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Afforestation is an important human activity that changes land use, and would affect regional climate. However, the impact of afforestation on the regional climate in arid and semi-arid regions remains a subject of controversy. By ensemble sensitivity simulations using Advanced Research Weather Research and Forecasting (WRF) model, this study investigates the influence of vegetation category change within a 100 km width along the middle and upper reaches of the Yellow River (YR) on regional climate. Results suggest that afforestation would reduce summer surface temperature by -0.05 ~ -0.8℃ and increase summer convective precipitation by 3 ~ 45 mm in the afforestation regions, instead, summer non-convective precipitation would increase (3 ~ 35 mm) in the upper reaches of the YR. The increased non-convective precipitation in the upper reaches of the YR could be attributed to the increased integral water vapor convergence, cloud hydrometeor, and decreased cloud top temperature. In the afforested region, the increased convective precipitation could be related to the enhanced thermodynamic and water vapor conditions. These findings emphasize that even on regional scale in semi-arid and arid regions, greening can lead to evident regional climate impacts, these also provide insight for policymakers in formulating sustainable afforestation strategies. [ABSTRACT FROM AUTHOR]

Arid, semi-arid, and semi-humid regions (drylands) with fragile ecological balance have undergone dramatic climate change in past decades, and how the desertification will change under a continuous warming background still remain uncertain. In this study, the bias-corrected Community Earth System Model outputs from Coupled Model Intercomparison Project phase 5 were dynamically downscaled using the Weather Research and Forecasting (WRF) model, based on which the evolution trend of desertification over northern China (NC) in the past (1972–2000) and the near future period (2021–2050) under the RCP8.5 scenario were analyzed using the dune mobility index, and the impacts of climate change on the intensification or reversal of desertification over NC in the near future were explored. The results show that WRF downscaling can reproduce the desertification changes over NC in the past. The regions with a high risk of desertification are mainly located on the border of the desert and gobi. Under a rapidly warming climate in the near future, desertification will likely reverse in most regions of NC, especially for regions north of 40°N over NC. Potential evapotranspiration changes will exacerbate desertification, while precipitation changes will promote rehabilitation, and wind speed changes show obvious local impacts on desertification. The results in this study imply that, with rising temperatures in the future, the extent of desertification will not always continue, desertification will likely reverse at the front and margin of deserts and gobi, and responses of desertification to climate change have significant spatial differences. [ABSTRACT FROM AUTHOR]

The snow melting and frozen soil thawing processes in spring could lead to the significant anomaly in soil moisture (SM) over the Tibetan Plateau (TP), which has a strong influence on the interaction between land and atmosphere, and influencing the subsequent summer precipitation in China. However, there is distinct bias in SM in model simulation, influencing the surface energy and mass balance. In this study, the impacts of assimilating spring SM over the TP with indirect soil nudging (ISN) scheme on the simulation of summer precipitation in China are investigated. The results show that using the ISN method can reduce the spring SM bias over the TP, which further decreases the root mean square error (RMSE) of summer precipitation simulation, especially in South China, North China, northeast China, and the middle and lower Yangtze River. Compared to the experiment without assimilation, the assimilation of spring SM evidently modulates the distribution and intensity of surface sensible and latent heating over the TP, which leads to the change of Rossby wave train's (RWT) shape and propagation direction over the TP and the downstream region, affecting the general circulation over East Asia. Specifically, the upward movement over the TP gets stronger and leads to the position of 200 hPa westerly jet which shifts to west and is closer to ERA5 reanalysis; meanwhile the location of the 500 hPa western Pacific subtropical high (WPSH) and the distribution of water vapor in Eastern China are also corrected. As a result, the precipitation simulation biases are alleviated. This study suggests that decreasing SM bias caused by the snow melting and frozen soil thawing can be an effective approach to improve the seasonal prediction skills of precipitation in China. [ABSTRACT FROM AUTHOR]

