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1.
R. García-Herrera D. Barriopedro E. Hernández D. Paredes J. F. Correoso L. Prieto 《Meteorology and Atmospheric Physics》2005,90(3-4):225-243
Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological
settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region
between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics.
Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period,
with September being the most active period. They tend to develop during the late afternoon or early night, with preferred
eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km2 is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity
was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average
speeds. The average MCS precipitation rate was 3.3 mm·h−1 but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h−1 (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation
in the Mediterranean region during the convective season, but its impact varied according to the location.
Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean
Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern
portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h−1 towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θe) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern
Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10−10 s−2 over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection
were the most relevant forcing mechanisms during 2001. 相似文献
2.
M. P. Guishard E. A. Nelson J. L. Evans R. E. Hart D. G. O’Connell 《Meteorology and Atmospheric Physics》2007,97(1-4):239-253
Summary This investigation focuses on North Atlantic subtropical cyclones which tracked within 100 nautical miles (185 km) of Bermuda
from 1957 to 2005, identified through subtropical structural characteristics distinguished using Cyclone Phase Space, from
the European Centre for Medium-Range Weather Forecasts 45-year reanalyses. The study assesses the characteristics of these
hybrid storms that affect the Island, in order to aid the local forecaster. Reanalysis charts, surface analyses, local observations,
HURDAT tracks, and satellite pictures, where available, were examined. This data shows that subtropical cyclones affecting
Bermuda usually form in close proximity, to the south-southwest, over water of an average of 26 °C, under moderate vertical
wind shear, with an upper trough lying to the west-north-west. They then move in a north-northeastward direction, intensifying
quickly, but not often reaching a peak intensity of more than 26 m s−1. They generally have their beginnings along old baroclinic zones. September is the peak month of occurrence. A direct hit
by a severe subtropical cyclone, producing locally observed winds of over 26 m s−1, appears to be a rare event. However, these storms are certainly a threat to the Island, particularly due to their lack of
predictability, and conditions conducive to an incipient subtropical cyclone with potential to affect the Island should always
be closely monitored. 相似文献
3.
Both aerosol and rainwater samples were collected and analyzed for ionic species at a coastal site in Southeast Asia over
a period of 9 months (January–September 2006) covering different monsoons. In general, the occurrence and distribution of
ionic species showed a distinct seasonal variation in response to changes in air mass origins. Real-time physical characterization
of aerosol particles during rain events showed changes in particle number distributions which were used to assess particle
removal processes associated with precipitation, or scavenging. The mean scavenging coefficients for particles in the range
10–500 nm and 500–10 μm were 7.0 × 10−5 ± 2.8 × 10−5 s−1 and 1.9 × 10−4 ± 1.6 × 10−5 s−1, respectively. A critical analysis of the scavenging coefficients obtained from this study suggested that the wet removal
of aerosol particles was greatly influenced by rain intensity, and was particle size-dependent as well. The scavenging ratios,
another parameter used to characterize particle removal processes by precipitation, for NH4
+, Cl−, SO4
2−, and NO3
− were found to be higher than those of Na+, K+, and Ca2+ of oceanic and crustal origins. This enrichment implied that gaseous species NH3, HCl, and HNO3 could also be washed out readily. These additional sources of ions in precipitation presumably counter-balanced the dilution
effect caused by high total precipitation volume in the marine and tropical area. 相似文献
4.
Summary. Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data
from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data
restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this
region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the
years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation.
They reach sizes of 2 − 6 · 104 km2. MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data
indicates that a giant MCS (covering more than 4 · 104 km2 with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine
region.
Received February 25, 1999/Revised June 29, 1999 相似文献
5.
Annual methane emission from Finnish mires estimated from eddy covariance campaign measurements 总被引:4,自引:0,他引:4
K. J. Hargreaves D. Fowler C. E. R. Pitcairn M. Aurela 《Theoretical and Applied Climatology》2001,70(1-4):203-213
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 CH4 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 CH4 m−2 y−1 of which 0.6 ± 0.1 g CH4 m−2 y−1 (11%) was released during the spring thaw which lasted 20 to 30 days.
The effect of global warming on the CH4 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 CH4 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.
