Southwesterly Monsoon Flow Research Articles

Cycles and Propagation of Deep Convection over Equatorial Africa

Long-term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales. Satellite observations are used to understand the timing, duration, and frequency of deep convection in equatorial Africa, a region with some of the most intense thunderstorms. Yet little has been published about the propagation characteristics of mesoscale convection in that region. Diurnal, subseasonal, and seasonal cycles of cold cloud (proxy for convective precipitation) are examined on a continental scale. Organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions. Convection is triggered by heating of elevated terrain, sea/land breezes, and lake breezes. Coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward. They have an average 17.6-h duration and 673-km span; most have zonal phase speeds of 8–16 m s−1. Propagating convection occurs in the presence of moderate low-level shear that is associated with the southwesterly monsoonal flow and midlevel easterly jets. Convection is also modulated by eastward-moving equatorially trapped Kelvin waves, which have phase speeds of 12–22 m s−1 over equatorial Africa. Westward propagation of mesoscale convection is interrupted by the dry phase of convectively coupled Kelvin waves. During the wet phase, daily initiation and westward propagation continues within the Kelvin wave and the cold cloud shields are larger. Mesoscale convection is more widespread during the active phase of the Madden–Julian oscillation (MJO) but with limited westward propagation. The study highlights multiscale interaction as a major source of variability in convective precipitation during the critical rainy seasons in equatorial Africa.

Improvement of Statistical Typhoon Rainfall Forecasting with ANN-Based Southwest Monsoon Enhancement

Typhoon Morakot 2009, with significant southwest monsoon flow, produced a record-breaking rainfall of 2361 mm in 48 hours. This study hopes to improve a statistical typhoon rainfall forecasting method used over the mountain region of Taiwan via an artificial neural network based southwest monsoon enhancement (ANNSME) model. Rainfall data collected at two mountain weather stations, ALiShan and YuShan, are analyzed to establish the relation to the southwest monsoon moisture flux which is calculated at a designated sea area southwest of Taiwan. The results show that the moisture flux, withsouthwest monsoon flow, transported water vapor during the landfall periods of Typhoons Mindulle, Bilis, Fungwong, Kalmaegi, Haitaing and Morakot. Based on the moisture flux, a linear regression is used to identify an effective value of moisture flux as the threshold flux which can enhance mountain rainfall in southwestern Taiwan. In particular, a feedforward neural network (FNN) is applied to estimate the residuals from the linear model to the differences between simulated rainfalls by a typhoon rainfall climatology model (TRCM) and observations. Consequently, the ANNSME model integrates the effective moisture flux, linear rainfall model and the FNN for residuals. Even with very limited training cases, our results indicate that the ANNSME model is robust and suitable for improvement of TRCM rainfall prediction. The improved prediction of the total rainfall and of the multiple rainfall peaks is important for emergency operation.

Observation of entrainment at the interface between monsoon flow and the Saharan Air Layer

AbstractDuring the AMMA experiment, the French ATR aircraft flew in the lower troposphere in south Niger, with instruments that allow fast measurement of wind, temperature and humidity. Stacked legs flown between early June and mid‐August into the moist southwesterly monsoon flow and the dry northeasterly flow of the Saharan Air Layer (SAL) above, enable us to study the interaction between the two flows at the turbulence scale and the role of the planetary boundary layer (PBL) in the transfers between them. We describe the fluctuations observed at the top of the PBL and their evolution during the season. Dry tongues approximately 600 m wide and a few kilometres apart, intruding from the overlying SAL down into the PBL, are characteristic of the Sahelian region at that time of year. They are fully part of the entrainment process at the PBL top and responsible for large moisture fluxes in the upper PBL. They also skew the water vapour distribution which in turn impacts the water vapour scales. We evaluate the impact of the entrainment by use of a conditional analysis on combined fluctuations. The dry tongues play the largest role down to two thirds of the PBL depth. We find larger entrainment during early monsoon than during its active phase, and a large variability of the ratio of entrainment buoyancy flux over surface flux. Copyright © 2009 Royal Meteorological Society

