首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Summary In this paper, the relationship between seasonal mean (June, July, August and September) monsoon circulation features and the midlatitude circulations in winter and spring seasons have been examined during contrasting years of more (less) number of snow days in winter/spring followed by deficient (excess) Indian Summer Monsoon Rainfall (ISMR) using NCEP/NCAR reanalyzed data for the period 1966–1994. The Historical Soviet Daily Snow Depth (HSDSD) version II data set has been used to calculate the number of days of snow over west and east Eurasia separately under three classes: class 1 for SD>5cm, class 2 for SD>10cm and class 3 for SD>50cm where SD stands for snow depth. Correlation coefficients are computed between the anomaly in the number of days of snow depth under the above three classes during winter/spring over west and east Eurasia and the subsequent ISMR. HSDSD data show that difference in the number of days of SD>10cm in two extreme years is most prominent in the west Eurasia in the months of January and April. Also the anomaly in the number of days of snow in January and April over west Eurasia has correlation coefficients of –0.69 and –0.56 with the following ISMR, respectively at 0.1% significance level when the SD is more than 10cm at all the stations. Results also show that low-level atmospheric temperature difference between two extreme years of snow days in winter is up to 10°C and the cooling persists up to spring season with a difference of 2°C. This cooling persistence may give rise to anomalous cyclonic circulations over the midlatitudes and tropics which may be responsible for weakening the monsoon circulation over India during the year of more snow days over west Eurasia.  相似文献   

2.
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   

3.
Summary This study used monthly rainfall totals for the period 1961 to 1988 and pentad OLR values for the period 1974 to 1991 to study the structure and transition of active convention across the Congo Basin (10°S–5°N, 15°E–35°E) from the southern to the northern hemisphere summer. This involved the examination of map patterns and cross-sections of monthly rainfall and pentad OLR data.The results from the study indicated that there were two seasons observed over the Congo Basin; one is the wet season lasting from September to April and the other a dry season covering the rest of the year. The onset of the wet season takes place rapidly with active convection spreading very quickly to the south near latitude 20°S. This is due to the formation of the meridional (north–south) branch of the ITCZ over this region.This study has confirmed that the annual rainfall over the Congo Basin is reliable with the coefficient of variation of less than 30%. The wet seasons (e.g., SON and DJF) also show reliable rainfall occurrence but the dry season (e.g., JJA) has low reliability.The anomalously wet seasons are characterised by a relatively slow transition rate (1° latitude per pentad) of zones of active convection resulting in a late withdrawal of the rainy season while the dry seasons show a rapid transition rate with an early withdrawal of zones of active convection.High-rainfall months (>200mm) are concentrated within the Southern Hemisphere summer months. These high-rainfall months progress from the equator to the southern latitude following generally the movement of the overhead sun.The results further revealed that the years 1987/1984 had the lowest/highest mean OLR values over the Congo Basin within the period 1979 to 1991. The rates of transition of the zones of low OLR values were 0.9/5.0 degrees of latitude during 1987/1984, respectively.Received June 18, 2002; revised September 30, 2002; accepted November 21, 2002 Published online: June 12, 2003  相似文献   

4.
Summary ¶The 0°C isotherm height, a parameter needed for the estimation of attenuation of microwave and millimetre wave for earth-space communication, has been estimated for different stations spread over India. The variations of 0°C isotherm height for different seasons over these stations are presented. Attenuations of radio wave due to rain at frequencies 10GHz and above have also been estimated for few stations using the 0°C isotherm height so derived. The results are useful for radio systems designers.  相似文献   

5.
Summary Based on observed rainfall data of India Meteorological Department (IMD), correlation coefficients (CCs) have been computed between Indian summer monsoon rainfall (ISMR) and sea surface temperature (SST) anomalies over different Nino regions and standardised pressure difference between Tahiti and Darwin. Significant positive CCs are found between the Southern Oscillation Index (SOI) in winter and subsequent June rainfall over India. Concurrent with and subsequent to Indian summer monsoon, SOI shows significant positive CC with the mean rainfall of July to September (JAS). Significant negative CCs are found between JAS mean rain and the concurrent and following SST anomalies over Nino-3.4 region. On the basis of these correlations, it is proposed that the entire period of summer monsoon from June to September could be divided into two sub-periods such as: early summer (June) and mid-late summer (July to September) monsoon for prediction of ISMR in the extended range.In order to examine the characteristics of atmospheric circulation during some El-Nino years, divergent flow at 200hPa and omega field at 500hPa based on NCEP/NCAR reanalysis have been studied in detail. Major significant southward shift of upper level divergent field from India are related to El-Nino and this shift may be responsible for causing droughts during several El-Nino years over India. Also vertical wind fields at 500hPa show sinking motion over large parts of India and west Pacific and ascending motion over southern Indian Ocean, central and eastern Pacific during major drought years.  相似文献   

