Snow Forcing Research Articles

The associations of Tibetan Plateau spring snow cover with East Asian summer monsoon rainfall before and after 1990

In recent decades, the existence of a relationship between snow cover on the Tibetan Plateau (TP) and East Asian summer monsoon (EASM) rainfall has been emphasized. According to recently published studies this snow-monsoon relationship experienced a shift after 1990. Although the changing snow-monsoon relationship has been studied, the causes of the interdecadal changes remain unclear. This study assesses the associations of TP spring snow cover with EASM rainfall before and after 1990 and explores what possible mechanisms could be responsible for the interdecadal changes. Correlation and composite analyses were used to assess the strength of the relationship between TP spring snow cover and EASM rainfall and to analyze the atmospheric and land surface patterns associated with high snow cover. The outcomes suggest that the relationship between TP spring snow cover and EASM rainfall changes from partially negative to positive over all regions of the TP from 1968–1990 (P1) to 1991–2019 (P2), implying that more snow cover is associated with less (more) EASM rainfall during P1 (P2). In P1, years with high snow cover are associated with an anomalous cyclone southwest of the TP (positioned over Iran and Pakistan) in spring, which persists into the following summer, partly due to the underlying snow cover. The anomalous cyclone is accompanied by downstream anomalies over East Asia, which form a strong east-west oriented wave pattern and induce a northerly inflow of dry air over East Asia, reducing rainfall over the northern EASM domain. In P2, high snow cover years are associated with an anomalous cyclone over the western TP, which weakens and loses its significance in May-June and summer, partly due to a decline in snow forcing. Southeastward propagation of wave energy in May-June initiates the formation of an anomalous anticyclone over southeastern China and the western North Pacific. Concurrently, a meridional circulation develops over East Asia that enhances the southwesterly moisture inflow, resulting in increased EASM rainfall. The changing snow-monsoon relationship can be linked to different wave train patterns resulting from changes in the background zonal wind and meridional temperature gradients. This research contributes to a better understanding of the changing snow-monsoon relationship.

A cleaner snow future mitigates Northern Hemisphere snowpack loss from warming

Light-absorbing particles (LAP) deposited on seasonal snowpack can result in snow darkening, earlier snowmelt, and regional climate change. However, their future evolution and contributions to snowpack change relative to global warming remain unclear. Here, using Earth System Model simulations, we project significantly reduced black carbon deposition by 2081-2100, which reduces the December-May average LAP-induced radiative forcing in snow over the Northern Hemisphere from 1.3 Wm−2 during 1995-2014 to 0.65 (SSP126) and 0.49 (SSP585) Wm−2. We quantify separately the contributions of climate change and LAP evolution on future snowpack and demonstrate that projected LAP changes in snow over the Tibetan Plateau will alleviate future snowpack loss due to climate change by 52.1 ± 8.0% and 8.0 ± 1.1% at the end of the century for the two scenarios, mainly due to reduced black carbon contamination. Our findings highlight a cleaner snow future and its benefits for future water supply from snowmelt especially under the sustainable development pathway of SSP126.

The Role of Coupled North Pacific Atmosphere–Ocean Interactions in Impacts of Tibetan Plateau Snow Anomalies on the Northern Hemisphere Winter Atmospheric Circulation

Abstract Previous studies indicate observed influences of autumn and winter Tibetan Plateau (TP) snow-cover anomalies on the winter Pacific–North American (PNA) teleconnection. This study simulates atmospheric and oceanic responses to persistent autumn–winter TP snow forcing using an atmospheric general circulation model (AGCM) and a coupled atmospheric-oceanic general circulation model (AOGCM), and quantifies the role of atmosphere–ocean interactions over North Pacific in TP snow effects. The AOGCM experiment induces a stronger and more realistic remote PNA response to heavy TP snow anomalies, and also a significant winter horseshoe-like North Pacific sea surface temperature (SST) pattern resulting from an anomalous equivalent barotropic cyclone, or a strengthened Aleutian low, with associated cyclonic wind stress anomalies. The horseshoe-like SST anomaly pattern is used as boundary forcing (without prescribed heavy TP snow) in another AOGCM experiment, which simulates an enhanced winter Aleutian low and a PNA-like response similar to the original AOGCM responses, indicating that that the direct Pacific–North American atmospheric response to persistent TP snow forcing in the AGCM is amplified in the AOGCM by the North Pacific midlatitude atmosphere–ocean interactions. This suggests that the mechanisms of the winter PNA responses to TP snow forcing involve dynamical atmospheric processes such as horizontal propagation of Rossby wave energy and transient eddy feedbacks, and also North Pacific atmosphere–ocean interactions, which provide a positive feedback on the development of the remote PNA teleconnection. Significance Statement The Tibetan Plateau has a 2.5 million km2 area and a 4000 m average elevation, and is often called “the third pole” because of its polar-like climate in midlatitude Eurasia. A heavy Tibetan Plateau snow cover increases the albedo effect and causes tropospheric cold temperature and low height changes that can be carried or advected to the North Pacific by the prevailing westerlies. The purpose of this study is to better understand how Tibetan Plateau snow anomalies influence the North Pacific and the remote atmospheric circulation through numerical simulations. Our results find that those North Pacific responses amply the direct atmospheric response to Tibetan Plateau snow-cover anomalies. This greatly improves our understanding of physical mechanisms of Tibetan Plateau snow impacts.

