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1.
 NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude 17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics. The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3–5-day regime over the Atlantic. Received: 28 May 1998 / Accepted: 2 May 1999  相似文献   

2.
一次梅雨锋气旋波雷暴天气生成的剖析   总被引：1，自引：1，他引：1  
魏绍远  林锡怀 《气象科学》1995,15(3):209-218
本文利用常规观测资料，并采用滤波方法，仔细地分析了１９８０年６月２４日一次江淮气旋中发生物强对流性质的暴雨天气。分析表明，强对流性质的暴雨天气之所发发生，是由于低层的西南风急流轴与Ｔｄ的湿舌轴重合，极有利于水汽的不断输送，８５０－７００ｈＰａ之间较长时间对流不稳定居存在，雷暴区又正好与Ｓ〈０的对称不稳定区重合，对称不稳定产生了倾斜对流，使雷暴天气得以维持；  相似文献   

3.
In correspondence with the establishment of the "upper high and lower high" pressure pattern due to the activities of 500 hPa high over the Tibetan Plateau in summer,a series of changes of the East Asia atmospheric circulation will take place.In this paper,the distributions of divergence and vertical velocity of 500 hPa high,the evolutions of atmospheric heat source,the variations of vorticity and zonal wind at 100 hPa level and vertical meridional cell over the Tibetan Plateau etc.are statistically analyzed.Thus,we can see that the ascending motion and the convective heating over the Tibetan Plateau,the South Asia high and the westerly jet on the north of the Plateau at 100 hPa level are weakned.The northern branch and the southern branch of the easterly jet on the south of the Plateau merge into a single whole and situate on the south of the former northern branch.In the meantime,thermodynamic land-sea discrepancy in South Asia and the convective heating over the Bay of Bengal is enhanced.It will play an important role in the maintenance of the easterly jet and the South Asia monsoon.  相似文献   

4.
Climatic trends over sub-Saharan Africa are described using major river flows, European Community Medium-Range Weather Forecasts, Coupled Forecast System, global land surface data assimilation and National Center for Environmental Prediction reanalysis, Global Precipitation Climate Center gauge data, and satellite observations in the period 1995–2010. The Niger and Zambezi rivers reached flow levels last seen in the 1950s (2,000 and 5,000 m3 s?1, respectively), and rainfall across the Congo Basin increased steadily ~+0.16 mm day?1 year?1. Weather events that contributed to flooding are studied and include the Zambezi tropical trough of 4 January 2008 and the Sahelian easterly wave of 19 July 2010. Diurnal summer rainfall increased threefold over the 1995–2010 period in conjunction with a strengthened land–sea temperature contrast, onshore flow, and afternoon uplift. 700 mb zonal winds over East Africa became easterly after 2001, so clean Indian Ocean air was entrained to the Congo, improving convective efficiency. Relationships between the African monsoon circulation and global teleconnections are explored. Zonal wind convergence around the Congo appears related with the tropical multi-decadal oscillation and signals in the Atlantic during the study period.  相似文献   

5.
利用京津冀区域加密自动气象站、SA多普勒天气雷达、L波段风廓线雷达、NCEP 0.25° 再分析资料及0.03° 高分辨率地形资料研究了北京2018年7月15—16日暖区特大暴雨特征和形成机制。结果表明:（1）这次暖区特大暴雨发生在副热带高压边缘的暖气团（θse高能区）中,无明显冷空气强迫,斜压性弱,有丰沛的水汽,850 hPa以下出现强水汽辐合。（2）暴雨的中尺度对流系统发展有3个过程:带状对流建立和局地强雨团影响、北京北部“列车效应”南部雷暴冷池出流造成对流加强和移动、平原地区线状对流重建。（3）暴雨发生前,低层西南风出现风速脉动,低空急流建立。首先在2500—3500 m高度形成低空急流,2 h后2500 m以下风速显著增大,5 h后急流厚度由边界层伸展到700 hPa。急流出口区降压,低层出现气旋性风场或切变,有利于垂直上升运动发展,触发和加强对流。（4）西南低空急流暖湿输送导致高温、高湿、高能的对流不稳定层结反复重建,这是对流发展加强的重要原因。（5）地面辐合线是对流触发并逐渐组织成带状对流系统的关键影响因素。地面辐合线方向、低空急流轴、回波移动方向三者几乎重叠是造成对流后向传播和“列车效应”的有利条件。（6）太行山和燕山地形对对流触发和暴雨增幅有重要影响。北京最大雨强≥40 mm/h站点中的77.4%位于西南部和东北部200—600 m海拔高度处。偏东风在华北西部太行山局地迎风坡触发对流,西南低空急流在北京北部迎风坡和喇叭口地形处辐合和抬升更为显著,造成局地特大暴雨。   相似文献   

