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     

Simulation of a long-lived meso-β scale convective system over the mediterranean coast of spain. Part I: Numerical predictability

Summary An unusually long-lived (33 hours), devastating (local maximum rainfall rate over 800 mm/24 hr) meso--scale (diameter smaller than 200 km) convective system that occurred over the Mediterranean coast of Spain has been simulated reasonably well by means of a regional numerical model. Several runs of the model including parameterized convection and boundary conditions of varying degrees of complexity have been performed. In most of these experiments, the main characteristics of the event, namely its, stationarity and duration, are captured. The direct relationship between the Lagrangian lifetime of a meteorological system and its degree of deturministic predictability seems to be corroborated by the results: It appears that the meso--scale forcing that preceded and favoured the MCS was especially well predictable, and once initiated, the simulated MCS seems to have several feedback mechanisms helping to extend its life. Results are encouraging, because they reveal that it might be possible to predict very severe episodes of small MCSs such as the one shown here sufficiently in advance.With 15 Figures     

A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview,damage survey and radar analysis

This study presents an analysis of a severe weather case that took place during the early morning of the 2nd of November 2008, when intense convective activity associated with a rapidly evolving low pressure system affected the southern coast of Catalonia (NE Spain). The synoptic framework was dominated by an upper level trough and an associated cold front extending from Gibraltar along the Mediterranean coast of the Iberian Peninsula to SE France, which moved north-eastward. South easterly winds in the north of the Balearic Islands and the coast of Catalonia favoured high values of 0–3 km storm relative helicity which combined with moderate MLCAPE values and high shear favoured the conditions for organized convection. A number of multicell storms and others exhibiting supercell features, as indicated by Doppler radar observations, clustered later in a mesoscale convective system, and moved north-eastwards across Catalonia. They produced ground-level strong damaging wind gusts, an F2 tornado, hail and heavy rainfall. Total lightning activity (intra-cloud and cloud to ground flashes) was also relevant, exhibiting several classical features such as a sudden increased rate before ground level severe damage, as discussed in a companion study. Remarkable surface observations of this event include 24 h precipitation accumulations exceeding 100 mm in four different observatories and 30 minute rainfall amounts up to 40 mm which caused local flash floods. As the convective system evolved northward later that day it also affected SE France causing large hail, ground level damaging wind gusts and heavy rainfall.     

Water vapour flux divergence over the Arabian Sea during 1987 summer monsoon using satellite data

The evaporation rates over the Arabian Sea (AS) for the summer monsoon months (June to September) of 1987 have been computed using the bulk-aerodynamic formula. The satellite derived precipitation from the INSAT-1B VHRR (Very High Resolution Radiometer) sensor operating in the wavelength 10.5–12.5 m has been used for computing the precipitation over the AS. The net water vapour flux divergence (NFD) over AS has been computed as the difference between evaporation and precipitation. The estimates being -0.02 × 10¹⁰, 2.55 × 10¹⁰, 0.70 × 10¹⁰ and 0.44 × 10¹⁰ tons/day respectively for the months June, July, August and September. The NFD over AS was found to be positively and significantly correlated with the mean monsoon rainfall along the west coast of India.     

Synoptic and thermodynamic characteristics of 30 March–2 April 2009 heavy rainfall event in Iran

A heavy rainfall event during the period from 30th of March to 2nd of April 2009 has been studied using upper air and surface data as well as NOAA HYSPLIT model. This observational study attempts to determine factors responsible for the occurrence of heavy rainfall over Iran induced by Mediterranean cyclone, a western severe sub-tropical storm that made rainfall on most regions of the country. On the surface chart, cyclones, anticyclones and weather fronts were identified. The positions of the cold and warm fronts, which extended from a two-core low pressure center, were quite in good agreements with directions of winds i.e., westerly, southerly and easterly flows as well as the regions of precipitation. The heavy rain event occurred due to a Mediterranean cyclone’s activity over the study area, while other conditions were also responsible for this event such as an unstable atmosphere condition with abundant low-level moisture, which the warm and moist air parcels were brought by the southwesterly low-level jet into the country from Persian Gulf, Oman Sea, Indian Ocean and Caspian Sea at lower levels as well as Mediterranean Sea, Red Sea and Persian Gulf at upper levels over the examined period. A strong low-level convergence zone was observed along the wind-shift line between the southwesterly flow because of the low-level jet and the northeasterly flow due to the Russian high pressure. The amount of precipitable water varied between 20 and 24 kg m^?2, surface moisture convergence exceeded 2.5 g kg^?1 s^?1 and the highest CAPE value in the sounding profiles was observed in Birjand site with 921 J kg^?1 during the study period. The HYSPLIT model outputs confirmed the observed synoptic features for the examined system over the country.     

