Li et al., Influence of Tibetan Plateau snow cover on East Asian atmospheric circulation at medium-range time scales, Nature Communications, 2018
The responses of atmospheric variability to Tibetan Plateau (TP) snow cover (TPSC) at seasonal, interannual and decadal time scales have been extensively investigated. However, the atmospheric response to faster subseasonal variability of TPSC has been largely ignored. Here, we show that the subseasonal variability of TPSC, as revealed by daily data, is closely related to the subsequent East Asian atmospheric circulation at medium-range time scales (approximately 3-8 days later) during wintertime. TPSC acts as an elevated cooling source in the middle troposphere during wintertime and rapidly modulates the land surface thermal conditions over the TP. When TPSC is high, the upper-level geopotential height is lower, and the East Asia upper-level westerly jet stream is stronger. This finding improves our understanding of the influence of TPSC at multiple time scales. Furthermore, our work highlights the need to understand how atmospheric variability is rapidly modulated by fast snow cover changes.
Li et al., Rapid response of the East Asian trough to Tibetan Plateau snow cover, International Journal of Climatology, 2020
Tibetan Plateau snow cover (TPSC) has subseasonal variations and rapidly influences the atmosphere. In this study, we present the rapid response of the East Asian trough (EAT) within a week to subseasonal variations in TPSC during the boreal winter. Using snow cover analysis obtained from the daily interactive multisensor snow and ice mapping system and the ERA-Interim reanalysis, a considerable relationship between TPSC and 500-hPa geopotential height anomalies over the downstream EAT region is found. Significant negative (positive) 500-hPa geopotential height anomalies originating from the Tibetan Plateau and moving into the EAT region appear within a week following anomalous positive (negative) TPSC events, which lead to changes in EAT strength. Thus, a significantly enhanced (reduced) intensity of the EAT occurs approximately 5-6 days after increased (decreased) TPSC. Numerical experiments confirm the causality of this relationship. Further analysis of the quasi-geostrophic geopotential height tendency equations in numerical experiments indicates that such EAT variations result from anomalous thermal advection from the Tibetan Plateau forced by TPSC.
Qiu et al., Responses of Australian dryland vegetation to the 2019 heat wave at a subdaily scale, Geophysical Research Letters, 2020
Satellite solar-induced chlorophyll fluorescence (SIF) products from the Global Ozone Monitoring Experiment 2 (GOME-2) and Orbiting Carbon Observatory 2 (OCO-2) are used to investigate the responses of vegetation growth to the 2019 heat wave in Australia. Both satellite SIF data sets are more sensitive to water and heat stress than is the greenness-based vegetation index (enhanced vegetation index). Moreover, the OCO-2 SIF observations show a more significant reduction and earlier response to the heat stress than does GOME-2 SIF, indicating that the two satellite SIF data sets differ in how they monitor the drought and heat wave event due to the different timing of observations. Eddy covariance measurements confirm the different responses of dryland vegetation to the 2019 heat wave at a subdaily time scale. The differences in the timing of the satellite SIF products can be used to assess different elements of the impact of heat and water stress on Australian dryland ecosystems.
Ge et al., Impact of revegetation of the Loess Plateau of China on the regional growing season water balance, Hydrology and Earth System Sciences, 2020
To resolve a series of ecological and environmental problems over the Loess Plateau, the "Grain for Green Program" (GFGP) was initiated at the end of 1990s. Following the conversion of croplands and bare land on hill-slopes to forests, the Loess Plateau has displayed a significant greening trend, which has resulted in soil erosion being reduced. However, the GFGP has also affected the hydrology of the Loess Plateau, which has raised questions regarding whether the GFGP should be continued in the future. We investigated the impact of revegetation on the hydrology of the Loess Plateau using relatively high-resolution simulations and multiple realizations with the Weather Research and Forecasting (WRF) model. Results suggest that revegetation since the launch of the GFGP has reduced runoff and soil moisture due to enhanced evapotranspiration. Further revegetation associated with the GFGP policy is likely to further increase evapotranspiration, and thereby reduce runoff and soil moisture. The increase in evapotranspiration is associated with biophysical changes, including deeper roots that deplete deep soil moisture stores. However, despite the increase in evapotranspiration, our results show no impact on rainfall. Our study cautions against further revegetation over the Loess Plateau given the reduction in water available for agriculture and human settlements and the lack of any significant compensation from rainfall.
