Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over theGreenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievalswith respect to remaining ground-clutter issues. Comparing CloudSatobservations to the GrIS topography (obtained from airborne altimetrymeasurements during IceBridge) we find that at the edges of the GrISspurious high-snowfall retrievals caused by ground clutter occasionallyaffect the operational snowfall product. After correcting for this effect,the height of the lowest valid CloudSat observation is about 1200 mabove the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple,empirical correction to account for precipitation processes occurringbetween the height of the observed CloudSat reflectivities and the snowfallnear the surface. Using the height-corrected, clutter-cleared CloudSatreflectivities we next evaluate various Z–S relationships in terms ofsnowfall accumulation at Summit through comparison with weekly stake fieldobservations of snow accumulation available since 2007. Using a set of threeZ–S relationships that best agree with the observed accumulation at Summit,we then calculate the annual cycle snowfall over the entire GrIS as well asover different drainage areas and compare the derived mean values and annualcycles of snowfall to ERA-Interim reanalysis. We find the annual meansnowfall over the GrIS inferred from CloudSat to be 34±7.5 cm yr−1liquid equivalent (where the uncertainty is determined by the range invalues between the three different Z–S relationships used). In comparison,the ERA-Interim reanalysis product only yields 30 cm yr−1 liquid equivalentsnowfall, where the majority of the underestimation in the reanalysisappears to occur in the summer months over the higher GrIS and appears to berelated to shallow precipitation events. Comparing all available estimatesof snowfall accumulation at Summit Station, we find the annually averagedliquid equivalent snowfall from the stake field to be between 20 and 24 cm yr−1, depending on the assumed snowpack density and from CloudSat 23±4.5 cm yr−1. The annual cycle at Summit is generally similar betweenall data sources, with the exception of ERA-Interim reanalysis, which showsthe aforementioned underestimation during summer months.
Abstract. Radiation fogs at Summit Station, Greenland (72.58∘ N,38.48∘ W; 3210 m a.s.l.), are frequently reported by observers. Thefogs are often accompanied by fogbows, indicating the particles are composedof liquid; and because of the low temperatures at Summit, this liquid issupercooled. Here we analyze the formation of these fogs as well as theirphysical and radiative properties. In situ observations of particle size anddroplet number concentration were made using scattering spectrometers near 2 and 10 m height from 2012 to 2014. These data are complemented bycolocated observations of meteorology, turbulent and radiative fluxes, andremote sensing. We find that liquid fogs occur in all seasons with thehighest frequency in September and a minimum in April. Due to thecharacteristics of the boundary-layer meteorology, the fogs are elevated,forming between 2 and 10 m, and the particles then fall toward the surface.The diameter of mature particles is typically 20–25 µm in summer.Number concentrations are higher at warmer temperatures and, thus, higher insummer compared to winter. The fogs form at temperatures as warm as −5 ∘C, while the coldest form at temperatures approaching −40 ∘C. Facilitated by the elevated condensation, in winter two-thirds offogs occurred within a relatively warm layer above the surface when thenear-surface air was below −40 ∘C, as cold as −57 ∘C,which is too cold to support liquid water. This implies that fog particlessettling through this layer of cold air freeze in the air column beforecontacting the surface, thereby accumulating at the surface as ice withoutriming. Liquid fogs observed under otherwise clear skies annually imparted1.5 W m−2 of cloud radiative forcing (CRF). While this is a smallcontribution to the surface radiation climatology, individual events areinfluential. The mean CRF during liquid fog events was 26 W m−2, andwas sometimes much higher. An extreme case study was observed toradiatively force 5 ∘C of surface warming during the coldest partof the day, effectively damping the diurnal cycle. At lower elevations ofthe ice sheet where melting is more common, such damping could signal a rolefor fogs in preconditioning the surface for melting later in the day.
more » « less- NSF-PAR ID:
- 10108872
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 19
- Issue:
- 11
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 7467 to 7485
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
more » « less
Abstract. Triplet excited states of organic matter are formed when colored organicmatter (i.e., brown carbon) absorbs light. While these “triplets” can beimportant photooxidants in atmospheric drops and particles (e.g., theyrapidly oxidize phenols), very little is known about their reactivity towardmany classes of organic compounds in the atmosphere. Here we measure thebimolecular rate constants of the triplet excited state of benzophenone(3BP∗), a model species, with 17 water-solubleC3–C6 alkenes that have either been found in theatmosphere or are reasonable surrogates for identified species. Measured rateconstants (
-
more » « less
Abstract. The magnitude and controls of particulate carbon exported from surface watersand its remineralization at depth are poorly constrained. The Carbon FluxExplorer (CFE), a Lagrangian float-deployed imaging sediment trap, has beendesigned to optically measure the hourly variations of particle flux tokilometer depths for months to seasons while relaying data in near-real timeto shore via satellite without attending ships. The main optical proxy forparticle load recorded by the CFE, volume attenuance (VA; units ofmATN cm2), while rigorously defined and highly precise, has not beenrobustly calibrated in terms of particulate organic carbon (POC), nitrogen(PN) and phosphorus (PP). In this study, a novel 3-D-printed particle samplerusing cutting edge additive manufacturing was developed and integrated withthe CFE. Two such modified floats (CFE-Cals) were deployed a total of15 times for 18–24 h periods to gain calibration imagery and samples atdepths near 150 m in four contrasting productivity environments during theJune 2017 California Current Ecosystem Long-Term Ecological Research (LTER)process study. Regression slopes for VA : POC and VA : PN (unitsmATN cm2: mmol; R2, n, p value in parentheses) were1.01×104 (0.86, 12, < 0.001) and 1.01×105(0.86, 15, < 0.001), respectively, and were not sensitive toparticle size classes or the contrasting environments encountered. PP was notwell correlated with VA, reflecting the high lability of P relative to C andN. The volume attenuance flux (VAF) to POC flux calibration is compared toprevious estimates.
