This paper has investigated the structure, propagation, orientation, and rainfall of precipitation systems and environmental wind and thermodynamic structures observed by the Doppler radar and the radiosonde system on board the R/V Mirai around a fixed site (12°N, 135°E) during the onset of the western North Pacific summer monsoon in 2008. The monsoon onset occurred in mid-June and was accompanied by a remarkable change in large-scale circulation over the observational area. Statistical analyses indicate that precipitation systems evolved deeply and widely upon the monsoon onset. The development of mesoscale convective systems contributed to the formation of much more rainfall. Precipitation systems tended to move along with low-level winds during the pre-onset and the early-onset periods and with mid-level winds during the latter-onset period. The increase in the height and the area of precipitation systems possessed much stronger correlation with the evolution of the low-level shear and the mid-level humidity than with other environmental variables. The internal structure of two precipitation systems that occurred in the environment with the distinct low-level shear and mid-level humidity has also been analyzed. The shear-parallel precipitation system during the early-onset period occurred in the environment with the dry middle troposphere and the weak low-level shear and was characterized by the descending rear inflow and weaker updrafts. On the other hand, the shear-normal precipitation system during the latter-onset period occurred in the environment with the moist middle troposphere and the strong low-level shear and was featured with the elevated rear inflow and stronger updrafts. According to the observational results, possible interaction processes between precipitation systems and their environment during the monsoon onset are discussed.
This study describes the characteristics of the cross-equatorial northerly surge (CENS), a northerly flow that appears intermittently over the equatorial region between October and April, and its relationship to the precipitation patterns over the Maritime Continent. The CENS in this study was defined as the area-averaged northerly wind exceeding 5 m s-1 over 105°E-115°E, 5°S-EQ based on the QuikSCAT sea surface wind data. During the 10 winters from December 2000 to March 2009, 62 CENS events were extracted and classified into the following patterns: 11 events were associated with cold surges and termed the CS pattern; 20 events were associated with tropical intra-seasonal variations and termed the MJO pattern; 16 events were associated with both cold surges and tropical intra-seasonal variations and termed the CS-MJO pattern; and other 15 events were not associated with these patterns. In the CS pattern, the development and dissipation processes of the cold surge appeared, and the increased precipitation to the north of the island of Java was significant. In the MJO pattern, a wide area of northerly winds in the vicinity of the depression around 10°S continued for a longer period than in the CS pattern, and the increased precipitation west of Sumatra and south of Java was significant. The CS-MJO pattern showed features of the northerly wind fields of both the CS and MJO patterns and was associated with the greatest increase in precipitation of the three patterns in the Maritime Continent, in particular, in northwestern Java and both north and south of Java. The generation of CENS was an important environmental factor for inducing the wide positive precipitation anomaly compared to the climatological mean over the Maritime Continent.
Isotopic and meteorological observations in November 2006 on the west coast of Sumatera, Indonesia during the intense observation period of the Hydrometeorological ARray for Intraseasonal Variation-Monsoon AUto-monitoring (HARIMAU2006), revealed the impacts of large-scale moisture transport and mesoscale processes on precipitation isotope ratios. Intraseasonal changes in the precipitation δ2H in November had large variability ranging from +10 to -65 per mil, as a result of the changes in the large-scale moisture transport associated with the intraseasonal oscillation with a time-scale of 10-15 day over Sumatera. The isotopic composition of precipitation was independent from difference in precipitation type (convective or stratiform precipitation). An isotope circulation model reproduced the observed isotopic changes, supporting that the isotopic effect of large-scale moisture transport was the main contributor to intraseasonal isotopic changes. In high-frequency samples taken over a shorter time scale, isotopic variability was related to event type classified by the analysis of radar observations, although the isotopic effects of mesoscale processes on the isotopic averages of each precipitation event were almost masked by the isotopic effect of large-scale moisture transport. The precipitation δ2H accompanying the well-organized convection type decreased significantly by about 20 per mil. Drastic changes in isotope ratios could be described by the Rayleigh distillation process. Isotope ratios of precipitation gently decreased and subsequently increased in the unorganized convection type since the water vapor in surrounding convectively rising air isotopically enriched the remaining low-isotope water vapor advected from the precedent clouds. Isotope ratios in the stratiform precipitation remained steady, possibly attributable to the homogeneous moisture of stratiform clouds.
