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1.
Using atmospheric forcing data generated from a general circulation climate model, sixteen land surface schemes participating
in the Project for the Intercomparison of Land-surface Parametrization Schemes (PILPS) were run off-line to equilibrium using
forcing data from a GCM representative of a tropical forest and a mid-latitude grassland grid point. The values for each land
surface parameter (roughness length, minimum stomatal resistance, soil depth etc.) were provided. Results were quality controlled
and analyzed, focusing on the scatter simulated amongst the models. There were large differences in how the models’ partitioned
available energy between sensible and latent heat. Annually averaged, simulations for the tropical forest ranged by 79 1 3;W m -2 for the sensible heat flux and 80 W m -2 for the latent heat flux. For the grassland, simulations ranged by 34 W m -2 for the sensible heat flux and 27 W m -2 for the latent heat flux. Similarly large differences were found for simulated runoff and soil moisture and at the monthly
time scale. The models’ simulation of annually averaged effective radiative temperature varied with a range, between all the
models, of 1.4 K for tropical forest and 2.2 K for the grassland. The simulation of latent and sensible heat fluxes by a standard
‘bucket’ models was anomalous although this could be corrected by an additional resistance term. These results imply that
the current land surface models do not agree on the land surface climate when the atmospheric forcing and surface parameters
are prescribed. The nature of the experimental design, it being offline and with artificial forcing, generally precludes judgements
concerning the relative quality of any specific model. Although these results were produced de-coupled from a host model,
they do cast doubt on the reliability of land surface schemes. It is therefore a priority to resolve the disparity in the
simulations, understand the reasons behind the scatter and to determine whether this lack of agreement in de-coupled tests
is reproduced in coupled experiments.
Received: 15 October 1997 / Accepted: 22 April 1999 相似文献
2.
Summary Turbulent fluxes of CO 2 were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment
at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover
of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period
with snow cover, fluxes of CO 2 to the atmosphere were small, typically 0.005 mg CO 2 m −2 s −1 (0.41 g CO 2 m −2 d −1), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the
onset of vegetation growth, fluxes of CO 2 to the atmosphere were typically 0.1 mg CO 2 m −2 s −1 in the afternoon, and daily sums reached values up to almost 9 g CO 2 m −2 d −1. After 4 July, downward fluxes of CO 2 increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values
of about −0.23 mg m −2 s −1 at midday and max values of daily sums of −12 g CO 2 m −2 d −1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO 2 m −2. Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements.
On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO 2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO 2 m −2 yr −1.
Received October 6, 1999 Revised May 2, 2000 相似文献
3.
Summary In this study, we employed a regional model to simulate the impact of urban expansion on monthly climate in Pearl River Delta
(PRD) region. Two experiments were performed by prescribing two different land covers in the PRD region. One land cover represents
vegetation in the 1970s which is derived from the United States Geological Survey (USGS) data with 24-category (hereafter
referred to as NU). The other land cover represents the current urban condition which is derived from remote sensing data
acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) in 2004 (hereafter referred to as HU). Using the two
land cover datasets, monthly climate of October 2004 was simulated, which was a very dry season in the PRD region. The results
obtained from the numerical simulation show a distinct difference in simulated shelter-level temperature, humidity, surface
fluxes and the height of planetary boundary layer (PBL) with two different land cover data sets being specified. The maximum
difference in simulated monthly mean temperature over urban areas was 0.9 °C. A large temperature difference was found in
urbanized area in Guangzhou, Dongguan, Zhongshan and Shenzhen. The monthly mean relative humidity in urban areas decreased
by 1.4% as a result of urban expansion (from 59.2% in NU to 57.8% in HU). The maximum decrease in mixing ratio was 0.4 g/kg
in Guangzhou and Dongguan, whereas the maximum decrease in relative humidity was 2.4%. There was an increase of sensible heat
flux in developed lands and the maximum increase was 90 W m −2. In contrast, latent hear flux in urban area decreased and the maximum decrease was 300 W m −2. In addition, the increase in mean height of PBL ranged from 20 to 80 m (HU compared with NU), and the maximum change of
the height was 180 m over urban area in city of Guangzhou. 相似文献
4.
Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change.
However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult
to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo
approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing
estimates to date. The resulting total radiative forcing has a 75–97% probability of being positive (or similarly a 3–25%
probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34 W m −2, and median radiative forcing ranging from +0.94 to +1.39 W m −2.
Received: 14 March 2001 / Accepted: 1 June 2001 相似文献
5.
Summary Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N,
27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position
on CH 4 emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with
surface temperature from which an activation energy of −99 kJ mol −1 was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in
three separate years, was calculated to be 5.5 ± 0.4 g CH 4 m −2 y −1 of which 0.6 ± 0.1 g CH 4 m −2 y −1 (11%) was released during the spring thaw which lasted 20 to 30 days.
The effect of global warming on the CH 4 budget of the site was estimated using the central scenario of the SILMU (Finnish Research Programme on Climate Change) model
which predicts annual mean temperature increases of 1.2, 2.4 and 4.4 °C in 2020, 2050 and 2100, respectively. Maximum enhancements
in CH 4 emission due to warming were calculated to be 18, 40 and 84% for 2020, 2050 and 2100, respectively. Actual increases may
be smaller because prediction of changes in water table are highly uncertain.
Received September 17, 1999 Revised October 16, 2000 相似文献
6.
大西洋经向翻转环流(the Atlantic Meridional Overturning Circulation,AMOC)由低纬输送大量热量至高纬度北大西洋海区,并通过热通量由海洋输送给大气,主导了附近区域的气候形态,并对北半球尺度的气候变化产生显著影响。本文根据CMIP5多模式多增暖情景的预估模拟结果,通过与增暖前控制试验的对比发现,全球增暖可导致该海区湍流热通量的减小,且减小的幅度随增暖强度增大,模拟结果与观测一致。进一步研究发现,热通量的减小存在季节差异,冬季的减小幅度远大于夏季。结合淡水扰动试验的分析表明,全球增暖下AMOC强度的减弱导致大西洋经向热输送减少,进而导致高纬度北大西洋海洋向大气的热输送减小。 相似文献
7.
The atmospheric general circulation model ECHAM-4 is coupled to a chemistry model to calculate sulfate mass distribution
and the radiative forcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle
and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scavenging. Two experiments
are performed, one with pre-industrial and one with present-day sulfur emissions. In the pre-industrial emission scenario
SO 2 is oxidized faster to sulfate and the in-cloud oxidation via the reaction with ozone is more important than in the present-day
scenario. The atmospheric sulfate mass due to anthropogenic emissions is estimated as 0.38 Tg sulfur. The radiative forcing
due to anthropogenic sulfate aerosols is calculated diagnostically. The backscattering of shortwave radiation (direct effect)
as well as the impact of sulfate aerosols on the cloud albedo (indirect effect) is estimated. The model predicts a direct
forcing of −0.35 W m -2 and an indirect forcing of −0.76 W m -2. Over the continents of the Northern Hemisphere the direct forcing amounts to −0.64 W m -2. The geographical distribution of the direct and indirect effect is very different. Whereas the direct forcing is strongest
over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing estimates
based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect
on the direct forcing.
Received: 16 October 1996/Accepted: 24 October 1996 相似文献
8.
