Contributions of aerosol composition and sources to particulate optical properties in a southern coastal city of China Publication date: 1 May 2020 Source: Atmospheric Research, Volume 235 Author(s): Jie Tian, Qiyuan Wang, Yongming Han, Jianhuai Ye, Ping Wang, Siwatt Pongpiachan, Haiyan Ni, Yaqing Zhou, Meng Wang, Youzhi Zhao, Junji Cao Abstract The contributions of chemical composition and emission sources to aerosol optical properties were evaluated for a coastal city in southern China. The average dry light scattering coefficient (bscat,dry) and light absorption coefficient (babs) were 32.5 ± 15.5 Mm−1 and 8.8 ± 4.7 Mm−1, respectively. Diurnal cycles in bscat,dry and babs were observed with peak values in the morning and at night. Both bscat,dry and babs varied with wind speeds and directions, and thus affected by transport pathways. Chemical composition data for 12-h PM2.5 samples were used with the revised IMPROVE algorithm and a Hybrid Environmental Receptor Model to evaluate aerosol composition and source contributions to dry light extinction (bext,dry = bscat,dry + babs), respectively. Ammonium sulfate and organic matter were the dominant contributors to bext,dry, followed by elemental carbon, sea salt, fine soil, and ammonium nitrate. The six PM2.5 sources identified were secondary sulfate source, biomass burning, marine emission, fugitive dust, traffic-related emission, and shipping emission. Marine emission and secondary sulfate source were the largest contributors of bext,dry during the daytime and nighttime, respectively. Backward trajectory analysis further explored the impact of potential sources to bext,dry at Sanya from surrounding regions. The results of our study would be useful for improving models of radiative effects from different sources in this area. Graphical abstract

Isotopic interaction and source moisture control on the isotopic composition of rainfall over the Bay of Bengal Publication date: 1 May 2020 Source: Atmospheric Research, Volume 235 Author(s): Nitesh Sinha, S. Chakraborty Abstract The isotopic interaction between ambient vapor and raindrops has been investigated for the first time at Port Blair, Andaman Islands, an environment having the minimal contribution of the continental moisture. Rainwater and ambient vapor samples were collected on a daily timescale during the Indian summer monsoon season of 2015. Oxygen (Hydrogen) heavy isotopic ratios of rainwater and ambient vapor are positively correlated and a difference in the seasonal average values was found to be 9.5‰ (65.8‰) at ~28 °C. On a daily scale, the oxygen isotope ratios of ambient vapor and rainwater are significantly correlated over a wide range of rainfall amount, but correlation weakens for rainfall exceeding ~36 mm/day. The isotopic variability of rainfall appears to be modulated by the interaction with the ambient vapor, which in turn is determined by the source moisture. In a case study, it is estimated that the vapor-rainwater isotopic exchange could lead to 18O enrichment (depletion) in raindrops (vapor) about 0.41‰ (approx. 30% of the total change), which is far above (>4σ) of the experimental uncertainty.

Characteristics of upper-tropospheric outflow-layer clouds of Typhoon Francisco (2013) observed by hydrometeor videosonde Publication date: 1 May 2020 Source: Atmospheric Research, Volume 235 Author(s): Tadayasu Ohigashi, Kazuhisa Tsuboki, Yuto Suzuki, Hiroyuki Yamada, Katsuhiro Nakagawa Abstract Ice crystals in upper-tropospheric outflow layers of typhoons are an important factor in determining typhoon intensity and track through radiative cooling of the secondary circulation. Quantitative representation of ice crystal distribution is fundamental for accurate reproduction of typhoon intensity and track in cloud-resolving numerical models. This study used hydrometeor videosondes (HYVISs) released from Okinawa Island (Japan) to conduct microphysical observations of ice crystals in the outflow layer of Typhoon Francisco (2013). During the approach of the typhoon center, seven HYVISs were released between 02:53 JST October 23, 2013 and 13:33 JST October 24, 2013 at distances of between 230 and 550 km from the typhoon center (r = 230 and 550 km). The observation site was located at the downshear left flank, and at least three HYVISs were released within 100 km of active convection in the eyewall from which outflow-layer clouds extended. During the observations, the central pressure of the typhoon increased slightly from 945 to 960 hPa. Ice particles were observed at heights of 7–14 km. The numbers of ice particles tended to decrease with distance from the typhoon center. The maximum number concentrations of ice crystals were 180 and 20 L−1 at r = 230 and 550 km, respectively. Ice particles with diameter of a few tens of micrometers were found dominant. Large particles with diameter > 100 μm were restricted to just outside the eyewall and the inner-core precipitation region. Ice supersaturation layers with thickness > 2 km were present from r = 230 to 360 km, i.e., reasonably close to the typhoon center. In addition, supercooled liquid droplets were observed in areas with relative humidity >95% in two profiles (r = 260 and 290 km). Conversely, there was no ice supersaturation layer in the outermost profile at r = 550 km. Saw-like radar echo patterns were common along the bottom of outflow-layer clouds, below which a considerably drier layer was evident. Saturation ratio minima relative to ice within the dry layer were 0.01–0.34 (for relative humidities of 1%–31%). It was considered that ice particles formed in the upper levels of rainbands near the typhoon center, and that they were sublimated in both the dry layer below the outflow layer and the ice subsaturation area in the outermost region of the typhoon, which resulted in decreased number concentrations of ice particles.

