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- Author or Editor: Carl S. Benson x
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Abstract
Two periods of very low (below −40C) surface temperature of Fairbanks, Alaska, were studied in detail as part of ice fog investigations during the 1961–1962 winter. The observed cooling rates from teh snow surface up to 3000 m were to large to be satisfactorily explained by advection and/or by radiative heat losses from the air and from the snow surface. The excess is shown to be due to radiation from ice crystals suspended in air.
The ice crystals, formed by overall cooling of the air, act as heat sinks. It is proposed that heat flows from the air to the crystals and is radiated away. This process results in strong temperature gradients in the air immediately adjacent to the crystals. It may also account for the fact that humidity measurements show less than saturation values during occurrences of ice fog, light snowfall or “diamond dust” crystal displays. The air temperature values used in determining humidity pertain to ambient air between the ice crystals, whereas the air in contact with crystals has a lower temperature and is saturated with respect to ice.
Abstract
Two periods of very low (below −40C) surface temperature of Fairbanks, Alaska, were studied in detail as part of ice fog investigations during the 1961–1962 winter. The observed cooling rates from teh snow surface up to 3000 m were to large to be satisfactorily explained by advection and/or by radiative heat losses from the air and from the snow surface. The excess is shown to be due to radiation from ice crystals suspended in air.
The ice crystals, formed by overall cooling of the air, act as heat sinks. It is proposed that heat flows from the air to the crystals and is radiated away. This process results in strong temperature gradients in the air immediately adjacent to the crystals. It may also account for the fact that humidity measurements show less than saturation values during occurrences of ice fog, light snowfall or “diamond dust” crystal displays. The air temperature values used in determining humidity pertain to ambient air between the ice crystals, whereas the air in contact with crystals has a lower temperature and is saturated with respect to ice.
Abstract
A previous attempt to calculate the temperature gradient around a growing ice crystal in clear air started with the radiation budget of the crystal plus the assumption that the measured frost point temperature (about 2C below air temperature) represented the crystal temperature. When the conductivity equation for the air around the crystal is fully solved with the radiation budget as the boundary condition, however, it is found that less than 0.03C temperature difference can be sustained by radiative cooling of the crystals. The most probable explanation of the difference between the two approaches is that the humidity measurements are in error, and that the error has been generally recognized only at measured humidities in excess of 100%.
Abstract
A previous attempt to calculate the temperature gradient around a growing ice crystal in clear air started with the radiation budget of the crystal plus the assumption that the measured frost point temperature (about 2C below air temperature) represented the crystal temperature. When the conductivity equation for the air around the crystal is fully solved with the radiation budget as the boundary condition, however, it is found that less than 0.03C temperature difference can be sustained by radiative cooling of the crystals. The most probable explanation of the difference between the two approaches is that the humidity measurements are in error, and that the error has been generally recognized only at measured humidities in excess of 100%.
Abstract
The production of the low temperatures which are responsible for ice fog in inhabited areas of interior Alaska would appear to be a classic example of clear sky radiative cooling under nearly polar night conditions. However, examination of the meteorological conditions associated with 15 periods of dense ice fog at Fairbanks indicates that local radiative cooling is important only in producing the observed steep ground inversion. The most rapid decreases in temperature at heights >1 km occurred with cloud cover and cold air advection preceding the cold weather at the ground. The most common synoptic pattern (observed for the 12 shortest events) consisted of the migration of a small high from Siberia across Alaska. Rapid growth of the high was common, and the resulting subsidence was strong enough to counterbalance not only radiative cooling, but further cold air advection as well. This resulted in an observed warming aloft during all but the first 12–24 hr of the clear, cold weather observed at the ground. Three of the 15 events did not follow this pattern. Two long and very cold events were associated with warm highs in northeastern Siberia, continuous belts of moderately high pressure extending from Siberia across the Bering Strait into Alaska, and advection from Siberia and the Arctic Ocean. The remaining long but relatively mild event was associated with a warm high north of Alaska and advection from Canada and the Arctic Ocean.
Abstract
The production of the low temperatures which are responsible for ice fog in inhabited areas of interior Alaska would appear to be a classic example of clear sky radiative cooling under nearly polar night conditions. However, examination of the meteorological conditions associated with 15 periods of dense ice fog at Fairbanks indicates that local radiative cooling is important only in producing the observed steep ground inversion. The most rapid decreases in temperature at heights >1 km occurred with cloud cover and cold air advection preceding the cold weather at the ground. The most common synoptic pattern (observed for the 12 shortest events) consisted of the migration of a small high from Siberia across Alaska. Rapid growth of the high was common, and the resulting subsidence was strong enough to counterbalance not only radiative cooling, but further cold air advection as well. This resulted in an observed warming aloft during all but the first 12–24 hr of the clear, cold weather observed at the ground. Three of the 15 events did not follow this pattern. Two long and very cold events were associated with warm highs in northeastern Siberia, continuous belts of moderately high pressure extending from Siberia across the Bering Strait into Alaska, and advection from Siberia and the Arctic Ocean. The remaining long but relatively mild event was associated with a warm high north of Alaska and advection from Canada and the Arctic Ocean.
Abstract
The stable isotopic composition of water in ice cores is an important source of information on past climate variability. At its simplest level, the underlying assumption is that there is an empirical relationship between the normalized difference in the concentration for these stable isotopes and a specified local temperature at the ice core site. There are, however, nonlocal processes, such as a change in source region or a change in the atmospheric pathway, which can impact the stable isotope signal, thereby complicating its use as a proxy for temperature. In this paper, the importance of these nonlocal processes are investigated through the analysis of the synoptic-scale circulation during a snowfall event at the summit of Mount Wrangell (62°N, 144°W; 4300 m MSL) in south-central Alaska. During this event there was, over a 1-day period in which the local temperature was approximately constant, a change in δ18O that exceeded half that normally seen to occur in the region between summer and winter. As shall be shown, this arose from a change in the source region, from the subtropical eastern Pacific to northeastern Asia, for the snow that fell on Mount Wrangell during the event.
Abstract
The stable isotopic composition of water in ice cores is an important source of information on past climate variability. At its simplest level, the underlying assumption is that there is an empirical relationship between the normalized difference in the concentration for these stable isotopes and a specified local temperature at the ice core site. There are, however, nonlocal processes, such as a change in source region or a change in the atmospheric pathway, which can impact the stable isotope signal, thereby complicating its use as a proxy for temperature. In this paper, the importance of these nonlocal processes are investigated through the analysis of the synoptic-scale circulation during a snowfall event at the summit of Mount Wrangell (62°N, 144°W; 4300 m MSL) in south-central Alaska. During this event there was, over a 1-day period in which the local temperature was approximately constant, a change in δ18O that exceeded half that normally seen to occur in the region between summer and winter. As shall be shown, this arose from a change in the source region, from the subtropical eastern Pacific to northeastern Asia, for the snow that fell on Mount Wrangell during the event.
