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1. The influence of sea- and land-breeze circulations on the diurnal variability in precipitation over a tropical island

   https://doi.org/10.5194/acp-17-13213-2017  

   Zhu, Lei ; Meng, Zhiyong ; Zhang, Fuqing ; Markowski, Paul M. ( January 2017 , Atmospheric Chemistry and Physics)

   Abstract. This study examines the diurnal variation in precipitation over Hainan Island in the South China Sea using gauge observations from 1951 to 2012 and Climate Prediction Center MORPHing technique (CMORPH) satellite estimates from 2006 to 2015, as well as numerical simulations. The simulations are the first to use climatological mean initial and lateral boundary conditions to study the dynamic and thermodynamic processes (and the impacts of land–sea breeze circulations) that control the rainfall distribution and climatology. Precipitation is most significant from April to October and exhibits a strong diurnal cycle resulting from land–sea breeze circulations. More than 60% of the total annual precipitation over the island is attributable to the diurnal cycle with a significant monthly variability. The CMORPH and gauge datasets agree well, except that the CMORPH data underestimate precipitation and have a 1h peak delay. The diurnal cycle of the rainfall and the related land–sea breeze circulations during May and June were well captured by convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model, which were initiated from a 10-year average ERA-Interim reanalysis. The simulations have a slight overestimation of rainfall amounts and a 1h delay in peak rainfall time. The diurnal cycle of precipitation is driven by the occurrence of moist convection around noontime owing to low-level convergence associated with the sea-breeze circulations. The precipitation intensifies rapidly thereafter and peaks in the afternoon with the collisions of sea-breeze fronts from different sides of the island. Cold pools of the convective storms contribute to the inland propagation of the sea breeze. Generally, precipitation dissipates quickly in the evening due to the cooling and stabilization of the lower troposphere and decrease of boundary layer moisture. Interestingly, the rather high island orography is not a dominant factor in the diurnal variation in precipitation over the island.

    

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2. The Importance of Scale‐Dependent Groundwater Processes in Land‐Atmosphere Interactions Over the Central United States

   https://doi.org/10.1029/2020GL092171  

   Barlage, Michael ; Chen, Fei ; Rasmussen, Roy ; Zhang, Zhe ; Miguez‐Macho, Gonzalo ( March 2021 , Geophysical Research Letters)

   This study explores the impacts of groundwater processes on the simulated land‐surface water balance and hydrometeorology. Observations are compared to multiscale Weather Research and Forecasting (WRF) simulations of three summer seasons: 2012, 2013, and 2014. Results show that a grid spacing of 3 km or smaller is necessary to capture small‐scale river and stream networks and associated shallow water tables, which supplies additional root‐zone water double that of simulations with 9‐km and 27‐km grid spacing and is critical to replenishing the depleted vegetation root zones and leads to 150 mm more evapotranspiration. Including groundwater processes in convection‐permitting models is effective to reduce: (1) 2‐m temperature warm biases from 5–6 to 2–3 °C and (2) the low precipitation bias by half. The additional groundwater supply to active soil flux in convection‐permitting simulations with groundwater for June‐August is nearly translated into the same amount of increased precipitation in the domain investigated.

    

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3. Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India

   https://doi.org/10.5194/cp-14-1869-2018  

   Kathayat, Gayatri ; Cheng, Hai ; Sinha, Ashish ; Berkelhammer, Max ; Zhang, Haiwei ; Duan, Pengzhen ; Li, Hanying ; Li, Xianglei ; Ning, Youfeng ; Edwards, R. Lawrence ( January 2018 , Climate of the Past)

   Abstract. A large array of proxy recordssuggests that the “4.2ka event” marks an approximately300-year long period (∼3.9 to 4.2ka) ofmajor climate change across the globe. However, the climatic manifestation ofthis event, including its onset, duration, and termination, remains lessclear in the Indian summer monsoon (ISM) domain. Here, we present new oxygenisotope (δ¹⁸O) data from a pair of speleothems (ML.1 and ML.2)from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record ofISM variability during a period (∼3.78 and 4.44ka) that fullyencompasses the 4.2ka event. The sub-annually to annually resolved ML.1δ¹⁸O record is constrained by 18 ²³⁰Th dates with anaverage dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability withunprecedented detail. The inferred pattern of ISM variability during theperiod contemporaneous with the 4.2ka event shares broad similarities andkey differences with the previous reconstructions of ISM from the MawmluhCave and other proxy records from the region. Our data suggest that the ISMintensity, in the context of the length of our record, abruptly decreased at∼4.0ka ($\sim \pm \mathrm{13}$ years), marking the onset of a multi-centennialperiod of relatively reduced ISM, which was punctuated by at least twomulti-decadal droughts between ∼3.9 and 4.0ka. The latter stands outin contrast with some previous proxy reconstructions of the ISM, in which the4.2ka event has been depicted as a singular multi-centennial drought.

