shi et al., 2009), but the ability of the model to simulate rainfall anomalies during ENSO decaying summers has never been examined. This study aims to answer the following questions: 1) How well does the CREM simulate the rainfall anomalies in East China during ENSO decaying summers? Is there any added value in rainfall simulation, relative to rainfall products derived from reanalysis data? 2) Why does the model show bias in reproducing rainfall anomalies? The remainder of the paper is organized as follows: in Section 2, the experimental design, validation datasets, and methods are described; analyses of model results are presented in Section 3; and a summary is presented in Section 4. 2 Model, experiment, and methods description 2.1 Model and experiment The CREM was developed based on the Advanced Regional Eta-coordinate Model (AREM). An advanced radiation package (Sun and Rikus, 1999) and a common land surface scheme (Dickinson et al., 1993) were included in the model. Eta-coordinate was used as the vertical coordinate, and there were 32 uneven levels from surface to 10 hpa. The Arakawa E-grid was used as the horizontal coordinate, and the horizontal resolution was km. The modified Betts-Miller cumulus parameterization scheme was incorporated to compute convective precipitation. An explicit prognostic cloud scheme was used to represent the grid-scale moisture processes (Xu et al., 1998). More information about the model is contained in Shi et al. (2009). The model domain used in this study spans N, E. The initial and lateral boundary conditions for the CREM were obtained from National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) Reanalysis (hereafter, NCEP2) six-hourly data (Kanamitsu et al., 2002). The resolution of NCEP2 data is The SST forcing data were acquired from weekly Optimally Interpolated Sea Surface Temperature (OISST) data with a resolution of 1 1 (Reynolds et al., 2002). The model was integrated from April to August for each year from The observational or reanalysis data used to evaluate the model included: 1) grid precipitation datasets from (Xie et al., 2007), and 2) NCEP2 reanalysis data (Kanamitsu et al., 2002). 2.2 Methods According to Wu et al. (2009), the years of 1983, 1988, 1992, 1995, and 1998 are regarded as El Niño decaying years, and 1984, 1985, 1989, 1996, 2000, and 2001 are regarded as La Niña decaying years. The anomalies relative to the mean state of summers are synthesized as El Niño and La Niña decaying summer anomalies. A budget analysis of water vapor transport was used to examine model bias. The divergence of water vapor flux was divided into a moisture advection term and a wind divergence term (Huang et al., 1998). The water vapor values were vertically integrated from the surface to 300 hpa, which is the limit because there is negligible water vapor above 300 hpa (Zhou, 2003; Zhou and Yu, 2005). 3 Results 3.1 Precipitation The composite rainfall anomalies during ENSO decaying summers are shown in Fig. 1. Above-normal rainfall was observed along the YRV, and below-normal rainfall was seen along the Jiang-Huai valley during El Niño decaying summers (Fig. 1a). Meanwhile, this situation was reversed during La Niña decaying summers (Fig. 1b). The NCEP2 failed to reproduce these features (Figs. 1e and 1f). However, the observed features were reasonably well reproduced by the CREM, especially for the above-normal rainfall center during El Niño decaying summers (region A in Fig. 1c) and the below-normal rainfall center along the YRV during La Niña decaying summers (region C in Fig. 1d). It also captured the above-normal rainfall center along the HRV during La Niña decaying summers (region D in Fig. 1d). Thus, the CREM model exhibited added value in simulating rainfall anomalies relative to the reanalysis product. The deficiency of the model was that the simulated rainfall anomalies were stronger and weaker in magnitude than those observed during El Niño (Figs. 1c and 1a) and La Niña (Figs. 1d and 1b) decaying summers, respectively. In addition, an artificial above-normal rainfall center became evident in North China during El Niño decaying summers (region B in Fig. 1c). 3.2 Water vapor budget Water vapor transport is a pre-condition for a rainfall event because precipitation is directly determined by water vapor convergence. Compared with La Niña decaying summers, signals during El Niño decaying summers were stronger. As shown in Fig. 2, a stronger water vapor convergence in central-eastern China along 30 N during El Niño decaying summers was associated with abovenormal rainfall anomalies along the YRV (Fig. 2a). The total water vapor divergence was further separated into two parts: a moisture divergence term (the middle column of Fig. 2) and a wind divergence term (the right column of Fig. 2). In the reanalysis, the total water vapor convergence was determined by the moisture advection term (Fig. 2b), while the contribution of wind convergence was only evident in two small regions centered on 30 N, 105 E and 32 N, 117 E (Fig. 2c). The general feature of total water vapor divergence, along with the relative contributions from the moisture advection and wind divergence terms, were reasonably reproduced by the model along the YRV (Figs. 2b and 2e; Figs. 2c and 2f). How-

