An assessment of the latent and sensible heat flux on the simulated regional climate over Southwestern South Atlantic Ocean

A Regional Climate Model (RegCM3) 10-year (1990–1999) simulation over southwestern South Atlantic Ocean (SAO) is evaluated to assess the mean climatology and the simulation errors of turbulent fluxes over the sea. Moreover, the relationship between these fluxes and the rainfall over some cyclogenetic areas is also analyzed. The RegCM3 results are validated using some reanalyses datasets (ERA40, R2, GPCP and WHOI). The summer and winter spatial patterns of latent and sensible heat fluxes simulated by the RegCM3 are in agreement with the reanalyses (WHOI, R2 and ERA40). They show large latent heat fluxes exchange in the subtropical SAO and at higher latitudes in the warm waters of Brazil Current. In particular, the magnitude of RegCM3 latent heat fluxes is similar to the WHOI, which is probably related to two factors: (a) small specific humidity bias, and (b) the RegCM3 flux algorithm. In contrast, the RegCM3 presents large overestimation of sensible heat flux, though it simulates well their spatial pattern. This simulation error is associated with the RegCM3 underestimation of the 2-m air temperature. In southwestern SAO, in three known cyclogenetic areas, the reanalyses and the RegCM3 show the existence of different physical mechanisms that control the annual cycles of latent/sensible heating and rainfall. It is shown that over the eastern coast of Uruguay (35°–43°S) and the southeastern coast of Argentina (44°–52°S) the sea-air moisture and heat exchange play an important role to control the annual cycle of precipitation. This does not happen on the south/southeastern coast of Brazil.     

Joining Australia to Antarctica GCM implications for the Cenozoic record of Antarctic glaciation

A previous GCM study concerning the formation and maintenance of Antarctic glaciation is expanded to include the joining of Australia to Antarctica; the two continents were physically connected prior to about 40 million years ago. It has been proposed that the increased continentality resulting from the enlarged landmass inhibited glaciation by increasing the degree of summer heating. However, simulations with the NCAR CCM1 suggest little change in the net Antarctic snow accumulation when Australia is joined to Antarctica, even under extreme variations in SST and topography. If anything, there is a slight increase in the net accumulation with the larger landmass. The climate of Australia does change markedly, consistent with the roughly 30° poleward shift in latitude. These results may not be inconsistent with paleoclimatic data from the early Cenozoic and the Cretaceous, with temperate flora and fauna along the coast, and large ice sheets inland.     

A numerical modelling study of coastal flooding

Summary A coastal ocean model capable of modelling tides, storm surge and the overland flow of floodwaters has been further developed to include the flux of water from tributaries and the forcing from wave breaking that leads to wave setup in the nearshore zone. The model is set up over the Gold Coast Broadwater on the east coast of Australia. This complex region features a coastal lagoon into which five tributaries flow and is subject to flooding from extreme oceanic conditions such as storm surge and wave setup as well as terrestrial runoff. Weather conditions responsible for storm surge, waves and flooding include cyclones of both tropical and mid-latitude origin. Two events are modelled. The first is an east coast low event that occurred in April 1989. This event verified well against available observations and analysis of the model simulations revealed that wave setup produced a greater contribution to the elevated water levels than the storm surge. The second case to be modelled was tropical cyclone Wanda, responsible for the 1974 floods. Modelled water levels in the Broadwater were reasonably well captured. Sensitivity experiments showed that storm surge and wave setup were only minor contributors to the elevated sea levels and their contribution was confined to the earlier stage of the event before the runoff reached its peak. The contribution due solely to runoff exhibited a tidal-like oscillation that was 180° out-of-phase with the tide and this was attributed to the greater hydraulic resistance that occurs at high tide. A simulation of this event with present day bathymetry at the Seaway produced sea levels that were 0.3–0.4 m lower than the simulation with 1974 bathymetry highlighting the effectiveness of deepened Seaway channel to reduce the impact of severe runoff events in the Broadwater. Received October 16, 2001 Revised December 28, 2001     

Topography and the dynamical response to easterly flow in Southern Hemisphere subtropical West Coast regions