    

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4. Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry

   https://doi.org/10.5194/acp-18-427-2018  

   Xu, Jianzhong ; Zhang, Qi ; Shi, Jinsen ; Ge, Xinlei ; Xie, Conghui ; Wang, Junfeng ; Kang, Shichang ; Zhang, Ruixiong ; Wang, Yuhang ( January 2018 , Atmospheric Chemistry and Physics)

   Abstract. Recent studies have revealed a significant influx of anthropogenic aerosol from South Asia to the Himalayas and Tibetan Plateau (TP) during pre-monsoon period. In order to characterize the chemical composition, sources, and transport processes of aerosol in this area, we carried out a field study during June 2015 by deploying a suite of online instruments including an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) and a multi-angle absorption photometer (MAAP) at Nam Co station (90°57′E, 30°46′N; 4730ma.s.l.) at the central of the TP. The measurements were made at a period when the transition from pre-monsoon to monsoon occurred. The average ambient mass concentration of submicron particulate matter (PM₁) over the whole campaign was  ∼ 2.0µgm⁻³, with organics accounting for 68%, followed by sulfate (15%), black carbon (8%), ammonium (7%), and nitrate (2%). Relatively higher aerosol mass concentration episodes were observed during the pre-monsoon period, whereas persistently low aerosol concentrations were observed during the monsoon period. However, the chemical composition of aerosol during the higher aerosol concentration episodes in the pre-monsoon season was on a case-by-case basis, depending on the prevailing meteorological conditions and air mass transport routes. Most of the chemical species exhibited significant diurnal variations with higher values occurring during afternoon and lower values during early morning, whereas nitrate peaked during early morning in association with higher relative humidity and lower air temperature. Organic aerosol (OA), with an oxygen-to-carbon ratio (O∕C) of 0.94, was more oxidized during the pre-monsoon period than during monsoon (average O∕C ratio of 0.72), and an average O∕C was 0.88 over the entire campaign period, suggesting overall highly oxygenated aerosol in the central TP. Positive matrix factorization of the high-resolution mass spectra of OA identified two oxygenated organic aerosol (OOA) factors: a less oxidized OOA (LO-OOA) and a more oxidized OOA (MO-OOA). The MO-OOA dominated during the pre-monsoon period, whereas LO-OOA dominated during monsoon. The sensitivity of air mass transport during pre-monsoon with synoptic process was also evaluated with a 3-D chemical transport model.

    

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5. A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

   https://doi.org/10.5194/acp-17-6353-2017  

   Horowitz, Hannah M. ; Jacob, Daniel J. ; Zhang, Yanxu ; Dibble, Theodore S. ; Slemr, Franz ; Amos, Helen M. ; Schmidt, Johan A. ; Corbitt, Elizabeth S. ; Marais, Eloïse A. ; Sunderland, Elsie M. ( January 2017 , Atmospheric Chemistry and Physics)

   Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg⁰. Oxidation to water-soluble Hg^II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg⁰∕Hg^II redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg⁰∕Hg^II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg⁰ oxidant and that second-stage HgBr oxidation is mainly by the NO₂ and HO₂ radicals. The resulting chemical lifetime of tropospheric Hg⁰ against oxidation is 2.7 months, shorter than in previous models. Fast Hg^II atmospheric reduction must occur in order to match the  ∼ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg⁰+Hg^II(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg^II–organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere. The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM–ozone relationship indicative of fast Hg⁰ oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0–30%). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO₂ and HO₂ radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg^II reduction in the presence of high organic aerosol concentrations. We find that 80% of Hg^II deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg^II reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO₂ and NO₂ for second-stage HgBr oxidation.

    

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