6.
Summary The structure and variability of the inter-tropical convergence zone (ITCZ) in the SW Indian Ocean in the austral summer is investigated. The ITCZ is identified by satellite microwave (SSMI) precipitable water (PW) values > 5 g cm–2, minimum outgoing longwave radiation (OLR) values < 220 W m–2 and the pattern of convergence in the low level (850 hPa) winds. According to OLR climatology, the ITCZ lies over 15°S latitude to the west of Madagascar (40–50°E), but near 10°S to the east of 60°E. Inter-annual and intra-seasonal variability is induced by the interaction of the convective NW monsoon and subsident easterly trades. Symptoms of the structure and variability are presented using tropical cyclone (TC) tracks, axes of PW exceedences and OLR, 850hPa wind and PW fields in the period 1988–1990. The shape and intensity of the ITCZ is modulated by the strength of the NW monsoon off east Africa and by standing vortices in the SW Indian Ocean. The topography of Madagascar imparts a distinctive break in convective characteristics, and distinguishes the SE African ITCZ from its maritime counterpart.With 6 Figures  相似文献   

7.
对流性天气的气候背景分析是临近预报的基础,是做好北京2008年奥运气象服务必不可少的准备工作。对北京地区暖季(5—9月)发生雷暴、冰雹、暴雨和大风4种对流天气当日08时高空探测资料(2002—2005年)的压、温、湿、风等气象参数进行气候统计和分析,并与气候平均值进行比较,以揭示发生对流天气的环境条件及规律,为预报业务提供参考。结果表明:4种对流天气日所在各月高空要素均表现出比较典型的特征;500 hPa急流和对流产生有一定的对应关系:急流存在时,容易产生对流性天气,且雷暴、大风、暴雨甚至冰雹同时发生的概率增大;对流性天气过程与风向垂直切变有很好的对应关系,强雷暴、冰雹、暴雨和大风4种对流日500 hPa以下出现顺转平均发生率为84.2%。风向较强的逆转常常发生混合性对流天气,且逆转高度越高、对流越强。整层顺转容易发生多站暴雨,但雷暴发生较少。  相似文献   

8.
In spring the inland penetration of the West African Monsoon (WAM) is weak and the associated rainband is located over the Guinean coast. Then within a few days deep convection weakens considerably and the rainband reappears about 20?days after over the Sahel, where it remains until late September signalling the summer rainy season. Over the period 1989–2008 a teleconnection induced by the Indian monsoon onset is shown to have a significant impact on the WAM onset, by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the Indian monsoon sector. The initiation of convective activity over the Indian subcontinent north of 15°N at the time of the Indian monsoon onset results in a westward propagating Rossby wave establishing over North Africa 7–15?days after. A back-trajectory analysis shows that during this period, dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over West Africa inhibiting convection there. At the same time the low-level pressure field over West Africa reinforces the moisture transport inland. After the passage of the wave, the dry air intrusions weaken drastically. Hence 20?days after the Indian monsoon onset, convection is released over the Sahel where thermodynamic conditions are more favourable. This scenario is very similar in the observations and in the nudged simulations, meaning that the Indian monsoon onset is instrumental in the WAM onset and its predictability at intraseasonal scale.  相似文献   