An overview of convective activity during the HARIMAU2006 campaign conducted from 26 October to 27 November 2006 was presented, focusing on the differences between coastal land/sea and inactive/active phases of intraseasonal variation (ISV) based on observations using an X-band Doppler radar (XDR) and intensive soundings at Sumatera Island. Diurnal variation (DV) in coastal convections and formation of the coastal heavy rainband (CHeR) along Sumatera Island were also examined in terms of diurnal land-sea migration of coastal convective systems. Convection in the ISV inactive period (PP1) contained convective rain fractions nearly twice as much as stratiform rain fractions, whereas that in the ISV active period (PP2) comprised convective and stratiform elements almost equally. Vertical profiles of radar echo coverage for stratiform rain during PP2 were greater than those during PP1, especially in the lower troposphere over the sea. The radar echo coverage for convective rain over the sea during both periods was nearly double that over land from the near surface up to 6 km high. Convection was generated in the southwestern foothills of the mountain range in the early afternoon (12-15 Local Time, LT). Part of the convective system remained over the coastal land and exhibited weak reflectivity until the next morning. The other part migrated offshore at a speed of approximately 4 m s-1 and intensified until around 21 LT while still offshore. Additional convective cells also developed offshore in the early morning hours, independent of those that formed over land. Results suggested that the CHeR along Sumatera Island is dictated by diurnal variations in coastal convective development and consists of the following phases: 1) migration of convection away from the coastal land and its redevelopment in the late evening, and 2) additional generation of convection just offshore during the early morning hours.
Temperature data from Global Positioning System based Radio Occultation (GPS RO) soundings of the Formosa Satellite mission 3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F-3/C) micro satellites has been investigated in detail to study the Kelvin wave properties. The high temporal and spatial resolution satellite data from August 2006 to August 2009 have enabled the investigation of Kelvin wave activity on each day. The dominant waves of wave numbers 1 and 2 (W1 and W2) have been investigated in detail at three altitudes—19, 25 and 30 km, and it is found that the amplitude of W1 is greater than that of W2 during 60% of the time. A statistical study of the amplitudes of W1 and W2 is also presented and it is found that the dominant amplitudes are 0.5 to 1.0 K for both waves. At lower altitudes (19 km), the amplitudes of W1 are larger and the distribution is also broader. The amplitudes of both waves in the stratosphere are higher during the easterlies of the quasi-biennial oscillation (QBO) and are maximum when the zonal wind changes from easterlies to westerlies. In the lower altitudes near the tropopause they vary in consonance with the outgoing long wave radiation, a proxy of deep convection. Deduction of the Kelvin wave periods and phase velocities has been possible with better accuracy with the use of the F-3/C data. The average periods of W1 for all years are 15 ± 3, 13 ± 4, and 10 ± 3 days at altitudes 19, 25, and 30 km, respectively and the average periods of W2 for all years are 10 ± 2, 7 ± 2, and 6 ± 2 days, respectively. These standard deviations are geophysical and are due to the variation in the periods of the individual Kelvin wave events and identification of the period for a single Kelvin wave event is correct to within ± one day. We found that the Kelvin waves of both the zonal wave numbers are slow in the lower altitudes and fast in the higher altitudes. Also, the periods decrease gradually with height. This is the most important result of the present study.