Summary The response of the coastal atmosphere in subtropical Western Australia and south-western Africa to easterly flow is considered. Easterly flow, arising from the ridging of a large scale anticyclone near the southern extremity of these land-masses, is a common synoptic pattern, particularly during the summer half of the year. Despite similar synoptic forcing, coastline orientation and latitude, there are significant differences in the response. In Western Australia, the typical response to this easterly flow is a synoptic, non-propagating feature (the West Coast Trough) which may be located on- or offshore. The response in southern Africa is typically a mesoscale, propagating feature (the coastal low) which is trapped against the coastal mountains.It is argued that the steep coastal mountain ranges (about 1 km height) in southern Africa compared to the gentle, low-lying Western Australian topography combined with the mean coastal stratification contribute significantly towards the differences between the coastal low and West Coast Trough. A secondary feature associated with the regional topography is the existence of an oceanic throughflow north of Western Australia from the western equatorial Pacific Ocean with associated flow of the warm Leeuwin Current polewards along the Western Australian coast. It is suggested that this current and the associated lack of coastal upwelling may play a role in the location and intensity of the West Coast Trough.With 6 Figures     

1617号鲇鱼台风登陆后引发不同性质暴雨的成因对比分析

2016年9月28日1617号台风"鲇鱼"登陆后由台风本体环流和外围环流引发了不同性质的暴雨,这是本次秋季登陆台风暴雨预报的难点。利用常规气象观测资料以及NCEP的1°×1°再分析资料等,对不同性质暴雨的成因进行了诊断对比分析。结果表明:浙江东南部对流性降水和江西南部稳定性降水的大气层结结构具有明显的差异。中高纬度低槽距离台风较远,冷空气主要从低层入侵台风西北侧,破坏台风低层的暖心结构。台风外围中层干冷空气随东南风向浙江东南部输送,并叠加在低层暖湿气流之上,形成上冷下暖的不稳定层结,同时在对流层上层有干冷空气下沉至台风环流中下层(干侵入),导致浙江文成附近出现了局地特大暴雨。江西南部由于低层被湿冷空气占据,层结较为稳定,降水发展平缓。低空东南急流为台风外围环流暴雨提供了充足的水汽,浙江东南部地形对降水起到了增幅作用。不稳定层结及中层冷空气的输送对对流性暴雨的激发作用可以为登陆台风降水性质和强度预报提供参考依据。     

地形对豫东南一次极端暴雪影响分析

为了更全面地认识豫东南极端暴雪发生发展的机制,利用常规探空和地面观测资料以及NCEP 1°×1°再分析资料,对2018年1月3-4日豫东南极端暴雪过程从环流背景,高低空急流配置及水汽输送等方面进行分析。结果表明：高空低槽、中低空急流及切变线同时配合地面强冷空气南下,是形成本次极端暴雪事件最根本的天气尺度条件;豫东南位于高低空急流耦合作用形成的强烈上升区,西南急流不但为暴雪区提供源源不断的水汽条件,还起到了动力抬升及不稳定能量释放等作用,有利于暴雪发生发展与维持。豫东南地处大别山脉与桐柏山脉东北侧迎风坡处,地形较复杂,为了探究地形对本次极端暴雪的影响,本文尝试通过WRF模式模拟豫东南地形对降水的影响,通过控制试验和敏感试验对比分析表明,山体的迎风坡一侧对降水的增幅作用达4-6 mm,而背风坡一侧对降水的减幅作用约为6 mm;低层的U风分析显示,地形消减后U风风速将减小2-4 m·s^-1,并且中心位置东南移1-2个纬距,强降水中心随之东南移;垂直速度沿经度及纬度剖面均显示山体迎风坡处对垂直上升运动增幅达0.1×10^-2 m·s^-1;地形对水汽辐合起到明显增强作用,尤其在山体上方850-700 hPa高度辐合更加显著。     

Biases in AMIP model simulations of the east China monsoon system

 AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence, produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions. In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct atmospheric response to tropical convective heating. Received: 20 June 2000 / Accepted: 17 September 2000     

