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Mu Xiao
,
Sarith P. Mahanama
,
Yongkang Xue
,
Fei Chen
, and
Dennis P. Lettenmaier

component ( Mote et al. 2005 ). Li et al. (2017) found that 53% of the runoff over the western United States originates from melting snowpacks, a number that increases to 70% in the mountainous parts of the region. In relatively dry and heavily populated Southern California, more than half the water supply is derived from snowmelt from remote mountainous sources ( Waliser et al. 2011 ). As temperatures have warmed in recent decades, snowpack behavior and corresponding hydrological processes have been

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Wenfang Xu
,
Lijuan Ma
,
Minna Ma
,
Haicheng Zhang
, and
Wenping Yuan

recent decades. Based on passive microwave satellite data collected since 1979, other studies support a significant trend toward a shortening of the snowmelt season over much of Eurasia ( Takala et al. 2009 ) and the pan-Arctic region ( Tedesco and Monaghan 2009 ). Snowmelt mainly occurs in spring when the temperature is closer to the freezing point; therefore, changes in air temperature are the most effective at increasing snowmelt. Our results also highlighted that snow cover phenology is more

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David W. Clow

. Observed changes include decreases in the proportion of precipitation falling as snow ( Knowles et al. 2006 ), decreases in 1 April snow-water equivalent (SWE) in snowpacks ( Mote 2006 ), and earlier runoff during the spring snowmelt period ( Cayan et al. 2001 ; McCabe and Clark 2005 ; Regonda et al. 2005 ; Stewart et al. 2005 ). These studies indicated that in the west, changes were most pronounced in the Cascade Mountains, the northern Sierra Nevada, and the northern Rocky Mountains, where

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Zachary M. Seligman
,
Joel T. Harper
, and
Marco P. Maneta

1. Introduction As atmospheric conditions warm during the spring snowmelt season, the energy gains by the snowpack surface outweigh the energy losses. Heat is transferred through the snowpack by diffusive processes and by advection of latent heat, as meltwater generated at the surface percolates downward either along a uniform wetting front or as discrete “pipes,” and refreezes (e.g., Marsh and Woo 1985 ). The added heat initially removes cold content and drives snow grain metamorphism. Once

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Steven J. Fletcher
,
Glen E. Liston
,
Christopher A. Hiemstra
, and
Steven D. Miller

assimilate SWE ground and remotely sensed observations. Molotch and Margulis (2008) combined various sources of snow-covered area (SCA) [MODIS, Enhanced Thematic Mapper (ETM), and the Advanced Very High Resolution Radiometer (AVHRR)] with a snow depletion curve and snowmelt model to calculate SWE. Ensemble Kalman filters have been used to assimilate MODIS snow-covered fraction ( Andreadis and Lettenmaier 2006 ) to update SWE estimates or SWE estimates from passive microwave sensors ( De Lannoy et al

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Matthew C. Sanders
,
Jason M. Cordeira
, and
Nicholas D. Metz

-up, grounded, and floating ice jams; e.g., FEMA 2018 ) and modeling their formation and break up (e.g., Rokaya et al. 2018 , and references therein). Breakup ice jams typically form during the second half of winter as river ice breaks off the river bed, fractures, and accumulates downstream due to enhanced streamflow and increased discharges associated with warming temperatures, snowmelt, and rainfall (e.g., Bates and Brown 1982 ). Freeze-up ice jams typically form during the first half of winter as

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Christopher A. Shuman
,
Dorothy K. Hall
,
Nicolo E. DiGirolamo
,
Thomas K. Mefford
, and
Michael J. Schnaubelt

Abstract

The stability of the Moderate Resolution Imaging Spectroradiometer (MODIS) ice-surface temperature (IST) product from Terra was investigated for use as a climate-quality data record. The availability of climate-quality air temperature data TA from a NOAA observatory at Greenland’s Summit Station has enabled this high-temporal-resolution study of MODIS ISTs. During a >5-yr period (July 2008–August 2013), more than 2500 IST values were compared with ±3-min-average TA values from NOAA’s primary 2-m temperature sensor. This enabled an expected small offset between air and ice-sheet surface temperatures (TA > IST) to be investigated over multiple annual cycles. The principal findings of this study show 1) that IST values are slightly colder than the TA values near freezing but that this offset increases as temperature decreases and 2) that there is a pattern in IST–TA differences as the solar zenith angle (SoZA) varies annually. This latter result largely explains the progressive offset from the in situ data at colder temperatures but also indicates that the MODIS cloud mask is less accurate approaching and during the polar night. The consistency of the results over each year in this study indicates that MODIS provides a platform for remotely deriving surface temperature data, with the resulting IST data being most compatible with in situ TA data when the sky is clear and the SoZA is less than ~85°. The ongoing development of the IST dataset should benefit from improved cloud filtering as well as algorithm modifications to account for the progressive offset from TA at colder temperatures.

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Ken X. Zhao
,
Andrew L. Stewart
,
James C. McWilliams
,
Ian G. Fenty
, and
Eric J. Rignot

Abstract

Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.

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Constantin W. Arnscheidt
,
John Marshall
,
Pierre Dutrieux
,
Craig D. Rye
, and
Ali Ramadhan

Abstract

Antarctic glacial meltwater is thought to play an important role in determining large-scale Southern Ocean climate trends, yet recent modeling efforts have proceeded without a good understanding of how its vertical distribution in the water column is set. To rectify this, here we conduct new large-eddy simulations of the ascent of a buoyant meltwater plume after its escape from beneath an Antarctic ice shelf. We find that the meltwater’s settling depth is primarily a function of the buoyancy forcing per unit width of the source and the ambient stratification, consistent with the classical theory of turbulent buoyant plumes and in contrast to previous work that suggested an important role for centrifugal instability. Our results further highlight the significant role played by localized variability in stratification; this helps explain observed interannual variability in the vertical meltwater distribution near Pine Island Glacier. Because of the vast heterogeneity in mass loss rates and ambient conditions at different Antarctic ice shelves, a dynamic parameterization of meltwater settling depth may be crucial for accurately simulating high-latitude climate in a warming world; we discuss how this may be developed following this work, and where the remaining challenges lie.

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Bei Xu
and
Gen Li

processes. For example, Halder and Dirmeyer (2017) suggested that the delayed hydrological impact of snow cover anomalies could lead to abnormal soil moisture, thereby inducing surface temperature and geopotential height anomalies through strong land–atmosphere interactions to affect the Indian summer monsoon rainfall. The numerical experiments showed that the snow cover albedo effect is found to be active before and during the snowmelt period. After the snow melts, the hydrological effect increases

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