This study aims to describe the influence of soil moisture on deep convection around Ulaanbaatar, Mongolia, as an arid environment based on observation data. Variation in the volumetric soil moisture was evaluated using the AMSR-E microwave data, and the convective activity was estimated using C-band airport radar. The following results were obtained for the influence of soil moisture on deep convection. 1. Soil moisture averaged over the analysis region was strongly influenced by the variations in the precipitation amount. 2. Onset of deep convection and the peak of convective activity were delayed for 1-2 h on moist soil days as compared to those on dry soil days. 3. Deep convection tended to develop over an area with positive anomalies of soil moisture under dry soil moisture conditions. 4. Deep convection could occur even for relatively high stability of the SSI (Showalter’s stability index) values ranging from 1 to 5°C on dry soil days, whereas the SSI values of less than 1°C were necessary for the occurrence of deep convection on the moist soil days. 5. Deep convection during the early afternoon over the mountains tended to be weak when the soil moisture content over the mountains was higher than 11%.
The authors investigate the interannual rainfall variability over the eastern maritime continent using station rainfall data in the Republic of Palau during 1923-2009 and Eastern Indonesia during 1973-2008. Two possible mechanisms are proposed to explain the interannual rainfall variability associated with the El Niño/Southern Oscillation over the surrounding region of the Banda and Arafura seas. During the El Niño developing year, rainfall starts to decrease, first in region close to the equator in summer than in off-equatorial region in autumn, such as in mainland Palau. Eastern Indonesia has a dry season during boreal summer in general, and the dry season becomes a significantly drier condition during El Niño years. The start of increasing rainfall is delayed. Cloud data show a convection-suppressed region from the east coast of the Kalimantan to the west coast of the New Guinea during July to November of El Niño years. One of the mechanisms for these changes is the air-sea interaction over the Banda and Arafura seas. Intensification of seasonal southerly wind associated with the western North Pacific monsoon decreases sea-surface temperature over the Banda and Arafura seas, and convection is more suppressed during El Niño years than other years. Another mechanism is the subsidence over the Banda and Arafura seas during the pre-monsoon of the Australian summer monsoon. Land-ocean thermal contrast between Australia and its surrounding seas is intensified during El Niño years. Shallow thermal convection produces subsidence over the Banda and Arafura seas, and extremely dry condition spreads over the surrounding region in October during El Niño years.
Intense southwest monsoon (SWM) rainfall events causing massive landslides and flash floods along the western sections of the Philippines were studied. These rainfall events, are not directly coming from the tropical cyclones (TCs) for they are situated far north to northeast of Luzon Island. The heavy rainfall is hypothesized as caused by the interaction of strong westerlies with the mountain ranges along the west coast of Luzon that produces strong vertical motion and consequently generates heavy rainfall. Four of heavy SWM rainfall cases were examined to determine how the presence and position of tropical cyclones in the Philippine vicinity affect these SWM rainfall events; three cases with TC of varying positions within the Philippine area of responsibility (PAR) and the fourth case without TC. Using a spatial Fourier decomposition approach, the total streamfunction is decomposed into two flow regimes: monsoon basic flow (Waves 0-1) and tropical cyclone perturbation flow (Waves 2-23) over a domain of (20°E-140°W, 5°S-35°N). The purpose of this flow decomposition is to determine the latter’s effect on or contribution to the monsoon activity. The analysis utilized the NCEP Final (FNL) data with 1° long. × 1° lat. resolution. Results show that the tropical cyclones over the Pacific Ocean located northeast of Luzon generate strong southwesterly winds over the west coast of Luzon. These in addition to the southwesterlies from the basic flow strengthened the southwest winds that interact with the high Cordillera Mountain ranges
The relationship between the Meiyu precipitation over the Yangtze-Huaihe River Basins (YHRB) and the frequencies of tropical cyclone (TC) genesis in the western North Pacific (WNP) for the period 1951-2006 has been investigated in the present paper. The results have shown that there exists a significant negative correlation between them, which is due to the fundamental changes in monsoonal airflows and associated moisture transports over East Asia and the WNP. In the years with fewer TCs genesis, the location of the main body of the subtropical high over the WNP is farther west and south, and its intensity is abnormally stronger. The monsoon trough also shifted farther west, with a weaker intensity. Under this condition, the monsoon flow and the corresponding moisture transport can not extend eastward down to the WNP, but turns to the East Asian region along the western flank of the subtropical anticyclone over the South China Sea (SCS). Meanwhile, the westerlies at mid-latitudes are located farther south with a stronger intensity. This kind of circulation conditions is favorable for the convergence of the dry and cold air from the mid- and high latitudes and the warm and moist air from the low latitudes over the YHRB, and leads to more precipitation there. In the years with more TCs, the subtropical high over the WNP is located farther north and east, with its intensity being weaker. The monsoon trough zonally extends eastward down to the open sea of the WNP. Most of the monsoon flow and the associated moisture transport from the tropical Indian Ocean can all the way reach the WNP, with a smaller proportion turning northward to the East Asian Meiyu region over the SCS, which is favorable for the genesis and development of TC in the WNP. At the same time, the location of the mid-latitude westerlies is farther north and its intensity is weaker, which is unfavorable for the occurrence of the precipitation over the YHRB. This kind of negative correlation relationship will be conducive to the seasonal forecast of the TC and the Meiyu during the rainy season in China.
