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    (a) Topography (unit: m) and (b) land cover types (LU_index) and leaf area index (LAI) over the TP. In (a), 1 stands for the Tarim River basin (TRB), 2 for Qilian Mountain (QLB), 3 for the Qaidam basin (QDB), 4 for the Chang Tang Plateau (CTB), 5 for the Yangtze River (YTR), 6 for the Yellow River (YLR), 7 for the Mekong River (MKR), 8 for the Salween River (SWR), and 9 for the Brahmaputra River (BPR). In (b), land cover types 7, 9, and 19 respectively denote grassland, shrubland, and sparse vegetation or barren ground.

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    Changes in (a)–(c) P, (d)–(f) E, and (g)–(i) PE (unit: mm day−1) of 1998–2011 compared to 1979–97 for OBS/GLDAS, WRF simulation, and ERA-Int, respectively, over the TP.

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    Time series of annual mean P, E, and PE (unit: mm day−1) for nine subregions labeled in Fig. 1a corresponding mostly to the headwater of major rivers in Asia that originate from the TP.

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    Distributions of annual (a),(b) PE changes and contributions from (c),(d) mean circulation dynamics (δMCD), (e),(f) thermodynamics (δTH), and (g),(h) transient eddy (δTE) from (left) the WRF simulation and (right) ERA-Int in 1998–2011 compared to 1979–1997 (Unit: mm day−1).

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    Contributions to PE changes from (a)–(d) mean circulation dynamics and (e)–(h) thermodynamic due to advection [in (a),(b),(e),(f)] and convergence or divergence [in (c),(d),(g),(h) ] from (left) the WRF simulation and (right) ERA-Int in 1998–2011 compared to 1979–97 in WRF simulation (Unit: mm day−1).

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    Distributions of changes in circulation (unit: m s−1) at 200 mb in 1998–2011 compared to 1979–97 in (a) the WRF simulation and (b) ERA-Int.

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    Distributions of changes in (a),(b) divergence (unit: 10−6 s−1) at 500 and 100 mb, (c) vertical motion (unit: mm s−1), (d) column integrated moisture (unit: g kg−1), and (e) microphysics diabatic latent heat (unit: K s−1) in 1998–2011 compared to 1979–97 in the WRF simulation.

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Changes in Moisture Flux over the Tibetan Plateau during 1979–2011: Insights from a High-Resolution Simulation

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  • 1 Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China
  • 2 Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington
  • 3 Research Applications Laboratory, National Center for Atmospheric Research, Boulder, Colorado
  • 4 Key Laboratory of Tibetan Environmental Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China
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Abstract

Net precipitation [precipitation minus evapotranspiration (PE)] changes between 1979 and 2011 from a high-resolution regional climate simulation and its reanalysis forcing are analyzed over the Tibetan Plateau (TP) and compared to the Global Land Data Assimilation System (GLDAS) product. The high-resolution simulation better resolves precipitation changes than its coarse-resolution forcing, which contributes dominantly to the improved PE change in the regional simulation compared to the global reanalysis. Hence, the former may provide better insights about the drivers of PE changes. The mechanism behind the PE changes is explored by decomposing the column integrated moisture flux convergence into thermodynamic, dynamic, and transient eddy components. High-resolution climate simulation improves the spatial pattern of PE changes over the best available global reanalysis. High-resolution climate simulation also facilitates new and substantial findings regarding the role of thermodynamics and transient eddies in PE changes reflected in observed changes in major river basins fed by runoff from the TP. The analysis reveals the contrasting convergence/divergence changes between the northwestern and southeastern TP and feedback through latent heat release as an important mechanism leading to the mean PE changes in the TP.

Corresponding author address: Yanhong Gao, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 320 Donggang West Rd., Lanzhou 730000, China. E-mail: gaoyh@lzb.ac.cn

Abstract

Net precipitation [precipitation minus evapotranspiration (PE)] changes between 1979 and 2011 from a high-resolution regional climate simulation and its reanalysis forcing are analyzed over the Tibetan Plateau (TP) and compared to the Global Land Data Assimilation System (GLDAS) product. The high-resolution simulation better resolves precipitation changes than its coarse-resolution forcing, which contributes dominantly to the improved PE change in the regional simulation compared to the global reanalysis. Hence, the former may provide better insights about the drivers of PE changes. The mechanism behind the PE changes is explored by decomposing the column integrated moisture flux convergence into thermodynamic, dynamic, and transient eddy components. High-resolution climate simulation improves the spatial pattern of PE changes over the best available global reanalysis. High-resolution climate simulation also facilitates new and substantial findings regarding the role of thermodynamics and transient eddies in PE changes reflected in observed changes in major river basins fed by runoff from the TP. The analysis reveals the contrasting convergence/divergence changes between the northwestern and southeastern TP and feedback through latent heat release as an important mechanism leading to the mean PE changes in the TP.

