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1.
Summary ¶In order to better understand land-atmosphere interactions and increase the predictability of climate models, it is important to investigate the role of forest representation in climate modeling. Corresponding to the big-leaf model commonly employed in land surface schemes to represent the effects of a forest, a so called big-tree model, which uses multi-layer vegetation to represent the vertical canopy heterogeneity, was introduced and incorporated into the National Center for Atmospheric Research (NCAR) regional climate model RegCM2, to make the vegetation model more physically based. Using this augmented RegCM2 and station data for China during 1991 Meiyu season, we performed 10 experiments to investigate the effects of the application of the big-tree model on the summer monsoon climate.With the big-tree model incorporated into the regional climate model, some climate characteristics, e.g. the 3-month-mean surface temperature, circulation, and precipitation, are significantly and systematically changed over the model domain, and the change of the characteristics differs depending on the area. Due to the better representation of the shading effect in the big-tree model, the temperature of the lower layer atmosphere above the plant canopy is increased, which further influences the 850hPa temperature. In addition, there are significant decreases in the mean latent heat fluxes (within 20–30W/m2) in the three areas of the model domain.The application of the big-tree model influences not only the simulated climate of the forested area, but also that of the whole model domain, and its impact is greater on the lower atmosphere than on the upper atmosphere. The simulated rainfall and surface temperature deviate from the originally simulated result and are (or seem to be) closer to the observations, which implies that an appropriate representation of the big-tree model may improve the simulation of the summer monsoon climate.We also find that the simulated climate is sensitive to some big-tree parameter values and schemes, such as the shape, height, zero-plane displacement height and mixing-length scheme. The simulated local/grid differences may be very large although the simulated areal-average differences may be much lower. The area-average differences in the monthly-mean surface temperature and heat fluxes can amount to 0.5°C and 4W/m2, respectively, which correspond to maximum local/grid differences of 3.0°C and 40W/m2 respectively. It seems that the simulated climate is most sensitive to the parameter of the zero-plane displacement among the parameters studied.  相似文献   

2.
Summary Trends of monthly air temperature extremes were investigated in five meteorological stations of the Grand-Duchy of Luxembourg during the period 1949–1998. The application of an innovative homogenization method based on the concept of relative homogeneity to climatic time series allows identifying multiple break points, as well as correcting data series in an objective and robust statistical way. The rise of maximum temperature (Tmax) has occurred at a rate of 1.5 times that of the minimum temperature (Tmin) in winter (+1.4°C versus +0.9°C) and summer (+1.4°C versus +0.8°C). No trend in temperature extremes was found in autumn, while spring was affected by a small warming (+0.3°C) of Tmin and no change in Tmax resulting in a decrease of the diurnal temperature range (DTR) (–0.3°C). In spring, a strong positive linear relationship between Tmin warming and local terrain slope could be found. Comparison to new-gridded large-scale climatologies indicates generally close agreement to temperature trends during the 1949–1998 period, while a lower local warming was observed in summer during the post-1975 period following the changing-point year of atmospheric circulation over North-western Europe. This study shows that the question of data homogeneity is not trivial and should receive careful attention before quantifying historical temperature trends and identifying their spatial patterns at regional scale.  相似文献   

3.
Thermal comfort of man in different urban environments   总被引：2，自引：4，他引：2  
Summary On July 29, 1985, a hot summer day, biometeorological measurements were performed simultaneously in three different urban structures within the city of Munich and in the trunk space of a nearby tall spruce forest. Based on the results of these experiments the following thermophysiologically relevant biometeorological indices were calculated: Predicted mean vote, skin wettedness and physiologically equivalent temperature. These three indices are derived from different models for the human energy balance. They allow the assessment of the thermal components of the microclimates at the selected sites with regard to application in urban planning. The results quantitatively show the great heat stress in the urban structure street canyon, exposed to south, whereas in the trunk space of the tall spruce forest there is nearly an optimal climate even on hot summer days. Between these extremes the results for street canyon, exposed to north show a little higher heat load than for backyard with trees.
Zusammenfassung An einem heißen Sommertag, dem 29. Juli 1985, wurden in drei Stadtstrukturen in München und im Stammraum eines nahegelegenen Fichtenhochwaldes zeitgleich biometeorologische Messungen durchgeführt. Mit den Meßergebnissen wurden folgende thermophysiologisch relevante biometeorologische Indizes berechnet: Predicted mean vote, Hautbenetzungsgrad und physiologisch äquivalente Temperatur. Diese drei Indizes beruhen auf verschiedenen Modellen zur menschlichen Energiebilanz. Mit den drei Indizes wurden die thermischen Komponenten der Mikroklimate an den ausgewählten Meßplätzen im Hinblick auf Stadtplanungsaufgaben bewertet. Die Ergebnisse zeigen quantitativ die relativ große Hitzebelastung bei der Stadtstruktur Straßenschlucht, nach Süd exponiert, während im Stammraum des Fichtenhochwaldes selbst an heißen Sommertagen nahezu optimale Bedingungen herrschen. Zwischen diesen Extremen liegen die Ergebnisse für die anderen Meßplätze, wobei für Straßenschlucht, nach Nord exponiert die Wärmebelastung etwas höher als für Innenhof mit Bäumen ist.