The South China Heavy Rainfall Experiments (SCHeREX) was staged during 2008 and 2009 in the southern part of China by the Chinese Academy of Meteorological Sciences under the support of the Chinese Ministry of Science and Technology and China Meteorological Administration. SCHeREX aims at obtaining abundant observational datasets at the meso-β scale, better understanding of the structure and evolution of heavy precipitation systems in south China, exploring establishment of an operational platform for heavy rainfall monitoring and prediction, and improving the ability of heavy rainfall monitoring and prediction. Four zones were selected in SCHeREX, namely, the southern China, the middle reaches of the Yangtze River valley, the Huai River valley, and the lower reaches of the Yangtze River valley. The observation phases were May 1-June 10 in the southern China zone and June 10-July 20 in the other three zones. The efforts have led to the establishment of the meso-scale observing networks with enhanced capacity to observe precipitation systems at the meso-β scale level. The collected data have been utilized in meso-scale reanalysis not only to reveal the fine structures of the precipitation systems but also to provide better initial conditions for meso-scale numerical models to make short-term forecasts. Assimilation of the dropsonde data has improved the analysis of the locations and intensities of typhoons Goni and Morakot. With the real-time field data being part of the forecast system, the experiments have allowed more efficient interactions between the observing system and the forecast system and thus improve the performance of meso-scale heavy rainfall forecasts.
Simultaneous intensive radiosonde observation at three stations over northeast (Sylhet), central (Dhaka), and northwest (Bogra) Bangladesh was analyzed to describe a three-dimensional thermodynamic and airflow struc-tures on 25 April 2010, in which a distinct nocturnal rainfall occurred at Sylhet. As the observation was con-ducted in the daytime, the observed structures represent convectively inactive condition in a diurnal cycle. Local circulation was unclear and larger-scale airflow was dominated over Sylhet. The radiosonde observation indicated strong subsidence within 1-5 km over Dhaka and Bogra and within 4-5 km over Sylhet, whereas topographic uplifting of southwesterly airflow was observed within 1-3 km over Sylhet. Temperature profiles of Sylhet up to 3 km were lower than that of other stations, and easterly wind was observed at the surface.
The impact of northerly surges of the East Asian winter monsoon on tropical cyclogenesis over the eastern Indian Ocean and maritime continent was examined for the 6-month period of October to March from 1979/1980 to 2006/2007 based on case studies and lag-composite analysis. We focused on long-lasting northerly surge events at 6- to 30-day (sub-monthly or intraseasonal) time scales over the South China Sea. In addition, we examined seasonal differences in the impact of northerly surges over the South China Sea on tropical atmospheric circulation. The results show that northerly surges occur frequently in the period from October to March. Long-lasting northerly surges over the South China Sea intrude into tropical regions. Over the eastern Indian Ocean and maritime continent, the surges are associated with the appearance of tropical cyclones. However, the impact of these surges varies with the seasonal march. In October and November, tropical cyclones occur over the South China Sea during the northerly surge events, enhancing positive vorticity over the South China Sea. A cyclone pair symmetric with respect to the equator also appears over the eastern Indian Ocean in November and is responsible for the enhancement of the horizontal gradient of zonal wind by the northerly surge. In contrast, in December, January, and February, an asymmetric cyclone pair (the so-called Borneo vortex) develops around Borneo. The asymmetric cyclone pair around the maritime continent is associated with intensification of low-level wind along a channel between the islands of Borneo, Sumatra, and Java. In March, no clear tropical cyclone appears over the tropical regions in association with the northerly surge.