Summary ?Microclimatological data obtained during a field experiment in the nongrowing winter period were used to study the microclimatologically
stable night conditions of a 200 × 150 m miscanthus ( Miscanthus cv. giganteus) stand and compared to open field conditions. The microclimatological pattern within the miscanthus canopy
was characterized by long-wave radiative cooling of the plant stand and by an established temperature inversion within the
canopy at calm nights. The results show that there are significant differences in air temperature and energy balance components
between the open field and the miscanthus field during calm and clear nights. In general, net radiation difference during
the cold and calm nights was relatively constant and about 20 W m −2 less negative in miscanthus (because of lower surface temperatures) than at the open field. Air temperature differences also
remained fairly constant and were up to 3 °C lower than at the open field (at the height of 1 m). Through thermal inversion
cold air accumulated in the lower parts of the canopy as shown by the vertical air temperature profiles. They showed a greater
amplitude within the diurnal cycle in the miscanthus stand than in the open field. Through the onset of wind, temperature
profiles changed rapidly and differences diminished. Vertical katabatic air drainage into the canopy layers was estimated
indirectly by using the energy balance approach. It was calculated from the significant energy balance closure gap and showed
a mean air exchange rate of up to 22 m 3 m −2 h −1, related to a stand volume of 1 m 2 area and 4 m height, during the mostly calm and clear nights, depending on the canopy net radiation and turbulent heat exchange
forced by slight wind spells. Quantitative uncertainties in calculated cold air drainage which are introduced by the measurement
method and certain assumptions in the calculations, were considered in a sensitivity analysis. In spite of these uncertainties
evidence of katabatic cold air flow is given.
Received July 29, 1999; revised June 11, 2001; accepted March 14, 2002 相似文献
9.
Summary One of the great unknowns in climate research is the contribution of aerosols to climate forcing and climate perturbation.
In this study, retrievals from AERONET are used to estimate the direct clear-sky aerosol top-of-atmosphere and surface radiative
forcing effects for 12 multi-site observing stations in Europe. The radiative transfer code sdisort in the libRadtran environment is applied to accomplish these estimations. Most of the calculations in this study rely on observations which
have been made for the years 1999, 2000, and 2001. Some stations do have observations dating back to the year of 1995. The
calculations rely on a pre-compiled aerosol optical properties database for Europe. Aerosol radiative forcing effects are
calculated with monthly mean aerosol optical properties retrievals and calculations are presented for three different surface
albedo scenarios. Two of the surface albedo scenarios are generic by nature bare soil and green vegetation and the third relies on the ISCCP (International Satellite Cloud Climatology Project) data product. The ISCCP database has
also been used to obtain clear-sky weighting fractions over AERONET stations. The AERONET stations cover the area 0° to 30° E
and 42° to 52° N. AERONET retrievals are column integrated and this study does not make any seperation between the contribution
of natural and anthropogenic components. For the 12 AERONET stations, median clear-sky top-of-atmosphere aerosol radiative
forcing effect values for different surface albedo scenarios are calculated to be in the range of −4 to −2 W/m 2. High median radiative forcing effect values of about −6 W/m 2 were found to occur mainly in the summer months while lower values of about −1 W/m 2 occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m 2. Individual stations often have much higher values by a factor of 2. The median top-of-atmosphere aerosol radiative forcing
effect efficiency is estimated to be about −25 W/m 2 and their respective surface efficiency is around −35 W/m 2. The fractional absorption coefficient is estimated to be 1.7, but deviates significantly from station to station. In addition,
it is found that the well known peak of the aerosol radiative forcing effect at a solar zenith angle of about 75° is in fact
the average of the peaks occurring at shorter and longer wavelengths. According to estimations for Central Europe, based on
mean aerosol optical properties retrievals from 12 stations, the critical threshold of the aerosol single scattering albedo,
between cooling and heating in the presence of an aerosol layer, is close between 0.6 and 0.76. 相似文献
10.
Summary This paper summarizes results from a mesoscale modeling study to quantify the possible meteorological and energy-use impacts
of large-scale increases in surface albedo and vegetative fraction. Ten regions in the U.S. were characterized and simulated
in base- and modified-surface conditions. Time- and space-dependent meteorological variables were simulated for each region
in four 3-day episodes to represent a range of seasonal variations.
Using a simple interpolative procedure, a complete year of hourly weather data was created for each region (based on episodic
meteorological simulation results) and input into energy-use models. The modified weather input was used to assess the effects
of large-scale albedo and vegetative fraction changes on annual energy consumption in each of the ten areas targeted in this
study. The simulations suggest annual electricity savings of between 1and 6.7 kWh m −2 (of roof area) in residential neighborhoods and between 2 and 6.1 kWh m −2 in office areas, depending on region. Annual gas penalties amount to up to 34.8 MJ m −2 (of roof area) in residential neighborhoods and up to 21.1 MJ m −2 in office areas.