Kinetic Study of the Reactions of Ozone with Polycyclic Aromatic Hydrocarbons Adsorbed on Atmospheric Model Particles 总被引:1,自引:0,他引:1
In this experimental study, rate constants were measured for the reactions of ozone with 13 polycyclic aromatic hydrocarbons (PAHs) adsorbed on different types of particles. Graphite and silica were chosen to model, respectively, carbonaceous and mineral atmospheric particles. The pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate PAH concentrations versus time. Second order rate constants were calculated considering the ozone gaseous concentration. At room temperature, rate constants varied, in the case of graphite particles, between (1.5 ± 0.5) × 10−17 and (1.3 ± 0.7) × 10−16 cm3 molecule−1 s−1 for chrysene and dibenzo[a,l]pyrene, respectively, and, in the case of silica particles, between (1.5 ± 0.3) × 10−17 and (1.4 ± 0.3) × 10−16 cm3 molecule−1 s−1 for fluoranthene and benzo[a]pyrene, respectively. Different granulometric parameters (particle size, pore size) and different PAH concentrations were tested in the case of silica particles. Heterogeneous reactions of ozone with particulate PAHs are shown to be more rapid than those occurring in the gas-phase, and may be competitive with atmospheric photodegradation. 相似文献
7.
Seasonal carbon dioxide balance and respiration of a high-arctic fen ecosystem in NE-Greenland 总被引:2,自引:0,他引:2
C. Nordstroem H. Soegaard T. R. Christensen T. Friborg B. U. Hansen 《Theoretical and Applied Climatology》2001,70(1-4):149-166
Summary Turbulent fluxes of CO2 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 CO2 to the atmosphere were small, typically 0.005 mg CO2 m−2 s−1 (0.41 g CO2 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 CO2 to the atmosphere were typically 0.1 mg CO2 m−2 s−1 in the afternoon, and daily sums reached values up to almost 9 g CO2 m−2 d−1. After 4 July, downward fluxes of CO2 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 CO2 m−2 d−1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO2 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 CO2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO2 m−2 yr−1.
Received October 6, 1999 Revised May 2, 2000 相似文献
8.
Summary ?For the LITFASS-98 experiment, from June 1 until June 30, 1998, the spatially resolved insolation at surface could be computed
from NOAA-14 AVHRR data applying the modular analysis scheme SESAT (Strahlungs- und Energiebilanzen aus Satellitendaten). The satellite inferred insolation for this period shows for clear-sky regions a good agreement with surface
based observations with a rms error of 76 Wm−2. For cloudy conditions the insolation is overestimated with respect to ground based observations, with a rms error between
83 and 118 Wm−2, depending on the cloud optical thickness. This overestimation can be explained by the surface heterogeneity, leading to
underestimated cloud optical thickness, and also by a fixed relative humidity below clouds (55%, dry atmosphere) and a fixed
horizontal visibility (50 km, clear atmosphere). A detailed study of comparable scales in space and time, considering the
different observation geometries and sampling intervals, shows that a 30 min ground based observation can be compared with
a 8 × 8 km2 mean by the satellite data.
Received July 12, 2001; revised April 29, 2002; accepted June 7, 2002 相似文献
9.
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
10.
Rebecca S. Anderson Lin Huang Richard Iannone Jochen Rudolph 《Journal of Atmospheric Chemistry》2007,56(3):275-291
The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured
at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All
measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k
12/k
13, are greater than unity or normal KIEs. The KIEs, reported in per mil according to Cl
ɛ = (k
12/k
13−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene,
5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31
(2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC
with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene
and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are:
cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10−10 cm3 molecule−1 s−1. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of
alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the
KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing
contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries
of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope
ratios of atmospheric NMHC are discussed. 相似文献
11.
Yang Zhao Deliang Chen Jiao Li Dandan Chen Yi Chang Juan Li Rui Qin 《Climate Dynamics》2020,54(5):2713-2730
This study investigates the roles of atmospheric moisture transport under the influence of topography for summer extreme precipitation over North China (NC) during 1979–2016. Based on rain gauge precipitation data and a reanalysis, 38 extreme precipitation days in NC during the 38 years were selected and associated moisture fluxes estimated. The results show that there is a dominant moisture influx of 311.8 kg m−1 s−1 into NC along its southern boundary from tropical oceans, and a secondary influx of 107.9 kg m−1 s−1 across its western boundary carried by mid-latitude westerlies. The outflux across the eastern boundary is 206.9 kg m−1 s−1 and across the northern boundary is 76.0 kg m−1 s−1, giving a net moisture gain over NC of 136.8 kg m−1 s−1. During extreme precipitation days, the moisture flux convergence (MFC) was much larger, exceeding 4 × 10−5 kg m−1 s−1. The MFC maximum core, the pronounced moisture transport, and the striking extreme precipitation zone over NC are all anchored to the east of the steep slopes of the surrounding topography. Moreover, a remarkably high humidity and strong upward motion also occur near steep slopes, indicating the critical role of the adjacent topography on the extreme precipitations. Simulations with and without the topography in NC using the Weather and Research Forecasting model for six selected out of the 38 extreme precipitation days demonstrate that the surrounding topography reinforces the MFC over NC by 16% relative to the case without terrain, primarily through enhanced wind convergence and higher moisture content, as well as stronger vertical motion induced by diabatic heating. The interactions between moisture convergence and topographic settings strengthen the extreme precipitation over NC. 相似文献
12.