Ge et al., The nonradiative effect dominates local surface temperature change caused by afforestation in China, Journal of Climate, 2019
China is several decades into large-scale afforestation programs to help address significant ecological and environmental degradation, with further afforestation planned for the future. However, the biophysical impact of afforestation on local surface temperature remains poorly understood, particularly in midlatitude regions where the importance of the radiative effect driven by albedo and the nonradiative effect driven by energy partitioning is uncertain. To examine this issue, we investigated the local impact of afforestation by comparing adjacent forest and open land pixels using satellite observations between 2001 and 2012. We attributed local surface temperature change between adjacent forest and open land to radiative and nonradiative effects over China based on the Intrinsic Biophysical Mechanism (IBM) method. Our results reveal that forest causes warming of 0.23 degrees C (+/- 0.21 oC) through the radiative effect and cooling of -0.74 degrees C (+/- 0.50 oC) through the nonradiative effect on local surface temperature compared with open land. The nonradiative effect explains about 79% (+/- 16%) of local surface temperature change between adjacent forest and open land. The contribution of the nonradiative effect varies with forest and open land types. The largest cooling is achieved by replacing grasslands or rain-fed croplands with evergreen tree types. Conversely, converting irrigated croplands to deciduous broadleaf forest leads to warming. This provides new guidance on afforestation strategies, including how these should be informed by local conditions to avoid amplifying climate-related warming.
Ge et al., Do uncertainties in the reconstruction of land cover affect the simulation of air temperature and rainfall in the CORDEX region of East Asia? Journal of Geophysical Research-Atmospheres, 2019
Land cover type reconstructions, required in climate models, commonly utilize remote sensing products. There are inevitable misclassifications in land cover reconstructions due to the retrieval process. We use the Weather Research and Forecasting model to determine whether these misclassifications can affect the simulations of air temperature and rainfall over the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia region, where the accuracy of the land cover classification is low. The Moderate Resolution Imaging Spectroradiometer land cover map is used for the control simulations and is then replaced by the most likely alternative land cover type at pixels where the classification confidence is below various threshold values. Results show that misclassification-induced land cover change can affect key biogeophysical characteristics (albedo, leaf area index, and roughness length) and these can affect the sensible and latent heat fluxes at regional scales. However, the impact on air temperature is very limited and is restricted to the Tibetan Plateau where warming of up to 2 oC occurs associated with the replacement of barren or sparsely vegetated land to grassland. The impact on rainfall is negligible, and most changes are likely caused by model internal variability rather than land cover change. Overall, uncertainties in the reconstruction of land cover have negligible impacts, and the Moderate Resolution Imaging Spectroradiometer land cover product can be used in regional simulations over East Asia. However, we note that land cover change experiments incorporating uncertainties must utilize large numbers of simulations if air temperature and rainfall changes are to be examined robustly.
Wang et al., Quantifying the contribution of land use change to surface temperature in the lower reaches of the Yangtze River, Atmospheric Chemistry and Physics, 2017
Anthropogenic land use has a significant impact on climate change. Located in the typical East Asian monsoon region, the land-atmosphere interaction in the lower reaches of the Yangtze River is even more complicated due to intensive human activities and different types of land use in this region. To better understand these effects on micro-climate change, we compare differences in land surface temperature (Ts) for three land types around Nanjing from March to August, 2013, and then quantify the contribution of land surface factors to these differences (ΔTs) by considering the effects of surface albedo, roughness length, and evaporation. The atmospheric background contribution to ΔTs is also considered based on differences in air temperature ( ΔTa. It is found that the cropland cooling effect decreases Ts by 1.76 oC and the urban heat island effect increases Ts by 1.25 oC. They have opposite impacts but are both significant in this region. Various changes in surface factors affect radiation and energy distribution and eventually modify Ts. It is the evaporative cooling effect that plays the most important role in this region and accounts for 1.40 oC of the crop cooling and 2.29 oC of the urban warming. Moreover, the background atmospheric circulation is also an indispensable part in land-atmosphere feedback induced by land use change and reinforces both these effects.
