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1.
Surface fluxes of heat and water vapour from sites in the European Arctic   总被引：1，自引：0，他引：1  
Summary  Measurements of the surface fluxes of heat and water vapour were taken at four sites across the European Arctic as part of the EU funded LAPP project. The sites cover a range of latitudinal, altitudinal and climatic conditions. The most northerly site is near Ny-?lesund, Svalbard, a polar semi-desert with continuous permafrost. A second permafrost site is a fen area in the Zackenberg valley, East Greenland. Finally two sites in northern Finland, Skalluvaara and Kaamanen are on the southern boundary of the region affected by permafrost. At all sites measurements were made of the turbulent fluxes of heat and water vapour using eddy correlation equipment for at least one active season. The net radiation totals for July and August are similar at all sites. At the sites with permafrost a substantial proportion (over 20%) of the net radiation goes into soil heat flux, to thaw the soil moisture in the top metre. Of the remaining energy just over half is used for evaporation. At the Finnish sites the vegetation is largely deciduous and this is seen in the record with higher evaporative ratios in July and August, after the vegetation becomes green. The Finnish sites tend to have higher surface resistance to evaporation; however, the evaporative demand is greater leading to slightly higher evaporation rates. The two Finnish sites have a similar seasonal pattern determined by the water table and seasonality of the vegetation. The two northern sites show a pattern that is determined primarily by the variation of water table only. It is concluded that the water balance through the active season is influenced primarily by the history of snow cover. The seasonality of the vegetation, the permafrost and the depth of water table are also important influences. Received November 1, 1999 Revised April 17, 2000  相似文献   

2.
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   

3.
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   

4.
Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH4+ and ~32% NO3– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH4+ and NO3− in rainwater ranged from 1.7–55 μmol L−1 and 0.16–18 μmol L−1, respectively, accounting for ~87% by NH4+ and ~13% by NO3− of total inorganic N (median values for all data). A significant correlation (r = 0.74, p < 0.05, n = 10) between NH4+ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations were found between NH4+ and MSA in rainwater collected over the subarctic (r = 0.42, p > 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH3) by marine biological activity from the ocean could become a significant source of NH4+ over the South Pacific. While NO3− was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH4+ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m−2 d−1, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.  相似文献   

5.
Summary  Within the framework of the European LAPP-project (Land Arctic Physical Processes) and as part of the Danish Research Council’s Polar Programme, studies on water- and surface energy balance in NE Greenland were conducted in 1996 and 1997. Eddy correlation measurements of water vapour and sensible heat fluxes above the three dominant vegetation types: fen, willow snowbed, and heath were conducted for the entire growing season. This was supplemented by measurements of evaporation from snow covered areas and from a small pond. The evapotranspiration was found to be relatively high with the maximum from the fen (≈86 mm per season). For the two other vegetation types the evapotranspiration was less, for heath 61 mm per season, while willow snowbed had evaporation rates on intermediate level. By use of the Penman-Monteith equation it was possible to estimate the altitude dependence of the evapotranspiration and calculate the annual evaporation for the whole area to 80 mm per year. By applying a bucket model the evaporation was found to be in accordance with changes in soil moisture as monitored with TDR. The observed surface water balance was compared to river discharge, which shows a glacio-nival regime with an early spring flow (June), determined by the snow melt in the main valley and an July–August maximum determined by melt on higher plateau areas. When balancing the individual hydrological components an annual deficit of 180 mm was observed, but it was found that this deficit could be reduced by correcting for aerodynamic and altitude effects on the precipitation. Finally some of the possible consequences of a global warming is discussed in relation to the water and energy balance in the high-arctic ecosystem. Received November 1, 1999 Revised May 15, 2000  相似文献   

