39 Snow cover and its melt dominate regional climate and water resources in many of the 40 world's mountainous regions. Snowmelt timing and magnitude in mountains are controlled 41 predominantly by absorption of solar radiation and the distribution of snow water equivalent 42 (SWE), and yet both of these are very poorly known even in the best-instrumented mountain 43 regions of the globe. Here we describe and present results from the Airborne Snow 44 Observatory (ASO), a coupled imaging spectrometer and scanning lidar, combined with 45 distributed snow modeling, developed for the measurement of snow spectral albedo/broadband 46 albedo and snow depth/SWE. Snow density is simulated over the domain to convert snow 47 depth to SWE. The result presented in this paper is the first operational application of remotely 48 sensed snow albedo and depth/SWE to quantify the volume of water stored in the seasonal 49 snow cover. The weekly values of SWE volume provided by the ASO program represent a 50 critical increase in the information available to hydrologic scientists and resource managers in 51 mountain regions. 52 53 3 54 55Introduction 56Snow cover and its melt dominate sources in many of the world's mountainous regions, and 57 in adjacent areas dependent on river flows originating from mountain basins. In the western 58 United States, snowmelt runoff dominates the surface water hydrology, providing more than 59 75% of the total freshwater (Bales et al., 2006). However, the region faces significant water 60 resource challenges due to the intersection of increasing demand from population growth and 61 changes in runoff volume and timing due to climate change (Christensen et al., 2004; 62 Christensen and Lettenmaier, 2007). 63Observations indicate an ongoing reduction in the seasonal duration of mountain snowpacks 64 Mote et al., 2005;Hamlet et al., 2007;Clow, 2010), a trend likely to 65 continue under unimpeded warming associated with climate change (Christensen and 66 Lettenmaier, 2007;Deems et al., 2013b). Moreover, increasing temperatures in desert systems 67 will increase dust loading to mountain snow cover (Munson et al., 2011), thus reducing the 68 snow cover albedo and accelerating snowmelt runoff (Painter et al., 2007;Painter et al., 2010; 69 Skiles et al., 2012). 70The two most critical properties for understanding timing and magnitude of snowmelt runoff 71 are the spatial distributions of snow albedo and snow water equivalent (SWE) (Blöschl, 1991; 72 Kirnbauer et al., 1994). Despite the importance of these properties in controlling volume and 73 timing of runoff, the mountain snowpack remains poorly quantified around the globe (Bales et 74 al., 2006) (Barnett et al., 2005, leaving runoff and climate models poorly constrained and our 75 physical understanding of mountain snowmelt driven systems incomplete. 76 4In the western US, we have relatively sparse measurements of SWE, mostly at lower and 77 middle elevations and only a few per basin (Bales et al., 2006). These measurements are used as 78 in...

[1] Dust in snow accelerates snowmelt through its direct reduction of snow albedo and its further indirect reduction of albedo by accelerating the growth of snow grains. Since the westward expansion of the United States that began in the mid-19th century, the mountain snow cover of the Colorado River Basin has been subject to five-fold greater dust loading, largely from the Colorado Plateau and Great Basin. Radiative forcing of snowmelt by dust is not captured by conventional micrometeorological measurements, and must be monitored by a more comprehensive suite of radiation instruments. Here we present a 6 year record of energy balance and detailed radiation measurements in the Senator Beck Basin Study Area, San Juan Mountains, Colorado, USA. Data include broadband irradiance, filtered irradiance, broadband reflected flux, filtered reflected flux, broadband and visible albedo, longwave irradiance, wind speed, relative humidity, and air temperatures. The gradient of the snow surface is monitored weekly and used to correct albedo measurements for geometric effects. The snow is sampled weekly for dust concentrations in plots immediately adjacent to each tower over the melt season. Broadband albedo in the last weeks of snow cover ranged from 0.33 to 0.55 across the 6 years and two sites. Total end of year dust concentration in the top 3 cm of the snow column ranged from 0.23 mg g À1 to 4.16 mg g À1 . These measurements enable monitoring and modeling of dust and climate-driven snowmelt forcings in the Upper Colorado River Basin.