3 NO. 2 ZENG ET AL.: CREM SUMMER RAINFALL DURING ENSO DC IN EAST CHINA 93 Figure 1 Composite rainfall anomalies (mm d 1 ) derived from (a, b) Xie, (c, d) CREM, and (e, f) NCEP2 during ENSO decaying summers, including El Niño decaying summers (left column) and La Niña decaying summers (right column). Coordinates of A, B, C, and D region boxes are N, E; N, E; N, E; N, E, respectively. ever, the magnitudes of the two terms in the simulation were stronger than in the observations, especially for the wind convergence term along the YRV. The wet moisture advection in North China was also stronger in the simulation, leading to an artificial above-normal rainfall center over the area. During La Niña decaying summers, the moisture advection term made greater contributions and caused the anomalous water vapor convergence center to move to the HRV. However, wind divergence made more contributions in the model. A quantitative comparison of water vapor divergence terms over four regions (regions A, B, C, and D of Fig. 1) is shown in Table 1. The observed rainfall data and NCEP2 circulation data were used for validation. In the reanalysis, the net water vapor convergence in region A was 0.55 mm d 1 and the contribution of moisture advection term was 0.61 mm d 1 during El Niño decaying summers. The model was able to reproduce the contribution of the moisture advection term. However, the net convergence of water vapor was 2.32 mm d 1 in the model, approximately 1.63 mm d 1 of which comes from the wind divergence term. In region B, the contribution of the wind divergence term was greater than the moisture advection term, at 0.26 mm d 1 versus 0.08 mm d 1. In the simulation the moisture advection term made a contribution of 0.52 mm d 1, leading to a net moisture divergence of 0.57 mm d 1. This resulted in the artificial excessive rainfall anomaly of 0.53 mm d 1 seen in region B (Fig. 1). During La Niña decaying summers, a below-normal rainfall center along the YRV and an above-normal rainfall center along the HRV were evident. In the reanalysis, the wind divergence term made contributions equal to those made by the moisture advection term, and the net water vapor divergence over region C was 0.37 mm d 1. The model captured these features, except at a stronger intensity. For region D, the moisture advection term made a

4 94 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 Figure 2 Vertically integrated water vapor divergence anomalies (mm d 1 ) during El Niño decaying summers (first two rows) derived from NCEP2 forcing (first row) and CREM (second row). Vertically integrated water vapor divergence anomalies (mm d 1 ) during La Niña decaying summers (second two rows) derived from NCEP2 forcing (third row) and CREM (fourth row). (a, d, g, j) Divergence, (b, e, h, k) moisture advection term, and (c, f, i, l) wind divergence term values are illustrated in columns from left to right, respectively. Table 1 Budget analysis of total water vapor convergence (mm d 1 ). Regions Data Rainfall Total divergence Moisture advection Wind divergence Region A NCEP2/Xie CREM Region B NCEP2/Xie CREM Region C NCEP2/Xie CREM Region D NCEP2/Xie CREM greater contribution, and the net moisture convergence was 0.22 mm d 1, but the wind divergence term was balanced by the moisture advection term, with values of 0.80 mm d 1 and 0.99 mm d 1 ; thus, there were fewer rainfall anomalies. Water vapor transport is dominated by atmospheric circulation. Vertically integrated water vapor transport during ENSO decaying summers are shown in Fig. 3.