Investigation of a heavy rainfall event over southwestern Taiwan associated with a subsynoptic cyclone during the 2003 Mei-Yu season

The objective of this study is to perform observational data analyses, including European Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite imagery, radar reflectivity, and rainfall data, and numerical simulations with the Weather Research and Forecast (WRF) model to investigate both the synoptic and mesoscale processes responsible for causing the heavy rainfall event which produced up to 379.5 mm over southwestern Taiwan on 7 June 2003. We found that an 850 hPa subsynoptic cyclone, composed of both a low-pressure center and a vortex, formed over the eastern Tibetan Plateau and moved with a 500 hPa shortwave trough toward a wind shear zone over southeastern China in the early morning of 7 June. The wind shear zone was generated by a southwesterly monsoonal flow and a northeasterly flow associated with a high pressure in eastern China. The intensifying 850 hPa subsynoptic cyclone served as a precursor of the heavy rainfall episode in Taiwan. The 850 hPa subsynoptic cyclone extended downward to form a surface subsynoptic cyclone over the southeastern coast of China by coupling the ascending motions associated with the 500 hPa shortwave trough and the 850 hPa subsynoptic cyclone. The mesoscale convective system associated with this surface subsynoptic cyclone migrated from southeastern China to the southwestern Taiwan Strait and then toward southwestern Taiwan. Meanwhile, the speed of the 850 hPa low-level jet on the southern side of the subsynoptic cyclone exceeded 20 m s − 1 over the southern Taiwan Strait. This jet then served as a conveyor belt of the moist airstream and enhanced the inland-moving convection over the mountain slopes. In addition, the low-level convergence resulted from the confluence of flow due to flow deflection over southern Taiwan topography also facilitated rainfall there.

On the late northward propagation of the West African monsoon in summer 2006 in the region of Niger/Mali

This paper investigates the fine‐scale dynamical processes at the origin of the late northward migration of the monsoon flow in summer 2006 in the region of Niger and Mali (onset on 3 July 2006 compared to the climatological onset date, 24 June). Compared to a 28‐year climatology, 2006 NCEP‐2 reanalyses show evidence of an anomalous pattern during 10 days between 25 June and 3 July 2006, characterized by the African Easterly Jet (AEJ) blowing from the northeast along a narrow northeast/southwest band located over the Hoggar and Air mountains associated with an unusually strong northeasterly harmattan in the lee of the mountains. Using data collected during the African Monsoon Multidisciplinary Analysis (AMMA) experiment and mesoscale numerical simulations, this study shows evidence of interaction between the AEJ and the orography supported by the reduced gravity shallow water theory which explains the enhancement of the harmattan downstream of the Hoggar and Air mountains in summer 2006. The enhanced harmattan contributes to move southward the intertropical discontinuity (ITD) defined as the interface between the cool moist southwesterly monsoon flow and the warm dry harmattan. Finally, an interaction between the ITD and African Easterly waves contributes to propagate the ITD southward retreat about 1500 km to the west of the Hoggar and Air mountains.

Annual Cycle of Rainfall in the Western North Pacific and East Asian Sector

Abstract The annual cycle of precipitation over the western North Pacific and East Asian (WNP–EA) sector has five major periods: spring, the first and second wet periods, fall, and winter. In this study, processes that induce precipitation in each period are examined from a large-scale point of view. The wet phase over this sector has two distinct periods, which are dominated by the Asian summer monsoon circulation induced by the land–ocean contrast of net energy into the atmospheric column (Fnet). In the first wet period, the pre-mei-yu/mei-yu rainband is directly associated with a moisture flux convergence caused by the southwesterly Asian summer monsoon flow and the southeasterly trade winds, and indirectly associated with a dynamic feedback induced by this horizontal moisture convergence. The tropical convection, in the meantime, is associated with a rising motion that is induced by positive Fnet. In the second wet period, the WNP summer monsoon gyre dominates the rainfall of this region, which is partially associated with warmer local sea surface temperature (SST) via positive Fnet. The land–sea contrast of Fnet and the atmosphere–ocean interaction also play an important role in establishing the monsoon gyre. The dry phase over the WNP–EA region is the winter period in which precipitation is associated with winter storm activities and large-scale lifting associated with a pressure surge. In the two transition phases, due to a difference in heat capacity, the atmosphere and ocean have distinct impacts on precipitation, albeit similar solar insolations during the two periods. In the spring period, the atmospheric condition is favorable for convection, while the ocean surface is relatively colder, so the horizontal moisture advection associated with the westward extent of the Pacific subtropical high, which is different from a typical winter frontal system, is a major source for the spring rain. In the fall period, however, the atmospheric conditions dominated by the Asian winter monsoon circulation suppress convection, while relatively warmer SST still maintains tropical convection over the southern part of the WNP–EA region. Over the northern part of the WNP–EA region, the fall precipitation is associated with frontal systems, similar to those in winter.