9.
Summary In the Northern Summer, Kenya is located under the influence of the divergent Indian monsoon flow, and therefore is dry except for two separate areas: the coastal strip and the western regions. Analysis of daily rainfall data for June–September 1982 to 1988 has revealed that, although there are many distinct rainfall events between the two regions, an out-of-phase relationship is also evident, rain on the Coast being frequently accompanied by a drop in the precipitation over the Rift Valley area. It is shown that two types of wind forcing accompany these patterns. Alternating westerly and easterly anomalies at the 700 hPa level are associated with persistent wet and dry conditions (respectively) in western Kenya, and the opposite along the Coast. Large speed increases of the cross-equatorial low-level jet over Mombasa are followed by short rain spells in this latter region. These observations are thought to reflect the importance of an influx of moist unstable air from the west, linked to the West African monsoon, to ensure heavy rainfall over the Highlands. Variations in the low-level jet speed, which cannot be easily followed downstream, also have a significant, but less persistent impact on rainfall in the two regions.With 7 Figures  相似文献   

10.
This study implements the parameterizations of convective and frontal gravity wave drag (GWD) with wide phase speed spectra into a global forecast model with a model top near 0.3 hPa. The new convective GWD scheme replaces the existing one that considers only a stationary convective GW, and the frontal GWD scheme is newly introduced. When the new GWD schemes are used, the Rayleigh friction, applied above 2 hPa to mimic the effects of missing GWD, is removed. The convective (frontal) GWs are generated mainly in the Intertropical Convergence Zone and winter extratropical storm track regions (extratropics where strong baroclinicity exists). The convective and frontal GWD derived from the new schemes are significant near the model top, with maxima of ~2-4 and ~26-58 m s?1 day?1, respectively. The differences in convective GWD between the stationary and non-stationary schemes appear mainly in the tropics and summer hemisphere, where stationary GWs cannot propagate upward. The new schemes improve the seasonal representation of stratospheric wind, through changes in both the GWD and the resolved wave forcing, which is modulated by the changed large-scale wind due to the GWD. The downward influence, in response to the changed GWD, is also positive in the tropospheric fields, such as subtropical jet and planetary-scale disturbances. For the medium-range forecasts, improved skill scores on wind speed are achieved globally with the new schemes. The improvements mostly appear only in the stratosphere during the early forecast period (~3 days) but expand to the troposphere as forecast time increases.  相似文献   

11.
The Webster and Yang monsoon index (WYI)-the zonal wind shear between 850 and 200 hPa was calculated and modified on the basis of NCEP/NCAR reanalysis data. After analyzing the circulation and divergence fields of 150-100 and 200 hPa, however, we found that the 200-hPa level could not reflect the real change of the upper-tropospheric circulation of Asian summer monsoon, especially the characteristics and variation of the tropical easterly jet which is the most important feature of the upper-tropospheric circulation. The zonal wind shear U850-U(150 100) is much larger than U850-U200, and thus it can reflect the strength of monsoon more appropriately. In addition, divergence is the largest at 150 hPa rather than 200 hPa, so 150 hPa in the upper-troposphere can reflect the coupling of the monsoon system. Therefore, WYI is redefined as DHI, i.e., IDH=U850* - U(150 100)*, which is able to characterize the variability of not only the intensity of the center of zonal wind shear in Asia, but also the monsoon system in the upper and lower troposphere. DHI is superior to WYI in featuring the long-term variation of Asian summer monsoon as it indicates there is obvious interdecadal variation in the Asian summer monsoon and the climate abrupt change occurred in 1980. The Asian summer monsoon was stronger before 1980 and it weakened after then due to the weakening of the easterly in the layer of 150-100 hPa, while easterly at 200 hPa did not weaken significantly. After the climate jump year in general, easterly in the upper troposphere weakened in Asia, indicating the weakening of summer monsoon; the land-sea pressure difference and thermal difference reduced, resulting in the weakening of monsoon; the corresponding upper divergence as well as the water vapor transport decreased in Indian Peninsula, central Indo-China Peninsula, North China, and Northeast China, indicating the weakening of summer monsoon as well. The difference between NCEP/NCAR and ERA-40 reanalysis data in studying the intensity and long-term variation of Asian summer monsoon is also compared in the end for reference.  相似文献   