Evaluation of E3SM land model snow simulations over the western United States

Abstract. Seasonal snow has crucial impacts on climate, ecosystems, and humans, but it is vulnerable to global warming. The land component (ELM) of the Energy Exascale Earth System Model (E3SM) mechanistically simulates snow processes from accumulation, canopy interception, compaction, and snow aging to melt. Although high-quality field measurements, remote sensing snow products, and data assimilation products with high spatio-temporal resolution are available, there has been no systematic evaluation of the snow properties and phenology in ELM. This study comprehensively evaluates ELM snow simulations over the western United States at 0.125∘ resolution during 2001–2019 using the Snow Telemetry (SNOTEL) in situ networks, MODIS remote sensing products (i.e., MCD43 surface albedo product), the spatially and temporally complete (STC) snow-covered area and grain size (MODSCAG) and MODIS dust and radiative forcing in snow (MODDRFS) products (STC-MODSCAG/STC-MODDRFS), and the snow property inversion from remote sensing (SPIReS) product and two data assimilation products of snow water equivalent and snow depth – i.e., University of Arizona (UA) and SNOw Data Assimilation System (SNODAS). Overall the ELM simulations are consistent with the benchmarking datasets and reproduce the spatio-temporal patterns, interannual variability, and elevation gradients for different snow properties including snow cover fraction (fsno), surface albedo (αsur) over snow cover regions, snow water equivalent (SWE), and snow depth (Dsno). However, there are large biases of fsno with dense forest cover and αsur in the Rocky Mountains and Sierra Nevada in winter, compared to the MODIS products. There are large discrepancies of snow albedo, snow grain size, and light-absorbing particle-induced snow albedo reduction between ELM and the MODIS products, attributed to uncertainties in the aerosol forcing data, snow aging processes in ELM, and remote sensing retrievals. Against UA and SNODAS, ELM has a mean bias of −20.7 mm (−35.9 %) and −20.4 mm (−35.5 %), respectively, for spring, and −13.8 mm (−27.8 %) and −10.2 mm (−22.2 %), respectively, for winter. ELM shows a relatively high correlation with SNOTEL SWE, with mean correlation coefficients of 0.69 but negative mean biases of −122.7 mm. Compared to the snow phenology of STC-MODSCAG and SPIReS, ELM shows delayed snow accumulation onset dates by 17.3 and 12.4 d, earlier snow end dates by 35.5 and 26.8 d, and shorter snow durations by 52.9 and 39.5 d, respectively. This study underscores the need for diagnosing model biases and improving ELM representations of snow properties and snow phenology in mountainous areas for more credible simulation and future projection of mountain snowpack.

The Tremulous Resonance of Yves Bonnefoy’s Textual Présence

The Tremulous Resonance of Yves Bonnefoy’s Textual <i>Présence</i>

Modeled Climate Responses to Realistic Extremes of Northern Hemisphere Spring and Summer Snow Anomalies