Statistics of Heavy Rainfall Occurrences in Taiwan

Abstract The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997–2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate >15 mm h−1 and daily accumulation >50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nontyphoon heavy rainfall days are frequent (>60%), but only a few stations recorded extremely heavy rainfall (>130 mm day−1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spatial distribution of heavy rainfall occurrences with a pronounced afternoon maximum, especially during the summer months under the southwesterly monsoon flow. After the summer–autumn transition, heavy rainfall days are most frequent over northeastern Taiwan under the northeasterly monsoon flow. Extremely heavy rainfall events (>130 mm day−1) are infrequent during the winter months because of stable atmospheric stratification with a low moisture content. Typhoon heavy rainfall events start in early May and become more frequent in late summer and early autumn. During the analysis period, heavy rainfall occurrences are widespread and dominated by extremely heavy rainfall events (>130 mm day−1) on the windward slopes of the storm circulations. The spatial distribution of typhoon heavy rainfall occurrences depends on the typhoon track with very little diurnal variation.

Analysis of Influenza Cases and Seasonal Index in Thailand

This study investigated the pattern and seasonal index of influenza cases in Thailand. Our results showed that southern Thailand had the highest influenza incidence among the four regions of Thailand (i.e. north, northeast, central and southern Thailand). The influenza pattern in southern Thailand was similar to that of north- eastern Thailand. Seasonal index values of influenza cases in Thailand were higher in the hot season than in the wet season. Influenza cases started to increase at the beginning of the hot season (April), reached a maximum in August, rapidly declined in the middle of the wet season and reached the lowest value in December. Seasonal index values for northern Thailand differed from other regions of Thailand. region of Thailand. The major influences on Thailand's climate are its location in the tropical monsoon zone of mainland Southeast Asia and certain topographic features that affect the distribution of precipitation. Beginning in May, the warm, humid air masses of the southwest monsoon flow northeastward over the region from the Indian Ocean, depositing great quantities of precipitation; rainfall reaches a maximum in September. The wind pattern is reversed between November and February, when the northeast monsoon brings cool, relatively dry air masses in a southwesterly flow to create a seasonably cooler climate for much of the country. Stagnant air in March and April is associated with a distinct hot and dry inter-monsoonal period (4).

Sun-Earth System Interaction studies over Vietnam: an international cooperative project

