ALBERT

Description: The Equatorial Quasi Biennial Oscillation (QBO) is known to be an important source of interannual variability in the mid and high-latitude stratosphere. The influence of the QBO on the stratospheric polar vortex in particular has been extensively studied. However, the impact of the QBO on the winds of the mid-latitude mesosphere is much less clear. We have applied 13 years (2002-2014) of data from the Saskatoon SuperDARN HF radar to show that there is a strong QBO signature in the mid-latitude mesospheric zonal winds during the late winter months. We find that the Saskatoon mesospheric winds are related to the winds of the equatorial QBO at 50 hPa such that the westerly mesospheric winds strengthen when QBO is easterly, and vice-versa. We also consider the situation in the late-winter Saskatoon stratosphere using the ECMWF ERA-Interim reanalysis data set. We find that the Saskatoon stratospheric winds between 7 hPa and 70 hPa weaken when the equatorial QBO at 50 hPa is easterly, and vice-versa. We speculate that gravity wave filtering from the QBO-modulated stratospheric winds and subsequent opposite momentum deposition in the mesosphere plays a major role in the appearance of the QBO signature in the late winter Saskatoon mesospheric winds, thereby coupling the equatorial stratosphere and the mid-latitude mesosphere.

Description: Eastern Australia recently experienced an intense drought (Millennium Drought, 2003–2009) and record-breaking rainfall and flooding (austral summer 2010–2011). There is some limited evidence for a climate change contribution to these events, but such analyses are hampered by the paucity of information on long-term natural variability. Analyzing a new reconstruction of summer (December–January–February) Palmer Drought Severity Index (the Australia–New Zealand Drought Atlas; ANZDA, 1500–2012 CE), we find moisture deficits during the Millennium Drought fall within the range of the last 500 years of natural hydroclimate variability. This variability includes periods of multi-decadal drought in the 1500s more persistent than any event in the historical record. However, the severity of the Millennium Drought, which was caused by autumn (March–April–May) precipitation declines, may be underestimated in the ANZDA because the reconstruction is biased towards summer and antecedent spring (September-October-November) precipitation. The pluvial in 2011, however, which was characterized by extreme summer rainfall faithfully captured by the ANZDA, is likely the wettest year in the reconstruction for Coastal Queensland. Climate projections (RCP 8.5 scenario) suggest that eastern Australia will experience long-term drying during the 21 st century. While the contribution of anthropogenic forcing to recent extremes remains an open question, these projections indicate an amplified risk of multi-year drought anomalies matching or exceeding the intensity of the Millennium Drought.

Description: Along with air temperatures, the freezing level height (FLH) has risen over the last decades. The mass balance of tropical glaciers in Peru is highly sensitive to a rise in the FLH, mainly due to a decrease in accumulation and increase of energy for ablation caused by reduced albedo. Knowledge of future changes in the FLH is thus crucial to estimating changes in glacier extents. Since in-situ data are scarce at altitudes where glaciers exist (above ca. 4800 m asl.), reliable FLH estimates must be derived from multiple data types. Here, we assessed the FLHs and their spatiotemporal variability, as well as the related snow/rain transition in the two largest glacier-covered regions in Peru by combining ERA-Interim and MERRA2 reanalysis, TRMM Precipitation Radar Bright Band, Micro Rain Radar data and meteorological ground station measurements. The mean FLH lies at 4900 and 5010 m asl., for the Cordillera Blanca and Vilcanota, respectively. During the wet season, the FLH in the Cordillera Vilcanota lies ca. 150 m higher compared to the Cordillera Blanca, which is in line with the higher glacier terminus elevations. CMIP5 climate model projections reveal that by the end of the 21st century, the FLH will rise by 230 m (±190 m) for RCP2.6 and 850 m (±390 m) for RCP8.5. Even under the most optimistic scenario, glaciers may continue shrinking considerably, assuming a close relation between FLH and glacier extents. Under the most pessimistic scenario, glaciers may only remain at the highest summits above approximately 5800 m asl.