Numerical experiments investigating the mechanisms of a heavy rainfall event over northeastern Taiwan and a mesovortex during TAMEX

Summary This study analyzes the mechanisms of the development of a heavy rainfall event (17 June 1987) over the lee side of the Central Mountain Range (CMR) in northeastern Taiwan during the southwesterly monsoon. This heavy rainfall event was examined using gridded data from the European Centre for Medium-Range Weather Forecasts, surface rainfall data and numerical model results, employing a non-hydrostatic fifth-generation mesoscale model (MM5) developed by the National Center for Atmospheric Research and Pennsylvania State University. A tropical depression was simulated over the northern South China Sea on 16 June. Convergence, resulting from the southeasterly winds associated with the circulation from the tropical depression, and northeasterly winds over the Taiwan Strait, occurred over the northern Bashi Channel at 850 hPa. The convergence amplified planetary vorticity and the vorticity associated with the intensifying tropical depression. Consequently, a mesovortex with low pressure over the northeastern edge of the tropical depression near southern Taiwan was produced. Additional convergence over the ocean adjacent to southern Taiwan caused by the interaction between the northeasterly flow, which was deflected over the southeastern slope of the CMR, and the southeasterly flow of the tropical depression, also affected the intensity of the mesovortex. When the mesovortex moved northward and reached southern Taiwan, the southeasterly flow associated with it interacted with an east-southeasterly flow, which was related to the tropical depression, to form a mesoscale convective system (MCS) over the ocean adjacent to southeastern Taiwan. As the mesovortex moved northward, the MCS, which was embedded in the southeasterly flow, also drifted inland toward northeastern Taiwan. The orographic lifting and the ascending motion associated with the deceleration of the easterly flow near the CMR enhanced the MCS over northeastern Taiwan and produced heavy rainfall. To examine the role of Taiwan’s orography on the modelled rainfall, two simulations were conducted; one which included Taiwan’s orography and one which excluded it. In both simulations, the mesovortex in the northern Bashi Channel and the MCS near southeastern Taiwan were reproduced. However, in the simulation excluding the orography, the mesovortex was slightly less intense. In addition, without the extra orographic lifting and the ascending motion caused by flow deceleration, rainfall over northeastern Taiwan was weaker than in the simulation with the orography.     

Evaporation over the Arabian Sea during two contrasting monsoons

Summary Monthly mean surface fields of different meteorological parameters and evaporation are studied for the 1979 (poor monsoon) and 1983 (good monsoon) monsoon seasons over the Arabian Sea, in order to understand the role of evaporation on the Indian monsoon rainfall. It is noticed that in general, the sea surface temperatures are higher in 1983 throughout the monsoon season than in 1979 in the Arabian Sea excepting western region. The mean rates of evaporation on a seasonal scale are found to be equal in both years (3.66×10¹⁰ and 3.59×10¹⁰ tons/day in 1979 and 1983, respectively). No coherence is observed between the evaporation and the west coast rainfall within a season. It is also noted that the pressure distribution over the Arabian Sea is even important to advect the moisture towards the west coast of India, through winds.With 10 Figures     

Impact of land–sea breezes at different scales on the diurnal rainfall in Taiwan

The formation mechanism of diurnal rainfall in Taiwan is commonly recognized as a result of local forcings involving solar thermal heating and island-scale land–sea breeze (LSB) interacting with orography. This study found that the diurnal variation of the large-scale circulation over the East Asia-Western North Pacific (EAWNP) modulates considerably the diurnal rainfall in Taiwan. It is shown that the interaction between the two LSB systems—the island-scale LSB and the large-scale LSB over EAWNP—facilitates the formation of the early morning rainfall in western Taiwan, afternoon rainfall in central Taiwan, and nighttime rainfall in eastern Taiwan. Moreover, the post-1998 strengthening of a shallow, low-level southerly wind belt along the coast of Southeast China appears to intensify the diurnal rainfall activity in Taiwan. These findings reveal the role of the large-scale LSB and its long-term variation in the modulation of local diurnal rainfall.     