6.
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   

7.
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   

8.
Summary This study uses a 1°×1° lat/long dataset, extracted from ECMWF re-analyses for the 15-year period 1979–1993 (ERA-15), to diagnose the synoptic-scale kinematic, thermodynamic and moisture environments in the vicinity of named tropical cyclones (TCs) in the eastern North Pacific. Based on the NCDC best track dataset, TCs are partitioned into one of three categories: weak (W), strong (S) or intensifying (I). In total, 63TCs are examined: 8Ws and 20Is at point A (maximum intensification) and 11Ws, 13Ss and 11Is at point B (maximum frequency). Composite maps are compiled for all five groups, and six individual case studies are examined, four for extreme TC cases and two for cases involving dry air intrusions.For the most part, peak values and patterns of composited ERA-15 variables display circulation, thermodynamic and moisture characteristics that are compatible with the strength represented by a groups classification. Intercomparison between Ws and Is at points A and B yielded larger conditional instability of low-level air parcels and upper-level outflow within the region of maximum intensification (point A).The intrusions of dry versus moist mid-level air are addressed for each storm with the assistance of 72-hour backward trajectories. Trajectory density maps indicate two preferred paths of air parcels that reach the environment of W storms at point A on the 700 and 500hPa levels. The first one crossed Central America in the region of the Isthmus of Tehuantepec and the second one south of the Central American mountains. Several storms revealed that these trajectories were associated with dry air intrusions into the larger storm area, and this might be one reason for their weak status at point A. One documented example is Kevin (1985). By the time it reached point B, the dry air was replaced by air that was moist and Kevin intensified, although it remained a W system. In contrast, Narda (1989) received a dry air intrusion from Central Mexico at 500hPa as a weak storm at point B and did not intensify. Despite possible analyses problems, the documentation in this study of mid-level dry air intrusions into eastern Pacific TCs from the Mexican-Central American region suggests a hitherto unexploited forecast potential. Received January 15, 2002; revised November 28, 2002; accepted December 19, 2002 Published online: May 8, 2003  相似文献   

9.
Summary This study examines the impact of ice formation and growth processes on freezing drizzle formation in stably stratified clouds. In particular we investigate the reason why freezing drizzle is rarely observed in clouds with top temperatures less than –15°C. We also investigate the sensitivity of freezing drizzle formation to the Hallett Mossop secondary ice process (Hallet and Mossop, 1974). The evaluation is performed by simulating cloud formation over a two-dimensional idealized mountain using a detailed microphysical scheme. The height and width of the two-dimensional mountain were designed to produce an updraft pattern with extent and magnitude similar to documented freezing drizzle cases. The simulations show that: (i) drizzle formation is very sensitive to the ice crystal concentration, with a significant reduction in the area over which drizzle forms and the maximum drizzle water content as the cloud top temperature decreases below –10°C, and (ii) secondary ice crystal formation has a significant effect on drizzle formation at cloud top temperatures below –10°C.The above two factors are likely the main cause for the lack of freezing drizzle at cloud top temperatures less than –15°C. We also found that neglecting the depletion of ice forming nuclei resulted in considerable overestimation of the ice crystal concentration and suppression of drizzle, even for the –10°C case.  相似文献   

10.
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   

11.
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   

12.
Summary The Earths local fair-weather electric field is significantly affected by small ions present in the atmosphere. These ions are typically smaller than 0.001µm and occur in concentrations from 500 to 600cm–3 in air. Attachment to larger aerosol particles may severely decrease the mobility of these atmospheric ions resulting in an increased local electric field. The number concentration of environmental aerosol particles in the size range 0.1 to 5.0µm was measured with two automatic laser scattering particle counters. The Earths electric field was monitored with an electric fieldmeter. Measurements were made in clean air and in an environment highly polluted by wood smoke. The electric field was found to be positively correlated to the aerosol number concentration. During one 24-hour period of measurement, the electric field increased from 180 to about 280Vm–1 as the number concentration of aerosols larger than 0.1µm increased from about 2000 to 9000cm–3. The number concentrations of aerosols larger than 0.1 and 0.3µm were both found to be positively correlated with the Earths electric field with correlation coefficients of 70% and 61%, respectively.Present address: School of Physical Sciences, Queensland University of Technology, Brisbane 4001, Australia.  相似文献   