12.
Summary The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season to season, covering a longitude of 29° from its southernmost to the northernmost positions.  相似文献   

13.
Regional climate models, such as RegCM3, generally show large biases in the simulation of western North Pacific (WNP) summer monsoon (WNPSM). In this study, the authors improved the simulation of WNPSM by applying the convection suppression criterion based on the averaged relative humidity from cloud base to cloud top. The simulated rainfall and monsoon circulation are significantly improved. The suppressed convective heating associated with the decrease in convective rainfall simulates a low-level anomalous anticyclone to its north. The anomalous anticyclone reduces the intensity of low-level southwesterly flow and the wind speed at 10 m. The reduction in wind speed at 10 m decreases the evaporation at sea surface. The less supply of water vapor from underlying ocean in turn favors less convective rainfall. The overestimation of simulated convective percentages and the cold bias of 2 m air temperature are also reduced. The different effects of convection suppression criterion in stand-alone RegCM3 and corresponding regional air–sea coupled model are also discussed.  相似文献   

14.
The interannual variability in the formation of mini warm pool (MWP, SST ≥ 30.5°C) and its impact on the formation of onset vortex (OV) over the east-central Arabian Sea (ECAS) are addressed by analyzing the NCEP OIV 2-weekly SST data and NCEP–NCAR reanalysis 850 hPa wind fields from May to June (prior to the onset of monsoon) over the north Indian Ocean for a period of 12 years from 1992 to 2003. Strong interannual variability in the formation and intensification of MWP was observed. Further, the 850 hPa wind fields showed that OV developed into an intense system only during 1994, 1998 and 2001. It formed in the region north of the MWP and on the northern flank of the low-level jet axis, which approached the southern tip of India just prior to the onset of monsoon, similar to the vortex of MONEX-79. The area-averaged zonal kinetic energy (ZKE) over the ECAS (8–15°N, 65–75°E) as well as over the western Arabian Sea (WAS, 5°S–20°N, 50–70°E) showed a minimum value of 5–15 m2 s?2 prior to monsoon onset over Kerala (MOK), whereas a maximum value of 280 m2 s?2 (40–70 m2 s?2) was observed over the ECAS (WAS) during and after MOK. The study further examined the plausible reasons for the occurrence of MWP and OV.  相似文献   

15.
积云参数化方案对热带降水年循环模态模拟的影响   总被引：6，自引：5，他引：1  
本文利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室( LASG)发展的大气环流模式(SAMIL),采用Zhang-McFarlane (ZM)和Tiedtke (TDK)两种积云对流参数化方案,讨论了积云对流参数化方案对热带降水年循环模态模拟的影响.结果表明,两种积云对流参数化方案均能合理再现...  相似文献   

16.
Summary It is proposed that squall lines are initiated through the growth (with time) of wave-like perturbations along the surface of discontinuity between the monsoon southwesterlies and the dry northeasterlies in West Africa. Out of the many possible modes of different growth rates, the modes with the largest amplification could block the 650-mb mid-tropospheric jet which, because it is cold, progressively sinks as it traverses West Africa from east to west. The distortion created by the blockage forces up parcels of convectively-unstable southwesterlies resulting in precipitation. Precipitation falls or partly evaporates into the underlying jet the latent heat of vaporisation being supplied by the jet. The jet, now cooler, sinks. While sinking, it can reach the surface of the earth and, due to the strong convergence created, a gust front is formed. The front, as a result of ascending southwesterlies, constitutes an area of vigorous convective activity which triggers off a self-regenerative mechanism of condensation, evaporation and sinking. This hypothesis, with others, is able to explain the predominance of highlands as source regions of squall lines, the close association between the propagation speed of squall lines and that of the mid-tropospheric jet, the observed overturning of the atmosphere after the passage of squalls and the possible effects of insolation and African easterly waves on the initiation process.With 2 Figures  相似文献   