Guo et al., Comparison of land-atmosphere interaction at different surface types in the mid- to lower reaches of the Yangtze River valley, Atmospheric Chemistry and Physics, 2016
The mid-to lower reaches of the Yangtze River valley are located within the typical East Asian monsoon zone. Rapid urbanization, industrialization, and development of agriculture have led to fast and complicated land use and land cover change in this region. To investigate land-atmosphere interaction in this region where human activities and monsoon climate have considerable interaction with each other, micrometeorological elements over four sites with different surface types around Nanjing, including urban surface at Dangxiao (hereafter DX-urban), suburban surface at Xianling (XL-suburb), and grassland and farmland at Lishui County (LS-grass and LS-crop), are analyzed and their differences are revealed. The impacts of surface parameters of different surface types on the radiation budget and land surface-atmosphere heat, water, and mass exchanges are investigated and compared. The results indicate the following. (1) The largest differences in daily average surface air temperature (Ta), surface skin temperature (Ts), and relative humidity (RH), which are found during the dry periods between DX-urban and LS-crop, can be up to 3.21 oC, 7.26 oC, and 22.79 %, respectively. The diurnal ranges of the above three elements are the smallest at DX-urban and the largest at LS-grass, XL-suburb, and LS-crop. (2) Differences in radiative fluxes are mainly reflected in upward shortwave radiation (USR) that is related to surface albedo and upward longwave radiation (ULR) that is related to Ts. When comparing four sites, it can be found that both the smallest USR and the largest ULR occur at the DX-urban site. The diurnal variation in ULR is same as that of Ts at all four sites. (3) The differences in daily average sensible heat (H) and latent heat (LE) between DX-urban and LS-crop are larger than 45 and 95 Wm-2, respectively. The proportion of latent heat flux in the net radiation (LE/Rn) keeps increasing with the change in season from the spring to summer. (4) Human activities have obvious effects on microclimate. The urban heat island (UHI) effect results in a Ta 2 oC higher at the urban site than other sites in the nighttime. At the crop site, LE is dominant due to irrigation, and negative H is observed since evaporation cooling leads to low Ts. Although Ts is higher at XL-suburb than that at LS-grass, there is no large difference in Ta between the two sites due to the distinct effects of the planted forest.
Li et al., Sensitivity of a regional climate model to land surface parameterization schemes for East Asian summer monsoon simulation, Climate Dynamics, 2016
Land surface processes play an important role in the East Asian Summer Monsoon (EASM) system. Parameterization schemes of land surface processes may cause uncertainties in regional climate model (RCM) studies for the EASM. In this paper, we investigate the sensitivity of a RCM to land surface parameterization (LSP) schemes for long-term simulation of the EASM. The Weather Research and Forecasting (WRF) Model coupled with four different LSP schemes (Noah-MP, CLM4, Pleim-Xiu and SSiB), hereafter referred to as Sim-Noah, Sim-CLM, Sim-PX and Sim-SSiB respectively, have been applied for 22-summer EASM simulations. The 22-summer averaged spatial distributions and strengths of downscaled large-scale circulation, 2-m temperature and precipitation are comprehensively compared with ERA-Interim reanalysis and dense station observations in China. Results show that the downscaling ability of RCM for the EASM is sensitive to LSP schemes. Furthermore, this study confirms that RCM does add more information to the EASM compared to reanalysis that imposes the lateral boundary conditions (LBC) because it provides 2-m temperature and precipitation that are with higher resolution and more realistic compared to LBC. For 2-m temperature and monsoon precipitation, Sim-PX and Sim-SSiB simulations are more consistent with observation than simulations of Sim-Noah and Sim-CLM. To further explore the physical and dynamic mechanisms behind the RCM sensitivity to LSP schemes, differences in the surface energy budget between simulations of Ens-Noah-CLM (ensemble mean averaging Sim-Noah and Sim-CLM) and Ens-PX-SSiB (ensemble mean averaging Sim-PX and Sim-SSiB) are investigated and their subsequent impacts on the atmospheric circulation are analyzed. It is found that the intensity of simulated sensible heat flux over Asian continent in Ens-Noah-CLM is stronger than that in Ens-PX-SSiB, which induces a higher tropospheric temperature in Ens-Noah-CLM than in Ens-PX-SSiB over land. The adaptive modulation of geopotential height gradients affects wind field (through geostrophic balance) simulation especially at lower levels, which subsequently affects the simulation of large-scale circulation, 2-m temperature and monsoon precipitation as well as RCM's downscaling ability.
