Initial Soil Moisture Effects on the Climate in China

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1 J. Ocean Univ. China (Oceanic and Coastal Sea Research) DOI /s z ISSN , (2): xbywb@ouc.edu.cn Initial Soil Moisture Effects on the Climate in China A Regional Climate Model Study SHI Xueli * Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing , P. R. China (Received March 3, 2008; revised March 28, 2008; accepted November 10, 2008) Abstract In this study, the effects of initial soil moisture (SM) in arid and semi-arid Northwestern China on subsequent climate were investigated with a regional climate model. Besides the control simulations (denoted as CTL), a series of sensitivity experiments were conducted, including the DRY and WET experiments, in which the simulated initial SM over the region N, E was only 5% and 50%, and up to 150% and 200% of the simulated value in the CTL, respectively. The results show that SM change can modify the subsequent climate in not only the SM-change region proper but also the far downstream regions in Eastern and even Northeastern China. The SM-change effects are generally more prominent in the WET than in the DRY experiments. After the SM is initially increased, the SM in the SM-change region is always higher than that in the CTL, the latent (sensible) heat flux there increases (decreases), and the surface air temperature decreases. Spatially, the most prominent changes in the WET experiments are surface air temperature decrease, geopotential height decrease and corresponding abnormal changes of cyclonic wind vectors at the mid-upper troposphere levels. Generally opposite effects exist in the DRY experiments but with much weaker intensity. In addition, the differences between the results obtained from the two sets of sensitivity experiments and those of the CTL are not always consistent with the variation of the initial SM. Being different from the variation of temperature, the rainfall modifications caused by initial SM change are not so distinct and in fact they show some common features in the WET and DRY experiments. This might imply that SM is only one of the factors that impact the subsequent climate, and its effect is involved in complex processes within the atmosphere, which needs further investigation. Key words soil moisture; regional climate; numerical model experiments 1 Introduction As a very important hydrological variable, soil moisture (SM) can impact the exchange of energy and hydrological flux at the land-surface boundary by changing the surface albedo, heat capacity, sensible and latent heat to the atmosphere (e.g., Pal and Eltahir, 2001; Shukla and Mintz, 1982). Results have shown that about 65% of the precipitation over land is from evaporation (even higher in the inland regions), which is closely related to the SM (Chahine, 1992). SM could not only affect the circulation and climate, but also complicate the effects via various feedbacks (such as albedo), especially over vegetated areas (Long et al., 2003). Additionally, as a low-pass filter of the land-atmosphere system, SM has a long memory (e.g., Pielke et al., 1999; Wu et al., 2002; Yeh et al., 1984), and the strength of land-atmosphere coupling is different around the world (e.g., Guo et al., 2006; Koster et al., 2004). Observations in China have shown that the SM east of * Corresponding author. Tel: shixl@cma.gov.cn 100 E is positively (negatively) related to precipitation (temperature) (Ma et al., 2000). The regional SM effects are more significant in the warm season (Liu, 2003), but the SM anomalies over the Eurasian region might contain information of snow anomalies in the preceding winter/spring (Qian et al., 2003). Model studies also show that SM modification could affect the rainfall simulations over East Asia (e.g., Kim et al., 1998; Li et al., 2007). The arid and semi-arid regions over Northwest China with scarce rainfall are far away from the ocean and affected by large-scale topography (Qian et al., 2001), but the conditions here have great impacts on eastern China climate (e.g., Hsu and Liu, 2003; Wu et al., 2004). However, such kind of studies are limited partly by the lack of SM observations and the low resolution and poor accuracy of the existing data sets, so numerical models become very useful as an alternative (e.g., Wang et al., 2007). For example, Zhu et al. (1996) studied the initial drier soil effects on north China climate with the OSU model by setting the initial SM equal to zero over the region N, E. Wang et al. (2004) studied the drier SM impacts on short-term climate with MM5 by decreasing the moisture of all the four soil layers to 5%. Chow et al. (2008) tested initial SM effects over the Ti-