Description: The so-called accumulation size range of airborne particles is the center of a continuing disagreement about the formulation of dry deposition. Some contemporary meteorological and air quality models use theoretical developments based on early wind tunnel and other controlled experiments, while other models consider the bulk properties of the underlying surface and the ability of atmospheric turbulence to deliver particles to it. This dichotomy arose when the first micrometeorological measurements of particle deposition velocities became available, yielding numbers exceeding the highest expectations of the then-current models based on assumptions about inertial impaction and interception. The model predictions had previously been shown to be in accord with theoretical treatments of filtration. A common reaction was to distrust the field experimental results, but the experimental findings were supported by subsequent studies. The difference between model predictions and field measurements appears greatest for densely vegetated canopies. Ongoing research is investigating factors that could give rise to the discrepancy, e.g. turbulence intermittency, leaf orientation, leaf morphology, leaf flutter, electrical charges, and a number of phoretic effects. In the meantime, many investigators are faced with a decision as to whether to make use of parameterized field results or theoretical descriptions of behaviors that are not yet well examined. Here, the history of the ongoing disagreement is reviewed and some possible resolutions are presented.

Description: [1]  The relationship between snow microstructure and its mechanical properties is crucial for modeling the microstructural cause of snow avalanches. In this paper, the knowledge gap between snow microstructural evolution and overburden is bridged by combining compression tests with X-ray computed micro-tomography (micro CT). Continuous compression tests and interrupted compression tests were performed on three types of snow (fresh low temperature snow, fresh high temperature snow, and sintered low temperature snow with densities ranging from 100-350 kg/m 3 ) to investigate the effects of initial structure on both the stress-displacement curve and structural changes during the compression test. 3D micro CT images and SEM images showed that well-connected structures of sintered snow contribute to a more rapid stress increase during deformation. By combining the analyses of the specific surface area, structure model index, and structure thickness, it is shown that compression tests at a fixed temperature, loading rate, and density snow metamorphism can be regarded as an accelerated pressure sintering process. However, when extremely low-density snow (50 kg/m 3 ) was subjected to compression testing, the same evolution of structure parameters was again found, but fracture of snowflakes was also observed. The overall process of low-density snow deformation can be explained as a combination of fracturing and sintering, but with the sintering predominating.

Description: We present variations of methyl chloride (CH 3 Cl) and nitrous oxide (N 2 O) in the lowermost stratosphere (LMS) obtained from air samples collected by the IAGOS-CARIBIC passenger aircraft observatory for the period 2008–2012. To correct for the temporal increase of atmospheric N 2 O, the CARIBIC N 2 O data are expressed as deviations from the long-term trend at the northern hemispheric baseline station Mauna Loa (MLO) Hawaii (ΔN 2 O). ΔN 2 O undergoes a pronounced seasonal variation in the LMS with a minimum in spring. The amplitude increases going deeper in the LMS (up to potential temperature of 40 K above the thermal tropopause), as a result of the seasonally varying subsidence of air from the stratospheric overworld. Seasonal variation of CH 3 Cl above the tropopause is similar in phase to that of ΔN 2 O. Significant correlations are found between CH 3 Cl and ΔN 2 O in the LMS from winter to early summer, both being affected by mixing between stratospheric air and upper tropospheric (UT) air. This correlation however disappears in late summer to autumn. The slope of the CH 3 Cl-ΔN 2 O correlation observed in the LMS allows us to determine the stratospheric lifetime of CH 3 Cl to be 35±7 yr. Finally, we examine the partitioning of stratospheric air and tropical/extra-tropical tropospheric air in the LMS based on a mass balance approach using ΔN 2 O and CH 3 Cl. This analysis clearly indicates efficient inflow of tropical tropospheric air into the LMS in summer and demonstrates the usefulness of CH 3 Cl as a tracer of tropical tropospheric air.