Corresponding author address: Yanhong Gao, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 320 Donggang West Rd., Lanzhou 730000, China. E-mail: gaoyh@lzb.ac.cn

1. Introduction

In the past three decades, the Tibetan Plateau (TP) has been warming at a faster pace than the global average temperature (Wu et al. 2007; Kang et al. 2010). The warming has already exerted great impacts on the cryosphere and water cycle in the TP (Solomon et al. 2007; Krause et al. 2010; Moore 2012; Gao et al. 2014) that feed major rivers supporting the large population and economic development in Asia. Complex topography has a dominating influence on the TP climatology (Xu et al. 2008; Wu et al. 2013), but observations are sparse in the expansive plateau due to the harsh conditions. With observation sites predominantly located in the eastern mountain valley with accessibility, the vast northwestern TP is mostly devoid of observations. Therefore, climate change analysis is severely limited by poorly gauged or ungauged conditions. Although global reanalyses are appropriate tools for assessing past climate change, they have limited value in the TP because global models are not well constrained by observations and they lack sufficient spatial resolution to represent the large spatial heterogeneity associated with topography (Fig. 1a), land cover (Fig. 1b), and soil that influence atmospheric and land processes. Land cover in the TP is dominated by grassland, shrubland, and sparse vegetation or barren ground, denoted as land cover types 7, 9, and 19, respectively, in Fig. 1b. These land copy types are distributed from the southeast to the northwest, with a corresponding decrease in leaf area index (LAI; Fig. 1c).

Fig. 1.
Fig. 1.

(a) Topography (unit: m) and (b) land cover types (LU_index) and leaf area index (LAI) over the TP. In (a), 1 stands for the Tarim River basin (TRB), 2 for Qilian Mountain (QLB), 3 for the Qaidam basin (QDB), 4 for the Chang Tang Plateau (CTB), 5 for the Yangtze River (YTR), 6 for the Yellow River (YLR), 7 for the Mekong River (MKR), 8 for the Salween River (SWR), and 9 for the Brahmaputra River (BPR). In (b), land cover types 7, 9, and 19 respectively denote grassland, shrubland, and sparse vegetation or barren ground.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

Observations and global reanalyses of the net precipitation [i.e., precipitation − evapotranspiration (PE)] revealed a trend toward drier and wetter conditions over the humid southeastern TP and the vast arid northwestern TP, respectively (Yang et al. 2011; Yin et al. 2012; Gao et al. 2014). Among four global reanalysis products analyzed, Gao et al. (2014) found that ERA-Interim (ERA-Int) best captured the general moistening in the northwestern TP and drying in the southeastern TP in the past three decades, but a large discrepancy is still notable. For example, ERA-Int does not capture the decrease of PE in the Yellow River headwater and the Rouergai plateau located in the east. Also the contrast in PE changes between the northwestern and southeastern TP is not well replicated. None of the global reanalyses yields a statistically significant pattern correlation with the observed changes at the 70% confidence level. Since the headwater of major rivers originating from the TP is widely distributed across the eastern and central plateau, uncertainty in estimating the spatial distribution of PE changes limits the usefulness of the global reanalysis products in understanding the mechanisms responsible for historical changes across the broad Asian region.