Numerical weather predictions of three heavy rainfall events in the northeast monsoon causing floods and damage in southern Thailand are reported in this paper. The Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model (MM5) was used with the Betts-Miller (BM), Grell (GR), and new Kain-Fritsch (KF2) convective parameterization (CP) schemes at 5 km resolution, and also with an unparameterized or explicit (EX) scheme, to look for a promising method for precipitation forecasting. The accumulated precipitation amounts predicted by the model were evaluated qualitatively by comparison with the Tropical Rainfall Measuring Mission (TRMM) maps, and quantitatively by calculating statistical score indices for different rainfall thresholds. The simulations were evaluated by comparing the synoptic near surface fields with the NCEP FNL objective analysis fields and also with the vertical profiles of wind speed, temperature and mixing ratio at Songkhla station. The simulations gave generally satisfactory predictions of the synoptic fields, except for an overprediction of the relative humidity. The heavy rainfall was associated in each case with a small vortex where the northeasterly wind interacted with a warm humid southerly wind. The model underestimated the heavy rainfall amounts, and widely different rainfall patterns were produced by the different schemes used. The KF2 and EX schemes generally gave better results than the BM and GR schemes.
Downward shortwave flux at the surface over China was derived from sunshine duration data using parameters of Jordan sunshine recorders. The sunshine duration data were obtained as routine meteorological data and ad-justed as necessary for the effects of topography, station surroundings, altitude, and atmospheric turbidity. Calculated flux was verified by in situ observations. Daily flux was calculated at more than 190 stations in China, Climatic maps (statistical period: 1971-2000) of surface solar radiation were produced and have been made public. The sunshine duration-based shortwave flux was used to check satellite observation-based datasets. Two ver-sions of satellite-based surface radiation budget datasets (SRB 2.0 and SRB 3.0) from NASA's Langley Research Center were investigated. Compared with the SRB 2.0 dataset, the SRB 3.0 dataset showed improved shortwave flux, especially over western China and the Tibetan Plateau.
In this study, the processes responsible for an intense wind event that occurred in west Sumatera on November 19, 2006, during the first campaign of the Hydrometeorological ARay for Isv-Monsoon AUtomonitoring (HARIMAU) were investigated. Strong winds of 17 m s-1 and a sudden temperature drop of 5 K were observed at an X-band Doppler radar site associated with the passage of a convective system, and some houses were severely damaged. The convective system developed under an environment of strong low-level easterly vertical shear associated with the easterly region of an equatorial Rossby wave. The northern part of the convective system possessed qualitatively similar structures to midlatitude bow echoes, including the convex shape of the convective line, a descending rear-inflow jet positioned at its apex, and mesoscale vortices on both sides of the rear-inflow jet. The low-level wind behind the convective system formed a channel of strong easterly wind as it passed through an area of relatively low topography in the mountain range. The enhanced easterly wind was thought to contribute to the formation of the bow echo-like structure in the northern part of the convective system. This easterly rear-inflow jet was further accelerated in the convective system and descended near the leading edge, forming divergent strong winds at the surface. The sounding data that were taken after the passage of the convective system indicated that dry air appeared in the lower troposphere associated with an enhancement of the southerly component of the wind. An analysis of objective reanalysis data suggests that the southerly was probably associated with westward-propagating mixed Rossby-gravity waves with a period of approximately 5 days. It is suggested that the dry air intruded into the convective system across the back edge of the precipitation area and caused enhanced evaporative cooling, which resulted in the effective downward transport of the enhanced easterly momentum.