AbstractNorthern Hemisphere (NH) snow cover extent (SCE) has diminished in spring and early summer since the 1960s. Historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) estimated about half as much NH SCE reduction as observed, and thus underestimated the associated climate responses. This study investigates atmospheric responses to realistic decreasing snow anomalies using multiple ensemble transient integrations of climate models forced by observed light and heavy NH snow cover years, specifically satellite-based observations of NH SCE and snow water equivalent from March to August in 1990 (light snow) and 1985 (heavy snow), as a proxy for the trend. The primary atmospheric responses to March–August NH snow reduction are decreased soil moisture, increased surface air temperature, general tropospheric warming in the extratropics and the Arctic, increased geopotential heights, and weakening of the midlatitude jet stream and eddy kinetic energy. The localized response is maintained by persistent increased diabatic heating due to reduced snow anomalies and resulting soil moisture drying, and the remote atmospheric response results partly from horizontal propagation of stationary Rossby wave energy and also from a transient eddy feedback mechanism. In summer, atmospheric responses are significant in both the Arctic and the tropics and are mostly induced by contemporaneous snow forcing, but also by the summer soil moisture dry anomaly associated with early snow melting.

Impact of topography on black carbon transport to the southern Tibetan Plateau during the pre-monsoon season and its climatic implication

Abstract. Most previous modeling studies about black carbon (BC) transport and its impact over the Tibetan Plateau (TP) conducted simulations with horizontal resolutions coarser than 20 km that may not be able to resolve the complex topography of the Himalayas well. In this study, the two experiments covering all of the Himalayas with the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) at the horizontal resolution of 4 km but with two different topography datasets (4 km complex topography and 20 km smooth topography) are conducted for pre-monsoon season (April 2016) to investigate the impacts of topography on modeling the transport and distribution of BC over the TP. Both experiments show the evident accumulation of aerosols near the southern Himalayas during the pre-monsoon season, consistent with the satellite retrievals. The observed episode of high surface BC concentration at the station near Mt. Everest due to heavy biomass burning near the southern Himalayas is well captured by the simulations. The simulations indicate that the prevailing upflow across the Himalayas driven by the large-scale westerly and small-scale southerly circulations during the daytime is the dominant transport mechanism of southern Asian BC into the TP, and it is much stronger than that during the nighttime. The simulation with the 4 km topography resolves more valleys and mountain ridges and shows that the BC transport across the Himalayas can overcome the majority of mountain ridges, but the valley transport is more efficient. The complex topography results in stronger overall cross-Himalayan transport during the simulation period primarily due to the strengthened efficiency of near-surface meridional transport towards the TP, enhanced wind speed at some valleys and deeper valley channels associated with larger transported BC mass volume. This results in 50 % higher transport flux of BC across the Himalayas and 30 %–50 % stronger BC radiative heating in the atmosphere up to 10 km over the TP from the simulation with the 4 km complex topography than that with the 20 km smoother topography. The different topography also leads to different distributions of snow cover and BC forcing in snow. This study implies that the relatively smooth topography used by the models with resolutions coarser than 20 km may introduce significant negative biases in estimating light-absorbing aerosol radiative forcing over the TP during the pre-monsoon season. Highlights. The black carbon (BC) transport across the Himalayas can overcome the majority of mountain ridges, but the valley transport is much more efficient during the pre-monsoon season. The complex topography results in stronger overall cross-Himalayan transport during the study period primarily due to the strengthened efficiency of near-surface meridional transport towards the TP, enhanced wind speed at some valleys and deeper valley channels associated with larger transported BC mass volume. The complex topography generates 50 % higher transport flux of BC across the Himalayas and 30 %–50 % stronger BC radiative heating in the atmosphere up to 10 km over the Tibetan Plateau (TP) than the smoother topography, which implies that the smooth topography used by the models with relatively coarse resolution may introduce significant negative biases in estimating BC radiative forcing over the TP during the pre-monsoon season. The different topography also leads to different distributions of snow cover and BC forcing in snow over the TP.

Toward Understanding Direct Absorption and Grain Size Feedbacks by Dust Radiative Forcing in Snow With Coupled Snow Physical and Radiative Transfer Modeling

AbstractThe darkening of the snow surface by light‐absorbing particles impacts snow albedo directly by increasing absorption of shortwave radiation in the visible wavelengths. This indirectly enhances the rate of snow grain coarsening, which determines absorption in the near‐infrared wavelengths. In combination, these processes reduce snow albedo over the full range of snow reflectance, accelerating melt, and impacting regional climate and hydrology. Accurate parameterizations of snow albedo should represent both the direct and indirect radiative impacts. Here dust‐influenced snow cover evolution was simulated at Senator Beck Basin Study Area, San Juan Mountains, CO with a multilayer physically based snow process model. The model was modified to track dust stratigraphy, and coupled to a snow/aerosol radiative transfer model to inform reflected shortwave radiation based on snow properties, dust concentrations, and region‐specific dust optical properties. This varies from previous efforts to constrain the magnitude of accelerated melt due to dust by directly and physically representing the processes that determine the radiative impacts. Model outputs, including effective grain size, dust stratigraphy, timing of dust emergence, and albedo, were validated with a near daily snow and light‐absorbing particle physical and optical property data set, and were well simulated. Daily mean radiative forcing ranged from 2 to 109 W/m2 and was 30 W/m2 on average over the full simulation, advancing snowmelt by 30 days. A partitioning of direct and indirect radiative impacts shows that direct absorption by dust contributes ~80% of total radiative forcing, with grain coarsening accounting for ~20%.