Received December 1, 1996 Revised May 11, 1998 相似文献
11.
Summary ?Observations of sensible and latent heat flux above forest downwind of a forest edge show these fluxes to be larger than
the available energy over the forest. The enhancement averages to 56 W m −2, or 16% of the net radiation, at fetches less than 400 m, equivalent to fetch to height ratios less than 15. The enhancement
of turbulent energy fluxes is explained by advection and increases with the difference in temperature and humidity of the
air over the upwind area as compared to the forest. The relatively high temperature and humidity of the upwind air are not
caused by high surface heat fluxes, but are explained by the relatively low aerodynamic roughness of the upwind surface. Although
the heat fluxes over forest are enhanced, the momentum fluxes are almost adjusted to the underlying forest. The different
behaviour of heat and momentum fluxes is explained by absorption of momentum by pressure gradients near the forest edge. It
is concluded that fetch requirements to obtain accurate surface fluxes from atmospheric observations need to be more stringent
for scalar fluxes as compared to momentum fluxes.
Received November 23, 2001; accepted May 13, 2002 相似文献
12.
Whether the stratospheric radiative feedback amplifies the global warming remains under debate. The stratospheric water vapor (SWV), one of the primary feedbacks in the stratosphere, is argued to be an important contributor to the global warming. On the other hand, the overall stratospheric feedback, which consists of both the SWV feedback and the stratospheric temperature (ST) feedback, does not amount to a significant value. The key to reconciling these seemingly contradictory arguments is to understand the ST change. Here, we develop a method to decompose the ST change and to quantify the decomposed feedbacks. We find that the SWV feedback, which consists of a 0.04 W m−2 K−1 direct impact on the top-of-the-atmosphere radiation and 0.11 W m−2 K−1 indirect impact via ST cooling, is offset by a negative ST feedback of − 0.13 W m−2 K−1 that is radiatively driven by the tropospheric warming. This compensation results in an insignificant overall stratospheric feedback. 相似文献
13.
An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing
by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel
burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation
at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly
on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol
at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m -2 to −0.55 Wm -2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel
emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm -2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of
carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm -2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple
scattering associated with clouds and high surface albedos can change the forcing from negative to positive.
Received: 29 September 1997 / Accepted: 10 June 1998 相似文献
14.
A time series of microwave radiometric profiles over Arctic Canada’s Cape Bathurst (70°N, 124.5°W) flaw lead polynya region
from 1 January to 30 June, 2008 was examined to determine the general characteristics of the atmospheric boundary layer in
winter and spring. A surface based or elevated inversion was present on 97% of winter (January–March) days, and on 77% of
spring (April–June) days. The inversion was the deepest in the first week of March (≈1100 m), and the shallowest in June (≈250 m).
The mean temperature and absolute humidity from the surface to the top of the inversion averaged 250.1 K (−23.1°C), and 0.56 × 10 −3 kg m −3 in winter, and in spring averaged 267.5 K (−5.6°C), and 2.77 × 10 −3 kg m −3. The median winter atmospheric boundary-layer (ABL) potential temperature profile provided evidence of a shallow, weakly
stable internal boundary layer (surface to 350 m) topped by an inversion (350–1,000 m). The median spring profile showed a
shallow, near-neutral internal boundary layer (surface to 350 m) under an elevated inversion (600–800 m). The median ABL absolute
humidity profiles were weakly positive in winter and negative in spring. Estimates of the convergence of sensible heat and
water vapour from the surface that could have produced the turbulent internal boundary layers of the median profiles were
0.67 MJ m −2 and 13.1 × 10 −3 kg m −2 for the winter season, and 0.66 MJ m −2 and 33.4 × 10 −3 kg m −2 for the spring season. With fetches of 10–100 km, these accumulations may have resulted from a surface sensible heat flux
of 15–185 W m −2, plus a surface moisture flux of 0.001–0.013 mm h −1 (or a latent heat flux of 0.7–8.8 W m −2) in winter, and 0.003–0.033 mm h −1 (or a latent heat flux of 2–22 W m −2) in spring. 相似文献
15.