With 6 Figures  相似文献   

4.
A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO3 (0.01), N2O5 (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.  相似文献   

5.
In the summer of 1988/89 flights were carried out in the Coorong coastal area of South Australia to investigate sea-breeze fronts. The flights yielded data sets of the structure of the fronts in the cross-frontal direction with a spatial resolution of approximately 3 m. The study is focused on the budgets of sensible and latent heat in the vicinity of the front and on frontogenesis/frontolysis processes which are closely related to budget considerations.The frontogenesis relationships and the budgets were established on a 2 km length scale by low-pass filtering of the space series. As the wind components were measured with high accuracy, all processes which determine frontogenesis could be evaluated and are displayed in x,z-cross-sections: these are the confluence, shear and diabatic effects, all of which play a role in q/x-, q/z-, /x- as well as /z-frontogenesis. A detailed analysis is given for two different states of frontal development. The presented results shed much light on the governing physical processes in the frontal region with strong emphasis on the effects of confluence-generated updrafts, on shear instabilities causing bulges and clefts in the frontal surface as well as producing the elevated frontal head, and on processes related to differential heating and moistening.  相似文献   

6.
Parameterization of evaporation from a non-plant-covered surface is very important in the hierarchy strategy of modelling land surface processes. One of the representations frequently used in its computation is the resistance formulation. The performance of the evaporation schemes using the , , and their combination resistance approaches to parameterize evaporation from bare soil surfaces is discussed. For that purpose, the nine schemes, based on a different dependence of and on volumetric soil moisture content and its saturated value, are used.The tests of performances of the considered schemes are based on time integrations by the land surface module (BARESOIL) using observed data. The 23 data sets at a bare surface experimental site in Rimski anevi, Yugoslavia on chernozem soil, were used for the resistance algorithm evaluation. The quality of the schemes was compared with the observed values of the latent heat flux using several statistical parameters.  相似文献   

7.
Airborne measurements made during August 1985 over Greenland and its environs show that both accumulation-mode (0.1 m D2.0 m) and giant (D2 m) particles were present in relatively high concentrations in arctic haze layers and that the accumulation-mode particles dominated light scattering. Particles with diameters (D) between 1 and 4 m consisted predominately of mixed materials, small and dense inclusions, and probably organic compounds containing sulfur. Many of the particles from 0.1 to 1 m in diameter were also of mixed composition, with sulfuric acid, ammonium sulfate and organics probably the dominant constituents.  相似文献   

8.
Summary Statistical characteristics of extremely low and high daily mean temperatures in summer (June, July and August) in eastern China have been investigated. The extremely low temperatures are defined as those days with temperatures not exceeding the 10th percentile with respect to the reference period of 1961–90; similarly the extremely high temperatures are defined as those exceeding the 90th percentile. There are well-defined spatial structures in trends of the frequency of extremely low temperatures as well as of high temperature extremes. In the north region (i.e. northern and northeastern China) the linear trends of frequency of low and high temperature extremes are –1.09 and +1.23 days/10yr, respectively. For the southern portion of the study area, the trends are –1.32 and –2.32 days/10yr. Taking the study area as a whole, the linear trends are –0.76 days/10yr and +1.08 days/10yr, respectively. The changes of frequency of extreme temperatures are mainly related to the shift in the temperature means. There is a dominant anticyclonic pattern in the lower- to middle troposphere over East Asia in association with warmer conditions in the north region. For the south region there is a jump-like change in the summer mean temperature and the extreme temperature events in around 1976. The large-scale northwestern Pacific subtropical high plays an important role in the jump-like changes of the temperature extremes.  相似文献   

9.
Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.  相似文献   

10.
Summary The influence of agricultural management on the CO2 budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO2 fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm–2 on the managed meadow, and 270±24g Cm–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO2 uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.  相似文献   

