Spatial energy budgets in alpine tundra

Summary Modelling and Geographic Information System (GIS) technology are employed in order to extend spatially, estimated and observed growing season values of the components of the surface heat energy budget for an area of alpine tundra in the Colorado Front Range. A surface equilibrium temperature model is calibrated for one sub-class of vegetation surface and is used to model surface heat energy budget component values for other sub-classes of vegetation. The model values compare favorably with values independently estimated or observed. The data are spatially displayed using the Idrisi GIS. At the microclimatic scale the presence of different sub-classes of vegetation plays a large role in controlling the actual values of the surface heat budget components. This is in contrast to the larger scale at which climatic variables such as air temperature control the overall vegetation type found in the area.With 6 Figures     

高寒地区冻土活动层变化特征分析

利用1960-2010年黑龙江省83个气象站的冻土和0 cm地温资料,采用线性回归和多项式回归方法,分析了黑龙江省冻土活动层的时空变化特征,揭示了黑龙江省五个典型气候区域最大冻土深度的变化趋势与特征,讨论了黑龙江省冻土活动层的影响因子。结果表明：黑龙江省冻土活动层冻结开始于9月份,至冬季3月份冻土深度达到最大值,8月份时冻土厚度接近于0 cm。由北向南,最大冻土深度逐渐变小,冻结开始时间逐渐推迟,融化结束时间逐渐提前。黑龙江省最大冻土深度均呈显著减小趋势,存在明显的退化趋势。从年代际变化上看,20世纪90年代前黑龙江省最大冻土深度变化不大,最大冻土深度较深,90年代后最大冻土深度呈显著减小趋势。高纬度地区地温低,在同等条件下冻土深度较低纬度地区大。     

Thermal asymmetry and cross-valley circulation in a small alpine valley

Wind observations from a small Alpine valley are used to investigate the problem of cross-valley winds. The observed daytime windfield is a superposition of dynamically and thermally forced cross-winds. Prevailing cross-winds above the valley result in a recirculation cell above the lee slope. The return flow is strengthened or weakened by thermal effects which induce a wind that blows from the shaded to the sunny side of the valley. The reaction time of the thermally induced cross-winds is only 4 min. The horizontal and vertical motions of the cross-valley circulation transport heat in such a way that the insolation differences between the two sides of the valley are nearly equalized.     

Magnitude and sources of variation in albedo within an alpine tundra

Summary Temporal and spatial variations of albedo in a mid-latitude alpine tundra are assessed in order to develop a classification of surface cover mapping units which is useful for surface climate simulations. The largest temporal changes in albedo result from alterations in moisture conditions at the tundra surface associated with snowpack ripening and precipitation. Surface albedo varies under high atmospheric transmission conditions (clear skies) from 0.168 to 0.205; under low transmission conditions (cloudy) there was little variation in the surface albedo with the solar zenith angle and the value of the albedo was approximately equal to that under clear skies when 45° >z > 30°.Spatial variation of albedo within commonly used alpine surface cover mapping units is large, due to the roughness and heterogeneity of the tundra surface. Small differences between mean albedo among vegetated units (mean values range from 0.15 to 0.19) and large ranges of values within units (average 25% of the mean value) preclude differentiation of the commonly used surface cover mapping units (except snow) on the basis of shortwave reflectivity. Aggregation of the vegetated surface cover units based on height and density of plants yields three classes (krummholz, dense low vegetation, and sparse low vegetation) for which differences between mean albedo among all combinations of pairs of mapping units (4 units, 3 vegetative plus snow) are statistically significant.