Abstract. During many past decades, scientists from various countries have studied separately the atmospheric motions in the lower atmosphere, in the Earth's magnetic field, in the magnetospheric currents, etc. All of these separate studies lead today to the global study of the Sun and Earth connections, and as a consequence, new scientific programs (IHY- International Heliophysical Year, CAWSES- Climate and Weather in the Sun-Earth System) are defined, in order to assume this new challenge. In the past, many scientists did not have the possibility to collect data at the same time in the various latitude and longitude sectors. Now, with the progress of geophysical sciences in many developing countries, it is possible to have access to worldwide data sets. This paper presents the particularities of geophysical parameters measured by the Vietnamese instrument networks. It introduces a cooperative Vietnamese-IGRGEA (International Geophysical Research Group Europe Africa) project, and presents, for the first time, to the international community, the geophysical context of Vietnam. Concerning the ionosphere: since 1963, during four solar cycles, the ionosonde at Phu Thuy (North Vietnam) was operating. The Phu Thuy data exhibits the common features for the ionospheric parameters, previously observed in other longitude and latitude sectors. The critical frequencies of the E, F1 and F2 ionospheric layers follow the variation of the sunspot cycle. F2 and E critical frequencies also exhibit an annual variation. The first maps of TEC made with data from GPS receivers recently installed in Vietnam illustrate the regional equatorial pattern, i.e. two maxima of electronic density at 15° N and 15° S from the magnetic equator and a trough of density at the magnetic equator. These features illustrate the equatorial fountain effect. Concerning the Earth's magnetic field: a strong amplitude of the equatorial electrojet was first observed by the CHAMP satellite at the height of 400 km in the Vietnamese longitude sector. In this paper we compare the ground magnetic observations of the Indian and Vietnamese magnetometer networks. This comparison highlights the regional structure of the amplitude of the equatorial electrojet, which is stronger in Vietnam than in India. Concerning the monsoon: Vietnam exhibits a strong monsoon and has mainly one rainy season peaking in August, hence associated with the southwest monsoon flow. But some monsoon variability from one place to another is related to the orography. In the mountainous northern regions of Vietnam, there is an "early" monsoon peaking in July. In the coastal regions between 12° N and 19° N the monsoon season is centered on October. Concerning lightning: Vietnam is a country of strong atmospheric storms with some areas of very intense lightning in North Vietnam (22,5° N, 105° E) and in South Vietnam (11° N, 107° E). In North Vietnam strong lightning is associated with the most intense rainy region.

Abrupt transition from natural to anthropogenic aerosol radiative forcing: Observations at the ABC‐Maldives Climate Observatory

Using aerosol‐radiation observations over the north Indian Ocean, we show how the monsoon transition from southwest to northeast flow gives rise to a similar transition in the direct aerosol radiative forcing from natural to anthropogenic forcing. These observations were taken at the newly built aerosol‐radiation‐climate observatory at the island of Hanimaadhoo (6.776°N, 73.183°E) in the Republic of Maldives. This observatory is established as a part of Project Atmospheric Brown Clouds (ABC) and is referred to as the ABC‐Maldives Climate Observatory at Hanimaadhoo (ABC_MCOH). The transition from the southwest monsoon during October to the northeast monsoon flow during early November occurs abruptly over a period of few weeks over ABC‐MCOH and reveals a dramatic contrast between the natural marine aerosols transported from the south Indian Ocean by the southwest monsoon and that of the polluted aerosols transported from the south and Southeast Asian region by the northeast monsoon. We document the change in the microphysical properties and the irradiance at the surface, to identify the human signature on aerosol radiative forcing. We first establish the precision of surface radiometric observations by comparing simultaneous observations using calibrated Kipp & Zonen and Eppley pyrheliometers and pyranometers for direct, diffuse and global solar radiation. We show that the direct, diffuse and global radiation can be measured within a precision of about 3 to 5 Wm−2. Furthermore, when we include the observed aerosol optical properties as input into the Monte Carlo Aerosol Cloud Radiation (MACR) model (developed by us using Indian Ocean Experiment data), the simulated fluxes agree with the observed direct, diffuse and global fluxes within the measurement accuracy. A steady southwest monsoon flow of about 5 to 7 ms−1 persists until middle of October which switches to an abrupt change in direction to northeast flow of similar speeds bringing in polluted air from south Asia. However, it is not until end of November that a steady northeasterly flow is well established. The abrupt transition is accompanied by a large increase in aerosol optical depth from about 0.1 in October to as high as 0.4 during January, the SSA decreases from 1 to about 0.9, and the Angstrom coefficient increases from about 0.5 (suggesting large particles > 1 micron) to about 1.2 in January (submicron particles) and an increase in aerosol extinction below 3 km altitude. These changes are consistent with the transport of continental pollution from south and Southeast Asia (about 1000 to several 1000 km away from ABC_MCOH) to the north Indian Ocean during the northeast monsoon. The direct aerosol forcing, determined solely from radiometric observations without resorting to models, changes from −5 Wm−2 during October to −22 Wm−2 during January. About 50% of this forcing occurs in the photosynthetically active part of the solar spectrum (0.4 to 0.7 micron). MACR shows that the decrease in SSA from 1 to 0.9 changes the aerosol forcing efficiency by a factor of about 2 from about −40 Wm−2 (per AOD) in October to −80 Wm−2 (per AOD) in January. Thus the arrival of the brown clouds from south and Southeast Asia has a large seasonal dimming effect over remote parts of the north Indian Ocean. The observational results presented here should be used for validating climate models that attempt to simulate the anthropogenic effects of aerosol forcing on climate. The observational and model results presented in this study shows how near continuous surface based observations can be used to differentiate the human impact on aerosol forcing which is a major challenge for models.