5 NO. 2 ZENG ET AL.: CREM SUMMER RAINFALL DURING ENSO DC IN EAST CHINA 95 During El Niño decaying summers (Fig. 3a), the enhanced western Pacific anticyclone anomaly led to westward expansion of the WNPSH. It induced anomalous water vapor transport convergence along the YRV, leading to above-normal rainfall. The pattern of water vapor transport was generally well simulated (Fig. 3c). However, over the region south to 30 N, the westerly flow was stronger than in the reanalysis (Fig. 3e), leading to more moisture convergence along the YRV. In the region north to 30 N, there was an anticyclone bias centered on 40 N, 124 E, which led to more northward water vapor transport; hence, an excessive rainfall center appeared in region B (Fig. 1c). During La Niña decaying summers, anomalous water vapor transport diverged at the middle and lower branches of the YRV. The model reduced these features. However, over the region south to 30 N, the northeasterly flow was stronger than in the reanalysis (Fig. 3f), leading to moisture divergence along the YRV, which induced more rainfall in the south part of the region C. We also checked the wind bias at 700 hpa during El Niño decaying summers (figure not shown), and we found that the pattern was nearly the same as Fig. 3e. An artificial anticyclone dominated the region north to 30 N and east to 110 E, which is understandable because water vapor concentrates in the lower troposphere and the vertically integrated water vapor transport value is mainly derived from the lower troposphere. Further examination of the bias created by specific humidity reveals that most of continental China (north to 30 N and west to 120 E) was dominated by positive anomalies (figure not shown), indicating that the model atmosphere was too wet, which also contributed to stronger water vapor transport. To present a clear picture of the vertical distribution of water vapor transport, the whole air column was further divided into three layers: lower ( hpa), middle ( hpa) and upper ( hpa) layer. A budget analysis of water vapor transport across boundaries during El Niño decaying summers was performed for regions A and B. As is shown in Fig. 4a, in the reanalysis, the main water vapor input channels were through the southern boundary in the lower layer ( kg s 1 ) and the western boundary in low-middle layer ( kg s 1 ) for region A. The output of water vapor transport was mainly through the eastern boundary in the lower layer Figure 3 Vertically integrated water vapor transport anomalies (kg m 1 s 1 ) during El Niño decaying summers (left column) and La Niña decaying summers (right column). From top to bottom, rows depict results from (a, b) NCEP2, (c, d) CREM, and (e, f) bias.

6 96 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 4 ( kg s 1 ). Clearly, the water vapor transport in the lower layer plays a key role. The vertical features of water vapor transport for region A were generally well simulated by the CREM (Fig. 4c), except that the inputs from the western and southern boundaries were stronger in magnitude than in the reanalysis, with values of kg s 1 versus kg s 1, respectively, from the southern boundary, and kg s 1 versus kg s 1, respectively, in low-middle layer from the western boundary. Thus, the excessive rainfall in the simulation was associated with excessive water vapor input in the lower layer. For region B, the main output of water vapor transport in the lower layer of the eastern boundary was greater in magnitude than the main input in the middle layer of the western boundary, with values of kg s 1 and kg s 1. Thus, the net water vapor budget was negative, inducing nearly no rainfall anomalies (Fig. 4b). However, a strong northward water vapor transport ( kg s 1 ) across the southern boundary in the lower layer of the model (Fig. 4d) led to a net gain in the water vapor budget and thus, to artificial excessive rainfall anomalies. For La Niña decaying summers, the model reproduced the basic features, except that it produced stronger southward water vapor transport output from the southern and western boundaries in regions C and D. In the mean time, the main output in region D of the reanalysis was from the western boundary in the lower layer (3.04 mm d 1 ), while the main output in the model was from the southern boundary in the lower layer (7.29 mm d 1 ; figure not shown). 4 Summary The rainfall anomalies during El Niño and La Niña decaying summers over eastern China were dynamically downscaled by the NCEP2 reanalysis for the period of The regional climate model, CREM, developed by LASG/IAP was used as a dynamical downscaling tool. The results demonstrated that rainfall patterns added significant value compared to the rainfall product derived from the driving NCEP2 reanalysis. Rain gauge observations showed an above-normal rainfall center along the YRV during El Niño decaying summers, while the simulation showed a below-normal rainfall center along the YRV and an above-normal rainfall center along the HRV during La Niña decaying summers. This feature was not evident in the reanalysis rainfall product, suggesting added values of dynamical downscaling. The deficiencies of the simulation were that the rainfall anomalies along the YRV were stronger and weaker in magnitude than those observed during El Niño and La Niña decaying summers, respectively. Analysis of the water vapor budget indicated that these deficiencies were due to a stronger water vapor convergence during El Niño decaying summers, which is determined by the moisture advection term mostly in the lower layer of the tropo- Figure 4 Anomalous water vapor budgets (units: 10 6 kg s 1 ) during El Niño decaying summers in the YRV (left column, Region A in Fig. 1) and North China (right column, Region B in Fig. 1). The top and bottom rows illustrate water budgets derived from NCEP2 reanalysis and CREM.

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