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Zusammenfassung Zeitliche und räumliche Unterschiede der Albedo einer alpinen Tundra der mittleren Breiten werden herangezogen, um eine kartographische Klassifikation der Oberflächendecke zu entwickeln, die für klimatische Simulationen von Nutzen sein kann.Die auffälligsten zeitlichen Schwankungen der Albedo resultieren aus einer Veränderung der Feuchtigkeitsbedingungen an der Tundraoberfläche in Verbindung mit der Alterung der Schneedecke und Niederschlag. Die Oberflächenalbedo schwankt bei starker atmosphärischer Transmission (klarer Himmel) von 0.168 bis 0.205; bei geringer atmosphärischer Transmission (wolkig) zeigt sich nur wenig Veränderung der Oberflächenalbedo, wobei der Sonnenzenithwinkel und der Albedowert bei 45° >z > 30° in etwa den Werten bei klarem Himmel gleichen.Die räumliche Variation der Albedo innerhalb der üblicherweise herangezogenen kartographischen Einheiten der alpinen Oberflächendecke ist aufgrund der Rauhigkeit und Heterogenität der Tundraoberfläche groß. Bei Vergleich der Vegetationseinheiten untereinander zeigen sich nur geringfügige Unterschiede der durchschnittlichen Albedo (die Durchschnittswerk bewegen sich zwischen 0.15 und 0.19), während die Bandbreite der Werte innerhalb der jeweiligen Einheiten (im Durchschnitt 25% des Mittelwerts) hoch ist. Dies verhindert eine Differenzierung der allgemein angewandten kartographischen Klassifizierung der Oberflächendecke (Schnee ausgenommen) auf der Grundlage kurzwelliger Reflektivität.Eine Einteilung der Vegetationseinheiten hinsichtlich Höhe und Pflanzendichte ergibt drei Klassen (Krummholz, dichter Niederbewuchs und spärlicher Niederbewuchs), wobei die Unterschiede der Albedomittelwerte aller paarweisen Kombinationen der kartographischen Einheiten (4 Einheiten: 3 Vegetationstypen und Schnee) statistische Signifikanz aufweisen.

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With 3 Figures     

Measurement and modelling of dew in island,coastal and alpine areas

We compare the characteristics of dew at nearly the same latitude (42–45°N) for the Mediterranean island of Corsica (Ajaccio, France) and two continental locations (Bordeaux, France, Atlantic coastal area; Grenoble, France, alpine valley). Dew amount was measured on a horizontal reference plate made of polymethylmethacrylate (PMMA) and placed at 1 m above the ground. Data are correlated with plate and air temperature, air relative humidity, wind speed and cloud cover during the period from 14-08-1999 to 15-01-2003.General features as well as particularities of the sampling sites are discussed. The average daily dew yield is higher for the island station at Ajaccio (0.070 mm) than the Bordeaux coastal area (0.046 mm) or the Grenoble valley (0.036 mm). However, the accumulated dew yield was highest for the coastal station (9.8 mm/year) as compared to the island (8.4 mm/year), and much larger than in the alpine valley (4 mm/year). The difference between cumulated and average dew yield stems from the greater number of dew days in the coastal area (58%) versus 33% for the island and 30% in the valley. The higher wind speeds at the island station (average wind during dew is 2 m/s) and lower relative humidity explain the smaller number of dew days. The dew rate seasonal variation is negligible in Bordeaux and exhibits during summer a maximum in Ajaccio and a minimum in Grenoble.A computer model that includes simple meteorological data (air temperature and relative humidity, wind speed, cloud cover) is used to determine the thermal balance and fit to dew mass evolution. Two parameters that account for heat and mass exchange can be adjusted. It was found that, within the uncertainties, these two numbers are the same for the two continental sites, thus allowing dew formation on plates to be evaluated from only simple meteorological measurements. Somewhat larger values are found for the island, due to limitations in the model, which are discussed.     

An orographically split mixed-layer and the alpine low-level jet

Summary A study is undertaken of the semi-geostrophic flow of shallow water incident tangentially upon a sharp leading edged, slender, orographic feature. The nature of the response is shown to depend upon the Froude number of the upstream flow and upon the orographic shape. In particular it is deduced that the orographically split flow has a laterally asymmetric response, and that for sub-critical upstream settings a jet can develop on one particular side of the orography. Features of an easterly low-level jet observed on the Alpine northside during the ALPEX project are compared with the theory.With 5 FiguresJosef Horn died in an Alpine accident in September 1987.     