2 112 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : betan Plateau in early spring on East China climate. Methods of modifying the initial SM and the areas of interest are not the same among these studies. In this study, the initial SM over part of Northwestern China, including most of the Tibetan Plateau (30 50 N, E), was investigated for the first time with a regional climate model to test the initial SM effects. A brief description of the model and the numerical experiments will be presented in Section 2, followed by the numerical experiment results in Section 3. Finally, a summary and discussion are given in Section 4. 2 Model Description and Experimental Design 2.1 Model and Data Information The model used in this study was the National Climate Center regional climate model, which is based on the RegCM2/NCAR (Giorgi et al., 1993a, b) and developed by improving and assembling various physical process parameterizations. The detailed information of the model can be found in Chan et al. (2004) and Ding et al. (2006). The model has a 60 km horizontal resolution, with 91 and 151 grids in the longitudal and latitudal directions, respectively, which include almost the entire Tibetan Plateau (Fig.1). The year 1998 was selected for the case study, which was especially abnormal because there was a severe snowfall disaster over the Tibetan Plateau in the winter season and a catastrophic flood over the Yangtze River Valley in the summer. The large-scale data was from the 6-hour interval NCEP/NCAR reanalysis II dataset (Kanamitsu et al., 2002), which was also used to validate the simulation results. The monthly Global Precipitation Climatology Project (GPCP) version 2 data (Huffman et al., 1997) was used for precipitation comparison. Fig.1 Terrain height of the model domain (unit: m). Left dashed rectangle indicates the soil moisture change region N, E, right solid rectangle shows part of the downstream regions N, E. Shaded area denote height 5000 m. 2.2 Experimental Design The SM initialization is still an open question in land surface and atmosphere models because of the limitation of observed SM and great differences among various observed or simulated datasets (e.g., Dirmeyer et al., 2004), and it is not easy to judge which one is the best. In this study, the method of the RegCM2 model was still used, i.e., the initial soil water content is related to land cover type, soil property, and season. Besides the control experiments (denoted as CTL), four sensitivity experiments were conducted by setting the simulated initial SM at different percentages to represent SM increase (WET) and decrease (DRY). The initial here does not mean the SM value at the right beginning of the model integration (like in Chow et al., 2008; Li et al., 2007; Wang et al., 2004; Zhu et al., 1996), but the simulated SM at a certain time step as in Pielke et al. (1999), i.e., the last time slice at 15th April 1998 in this study. This would effectively diminish the large spread between the real condition and the model s internal consistency. The simulated SM was changed to 5%, 50%, 150% and 200% of the original simulated value, which are denoted, respectively, as SM05, SM50, SM150 and SM200. Additionally, in order to keep the SM within reasonable limits, the modified SM must not be less than the wilt point and not larger than the field capacity (related to the soil properties). Except for the initial SM change, all the other variables and configurations were the same as in the CTL. 3 Numerical Experiment Results The model integration periods were from 00UTC of 1st

3 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : April to 18UTC of 30th June 1998, with the first month being considered as the spin-up period. The results in May and June of 1998 were analyzed in this section. 3.1 Control Runs Fig.2 shows the observed and simulated precipitation in May June The heavy rainfall centers in South China, the South and East China Seas, with a continuous belt stretching from the western Indochina Peninsula through regions south of the Yangtze River Valley to the north Western Pacific (Fig.2a), which is well captured by the CTL (Fig.2b). But the model obviously overestimated the rainfall amount in most of the model domain except part of Yunnan Province and the southwest of the Tibetan Plateau. This is a common weakness of all regional climate models which is related to the cloud and radiation process parameterization schemes used and needs to be further improved. The surface air temperature in May June is characterized by the high centers at the southwestern tip of the Tibetan Plateau and the South China Sea (about 35 ), and the lower areas in the mid-western part of the Tibetan Plateau (lower than 5 ) (Fig.3a). These are well simulated by the CTL in both distribution pattern and intensity (Fig.3b), except that the simulated temperature is slightly higher than the NCEP/NCAR reanalyzed in eastern China and the northern Tibetan Plateau. The mean atmospheric circulation is featured by a steady westerly and weak trough over Northwestern China at 200 and 500 hpa (Figs.4a d), a low trough over Northeast China and a subtropical high pressure over the south- east of the model domain at 850 hpa (Figs.4e f). The NCAR/NCEP reanalyzed and CTL simulated circulation features are mostly consistent with each other at various layers in both pattern and intensity, even with different horizontal resolutions. In summary, we have shown that the regional climate model can well reproduce the basic climate features in May June 1998, and therefore is suitable for further sensitivity experiments. Fig.2 Rainfall in May June 1998 (unit: mm). a. GPCP. b. CTL. Shaded areas denote rainfall 300 mm.

4 114 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : Fig.3 Mean temperature in May June 1998 ( ). a. NCEP/NCAR reanalysis. b. CTL simulation. Shaded areas from light to deep denote temperatures 20, 24 and 28, respectively. Fig.4 NCEP/NCAR reanalyzed (left) and CTL simulated (right) geopotential height (m) and wind vector (m s -1 ) in May June 1998 at different levels. (a), (b): 200 hpa; (c), (d): 500 hpa; (e), (f): 850 hpa.

