Mark Flanner - Drivers of snow reflectance variability and associated climate feedback
Snow cover plays an important role in climate because it is highly reflective and generally exists near its melting point. When snow melts, it exposes darker underlying substrates (rock, soil, vegetation, or water), leading to increased absorption of solar energy by the surface. This heating, in turn, can lead to regional warming that melts snow elsewhere. This sequence of events constitutes a positive feedback mechanism, known as "snow-albedo feedback," and is arguably the strongest positive feedback mechanism currently operating in Earth's climate system. Positive feedback mechanisms are of interest because they amplify perturbations to the climate system, such as those caused by increased levels of greenhouse gases.
For these reasons, it is important to understand factors influencing snowpack evolution. Mark's research focuses on controls of snow reflectance. The main processes governing snow reflectance are deposition from the atmosphere of impurities like soot ("black carbon") and desert dust, and snow aging, which determines the ice crystal size. While these processes only alter snow reflectance by about 1-10%, such changes can translate into large increases in absorbed solar energy, hastening snowmelt. This is especially true during local springtime when sunlight is relatively intense and day-length becomes longer. Moreover, the processes of snow aging and darkening from impurities are intertwined. This can be seen in Figure 1, which shows the change in spectral snow reflectance caused by a given mass of black carbon mixed with snow composed of two different grain sizes.
The snowpack composed of coarser-grained snow is darkened much more than the fine-grained snowpack, when equal mixing ratios of black carbon are present. This happens because sunlight penetrates more deeply in coarser-grained snow, allowing a greater amount of black carbon to aid in the absorption-enhancement process. Because snow aging is determined by temperature, this connection highlights interplay between global climate, snow aging, and darkening from impurities.
Figure 1: Snow albedo (or reflectance) as a function of wavelength. For reference, visible light is between 0.4 and 0.7 µm. The two blue curves show albedo of a snowpack composed of fine ice crystals (effective radius of 50 µm), with and without black carbon (BC) mixing ratios of 200 nanograms/gram. The two green curves show albedo for coarse-grained snow (effective radius of 1000 µm), with and without black carbon. This figure shows that BC-induced albedo perturbation is greater in larger-grained snow, and therefore that snow darkening is linked to snow aging.
Global climate models simulate the atmospheric transport and deposition of black carbon (BC) to snowpack, resulting changes in snow reflectance and influence on climate, and removal of BC from snow with meltwater. Figure 2 shows global estimates of BC mixing ratios in surface snow and the subsequent increase in absorbed solar energy. Large amounts of black carbon are simulated for snow in East Asia, where BC emissions are large. The greatest change in solar absorption, however, occurs over the Tibetan Plateau. While Tibetan snow is less polluted, sunlight is very intense in this region because of its high altitude and low latitude. Therefore, snowpack in this region is more susceptible to darkening from impurities, as shown in the bottom panel of Figure 2.
Figure 2: Top: Annual-mean mixing ratio of black carbon (BC) in surface snow, in units of nanograms black carbon per gram of ice. Bottom: Annual-mean surface forcing, or change in absorbed solar energy at the surface, caused by black carbon in snow, averaged only when snow is present. These are results from global climate simulations applying 1998 black carbon emissions.
ASP Spotlight September 2008
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