11.
Three million years of monsoon variability over the northern Sahara   总被引：8，自引：0，他引：8  
We present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of 400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the eastern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (21°N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan watershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from green Sahara climate models. Our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years. As would be expected, this orbital precession-scale mechanism is modulated on both short (100-kyr) and long (400-kyr) eccentricity time scales. We also observe a strong expression of the 41-kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also observe a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (0.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.  相似文献   

12.
Parameterizing turbulent diffusion through the joint probability density   总被引：3，自引：3，他引：0  
The convective mass flux parameterization often used in meteorological modeling expresses the vertical flux of a transported scalar as proportional to the product of the difference in mean values of the scalar in updrafts and downdrafts and their characteristic velocity. The proportionality factor is a constant to be specified. We show that this proportionality factor also appears in the relaxed eddy accumulation technique of Businger and Oncley. That associates the surface-layer flux of a scalar with the product of the standard deviation of vertical velocity and the mean concentration difference between updrafts and downdrafts.We show that this constant (b) is determined uniquely by the joint probability density (jpd) of vertical velocity and the scalar. Using large-eddy simulation, we generate this jpd for a conservative scalar diffusing through a convective boundary layer. It has quite different forms in top-down and bottom-up diffusion geometries. The bottom-up jpd is fairly well represented by a jointly Gaussian form and implies b ~ 0.6, in good agreement with the surface-layer value reported by Businger and Oncley. The top-down jpd is strikingly non-Gaussian and gives b ~ 0.47. Updrafts carry the bulk of the scalar flux - 70% in the bottom-up case, 60% in the top-down case.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

13.
Summary A simple parameterization for the estimation of turbulent kinetic energy (TKE) and momentum flux profiles under near-neutral stratification based on sodar measurements of the vertical velocity variance has been tested using data from the LINEX-2000 experiment. Measurements included operation of a phased-array Doppler sodar DSDPA.90 and of a sonic anemometer USA-1 mounted at a meteorological tower at a height of 90m. Good agreement has been found between the TKE and momentum flux values derived from the sonic and sodar data (with correlation coefficients r>0.90 and a slope of the regression lines of about 1.01.1) suggesting the possible use of sodar measurements of w 2 to derive turbulence parameter profiles above the tower range.  相似文献   

14.
Summary Mechanisms associated with Alpine lee cyclogenesis during the early phase of their generation are investigated using a variational quasigeostrophic filter technique. It was possible to extract the quasigeostrophic signal from the available analyzed real data set.The results presented here are for the 11–12 March 1982, an example of so-called orographically induced lee cyclogenesis. Non-quasigeostrophic fields, calculated as a difference between observations and the quasigeostrophic fields, show significant magnitudes indicating the possible importance of non-quasigeostrophic processes. A dipole structure in the residual geopotential field was observed, similar to the results of numerical model experiments. Also, a strong upper-level non-quasigeostrophic divergence was found in the Alpine region 24 hours prior to lee cyclogenesis, lasting for 6–12 hours. On the other hand, quasigeostrophic results indicate only a local effect of mountain slopes, suggesting possibly a dominant role of the low-level blocking. A hypothetical scenario of Alpine lee cyclogenesis is proposed, based on results obtained here.With 14 Figures  相似文献   

15.
Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and aerosol carbon to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (SO) ranged from 0.52 to 0.99 with an average of 0.80. Aerosol carbon was scavenged less efficiently with an average value (AC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both SO and AC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, SO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of 0.3 g/m3. In the case of aerosol carbon, we obtained a more gradual increase of AC up to an LWC of 0.5 g/m3. At higher LWCs, _ remained relatively constant at 0.60. As the differences between SO and A varied across the LWC range observed at SBO, we assume that part of the aerosol carbon was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.  相似文献   