Assessing Satellite‐Derived Radiative Forcing From Snow Impurities Through Inverse Hydrologic Modeling

AbstractLight‐absorbing impurities in snow and ice (LAISI) lower the snow albedo and cause accelerated snowmelt. The radiative forcing caused by LAISI is in this connection the key variable in understanding LAISI‐snowpack dynamics. Here we present an approach combining distributed hydrologic model simulations and remotely sensed radiative forcing from LAISI in order to improve model predictions of radiative forcing impacts. In a case study, we assess the seasonal cycle of instantaneous at‐surface clear‐sky radiative forcing from LAISI as predicted by model and satellite observations for a river basin located at the southern slope of the Himalayas. By scaling dust depositions, we optimize simulated radiative forcing conditioned on satellite observations. The optimized model predicts that LAISI‐induced radiative forcing in snow contributes to 4.1% to 5.8% of the annual discharge. The presented approach has a wide range of applications as it provides a novel method to constrain and evaluate measures of LAISI‐induced radiative forcing.

Variation in Rising Limb of Colorado River Snowmelt Runoff Hydrograph Controlled by Dust Radiative Forcing in Snow

AbstractCommon practice and conventional wisdom hold that fluctuations in air temperature control interannual variability in snowmelt and subsequent river runoff. However, recent observations in the Upper Colorado River Basin confirm that net solar radiation and by extension radiative forcing by dust deposited on snow cover exerts the primary forcing on snowmelt. We show that the variation in the shape of the rising limb of the annual hydrograph is controlled by variability in dust radiative forcing and surprisingly is independent of variations in winter and spring air temperatures. These observations suggest that hydroclimatic modeling must be improved to account for aerosol forcings of the water cycle. Anthropogenic climate change will likely reduce total snow accumulations and cause snowmelt runoff to occur earlier. However, dust radiative forcing of snowmelt is likely consuming important adaptive capacity that would allow human and natural systems to be more resilient to changing hydroclimatic conditions.

Modeled Northern Hemisphere Autumn and Winter Climate Responses to Realistic Tibetan Plateau and Mongolia Snow Anomalies

Observational studies link a persistent dipole of autumn and winter snow cover anomalies over the Tibetan Plateau (TP) and Mongolia with winter Pacific–North American (PNA)-like atmospheric variations. This study investigates atmospheric responses to such snow forcings using multiple ensemble transient integrations of the CAM4 and CLM4.0 models. Model boundary conditions are based on climatological sea ice extent and sea surface temperature, and satellite observations of snow cover extent (SCE) and snow water equivalent (SWE) over the TP and Mongolia from October to March in 1997/98 (heavy TP and light Mongolia snow) and 1984/85 (light TP and heavy Mongolia snow), with model-derived SCE and SWE elsewhere. In various forcing experiments, the ensemble-mean difference between simulations with these two extreme snow states identifies local, distant, concurrent, and delayed climatic responses. The main atmospheric responses to a dipole of high TP and low Mongolia SCE persisting from October to March (versus the opposite extreme) are strong TP surface cooling, warming in the surrounding China and Mongolia region, and a winter positive PNA-like response. The localized response is maintained by persistent diabatic cooling or heating, and the remote PNA response results mainly from the increased horizontal eastward propagation of stationary Rossby wave energy due to persistent TP snow forcing and also a winter transient eddy feedback mechanism. With a less persistent dipole anomaly in autumn or winter only, local responses are similar depending on the specific anomalies, but the winter PNA-like response is nearly absent or noticeably reduced.