[1] Here we present the radiative and snowmelt impacts of dust deposition to snow cover using a 6-year energy balance record (2005)(2006)(2007)(2008)(2009)(2010) at alpine and subalpine micrometeorological towers in the Senator Beck Basin Study Area (SBBSA) in southwestern Colorado, USA. These results follow from the measurements described in part I. We simulate the evolution of snow water equivalent at each station under scenarios of observed and dust-free conditions, and þ2C and þ4 C melt-season temperature perturbations to these scenarios. Over the 6 years of record, daily mean dust radiative forcing ranged from 0 to 214 W m À2, with hourly peaks up to 409 W m À2 . Mean springtime dust radiative forcings across the period ranged from 31 to 49 W m À2 at the alpine site and 45 to 75 W m À2 at the subalpine site, in turn shortening snow cover duration by 21 to 51 days. The dust-advanced loss of snow cover (days) is linearly related to total dust concentration at the end of snow cover, despite temporal variability in dust exposure and solar irradiance. Under clean snow conditions, the temperature increases shorten snow cover by 5-18 days, whereas in the presence of dust they only shorten snow duration by 0-6 days. Dust radiative forcing also causes faster and earlier peak snowmelt outflow with daily mean snowpack outflow doubling under the heaviest dust conditions. On average, snow cover at the towers is lost 2.5 days after peak outflow in dusty conditions, and 1-2 weeks after peak outflow in clean conditions.

[1] The episodic deposition of dust and carbonaceous particles to snow decreases snow surface albedo and enhances absorption of solar radiation, leading to accelerated snowmelt, negative glacier mass balance, and the snow-albedo feedback. Until now, no remote sensing retrieval has captured the spatial and temporal variability of this forcing. Here we present the MODIS Dust Radiative Forcing in Snow (MODDRFS) model that retrieves surface radiative forcing by light absorbing impurities in snow cover from Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data. Validation of MODDRFS with a 7-year record of in situ measurements indicates the radiative forcing retrieval has positive bias at lower values and slight negative bias above 200 W m À2 , subject to mixed pixel uncertainties. With bias-correction, MODDRFS has a root mean squared error of 32 W m À2 and mean absolute error of 25 W m À2 . We demonstrate MODDRFS in the Upper Colorado River Basin and Hindu Kush-Himalaya.

Abstract. Black carbon (BC) and dust deposited on snow and glacier surfaces can reduce the surface albedo, accelerate snow and ice melt, and trigger albedo feedback. Assessing BC and dust concentrations in snow and ice in the Himalaya is of interest because this region borders large BC and dust sources, and seasonal snow and glacier ice in this region are an important source of water resources. Snow and ice samples were collected from crevasse profiles and snow pits at elevations between 5400 and 6400 m a.s.l. from Mera glacier located in the Solu-Khumbu region of Nepal during spring and fall 2009, providing the first observational data of BC concentrations in snow and ice from the southern slope of the Himalaya. The samples were measured for Fe concentrations (used as a dust proxy) via ICP-MS, total impurity content gravimetrically, and BC concentrations using a Single Particle Soot Photometer (SP2). Measured BC concentrations underestimate actual BC concentrations due to changes to the sample during storage and loss of BC particles in the ultrasonic nebulizer; thus, we correct for the underestimated BC mass. BC and Fe concentrations are substantially higher at elevations < 6000 m due to post-depositional processes including melt and sublimation and greater loading in the lower troposphere. Because the largest areal extent of snow and ice resides at elevations < 6000 m, the higher BC and dust concentrations at these elevations can reduce the snow and glacier albedo over large areas, accelerating melt, affecting glacier mass balance and water resources, and contributing to a positive climate forcing. Radiative transfer modeling constrained by measurements at 5400 m at Mera La indicates that BC concentrations in the winter-spring snow/ice horizons are sufficient to reduce albedo by 6-10 % relative to clean snow, corresponding to localized instantaneous radiative forcings of 75-120 W m −2 . The other bulk impurity concentrations, when treated separately as dust, reduce albedo by 40-42 % relative to clean snow and give localized instantaneous radiative forcings of 488 to 525 W m −2 . Adding the BC absorption to the other impurities results in additional radiative forcings of 3 W m −2 . The BC and Fe concentrations were used to further examine relative absorption of BC and dust. When dust concentrations are high, dust dominates absorption, snow albedo reduction, and radiative forcing, and the impact of BC may be negligible, confirming the radiative transfer modeling. When impurity concentrations are low, the absorption by BC and dust may be comparable; however, due to the low impurity concentrations, albedo reductions are small. While these results suggest that the snow albedo and radiative forcing effect of dust is considerably greater than BC, there are several sources of uncertainty. Further observational studies are needed to address the contribution of BC, dust, and colored organics to albedo reductions and snow and ice melt, and to characterize the time variation of radiative forcing.