Impacts of the MJO in the Indian Ocean and on the Western Australian coast

The large sea surface temperature variations induced by the Madden-Julian Oscillation (MJO) on the northwest shelf of Australia and the remote influence of the MJO on the subtropical Western Australian coast are explored using the POAMA Ensemble Ocean Data Assimilation System reanalyses (PEODAS) for the period 1980–2010. The focus here is during the November–April extended summer season when the impacts of the MJO on and along the west coast of Australia are greatest. The MJO is well known to force equatorial Kelvin and Rossby waves in the Indian Ocean, and these are well depicted in the PEODAS reanalyses. When the downwelling Kelvin waves (forced by the westerly-convective phase of the MJO) reach the Indonesian region at the eastern boundary of the Indian Ocean, a coastally trapped Kelvin wave appears to propagate southeast along the Indonesian coastline. At the same time, the suppressed convection/easterly phase of the MJO arrives in the eastern Indian Ocean, with increased heat flux into the ocean due to reduced latent heat flux and increased insolation. The coastally trapped Kelvin waves do not appear to get onto the Western Australian coast. Rather, the increased heat flux and Ekman-induced downwelling onto the northwest (NW) coast in the suppressed/easterly phase of the MJO drive an increase in sea surface temperature on the NW Australian shelf. The piling up of warm water and associated sea level rise on the NW shelf is then communicated down the Western Australian coast as a coastally trapped wave, resulting in an increase in the Leeuwin current. Thus we conclude that the MJO signal in sea level along the west coast of Australia does not result from transmission of equatorial waves onto the Western Australian coast, but rather a southward-propagating coastal trapped wave that is directly forced on the NW shelf through Ekman-induced vertical advection and surface heat fluxes in the easterly phase of the MJO. Additionally, subtropical coastal sea level variability is reinforced locally via a teleconnection of the MJO to the local meridional wind off the southwest Australian coast. Considering the capability to predict the MJO to about 4 weeks lead time plus the 2 weeks taken for the MJO signal on the NW shelf to influence sea level at Fremantle, the use of MJO forecasts in management of the Western Australian marine environment should be considered for future application.     

The effect of topography and sea surface temperature on heavy snowfall in the Yeongdong region: A case study with high resolution WRF simulation

An analysis of the heavy snowfall that occurred on 11?C14 February 2011 in the Yeongdong region along the eastern coast is presented. Relevant characteristics based on observation and model simulations are discussed with a focus on the times of maximum snowfall in Gangneung (GN) and Daegwallyong (DG). This event was considered part of the typical snowfall pattern that frequently occurs in the Yeongdong region due to the prevailing northeasterly flow. The control simulation using the high resolution Weather Research and Forecasting (WRF) model (1 km × 1 km) showed reasonable performance in capturing the spatial distribution and temporal evolution of precipitation. The area of precipitation maxima appeared to propagate from the plain coastal region further into the inland mountainous region, in relation to the location of convergence zone. In addition, a series of sensitivity experiments were performed to investigate the effect of topography and sea surface temperature (SST) on the formation of heavy snowfall. The change of topography tended to modulate the topographically induced mechanical flow, and thereby modify the precipitation distribution, which highlights the importance of an elaborate representation of the topography. On the other hand, the sensitivity experiment to prescribe positive (negative) SST forcing shows the enhanced (suppressed) precipitation amount due to the change of the sensible and latent heat fluxes.     

Complexity analysis of the air temperature and the precipitation time series in Serbia

Regional Atmospheric Modeling System (RAMS) was applied to the study of the effect of the topographical altitude of the Tibetan Plateau (TP) on a severe drought event which took place in eastern China from November 2008 to January 2009. Two simulations of this drought event were conducted: a control simulation (CNTRL run) using original model settings and a sensitive simulation (TOPO run), where no change other than to reduce the TP topography by 50 %. The results show that the CNTRL simulation validates RAMS by reproducing this drought event fairly accurately. However, as part of the TOPO simulation, the total heat flux showed a decrease over most parts of the TP, latent heat flux underwent a significant increase over the southeastern TP, contrary to sensible heat, and a universal decrease over eastern China; this led to an increase in precipitation over the southeastern TP and a decrease in precipitation over eastern China. The decrease of total heat flux over the TP is collocated with an anomalous anticyclonic circulation from the TP to the coasts of southeastern China. Changes in atmospheric circulation and low-level water vapor transport pathways were consistent with changes in precipitation. In general, reducing the topographical altitude of the TP worsens drought in eastern China and moreover causes a significant decrease in precipitation over southern China.     

