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Gerd Wendler

Abstract

The heat balance was estimated and climatic elements were compared for two stations in the Fairbanks area on a year-round basis, the first study of this kind carried out in the interior of Alaska. Although the stations were situated near to each other (within 5 km and 200 m altitude), great differences were observed. For the valley station the heat source is mainly radiation with 61 cal cm−2 for an average day of the year August 1966 to July 1967, and to a smaller extent, sensible heat flux (17 cal cm−2). Most of this energy is needed for evaporation (−74 cal cm−2). The heat flow in the soil is about zero over the year and the actual value of −4 cal cm−2 was mainly used for melting of the snow cover.

For the hill station the heat fluxes are generally smaller. The only source is the radiation balance (24 cal cm−2), which is only 40% of the value found for the valley station. The latent heat flux is much smaller (−19 cal cm−2), and the surface temperature on the hill is higher. The sensible heat flux is slightly negative (−2 cal cm−2), meaning that the air will be warmed by the surface only slightly for an average day over a year in contrast to the valley station. This higher surface temperature also raises the outgoing longwave radiation, and can explain partially the less positive radiation balance for the hill station. The heat flux in the soil is again near zero and the value of −3 cal cm−2 represents the energy needed to melt the snow cover.

The agreement of the radiation balance with long-term means calculated by Gavrilova was found to be satisfactory.

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GERD WENDLER

Abstract

Ice fog, which is in a way similar to a dense cirrostratus cloud near the surface, occurs in Fairbanks when the temperature drops below −35°C. The heat balance of two stations, one below the ice fog in the valley and the other one on Birch Hill above the ice fog, are compared for the longest cold spell in winter 1966/67. The valley station shows a relatively small radiative loss of 47 cal cm−2 day−1, as the ice fog shelters the surface against a highly negative long-wave radiation balance. The energy for compensation of this loss comes mostly (25 cal cm−2 day−1) out of the soil. On Birch Hill the radiative loss (124 cal cm−2 day−1) is nearly three times larger, mostly due to the smaller amount of the incoming long-wave radiation, and here the sensible heat flux (102 cal cm−2 day−1) provides most of the energy.

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Gerd Wendler

Detailed insolation measurements have been carried out in Fairbanks for the last four years. Beginning on 15 November 1982, these measurements showed substantial changes, believed to be due to the dust cloud of El Chichón. The volcano, situated in Mexico, erupted most intensely on 4 April 1982, putting a large amount of material into the atmosphere. The long traveling time to the North is in line with results found by Rao and Bradley (1983). Compared to clear-day data for previous years, clear days for the time period 15 November 1982–31 May 1983 showed a decrease in the direct beam of 24.8%, an increase in the ratio of diffuse to global radiation of 76%, and a decrease in the global radiation of about 5%. A decrease in the direct beam, a substantial increase in the diffuse radiation, and a small decrease in the global radiation are typical for increased turbidity of the atmosphere, but the volcanic cloud caused changes greater than those due to “normal” turbidity changes.

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Gerd Wendler and Yuji Kodama

Abstract

Five years of detailed radiation measurements were taken at Fairbanks, Alaska. These data showed that the El Chichón volcanic cloud had a major impact on the surface radiative regime, the maximum of which was observed in winter 1982/83, about 9 months after the eruption. The direct beam of the solar radiation was reduced by as much as 38% (3-month mean value), the ratio of diffuse over global radiation was increased by 91%, and the global radiation was reduced by about 5%. These values show that the volcanic cloud was a strong forward scatterer, while relatively little energy was absorbed or reflected back to space. Further, the aerosol optical depth and Linke's turbidity factor were calculated, and both displayed substantial increases. Effects of the stratospheric dust cloud were seen all through 1983 and the spring of 1984. In summer 1984, however, the radiative values were back to “normal.” Our values were compared to other observations at lower latitudes and, in general, a good agreement was found.

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Gerd Wendler and Nobuyoshi Ishikawa

Abstract

Short-term heat balance studies were carried out for three surfaces—moraine, ice and snow—in a partly glacierized watershed in Arctic Alaska, for an 11-day period in summer 1971. It was found that the radiation balances for the moraine and the ice surface were very similar, as the reduced shortwave radiation balance resulting from the higher albedo of the ice (33%) compared to the moraine surface (19%) was compensated for by the less negative longwave radiation balance resulting from the lower surface temperature of the ice surface compared to the moraine. However, the less negative longwave radiation balance for the snow surface was not able to compensate for the loss of shortwave radiation resulting from the high albedo (59%) of the snow surface. For the moraine, the radiation balance (146 ly day−1) was the only energy source. It was used to warm the air (49%), for evaporation (43%), and to heat the ground (8%). For the ice surface the energy sources were the radiation (149 ly day−1) and the sensible heat flux (116 ly day−1). Most of this energy was utilized to melt ice (89%), while only small fluxes were found for evaporation (4%) and warming the ice (7%). For the snow surface, the portion of the radiation balance (88 ly day−1) and sensible heat flux (52 ly day−1) are similar; however, the fluxes are smaller. Again, most of the energy is used for ablation (59%), but evaporation (40%) is also of substantial importance. The heat flux into the snow by conductivity is very small for a snow surface (1%).

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Gerd Wendler and Philip Nicpon

Abstract

Low-level inversions up to 200 m were investigated on a statistical basis for Fairbanks, Alaska, using hourly data for the year March 1967 to February 1968. Surface inversions were found to be present for more than 50% of the time. In winter (November to February) there is an inversion for more than 95% of the time; maximum values of the inversion strength were 20°C in 200 m altitude difference. In summer (June to August) inversions occur relatively seldom. For the rest of the year, inversions are normally established at night, but are destroyed by day.

