ALBERT

Description: Frequently used gridded meteorological datasets poorly represent precipitation in the Himalayas because of their relatively low spatial resolution and the associated representation of the complex topography. Dynamical downscaling using high-resolution atmospheric models may improve the accuracy and quality of the precipitation fields. However, most physical parameterization schemes are designed for a spatial resolution coarser than 1 km. In this study the Weather Research and Forecasting (WRF) Model is used to determine which resolution is required to most accurately simulate monsoon and winter precipitation, 2-m temperature, and wind fields in the Nepalese Himalayas. Four model nests are set up with spatial resolutions of 25, 5, 1, and 0.5 km, respectively, and a typical 10-day period in summer and winter in 2014 are simulated. The model output is compared with observational data obtained from automatic weather stations, pluviometers, and tipping buckets in the Langtang catchment. Results show that, despite issues with the quality of the observational data due to undercatch of snowfall, the highest resolution of 500 m does provide the best match with the observations and gives the most plausible spatial distribution of precipitation. The quality of the wind and temperature fields is also improved, whereby the cold temperature bias is decreased. Our results further elucidate the performance of WRF at high resolution and demonstrate the importance of accurate surface boundary conditions and spinup time for simulating precipitation. Furthermore, they suggest that future modeling studies of High Mountain Asia should consider a subkilometer grid for accurately estimating local meteorological variability.

Description: A dust storm simulation chamber has been designed based on worst-case conditions to simulate the effects of blowing dust and sand on candidate solar concentrator materials. After evaluation with common glass mirrors, the system was used to simulate field exposure of Carolina glass mirrors in the West Texas environment in terms of loss of reflectivity. System calibration was achieved by comparing actual exposure data over a three-year period to results obtained in the simulation chamber. It was found that a twenty-year service life can be simulated by 80 hrs. exposure in the dust storm chamber. Degradation of the Carolina glass mirrors has been shown to be primarily dependent on momentum transport to and kinetic energy deposition on the mirror surface during dust storms: annual reflectivity losses of 2.4 percent and 1.4 percent may be expected, respectively, from dust storm and the sum of all other effects.

Description: Tropical cyclones represent an important component of intraseasonal atmospheric variability in the southwest Indian Ocean, and their landfall can be devastating to coastal communities. However, little is known about their impact on precipitation in the high-elevation regions of East Africa. Here we combine in situ measurements from the summit of Kilimanjaro and subkilometer atmospheric modeling of the region to investigate the impact of these storms during the 2006–2007 cyclone season, which was characterized by anomalously positive snow accumulation at the summit coinciding with cyclone activity. Observations indicate that two storms are associated with snowfall amounts exceeding 10 cm per day, an amount representing ~5% of the observed average annual total. Numerical simulations show that some tropical cyclones transport moisture directly to the region while others induce moisture transport from the continental interior. Our study suggests that these synoptic-scale phenomena have the potential to complicate the extraction of climate signals from glaciers. ©2019. American Geophysical Union. All Rights Reserved.

Description: The traditional approach to simulations of alpine glacier mass balance (MB) has been one-way, or offline, thus precluding feedbacks from changing glacier surface conditions on the atmospheric forcing. In addition, alpine glaciers have been only simply, if at all, represented in atmospheric models to date. Here, we extend a recently presented, novel technique for simulating glacier–atmosphere interactions without the need for statistical downscaling, through the use of a coupled high-resolution mesoscale atmospheric and physically-based climatic mass balance (CMB) modelling system that includes glacier CMB feedbacks to the atmosphere. We compare the model results over the Karakoram region of the northwestern Himalaya with remote sensing data for the ablation season of 2004 as well as with in situ glaciological and meteorological measurements from the Baltoro glacier. We find that interactive coupling has a localized but appreciable impact on the near-surface meteorological forcing data and that incorporation of CMB processes improves the simulation of variables such as land surface temperature and snow albedo. Furthermore, including feedbacks from the glacier model has a non-negligible effect on simulated CMB, reducing modelled ablation, on average, by 0.1 m w.e. (−6.0%) to a total of −1.5 m w.e. between 25 June–31 August 2004. The interactively coupled model shows promise as a new, multi-scale tool for explicitly resolving atmospheric-CMB processes of mountain glaciers at the basin scale.