17.
Janicot and Sultan (Geophys Res Lett 28(3):523–526, 2001) and Sultan et al. (J Clim 16(21):3389–3406, 2003) showed evidence of an intraseasonal signal of Sahelian rainfall corresponding to wet and dry sequences of the West African Monsoon. Using NCEP/NCAR reanalysis, NOAA outgoing longwave radiation (OLR) and observed daily rainfall over West Africa from 1968 to 1990, this paper investigates the variability of 3 to 5-day African Easterly Waves (AEWs), convection and their relationship with rainfall in these wet and dry sequences. The mean daily value rainfall during wet sequences is twice the mean value during dry sequences but the number of dry or wet sequences per year is not correlated with the annual rainfall. Wet sequences account for 39% of the annual accumulated rainfall while dry sequences account for 22%. The number of 3 to 5-day AEWs increases during wet years in wet sequences and the activity tends to be larger during wet years in both wet and dry sequences. These AEWs explain 40% of the accumulated rainfall during wet sequences whereas they contribute to 26% of the accumulated rainfall observed during dry sequences. Generally, they contribute to the increase of rainfall during these sequences. Mean convection is stronger and there are twice as many low OLR days (<225W/m2) during wet than dry sequences. The mean rainfall for days with high convective activity (convective days) is also twice as great during wet sequences. Rainfall that occurs during days without low OLR (weak convection with warm cloud tops or isolated deep convection) contributes to 69% of the total rainfall during dry sequences and 45% during wet sequences. A composite study was performed from day D 0−10 to day D 0+10 in each sequence. Wet (dry) sequences of the African monsoon start with a decrease (slight increase) of the negative meridional Ertel Potential Vorticity (PV) gradient at 700 hPa, associated with an increase (decrease) of the spectral density of AEWs. During the wet sequence, the African Easterly Jet (AEJ), detected by 700 hPa zonal wind, decreases and moves northward, whereas the Tropical Easterly Jet (TEJ), detected at 200 hPa, increases and shifts southward. Convective activity increases from D 0−6 to D 0−3 and remains high for 4 days in wet sequences. The daily rainfall increases (decreases) between D 0−6 and D 0 and returns to the mean value at D 0+4 for wet (dry) sequences.  相似文献   

18.
In June 2013, a field experiment was conducted in Southeast Tibet in which the air temperature, moisture, and wind were measured by using a GPS sounding system. In the present study, based on these observations and ERA-Interim reanalysis data, the vertical structures of these atmospheric properties and the possible influence of the South Asian summer monsoon (SASM) were investigated. On average, the temperature had a lapse rate of 6.8℃km?1 below the tropopause of 18.0 km. A strong moisture inversion occurred at the near-surface, with a strength of 1.7 g kg?1 (100 m)?1 for specific humidity. During the observation period, the SASM experienced a south phase and a north phase in the middle and by the end of June, respectively. The monsoon’s evolution led to large changes in convection and circulation over Southeast Tibet, which further affected the local thermal, moisture, and circulation conditions. The strong convection resulted in an elevated tropopause height over Southeast Tibet during the north phase of the SASM, and the large-scale warm and wet air masses delivered by the monsoon caused high local temperature and moisture conditions.  相似文献   