The strength of the inversion was analyzed and shown graphically as a function of different independent meteorological parameters for the four seasons and annually. Although there are some differences depending on the season, the strength of the inversions was observed to increase with a) negative net radiation, b) decreasing cloudiness, and c) decreasing windspeed. Furthermore, during the winter a northerly wind direction, probably of orographic origin, was associated with stronger inversions. Such graphical data for a typical subarctic community should be useful in local forecasting and pollution control planning.

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Brian Hartmann and Gerd Wendler

Abstract

The 1976 Pacific climate shift is examined, and its manifestations and significance in Alaskan climatology during the last half-century are demonstrated. The Pacific Decadal Oscillation index shifted in 1976 from dominantly negative values for the 25-yr time period 1951–75 to dominantly positive values for the period 1977–2001.

Mean annual and seasonal temperatures for the positive phase were up to 3.1°C higher than for the negative phase. Likewise, mean cloudiness, wind speeds, and precipitation amounts increased, while mean sea level pressure and geopotential heights decreased. The pressure decrease resulted in a deepening of the Aleutian low in winter and spring. The intensification of the Aleutian low increased the advection of relatively warm and moist air to Alaska and storminess over the state during winter and spring.

The regime shift is also examined for its effect on the long-term temperature trends throughout the state. The trends that have shown climatic warming are strongly biased by the sudden shift in 1976 from the cooler regime to a warmer regime. When analyzing the total time period from 1951 to 2001, warming is observed; however, the 25-yr period trend analyses before 1976 (1951–75) and thereafter (1977–2001) both display cooling, with a few exceptions. In this paper, emphasis is placed on the importance of taking into account the sudden changes that result from abrupt climatic shifts, persistent regimes, and the possibility of cyclic oscillations, such as the PDO, in the analysis of long-term climate change in Alaska.

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Zbigniew Sorbjan, Yuji Kodama, and Gerd Wendler

Abstract

During the austral summer of 1982/83, measurements of wind and temperature profiles were made through the atmospheric boundary layer in Adelie Land, East Antarctica, an area known for strong katabatic winds. It was found that a shallow but strong temperature inversion was developed at night, and destroyed during the day, resulting in the development of a well-mixed layer. Wind hodographs were quite regular and spiral-like at night, but irregular during the day. The mean wind direction was about 40° to the left, looking downslope, but more downslope at night and more cross-slope during the day.

The conclusion was derived that during the polar summer the flow over Antarctica is controlled by the gravitational factor (slope-induced baroclinicity), by the thermal stability (turbulent mixing), and also by the synoptic forcing.

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Martin Stuefer, Xiande Meng, and Gerd Wendler

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is being used for forecasting the atmospheric layers of aircraft condensation trail (contrail) formation. Contrail forecasts are based on a conventional algorithm describing the adiabatic mixing of aircraft exhaust with environmental air. Algorithm input data are MM5-forecasted temperature and humidity values at defined pressure or sigma levels, and an aircraft-relevant contrail factor that is derived statistically from a contrail observation database.

For comparison purposes a mean overlap (MO), which is a parameter quantifying the overlap between forecasted contrail layers and contrail layers derived from radiosonde measurements, is introduced. Mean overlap values are used to test for the altitude and thickness of forecasted contrail layers. Contrail layers from Arctic MM5 and Air Force Weather Agency (AFWA) MM5 models agree well with contrail layers derived from corresponding radiosonde measurements for certain forecast periods; a steady decrease of the MO shows a decrease of contrail forecast accuracy with the increasing forecast period. Mean overlaps around 82% indicate reasonable results for the 48-h forecasts. Verification of MM5 with actual contrail observations shows a slightly better performance of Arctic MM5. A possible dry bias might occur in humidity measurements at low temperature levels due to temperature-dependence errors of the humidity sensor polymer, which might also affect forecasts of humidity of the upper troposphere or lower stratosphere. Despite this fact, this contrail verification study shows hit rates higher than 82% within forecast periods up to 36 h using Arctic MM5.

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Gerd Wendler, Nobuyoshi Ishikawa, and Yuji Kodama

Abstract

A complete but budget investigation was carried out in summer at a site in Adelie Land, some 100 km from the edge of the Antarctic ice sheet. For an average day, the all wave radiation budget based on the fluxes toward the surface being positive was positive for about 11 h, which is a short time considering that the sun was above the horizon between 22 and 24 h a day during the observational period. It is a result of the high albedo, which, on average, was found to be about 83%. Furthermore, with increasing cloudiness, a more positive radiation budget was found, which is in contrast to most studies at lower latitudes. The heat flux in and out of the snow cover was small, and showed a typical sinusoidal diurnal variation. The mean daily values of snow heat flux were negative, as the snow cover was warmed during the observational period. The latent heat flux was negative, on the average, as sublimation took place for most of the time. Deposition was observed only on a few nights. The sensible heat flux was negative around noon, but positive for most of the day, which means that the air above the surface was cooled, an inversion developed, and as the surface is inclined, gravitational flow (katabatic wind) started to occur. While the all-wave radiation balance had its minimum around midnight, the minimum temperature was observed some 3 h law, and the maximum wind speed occurred about 2 additional hours later.

In summary, the mean warming of the snow, the sublimation and the negative all-wave radiation budget for most of the days were compensated by a positive sensible heat flux, which explains the frequent occurrence of the katabatic wind.

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