Regional climate models (RCMs) are useful for a host of climate change applications that require finer-scale information. They have been widely used to study historical and projected regional climate changes (Mearns et al. 2009; Nikulin et al. 2012; Déqué et al. 2005; Duffy et al. 2006; Giorgi et al. 1992; Kim et al. 2002; Leung et al. 2003a,b; Plummer et al. 2006; Zhang et al. 2009; Laprise et al. 1998) and are generally shown to provide useful skill in simulating surface air temperature, precipitation, and water resources in East Asia (Gao et al. 2008, 2011; X. Gao et al. 2012; Xu and Gao 2014). A recent 33-yr-long regional climate simulation has been conducted over East Asia and comparisons with observations show that the high-resolution simulation not only improves the pattern correlations but also reproduces finer-scale changes in the surface air temperature in the wet season over the TP compared to the global reanalyses (Gao et al. 2015). This study analyzes the precipitation (P), evapotranspiration (E), and net precipitation (PE) changes from the regional simulation between 1979 and 2011. Differences between the high-resolution simulation and the coarse-resolution reanalysis that provided meteorological forcing are investigated to explore the mechanisms behind the regional-scale PE changes. Section 2 introduces the methods and data used. Results are presented in section 3, with conclusions and discussion provided in section 4.

2. Data and methods

The Weather Research and Forecasting (WRF) Model (http://www.wrf-model.org/index.php; Skamarock et al. 2005) was used in the high-resolution regional climate simulation. WRF is a nonhydrostatic model with various choices of physics parameterizations suitable for applications across a wide range of scales. The simulation was performed at 30-km horizontal grid resolution with 159 × 196 grids cells covering East Asia for 1 January 1979 to 31 December 2011 using lateral boundary conditions and SST interpolated from ERA-Int (Dee and Uppala 2009; Dee et al. 2011) since ERA-Int ranks the best among the examined reanalyses in describing temperature and elements of the water cycle over the TP (Wang and Zeng 2012; Li et al. 2012; Gao et al. 2014). ERA-Int is an improved version of the ERA-40 reanalysis (Simmons et al. 2006). The ERA-Int archive maintained by the Computational and Information Systems Laboratory (CISL) at the National Center for Atmospheric Research (NCAR) has a horizontal resolution of 0.7° and available at 6-hourly intervals. The NCAR Community Atmospheric Model (CAM) shortwave and longwave scheme (Collins et al. 2004), the single-moment three-class cloud microphysics (WSM3), Grell–Devenyi ensemble cumulus convective scheme (Grell 1993), the Yonsei University (YSU) boundary layer scheme (Hong and Pan 1996), and the Noah land surface model (Chen and Dudhia 2001) were used in this simulation. The regional simulation produced model outputs at 3-hourly intervals for analysis of moisture budgets. Results of the regional simulation are interpolated onto the grid cells of ERA-Int for plotting.

The Global Land Data Assimilation Systems dataset (GLDAS; Rodell et al. 2004) with a spatial resolution of 1 degree is used as a reference for PE changes. GLDAS assimilated a variety of conventional data (radiosonde, buoy, ship, and airborne) and satellite-derived observations using a four-dimensional multivariate approach (Rodell et al. 2004). Near-real-time satellite-derived precipitation data were obtained from the U.S. Naval Research Laboratory (NRL) and Goddard Space Flight Center (GSFC). NRL’s precipitation fields were based on geostationary satellite infrared (IR) cloud-top temperature measurements and microwave observation techniques (Turk et al. 2000). The microwave product merged data from the Special Sensor Microwave/Imager (SSM/I), NASA–JAXA (Japan Aerospace Exploration Agency)’s Tropical Rainfall Measuring Mission (TRMM), and the Advanced Microwave Sounding Unit (AMSU) instruments. GSFC’s precipitation fields were based on optimal merging of microwave measurements from the Advanced Microwave Scanning Radiometer (AMSR) and the Advanced Earth Observation Satellite II (AdEOS-II) besides SSM/I and TRMM with the more frequent IR measurements from the international constellation of geosynchronous-Earth-orbit (GEO) satellites (Huffman et al. 2003). Driven by observed temperature and precipitation, GLDAS captures both daily and monthly precipitation variability and exhibits satisfactory performance for many surface variables (Wang and Zeng 2012). Most importantly, the GLDAS product replicates the observed contrast in PE changes between the northwestern and southeastern TP (Gao et al. 2014). The ensemble mean of the multiland model products is used as the reference E in this study.