6.
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   

7.
Summary ?The LITFASS project (‘Lindenberg Inhomogeneous Terrain – Fluxes between Atmosphere and Surface: a Long-term Study’) of the Deutscher Wetterdienst (DWD, German Meteorological Service) aims to develop and to test a strategy for the determination and parameterisation of the area-averaged turbulent fluxes of heat, momentum, and water vapour over a heterogeneous land surface. These fluxes will be representative for an area of about 10 * 10 km2 (while the typical patch size is between 10−1 to 100 km2) corresponding to the size of a grid cell in the present operational numerical weather prediction model of the DWD. LITFASS consists of three components: – the development of a non-hydrostatic micro-α-scale model (the LITFASS local model – LLM) with a grid-size of about 100 * 100 m2, – experimental investigations of land surface – atmosphere exchange processes and boundary layer structure within a 20 * 20 km2 area around the Meteorological Observatory Lindenberg, – the assimilation of a data base as an interface between measurements and modelling activities. The overall project strategy was tested over a three-week period in June 1998 during the LITFASS-98 field experiment. This paper gives an overview on the LITFASS project, on the design and measurement program of the LITFASS-98 experiment, and on the weather conditions during the period of the experiment. Conclusions are formulated for the operational realisation of the LITFASS measurement concept and for future field experiments aimed at studying the land surface – atmosphere interaction in the Lindenberg area. Selected results from both experimental and modelling activities are presented in a series of companion papers completing this special issue of the journal. Received June 18, 2001; revised March 18, 2002; accepted April 2, 2002  相似文献   

8.
Summary The qualitative agreement of two climate models, HADCM2 and ECHAM3, on the response of surface climate to anthropogenic climate forcing in the period 2020 – 2049 is studied. Special attention is paid to the role of internal climate variability as a source of intermodel disagreement. After illustrating the methods in an intermodel comparison of simulated changes in June–August mean precipitation, some global statistics are presented. Excluding surface air temperature, the four-season mean proportion of areas in which the two models agree on the sign of the climatic response is only 53 – 60% both for increases in CO2 alone and for increases in CO2 together with direct radiative forcing by sulphate aerosols, but somewhat larger, 59 – 70% for the separate aerosol effect. In areas where the response is strong (at least twice the standard error associated with internal variability) in both models, the agreement is better and the contrast between the different forcings becomes more marked. The proportion of agreement in such areas is 57 – 75% for the response to increases in CO2 alone, 64 – 84% for the response to combined CO2 and aerosol forcing, and as high as 88 – 94% for the separate aerosol effect. The relatively good intermodel agreement for aerosol-induced climate changes is suggested to be associated with the uneven horizontal distribution of aerosol forcing. Received December 2, 1998 Revised May 5, 1999  相似文献   

9.
Summary.  Using 9 years (1985–1993) data, final stratospheric warmings in the Southern Hemisphere are studied. Interannual variations in the onset date and the temperatures are noted. In 1985 the stratosphere was colder by about 5 K and the wave activity was less. This year the final warming got delayed. In contrast in 1988 the final warming occurred earlier when compared with the mean picture and the wave activity was more. An examination of Eliassen-Palm fluxes showed the important role of planetary waves in the wave-mean flow interaction. In the energetics the most spectacular change is the reduction of zonal kinetic energy. Before the warming the energy exchanges were Pz → Pe → Ke → Kz ← Pz and after the warming they were Pz ← Pe ← Ke → Kz ← Pz. The dramatic reduction of zonal kinetic energy seems to be due to two effects: the reduction in Ke → Kz conversion and the weakening of direct meridional circulation. Received October 3, 2001; revised June 5, 2002  相似文献   