Summary Based on the six-hourly re-analysis sea-level pressure data of the European Centre for Medium-Range Weather Forecast (ECMWF)
a cyclone statistics for the Arctic region north of 60° is elaborated for the period 1 November 1986 to 31 October 1991. For
each low pressure center on a weather map its location, central pressure and horizontal pressure gradients in E, W, N, and
S direction are determined. Furthermore, cyclone centers are followed with time to calculate trajectories, pressure tendencies,
and lifetimes.
A horizontal grid of 300 km × 300 km is used as unit area for the statistical computations. A unit area experiences about
20 cyclone passages per year (range 5–40). On the average, six cyclones occur simultaneously in the Arctic region. Lifetimes
vary from 6 h to 15 days.
The annual cyclone activity over the 5-year period is nearly the same. Cyclones are more frequent in summer (about 94 per
month) than in winter (77 per month). In general summer cyclones are weaker than winter cyclones. On the average, the minimum
central pressure during the lifetime of a cyclone is about 1000 hPa (typical range 980–1020) in summer and about 988 hPa (typical
range 940–1030) in winter.
In winter, a zone of high cyclone frequency extends from the region near Iceland over the Greenland Sea, Barents Sea, and
Kara Sea to the Laptev Sea while the interior of the Arctic shows little cyclone frequency. In summer, the region near Iceland
and the interior of the Arctic are separate centers of high cyclone frequency. Both in winter and summer very high cyclone
frequencies are observed over the northern Baffin Bay. The regional distribution of mean central pressures and maximum pressure
gradients roughly follows the distribution of cyclone frequencies except for the Baffin Bay cyclones which are generally weak.
Cyclolysis dominates cyclogenesis over largest parts of the Arctic. Regions of high cyclone frequency are also regions of
frequent cyclogenesis and frequent cyclolysis. One third of all cyclones is generated in a region with an already existing
cyclonic circulation.
Cyclones in the Fram Strait are studied in more detail because of their special impact on the ice export from the Arctic Ocean
to the Atlantic Ocean. On the average, there are 5 cyclones per month. the cyclone frequency in the Fram Strait is higher
during the winter period than during the summer period. This is in contrast to the overall Arctic frequency which is higher
in summer than in winter. Cyclogenesis predominates in winter and cyclolysis in summer in the Fram Strait. The most frequent
direction of motion is from SW to NE.
Received November, 1999 Revised June 22, 2000 相似文献
13.
Summary Net ecosystem CO2 exchange was measured over a mountain birch forest in northern Finland throughout the growing season. The maximal net CO2 uptake rate of about − 0.5 mg(CO2) m−2 s−1 was observed at the end of July. The highest nocturnal respiration rates in early August were 0.2 mg(CO2) m−2 s−1. The daily CO2 balances during the time of maximal photosynthesis were about −15 g(CO2) m−2 d−1. The mountain birch forest acted as a net sink of CO2 from 30 June to 28 August. During that period the net CO2 balance was −448 g(CO2)m−2. The interannual representativeness of the observed balances was studied using a simplified daily balance model, with daily
mean global radiation and air temperature as the input parameters. The year-to-year variation in the phenological development
was parameterised as a function of the cumulative effective temperature sum. The daily balance model was used for estimating
the variability in the seasonal CO2 balances due to the timing of spring and meteorological factors. The sink term of CO2 in 1996 was lower than the 15-year mean, mainly due to the relatively late emergence of the leaves.
Received October 11, 1999 Revised April 25, 2000 相似文献
14.
Summary ?The analysis of ground-based measurements of solar erythemal ultraviolet (UV) irradiance with a Solar Light 501 biometer,
and total (300–3000 nm) irradiance with an Eppley B&W pyranometer at the Argentine Antarctic Base “Almirante Brown”, Paradise
Bay (64.9° S, 62.9° W, 10 m a.s.l.) is presented. Measurement period extends from February 16 to March 28 2000. A relatively
high mean albedo and a very clean atmosphere characterise the place. Sky conditions were of generally high cloud cover percentage.