Li et al., Influence of the Madden-Julian oscillation on Tibetan Plateau snow cover at the intraseasonal time-scale, Scientific Reports, 2016
The Tibetan Plateau (TP), known as the third pole of the Earth, has snow cover with intraseasonal to decadal variability that affects weather and climate both inside and outside the TP. However, the factors that generate the TP snow cover (TPSC) anomalies at the intraseasonal time-scale are unclear. This report reveals the influence of the Madden. Julian oscillation (MJO), which is the most dominant component of the tropical intraseasonal variability, on TPSC. We focus on wintertime snow cover over the central and eastern TP, where the intraseasonal variability is large. TPSC increases/ decreases in the MJO phases 8-1/4-5, when the eastward-propagating MJO suppressed/ enhanced convection locates over the Maritime Continent. Such a change in TPSC leads to the most dominant positive/ negative anomalies of TPSC in the following phases 2-3/6-7 due to the non-significant change of TPSC in these phases. There is anomalous moisture advection over the upstream of the TP caused by MJO-excited large-scale atmospheric circulation. The advection process generates the low-frequency eastward-propagating anomalous water vapour from upstream to the TP that influences precipitation and, eventually, TPSC.
Zhang et al., The impacts of land-use and land-cover change on tropospheric temperatures at global and regional scales, Earth Interactions, 2016
The impacts of land-use and land-cover change (LULCC) on tropospheric temperatures are investigated in this study using the fully coupled Community Earth System Model. Two simulations are performed using potential and current vegetation cover. The results show that LULCC can induce detectable changes in the tropospheric air temperature. Although the influence of LULCC on tropospheric temperature is weak, a significant influence can still be found below 300 hPa in summer over land. Compared to the global-mean temperature change, LULCC-induced changes in the regional-mean air temperature can be 2-3 times larger in the middle-upper troposphere and approximately 8 times larger in the lower troposphere. In East Asia and South Asia, LULCC is shown to produce significant decreases (0.2 to 0.4 oC) in air temperature in the middle-upper troposphere in spring and autumn due to the largest decrease in the latent heat release from precipitation. In Europe and North America, the most significant tropospheric cooling occurs in summer, which can be attributed to the significant decrease in the absorbed solar radiation and sensible heat flux during this season. In addition to local effects, LULCC also induces nonlocal responses in the tropospheric air temperature that are characterized by significant decreases over the leeward sides of LULCC regions, which include East Asia-western North Pacific Ocean, Mediterranean Sea-North Africa, North America-Atlantic Ocean, and North America-eastern Pacific. Cooling in the leeward sides of LULCC regions is primarily caused by an enhanced cold advection induced by LULCC.
Ling et al., Composite analysis of impacts of dust aerosols on surface atmospheric variables and energy budgets in a semiarid region of China, Journal of Geophysical Research-Atmospheres, 2014
Loess Plateau is one of the dust aerosol source regions featured by its sandy underlying surface and affected significantly by dust events. In order to investigate climatic forcing of dust aerosols in semiarid region, continuous observations of particulate matter (PM10 concentration), meteorological elements, and energy fluxes were collected at Semiarid Climate Observatory and Laboratory in northwestern China from March to May during 2007-2010. The result shows that dusty days are often evoked under the condition when a strengthening trough is combined with the development of strong convection. During dusty days, the frequency of northerly winds increases significantly with the average wind velocity to be around 4.0 m/s; temperature is low during the daytime and high at nighttime. Relative humidity and surface pressure, however, are about 15% and 70% lower than average in dusty days, respectively. Energy balance closure is typically poor in dusty days. During daytime, the downward/upward solar radiation at land surface is less in dusty days than in nondusty days with the largest difference of 206.7W/m2 and 33.25 W/m2, respectively. Difference in downward longwave radiation between dusty and nondusty days is 35 W/m2, accounting for 11.7% and 14% of the daily mean for dusty and nondusty days, respectively. The net radiation flux, as well as sensible/latent heat fluxes at surface is smaller during the daytime but larger at nighttime in dusty days. The maximum differences of sensible/latent heat fluxes between dusty and nondusty days can reach for 41.9% and 12.1% of the maximum net radiation, respectively.