The twenty nine years (1979-2007) rain-gauge based gridded precipitation data generated by the Asian Precipitation−Highly Resolved Observational Data Integration Towards Evaluation of water resources (APHRODITE) are used to depict the rainfall climatology in Vietnam. The rain gauge observations of 163 stations in Vietnam for year 2007 are employed to validate the analysis results of the APHRODITE precipitation and to verify two distinct rainfall regimes: the October-November regime in central Vietnam and the May- October regime in the northern and southern part of this country identified with APHRODITE data. It appears that the Truong Son Range along the western border of Vietnam with Laos and Cambodia provides a natural separation of the October-November rainfall regime in central Vietnam from others. The interannual variation of the October-November rainfall regime can be well depicted by a principal mode obtained from the empirical orthogonal function analysis on the 29-year-APHRODITE precipitation. Time variation of this interannnual mode is out-of-phase with the SST(NINO3.4) index. It is inferred from this negative correlated relationship that central Vietnam is drier (wetter) when the SSTs over the NINO3.4 region is warmer (colder). It is found from the water vapor transport analysis that an anomalous cyclonic (anticyclonic) circulation over south Asia is paired with an anomalous anticylonic (cyclonic) circulation over the western north Pacific during cold (warm) episodes. Water vapor is converged (diverged) by these two anomalous circulations toward (out of ) the South China Sea and Philippine Sea west of 150°E during cold (warm) years. In turn, the anomalous cyclonic (anticyclonic) circulation in South Asia enhances (reduces) the water vapor supply to Indochina, particularly to Vietnam. Coupling with this pair of anomalous circulations, water vapor is converged (diverged) by the anomalous divergent circulation, coupling with the aforementioned pair of anomalous circulation, toward (out of ) Southeast Asia to maintain excessive (deficient) rainfall during cold (warm) episodes. Evidently, the response of the divergent circulation to the tropical Pacific SST anomalies contributes to the interannual variation of the October-November rainfall in central Vietnam.
This article investigated the long-term variation of the boreal summer (JJA) moisture circulation over South China (SC). The total precipitable water (TPW) and moisture convergence over SC have increased obviously, such trend is not a local phenomenon, but the result of large-scale circulation anomaly. In the past few decades, the water vapor transport from southern hemisphere and subtropical western Pacific is strengthened, while the northward transport over East Asia is weakened, leading enhanced moisture convergence over the Philippines and adjacent region. The net flux of SC shows no significant shift in the end of the 1970s which is widely acknowl-edged as “climate shift”, but ascends remarkably after the early 1990s. This doesn't mean the moisture circulation over SC experiences no variation with the ''climate shift''. In fact, the relationship between total net flux with budgets of four boundaries had been modified in the end of the 1970s. The correlation of the net flux is highly negative with the budget of north boundary, and positive with the budget of west and east boundaries before 1979/80, but only highly positive with the budget of south boundary after 1979/80. This modification is closely related to the north-south migration of western Pacific subtropical high (WPSH). Before 1979/80, the ridge of WPSH lies northward, resulting in stronger output of water vapor via the north boundary which may probably lead to the significant correlation between flux of north boundary and net flux. After 1979/80, the ridge of WPSH shifts southward, inducing stronger water vapor input through the south boundary, which is probably the causation of significant correlation between flux of south boundary and net flux.
A possible role of the air-sea coupling in the South China Sea (SCS) summer monsoon variability is studied using a coupled general circulation model (CGCM) and its atmospheric component (AGCM). The 50-year integration of the CGCM well reproduces the summer monsoon variability over SCS, where the precipitation anomaly is positively correlated with the low-level cyclonic circulation anomaly that accompanies enhanced surface westerlies. Negative sea surface temperature (SST) anomaly is found in SCS during the strong monsoon years, indicating the atmospheric driving SST through wind-induced evaporation. The 50-year AGCM run forced by historical SST obtained from the CGCM reveals the monsoon variability amplified by about 50 percent as compared with the CGCM. The absence of the air-sea coupling keeps SST warm in SCS, which increases the local evaporation and precipitation. The enhanced precipitation over SCS may intensify surface westerly over the remote regions, resulting in an increase in the moisture flux convergence that in turn contributes to the positive precipitation anomaly. This result suggests that the air-sea coupling works to stabilize the monsoon and hence suppress the variability via the large-scale moisture transport and the wind-induced local evaporation.