以通用陆面模式CLM 3.0(Community Land Model 3.0)为模型算子,基于集合卡尔曼滤波(Ensemble Kalman Filter,En KF)发展了一个土壤温湿度同化系统,主要用于改进模式对土壤温湿度和地表水热通量的模拟精度,并考察集合样本数、同化频率及不同观测量的组合对同化效果的影响。该系统同化了FLUXNET两个站点(阿柔和Bondville)不同土壤深度、不同时间频率的土壤温度和湿度数据。通过对阿柔站不同集合样本数的设计,综合考虑计算成本和计算精度,最终将集合样本数设置为40。通过分析三种同化方案对同化频率的敏感性得出,同化土壤温度最为敏感,同时同化土壤温湿度次之,同化土壤湿度最不敏感。对于阿柔站点,同化系统对不同土壤深度温度和湿度的模拟精度均能提高90%,潜热通量的均方根误差由94.0 W·m~(-2)降为46.3 W·m~(-2),感热通量均方根误差由55.9 W·m~(-2)降为24.6 W·m~(-2)。Bondville站点浅层土壤温度的改进在30%左右,深层土壤温度改进达到60%,对土壤湿度的改进均在70%以上,潜热通量和感热通量的均方根误差分别从57.4 W·m~(-2)和54.4 W·m~(-2)降为51.0 W·m~(-2)和42.5 W·m~(-2)。试验结果表明,同化站点土壤温湿度数据对土壤水热状况及通量的模拟改进非常有效,同时也验证了同化土壤水分遥感产品的可行性和必要性。 相似文献
16.
Summary A measurement programme was conducted in G?teborg Sweden, to examine the spatial variations of incoming longwave irradiance
on calm, cloudless nights. Both regional and local spatial variations were examined. Incoming longwave irradiance data was
obtained from mobile car transects, and at a fixed site on a building roof at the city centre. Ancillary data included sky
view factor at various transect locations, and balloon soundings of air temperature and humidity on one night.
Measurements revealed that on average, incoming longwave irradiance at the fixed urban site was 11 W m
−2 higher than at the rural station, with varying differences for intervening sites. Bulk apparent sky emissivity was higher
at the most rural station compared to the fixed urban site, by about 0.03 on average. Nighttime balloon measurements and a
sensitivity analysis with a radiative transfer model argue that the bulk apparent sky emissivity differences stem mainly from
the temperature structure of the lower boundary layer which changes markedly from rural to urban areas.
A good relationship was found between sky view factor and incoming longwave irradiance for a range of urban and park locations.
The relationship applies to both individual nights and average data. Using a simple obstruction model, canyon wall temperatures
are derived, and the relationship between sky view factor and wall temperature is examined.
Received December 23, 1999 Revised May 5, 2000 相似文献
17.
In the present study, an attempt has been made to examine the governing photochemical processes of surface ozone (O 3) formation in rural site. For this purpose, measurements of surface ozone and selected meteorological parameters have been
made at Anantapur (14.62°N, 77.65°E, 331 m asl), a semi-arid zone in India from January 2002 to December 2003. The annual
average diurnal variation of O 3 shows maximum concentration 46 ppbv at noon and minimum 25 ppbv in the morning with 1σ standard deviation. The average seasonal
variation of ozone mixing ratios are observed to be maximum (about 60 ppbv) during summer and minimum (about 22 ppbv) in the
monsoon period. The monthly daytime and nighttime average surface ozone concentration shows a maximum (55 ± 7 ppbv; 37 ± 7.3 ppbv)
in March and minimum (28 ± 3.4 ppbv; 22 ± 2.3 ppbv) in August during the study period. The monthly average high (low) O 3 48.9 ± 7.7 ppbv (26.2 ± 3.5 ppbv) observed at noon in March (August) is due to the possible increase in precursor gas concentration
by anthropogenic activity and the influence of meteorological parameters. The rate of increase of surface ozone is high (1.52 ppbv/h)
in March and lower (0.40 ppbv/h) in July. The average rate of increase of O 3 from midnight to midday is 1 ppbv/h. Surface temperature is highest (43–44°C) during March and April months leading to higher
photochemical production. On the other hand, relative humidity, which is higher during the rainy season, shows negative correlation
with temperature and ozone mixing ratio. It can be seen that among the two parameters are measured, correlation of surface
ozone with wind speed is better ( R
2=0.84) in compare with relative humidity ( R
2=0.66). 相似文献
18.