Generation and termination of Indian Ocean dipole events in 2003, 2006 and 2007

Evolution of Indian Ocean Dipole (IOD) events in 2003, 2006 and 2007 is investigated using observational and re-analysis data products. Efforts are made to understand various processes involved in three phases of IOD events; activation, maturation and termination. Three different triggers are found to activate the IOD events. In preceding months leading to the IOD evolution, the thermocline in southeastern Indian Ocean shoals by reflection of near equatorial upwelling Rossby waves at the East African coast into anomalous upwelling equatorial Kelvin waves. Strengthening (weakening) of northern (southern) portion of ITCZ in March/April and May/June of IOD years, leads to strengthening of alongshore winds along Sumatra/Java coasts. With the combined shallow thermocline and increased latent heat flux due to enhanced wind speeds, the SST in the southeastern Indian Ocean cools in following months. On intraseasonal time scales convection-suppressing phase of Madden-Julian oscillation (MJO) propagates from west to east in May/June of IOD year, and easterlies associated with this phase of MJO causes further shoaling of thermocline in southeastern Indian Ocean, through anomalous upwelling Kelvin wave. All these three mechanisms appear to be involved in initiating IOD event in 2006. On the other hand, except the strengthening/weakening of ITCZ, all other mechanisms are involved in activation of 2003 IOD event. Activation of 2007 IOD event was due to propagation of convection-suppressing MJO in May/June and strengthening of mean winds along Sumatra/Java coast from March to June through changes in convection. The IOD events matured into full-fledged events in the following months after activation, by surface heat fluxes, vertical and horizontal advection of cool waters supported by local along-shore upwelling favorable winds and remote equatorial easterly wind anomalies through excitation of upwelling Kelvin waves. Propagating MJO signals in the tropical Indian Ocean brings significant changes in evolution of IOD events on MJO time scales. Termination of 2003 and 2007 IOD events is achieved by strong convection-enhancing MJOs propagating from west to east in the tropical Indian Ocean which deepen the thermocline in the southeastern equatorial Indian Ocean. IOD event in 2006 was terminated by seasonal reversal of monsoon winds along Sumatra/Java coasts which stops the local coastal upwelling.     

Numerical case study of an extreme rainfall event during 9–11 December 2004 over the east coast of Peninsular Malaysia

Summary An exceptional rainstorm affected the eastern coast of Peninsular Malaysia during 9–11 December 2004 as a result of a westward propagating tropical disturbance known as the Borneo vortex. Rainfall totals near the storm center exceeded 600 mm and led to flash floods, loss of life and severe damage in the area. This study presents the results of a numerical simulation of this event using the fifth generation of the Penn State – NCAR Mesoscale Model (MM5). The model successfully simulated the synoptic circulation and reproduced the episode with comparable spatial patterns and total accumulated amount of precipitation to the observed. Various sensitivity experiments showed that the local topography is decisive in shaping the rainfall distribution during the storm episode. The role of the terrain elevation appears to be to block the westward progression of the system and inhibit excessive rainfall in the inland areas of Peninsular Malaysia. To the north of the storm center where coastal terrain elevation is relatively high, orography plays an important role in the rainfall by providing an additional forcing for moist air lifting. An additional fake dry simulation suggested that latent heat release is crucial for the development of the storm. Without latent heating, the vertical coupling of low-level convergence and upper level divergence is weakened and the vertical motion associated with the storm is suppressed.     

The Freezing Precipitation over the Middle-Lower Reaches of the Yangtze River during February-March 2009

In this paper, the authors analyze the quasistationary fronts, surface conditions, and atmospheric stratification processes associated with a freezing precipitation event over the middle-lower reaches of the Yangtze River, especially in the Dabie mountain during February-March 2009. The long duration of freezing precipitation was primarily caused by stationary and anomalous synoptic weather patterns, such as a blocking high pressure in the northern branch and a trough in the southern branch of the westerlies, which resulted in the encounter cold air from northern China and warm moisture from the south. The east-west-oriented, quasi-stationary front （or shear line） found in central China was mostly responsible for producing the precipitation. The warm layer and nearsurface frozen layer were located in the lower troposphere along the front zone. Although the warm layer （〉 0℃） existed along the whole front, a surface temperature less than 0℃ appeared only over the lower-middle reaches of the Yangtze River, especially in the Dabie mountain. Therefore, the surface temperature was the main influencing factor, as the freezing precipitation only happened over the Dabie mountain.     

