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1.
Summary  At times, a pronounced trough of low barometric pressure extends from equatorial Africa northward, over the Red Sea and the eastern Mediterranean countries, i.e., the Red Sea Trough. The associated weather is usually hot and dry, and consequently the atmosphere becomes conditionally unstable. In cases in which additional moisture is supplied and dynamic conditions become supportive, as the case analyzed here, intense thunderstorms occur, with extreme rain rates, hail and floods. The storm herein analyzed caused extensive damage both in casualties and property and evolved in two main consecutive phases: In the first a Mesoscale Convective System that moved from Sinai northward over Israel dominated, and in the second deep convection was organized mainly along a cold front. Data analysis indicates several synoptic-scale factors that had a supportive effect on the storm formation and intensification: Conditional instability established by the Red Sea trough, mid-level moisture transport from Northern Africa, and upper-level divergence imparted by both polar and subtropical jet streams over the Middle-East. Mesoscale features were further investigated by means of a hydro-meteorological observational analysis with high spatio-temporal resolution using raingauge and radar data, and satellite imagery. It is shown that local factors, particularly topographic effects, play a major role in the evolution, intensity and spatial organization of the convective activity. Our findings support results of a numerical study of another autumn rainstorm associated with the Red Sea trough. In the present case we identify an additional contributing factor, i.e., a mid-latitude upper-level trough that further intensified the storm as it was approaching the Middle-East. Received July 4, 2000 Revised January 16, 2001  相似文献   

2.
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   

3.
Summary The study examines regional atmospheric model (RM) simulations of the mean June–September (JJAS) climate and the implications of the mean state for the model representation of African wave disturbances (AWD). Two simulations are made with a version of the RM that computes soil moisture with multivariate functions that statistically relate it to rainfall, surface temperature, albedo, vegetation and terrain slope. These simulations differ according to the assignment of ground temperatures (Tg). While the control experiment is based on the fully interactive computation of soil moisture, the second experiment tests the response to fixed Tg whose seasonal means are more realistic than in the control within a swath along the Gulf of Guinea coast. A third simulation is made with the RM coupled to a sophisticated land surface process model (RM2). Results show a rather acute sensitivity of the mean circulation to land surface processes. The more realistic meridional temperature gradient created by fixing Tg in turn increased the vertical wind shear over West Africa and eliminated unrealistic westerly circulation at 700mb. AWD composites were transformed from intense closed cyclonic circulations with copious rainfall to more realistic open waves that organized more moderate precipitation maxima. Lower vorticity variances in the specified change experiment imply that the open waves were characterized by more moderate vorticity extremes. Corresponding spectral amplitudes for 3–6 day periodicities of the 700mb meridional wind were 40–80% of control values within the swath of maximum AWD activity. From among the three simulations, RM2 achieved seasonal mean precipitation, temperature, energy flux and circulation distributions that, despite some unrealistic features, were closest to observational evidence. RM2 AWD were much less intense and favored slightly longer periods. Results demonstrate that discrepancies in modeled ground temperatures caused by underestimating the cloudiness that intercepts short wave solar flux along a narrow swath of the West African coast have far-reaching consequences for the simulation of both the mean summer climate and individual synoptic disturbances. The study implies that this cooling along the Gulf of Guinea coast prevents AWD from developing into more intense storms with heavier precipitation.  相似文献   

4.
Observations from research ships which took part in the Indo-Soviet Monsoon Experiment of 1977 (MONSOON 77) and the International Monsoon Experiments (MONEX 79) over the central Arabian Sea and the north central Bay of Bengal were analyzed to study the mean wind and temperature structure of the monsoon boundary layer during active and break conditions. Mean profiles of wind speed and direction along with virtual potential temperature obtained by averaging data from several research ships during 1977 and 1979 indicate that onset conditions were associated with substantial increases in wind speed over the Arabian Sea and a shift to strong southwest flow. Monsoon onset was also characterized by near-neutral to slightly unstable temperature profiles in the lowest kilometer. Break conditions in 1977 in which the monsoon trough moved northward and substantial (5 mb) pressure rises were noted over the Arabian Sea show wind speeds typically decreasing from approximately 18 m s–1 during active conditions to roughly 8 m s –1. Temperature profiles during break conditions are similar to those observed in pre-monsoon conditions in that the boundary layer is observed to be generally much more stable up to 900 mb. Above 900 mb, profiles of virtual potential temperature show little variation.Analysis of latent and sensible heat fluxes during June 1977 calculated by the bulk aerodynamic method indicates values of latent heat flux during active conditions to be roughly two to three times larger than those during break conditions. Sensible heat flux shows an increase from approximately 20 to 80 W m –1 during the onset of the monsoon. Surface fluxes of water vapor indicate the importance of water vapor transport over the ship observation region in the central Arabian Sea during active conditions. Onset of the monsoon over the Arabian Sea is accompanied by an increase in the surface moisture flux by a factor of about two. Time histories of precipitable water show decreases of approximately 15% from active to break periods.  相似文献   