5 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : Sensitivity Experiment Results From the view of temporal evolution, the trends of various variables are generally consistent between the CTL and the initial SM-change sensitivity experiments (figures omitted), except for the magnitude differences. Therefore in the following sections, the differences be- tween the sensitivity experiments and the CTL are analyzed to investigate the initial SM effects on the subsequent climate. Fig.5 lists the temporal evolution of the regional mean differences between the sensitivity experiments and the CTL. It shows that for the SM-change area N, E, the differences for the WET experiments are Fig.5 Differences between sensitivity experiments and CTL in the SM-change area (left) and downstream regions (right) in May-June (a), (g): Latent heat (W m -2 ); (b), (h): Sensible heat (W m -2 ); (c), (i): Precipitation (mm); (d), (j): Surface soil moisture (mm); (e), (k): Net long wave radiation at the surface (W m -2 ); (f), (l): Net short wave radiation at the surface (W m -2 ). Solid line with pluses: SM200 CTL, dashed line with crosses: SM150 CTL, dashed line with rectangles: SM50 CTL, dashed line with circles: SM05 CTL.

6 116 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : larger than those for the DRY experiments. Wetting SM causes the increase (decrease) of the latent (sensible) heat flux, the largest magnitude of which being around 65 ( 40) W m -2. The differences for the DRY experiments are much less (generally less than 5 W m -2 ), and they fluctuate around zero (Figs.5a b). The precipitation is generally increased in the WET experiments and decreased in the DRY ones (Fig.5c). The surface SM remains positive in the WET experiments, while it slightly changes around zero in the DRY ones (Fig.5d). Similar patterns also appear in the net long-wave and short-wave radiation at the surface (Figs.5e 5f). Differences in the downstream regions (east of the SM-change area) are generally consistent with and larger than those in the SM-change area; for instance, for the WET experiments, there are the increase (decrease) of the latent (sensible) heat flux (Figs.5g h), increase of surface SM (Fig.5j), and decrease of net short-wave and long-wave radiation at the surface (Figs.5k l). On the other hand compared with the SM-change area (c.f., Fig.5c), different patterns in rainfall appear in the downstream areas, where the rainfall is decreased at the end of June (Fig.5i) Spatial distribution of precipitation and temperature In the DRY experiments, precipitation in May June 1998 is decreased in the Yangtze River Valley, part of the South and East China Sea, as well as the south tip of the Tibetan Plateau (Figs.6a b). In the WET ones, rainfall is significantly increased in regions north and east of the SM-change area and part of South China, while it is decreased at the south tip and southeast of the Tibetan Plateau, part of the East China Sea, far regions of northeast China and the Korean Peninsula (Figs.6c d). So generally speaking, the precipitation differences between sensitivity experiments and CTL are locally confined. The patterns in the WET and DRY experiments are not totally opposite, on the contrary, they are somewhat similar. This is related to the complex physical processes within the atmosphere. The changes in atmospheric temperature at 2 m because of initial SM modification present a notably different pattern in the WET and DRY experiments. The surface air temperature differences are not prominent in the DRY experiments (less than 0.5, Figs. 7a b), but anomalous increase (decrease) appears in eastern and northeastern China (immediate vicinity of the eastern Tibetan Plateau and southwest tip of the Plateau). On the contrary, temperature is significantly decreased in the WET experiments, especially on the south and northeastern sides of the Tibetan Plateau (Figs.7c d), where the difference can reach 8 ( 7 ) at the south tip (in the north part) of the Tibetan Plateau. Fig.6 Total precipitation differences between sensitivity experiments and CTL in May June Unit: mm. (a): SM05 CTL, (b): SM50 CTL, (c): SM150 CTL, (d): SM200 CTL. Shaded areas denote rainfall differences 50 mm. The long-short dashed lines represent terrain heights 4500 m.