Observed annual surface air temperature and precipitation averaged across the TP showed abrupt changes around 1998 (Fig. 2 in Gao et al. 2014). Following Gao et al. (2014), 1998 is used as the pivotal year to study changes in PE from 1979 to 2011, using the difference defined as
e1
where the subscripts “pre” and “post” represent the average in 1979–97 (prephase) and 1998–2011 (postphase), respectively. Terrestrial P, E, and PE changes are compared with GLDAS. For the mechanism behind the PE changes, we analyze the column integrated moisture flux convergence. To reduce uncertainty in postprocessing, vertical integration of the moisture budget terms is performed at the original model levels assuming a piecewise linear profile from the surface to the top level of the datasets to give a second-order approximation (Y. Gao et al. 2012). Changes of PE in the postphase relative to the prephase can be decomposed into mean moisture flux convergence and transient eddy moisture convergence:
e2
where represents the changes in surface contributions; Ps is the surface pressure, qs is the surface specific humidity, and Vs is the wind vector. Since the surface contributions are a few times smaller than the other terms in Eq. (2) [for details, please see Seager et al. (2010) and Gao et al. (2014)], they are neglected in subsequent analysis.
The mean moisture flux convergence change is further separated into the thermodynamic contributor (TH) due to changes in mean specific humidity () and the dynamic contributor due to changes in mean circulation (). Consequently, PE changes are attributed to a combination of changes in thermodynamics (TH), dynamics (MCD), and transient eddy (TE) components as denoted in Gao et al. (2014):
e3

3. Results

a. P − E changes

For comparison, PE changes from GLDAS and ERA-Int are presented alongside those from WRF and the results are shown in Fig. 2. The difference PE exhibits general increases in the vast northwestern TP and decreases in the southeastern TP in all three datasets. However, the pattern of PE changes in the WRF simulation (Fig. 2h) resembles GLDAS (Fig. 2g) more than ERA-Int (Fig. 2i) does. The pattern correlation of PE changes between GLDAS and WRF is 0.17 (Table 1), which passes the statistical significance t test at a 98% confidence level. In testing the statistical significance of the pattern correlation, we estimated the degree of freedom based on the autocorrelation length of the dataset following Leith (1973). In contrast, the correlation (0.04) between GLDAS and ERA-Int does not pass statistical significance even at 70% confidence level, although ERA-Int is already reproducing the observed PE climatology and trend in the TP better than other popular reanalyses (Gao et al. 2014). Most notably, the WRF simulation captures the larger increase in the Qiangtang plateau over the northwest comparable to GLDAS, but ERA-Int shows increases that are more evenly distributed over the TP and a slight positive gradient from the northwestern to southeastern TP (Fig. 2i). Furthermore, the decrease in the southeastern TP has smaller values and spreads across the lower reaches of the Upper Brahmaputra River basin in WRF (Fig. 2h) compared to the larger, more concentrated decreases in ERA-Int (Fig. 2i). Interestingly, the pattern correlation coefficients between PE in GLDAS with HGT and MODIS annual mean LAI averaged from 2001 to 2012 are 0.20 and −0.27, respectively. The positive correlation of the GLDAS PE change with topography indicates larger PE increase at higher elevation, while the negative correlation of the GLDAS PE change with LAI suggests that PE increases more over the dry land cover type (type 19) with smaller LAI in GLDAS. These relationships of PE changes in GLDAS with elevation and LAI are well captured by the WRF simulations showing high pattern correlations of PE with topography (Fig. 1a) and LAI (Fig. 1c). However none of these relationships is captured in ERA-Int (Table 1). This suggests a potential shift toward topography and vegetation distribution as a stronger driver of PE changes at regional scale, and further supports the importance of resolving topography and land cover using models with higher resolution to simulate regional changes in PE.

Fig. 2.
Fig. 2.

Changes in (a)–(c) P, (d)–(f) E, and (g)–(i) PE (unit: mm day−1) of 1998–2011 compared to 1979–97 for OBS/GLDAS, WRF simulation, and ERA-Int, respectively, over the TP.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

Table 1.

Pattern correlations between annual PE change in GLDAS and ERA-Int/WRF simulation in 1998–2011 compared to 1979–97 and pattern correlations between ERA-Int/WRF simulation with topography (HGT) and leaf area index (LAI) climatology in 2000–11. The degree of freedom is 189. Correlation coefficients with an asterisk are statistically significant at 98% confidence level based on a two-tailed t test, and the same applies in other tables.

Table 1.