5.
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   

6.
Summary In this paper, the interannual variability of satellite derived outgoing longwave radiation (OLR) is examined in relation to the Indian summer monsoon rainfall (June to September total rainfall; ISMR). Monthly grid point OLR field over the domain i.e. the tropical Pacific and Atlantic region (30°N to 30°S, 110°E to 10°W) and the ISMR for the period 1974–2001 are used for the study. A strong and significant north–south dipole structure in the correlation pattern is found between the ISMR and the OLR field over the domain during January. This dipole is located over the west Pacific region with highly significant negative (positive) correlations over the South China Sea and surrounding region (around north-east Australia). The dipole weakens and moves northwestward during February and disappears in March. During the month of May, the OLR over the central Atlantic Ocean shows a significant positive relationship with the ISMR. These relationships are found to be consistent and robust during the period of analysis and can be used in the prediction of the ISMR.A multiple regression equation is developed, using the above results, for prediction of the ISMR and the empirical relationships are verified using an independent data set. The results are encouraging for the prediction of the ISMR. The composite annual cycle of the OLR, over the west Pacific regions during extreme ISMR is found to be useful in the prediction of extreme summer monsoon rainfall conditions over the Indian subcontinent.  相似文献   

7.
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   

8.
In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.  相似文献   

9.
许乐心  张人禾  齐艳军 《大气科学》2017,41(6):1125-1140
利用1979~2013年中国站点逐日降水资料和NCEP/NCAR再分析资料,对长江中下游夏季降水的季节内振荡最显著周期进行了分析研究。结果表明长江中游最显著周期为10~30天,长江下游最显著周期为30~60天。为了揭示这种差异产生的物理原因,进一步利用位相合成的方法对这两个区域不同周期的季节内振荡降水、高低空风场和高度场以及垂直结构和水汽等循环过程的演变特征进行分析。在200 hPa环流场上,长江中游的降水主要受到高纬度自西向东传播的波列影响,而长江下游的降水与鄂霍次克海的高度场的变化相关。在风场的垂直涡度和散度的位相结构演变过程中,10~30天的垂直涡度和散度有自北向南的移动,30~60天的垂直涡度和散度在长江以南地区有自南向北的传播。水汽输送的位相发展过程表明,长江中游的水汽分别来自于南海的向北输送和长江以北地区向南的水汽输送;长江下游地区的水汽则主要来自于热带东印度洋经孟加拉湾的向东输送并在南海的北向输送,以及西太平洋水汽向西输送到南海再向长江下游的输送。从高层大尺度环流场和整层积分的水汽通量输送上解释了长江中游10~30天降水的自北向南移动,和长江下游30~60天降水自南向北传播的原因。  相似文献   

10.
用加密气象站降水资料、NCEP再分析资料以及WRF模式的精细化模拟产品资料,对2011年6月24日20时-25日20时,由强热带风暴“米雷”与西风槽结合造成的江淮区域暴雨过程进行分析和诊断.结果表明:西风槽温压场斜压性显著,强热带风暴温压场正压性显著,构成了有利于中小尺度系统和暴雨发生发展的环流背景.由强热带风暴携带而来的水汽,路程近、速度快,在暴雨区形成深厚的水汽层.暴雨区具有两个上下叠置的垂直上升运动中心,保持对水汽的深厚强抬升,维持暴雨环流系统的强度.暴雨区环境大气流场动力正、斜压分解显示,此次暴雨过程大气流场的斜压成分占显著的主导地位;暴雨开始阶段,正压动能向斜压动能的转换迅速增强,各分项和总项都达到最大值;其后的暴雨阶段,转换强度逐渐减弱,暴雨结束时各项都接近0值,甚至出现弱的斜压动能向正压动能的转换.  相似文献   

11.
Summary ¶This study presents a numerical simulation of the bora wind as it occurs in form of a severe wind blowing down coastal mountains and over the Adriatic Sea. A typical cyclonic bora event, occurring during the period January 3–6, 1995, is simulated using a nested limited area model. An integration, with horizontal resolution of about 14km, and a nested one, with higher resolution, about 5.5km, are presented. The 1997 version of the Eta Model is used for both the lower resolution and the higher resolution runs. Numerous details of the simulation are found to be in good agreement with the understanding as well as the observational knowledge of the bora, thus supporting confidence in the realism of the results. In particular, features of the simulated flow are seen strongly indicative of some basic characteristics of the hydraulic model of the phenomenon, such as the mountain wave breaking and the upstream flow acceleration. Moreover, the increase in horizontal resolution, in combination with an improvement of the coastal SST information, led to a still improved realism of the low-level wind representation over the Adriatic Sea.Received December 31, 2001; revised March 25, 2002; accepted July 19, 2002 Published online: February 20, 2003  相似文献   

