ALBERT

Description: Limonene, a typical proxy of monoterpenes emitted from biogenic sources, plays an important role in secondary organic aerosol (SOA) formation. However, the optical properties of SOA generated from limonene under various oxidation pathways remain poorly understood. In this study, we investigate the refractive index (RI) of limonene SOA produced from four oxidation conditions with cavity ring-down spectrometer (CRDS) and photoacoustic extinctiometer (PAX) operated at 532 and 375 nm. Our results show that there is a significant difference in RI values of SOA produced from NO 3 oxidation compared to other oxidation pathways. The mean values of refractive index of SOA produced from NO 3 oxidation, NO x oxidation, OH oxidation with NO x free, and O 3 oxidation experiments are 1.578, 1.469, 1.495, and 1.494 at 532 nm; and 1.591, 1.527, 1.513, and 1.537 at 375 nm, respectively, while no detectable absorption is found in all oxidation conditions. We attribute the high RI values of SOA by NO 3 oxidation to two factors: a large proportion of organic nitrates and high-molecular-weight dimers/oligomers in the SOA. Our study results indicate that the nighttime chemistry may significantly influence the optical properties of limonene oxidation products. The refractive index values of limonene SOA generated under various oxidation conditions at different wavelengths retrieved in our laboratory experiments could help improve the model predictions for evaluating the effect of biogenic SOA on the global radiative forcing as well as climate change.

Description: Lake surface water temperature (LSWT) measurements from various sources illustrate that lakes are warming in response to climate change. Most previous studies of geographical distributions of lake warming have tended to utilize data with limited spatial resolution of LSWTs, including single-point time series. Spatially resolved LSWT time-series are now available from satellite observations and some studies have investigated previously the intra-lake warming patterns in specific lakes (e.g., North American Great Lakes). However, across-lake comparisons of intra-lake warming differences have not yet been investigated at a large, across-continental scale, thus limiting our understanding of how intra-lake warming patterns differ more broadly. In this study, we analyze up to 20 years of satellite data from 19 lakes situated across the Northern Hemisphere, to investigate how LSWT changes vary across different lake surfaces. We find considerable intra-lake variability in warming trends across many lakes. The deepest areas of large lakes are characterized by a later onset of thermal stratification, a shorter stratified warming season and exhibit longer correlation timescales of LSWT anomalies. We show that deep areas of large lakes across the Northern Hemisphere as a result tend to display higher rates of warming of summer LSWT, arising from a greater temporal persistence in deep areas of the temperature anomalies associated with an earlier onset of thermal stratification. Utilization of single-point LSWT trends to represent changes in large lakes therefore suppresses important aspects of lake responses to climate change, whereas spatially resolved LSWT measurements can be exploited to provide more comprehensive understanding.

Description: Sumatra Squalls, organized bands of thunderstorms, are the dominant mesoscale convective systems during the inter-monsoon and Southwest Monsoon seasons in Singapore. To understand how they affect precipitation isotopes, we monitored the δ-value of precipitation daily and continuously (every second and integrated over 30 seconds) during all squalls in 2015. We found that precipitation δ 18 O values mainly exhibit a “V” shape pattern and less commonly a “W” shape pattern. Variation in δ 18 O values during a single event is about 1 to 6‰ with the lowest values mostly observed in the stratiform zone, which agrees with previous observations and modeling simulations. Re-evaporation can significantly affect δ-values, especially in the last stage of the stratiform zone. Daily precipitation is characterized by periodic negative shifts in δ-value, largely associated with squalls rather than moisture source change. The shifts can be more than 10‰, larger than intra-event variation. Initial δ 18 O values of events are highly variable, and those with the lowest values also have the lowest initial values. Therefore, past convective activities in the upwind area can significantly affect the δ 18 O, and convection at the sampling site has limited contribution to isotopic variability. A significant correlation between precipitation δ 18 O value and regional Outgoing Longwave Radiation and rainfall in the Asian monsoon region and western Pacific suggests that regional organized convection probably drives stable isotopic compositions of precipitation. A drop in the frequency of the squalls in 2015 is related to weak organized convection in the region caused by El Niño.