Comparing the P and E changes with PE changes separately for GLDAS, WRF, and ERA-Int (Figs. 2a–f), P changes significantly contribute to the PE change pattern in all three datasets (Table 2). Furthermore, Table 2 shows that the PE changes are explained much more by the P changes than the E changes in GLDAS and WRF, but in ERA-Int the PE changes explained by the P and E changes are more comparable. Like most GCMs, ERA-Int greatly overestimates P in the TP (Fig. 2c) but the wet biases at high elevation are reduced in WRF at high resolution (Fig. 2b). Similar bias reduction is also found in PE in many subregions of the TP except QDB and QLB (Figs. 2h, 3, and 1a; regions are defined in Fig. 1). Larger improvements are most notable in the southern (Figs. 3a–d) rather than the northern TP (Figs. 3e–i), hinting that the WRF improvements may be related to better representation of the sharp topographic gradient associated with the Himalayas in the southern slope and its impacts on moisture transport and precipitation.

Table 2.

Pattern correlations between annual PE change with P and E changes in GLDAS and ERA-Int/WRF simulation in 1998–2011 compared to 1979–97. Correlation coefficients with an asterisk are statistically significant at 98% confidence level based on a two-tailed t test, and the same applies in other tables.

Table 2.
Fig. 3.
Fig. 3.

Time series of annual mean P, E, and PE (unit: mm day−1) for nine subregions labeled in Fig. 1a corresponding mostly to the headwater of major rivers in Asia that originate from the TP.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

b. P − E changes contributions

Figures 4a–h present the PE changes estimated from the column integrated moisture flux convergence and its three contributors (δMCD, δTH, and δTE) in the WRF simulation and ERA-Int. As demonstrated in Gao et al. (2014), changes in the mean circulation dynamics (δMCD) contribute the most to PE changes (Figs. 4b,d). The WRF simulation presents the same predominant contribution of δMCD to PE changes (Figs. 4a,c), which explains 41% of the PE change pattern (Table 3). Unlike ERA-Int, however, changes in the thermodynamics (δTH) and the transient eddy (δTE) also contribute importantly to PE changes in the high-resolution simulation (Figs. 4e,g and Table 3 vs Figs. 4f,h). Pattern correlations between δTH and δTE with the PE changes are 0.40 and −0.26, respectively, both statistically significant at 99.9% confidence level (Table 3).

Fig. 4.
Fig. 4.

Distributions of annual (a),(b) PE changes and contributions from (c),(d) mean circulation dynamics (δMCD), (e),(f) thermodynamics (δTH), and (g),(h) transient eddy (δTE) from (left) the WRF simulation and (right) ERA-Int in 1998–2011 compared to 1979–1997 (Unit: mm day−1).

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

Table 3.

Pattern correlations between annual PE changes and annual changes in mean circulation contribution (δMCD), thermodynamic contribution (δTH), and transient eddy contribution (δTE) in WRF simulation in 1998–2011 compared to 1979–97. Correlation coefficients with an asterisk are statistically significant at 98% confidence level based on a two-tailed t test, and the same applies in other tables.

Table 3.

1) δTE

Having demonstrated important differences between ERA-Int and the regional simulation in their depiction of PE changes and the contributors to those changes, it would be interesting to contrast how moisture budget terms differ between regional models and their global counterparts in the TP and the Rocky Mountains, two important mountain ranges in the midlatitudes. Y. Gao et al. (2012) found a similar opposite contribution of transient eddy changes to PE changes in the high-resolution simulation over the Rocky Mountains region, which offsets the drying trend in the southwestern United States under climate change, but in the global models the transient eddy changes contribute to the drying trend, exacerbating the vulnerability of the southwestern United States to droughts in the global model projections. For the TP, in contrast, δTE in the regional simulation offsets the wetter trend in the northwestern TP and drier trend in the southeastern TP in the historical period, but it has almost no contribution to the PE changes in ERA-Int. Previous studies of extreme precipitation changes have reported an upward trend in the southern TP and a downward trend in the central TP (Wu et al. 2013; You et al. 2008). Since transient eddy changes are associated with changes in storms, similarities between the distributions of δTE changes in the WRF simulation with observed pattern of extreme precipitation changes suggest that the WRF simulated δTE changes may be more realistic than those of ERA-Int.