13.
The convective boundary layer over pasture and forest in Amazonia   总被引：1，自引：0，他引：1  
Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.  相似文献   

14.
Trends of summer dry spells in China during the late twentieth century   总被引：5，自引：0，他引：5  
Summary In the present study the trends in frequency and duration of dry spells in six sub-regions of China were analyzed for the summer-half-year season (April–September) in period 1956–2000. A dry spell was defined as a number of consecutive days without measurable precipitation. For the frequency of short dry spells (length <10 days), significant changes are observed in the North, Northeast and Southwest China. For the frequency of long dry spells (length 10 days) there are significant trends in North and Northeast China; while no remarkable trends in frequency are found in other regions. There are also significant lengthening trends in dry spell duration in North and Northeast China, resulting mainly from the long-term changes in short dry spells. No significant change is observed for the maximum length in all regions. It is found that the temporal distribution of precipitation within the rainy season would notably impact the features of dry spells. An increase in the precipitation amount does not necessarily mean a synchronous reduction in dry spell frequency and/or duration. Seasonal mean anomalies of 500hPa heights in association with the long dry spells show similar spatial patterns over the middle to high latitudes for five of the six sub-regions (with exception of the case of Southwest China), resembling a west–east direction dipole in latitudes about 30°N northwards. For the case of Southwest China the dominant feature in 500hPa heights is the negative anomalies over most middle to high latitude Asia. Among these cases there are recognizable differences, particularly, in the tropical regions in western Pacific. That would provide useful information of circulation background for understanding the climate extremes.  相似文献   

15.
Summary The Tierras Bajas regions of eastern Santa Cruz, Bolivia have undergone among the most rapid rates of concentrated deforestation during the 1980s and 1990s. We investigate the sensitivity of local climate to these land cover changes as observed from Landsat images acquired between 1975 and 1999. The Simple Biosphere model (SiB2) is used to assess the effects of both morphological and physiological changes in vegetation and the implications for fluxes of water, energy and carbon between the vegetation and the atmosphere during the rainy season.Conversion from tropical forest to cropland implicates morphological changes in vegetation as the primary drivers for a daily maximum warming of about 2°C and a slight nighttime cooling, suggesting that clearing of tropical forests for agricultural use may increase the diurnal temperature range, mainly by increasing the maximum temperature. On the other hand, the conversion of wooded grassland to cropland resulted in a similar daily warming and drying but exclusively due to vegetation physiological activity.The area-averaged monthly mean response for each conversion type resulted in a warming of about 0.6°C for the conversion of broadleaf evergreen and 1.2°C for conversion of wooded grassland. These temperature differences represent an augmentation in the local heat source associated with a reduction in evapotranspiration due to land cover conversion and do not reflect variations forced by changes in atmospheric circulation.When averaged over the entire domain, the effect of landscape conversion results in a reduction of the latent heat flux and an increase in sensible heat flux, producing a large-scale apparent heat source of 0.5°C during January. This warming is in line with an increasing trend observed in monthly mean temperature in Santa Cruz, Bolivia during the same period.  相似文献   