Spatial variability in the chemical composition of the snowcover at high alpine sites

Summary For a comparison of snow chemistry data from different glaciers or snow fields it is important to have informations about the spatial representativeness of the data from each of the individual sites. In order to assess the representativeness of concentration data of major ions (volume weighted means of the winter accumulation) from glacier fields we investigated the variability in the average concentration of major ions from parallel samples within one snow pit and from different pits within one glacier field. The variabilities in the average concentrations of NO ₃ ^- , SO ₄ ^2- and NH ₄ ⁺ for three parallel profiles within one snow pit at Goldbergkees (Austria) were 1.2, 3.3 and 2.0% (coefficient of variation). Cl^– and Na⁺ showed larger variations (6.6 and 56.6%) possibly originating from contaminations. The variability of average concentration data from different snow pits within one glacier field was studied at La Grave (France) and at Goldbergkees (Austria). At La Grave 3 pits and at Goldbergkees 4 pits at distance of several hundred meters were sampled. Variabilities for SO ₄ ^2- and NO ₃ ^- were quite similar for the two sites (17% for both ions at La Grave, 12% and 15% at Goldbergkees). Whereas variabilities for Na⁺, NH ₄ ⁺ , Mg²⁺, Ca²⁺ and Cl^– were quite low at La Grave ( 12% and 27% for Cl^–), higher values were obtained at Goldbergkees for these ions (17–56%). Likely reasons for the higher variability observed at Goldbergkees are a) higher spatial variability of crustal aerosol species (Mg²⁺, Ca²⁺), b) problems with the detection limit of the analytical method (Ca²⁺), c) contaminations (Na⁺, Cl^–).With 4 Figures     

Analysis of flow and pollutant dispersion by tracer experiments in south alpine valleys

Summary Dispersion and transport over complex terrain have been recognized as an important research field. In July 1988, for studying atmospheric dispersion in a mountain area, a tracer experiment has been performed in south alpine valleys near a mountain (Campo dei Fiori, 1226 m.a.s.l.). In this area air masses circulation in wind breeze conditions is frequent. This paper analyses the meteorological situation and the tracer dispersion during those experiments. First a reconstruction of the three-dimensional wind fields overCampo dei Fiori area, taking into account the effects of orography, surface thermal gradients, atmospheric stability and energy of air masses, has been performed. Then the tracer concentrations have been evaluated by means of suitable models and the results have been discussed.With 11 Figures     

Effects of rainfall amount and frequency on vegetation growth in a Tibetan alpine meadow

Over the past decades, rainfall amount and frequency changed considerably on the Tibetan Plateau. However, how altered rainfall pattern affects vegetation growth and phenology in Tibetan alpine grasslands is poorly understood. In this study, we investigated the long-term effects of rainfall amount and frequency on production (i.e., aboveground biomass, AGB) and phenology of three perennial plants in a Tibetan alpine meadow from 1994 to 2005. Growth period (i.e., the dates from greening to senescence) was referred to plant phenology here. Our results showed that annual precipitation and total rainfall from large events (≥ 5 mm per day) were mainly distributed in the growing season, which increased significantly from 1994 to 2005 with more increment in May and July (p?<?0.05). Total AGB and growth periods of three plants were linearly correlated with annual precipitation and total rainfall from large events, but have insignificant correlations with total rainfall from small events (< 5 mm per day) and rainfall frequency (including small, large, and all events). The results suggest that aboveground plant production and phenology are more sensitive to changes in large rainfall events (≥ 5 mm per day) than small events (< 5 mm per day) in the alpine meadow ecosystems.     

Dynamics of alpine grassland NPP and its response to climate change in Northern Tibet

Based on the remote sensing and meteorological data collected from the 1981–2004 period, we calculated the alpine grassland Net Primary Productivity (NPP) in Northern Tibet using the Carnegie–Ames–Stanford Approach (CASA), and subsequently analyzed the trend of grassland NPP changes and its response to climate change from 1981 to 2004. The results show that alpine grassland NPP in Northern Tibet was very low in the last 24 years with a relatively large yearly variation. Most of the grassland area (88.61%) in Northern Tibet did not show a significant annual NPP change. The area with a significant decrease of annual NPP variation accounted for only 11.30% of the total grasslands surface, whereas that with a significant increase accounted for 0.09%. In recent years, the precipitation variation in Northern Tibet resulted in an increase of grassland NPP, though solar radiation resulted in decreased grassland NPP. During the 1981–2004 period, total solar radiation, precipitation and temperature, with a decreasing impact magnitude factor, have impacted the grassland NPP in Northern Tibet. The impact of regional climate change on grassland NPP was overall more detrimental than positive.