Description: Due to the complexity of treating moisture in supraglacial debris, surface energy balance models to date have neglected moisture infiltration and phase changes in the debris layer. The latent heat flux (QL) is also often excluded due to the uncertainty in determining the surface vapour pressure. To quantify the importance of moisture on the surface energy and climatic mass balance (CMB) of debris-covered glaciers, we developed a simple reservoir parameterization for the debris ice and water content, as well as an estimation of the latent heat flux. The parameterization was incorporated into a CMB model adapted for debris-covered glaciers. We present the results of two point simulations, using both our new "moist" and the conventional "dry" approaches, on the Miage Glacier, Italy, during summer 2008 and fall 2011. The former year coincides with available in situ glaciological and meteorological measurements, including the first eddy-covariance measurements of the turbulent fluxes over supraglacial debris, while the latter contains two refreeze events that permit evaluation of the influence of phase changes. The simulations demonstrate a clear influence of moisture on the glacier energy and mass-balance dynamics. When water and ice are considered, heat transmission to the underlying glacier ice is lower, as the effective thermal diffusivity of the saturated debris layers is reduced by increases in both the density and the specific heat capacity of the layers. In combination with surface heat extraction by QL, subdebris ice melt is reduced by 3.1% in 2008 and by 7.0% in 2011 when moisture effects are included. However, the influence of the parameterization on the total accumulated mass balance varies seasonally. In summer 2008, mass loss due to surface vapour fluxes more than compensates for the reduction in ice melt, such that the total ablation increases by 4.0%. Conversely, in fall 2011, the modulation of basal debris temperature by debris ice results in a decrease in total ablation of 2.1%. Although the parameterization is a simplified representation of the moist physics of glacier debris, it is a novel attempt at including moisture in a numerical model of debris-covered glaciers and one that opens up additional avenues for future research.

Description: In this study we simulate the climatic mass balance of Svalbard glaciers with a coupled atmosphere-glacier model with 3 km grid spacing, from September 2003 to September 2013. We find a mean specific net mass balance of −167 mm w.e. yr−1, corresponding to a mean annual mass loss of about 5.7 Gt, with large interannual variability. Our results are compared with a comprehensive set of mass balance, meteorological and satellite measurements. Model temperature biases of 0.17 and −1.9 °C are found at two glacier automatic weather station sites. Simulated climatic mass balance is mostly within about 0.1 m w.e. yr−1 of stake measurements, and simulated winter accumulation at the Austfonna ice cap shows mean absolute errors of 0.05 and 0.06 m w.e. yr−1 when compared to radar-derived values for the selected years 2004 and 2006. Comparison of surface height changes from 2003 to 2008 from model, and satellite altimetry reveals good agreement in both mean values and regional differences. The largest deviations from observations are found for winter accumulation at Hansbreen (up to around 1 m w.e. yr−1), a site where sub-grid topography and wind redistribution of snow are important factors. Comparison with simulations using a 9 km grid spacing reveal considerable differences on regional and local scales. In addition, the 3 km grid spacing allows for a much more detailed comparison with observations than what is possible with a 9 km grid spacing. Further decreasing the grid spacing to 1 km appears to be less significant, although in general precipitation amounts increase with resolution. Altogether, the model compares well with observations and offers possibilities for studying glacier climatic mass balance on Svalbard both historically as well as based on climate projections.

Description: The Karakoram range of the Hindu-Kush-Himalaya is characterized by both extensive glaciation and a widespread prevalence of surficial debris cover on the glaciers. Surface debris exerts a strong control on glacier surface-energy and mass fluxes and, by modifying surface boundary conditions, has the potential to alter atmosphere-glacier feedbacks. To date, the influence of debris on Karakoram glaciers has only been directly assessed by a small number of glaciological measurements over short periods. Here, we include supraglacial debris in a high-resolution, interactively coupled atmosphere-glacier modelling system. To investigate glaciological and meteorological changes that arise due to the presence of debris, we perform two simulations using the coupled model from 1 May to 1 October 2004: one that treats all glacier surfaces as debris-free and one that introduces an simplified specification for mapping debris thickness. The basin-averaged impact of debris is a reduction in ablation of ~7%, although the difference exceeds 2.5 m w.e. on the lowest-altitude glacier tongues. The modest reduction in mean mass loss results in part from non-negligible sub-debris melt rates under thicker covers and from compensating increases in melt under thinner debris, and may help to explain the lack of distinct differences in recent elevations changes between clean and debris-covered ice. The presence of debris also strongly alters the surface boundary condition and thus heat exchanges with the atmosphere; near-surface meteorological fields at lower elevations and their vertical gradients; and the atmospheric boundary layer development. These findings are relevant for glacio-hydrological studies on debris-covered glaciers and contribute towards an improved understanding of glacier behaviour in the Karakoram.