This study investigated the impacts of historical land use/cover changes (LUCC), from forest to cultivated land, on the seasonal cycle of the hydroclimate over the Indian subcontinent and southern China. The mechanism of these impacts was studied by conducting numerical experiments using an atmospheric general circulation model MIROC3.2 coupled with the land surface scheme MATSIRO and historical global land use/cover changes between 1700 and 1850. A previous study found a decrease in summer (JJA) precipitation over the Indian subcontinent and southern China induced by extended cultivation between 1700 and 1850. We further found that evapotranspiration in the Indian subcontinent notably decreased, particularly in the spring, while that in southern China discernibly decreased throughout the year. The difference in the changes in evapotranspiration in the spring over both regions could be explained by the amount of precipitation during the dry season. In the Indian subcontinent, the marked decrease in evapotranspiration in May due to LUCC caused the decrease in precipitation during the same season. However, in southern China, the decrease of precipitation from March to April was contributed rather by the decrease of water vapor flux convergence due to atmospheric circulation changes than by the decrease of evapotranspiration.
First results of diurnal variation of radar echo area observed at Vientiane, Lao PDR, in the middle of Indo-china are reported. Radar echo intensity data from April to October 2008 is analyzed. The monthly averaged diurnal cycles of radar echo area have a peak in the late afternoon in April and midnight in July. The echo area per one echo at the time of the maximum total echo area in the diurnal cycle in April (∼600 km2) is smaller than that in July (∼1,200 km2). The phases of diurnal cycle of echo area in 6 banded regions oriented from southwest to northeast are compared to examine spatial differences of diurnal cycles. It is found that the phase of each band delays from southwestern to northeastern banded regions. The propagation speed of phase of echo area diurnal cycle is estimated as about 14 m s-1.
An L-band wind profiler deployed at an industrial park (120.38°E, 22.6°N) close to Kao-Hsiung city in southern Taiwan is utilized to diagnose the vertical structure of Mei-yu frontal precipitating cloud systems observed during 9-11 June 2006. Observational results show several mesoscale convection systems associated with the Mei-yu frontal cloud produced heavy rainfall over Taiwan island. Mei-yu precipitating clouds are classified into three types: convective, mixed convective-stratiform and stratiform based on vertical profile of Signal-to-noise ratio (SNR)/Reflectivity and Doppler velocity during approaching, passing, and leaving stages of the cloud systems. In approaching and pre-passing stages of Mei-yu frontal clouds, a pre-dominant convective precipitation without a significant bright band is observed. In contrast, a pronounced bright band around 4 km was observed during post-passing and leaving stages of the frontal clouds with stratiform precipitation. Doppler velocity profiles show hydrometeors (ice/snow) at 5 km and rain below 4 km for stratiform precipitation. For Mei-yu frontal stratiform precipitation the melting particles accelerations of 3.3 ms-1 km-1 are observed for the bright band region.
The authors investigated diurnal convection peak characteristics over the eastern Indian Ocean off the island of Sumatra during different phases of the Madden-Julian oscillation (MJO). During MJO phases 2 to 3 (P2 and P3) defined by Wheeler and Hendon (2004), prominent diurnal variation in convection was observed by satellites when moderate low-level westerly winds were dominant over the eastern Indian Ocean. The diurnal convection peaks were prominent over the island of Sumatra in the evening, while migrations of the convection toward the Indian Ocean were observed in the early morning. By using the Global Positioning System around the western region offshore of Sumatra, a significant reduction in water vapor was observed from evening until midnight, compensating for the upward motion over the island. During midnight to early morning, the water vapor in-creased in the western offshore region as the convections migrated from the island. This prominent diurnal varia-tion confirmed the result from a numerical experiment by Miura et al. (2007) using the Nonhydrostatic ICosahe-dral Atmospheric Model (NICAM). During P2 to P3, the atmosphere over the eastern Indian Ocean contains abundant water vapor, while the Maritime Continent is fairly well heated by solar radiation under calm conditions. This situation should be favor-able for the development of two diurnal convection peaks: the evening convection over the land induced by solar radiative heating and the midnight convection over the ocean triggered by convergence of the low-level westerly wind and the land breeze.