-
more » « less
Abstract. Lidar observations of the mesospheric Na layer have revealed considerablediurnal variations, particularly on the bottom side of the layer, where morethan an order-of-magnitude increase in Na density has been observed below 80 kmafter sunrise. In this paper, multi-year Na lidar observations areutilized over a full diurnal cycle at Utah State University (USU) (41.8∘ N,111.8∘ W) and a global atmospheric model of Na with 0.5 kmvertical resolution in the mesosphere and lower thermosphere (WACCM-Na) to explorethe dramatic changes of Na density on the bottom side of the layer. Photolysis of the principal reservoir NaHCO3 is shown to beprimarily responsible for the increase in Na after sunrise, amplified by theincreased rate of reaction of NaHCO3 with atomic H, which is mainlyproduced from the photolysis of H2O and the reaction of OH withO3. This finding is further supported by Na lidar observation at USUduring the solar eclipse (>96 % totality) event on 21 August 2017, when a decrease and recovery of the Na density on thebottom side of the layer were observed. Lastly, the model simulation showsthat the Fe density below around 80 km increases more strongly and earlierthan observed Na changes during sunrise because of the considerably fasterphotolysis rate of its major reservoir of FeOH.
-
more » « less
Abstract. Urbanization and deforestation have important impacts on atmosphericparticulate matter (PM) over Amazonia. This study presents observations andanalysis of PM1 concentration, composition, and opticalproperties in central Amazonia during the dry season, focusing on theanthropogenic impacts. The primary study site was located 70 km downwind ofManaus, a city of over 2 million people in Brazil, as part of theGoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol massspectrometer (AMS) provided data on PM1 composition, and aethalometermeasurements were used to derive the absorption coefficient babs,BrC ofbrown carbon (BrC) at 370 nm. Non-refractory PM1 mass concentrationsaveraged 12.2 µg m−3 at the primary study site, dominated byorganics (83 %), followed by sulfate (11 %). A decrease inbabs,BrC was observed as the mass concentration of nitrogen-containingorganic compounds decreased and the organic PM1 O:C ratio increased,suggesting atmospheric bleaching of the BrC components. The organic PM1was separated into six different classes by positive-matrix factorization(PMF), and the mass absorption efficiency Eabs associated with eachfactor was estimated through multivariate linear regression ofbabs,BrC on the factor loadings. The largest Eabs values wereassociated with urban (2.04±0.14 m2 g−1) and biomass-burning(0.82±0.04 to 1.50±0.07 m2 g−1) sources. Together, these sources contributed at least 80 % ofbabs,BrC while accounting for 30 % to 40 % of the organic PM1 massconcentration. In addition, a comparison of organic PM1 compositionbetween wet and dry seasons revealed that only part of the 9-foldincrease in mass concentration between the seasons can be attributed tobiomass burning. Biomass-burning factor loadings increased by 30-fold,elevating its relative contribution to organic PM1 from about 10 % inthe wet season to 30 % in the dry season. However, most of the PM1mass (>60 %) in both seasons was accounted for by biogenicsecondary organic sources, which in turn showed an 8-fold seasonalincrease in factor loadings. A combination of decreased wet deposition andincreased emissions and oxidant concentrations, as well as a positivefeedback on larger mass concentrations are thought to play a role in theobserved increases. Furthermore, fuzzy c-means clustering identified threeclusters, namely “baseline”, “event”, and “urban” to representdifferent pollution influences during the dry season. The baseline cluster,representing the dry season background, was associated with a mean massconcentration of 9±3 µg m−3. This concentration increasedon average by 3 µg m−3 for both the urban and the event clusters.The event cluster, representing an increased influence of biomass burningand long-range transport of African volcanic emissions, was characterized byremarkably high sulfate concentrations. The urban cluster, representing theinfluence of Manaus emissions on top of the baseline, was characterized byan organic PM1 composition that differed from the other two clusters.The differences discussed suggest a shift in oxidation pathways as well asan accelerated oxidation cycle due to urban emissions, in agreement withfindings for the wet season.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-19-7467-2019