Contrasting frontal and warm-sector heavy rainfalls over South China during the early-summer rainy season Publication date: 1 May 2020 Source: Atmospheric Research, Volume 235 Author(s): Naigeng Wu, Xi Ding, Zhiping Wen, Guixing Chen, Zhiyong Meng, Liangxun Lin, Jinzhong Min Abstract Heavy rainfalls occur frequently during early summer (April–June) over South China, causing severe floods. Using 12 years of hourly rain-gauge data, we identify a large number of regional heavy rainfall events and categorize them into two major types based on the strength of synoptic forcing. It is shown that frontal heavy rainfalls associated with fronts or shear lines mainly occurs over inland regions, whereas warm-sector heavy rainfall under weakly forced synoptic environment is observed in coastal areas. The rainfall maxima of both types tend to form over the low-lying plains or sea surfaces adjacent to windward mountains. Frontal heavy rainfall usually propagates southwards with a cold front, while warm-sector events move relatively slowly and produce coherent patterns of rainfall. The occurrence of warm-sector rainfall increases markedly from April to June in a close association with the onset of summer monsoon, in contrast to frontal rainfall with less monthly variation. Warm-sector rainfall also exhibits pronounced diurnal variation, with a peak in the early morning, as a result of intensified convergence between nocturnal low-level jets in southerly monsoon and land breezes. In contrast, frontal rainfall has an afternoon peak, due to the arrival of eastward-propagating rain systems and daytime heating on land. As frontal (warm-sector) heavy rainfall has relatively high (low) predictability in numerical models, understanding their occurrence and formation is of benefit to operational forecasting and decision-making for disaster prevention.

Exploring large-scale black‑carbon air pollution over Northern Eurasia in summer 2016 using MERRA-2 reanalysis data Publication date: 1 May 2020 Source: Atmospheric Research, Volume 235 Author(s): S.A. Sitnov, I.I. Mokhov, A.A. Likhosherstova Abstract The abnormally high temperature and the deficit of precipitation in Siberia in summer 2016 contributed to the development of massive forest fires, which resulted in pyrogenic emission in the atmosphere of various biomass burning products. This paper presents the analysis results of the spatial and temporal variations of black carbon (BC) in the atmosphere over Northern Eurasia during the 2016 Siberian wildfires using the MERRA-2 reanalysis data. The spatiotemporal evolution of BC anomalies is presented, with estimates of the increase in BC mass during wildfire period. The peculiarities of large-scale atmospheric circulation responsible for BC transport from Siberia to Central Europe are discussed. The estimates of spatial scale, effective height and speed of the long-range anomalous (from east to west) transfer of air masses in the atmosphere over Northern Eurasia in July 2016 are obtained. Changes in the optical and microphysical characteristics of biomass-burning aerosol during its long-range transport are also discussed. A comparison of daily variations in surface BC concentrations at the polar observatory Tiksi with those in BC column mass density from MERRA-2 revealed a high correlation between local and integral characteristics of BC during arriving air masses from Siberian wildfires.

Corrigendum to “Global climatology of nocturnal low-level jets and associated moisture sources and sinks” [Atmospheric Research (2019) 39–59] Publication date: April 2020 Source: Atmospheric Research, Volume 234 Author(s): Iago Algarra, Jorge Eiras-Barca, Raquel Nieto, Luis Gimeno

On the chemical nature of wet deposition over a major desiccated lake: Case study for Lake Urmia basin Publication date: April 2020 Source: Atmospheric Research, Volume 234 Author(s): Hesam Ahmady-Birgani, Parisa Ravan, Joseph Simon Schlosser, Alberto Cuevas-Robles, Mojtaba AzadiAghdam, Armin Sorooshian Abstract One of the largest hyper-saline lakes in the world is Lake Urmia, which is in the northwest region of Iran. Lake Urmia has experienced extreme desiccation in recent years. This study was conducted to investigate possible impacts of aerosol emitted by Lake Urmia on rainwater properties. Rainwater samples were collected from September 2017 to September 2018 at 13 sites spanning different areas of the Lake Urmia basin. Samples were analysed for acidity/alkalinity (pH), electrical conductivity (EC), total suspended solids (TSS) and elemental composition (Na, Mg, K, Ca, Al, Fe, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, and Pb). The results reveal that rainwater around Lake Urmia is characterized by high concentrations of heavy metals and toxic elements (e.g., Fe, Al, Ni, Cu, Cr, Co, Ti), especially in the outer ring of the study region to the north. Chemical signatures of anthropogenic pollution were most evident at sites near populated and manufacturing centers. The average pH of rainwater was enhanced relative to other regions, with the lowest and highest values of the 13 sites being 7.7 and 8.6, respectively. Concentrations of marine elements (Na+, K+, Mg2+, Ca2+) decreased with increasing distance from Lake Urmia, suggestive of possible impacts of the lakebed's emission on rain composition via likely below-cloud scavenging. The area impacted by the marine aerosol emitted by Lake Urmia has a diameter of 30 to 40 km surrounding the shoreline. The results reveal a stronger signature of crustal species in the north owing to desiccated lakebed hotspots located in the north.

