Journal of the Meteorological Society of Japan. Ser. II
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Volume 82 , Issue 6
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Articles
  • Kensuke NAKAJIMA, Eizi TOYODA, Masaki ISHIWATARI, Shin-ichi TAKEHIRO, ...
    Volume 82 (2004) Issue 6 Pages 1483-1504
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    For the purpose of examining the initial development of the atmospheric response to a warm SST anomaly placed at the equator, an ensemble switch-on experiment is conducted with an aqua-planet GCM. An ensemble average of the size of 128 significantly reduces the transient noises caused by both small scale convective activity and large scale intraseasonal variability.
    In the first three days after the switch-on of the SST anomaly, a convection center develops above the warm SST area. As a barotropic response to the heating of convection center, a global increase of surface pressure occurs outside the low pressure region around the warm SST area. The response after the emergence of the high pressure anomaly is consistent with Gill (1980); a warm Kelvin wave-like anomaly is emitted to the east of the convection center, while a warm Rossby wave-like anomaly is emitted to the west.
    The Kelvin wave-like signal propagates at a speed slower than that of free Kelvin wave expected from its vertical wavelength, suggesting that the signal is a “moist” Kelvin wave. Transient decrease of precipitation occurs at the moist Kelvin wave front; a decrease of convection associated with the downward motion at the wave front is consistent with its slow propagation. After several days, precipitation recovers and is even intensified because of the surface frictional convergence associated with the Kelvin wave-like equatorial low pressure anomaly. To the west of the warm SST area, on the other hand, precipitation decreases monotonically. The continuous reduction of precipitation is caused by the equatorial surface frictional divergence associated with the relatively high pressure anomaly at the equator of the Rossby wave structure.
    Finally, there appears a slow zonally symmetric response within the Hadley cell characterized with surface pressure rise in the tropics and westerly wind anomaly in the troposphere. The change of eddy zonal momentum transport, together with the transport toward the lower level by the Hadley circulation and the geostrophic adjustment to the resulting low level westerly acceleration, seems to be responsible for the response.
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  • Nobumitsu TSUNEMATSU, Kenji KAI
    Volume 82 (2004) Issue 6 Pages 1505-1520
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    The present study investigates the relationship between surface wind convergence and clouds by performing statistical analyses of time variations in surface wind field, its horizontal divergence, cloud distribution, and appearance frequency of radar echoes in the Nobi Plain in the daytime on summer sunny days using various observation data from 1990 to 1999. The 99 summer sunny days when general wind was weak were selected objectively in advance of the analyses. The results of analyses show that a westerly wind from the Biwa Lake direction and a sea breeze from Ise Bay converge in the north of Nagoya after 1300 JST on summer sunny days. In addition, clouds (cumulus) tend to be formed all over the Nobi Plain from late in the morning to about noon by thermal convections, and the formation of clouds continues near strong surface wind convergence zones until around 1500 JST on summer sunny days, although clouds are hardly formed in the afternoon in almost all parts of the plain due to the decline of the thermal convections. The continuation of the cloud formation is especially remarkable at the northeast of Nagoya. Thus, in the Nobi Plain on summer sunny days, the conditions where clouds are easily formed continue at the northeast of Nagoya until around 1500 JST, because of a strong surface wind convergence between a westerly wind from the Biwa Lake direction and a sea breeze from Ise Bay. The analysis of radar echoes observed around 1500 JST shows that the echoes occasionally appear near the northeast of Nagoya on summer sunny days. These results indicate that the strong surface wind convergence plays an important role in the formation of clouds, after thermal convection has weakened, on summer sunny days.
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  • Ching-Sen CHEN, Wan-Chin CHEN, Wei-Kuo TAO
    Volume 82 (2004) Issue 6 Pages 1521-1543
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    About a quarter to a half of all rainfall from mid-July through August from 1994 to 2000, on the windward side of southwestern Taiwan, came from convective systems embedded in the monsoon flow associated with the southwesterly monsoon. In this study, the causes of two heavy rainfall events (daily rainfall exceeding 100 mm day−1 over at least three rainfall stations), observed over the slopes and/or lowlands of southwestern Taiwan were examined. Data from the 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, on 9 August 1999, heavy rainfall of up to 393 mm 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 as preexisting convection did not drift into western Taiwan from the Taiwan Strait.