7 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : Fig.7 Same as Fig.6, but for surface air temperature (unit: ). Shaded areas denote temperatures Atmospheric circulation The troposphere circulations are also modified with the SM change. For instance, at 200 hpa altitude, the differences in geopotential height are limited within 2 m (Fig. 8a) in the SM50 experiment (and the SM05 experiment, figures for which are omitted), but large differences occur in the SM150 (and SM200) experiment, with the largest differences being 24 m centered in the SM-change areas (Fig.8b). Similar patterns appear at 500 hpa altitude, featured by a broad negative height (as low as 20 m) and correspondingly cyclonic anomaly (Figs.8c d). While in the lower troposphere (850 hpa altitude), the differences are generally small in both WET and DRY experiments (Figs.8e f): less than 5 m in geopotential height. The wind vector differences appear to be mostly larger in the WET experiments than those in the DRY counterparts. Vertically along the SM-change latitude zone, the initial SM wetting, such as that in the SM150 experiment, causes prominent changes in the atmosphere. Temperature is generally decreased below 300 hpa with the largest difference around 3.5 near the surface (Fig.9a). Geopotential height is increased (decreased) below (above) 700 hpa (Fig.9b). Atmosphere moisture is mainly increased in the mid-lower troposphere layers of the SM-change area and little changes occur east of 110 E, therefore the moisture is generally locally modified (Fig.9c). The vertical wind velocity is not changed consistently between the two regions. For example, abnormal upward motions appear at the eastern tip of the Tibetan Plateau but abnormal downward motions occur in the SM-change area (Fig.9d). Therefore, by changing the initial SM conditions, the differences caused in the WET experiments are generally larger than those in the DRY experiments, which might be related to the large-scale topography of the Tibetan Plateau and moistening in the arid and semi-arid regions (e.g., Song and Zhang, 2003). The SM in such regions is always very small, thus the actual available soil moisture content is larger in a WET (such as 50% increase in SM150) experiment than that in a DRY (50% decrease in SM50) experiment. To summarize, SM wetting can cause increase (decrease) of the latent (sensible) heat flux, broad decrease of surface air temperature, and certain changes in rainfall. Particularly, the temperature undergoes more prominent changes than rainfall, the magnitudes in the WET experiments (larger than 3 ) being approximately 10 times those in the DRY experiments (about 0.3 ). The changes at the surface then impact the atmosphere above, i.e., the increase of initial SM directly causes temperature decrease and moisture increase. The geopotential height decreases in the middle and upper troposphere layers. However, some inconstant changes appear in other variables, such as the wind velocity (especially the vertical velocity). The changes within the atmosphere should be the reason for the non-distinct changes in precipitation, which is not discussed here. But previous studies have shown that the initial SM can be influential through planetary boundary layer processes, cumulus convective, and so on.

8 118 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : Fig.8 Geopotential height (m) and wind vector (m s -1 ) differences between SM50 (left), SM150 (right) and CTL in May June (a), (b): 200 hpa; (c), (d): 500 hpa; (e), (f): 850 hpa. 4 Conclusions and Discussion The effects of initial soil moisture in arid and semiarid regions on subsequent climate were tested through sensitivity experiments with a regional climate model. The results show that initial SM change can impact the climate in not only the SM-change region proper but also the far downstream regions both at the land surface and in the atmosphere above. The differences caused in the WET experiments are generally significantly larger than those in the DRY experiments. When the SM is initially increased on 15th April 1998, the subsequent SM in May and June is always larger than that in the CTL, the latent (sensible) heat flux is increased (decreased), and the surface temperature and net radiations are generally decreased in the SM-change region. Generally, changes consistent with those in the SMchange region appear also in the downstream regions, with comparable or even larger intensities. Spatially, the most prominent changes in the WET experiments are surface temperature decreases, geopotential height decreases and correspondingly cyclonic circulation abnormalities at the middle and upper troposphere levels. Generally opposite patterns and much smaller intensities occur in the DRY experiments. No definite

9 SHI / J. Oean Univ. China (Oceanic and Coastal Sea Research) : Fig. 9 Longitude-height sections of SM150 CTL along N. (a): temperature ( ), (b): geopotential height (m), (c): moisture (kg kg -1 ), (d): vertical velocity (0.01 Pa s -1 ). Shaded area denotes temperature 0.5, Height 30 m, Moisture 31 kg kg -1, and vertical velocity 0 Pa s -1 relationship is shown between the variation of the SM and the effects it produces. The initial SM changes impact the atmospheric processes over the model domain through modifying the surface temperature, moisture and various fluxes (like latent/sensible heat), causing consistent changes such as the decrease (increase) of temperature (moisture) and decrease of geopotential height at middle and lower troposphere layers, which take place in the WET experiments. But some inconsistent features also appear, such as the vertical velocity, which are related to complex dynamic circulation responses within the atmosphere and need to be further investigated. Compared with the prominent and consistent changes (especially in the WET experiments) in temperature, the variation in rainfall is not so distinctly affected in both the WET and DRY experiments. As indicated above, the first changes due to land surface variable changes caused by SM variation seem to be those in the atmospheric temperature and moisture, which lead to changes of other relevant variables and processes. This implies that SM is only one of the factors that affect the subsequent climate, and it is related to a series of complex processes and factors in the atmosphere, such as the planetary boundary, cumulus convective rainfall, and so on (e.g., Eltahir, 1998), which need further investigation. In addition, only the climate changes in May and June 1998 were investigated in this study with a 15-day spinup, which may not be long enough. Therefore, more cases with longer spin-up periods are needed for further study. Finally, because of the complexity of SM variation and scarce of available observations in the arid and semi-arid regions, new datasets obtained by other means (e.g., remote sensing) are most urgently needed to deepen our understanding of SM and its effects (e.g., Song et al., 2007). Acknowledgements The author would thank the anonymous reviewers for their helpful comments. This work is jointly supported by the Ministry of Science and Technology of China public welfare funding (No. 2002DIB20070) and the National Basic Research Program of China (973 Program) (No. 2007CB411505). References Chahine, T. M., The hydrological cycle and its influence

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