12.
Summary ¶An Objective 500-hPa cyclone detection and analysis is performed during the warm-dry period (16 April to 15 October) of the year for the Central and Eastern Mediterranean Region (30°N–50°N, 5°E–35°E). The 40-year NCEP/NCAR reanalyzes gridded data of geopotential height and temperature employed in the study, enable a climatic approach with a spatial (2.5°×2.5°) and a 6-hour temporal (00, 06, 12, 18 UTC) resolution. The occurrence frequency for the entire region shows a high interannual variability without significant trends. The frequency maximization over land in the middle of the warm period indicates a possible relationship with land-air temperature difference. Three primary activity centers (Turkey, Black Sea and Genoa) are objectively determined in the frequency domain as local maxima and their intensity is assessed in terms of average geopotential height and temperature values. The Turkey center, though the most frequent, consists of shallow lows in contrast to the Black Sea center, which is second in frequency, but on average, contains the deepest lows amongst the three. The Genoa center, which comes third in frequency, shows the highest variability in the intensity of the lows. Frequency distributions in the area, during the three sub periods (Early, Middle and Late Warm Period) and during the four synoptic hours, provide insight in to the origins of these systems at the three locations and reveal secondary centers, such as the centers in the Adriatic (at 18 UTC), Northern Greece (during Middle Warm Period) and the South Aegean Sea (during Late Warm Period).Received February 20, 2002; revised November 4, 2002; accepted January 16, 2003 Published online July 30, 2003  相似文献   

13.
Summary The ITCZ (Intertropical Convergence Zone) is an important parameter for climatic studies in tropical areas, and meteorological satellite imagery provides an original way to follow its location. Using archive imagery covering the 1971–1987 period, we attempted to study further some of the relationships (suggested by former studies) between ITCZ locations (followed here over the Atlantic ocean at 28°W), and climate anomalies in the Sahel, an area affected by periodic drought for the last seventeen years. We also paid close attention to more frequently studied parameters, such as upper air data, wind at sea level, and sea surface temperature. As for relative drought estimates, we assumed that runoff from the Senegal River was representative of the sahelian area and we observed that its variations were consistent with the Lamb's rainfall index over the 1965–1987 period.Since the onset of the rainy season for West Africa responds to wind changes, we assessed the link between ITCZ and wind at sea level and found the timing of northward ITCZ migration to be highly correlated (r=0.84) with the date of zonal wind stress intensification.On a general point of view, the relationships we found between rainfall amount and ITCZ position anomalies (or SST anomalies) agree with known results of precedent works, though better fit is found with the seventies than the eighties. We think this discrepancy is due in part to the fact that the parameters studied were not identical and, perhaps to a possible change in climatic conditions (on a long term basis, the data show a continuous trend for less intense equatorial upwelling in the gulf of Guinea, and our time series covers a more recent period than referenced works).With a closer look on the first half of the year, it appears that typical (wet/dry) schemes of the ITCZ migration can be evidenced more clearly, than in reporting the northernmost ITCZ location, that we found to be a less significant index: in other words, a sooner (respectively later) northward ITCZ migration corresponds to dry (respectively wet) episodes during the rainy season in sahelian areas. Hence, we propose the speed of ITCZ northwards movement as a parameterization of this event.Moisture content of the lower troposphere revealed that steady anomalies of this parameter may last several years over sahelian areas. Taking into consideration the relative strength African tropical and easterly jets, some limited results were obtained, in regard of climatic anomalies.As first conclusions, moisture transportation over sahelian area (associated with larger negative SST anomalies) is more efficient for wetter rainy season, than the intensity of convective process linked to higher local SST in the equatorial Atlantic area. In joining moisture analysis and ITZ migration (1980–1987 period), wetter rainy seasons were observed each time that positive humidity anomalies coincided with a later northward ITCZ migration (or greater northward ITCZ speed).With 8 Figures  相似文献   