In the Rocky Mountains, the difference in the transient eddy changes dominates the difference in PE changes between the high- and coarse-resolution moisture budgets (Gao 2012a). However, in the TP, differences in the thermodynamic changes also contribute to differences in PE changes between the high-resolution simulation and the coarse-resolution forcing. In the narrower, north–south-oriented Rocky Mountains with prevailing westerly winds, higher resolution has important effects on a model’s ability to simulate orographic uplift and partitioning of precipitation between upwind and leeside along the winter storm tracks. Hence, it is not surprising that the primary difference between the high- and coarse-resolution moisture budgets is related to the transient eddy changes. However, the TP is a massive highland with altitude above 5000 m across a vast mountaintop, with its annual water cycle changes dominated more by the warm season changes (Gao et al. 2014). Unlike the Rocky Mountains where the main difference between the regional and global simulations lies in the transient eddies changes, in the TP the regional simulation and ERA-Int differ in both changes in transient eddies and thermodynamic effects.

2) δMCD and δTH

To provide further insights into the mechanisms of the projected hydrological changes and why the regional and global moisture budgets differ, we follow Seager et al. (2010) to further decompose the thermodynamic (δTH) and mean circulation dynamics contributions (δMCD) into terms due to advection of moisture (δTHA and δMCDA) and convergence or divergence of moisture (δTHD and δMCDD). The thermodynamic contribution can be written as
e4
in which Upre represents the wind components in 1979–97 (prephase), δq denotes the moisture change between postphase and prephase, and Ps is the surface pressure.
Similarly, the dynamic contribution can be written as
e5
in which qpre represents the moisture in 1979–97 (prephase), δU denotes the changes in wind components between postphase and prephase, and Ps is the surface pressure.

Figure 5 shows the annual changes of the four terms and Table 4 summarizes the pattern correlation coefficients between the changes in mean circulation/thermodynamics (δMCD/δTH) and the contributions due to the advection terms (δMCDA/δTHA) and the convergence or divergence terms (δMCDD/δTHD), respectively. The correlation between δMCD and δMCDA is negative (Table 4) so the changes in the advection of the mean circulation tend to offset the MCD changes. This is also apparent from Figs. 4c and 5a. However, such offsetting is absent in ERA-Int (Figs. 4d and 5b), which leaves a concentrated region of large MCD changes over the southern TP. Advection of the mean circulation change moisturizes almost the whole TP except for the upper reaches of the Tarim basin and lower reaches of the Brahmaputra River basin in the WRF simulation (Fig. 5a). Given the high pattern correlation of δ( PE) with δMCD (Table 3) and the high correlation of δMCD with δMCDD (Table 4), it is clear that changes in PE are largely attributed to the convergence or divergence changes of the mean circulation (Figs. 4a,c and 5c), consistent with analysis based on ERA-Int (Fig. 5d). Comparing the high- and coarse-resolution changes, the WRF simulation is closer to GLDAS than ERA-Int (Figs. 2a–i). Hence while WRF inherits the correct broad-scale mean circulation changes from ERA-Int through lateral boundary conditions, the added finer-scale circulation change pattern such as the wetter trend in the Qiangtang plateau gives WRF an overall more skillful representation of the PE changes seen in GLDAS.

Fig. 5.
Fig. 5.

Contributions to PE changes from (a)–(d) mean circulation dynamics and (e)–(h) thermodynamic due to advection [in (a),(b),(e),(f)] and convergence or divergence [in (c),(d),(g),(h) ] from (left) the WRF simulation and (right) ERA-Int in 1998–2011 compared to 1979–97 in WRF simulation (Unit: mm day−1).

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

Table 4.

Pattern correlations between annual changes in mean circulation/thermodynamic contribution (δMCD/δTH) and annual mean circulation dynamics/thermodynamic contributions due to the advection of moisture (δMCDA/δTHA), convergence or divergence of moisture (δMCDD/δTHD), and seasonal changes in mean circulation/thermodynamic contributions (δMCDwet/δTHwet and δMCDDRY/δTHDRY) in 1998–2011 compared to 1979–97 in the WRF simulation. Correlation coefficients with an asterisk are statistically significant at 98% confidence level based on a two-tailed t test, and the same applies in other tables.

Table 4.