16.
Summary Interannual modes are described in terms of three-month running mean anomaly winds (u,v), outgoing longwave radiation (OLR), and sea surface temperature (T * ). Normal atmospheric monsoon circulations are defined by long-term average winds (u n,v n) computed every month from January to December. Daily winds are grouped into three frequency bands, i.e., 30–60 day filtered winds (u L,v L); 7–20 day filtered winds (u M,v M); and 2–6 day filtered winds (u S,v S). Three-month running mean anomaly kinetic energy (signified asK L , K M , andK S , respectively) is then introduced as a measure of interannual variation of equatorial disturbance activity. Interestingly, all of theseK L , K M , andK S perturbations propagate slowly eastward with same phase speed (0.3 ms–1) as ENSO modes. Associated with this eastward propagation is a positive (negative) correlation between interannual disturbance activity (K L , K M , K S ) and interannualu (OLR) modes. Namely, (K L , K M , K S ) becomes more pronounced than usual nearly simultaneously with the arrival of westerlyu and negativeOLR (above normal convection) perturbutions. In these disturbed areas with (K L , K M , K S >0), upper ocean mixing tends to increase, resulting in decreased sea surface temperature, i.e.T * 0. Thus, groups (not individual) of equatorial disturbances appear to play an important role in determiningT * variations on interannual time scales. HighestT * occurs about 3 months prior to the lowestOLR (convection) due primarily to radiational effects. This favors the eastward propagation of ENSO modes. The interannualT * variations are also controlled by the prevailing monsoonal zonal windsu n, as well as the zonal advection of sea surface temperature on interannual time scales. Over the central Pacific, all of the above mentioned physical processes contribute to the intensification of eastward propagating ENSO modes. Over the Indian Ocean, on the other hand, some of the physical processes become insignificant, or even compensated for by other processes. This results in less pronounced ENSO modes over the Indian Ocean.With 10 FiguresContribution No. 89-6, Department of Meteorology, University of Hawaii, Honolulu, Hawaii.  相似文献   

17.
Summary The evolving modes of the sea-surface temperature (SST) in the Tropical Atlantic on the short interannual (IA) timescale were obtained by performing the extended empirical orthogonal function (EEOF) analyses on this variable separately for the 106-year (1871–1976) and 20-year (1881–1900; 1901–1920; 1921–1940; 1941–1960) periods. The equatorial and inter-hemispheric patterns manifest in the first EEOF mode of each analysis as part of the short IA evolution of the SST anomalies in the Tropical Atlantic. Another outstanding feature of the first EEOF mode of each analysis concerns the propagations of the SST anomalies in the meridional direction within the 20°N–20°S band and in the zonal direction in the sector 40°W–20°W. For all analyses, the SST anomalies propagate northward from the equator to 15°N and southward from 20°N to 15°N, with the same sign anomalies merging approximately at 15°N. On the other hand, the SST anomalies propagate westward in the sector 40°W–20°W with a propagation rate close to that of the phase speed of the fastest baroclinic Rossby wave in the ocean. So, the observed propagations of the SST anomalies in the 20°N–20°S band might result from the combined effect of the surface oceanic currents in this band and the baroclinic Rossby waves in the ocean.  相似文献   

18.
The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (Fa) and atmospheric moisture diffusivity (Kq). In a large ensemble of spinup experiments, the values of Fa and Kq are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of Fa and Kq, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.  相似文献   

19.
Periodicity of annual precipitation in different climate regions of Croatia   总被引：1，自引：1，他引：0  
Summary The periodicity of a 100-year series of annual precipitation over Croatia has been studied by means of power spectrum analysis at 3 stations representing the different climatic regions of Croatia. The annual precipitation variance spectra in the continental lowland (Osijek) and at the north East Adriatic coast (Crikvenica) can be fitted by Markov white noise continuum, but in the transitional region between the Dinaric Alps and the Pannonian lowland (Zagreb-Gri) a non-white noise continuum is necessary. Quasi-periodic oscillations appear in two spectra ranges: short (2.2 and 4.7 years) and medium (25.0 and 33.3 years). These results are compared with those of other authors for other parts of the Europe.With 2 Figures  相似文献   

20.
Influence of heterogeneous land surfaces on surface energy and mass fluxes   总被引：1，自引：0，他引：1  
Summary Land-surface heterogeneity affects surface energy fluxes. The magnitudes of selected land-surface influences are quantified by comparing observations with model simulations of the FIFE (First ISLSCP Field Experiment) domain. Several plausible heterogeneous and homogeneous initial and boundary conditions are examined, although soilmoisture variability is emphasized. It turns out that simple spatial averages of surface variation produced biased flux values. Simulated maximum latent-heat fluxes were approximately 30 to 40 W m–2 higher, and air temperatures 0.4 °C lower (at noon), when computations were initialized with spatially averaged soil-moisture and leaf-area-index fields. The planetary boundary layer (PBL) height and turbulent exchanges were lower as well. It additionally was observed that (largely due to the nonlinear relationship between initial soil-moisture availability and the evapotranspiration rate), real latent-heat flux can be substantially less than simulated latent-heat flux using models initialized with spatially averaged soil-moisture fields. Differences between real and simulated fluxes also vary with the resolution at which real soil-moisture heterogeneity is discretized.With 8 Figures  相似文献   

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