14.
本文试用综合研究方法探讨南海热带气旋发展各阶段的基本结构及其演变特征。 综合场分析表明:南海热带气旋的流场大体呈圆形分布。热带气旋的半径在地面图上约为4纬距左右,属天气尺度,垂直伸展范围一般可达300mb左右,涡度轴的方向从低压阶段至风暴阶段变化较大,而且是层和低层轴向不一致,强风暴阶段涡度轴方向基本垂直,径向流入层主要位于500mb以下,流入层达到的高度较西太平洋台风为低。温度场结构方面,各阶段气旋中心温度较周围高出2—6℃;强风暴阶段暖核出现在300mb附近,200mb附近为一冷中心;冷暖中心的数值和出现高度均小于西太平洋台风,此外各阶段的温湿分布清楚地表明,为热带气旋发展供应热量、水汽的暖湿空气,主要来自低层的中心以南地区。 文中还简单讨论了中、低空环境流场,热带气旋的暖心结构与垂直运动,稳定度等等影响南海台风发展加强的一些因子。   相似文献   

15.
The characteristics of moisture transport and budget of widespread heavy rain and local heavy rain events in Northeast China are studied using the NCEP--NCAR reanalysis 6-hourly and daily data and daily precipitation data of 200 stations in Northeast China from 1961--2005. The results demonstrate that during periods with widespread heavy rain in Northeast China, the Asian monsoon is very active and the monsoonal northward moisture transport is strengthened significantly. The widespread heavy rainfall obtains enhanced water vapor supply from large regions where the water vapor mainly originates from the Asian monsoon areas, which include the East Asian subtropical monsoon area, the South China Sea, and the southeast and southwest tropical monsoon regions. There are several branches of monsoonal moisture current converging on East China and its coastal areas, where they are strengthened and then continue northward into Northeast China. Thus, the enhanced northward monsoonal moisture transport is the key to the widespread heavy rain in Northeast China. In contrast, local heavy rainfall in Northeast China derives water vapor from limited areas, transported by the westerlies. Local evaporation also plays an important role in the water vapor supply and local recycling process of moisture. In short, the widespread heavy rains of Northeast China are mainly caused by water vapor advection brought by the Asian monsoon, whereas local heavy rainfall is mainly caused by the convergence of the westerly wind field.  相似文献   

16.
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   

17.
Summary In terms of heavy precipitation, the MAP IOP 5 was a two-phase event. During the first phase – on 3 October 1999 – there was strong precipitation in the Lago Maggiore MAP target area, while the prefrontal precipitation was mainly limited to the mountain ranges of the MAP mission area in the Julian and the Karnic Alps involving a series of thunderstorms developing continuously for about 15 hours and contributing most to precipitation levels. During the second phase – on 4 October – the main precipitation was limited to the Julian and the Karnic Alps where a frontal passage was noted by a squall line moving from Veneto region towards the east, accompanied by a strong SW upper-level jet. At the same time, a strong low-level cold flow invaded the region to the north of Adriatic Sea from the east as a significant amount of cold air moving ageostrophically around the eastern edge of the Alps was arriving in the area. To study MAP IOP 5 in detail, we describe the development for mesoscale features of the events radar images, time-height cross-sections and estimates of Convective Available Poteintial Energy (CAPE) based on radio-sounding data, and how surface-measured precipitation offers some smaller scale information. Surface potential temperature and winds are also studied. Very large precipitation accumulation gradients are diagnosed (150mm per day/25km in S–N direction) and time distributions of hourly precipitation shows completely diverse regimes in the Friuli plain and in the Alps with peak intensities in the Julian Alps. The mesometeorological mechanisms for high precipitation rate in the SE Alps are diagnosed and some characteristics of the squall line are discussed.  相似文献   

18.
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   

19.
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   

20.
Fine-resolution regional climate simulations of tropical cyclones (TCs) are performed over the eastern Australian region. The horizontal resolution (30 km) is fine enough that a good climatological simulation of observed tropical cyclone formation is obtained using the observed tropical cyclone lower wind speed threshold (17 m s–1). This simulation is performed without the insertion of artificial vortices (bogussing). The simulated occurrence of cyclones, measured in numbers of days of cyclone activity, is slightly greater than observed. While the model-simulated distribution of central pressures resembles that observed, simulated wind speeds are generally rather lower, due to weaker than observed pressure gradients close to the centres of the simulated storms. Simulations of the effect of climate change are performed. Under enhanced greenhouse conditions, simulated numbers of TCs do not change very much compared with those simulated for the current climate, nor do regions of occurrence. There is a 56% increase in the number of simulated storms with maximum winds greater than 30 m s–1 (alternatively, a 26% increase in the number of storms with central pressures less than 970 hPa). In addition, there is an increase in the number of intense storms simulated south of 30°S. This increase in simulated maximum storm intensity is consistent with previous studies of the impact of climate change on tropical cyclone wind speeds.  相似文献   

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