Divergence changes in the mean circulation could be separated into changes in the zonal and meridional wind components. Figure 6 shows the circulation changes at 200 mb in the high-resolution simulation and the coarse-resolution forcing. Similar to ERA-Int, the high-resolution simulation presents a weaker westerly, which controls the general wetter trend in the TP, as Gao et al. (2014) demonstrated. Differences exist in the meridional component changes. Stronger northwesterly changes over the northwestern TP are presented in the high-resolution simulation (Fig. 6a). The stronger northwesterly changes push the moisture air from the south to the northwestern TP in the high-resolution simulation, which causes the wetter trend in the Qiangtang plateau (Figs. 2b,h) than in the forcing from ERA-Int. In ERA-Int, circulation changes in the northwestern TP are almost subdued (Fig. 6b). In the southern TP, there are even slight increases in northeasterly winds. The almost subdued changes in the northwestern TP and slight increase in northeasterly winds in the southern TP hold the moisture-rich air in the south, which forms the pattern in P and PE changes in ERA-Int (Figs. 2c,i). As Table 4 demonstrated in Gao et al. (2014), the circulation changes in the upper level are highly correlated to the thermal wind changes, which are triggered by the thermal gradient changes above. The circulation changes in the high-resolution simulation are constrained by the same mechanism related to the thermal changes as in ERA-Int. However, major differences are found and interpreted in the following discussion.

Fig. 6.
Fig. 6.

Distributions of changes in circulation (unit: m s−1) at 200 mb in 1998–2011 compared to 1979–97 in (a) the WRF simulation and (b) ERA-Int.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

As mentioned above, a major difference between the WRF simulation and ERA-Int is the thermodynamic contribution. There is almost no contribution of thermodynamic change to PE changes in ERA-Int (Figs. 4b, 4f), but thermodynamic change contributes to PE changes in the WRF simulation, especially for the wetter trend in the Qiangtang plateau and Yangtze River headwater and the drying in the southern TP (Figs. 4a,e). When the thermodynamic change is further decomposed into δTHA and δTHD, the pattern correlation between δTHD and δTH is higher than the value between δTHA and δTH (Table 4), suggesting that moisture changes driven by the divergence are more responsible for the thermodynamic changes. Note that δTHD exhibits broad positive changes in the TP except for the upper reaches of the Tarim basin, Qaidam, and lower reaches of the Brahmaputra River basin (Fig. 5g). In contrast, δTHA exhibits mostly negative changes except for the Qiangtang plateau and Qaidam (Fig. 5e).

In recent decades, the largest and second largest inner river basin in the arid northern marginal TP, the Tarim basin and the Heihe River basin, experienced discharge increases (Figs. 2g, 3h,i) revealed by historical records (Wang and Meng 2008; Jiang and Xia 2007). In Figs. 4a,b and 5c,g, the PE increase over the upper reaches of Tarim basin comes from the divergence change of the mean circulation (Fig. 6a) while the PE increase over the Qilian Mountain stems from the moisture change indirectly driven by the convergence at low levels. The PE decrease in the Qaidam basin also comes from the moisture change.

Annual changes in MCD and TH are both more highly correlated with changes in the wet season than the dry season (Table 4; Gao et al. 2014). Therefore, changes in the wet season are further investigated. Changes in divergence at 500 and 100 mb, average vertical motion, moisture, and microphysics diabatic latent heat averaged between 500 and 200 mb are presented in Fig. 7. The patterns in Fig. 7 are all broadly similar to the δMCD (Fig. 4c) and δMCDD (Fig. 5c) patterns. More convergence and divergence are noted in the northwestern and southeastern TP, respectively (Fig. 7a). So moisture is advected to the northwestern TP from its surroundings and enhances moisture from the Qiangtang plateau to Qilian Mountain (Fig. 7d) as shown in Fig. 6a. Larger convergence triggers upward motion (Fig. 7c), resulting in condensation and release of diabatic latent heat by cloud microphysics processes (Fig. 7e) that further heats the air masses over the northwestern TP and enhances divergence at upper levels (Fig. 7b). The patterns of column integrated moisture, divergence, vertical motion, and microphysics diabatic latent heat all match the δMCDD and PE changes patterns very well.

Fig. 7.
Fig. 7.

Distributions of changes in (a),(b) divergence (unit: 10−6 s−1) at 500 and 100 mb, (c) vertical motion (unit: mm s−1), (d) column integrated moisture (unit: g kg−1), and (e) microphysics diabatic latent heat (unit: K s−1) in 1998–2011 compared to 1979–97 in the WRF simulation.