Satellite-Based Estimation of Temporally Resolved Dust Radiative Forcing in Snow Cover

AbstractRunoff from mountain snowpack is an important freshwater supply for many parts of the world. The deposition of aeolian dust on snow decreases snow albedo and increases the absorption of solar irradiance. This absorption accelerates melting, impacting the regional hydrological cycle in terms of timing and magnitude of runoff. The Moderate Resolution Imaging Spectroradiometer (MODIS) Dust Radiative Forcing in Snow (MODDRFS) satellite product allows estimation of the instantaneous (at time of satellite overpass) surface radiative forcing caused by dust. While such snapshots are useful, energy balance modeling requires temporally resolved radiative forcing to represent energy fluxes to the snowpack, as modulated primarily by varying cloud cover. Here, the instantaneous MODDRFS estimate is used as a tie point to calculate temporally resolved surface radiative forcing. Dust radiative forcing scenarios were considered for 1) clear-sky conditions and 2) all-sky conditions using satellite-based cloud observations. Comparisons against in situ stations in the Rocky Mountains show that accounting for the temporally resolved all-sky solar irradiance via satellite retrievals yields a more representative time series of dust radiative effects compared to the clear-sky assumption. The modeled impact of dust on enhanced snowmelt was found to be significant, accounting for nearly 50% of the total melt at the more contaminated station sites. The algorithm is applicable to regional basins worldwide, bearing relevance to both climate process research and the operational management of water resources.

Simulating black carbon and dust and their radiative forcing in seasonal snow: a case study over North China with field campaign measurements

Abstract. A state-of-the-art regional model, the Weather Research and Forecasting (WRF) model (Skamarock et al., 2008) coupled with a chemistry component (Chem) (Grell et al., 2005), is coupled with the snow, ice, and aerosol radiative (SNICAR) model that includes the most sophisticated representation of snow metamorphism processes available for climate study. The coupled model is used to simulate black carbon (BC) and dust concentrations and their radiative forcing in seasonal snow over North China in January–February of 2010, with extensive field measurements used to evaluate the model performance. In general, the model simulated spatial variability of BC and dust mass concentrations in the top snow layer (hereafter BCS and DSTS, respectively) are consistent with observations. The model generally moderately underestimates BCS in the clean regions but significantly overestimates BCS in some polluted regions. Most model results fall within the uncertainty ranges of observations. The simulated BCS and DSTS are highest with &gt; 5000 ng g−1 and up to 5 mg g−1, respectively, over the source regions and reduce to &lt; 50 ng g−1 and &lt; 1 μg g−1, respectively, in the remote regions. BCS and DSTS introduce a similar magnitude of radiative warming (~ 10 W m−2) in the snowpack, which is comparable to the magnitude of surface radiative cooling due to BC and dust in the atmosphere. This study represents an effort in using a regional modeling framework to simulate BC and dust and their direct radiative forcing in snowpack. Although a variety of observational data sets have been used to attribute model biases, some uncertainties in the results remain, which highlights the need for more observations, particularly concurrent measurements of atmospheric and snow aerosols and the deposition fluxes of aerosols, in future campaigns.

Snow gliding and loading under two different forest stands: a case study in the north-western Italian Alps

The presence of a thick snowpack could interfere with forest stability, especially on steep slopes with potential damages for young and old stands. The study of snow gliding in forests is rather complex because this phenomenon could be influenced not only by forest features, but also by snow/soil interface characteristics, site morphology, meteorological conditions and snow physical properties. Our starting hypothesis is that different forest stands have an influence on the snowpack evolution and on the temperature and moisture at the snow/soil interface, which subsequently could affect snow gliding processes and snow forces. The aim of this work is therefore to analyse the snowpack evolution and snow gliding movements under different forest covers, in order to determine the snow forces acting on single trees. The study site is located in a subalpine forest in Aosta Valley (NW-Italy) and includes two plots at the same altitude, inclination and aspect but with different tree composition: Larch (Larix decidua) and Spruce (Picea abies). The plots were equipped with moisture and temperature sensors placed at the snow/soil interface and glide shoes for continuous monitoring of snow gliding. The recorded data were related to periodically monitored snowpack and snow/soil interface properties. Data were collected during two winter seasons (2009–10 and 2010–11). The snow forces on trees were analytically calculated either from snowpack data and site morphology or also from measured snow gliding rates. Different snow accumulations were observed under the two different forest stands, with a significant effect on temperature and moisture at the snow/soil interface. The highest snow gliding rates were observed under Larch and were related to rapid increases in moisture at the snow/soil interface. The calculated snow forces were generally lower than the threshold values reported for tree uprooting due to snow gliding, as confirmed by the absence of tree damages in the study areas.