10.
Summary  Surface radiative fluxes play a major role in the energy exchange process between the atmosphere and earth surface and are thus very crucial to climatic processes within the atmospheric boundary layer. Based on four years REKLIP (REgio-KLIma-Project) data set of measured radiative fluxes and additional supporting meteorological variables, the surface radiation regime for selected lowland site (Bremgarten 212 m a.s.l.) and mountain sites (Geiersnest at 870 m a.s.l.; Feldberg 1489 m a.s.l.) in the southern Upper Rhine valley region (south-west Germany) has been reported. Time series of radiative fluxes and surface albedo showed significant inter-site differences. Possible reasons for the observed differences have been made. Downward atmospheric radiation A l at the study sites was parameterised in terms of air temperature, vapour pressure and cloud amount, all of which strongly govern the variation of A l . Effective terrestrial radiation amounted to about 50% of absorbed shortwave radiation at the study sites annually. During clear sky conditions, global solar irradiance G s constituted about 76.0% of the incident extraterrestrial solar irradiance at Feldberg mountain site but only 68.5% of that at Bremgarten lowland site. Annual cumulative of net radiative flux R n amounted to 1722 MJm−2 yr−1 at the lowland site, while that at Geiersnest and Feldberg mountain sites constituted 84% and 73% respectively of the corresponding magnitude for the lowland site. In the same vein, annual mean of radiation efficiency (defined here as R n /G s ) amounted to 0.32 in Feldberg, 0.37 in Geiersnest and 0.41 in Bremgarten. Consequently the annual available energy, of which net radiative flux is representative, was smaller at the mountain ous sites relative to the lowland site during the study period. Inter-annual variability of net radiative flux, its constituent variables and derivatives at the study sites were generally below 10%, with longwave fluxes showing the lowest fluctuation. This renders the measured data quite suitable for modelling purposes. In winter, mean daily sums of R n showed a slow rise with cloud amount N at the lowland site but a sharp rise with N at Feldberg mountain site. In summer however, mean daily sums of R n declined significantly with N as well as Linke turbidity factor at the study sites. Received June 24, 1999 Revised November 2, 2000  相似文献   

11.
Summary ?Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were performed on 18 June 1998 during the LITFASS-98 field experiment. The area-averaged turbulent vertical fluxes of momentum, sensible, and latent heat were determined on a 15 km × 15 km and a 10 km × 10 km flight pattern, respectively. The flights were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus clouds. Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was identified as a significant peak in some co-spectra. The turbulent fluxes of momentum and sensible heat at 80 m above the ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed for the latent heat flux, probably due to the vertical distribution of humidity in the boundary layer. Statistical error analysis of the fluxes F showed that the systematic statistical error ΔF was one order of magnitude smaller than the standard deviation σ F . The difference between area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements was much smaller than σ F , indicating that the systematic statistical error was possibly over-estimated by the usual method. In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data. A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements on various surface types. Systematic discrepancies between airborne and ground-based measurements were not found. Received June 18, 2001; revised December 21, 2001; accepted June 3, 2002  相似文献   

12.
Sonic anemometers are capable of measuring the wind speed in all three dimensions at high frequencies (10–50 Hz), and are relied upon to estimate eddy-covariance-based fluxes of mass and energy over a wide variety of surfaces and ecosystems. In this study, wind-velocity measurement errors from a three-dimensional sonic anemometer with a non-orthogonal transducer orientation were estimated for over 100 combinations of angle-of-attack and wind direction using a novel technique to measure the true angle-of-attack and wind speed within the turbulent atmospheric surface layer. Corrections to the vertical wind speed varied from −5 to 37% for all angles-of-attack and wind directions examined. When applied to eddy-covariance data from three NOAA flux sites, the wind-velocity corrections increased the magnitude of CO2 fluxes, sensible heat fluxes, and latent heat fluxes by ≈11%, with the actual magnitude of flux corrections dependent upon sonic anemometer, surface type, and scalar. A sonic anemometer that uses vertically aligned transducers to measure the vertical wind speed was also tested at four angles-of-attack, and corrections to the vertical wind speed measured using this anemometer were within ±1% of zero. Sensible heat fluxes over a forest canopy measured using this anemometer were 15% greater than sensible heat fluxes measured using a sonic anemometer with a non-orthogonal transducer orientation. These results indicate that sensors with a non-orthogonal transducer orientation, which includes the majority of the research-grade three-dimensional sonic anemometers currently in use, should be redesigned to minimize sine errors by measuring the vertical wind speed using one pair of vertically aligned transducers.  相似文献   