16.
Summary The temperature T of a black or gray body orbiting the Sun can be expressed in terms of spherical harmonics in latitude and longitude, its Keplerian orbital elements, and a variable describing rotation about its axis. Assuming that the Earth is a black or gray body without thermal inertia, the resulting equation for T exhibits previously unrecognized odd-degree zonal terms dubbed Seversmith psychroterms. They cause a hemispheric temperature difference which depends upon e sin S, where e is the orbital eccentricity and S is the Suns argument of perigee measured in an Earth-centered frame. The hemisphere containing perihelion is the cooler. For a gray body with the Earths average albedo of 0.3, an emissivity of unity, and an obliquity of 23.5°, the pole-to-pole temperature difference for the combined first and third degree spherical harmonic psychroterms can reach 3.4K for the present eccentricity of 0.016, and 12.9K for the maximum eccentricity of 0.06. While a thermally inertia-less black or gray body with boiling hot subsolar points and nights at absolute zero are poor models for the Earth, the Seversmith psychroterms will survive in more realistic models (although with smaller amplitudes) because the Earth radiates nonlinearly in T. The psychroterms acts in the direction opposite to the Milankovitch precession index, which also depends on e sin S: by warming the cool northern summers, the psychroterms make it harder for the traditional Milankovitch mechanism to operate. The Seversmith psychroterms could in fact be responsible for the ice sheets that cycle with e sin S, instead of the Milankovitch mechanism. By cooling the southern hemisphere for thousands of years when perihelion is in the south, the psychroterms may somehow cause the southern hemisphere to control the northern ice sheets associated with the 23kyr and 19kyr periods (kyr=103 years), possibly through ice-albedo feedback in the sea ice surrounding Antarctica. Two other unexpected features besides the psychroterms are: while the average insolation increases with increasing e, the average temperature of the Earth paradoxically decreases; and the equator-to-pole temperature difference of 21K on a gray body with an albedo equal to 0.3 and an emissivity of unity is actually smaller than the observed difference of 28K on the real Earth.  相似文献   

17.
Summary The skill of the FSU Superensemble technique as applied to global numerical weather prediction is evaluated extensively in this paper. The global mass and motion fields for year 2000 and precipitation over the domain 55S to 55N for year 2001, as predicted by the Superensemble, the ensemble member models, and the mean of the ensemble members, are evaluated by standard statistical measures of skill to determine the performance of the Superensemble in relation to the other models. The member models are global forecast models from 5 of the worlds operational forecast centers in addition to the FSU global spectral model. For precipitation 5 additional versions of the FSU global model are utilized in the ensemble, as defined by different initial conditions provided by various physical initialization algorithms. Statistical parameters calculated for the mass and motion fields include root mean square (RMS) error, systematic error (or bias), and anomaly correlation. These are applied to the mean sea level pressure, 500hPa heights, and the wind fields at 850hPa and 200hPa. Statistical parameters that were calculated for precipitation include RMS error, correlation, equitable threat score (ETS), and a special definition of bias appropriate for the precipitation field. For the mass and motion fields the performance of the Superensemble was considered for the annual global case, as well as for each hemisphere (north and south) and for each of the four seasons. For precipitation only the annual case was considered over the domain cited above.For the mass and motion fields the RMS calculations showed the Superensemble to be superior (to have the smallest total forecast error) in all comparisons to the ensemble member models, and to be superior to the ensemble mean in the vast majority of comparisons. Performance in comparison to the other models was generally better in the Southern Hemisphere than in the Northern Hemisphere, and better in the transition seasons of fall and spring than in the extreme seasons of winter and summer. The Superensemble had the best success with mean sea level pressure, followed in order by 500hPa geopotential heights, 850hPa winds, and 200hPa winds.In the calculations of 500hPa geopotential height anomaly correlation the Superensemble had higher scores in all comparisons to the ensemble member models, as well as higher scores in the majority of comparisons to the ensemble mean. As with the RMS error results, the Superensemble performed better in the Southern Hemisphere than in the Northern Hemisphere, and better in fall than in summer, in comparison to the other models. The superior anomaly correlation scores of the Superensemble attest to the ability of the model to forecast daily perturbations from the climatological means, perturbations that are associated with transient synoptic scale features, given the horizontal resolution in the forecast models.In terms of systematic error reduction the Superensemble produces its most impressive results. Annual global mean sea-level pressure systematic errors for day 5 forecasts are generally in the range of ±1hPa (compared to errors as high as 8hPa in other models), and day 2 forecasts of 500hPa geopotential height produced systematic errors generally in the range of ±10 meters (compared to errors as high as 60 meters in other models). The Superensemble was able to reduce systematic errors in forecasts of a variety of important features in the global mass and motion fields: surface equatorial trough, wave amplitude in geopotential heights at 500hPa, trade winds and Somali Jet at 850hPa, mid-latitude westerlies, subtropical jet, and Tropical Easterly Jet (TEJ) at 200hPa.In terms of forecasting precipitation the Superensemble outperforms all ensemble member models and the ensemble mean in terms of RMS error, correlation coefficient, equitable threat score, and bias. The superior correlation scores indicate that the Superensemble is more reliable than the other models in predicting perturbations in the area distribution of precipitation, perturbations that are essentially associated with migrant synoptic scale disturbances, considering the horizontal resolution of the forecast models.The Superensemble is a valuable tool for significantly improving upon the global model forecasts of the worlds operational forecast centers. These forecasts are used daily as important guidance in making weather forecasts in all regions of the world. This paper will demonstrate that the Superensemble improves upon the ensemble member model forecasts: (1) in a statistical sense considering broad areas of the globe, (2) in a synoptic climatology sense through focus on the improved forecasts of climatological features seen in the global mass and motion fields, (3) in a synoptic sense through use of anomaly correlation and correlation coefficient where improvement is demonstrated in the forecasts of perturbations from mean fields which are essentially associated with transient synoptic scale disturbances.  相似文献   