Description: Quantifying the carbonyl sulfide (OCS) land/ocean fluxes contributes to the understanding of both the sulfur and carbon cycles. The primary sources and sinks of OCS are very likely in a steady state because there is no significant observed trend or inter-annual variability in atmospheric OCS measurements. However, the magnitude and spatial distribution of the dominant ocean source is highly uncertain due to the lack of observations. In particular, estimates of the oceanic fluxes range from approximately 280 Gg-S yr −1 to greater than 800 Gg-S yr −1 , with the larger flux needed to balance a similarly sized terrestrial sink that is inferred from NOAA continental sites. Here we estimate summer tropical oceanic fluxes of OCS in 2006 using a linear flux inversion algorithm and new OCS data acquired by the Aura Tropospheric Emissions Spectrometer (TES). Modeled OCS concentrations based on these updated fluxes are consistent with HIPPO-4 airborne observations and improve significantly over the a priori model concentrations. The TES tropical ocean estimate of 70 ± 16 Gg-S in June, when extrapolated over the whole year (about 840 ± 192 Gg-S yr −1 ), supports the hypothesis proposed by Berry et al. (2013) that the ocean flux is in the higher range of approximately 800 Gg-S yr −1 .

Description: Modeled source attribution information from CMAQ was coupled with ambient data from the 2011 DISCOVER-AQ Baltimore field study. We assess source contributions and evaluate the utility of using aircraft measured CO and NO y relationships to constrain emission inventories. We derive ambient and modeled ΔCO:ΔNO y ratios which have previously been interpreted to represent CO:NO y ratios in emissions from local sources. Modeled and measured ΔCO:ΔNO y are similar; however, measured ΔCO:ΔNO y has much more daily variability than modeled values. Sector-based tagging shows that regional transport, onroad gasoline vehicles, and nonroad equipment are the major contributors to modeled CO mixing ratios in the Baltimore area. In addition to those sources, onroad diesel vehicles, soil emissions, and power plants also contribute substantially to modeled NO y in the area. The sector mix is important because emitted CO:NO x ratios vary by several orders of magnitude among the emission sources. The model-predicted gasoline/diesel split remains constant across all measurement locations in this study. Comparison of ΔCO:ΔNOy to emitted CO:NOy is challenged by ambient and modeled evidence that free tropospheric entrainment and atmospheric processing elevate ambient ΔCO:ΔNO y above emitted ratios. Specifically, modeled ΔCO:ΔNO y from tagged mobile source emissions is enhanced 5-50% above the emitted ratios at times and locations of aircraft measurements. We also find a correlation between ambient formaldehyde concentrations and measured ΔCO:ΔNO y suggesting that secondary CO formation plays a role in these elevated ratios. This analysis suggests ambient urban daytime ΔCO:ΔNO y values are not reflective of emitted ratios from individual sources.

Description: The Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) models were used to simulate a ten-day high-ozone episode observed during the 2013 Uinta Basin Winter Ozone Study (UBWOS). The baseline model had a large negative bias when compared to ozone (O 3 ) and volatile organic compound (VOC) measurements across the Basin. Contrary to other wintertime Uinta Basin studies, predicted nitrogen oxides (NO x ) were typically low compared to measurements. Increases to oil and gas VOC emissions resulted in O 3 predictions closer to observations, and nighttime O 3 improved when reducing the deposition velocity for all chemical species. Vertical structures of these pollutants were similar to observations on multiple days. However, the predicted surface layer VOC mixing ratios were generally found to be underestimated during the day and overestimated at night. While temperature profiles compared well to observations, WRF was found to have a warm temperature bias and too low nighttime mixing heights. Analyses of more realistic snow heat capacity in WRF to account for the warm bias and vertical mixing resulted in improved temperature profiles, although the improved temperature profiles seldom resulted in improved O 3 profiles. While additional work is needed to investigate meteorological impacts, results suggest that the uncertainty in the oil and gas emissions contributes more to the underestimation of O 3 . Further, model adjustments based on a single site may not be suitable across all sites within the Basin.