Impact of dust radiative forcing in snow on accuracy of operational runoff prediction in the Upper Colorado River Basin

Accurate prediction of snowmelt runoff is critical in the US Intermountain West, where water demand is increasing and snow patterns are shifting. Here, we show that errors in the National Weather Service Colorado Basin River Forecast Center's operational streamflow predictions are correlated with the interannual variability of dust radiative forcing in snow. With data from 2000–2010, we show that errors in snowmelt period streamflow prediction for the southern Colorado Rockies are linearly related to melt period dust radiative forcing in snow as inferred from NASA Moderate Resolution Imaging Spectroradiometer data, which ranged interannually from 20 to 80 W m−2. Each 10 W m−2 change of melt period dust forcing resulted in a corresponding change in runoff prediction bias of 10.0% ± 1.5% and a 1.5 ± 0.6 day shift in runoff center of mass. Accounting for bias introduced by dust forcing could improve streamflow prediction in regions prone to dust deposition in the snowpack.

Black carbon aerosol size in snow

The effect of anthropogenic black carbon (BC) aerosol on snow is of enduring interest due to its consequences for climate forcing. Until now, too little attention has been focused on BC's size in snow, an important parameter affecting BC light absorption in snow. Here we present first observations of this parameter, revealing that BC can be shifted to larger sizes in snow than are typically seen in the atmosphere, in part due to the processes associated with BC removal from the atmosphere. Mie theory analysis indicates a corresponding reduction in BC absorption in snow of 40%, making BC size in snow the dominant source of uncertainty in BC's absorption properties for calculations of BC's snow albedo climate forcing. The shift reduces estimated BC global mean snow forcing by 30%, and has scientific implications for our understanding of snow albedo and the processing of atmospheric BC aerosol in snowfall.

Circulation Response to Eurasian versus North American Anomalous Snow Scenarios in the Northern Hemisphere with an AGCM Coupled to a Slab Ocean Model

Abstract The difference between snow-covered and snow-free conditions is the most climatically significant natural seasonal change the land surface can experience. Most GCM studies investigating snow–atmosphere interactions have focused on impacts of Eurasian snow anomalies caused by the magnitude of snow mass, while North American snow has been shown to have a weaker relationship with downstream climate. Experiment design of recent snow–atmosphere interactions studies has been limited to atmosphere-only models, with sea surface temperature (SST) and sea ice extent represented as boundary conditions. The authors explore the circulation response to anomalous snow scenarios, for both North America and Eurasia, using a slab ocean model. Surface response include significant SST cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling under maximum snow conditions. Atmospheric response to anomalous snow conditions is consistent through multiple levels in the lower troposphere under maximum snow conditions throughout much of the midlatitudes in both experiments during early winter. Areas of strengthened midtropospheric eddy kinetic energy correlate well with steep geopotential height gradient differences and increased zonal wind at 250 hPa over the western Pacific. Both experiments show similar atmospheric response pathways; however, circulation response to maximum Eurasian snow is focused downstream in early winter, whereas upstream response is particularly evident from the North American experiment. This paper focuses on differences as a result of Eurasian versus North American snow forcing in atmospheric circulation response using an AGCM with a slab ocean model.

Radiative forcing by light absorbing impurities in snow from MODIS surface reflectance data

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−2, subject to mixed pixel uncertainties. With bias‐correction, MODDRFS has a root mean squared error of 32 W m−2 and mean absolute error of 25 W m−2. We demonstrate MODDRFS in the Upper Colorado River Basin and Hindu Kush‐Himalaya.

Dust radiative forcing in snow of the Upper Colorado River Basin: 1. A 6 year record of energy balance, radiation, and dust concentrations

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−1 to 4.16 mg g−1. These measurements enable monitoring and modeling of dust and climate‐driven snowmelt forcings in the Upper Colorado River Basin.

Dust radiative forcing in snow of the Upper Colorado River Basin: 2. Interannual variability in radiative forcing and snowmelt rates

Here we present the radiative and snowmelt impacts of dust deposition to snow cover using a 6‐year energy balance record (2005–2010) 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 +2°C and +4°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−2, with hourly peaks up to 409 W m−2. Mean springtime dust radiative forcings across the period ranged from 31 to 49 W m−2 at the alpine site and 45 to 75 W m−2 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.