Citation: Journal of Climate 28, 10; 10.1175/JCLI-D-14-00581.1

Comparing Figs. 7a and 7b herein to Figs. 11a and 11b in Gao et al. (2014), we see different divergence changes in the WRF simulation and ERA-Int, especially at low levels. Unlike the evenly increased convergence at low levels in ERA-Int, contrasting changes in divergence between the northwestern and southeastern TP are clear in the WRF simulation. This heterogeneity in divergence is likely related to heterogeneity in topography and overlaid vegetation captured by the finer-scale simulation but not the coarse forcing (Figs. 1, 2h, 2i). This uneven pattern is strengthened through feedback by the latent heat release associated with the convergence and condensation. Herein we identified the precipitation changes and subsequent changes in convergence (divergence) as an important mechanism leading to MCD increases (decreases) in the vast northwestern (southeastern) TP in the regional simulation (Fig. 2h). Furthermore, the larger convergence in the northwestern TP at low levels attracts more moisture to the Qiangtang plateau and Qilian Mountain from the surroundings, which enhances moisture in a swath extending from the Qiangtang plateau to Qilian Mountain, leading to TH changes in the TP (Fig. 4c). This mesoscale contrast in divergence at low levels is captured by the regional simulation but not coarse-resolution products like ERA-Int.

4. Conclusions

The PE changes from a high-resolution regional climate simulation and the global reanalysis that provided boundary forcing are evaluated by comparing them to GLDAS in the TP. The high-resolution WRF climate simulation not only improves the pattern of PE changes compared to GLDAS over the best available reanalysis, but it also provides new and substantial findings regarding the contributions of the thermodynamic and the transient eddy components of moisture flux convergence. Most notably, the WRF simulation better represents the observed positive (negative) changes in the vast northwestern (southeastern) TP than the coarse-resolution reanalysis forcing. The improved PE change pattern is attributed to improved P changes and reduced P biases at high elevations in the high-resolution simulation. Furthermore, the WRF simulation captures the correlation between PE changes with topography and LAI in the TP similar to that found in GLDAS. This demonstrates that over the TP with complex terrain, high-resolution climate simulations can provide important insights into regional water cycle changes.

Besides the predominant contribution of the mean circulation dynamic changes inherited from the large-scale forcing, thermodynamic and transient eddy changes also contribute importantly to PE changes in the WRF simulation but not the reanalysis. Contrasting divergence changes at low levels between the northwestern and southeastern TP induced by land surface heterogeneity trigger stronger (weaker) upward motion in the vast northwestern (southeastern) TP, leading to PE increases (decreases) by MCD changes. The larger convergence in the northwestern TP attracts more moisture to Qiangtang plateau and Qilian Mountain from the surroundings and strengthens PE changes through diabatic latent heat released and TH change contribution.

Although the WRF simulation provided improved depictions of PE changes in the TP, the PE increase in the Yellow River basin and Rouergai plateau is still uncertain because the horizontal resolution of 30 km used in the WRF simulation is still not fine enough to resolve the complexity in land surface properties such as the wetland in the Rouergai plateau and land and atmospheric processes over the TP. Furthermore, many studies reported that the Yellow River basin has experienced large land cover/land use changes in recent decades (Li and Liu 2004; Dong et al. 2005; Cuo et al. 2013), which are not included in our simulation. Increasing model resolution (Jiménez and Dudhia 2012; Ikeda et al. 2010; Rasmussen et al. 2011) and including variable land characteristics/parameters (Gao et al. 2008; Meng et al. 2009) might provide additional realisms to better understand PE changes in the eastern TP.

Acknowledgments

The reanalysis data used in this study are from the Research Data Archive (RDA), which is maintained by the Computational and Information Systems Laboratory (CISL) at the National Center for Atmospheric Research (NCAR). We acknowledge the Super Computing Center of Chinese Academy of Sciences for providing resources for conducting the simulations. This work is jointly supported by National Basic Research Program of China (2013CB956004), National Natural Science Foundation of China (41322033), and “100-Talent” program granted by the Chinese Academy of Sciences to Yanhong Gao and Lan Cuo. Ruby Leung is supported by the Office of Science of the U.S. Department of Energy through the Regional and Global Climate Modeling program. Pacific Northwest National Laboratory is operated for Department of Energy by Battelle Memorial Institute under Contract DE-AC05-76RL01830.

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