地形对门头沟一次大暴雨动力作用的数值研究

2002年6月24—25日,北京门头沟附近发生了一次大暴雨过程。为探讨地形在本次过程中的动力作用,采用美国俄克拉荷马大学风暴分析预测中心开发的ARPS模式,对大暴雨过程进行了数值试验。控制试验采用27、9 km双重单向嵌套网格,网格覆盖范围约为3000 km×3000 km、900 km×900 km。两层网格均采用全物理过程,使用的都是全球30″的地形资料。在控制试验的基础上,进行了3组敏感性试验:第1组试验采用干过程模拟,即不考虑凝结潜热的作用;第2组试验将地形整体向东/西平移1°;第3组试验是将门头沟西部的局地地形抠除一部分。试验结果表明,在不考虑凝结潜热作用时,东南风气流仍然可以爬升到2 km以上,超过了大气的抬升凝结高度,证实了地形的动力作用是本次大暴雨的触发机制;将地形向东/西平移1°后,由于大气的对流稳定度发生了改变,模拟的降水强度和落区也发生了变化,表明山坡和山顶的对流不稳定大气是导致本次大暴雨的必要条件;抠除局地地形后,模拟的降水量也发生了不同程度的改变,再次证明大暴雨是在多尺度地形以及一定的天气系统配置下产生的。     

A cyclone over Saudi Arabia on 5 January 2002: A case study

Summary A shallow and short-lived subsynoptic cyclone is discussed as one of typical events causing natural disasters over Saudi Arabia on 5 January 2002 using the Japan Meteorological Agency global analysis dataset. The main cause for its formation was vorticity stretching. On the same day widespread rainfall over Saudi Arabia with enhanced precipitation over the mountainous Jeddah region was also observed using the Global Precipitation Climatology Project dataset. Owing to eastward-moving wintertime synoptic-scale Mediterranean disturbances, this rainfall was associated with frontal activity, modified by the local orography.     

Rainfall patterns in the eastern United States

This investigation is an extension of earlier work on rainfall patterns in the western United States. In the present study, rainfall figures from World Weather Records for cities east of the Mississippi have been subjected to filter analysis using the four filters described in the earlier investigation.The results suggest substantial coherence of rainfall data in a broad central area of North America, from the Great Lakes to the Rockies and into southern parts of the Canadian Prairies and Ontario. In this entire region there appears to be a pronounced rainfall cycle, of about 22 yr, which exhibits a possible relationship with the double sunspot cycle. However, inland from the U.S. north-east coast and including southern Quebec and the Canadian maritime Provinces, the cycle is different and is closer to 16 yr.Although the earlier investigation pointed to a connection between the lunar cycle of 18.6 yr and rainfall behaviour in the far west of the United States, there is little evidence of a similar connection in the east.     

Simulation of lake ice and its effect on the late-Pleistocene evaporation rate of Lake Lahontan

A model of lake ice was coupled with a model of lake temperature and evaporation to assess the possible effect of ice cover on the late-Pleistocene evaporation rate of Lake Lahontan. The simulations were done using a data set based on proxy temperature indicators and features of the simulated late-Pleistocene atmospheric circulation over western North America. When a data set based on a mean-annual air temperature of 3° C (7° C colder than present) and reduced solar radiation from jet-stream induced cloud cover was used as input to the model, ice cover lasting 4 months was simulated. Simulated evaporation rates (490–527 mm a^–1) were 60% lower than the present-day evaporation rate (1300 mm a^–1) of Pyramid Lake. With this reduced rate of evaporation, water inputs similar to the 1983 historical maxima that occurred in the Lahontan basin would have been sufficient to maintain the 13.5 ka BP high stand of Lake Lahontan.     