Description: Changes in fire activity across regions around the Tibetan Plateau (TP) are poorly understood, especially under the recent warming and drying trends. In this work, we report records of the specific fire tracer levoglucosan in a central Tibetan ice core, indicating a rapid increase in wildfires across the Himalayas and surroundings at the beginning of the 21 st century. The climate system, especially precipitation changes, modulates the annual variability of wildfires in regions around the TP. Decreasing pre-monsoon precipitation has prolonged the dry seasons across Himalayan regions affected by the Indian summer monsoon; meanwhile, increasing precipitation over the arid and semiarid Indus River Plain promotes plant growth and thereby increases biofuel availability. These trends have therefore induced increased frequencies of strong wildfires in the Himalayas and surroundings. Increasing strong wildfire events can potentially enhance black carbon deposits on Himalayan glaciers, which would impact glacial melting during the pre-monsoon wildfire seasons in the near future.

Description: Cryospheric changes have great effects on alpine hydrology, but these effects are still unclear owing to rare observations and suitable models in the western cold regions of China (WCRC). Based on long-term field observations in the WCRC, a cryospheric basin hydrological model (CBHM) was proposed to evaluate the cryospheric effects on streamflow in the upper Hei river basin (UHR), and the relationship between the cryosphere and streamflow was further discussed with measured data. The Norwegian Earth System Model (NorESM1-ME) outputs were chosen to project future streamflow under scenarios RCP2.6, RCP4.5, and RCP8.5. The CBHM results were well validated by the measured precipitation, streamflow, evapotranspiration, soil temperature, glacier and snow cover area, and the water balance of land cover in the UHR. The moraine-talus region contributed most of the runoff (60%), even though it made up only about 20% of the area. On average, glacier and snow cover respectively contributed 3.5% and 25.4% of the fresh water to the streamflow in the UHR between 1960 and 2013. Because of the increased air temperature (2.9°C/54a) and precipitation (69.2 mm/54a) over the past 54 years, glacial and snowmelt runoff increased by 9.8% and 12.1%, respectively. The increase in air temperature brought forward the snowmelt flood peak and increased the winter flow due to permafrost degradation. Glaciers may disappear in the near future because of their small size, but snowmelt would increase due to increases in snowfall in the higher mountainous areas, and the basin runoff would increase slightly in the future.

Description: Using lightning mapping arrays (LMAs), lightning flash locations in three dimensions have been investigated using multiple methods. Approximately 500,000 flashes were analyzed to reveal the variability of lightning channel locations within convective storms. These flashes were produced by over 4000 isolated convective storms during one warm season across diverse weather regimes in northern Alabama, Washington, D.C., central Oklahoma and northeastern Colorado. Lightning locations are analyzed within the context of radar reflectivities and examined for vertical variability. Results show that storms in Colorado preferentially produced flashes at lower altitudes and in regions of higher reflectivity compared to the other regions. The regional differences in flash altitudes are largely attributed to the prevalence of anomalous polarity storms (mid- or low-level dominant positive charge) in Colorado, as anomalous storms produced a majority of flashes at lower altitudes compared to storms with normal polarity charge structures (mid-level negative charge). Conversely, anomalous storms are exceedingly rare in the other regions of study. The differences in flash altitudes are coincident with discrepancies between annual average densities estimated by satellite observations and LMA. Specifically, large differences in annual average flash density estimates exist in northeastern Colorado, that are not present in the other regions, suggesting the lower altitude flashes in Colorado may be more difficult to detect by satellites.

Description: The complete scattering matrix for cement dust was measured as a function of scattering angle from 5° to 160° at a wavelength of 532 nm, as a representative of mineral dust of anthropogenic origin in urban areas. Other related characteristics of cement dust, such as particle size distribution, chemical composition, refractive index and micro morphology were also analyzed. For this objective, a newly improved apparatus was built and calibrated using water droplets. Measurements of water droplets were in good agreement with Lorenz-Mie calculations. To facilitate the direct applicability of measurements for cement dust in radiative transfer calculation, the synthetic scattering matrix was computed and defined over the full scattering angle range from 0° to 180°. The scattering matrices for cement dust and typical natural mineral dusts were found to be similar in trends and angular behaviors. Angular distributions of all matrix elements were confined to rather limited domains. To promote the application of light scattering matrix in atmospheric observation and remote sensing, discrimination methods for various atmospheric particulates (cement dust, soot, smolder smoke, and water droplets) based on the angular distributions of their scattering matrix elements are discussed. The ratio -F 12 /F 11 proved to be the most effective discrimination method when a single matrix element is employed, aerosol identification can be achieved based on -F 12 /F 11 values at 90° and 160°. Meanwhile, the combinations of -F 12 /F 11 with F 22 /F 11 (or (F 11 -F 22 )/(F 11 +F 22 )) or -F 12 /F 11 with F 44 /F 11 at 160° can be used when multiple matrix elements at the same scattering angle are selected.