13.
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   

14.
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   

15.
The Mediterranean has been identified as particularly vulnerable to climate change, yet a high-resolution temperature reconstruction extending back into the Medieval Warm Period is still lacking. Here we present such a record from a high-elevation site on Mt. Smolikas in northern Greece, where some of Europe’s oldest trees provide evidence of warm season temperature variability back to 730 CE. The reconstruction is derived from 192 annually resolved, latewood density series from ancient living and relict Pinus heldreichii trees calibrating at r1911–2015 = 0.73 against regional July–September (JAS) temperatures. Although the recent 1985–2014 period was the warmest 30-year interval (JAS Twrt.1961–1990 = + 0.71 °C) since the eleventh century, temperatures during the ninth to tenth centuries were even warmer, including the warmest reconstructed 30-year period from 876–905 (+ 0.78 °C). These differences between warm periods are statistically insignificant though. Several distinct cold episodes punctuate the Little Ice Age, albeit the coldest 30-year period is centered during high medieval times from 997–1026 (− 1.63 °C). Comparison with reconstructions from the Alps and Scandinavia shows that a similar cold episode occurred in central Europe but was absent at northern latitudes. The reconstructions also reveal different millennial-scale temperature trends (NEur = − 0.73 °C/1000 years, CEur = − 0.13 °C, SEur = + 0.23 °C) potentially triggered by latitudinal changes in summer insolation due to orbital forcing. These features, the opposing millennial-scale temperature trends and the medieval multi-decadal cooling recorded in Central Europe and the Mediterranean, are not well captured in state-of-the-art climate model simulations.  相似文献   

16.
Summary  The winter wind regime of G?teborg, located on the West coast of Sweden, is composed of three different wind systems besides the ambient wind; a nocturnal low level jet (NLLJ), a winter land breeze (WLB) and an urban heat island circulation (UHIC). An inversion divides the air column into two layers, one between 10 – 50 m and one between 50 – 100 m. The UHIC is located in the lower layer, the WLB in the top layer and the NLLJ above the top layer. The intensity of the interacting processes depends on the stability of each layer as calculated from the bulk Richardson number (BRilow and BRihigh) using continuous data collected during four years (1991 – 94) from two sites (one within and one outside the urban area) and sampled at three levels. In the evening the WLB develops from the ground level and increases in height until after midnight. At about the same time an UHIC develops in the urban area, below the WLB and causing an uplift of the latter. However, at both sites the WLB does not exceed the 100 m level. At this time BRi in both layers are below one resulting in continuous coupling between the WLB, the UHIC layers and the regional wind. Consequently, the exchange of momentum is still effective between all layers and this is highlighted by a change in the wind direction and a regulation of wind-speed to more constant levels. When BRihigh≥1, the layers become frictionally decoupled, as indicated by a return in the wind direction in the top level to the regional wind, and an acceleration of the top wind. The top level then becomes incorporated in to a nocturnal low-level jet (NLLJ) system. The normally acknowledged development of the NLLJ, with a start around sunset, is in this case delayed for several hours at the top level. The reason for this is that there are meso-scale/local wind systems present in layers beneath the jet causing an interaction between the layers. In the morning, when the layers are again coupled the top layer wind is once more influenced by the WLB and therefore changes direction and speed. The local and meso-scale wind systems thus delay the current nocturnal wind development. Received August 24, 1998 Revised March 17, 1999  相似文献   

17.
Using the monthly mean NCEP/NCAR reanalysis dataset, the three-dimensional Eliassen-Palm (EP) fluxes of quasi-stationary wave propagation in the lower stratosphere were computed for each month from November to March for the period from 1958 to 2007. It is shown that the upward planetary wave propagation from the troposphere to the stratosphere generally occurs over the northern Eurasia, while their weak downward propagation is observed in Labrador and southern Greenland regions in the lower stratosphere. Interannual variations of the vertical EP fluxes also have the dipole-like spatial pattern with the opposite anomalies in the West and East hemispheres which are most prominent in January–February. Significant differences in the interaction of the zonal circulation of the stratosphere in the beginning of winter (November–December) and mid-to-late winter (January–March) are revealed. Intensification of the planetary waves’ penetration into the stratosphere in December causes changes in the stratospheric dynamics, creating the “preconditions” for the stratospheric warming appearances in January, but such a mechanism is not detected in February. In the years with the cold polar vortex, the “stratospheric bridge” is formed with the strengthening of the upward EP flux over the northern Eurasia and downward EP flux over the North Atlantic.  相似文献   