Nocturnal offshore precipitation near the Mediterranean coast of the Iberian Peninsula

While nocturnal offshore precipitation, which produces rain cells and bands, has been studied in tropical areas, few studies have analyzed the occurrence of this phenomenon at higher latitudes. Using radar reflectivity, nocturnal rainfall in the western Mediterranean area has been detected near the coast of the Iberian Peninsula and North Africa. More than 50 events have been recorded since 2009. MM5 mesoscale simulations of some of the recorded events allow us to establish that the most likely causes for these precipitation events are: (1) the interaction between cold air masses conducted by drainage and katabatic winds, and (2) a wetter and warmer synoptic wind. Two different episodes are presented: one in the northeastern Iberian Peninsula, caused by stratiform clouds, which occurred at the mouths of three rivers; the other case, produced by convective clouds, occurred at the southern Iberian Peninsula and was caused by the drainage winds flowing down from some mountain ranges located close to the coast.     

From the concept of “Kaltlufttropfen” (cold air pool) to the cut-off low. The case of September 1971 in Spain as an example of their role in heavy rainfalls

Summary Over the last 25 years the term “cold pool” has come to be used in many spheres as a synonym for floods. This has given rise to a major confusion that has even moved into international scientific and technical circles. In this paper we analyse how the concept of “cold air pool” has evolved from when it was defined at the beginning of the 20th century down to the present day, in which the Spanish term “DANA” (similar to a cut-off low) has been introduced in order to avoid existing confusions. In the course of the paper we take account of cold air pool genesis and their thermal and dynamic characteristics, and we discuss the factors that have led to them being identified (erroneously) with heavy rainfall events. The study takes as its basis a systematic analysis of all the cold air pools recorded in Europe, and particularly in the Iberian Peninsula, over the period 1974–1983, as well as in studies of the floods recorded in eastern Spain since 1950 until nowadays. The discussion done on the basis of this accurate analysis leads to the identification of a cold air pool as a type cut-off low (COL) and justifying the use of this more generic term when a structure like this is present in a heavy rainfall event. For a better illustration of the previous discussion and understanding of the role of COLs in intense rainfall events, we present the episode of September 1971 in Catalonia, in which over 400 mm were recorded. The analysis was carried out with the MM5 initialised with the ERA-40 re-analyses. The results show that the role of COLs in the heavy rainfall episodes recorded in Spain is mainly dynamic, both in terms of the circulation they create at low levels and the potential vorticity anomaly generated. This circulation draws in very warm, moist and potentially unstable air perpendicularly to the coast and the littoral mountain chains. The factor of thermal instability, owing to the presence of cold air at medium and higher levels, shows itself to be more important in zones where this warm moist advection at low levels is not as significant as in the Mediterranean zone.     

Investigation of orographic effects on two heavy rainfall events over southwestern Taiwan during the Mei-yu season

Orographic influences on the development of two heavy rainfall events over southwestern Taiwan during the Mei-yu season were analyzed. For the first event (5 June 1997), the maximum daily rainfall accumulation (∼371 mm) occurred over the southwest coast of Taiwan, whereas for the second event (20 May 2001), the maximum daily rainfall accumulation (∼535 mm) occurred over the southwestern windward slopes. The important processes for the development of heavy rainfall for these two events were studied from gridded data from the European Center for Medium-Range Weather Forecasts and from surface rainfall data and numerical modeling results using the nonhydrostatic fifth-generation mesoscale model from the National Center for Atmospheric Research and the Pennsylvania State University (MM5).     

A numerical case study of a cold air outbreak over East Asia during late spring

Summary A case study of a late spring cold air outbreak associated with heavy rainfall and snow storms over East Asia is carried out with a numerical model. This was the most severe case over northern China in 50 years. In the lower troposphere, the evolution of the edge anticyclone and the northerly low-level orographic jet east of Qinghai-Tibetan Plateau are well simulated in the ECMWF limited area model but the associated cold surge is under predicted. In the experiments with envelope and valley filled orography, results are improved. Neither cold surge nor strong frontogenesis is simulated in a no latent heating experiment, indicating that the feedback of latent heat release is quite essential to the burst of cold air in this case.With 9 Figures     

Red dust rain within the Spanish Mediterranean area

Dust rain belongs to the climatological conditions of the Iberian Peninsula's Mediterranean seaboard. Traditionally known as muddy' or bloody rains, red dust rain has become a topical issue as a result of more frequent rainfall of this kind in recent years. In spite of the difficulties that studying this phenomenon involves, owing to the lack of systematically kept records at meteorological observatories, details are provided of the high frequency of such phenomena within the Spanish Mediterranean area, using records from other phenological sources. The study analyzes the chemical composition of red dust rain and the atmospheric factors giving rise to such phenomena. Whether there is a relationship betwen the higher frequency of dust rain and possible changes in global atmospheric circulation is as yet unclear.     