Clear-sky irradiance for each day was estimated with model calculations, and the effect of the cloudiness was studied through
the ratio of measured to clear-sky value (r). Two particular cases were analysed: overcast sky without precipitation and overcast sky with rain or slight snowfall, the
last one presenting frequently dense fog. Total irradiance was more attenuated than UV by the homogeneous cloudiness, obtaining
mean r values of 0.54 for erythemal irradiance and 0.30 for total irradiance in the first case (without precipitation) and 0.27
and 0.17 respectively in the second case (with precipitation). Mean r values for the complete period were 0.58 for erythemal irradiance and 0.43 for total irradiance. Erythemal and total daily
insolations reduce quickly at this epoch due to the increase of the noon solar zenith angle and the decrease of daylight time.
Additionally, they were strongly modulated by cloudiness. Measured maxima were 2.71 kJ/m2 and 18.42 MJ/m2 respectively. Measurements were compared with satellite data. TOMS-inferred erythemal daily insolation shows the typical
underestimation with respect to ground measurements at regions of high mean albedo. Measured mean total daily insolation agrees
with climatological satellite data for the months of the campaign.
Received August 9, 2002; revised January 4, 2003; accepted January 28, 2003
Published online May 20, 2003 相似文献
15.
The uptake of water vapor on MgCl2×6H2O and NaCl salt dry solid films was studied over the temperature range 240 to 340 K and at 1 Torr pressure of helium using
a flow reactor coupled to a modulated molecular beam mass spectrometer. The H2O to salt uptake data were obtained from the kinetics of H2O loss on salt coated Pyrex rods. The following Arrhenius expression was obtained for the initial uptake coefficient of H2O on MgCl2×6H2O films: γ
0
(MgCl2) = (6.5 ± 1.0) × 10−6 exp[(470 ± 40)/T] (calculated with specific BET surface area, quoted uncertainties are 1σ statistical). The rate of H2O adsorption on NaCl was found to be much lower than on MgCl2×6H2O, and only an upper limit was determined for the corresponding uptake coefficient: γ (NaCl) ≤ 5.6 × 10−6 at T = 300 K. The results show that the rate of H2O adsorption to salt surfaces is drastically dependent on the salt sample composition. 相似文献
16.
Summary ?This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research
Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the
representation of the simulated rainband are discussed.
Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented
in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other
tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence
above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm
through its surroundings where horizontal wind has vertical shear.
The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation
scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a
precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering.
In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive,
suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud
microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result
suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud
water.
The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the
large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km
but decreased as the interval was further reduced from 10 to 5 km.
Received March 20, 2001; revised August 20, 2001 相似文献
17.
Summary Net Ecosystem CO2 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 CO2 at the heath. Our estimates suggest that net ecosystem CO2 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
18.
S. Gonzi O. Dubovik D. Baumgartner E. Putz 《Meteorology and Atmospheric Physics》2007,96(3-4):277-291
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/m2. High median radiative forcing effect values of about −6 W/m2 were found to occur mainly in the summer months while lower values of about −1 W/m2 occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m2. 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/m2 and their respective surface efficiency is around −35 W/m2. 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
19.
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
SO2 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
20.
Summary A set of the inhomogeneity factor for high-level clouds derived from the ISCCP D1 dataset averaged over a five-year period
has been incorporated in the UCLA atmospheric GCM to investigate the effect of cirrus cloud inhomogeneity on climate simulation.
The inclusion of this inhomogeneous factor improves the global mean planetary albedo by about 4% simulated from the model.
It also produces changes in solar fluxes and OLRs associated with changes in cloud fields, revealing that the cloud inhomogeneity
not only affects cloud albedo directly, but also modifies cloud and radiation fields. The corresponding difference in the
geographic distribution of precipitation is as large as 7 mm day−1. Using the climatology cloud inhomogeneity factor also produces a warmer troposphere related to changes in the cloudiness
and the corresponding radiative heating, which, to some extent, corrects the cold bias in the UCLA AGCM. The region around
14 km, however, is cooler associated with increase in the reflected solar flux that leads to a warmer region above. An interactive
parameterization for mean effective ice crystal size based on ice water content and temperature has also been developed and
incorporated in the UCLA AGCM. The inclusion of the new parameterization produces substantial differences in the zonal mean
temperature and the geographic distribution of precipitation, radiative fluxes, and cloud cover with respect to the control
run. The vertical distribution of ice crystal size appears to be an important factor controlling the radiative heating rate
and the consequence of circulation patterns, and hence must be included in the cloud-radiation parameterization in climate
models to account for realistic cloud processes in the atmosphere. 相似文献