Concentrations of manganese in 56 rain events in Wilmington, NC, USA rainwater from April 1, 2005 to March 31, 2006 were 11 ± 3 nM
for dissolved Mn and 1.2 ± 0.4 nM for particulate Mn. Concentrations of both forms of Mn were higher in terrestrial storms
relative to marine events. This observation along with the positive correlation of Mn with pollutant indicators suggests anthropogenic
inputs to rain at this location, as has been observed at other locations. The ratio of Mn part/Mn diss was threefold larger in summer relative to winter rain, which matched the increase of particulate to dissolved Fe in rainwater
suggesting influence of Saharan dust during the summer. Like Fe in rain, Mn undergoes photoreduction in rainwater, which has
also been shown to be important in Mn cycling in seawater. The flux of Mn removed from the atmosphere via wet deposition is
1.5 × 10 −5 moles m −2 yr −1 at this location, which is approximately twice the flux reported from two rainwater studies conducted in the early 1980s
on Bermuda. Atmospheric input of Mn to the oceans is important because Mn like Fe is an essential and potentially limiting
nutrient. Experiments mixing authentic rainwater and seawater demonstrate that rainwater dissolved Mn does not rapidly precipitate
in seawater suggesting wet deposition is an important source of soluble, stable Mn to surface seawater. 相似文献
19.
Summary As an aspect of the LINEX field studies (1996–1997; Lindenberg near Beeskow, Germany), the characteristics of the internal
boundary layer (IBL) that is associated with a step change of the surface roughnesses in neutral constant stress layers was
investigated and is reported in this paper. Both smooth to rough (in 1996) and rough to smooth (in 1997) types of flow, have
been studied based upon the profiles of mean wind and temperature realised from a 10-m mast and eddy correlation measurements
taken at two levels (2 m and 5 m). Depending upon wind direction, the fetch at the site varied between 140 m and 315 m within
the wind sector (200° to 340°) used for the field investigations. The height of the IBL, δ, had been determined from the intersect
of the logarithmic wind-profiles below (< 2 m) and above (> 6 ) the interface. Values of δ obtained at the experimental site
compared fairly well to the existing theoretical/empirical fetch-height relationships of the form: δ= aċ x
b
, where a, b, are empirical constants. The ratio for the friction velocities below and above the IBL as measured directly by the eddy
correlation techniques showed that for fetches less than 250 m there was an increase (decrease) of about 20% of the momentum
flux arising from the smooth to rough (rough to smooth) transitions. Influences of distant obstructions (e.g., bushes, pockets
of trees) on the surface flow were markedly important on the examined wind profiles and such can be indicative as multiple
IBLs.
Received September 1, 1997 Revised August 5, 1998 相似文献
20.
Summary Net Ecosystem CO 2 Exchange (NEE) was studied during the summer season (June–August) at a high Arctic heath ecosystem for 5 years in Zackenberg,
NE Greenland. Integrated over the 80 day summer season, the heath is presently a sink ranging from −1.4 g C m −2 in 1997 to −23.3 g C m −2 in 2003. The results indicate that photosynthesis might be more variable than ecosystem respiration on the seasonal timescale.
The years focused on in this paper differ climatically, which is reflected in the measured fluxes. The environmental conditions
during the five years strongly indicated that time of snow-melt and air temperature during the growing season are closely
related to the interannual variation in the measured fluxes of CO 2 at the heath. Our estimates suggest that net ecosystem CO 2 uptake is enhanced by 0.16 g C m −2 per increase in growing degree-days during the period of growth. This study emphasises that increased summer time air temperatures
are favourable for this particular ecosystem in terms of carbon accumulation. 相似文献
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