The thermodynamic and dynamical features of double front structures during 21–31 July 1998 in China

The daily 1° × 1° data of the Aviation (AVN) model, the black body temperature (TBB) data of cloud top, and cloud images by geostationary meteorological satellite (GMS) are used to identify a dew-point front near the periphery of the western Pacific subtropical high (WPSH). The results clearly demonstrate the existence of the dew-point front, and its thermodynamic and dynamic structural characteristics are analyzed in detail. The dew-point front is a transitional belt between the moist southwest monsoon flow and the dry adiabatic sinking flow near the WPSH, manifested by a large horizontal moisture gradient in the mid-lower troposphere and conjugated with the mei-yu front to form a predominant double-front structure associated with intense rainfall in the mei-yu period. The mei-yu front is located between 30° and 35°N, vertically extends from the ground level to the upper level and shifts northward. The dew-point front is to the south of the mei-yu front and lies up against the periphery of the WPSH. Generally, it is located between 850 hPa and 500 hPa. On the dew-point front side, the southwesterly prevails at the lower level and the northeasterly at the upper level; this wind distribution is different from that on the mei-yu front side. Vertical ascending motion exists between the two fronts, and there are descending motions on the north side of the mei-yu front and on the south side of the dew-point front, which form a secondary circulation. The dynamics of the double fronts also have some interesting features. At the lower level, positive vertical vorticity and obvious convergence between the two fronts are clearly identified. At the mid-lower level, negative local change of the divergence (corresponding to increasing convergence) is often embedded in the two fronts or against the mei-yu front. Most cloud clusters occur between the two fronts and propagate down stream in a wave-like manner.

Evolution and Structure of the Mesoscale Convection and Its Environment: A Case Study during the Early Onset of the Southeast Asian Summer Monsoon

Abstract The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM). For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow gener...

Effects of Mountain Uplift on East Asian Summer Climate Investigated by a Coupled Atmosphere–Ocean GCM

To study the effects of progressive mountain uplift on East Asian summer climate, a series of coupled general circulation model (CGCM) experiments were performed. Eight different mountain heights were used: 0% (no mountain), 20%, 40%, 60%, 80%, 100% (control run), 120%, and 140%. The land–sea distribution is the same for all experiments and mountain heights are varied uniformly over the entire globe. Systematic changes in precipitation pattern and circulation fields as well as sea surface temperature (SST) appeared with progressive mountain uplift. In summertime, precipitation area moves inland on the Asian continent with mountain uplift, while the Pacific subtropical anticyclone and associated trade winds become stronger. The mountain uplift resulted in an SST increase over the western tropical Pacific and the Maritime Continent and an SST decrease over the western Indian Ocean and the central subtropical Pacific. There is a drastic change in the East Asian circulations with the threshold value at the 60% mountain height. With the mountain height below 60%, the southwesterly monsoon flow from the Indian Ocean becomes strong by uplift and transports moisture toward East Asia, forming the baiu rainband. With higher mountain heights, intensified subtropical trade winds transport moisture from the Pacific into the Asian continent. In order to investigate how the SST change affected the results presented herein, additional experiments were performed with the same experimental design but with the atmospheric GCM (AGCM). A comparison between CGCM and AGCM experiments revealed that major features such as a shift in precipitation inland and an appearance of the baiu rainband by higher orography were reproduced similarly in both the AGCM and the CGCM. However, there was a qualitatively as well as quantitatively different feature. The anticyclonic circulation anomalies in the lower troposphere, which appeared by mountain uplift in the tropical western Pacific in the CGCM associated with lowered SST, fed more moisture over East Asia and resulted in a stronger baiu rainband in the CGCM than that in the AGCM. An extent of the monsoon westerly flow is regulated by competition between the Pacific subtropical anticyclone and the southwest monsoon. The confluence zone was located near the Philippines throughout the mountain uplift in the AGCM, but it shifted backward to the west via mountain uplift in the CGCM associated with simulated SST changes. Overall the CGCM showed a larger sensitivity to mountain uplift than the AGCM due to the SST changes, thus warranting an examination of the importance of air–sea coupling and a need for the use of coupled models for such sensitivity studies.