Local moisture recycling (precipitation recycling) plays a crucial role in precipitation. However, evapotranspiration, the key to precipitation recycling, is difficult to estimate. In this study, restrictions between the evapotranspiration ability and soil moisture (SM) supply are considered, and evapotranspiration estimations from 1981 to 2010 in Afro-Eurasia are examined. The results show that the improved evapotranspiration estimation obtained by correcting the potential evapotranspiration in Afro-Eurasia, especially in the Asia-Africa arid regions, ranges from 0.2 to 1.2 mm day−1, which was less than that obtained using the traditional Penman-Monteith evapotranspiration (PET, which ranges from 1.0 to approximately 11.0 mm day−1). Based on different evapotranspiration estimations, the characteristics of the precipitation recycling ratio (PRR) calculated using the dynamic recycling model (DRM) are analyzed and compared for three arid regions in the Asian-African continent, China-Mongolia (CM), West Asia (WA), and North Africa (NAF), during the precipitation season. A comparison with the results from the PET method reveals that the estimated evapotranspiration and precipitation recycling obtained using the corrected approach was more reasonable than that obtained using the Penman-Monteith method. Overall, the PRR in CM (about 0.7%) and NAF (about 0.5%) shows decreasing trends, whereas the PRR in WA (about 1.0%) increased, which implies that because the local moisture supply increased in WA and reduced in CM and NAF, the drought intensity increased in WA but weakened in CM and NAF. This trend was partially related to increased precipitation recycling that occurred with increased evapotranspiration in WA. Moreover, the negative PRR trend and alleviated drought intensity in CM and NAF implied that precipitation recycling had a negative effect when there was less local moisture supply in the region, and this alleviated the drought intensity in the Asian-African arid regions. [ABSTRACT FROM AUTHOR]

Changes of precipitation recycling (PR) in Northern Hemisphere from 1981 to 2010 are investigated using a water recycling model. The temporal and spatial characteristics of recycling in arid regions are analyzed. The results show that the regional precipitation recycling ratio (PRR) in arid regions is larger than in wet regions. PRR in arid regions has obvious seasonal variation, ranging from more than 25 % to less than 1 %. Furthermore, in arid regions, PRR is significantly negatively correlated with precipitation (correlation coefficient r = −0.5, exceeding the 99 % significance level). Moreover, the trend of PRR is related to changes in precipitation in two ways. PRR decreases with increasing precipitation in North Africa, which implies that less locally evaporated vapor converts into actual precipitation. However, in Asian arid regions, the PRR increases as precipitation reduces, which implies that more locally evaporated vapor converts into rainfall. Further, as PRR mainly depends on evapotranspiration, the PRR trend in Asian arid regions develops as temperature increases and more evaporated vapor enters the atmosphere to offset the reduced rainfall. [ABSTRACT FROM AUTHOR]

The statistical characteristics of the vertical structure of clouds in the Asian summer monsoon region are investigated using two CloudSat standard products (Geometrical Profiling Product (GEOPROF) and GEOPROF-lidar) during the pre- and post-onset periods of the Asian summer monsoon, from April to August in 2007-2010. The characteristics of the vertical structure of clouds are analyzed and compared for different underlying surfaces in four subregions during this period. Also analyzed are the evolution of precipitation and hydrometeors with the northward advance of the Asian summer monsoon, and different hydrometeor characteristics attributed to the underlying surface features. The results indicate that the vertical cloud amounts increase significantly after the summer monsoon onset; this increase occurs first in the upper troposphere and then at lower altitudes over tropical regions (South Asian and tropical Northwest Pacific regions). The heights of the cloud top ascend, and the vertical height between the top and the base of the whole cloud increases. Single-layer (SL) and double-layer (DL) hydrometeors contribute over half and one third of the cloudiness in these 5 months (April to August), respectively. The multilayer frequencies increase in four different regions, and cloud layer depths (CLD) increase after the summer monsoon onset. These changes are stronger in tropical regions than in subtropical regions, while the vertical distance between cloud layers (VDCL) deceases in tropical regions and increases in subtropical regions. [ABSTRACT FROM AUTHOR]