18.
Summary ¶During the Post-TAMEX forecast experiment of Taiwan in 1992, a mesoscale convective system (MCS) developed on June 5–6 over southern China. As this system matured, it produced readily apparent cirrus outflow on satellite imageries while the upper level flow also exhibited a diffluent pattern. The purpose of the current study is to examine the possible changes in its environment associated with the development of this MCS.By using 12-h data from 1200 UTC June 5 to 1200 UTC June 6, objective analyses were performed for a 1°×1° latitude/longitude grid using sounding data and a low-pass filter. To facilitate the diagnosis, a band-pass filter was further applied to separate mesoscale features from macroscale ones, while the apparent heat source and apparent moisture sink defined by Yanai et al (1973) were also calculated.Results suggest that the MCS exerted clearly discernable effects on its environment. The latent heat release led to the development of a warm core and mesoscale high-pressure disturbance at upper levels when the system matured. Ageostrophic winds and diffluent flow patterns together with strong anticyclonic vorticity at 200hPa near the MCS were associated with the mesohigh. After the mature stage, weak cooling occurred above 350hPa, likely due to radiative emission from the cloud top. However, a mid-level cyclonic vortex, often present in MCSs over the North America, was not apparent here due to weak environmental vorticity and small Coriolis parameter f. The level of maximum divergence was initially located at 500hPa, but rose to 200hPa as the MCS matured. In response, the upward motion not only intensified, but the level at which strongest rising occurred also ascended from 700 to 350hPa. Results from the apparent heat source and moisture sink calculation suggest that this slow ascent of maximum heating was partially due to vertical transport of sensible heat by updrafts.During the MCSs mature stage, under the stratiform clouds to the west of the strongest convection, a cold mesohigh formed at the surface due to evaporative cooling in downdrafts, and a gust front appeared along the leading edge of the outflow boundary. A trailing mesolow was also observed, likely due to near-adiabatic warming in drier downdrafts since no precipitation was associated with it.Received April 11, 2002; revised May 27, 2002; accepted July 14, 2002 Published online: April 10, 2003  相似文献   

19.
Summary The study on the characteristics of aerosol in Seoul during springtime from 1998 to 2003 is performed by the size-resolved number concentrations of aerosol. Asian dust events occur in spring most frequently, but it has been often observed in wintertime since 1999. Since 2000, the number of Asian dust days has been increasing, and the intensity has been more severe until 2002. However, there were only 3 dust days in Seoul during the spring of 2003, since the synoptic cyclone was relatively not intense enough to rise and transport dust to Korean peninsula, and the air stream was usually tiled to north of Korean peninsula. In addition, the precipitation was relatively plentiful and the air temperature was cold enough not to keep dry soil condition.Haze is the suspended particles in the air, reducing visibility by scattering light, and it is often a mixture of aerosols and photochemical smog. Dry particles with diameters of the order of 0.1µm, are small enough to scatter short wavelengths of light. Haze occurs well in winter and spring, and severe haze is observed in the afternoon. The occurrence frequency of haze has been decreasing since 2000 except in May of 2003.During Asian dust events from 1998 to 2003, the number concentration of aerosol with diameters from 0.3µm to 0.5µm decreases notably, but that larger than 1µm increases rapidly. On the other hand, for the haze events the number concentration from 0.3µm to 0.5µm increases notably, but that larger than 1µm decreases.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号