Description: For the first time, in-situ turbulence measurements collected in the vicinity of the Brazil-Malvinas Confluence are used to determine the influence of the ocean waves and atmospheric stability on the Marine Atmospheric Surface Layer. From the analysis of 187 high-frequency sampled segments of temperature and wind velocity, carefully selected from three ship campaigns of the Air-Sea Interaction at Brazil-Malvinas Confluence (INTERCONF) project, we found a particular behavior of the drag coefficient, with a negative trend for a calm wind speed up to 10 m s −1 when the significant wave height was lower than 2.5 m, and a continuous decrease of the drag coefficient with increasing wind speed for significant wave height higher than 2.5 m. The results suggest that waves act as roughness elements during high wave conditions, inducing a zero-plane displacement in the order of 0.1 to 1 m as an indication for a wave-induced roughness layer. In addition, the analysis of the Turbulent Kinetic Energy (TKE) budget indicates the occurrence of upward TKE transport mainly during stable conditions, while the general patterns of transport and dissipation of TKE are similar to observations taken over land surfaces.

Description: Volcanic lightning studies have revealed that there is a relatively long lasting source of very high frequency (VHF) radiation associated with the onset of explosive volcanic eruptions that is distinct from radiation produced by lightning. This VHF signal is referred to as "continual (or continuous) radio frequency (CRF)" due to its long-lasting nature. The discharge mechanism producing this signal was previously hypothesized to be caused by numerous, small (10-100 m) leader-forming discharges near the vent of the volcano. To test this hypothesis, a multiparametric data set of electrical and volcanic activity occurring during explosive eruptions of Sakurajima volcano in Japan was collected from May-June 2015. Our observations show that a single CRF impulse has a duration on the order of 160 ns (giving an upper limit on discharge length of 10 m) and is distinct from near-vent lightning discharges that were on the order of 30 meters in length. CRF impulses did not produce discernible electric field changes and occurred in the absence of a net static electric field. Lightning mapping data and infrared video observations of the eruption column showed that CRF impulses originated from the gas-thrust region of the column. These observations indicate that CRF impulses are not produced by small, leader-forming discharges, but rather are more similar to a streamer discharge, likely on the order of a few meters in length.

Description: We investigate the benefit of high altitude nudging in simulations of the structure and short-term variability of the upper mesosphere and lower thermosphere (UMLT) dynamical meteorology during boreal winter, specifically around the time of the January 2009 sudden stratospheric warming. We compare simulations using the Specified Dynamics, Whole Atmosphere Community Climate Model, extended version, nudged using atmospheric specifications generated by the Navy Operational Global Atmospheric Prediction System, Advanced Level Physics High Altitude. Two sets of simulations are carried out: one uses nudging over a vertical domain from 0-90 km; the other uses nudging over a vertical domain from 0-50 km. The dynamical behavior is diagnosed from ensemble mean and standard deviation of winds, temperature, and zonal accelerations due to resolved and parameterized waves. We show that the dynamical behavior of the UMLT is quite different in the two experiments, with prominent differences in the structure and variability of constituent transport. We compare the results of our numerical experiments to observations of carbon monoxide by the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer to show that the high-altitude nudging is capable of reproducing with high fidelity the observed variability, and traveling planetary waves are a crucial component of the dynamics. The results of this study indicate that to capture the key physical processes that affect short-term variability (defined as the atmospheric behavior within about 10 days of a stratospheric warming) in the UMLT, specification of the atmospheric state in the stratosphere alone is not sufficient, and upper atmospheric specifications are needed.