NUMERICAL EXPERIMENTS ON THE FORMATION OF THE EASTERN PACIFIC SUMMER MONSOON

In this paper,we use a two-dimensional primary equation model which contains (1) heating ofradiation,(2) heating of condensation,and (3) transfers of sensible and latent heat between air andthe underlying surface.To investigate the causes for the formation of the eastern North Pacific sum-mer monsoon,the data at 110°W are obtained and winds at underlying surface and at 200 hPa aremodified under the conditions (1) removing topography and (2) changing meridional sea surface tem-perature (SST) gradient.In the numerical modification,we find that by removing the topography,the center's location ofthe eastern North Pacific summer monsoon does not change,but the intensity of the summer monsoonis weakened.Also the onset of the summer monsoon is delayed to the end of May.The tropical east-erly jet is weakened obviously,even changes to westerly wind.On the other hand,we find that theSST gradient along 110°W influences the eastern North Pacific summer monsoon distinctly.If theSST gradient is decreased,the center of the southwest wind near 12°N does not exist any more.theintensity of the whole summer monsoon becomes very weak and the circulation pattern of the summermonsoon also changes a lot.Finally,we indicate that both topography and meridional SST gradient play important roles inthe occurrence of the eastern North Pacific summer monsoon.The meridional SST gradient is themost important factor that triggers the summer monsoon and the topography along 110°W influencesthe intensity and the onset time of the summer monsoon there mostly.     

Dynamical controls on the diurnal cycle of temperature in complex topography

We examine the climatological diurnal cycle of surface air temperature in a 6 km resolution atmospheric simulation of Southern California from 1995 to the present. We find its amplitude and phase both have significant geographical structure. This is most likely due to diurnally-varying flows back and forth across the coastline and elevation isolines resulting from the large daily warming and cooling over land. Because the region’s atmosphere is generally stably stratified, these flow patterns result in air of lower (higher) potential temperature being advected upslope (downslope) during daytime (nighttime). This suppresses the temperature diurnal cycle amplitude at mountaintops where diurnal flows converge (diverge) during the day (night). The nighttime land breeze also advects air of higher potential temperature downslope toward the coast. This raises minimum temperatures in land areas adjacent to the coast in a manner analogous to the daytime suppression of maximum temperature by the cool sea breeze in these same areas. Because stratification is greater in the coastal zone than in the desert interior, these thermal effects of the diurnal winds are not uniform, generating spatial structures in the phase and shape of the temperature diurnal cycle as well as its amplitude. We confirm that the simulated characteristics of the temperature diurnal cycle as well as those of the associated diurnal winds are also found in a network of 30 observation stations in the region. This gives confidence in the simulation’s realism and our study’s findings. Diurnal flows are probably mainly responsible for the geographical structures in the temperature diurnal cycle in other regions of significant topography and surface heterogeneity, their importance depending partly on the degree of atmospheric stratification.     

海陆分布对于大气环流的地形与热力的影响

关於海陆分布对大气环流的影响,有两派意见.一派著重於热力的影响,另一派则着重於动力的影响.对海洋而言,在冬季大陆可以看作冷源,在夏季则好似一热源.由於海洋上与大陆上受热不同的结果,冬季在西伯利亚可长期维持着一强大高压.关於冬夏季风的产生,一般也都认为是由於海陆受热不同的效果.     

1988年反厄尼诺(La Niña)事件的数值模拟

用中国科学院大气物理研究所高分辨率、自由表面热带海洋环流模式对厄尼诺/南方涛动(El/Southern Oscillation)循环中另一重要位相La Niña事件进行了数值模拟研究。模式区域为南北纬30°之间的热带太平洋,经纬圈水平方向分辨率分别为2°和1°,垂直方向分为不等距的14层;模式中考虑了盐度并引入与洋流切变和层结稳定度有关的垂直扩散参数化方案。在观测到的海表风应力、热量和淡水通量(蒸发与降水之差)驱动下,所发展的模式从1984年积分到1989年。本文给出模式对近十几年来最强的1988年La Niña事件进行数值模拟的结果,着重分析La Niña期间热带太平洋海面起伏、流场和温度场时空演变,以及La Niña和El期间变量场时空结构差别。文中还进行了模式结果和一些观测资料间的比较。     