Calculation of supersaturation maximum and droplet concentration at cloud boundaries Publication date: April 2020 Source: Atmospheric Research, Volume 234 Author(s): Mark Pinsky, Alexander Khain Abstract Most parameterizations of droplet concentration at cloud base in cloud models are derived under the assumption that all droplets at the cloud base form by nucleation of dry CCN crossing the lifting condensation level in updraft. In this study, a novel method of calculation of the supersaturation maximum and droplet concentration at cloud base is described that takes into account the existence of liquid at the level of droplet nucleation. Liquid exists in haze particles, as well as in the form of cloud droplets and raindrops. It is shown that the presence of liquid water inside non-activated CCN, as well as of cloud droplets and raindrops substantially decreases supersaturation maximum in case of high aerosol concentration. The possible effect of the supersaturation maximum decrease on formation of DSD in clouds is discussed. The method can be also applied to calculation of supersaturation and droplet nucleation at cloud edges.

Micrometeorological measurements in an arid environment: Diurnal characteristics and surface energy balance closure Publication date: April 2020 Source: Atmospheric Research, Volume 234 Author(s): Narendra Reddy Nelli, Marouane Temimi, Ricardo Morais Fonseca, Michael John Weston, Mohana Satyanarayana Thota, Vineeth Krishnan Valappil, Oliver Branch, Hans-Dieter Wizemann, Volker Wulfmeyer, Youssef Wehbe Abstract The goal of this study is to assess the surface energy balance closure (SEBC) in an arid environment through a comprehensive analysis of surface radiation, heat fluxes, and other micrometeorological measurements. These were taken from a 2.3-m eddy covariance (EC) flux tower at Al Ain (24°16′26.5535″ N, 55°37′03.2196″ E; 243 m a.m.s.l.) during the periods April – October 2017 and February 2018 – January 2019. The SEBC is investigated on seasonal and diurnal time-scales using the Energy Balance Ratio (EBR) and residual flux (ΔE) values. An analyses of the SEBC reveals that during daytime a three-way balance between the sensible heat, ground heat fluxes and the net surface radiative flux prevails, while at night the last two essentially balance each other. These fluxes are roughly 25% larger in the summer compared to the winter season. The surface downward shortwave radiation flux and latent heat flux peak in spring, despite the latter having rather small magnitudes (<20 W m−2) throughout the year. The EBR during daytime is in the range 0.60 to 0.75. The mean ΔE during daytime (nighttime) is 30 W m−2 (−14 W m−2) respectively. Further analysis of the SEBC with respect to the prevailing wind direction revealed that the maximum closure is observed for north-northwesterly winds and the minimum for southerly winds. This may be explained by the surface heterogeneity and the presence of a building to the south of the site. The SEBC is better for daytime and for periods when the friction velocity is higher.

Investigation to the relation between meteorological drought and hydrological drought in the upper Shaying River Basin using wavelet analysis Publication date: April 2020 Source: Atmospheric Research, Volume 234 Author(s): Qiongfang Li, Pengfei He, Yongchang He, Xingye Han, Tianshan Zeng, Guobin Lu, Hongjie Wang Abstract Investigation to the relation between meteorological drought and hydrological drought is important for facilitating early warning and mitigation of hydrological drought. Therefore, the propagation from the meteorological drought to hydrological drought in the upper Shaying River Basin, China was explored. Based on the goodness-of-fit test of monthly streamflow distribution from four hydrologic stations, the Pearson correlation analysis between monthly Standardized Precipitation Index accumulated periods of 1–24 months and monthly Standardizing Streamflow Index was applied to detect the propagation time between meteorological drought and hydrological drought; the continuous wavelet transform, cross wavelet transform, wavelet coherence and wavelet cross-correlation were utilized to depict the links between meteorological drought and hydrological drought in specific time-frequency bands. The results revealed: (1) different probability distributions were suitable for the Standardizing Streamflow Index at different gauging stations; (2) the propagation time from meteorological drought to hydrological drought notably varied with seasons, the longer in spring and winter and the shorter in summer and autumn; (3) hydrological drought and meteorological drought presented the similar patterns in term of phase shift; (4) close correlation existed between hydrological drought and meteorological drought with high absolute maximum and minimum wavelet cross-correlation coefficients, and changed with periodic scales and the lag time of hydrological drought to meteorological drought.