An extraordinarily heavy rain event occurred in northern Vietnam from 30 October to 1 November 2008. The three consecutive days of extremely heavy rain resulted in the worst flooding event in Hanoi, the capital of Vietnam, and its environs in 24 years. Results from analysis of the Japan Meteorological Agency Climate Data Assimilation System (JCDAS) re-analysis data show that a synoptic-scale tropical wave disturbance formed over the South China Sea and traveled northwestward over the eastern coast of the Indochinese Peninsula, brought the extremely heavy rainfall to northern Vietnam. In the mid-latitudes, a belt of surface high extended southeastward from western Mongolia to the East China Sea, causing a persistent northeasterly monsoonal flow on its south-eastern edge along the southern coast of China. The northeasterly flow worked together with the tropical disturbance, creating a strong low-level wind convergence in northern Vietnam. Examination of the structure of the tropical disturbance revealed that the meridional wind structure and cyclonic vorticity of the disturbance extended vertically to approximately 300 hPa, with the maximum fluctuations occurring at the 800-600 hPa layer and shows nearly vertical with height. A remarkable feature of the atmosphere is the large increases in the equivalent potential temperature in the upper troposphere higher than about 500 hPa, with strong southerly winds greater than 15 m s-1 in the 300-150 hPa layers for the three days in question. In conclusion, the tropical disturbance, which worked together with the persistent Asian winter monsoon, caused the extreme rainfall event.
The mountains of Himalayas are the important channels for the mass exchange between surface layer of Northern Hemisphere and troposphere. The mountains relate surface layer atmosphere over the Tibetan Plateau and the above free atmosphere through many atmospheric circulation systems including mountain-valley wind and glacier wind etc. Mt. Qomolangma (Mt. Everest) is the highest peak in the world, and its nearby region is a representative case of mountains of Himalayas. Firstly the back ground of the establishment of the Qomolangma Station for Atmospheric and Environmental Observation and Research, Chinese Academy of Sciences (QOMS/ CAS) and the role of the station in the study of the atmosphere-land interaction over Himalayan area were intro-duced in this paper. Then some preliminary observational analysis results, such as the structure of the Atmo-spheric Boundary Layer (ABL) and the characteristics of the atmospheric turbulence and the radiation energy distribution were shown. The results showed that: The excess resistance to heat transfer kB-1 has obvious diurnal variations with lower values in the night and higher values in the daytime; The low frequency perturbations have a large influence on the spectra variation of all wind components, but mid frequency perturbations have only in-fluence on the spectra variation of vertical wind components and also alter the co-spectra of momentum and sen-sible heat flux under near neutral stratification; The downward shortwave radiation over this area are obviously larger than that in other areas; There is very clear constant layer of potential temperature existing in the northern slope of Mt. Qomolangma area around 12:00 (Beijing Standard Time, BST) in May, and the layer is about 2,200 m during the period over this area. In comparison with study results from other areas, some uniqueness and commonness of the Himalayan region are clearly identified.
The empirical relationship between solar radiation and sunshine duration in Thailand is studied in this paper. Although regional long-term regional solar radiation has not yet been measured in Thailand, the data of sunshine duration measurements are available. Hence, measurement of global solar radiation is conducted to find the relationship between daily solar radiation and sunshine duration, which is mostly linear. The distribution of regression coefficients is examined and the formula that can be applied in Thailand is estimated. The efficiency of the proposed formula is validated through its prediction. The result shows that the accuracy of the country-wide regression equations can be comparable to that constructed at each station. Moreover, it is found that the proposed equations are more effective from May to November than from December to April. The proposed equations are applied to estimate solar radiation in Thailand and the differences in their prediction are found in the characteristics of statistical distributions between May-November and December-April.