中国沿海风能分布特征及其影响因子的数值模拟

利用欧洲中期天气预报中心0.75°×0.75°再分析资料,对中国海岸线两侧相邻区域内的风能、风速进行研究,讨论不同季节、不同区域风能、风速的分布特征;利用WRF(Weather Research Forecast)模式模拟海表面温度上升和城市化发展对中国东部沿海风能的影响。结果表明:1)中国沿海风能的时空分布不均一,季节变化明显。春季渤海湾区域风能明显大于其他三区(华东沿海、东南沿海和南海北部沿海区域)。夏季渤海湾区域风能显著小于其他三区,而华东沿海区域风能稍大。秋季东南沿海和南海北部沿海区域风能较大。冬季沿海四区风能大小接近。一般而言,秋冬季风能较大、春夏季风能较小,夏季风能显著小于冬季。2)不同区域、不同季节风速的年际变化存在明显差异。除冬季东南沿海区域风速有增大趋势外,其他区域各季节风速都呈缓慢减小趋势,但减小幅度很小。3)海表温度升高在不同季节对风速的影响不同。春季渤海湾和山东半岛、北部湾沿海及杭州湾风速随海温升高而增强。夏季海温升高幅度不同,则风速显著变化区域不同,但大部分沿海区域风速随海温升高而增强。秋冬季风速随海表温度升高而增强,影响区域较稳定:秋季东南沿海和华东沿海区域风速增强,冬季渤海湾和南海北部沿海区域风速增强。4)城市化发展增大了地表摩擦力,使得夏秋季登陆我国的热带气旋迅速减弱,沿海风速随之减小。     

On the role of resolution and topography in the simulation of East Asia precipitation

Summary In this paper, we investigate the role that horizontal resolution plays in the simulation of East Asia precipitation. Two sets of numerical experiments are performed using the Regional Climate Model (RegCM2) nested in one-way mode within the CSIRO global coupled atmosphere-ocean model. In the first set we use the actual RegCM2 topography at the selected model resolutions, which are 45, 60, 90, 120, 180, 240 and 360 km. In the second set of the experiments, the same coarse CSIRO model topography is used in all simulations using the different resolutions of the first set. The results demonstrate that the simulation of East Asian precipitation improves as the horizontal resolution is increased. Moreover, it is shown that the simulations using a higher resolution along with the coarse CSIRO topography perform better than the simulations using a coarser model resolution with corresponding model topography. This suggests that over East Asia adequate spatial resolution to resolve the physical and dynamical processes is more important than topography. Lastly, the results indicate that model resolutions of 60 km or higher are needed to accurately simulate the distribution of precipitation over China and East Asia.     

A Case of Convection Development over the Western Mediterranean Sea: A Study through Numerical Simulations

Summary  A convective case producing heavy precipitation in the western Mediterranean region, characterized by pronounced upper level forcing and main rainfall over the sea, is studied. On the day of the event (September 28^th, 1994), more than 140 mm of precipitation were recorded in coastal lands of eastern Spain, and 180 mm were estimated over the sea with radar data. Synoptically, the case appears to combine warm and moist easterly advection at low levels, typically observed in torrential rainfall events of the region, with a less common strong upper level dynamical forcing. A set of mesoscale numerical simulations using the Hirlam model is performed to investigate the mechanisms responsible for the convection development, and to assess the influence of the orography on the rainfall field. Model output diagnosis indicates that in addition to the lower level forcing, a two-jets interaction is decisive for the triggering and driving of the convection during the event. Moreover, a non-topographic simulation reveals a relatively weak influence of the orography on this event when compared with other similar heavy precipitation cases in eastern Spain. Previous studies have shown an orographic influence of more than 90% on the rainfall whereas in this case about 50% of the precipitation over the area is attributed to the orographic forcing. The study is extended with an analysis of the individual effects of the Atlas and Iberian Peninsula, by means of a factor separation technique. It is shown that the Atlas range induces a redistribution of the precipitation over the Mediterranean, whereas local enhancements can be attributed to the Iberian topography. Received March 2, 1999