Characteristics of Heavy Summer Rainfall in Southwestern Taiwan in Relation to Orographic Effects

On the windward side of southwestern Taiwan, about a quarter to a half of all rainfall during mid-July through August from 1994 to 2000 came from convective systems embedded in the southwesterly monsoon flow. k this study, the causes of two heavy rainfall events (daily rainfall exceeding 100 mm day over at least three rainfall stations) observed over the slopes and/or lowlands of southwestern Taiwan were examined. Data from European Center for Medium-Range Weather Forecasts /Tropical Ocean- Global Atmosphere (EC/TOGA) analyses, the rainfall stations of the Automatic Rainfall and Meteorological Telemetry System (ARMTS) and the conventional surface stations over Taiwan, and the simulation results from a regional-scale numerical model were used to accomplish the objectives. In one event (393 mm day on 9 August 1999), heavy rainfall was observed over the windward slopes of southern Taiwan in a potentially unstable environment with very humid air around 850 hPa. The extreme accumulation was simulated and attributed to orographic lifting effects. No preexisting convection drifted in from the Taiwan Strait into western Taiwan.

A Diagnostic Study of Formation and Structures of the Meiyu Front System over East Asia

The concept of a dew-point front was introduced, and its existence was identified near the periphery of the west Pacific subtropical anticyclone by using the daily 1 � � 1 � data of the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The dew-point front was a transitional belt between the moist southwest monsoon flow and the dry adiabatic sinking flow, marked by a large horizontal moisture gradient in the mid-lower troposphere. The dew-point front and the Meiyu front, to its north side, formed the Meiyu front system (MYFS). The Meiyu front was the north branch of the MYFS, and extended northward in the mid-lower troposphere. The dew-point front was the south branch of the MYFS, located between 500 hPa and 700 hPa. Along the dew-point front (Meiyu), westerly (easterly) and easterly (westerly) winds prevailed in the lower (upper) and upper (lower) troposphere respectively. Strong ascending motion in the passsageway between the two fronts was surrounded by subsidence. Further, a frontogenesis function was used to diagnose the frontogenesis of the MYFS. The analysis indicated that the convergence and deformation of the horizontal wind were important factors responsible for both the formation and development of the Meiyu front and the dew-point front.

Numerical Simulations of the Barrier Jet over Northwestern Taiwan during the Mei-Yu Season

Orographic effects on the airflow over Taiwan under the prevailing southwesterly monsoon flow are studied using the fifth-generation Pennsylvania State University‐National Center for Atmospheric Research (PSU‐ NCAR) Mesoscale Model (MM5). In particular, the orographically induced low-level strong winds along the northwestern coast of Taiwan are simulated in the absence of radiative forcing, thermal forcing from the surface, and synoptic forcing. The momentum balance for the low-level strong winds along the northwest coast of Taiwan shows that, above the mixed layer, the alongshore force balance is dominated by the inertial advection term and the pressure gradient force term. The significant Lagrangian flow acceleration of the northern branch of the deflected airflow along the northwest coast is mainly related to the pressure gradient force as the airflow moves down the orographically induced pressure ridge along the windward coast. Even though the alongshore pressure gradient force along the northwest coast is largest at the surface, the observed and simulated wind profiles show that the wind speed maximum in the vertical of the barrier jet occurs at a height of ;800 m primarily due to frictional retardation in the lowest levels. Within the well-mixed layer, the alongshore force balance is a threeway balance between inertial advection, pressure gradient force, and frictional force. Normal to the northwest coast, the force balance is approximately geostrophic. In addition, the location and the strength of the orographically induced low-level strong winds along the northwest coast of Taiwan also depend on the impinging angle and the wind speed of the prevailing winds.