19.
Development and structure of a maritime continent thunderstorm   总被引：4，自引：0，他引：4  
Summary The evaluation of a maritime continent thunderstorm complex (Hector) occurring over Bathurst and Melville Islands north of Darwin, Australia (12° S, 131° E) is investigated primarily using Doppler radar data. Thunderstorm formation follows the development of sea breeze circulations and a period of shallow non-precipitating convection. Evidence exists for initiation of long-lived and organised convection on the sea breeze fronts, although short-lived, scattered convection is apparent earlier in the day. Merging of the convective systems is observed in regions of enhanced low-level convergence related to sea breeze circulations. The merged convective complex is initially aligned in an almost east-west direction consistent with the low-level forcing. The merged complex results in rapid vertical development with updraughts reaching 40 m s– and echo tops reaching 20 km height. Maximum precipitation production occurs during this merger phase. On the perimeter of the merged convective complex, evidence exists for front-to-rear updraughts sloped over lower-level downdraughts with rear-to-front relative flow and forward propagating cold pools. The mature phase is dominated by this convection and the complex re-orientates in the prevailing easterly vertical shear to an approximate north-south direction, then moves westward off the islands with the classic multicellular squall-like structure.The one-dimensional cloud model of Ferrier and Houze (1989) used with a four class ice formulation reproduced the cloud top height, updraught structure and echo profile very well. To test the importance of ice physics upon thunderstorm development, several sensitivity tests were made removing the effects of the ice phase. All of these model clouds reached nearly 20 km, although simulations without the effects of ice had updraughts reduced from about 40 m s–1 to 30 m s–1. The simulated convection was more sensitive to changes in environmental conditions and parameterised cloud dynamics. The strong intensity of the convection was largely accounted for by increasing equivalent potential temperatures due to diurnal heating of the surface layer. The vertical velocity and radar structure of the island thunderstorm has more similarity with continental rather than oceanic convection. Maximum vertical velocities, in particular are almost an order of magnitude greater than typical of oceanic convection. With the intense updraughts, even in the low shear environment, there is evidence for mesoscale circulations within the convection.With 17 Figures  相似文献   

20.
Summary Observational studies indicate that the convective activity of the monsoon systems undergo intraseasonal variations with multi-week time scales. The zone of maximum monsoon convection exhibits substantial transient behavior with successive propagating from the North Indian Ocean to the heated continent. Over South Asia the zone achieves its maximum intensity. These propagations may extend over 3000 km in latitude and perhaps twice the distance in longitude and remain as coherent entities for periods greater than 2–3 weeks. Attempts to explain this phenomena using simple ocean-atmosphere models of the monsoon system had concluded that the interactive ground hydrology so modifies the total heating of the atmosphere that a steady state solution is not possible, thus promoting lateral propagation. That is, the ground hydrology forces the total heating of the atmosphere and the vertical velocity to be slightly out of phase, causing a migration of the convection towards the region of maximum heating. Whereas the lateral scale of the variations produced by the Webster (1983) model were essentially correct, they occurred at twice the frequency of the observed events and were formed near the coastal margin, rather than over the ocean.Webster's (1983) model used to pose the theories was deficient in a number of aspects. Particularly, both the ground moisture content and the thermal inertia of the model were severely underestimated. At the same time, the sea surface temperatures produced by the model between the equator and the model's land-sea boundary were far too cool. Both the atmosphere and the ocean model were modified to include a better hydrological cycle and ocean structure. The convective events produced by the modified model possessed the observed frequency and were generated well south of the coastline.The improved simulation of monsoon variability allowed the hydrological cycle feedback to be generalized. It was found that monsoon variability was constrained to lie within the bounds of a positive gradient of aconvective intensity potential (I). The function depends primarily on the surface temperature, the availability of moisture and the stability of the lower atmosphere which varies very slowly on the time scale of months. The oscillations of the monsoon perturb the mean convective intensity potential causing local enhancements of the gradient. These perturbations are caused by the hydrological feedbacks, discussed above, or by the modification of the air-sea fluxes caused by variations of the low level wind during convective events. The final result is the slow northward propagation of convection within an even slower convective regime. The ECMWF analyses show very similar behavior of the convective intensity potential. Although it is considered premature to use the model to conduct simulations of the African monsoon system, the ECMWF analysis indicates similar behavior in the convective intensity potential suggesting, at least, that the same processes control the low frequency structure of the African monsoon. The implications of the hypotheses on numerical weather prediction of monsoon phenomenon are discussed.  相似文献   

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