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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]

Soil moisture can be an important preceding signal in seasonal precipitation prediction because of its persistence and influence on the energy and water balance between land and atmosphere. The thawing process of frozen ground on the Tibetan Plateau (TP) during spring makes it difficult to accurately simulate soil variables (e.g., soil moisture and temperature) due to defective parameterizations in numerical models during spring. In this study, we investigate the effectiveness of soil moisture correction using an indirect nudging scheme to simulate the coupling between spring soil moisture and the subsequent precipitation with an advanced research version of the weather research and forecasting (WRF) model. The results without assimilation show that extreme cold and dry land surface states during spring cause a large bias in the spatial pattern of summer precipitation over almost the entire TP. However, the experiments with indirect assimilation in spring show that the spatial pattern of summer precipitation improved significantly. This comparison emphasizes the importance of correcting the spring soil moisture during the freeze–thaw period to significantly adjust the heat and water vapour exchange between the land and atmosphere. Additionally, changes in the circulation of the subtropical–tropical jet, vertical ascent, and region‐related moisture recycling can support an active convection environment over the eastern TP, thereby increasing summer precipitation. Furthermore, the pattern shift in water vapour convergence in summer highlights the regional horizontal transportation of water vapour. The differences between experiments with and without deep‐soil temperature nudging illustrate the significance of soil temperature in soil moisture simulation in the freeze–thaw process. More so, the deep‐soil temperature nudging scheme requires in‐depth studies to correct its variation and quantify its degree of impact on soil moisture. [ABSTRACT FROM AUTHOR]

In the context of climate change, extreme precipitation in semi-arid region happens frequently. How well models simulate extreme precipitation in semi-arid region remains unclear. Based on a WRF v4.3 simulation of a rainstorm event that occurred in Qingyang, China on 21 July 2019, applying Kain–Fritsch (KF), Grell–Devenyi (GD) and Bullock–Wang (BW) schemes, the impacts of different cumulus parameterizations on extreme precipitation simulations in semi-arid region were analyzed, and the possible causes of precipitation biases were explored. The results showed that the WRF with the three schemes essentially reproduced the location and structure of precipitation, but the intensity of precipitation in the central region was underestimated. Based on the structure-amplitude-location (SAL) method, the KF scheme exhibited better performance in precipitation simulation than the other two schemes, while there were significant intensity and location deviations of rain band occurrence between simulations using the GD, BW schemes and observations. Convection simulation using the GD and BW schemes was less effective than that using the KF scheme, compared to the observations. As a result, the GD and BW schemes simulated a larger geopotential height at 500 hPa over Qingyang and weaker upper-level low troughs than simulations using the KF scheme. This led to simulation of less water vapor transport into the front of the trough, resulting in a deficit in simulated precipitation. The study results highlight the impacts of convection on large-scale atmospheric circulation linked to extreme precipitation in semi-arid region. [ABSTRACT FROM AUTHOR]

This article discusses extreme precipitation events (EPEs) in arid regions, which are ecologically fragile and highly sensitive to climate change. Despite their aridity, EPEs are observed in these regions, and their increasing frequency due to global warming poses challenges for ecosystems and sustainable development. The article highlights the need for research on the characteristics and changing trends of EPEs, the sources of water vapor, the physical mechanisms driving EPEs, and the relationship between EPEs in arid regions and ocean/land signals. It also mentions the challenges in simulating EPEs in arid regions and the importance of improving simulations and understanding EPEs. The article provides a summary of eight original research papers included in the Research Topic, which cover changes in EPEs, physical processes and mechanisms, and simulations and parameterizations of extreme precipitation. The article concludes by emphasizing the need for further research to deepen our understanding of the complex mechanisms behind EPEs in arid regions. [Extracted from the article]

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]

Evaporation is a principal factor in the hydrological cycle and energy exchange; however, estimations of evaporation include large uncertainties. In this study, a modified estimation of evaporation based on empirical linearly simplified Penman evaporation (PES) is proposed, soil moisture and precipitation are used to correct the land surface evaporation estimation, and the temporal and spatial characteristics of the corrected evaporation (CE) are investigated globally. The results show that CE is strong at low latitudes and weak at high latitudes. CE has obvious seasonal variation, ranging from 0.2 to 4.0 mm day−1; CE is prominent in summer but feeble in winter. Compared to PES, CE is generally weaker in most regions, especially in arid regions, with differences of more than 9 mm day−1. CE agrees well with evaporation derived from FLUXNET-Model Tree Ensemble (FLUXNET-MTE), MERRA, and GLDAS. In general, the root-mean-square error (RMSE) between annual CE and FLUXNET-MTE is less than 0.2 mm day−1, and CE is about 5%–10% less than the evaporation of FLUXNET-MTE. In the arid regions, the maximum CE almost occurs in the month with the strongest precipitation; in the tropical regions, soil moisture enhances CE only when precipitation is less. In the context of global temperature rise, PES always shows an apparent increasing trend due to the water supply is not considered; however, CE decreases in western Asia, the western United States, the Amazon basin, and Central Africa, but weakly increases in the other study regions from 1984 to 2013. This study provides a method for estimating evaporation considering more restrictive factors on evaporation. [ABSTRACT FROM AUTHOR]

Short-term climate prediction based on a regional climate dynamical model heavily depends on atmospheric forcing and initial soil moisture state. In this study, the Weather Research and Forecasting (WRF) Model with different nudging schemes is used for approximate 2-yr simulations for investigating the importance of soil variables in seasonal temperature and precipitation simulations. The results show that the improvement of seasonal climate simulation (precipitation and air temperature) is more evident in the experiment of assimilating both soil and atmospheric variables than that in the experiments of assimilating atmospheric variables only. Further investigation of the impact of indirectly assimilating soil moisture on precipitation prediction with an indirect soil nudging (ISN) scheme shows that the precipitation reproducibility in summer is better than that in winter, and the effect of ISN is particularly prominent in the region where seasonal precipitation exceeds 200 mm. Moreover, statistical results also illustrate that initial soil moisture plays a crucial role in seasonal precipitation forecasts because of its slowly evolving nature, and its effect is more distinct in semiarid and semihumid regions than in arid and humid regions. The effects of indirectly assimilating soil moisture on precipitation can last two and three months in semiarid and semihumid areas, respectively. [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]