Simulation of the east asian summer monsoon using a variable resolution atmospheric GCM

The East Asia summer monsoon (EASM) is simulated with a variable resolution global atmospheric general circulation model (GCM) developed at the Laboratoire de Meteorologie Dynamique, France. The version used has a local zoom centered on China. This study validates the model's capability in reproducing the fundamental features of the EASM. The monsoon behaviors over East Asia revealed by the ECMWF reanalysis data are also addressed systematically, providing as observational evidence. The mean state of the EASM is generally portrayed well in the model, including the large-scale monsoon airflows, the monsoonal meridional circulation, the cross-equatorial low-level jets, the monsoon trough in the South China Sea, the surface cold high in Australia, and the upper-level northeasterly return flow. While the performance of simulating large-scale monsoonal climate is encouraging, the model's main deficiency lies in the rainfall. The marked rainbelt observed along the Yangtze River Valley is missed in the simulation. This is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western Pacific subtropical high, which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies. The excessively westward extension of the high, together with the distorted Indian low, also makes the contribution of the tropical southwest monsoon to the moisture convergence over the Yangtze River Valley too weak in the model. The insufficient plateau heating and the resulting weak land-sea thermal contrast are responsible for the weakly reproduced monsoon. It is the deficiency of the model in handling the low-level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating. Comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves, in a general manner, the EASM simulation.

A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales

AbstractThis article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important ‘local’ factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a ‘wet mode’ and a ‘dry mode’ in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958–1967 (representing the wet mode) and 1968–1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter‐tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so).The results of this study suggest that the key factor controlling the occurrence of the ‘wet Sahel’ mode versus the ‘dry’ mode is the presence of deep, well‐developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well‐known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the ‘wet Sahel’ and ‘dry Sahel’ modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well‐known dipole and no‐dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea‐surface temperatures (SSTs) in the Gulf of Guinea. Copyright © 2001 Royal Meteorological Society

A Report of the Field Operations and Early Results of the South China Sea Monsoon Experiment (SCSMEX)

The South China Sea Monsoon Experiment (SCSMEX) is an international field experiment with the objective to better understand the key physical processes for the onset and evolution of the summer monsoon over Southeast Asia and southern China aiming at improving monsoon predictions. In this article, a description of the major meteorological observation platforms during the intensive observing periods of SCSMEX is presented. In addition, highlights of early results and discussions of the role of SCSMEX in providing valuable in situ data for calibration of satellite rainfall estimates from the Tropical Rainfall Measuring Mission are provided. Preliminary results indicate that there are distinctive stages in the onset of the South China Sea monsoon including possibly strong influences from extratropical systems as well as from convection over the Indian Ocean and the Bay of Bengal. There is some tantalizing evidence of complex interactions between the supercloud cluster development over the Indian Ocean, advancing southwest monsoon flow over the South China Sea, midlatitude disturbances, and the western Pacific subtropical high, possibly contributing to the disastrous flood of the Yangtze River Basin in China during June 1998.

Characteristics of Rainfall Distributions over Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX)

Abstract Based on 808 daily rainfall stations, rainfall distributions over Taiwan during TAMEX (Taiwan Area Mesoscale Experiment) (10 May–27 June 1987) were analyzed. The total rainfall had a maximum on the windward slopes under the prevailing southwesterly monsoon flow, indicating that orographic lifting plays an important role in determining rainfall distribution. About 50% of the rainfall occurred as scattered orographic showers during nonfrontal periods. Along the northwestern coastal plain, about 80% of the rainfall was associated with the passage of surface fronts. Along the eastern coastal, mountainous region, which is on the lee side under the prevailing southwesterly monsoon flow, a large portion of the rainfall was produced by upper-level, cold-core lows and a midlevel vortex event. For these cases, low-level flow was southerly or southeasterly because Taiwan was on the western or southwestern periphery of the western Pacific subtropical high. In this situation, eastern Taiwan was on the windwar...