Diagnosis of mesoscale structures in cases of lee cyclogenesis during ALPEX

Summary Several cases of lee cyclogenesis that occurred during ALPEX-SOP have been analyzed, with the aim of separating the large scale structures from the subsynoptic/meso-a scale features that are characteristic of this meteorological phenomenon. The results presented here are mainly based on composites of the analyzed cases. We assume that the deformation caused by the orography can be, at least to a reasonable extent, isolated from the undisturbed state using scale separation. The analysis technique we employ provides the scale separation as built-in in the interpolation algorithm. The scale separation error due to the large inhomogeneities of the data density distribution is partly corrected using a method described in the text. The orographic disturbance appears in different mesoscale fields as a quasiantisymmetric dipolar structure. For example, high/low pressure, cold/warm temperature and anticyclonic/cyclonic couplets characterize the mesoscale fields near the Alps. A qualitative agreement is found with the structure of the orographically induced perturbations predicted by the normal mode theory of lee cyclogenesis.With 7 Figures     

Sea Level Pressure Departures in the Mediterranean and their Relationship with Monthly Rainfall Conditions in Israel

Summary Monthly rainfall conditions in Israel were determined, using data from 12 stations, during 30 years (1961–90). The definition of a month to be dry, normal or wet, was done using standardized rainfall totals. Pressure departures for each of the three rainfall categories for each month of the rainy season, were calculated and mapped. Correlation between rainfall totals at each of the 12 stations and monthly mean sea level pressure at 72 grid points in the area delimited by the 20° W and 50° E meridians and the 20° N and 60° N parallels, was performed. For each month, 12 correlation maps were prepared (one of each station). Similar maps were averaged together to form coherent rainfall regions. At the beginning of the rainy season (October) the rainfall in Israel is sporadic and spotty without a distinctable coherent region. At the end of the rainy season (April) the rainfall is more widespread, forming a large coherent region covering most of the country. Dry rainfall conditions in Israel, were found to be characterized by positive pressure departures in the eastern Mediterranean and over Israel and/or by easterly or southerly circulation over the eastern Mediterranean. Wet rainfall conditions in Israel, were found to be characterized by negative pressure departures in the eastern Mediterranean and over Israel and/or by westerly or northerly circulation over the eastern Mediterranean. Moreover, in many cases dry conditions in Israel, were associated with below normal pressure conditions over central or western Europe, while wet conditions in Israel, with above normal conditions over the same region, thus, reflecting the so-called Mediterranean Oscillation. Finally, normal rainfall conditions are characterized by very slight to negligible pressure departures over the entire Mediterranean and Europe. Received November 18, 1997 Revised March 3, 1998     

Synoptic and dynamic analysis of a flash flood-inducing heavy rainfall event in arid and semi-arid central-northern Iran and its simulation using the WRF model

The synoptic and dynamic aspects of heavy rainfall occurred on 5th May 2017 and caused flash flooding in arid and semi-arid central-northern Iran is analyzed by the Weather Research and Forecasting (WRF) model. This system synoptically is attributed to a surface low-pressure centered over southern Iran extended to the central parts, linking to a mid-tropospheric tilted-trough over western Iran, and advecting significant moisture from the Mediterranean Sea and the Red Sea to the studied area. The dynamical analysis revealed that the penetration of the upper-tropospheric potential vorticity streamer up to 300 hPa level was not related to such heavy rainfall. Contrarily, the low-level factors such as extensive moisture advection, mid-tropospheric diabatic processes such as the latent heat release, daytime deep convection, and topographical impact of Zagros Mountains were found as the key factors leading to this system. This study also examines 11 different convection schemes simulated by the WRF model and verified against rainfall observation. The forecast skills of the output simulations suggest the Grell-Devenyi scheme as the superior configuration in simulating observed precipitation of the event over the area.