18.
Palaeoclimates across Europe for 6000 y BP were estimated from pollen data using the modern pollen analogue technique constrained with lake-level data. The constraint consists of restricting the set of modern pollen samples considered as analogues of the fossil samples to those locations where the implied change in annual precipitation minus evapotranspiration (P–E) is consistent with the regional change in moisture balance as indicated by lakes. An artificial neural network was used for the spatial interpolation of lake-level changes to the pollen sites, and for mapping palaeoclimate anomalies. The climate variables reconstructed were mean temperature of the coldest month (T c ), growing degree days above 5  °C (GDD), moisture availability expressed as the ratio of actual to equilibrium evapotranspiration (α), and P–E. The constraint improved the spatial coherency of the reconstructed palaeoclimate anomalies, especially for P–E. The reconstructions indicate clear spatial and seasonal patterns of Holocene climate change, which can provide a quantitative benchmark for the evaluation of palaeoclimate model simulations. Winter temperatures (T c ) were 1–3 K greater than present in the far N and NE of Europe, but 2–4 K less than present in the Mediterranean region. Summer warmth (GDD) was greater than present in NW Europe (by 400–800 K day at the highest elevations) and in the Alps, but >400 K day less than present at lower elevations in S Europe. P–E was 50–250 mm less than present in NW Europe and the Alps, but α was 10–15% greater than present in S Europe and P–E was 50–200 mm greater than present in S and E Europe. Received: 3 January 1996 / Accepted: 15 July 1996  相似文献   

19.
Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from 212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from 19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27% and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and radiation efficiency declined with elevation at an annual mean of 1.15 Wm−2/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline of 1.54 Wm−2/100 m and 0.34 Wm−2/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm−2 for the lowland site and 73 Wm−2 for the highest elevated site. Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation, cloud amount and screen temperature. Received March 5, 2001 Revised October 29, 2001  相似文献   

20.
Summary An attempt has been made to simulate the unprecedented heavy precipitation of 94.4 cm in a day over Santacruz, Mumbai during 0300 UTC 26 July to 0300 UTC 27 July 2005. Three experiments have been conducted using Advanced Regional Prediction System model developed by Center for Analysis and Prediction of Storms of Oklahoma University, USA. In first experiment the model input at large domain size has been obtained using NCEP/NCAR reanalysis data at 2.5° × 2.5° lat.–lon. resolution. In other two experiments model input at large as well as at small domain sizes, have been obtained from NCEP/NCAR FNL data of 1° × 1° lat.–lon. resolution. In all three experiments model’s horizontal resolution is 40 km and integration period is 30 hours from 0000 UTC 26 July 2005. Based on the temporal distribution of observed rainfall rates it is considered that the rainfall of 38.1 cm during 0900–1200 UTC on 26 July could be due to cloud burst phenomenon and 56.3 cm from 1200 UTC of 26 July to 0300 UTC of 27 July has been due to continuous regeneration of thunderstorm activity under influence of mesoscale cloud complex. It is found that model forecast of rainfall in first experiment was qualitatively as well as quantitatively very poor. Among other two, experiment with large domain size has predicted better rainfall values and location compared to the experiment with small domain size. The larger domain has produced rainfall of 41 cm as against observed rain rate of 56.3 cm. during 1200 UTC of 26 July to 0300 UTC of 27 July. Divergence, vorticity, vertical velocity and moisture parameters are examined in relation with the various stages of the event. The maximum values of convergence, vorticity and moisture fluxes precede the initial phase of mature stage, however vertical velocity follows the later phase of mature stage. Vorticity budget over the location of maximum rainfall, revealed the significant role of tilting term in maintenance and dissipation of the cloud complex responsible for the event. The model has simulated mixing ratios of ice, snow and hail up to height of 15 km which matches with the observations that clouds reaching up to 15 km were present at the time of event of heavy precipitation.  相似文献   

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