    In another event, on 13 August 1994, with a maximum rainfall of 450 mm, westerly flow with potentially unstable and very moist air in the lowest layers, converged with northwesterly flow from the central Taiwan Strait over the southwestern coast. The westerly flow decelerated while approaching Taiwan, resulting in enhanced convergence and upward motion over the coastal area and a high amount of rainfall. In addition, a mesoscale convergence area was produced over the southern Taiwan Strait near the southwestern coast of Taiwan due to synoptic circulations. Some convection formed in the ocean near the southwest coast and drifted inland. This situation also enhanced precipitation over the coastal areas. Northwesterly flow formed over the southwestern slopes as weak westerly flow approached the mountains. As rainwater from the Strait and coastal areas moved eastward toward the slopes, the northwesterly flow transported the rainwater southeastward away from the southwestern slopes. Subsequently, less rainfall occurred over the southwestern slopes than over the coastal areas.
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  • Hatsuki FUJINAMI, Tetsuzo YASUNARI
    Volume 82 (2004) Issue 6 Pages 1545-1564
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Convective variability at submonthly timescales (7-20 days) over the Tibetan Plateau and the associated large-scale atmospheric circulation and convection were examined over regions affected by the Asian Monsoon. The mature phase of the Asian summer monsoon (July-August) was analyzed for those years (1986, 93, 98) in which convective variability on timescales of 14 days was notable over the Tibetan Plateau.
    Composite analyses of OLR, based on the filtered Tbb time series over the southern Tibetan Plateau, show that significant convective signals rotate clockwise around 28°N, 90°E, affecting the Tibetan Plateau, Indochina, the Bay of Bengal, and India. Significant signals also appear around the Philippines and the South China Sea. A well-developed wave train extending from North Africa to far-east Asia along the Asian subtropical jet is associated with convective fluctuations over the plateau. The waves are quasi-stationary and have a Rossby wave-like downward wave train with wavenumber 7.
    The waves control convective fluctuations over the plateau. During the transition to active (inactive) convection, an upper-level trough (ridge) develops west of the plateau. Simultaneously, cyclonic anomalies strengthen over India between the lower and middle troposphere. The development of the two troughs induces a southerly flow of moist air toward the plateau. Moistening of the lower atmosphere creates favorable conditions for subsequent active moist convection.
    Possible processes for forming the wave train over the subtropical jet and a link for convective signals between midlatitudes and the Asian monsoon are discussed.
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  • Yushu ZHOU, Shouting GAO, Samuel S.P. SHEN
    Volume 82 (2004) Issue 6 Pages 1565-1576
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    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.
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Notes and Correspondence
  • Seiji WAKABAYASHI, Ryuichi KAWAMURA
    Volume 82 (2004) Issue 6 Pages 1577-1588
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Four teleconnection patterns that are possibly associated with the anomalous summer climate in Japan and the surrounding regions, were extracted by applying empirical orthogonal function and regression analyses to stream function anomalies. The two teleconnection patterns prevailing over northern Eurasia, especially in early summer, called the Europe-Japan (EJ) 1 and EJ2, are linked with the variability of the Okhotsk high. The third teleconnection pattern, called the West Asia-Japan (WJ), is a stationary wave-train pattern along the upper-level subtropical jet from West Asia to the central North Pacific, which is possibly excited by the anomalous convective heating of the Indian summer monsoon. The final teleconnection pattern is identified with the Pacific-Japan (PJ), found by Nitta. Teleconnection indices that account for the variability of those patterns are also defined on a monthly basis. The PJ and WJ patterns, which are more influential teleconnection patterns than the others, are closely related to the summer temperature anomalies, especially in northern and western Japan, respectively. EJ1 and EJ2 were amplified in several extreme summers, and they played a vital role in the cool summer of 2003, along with PJ. A combination of two or three teleconnection patterns was also responsible for the occurrence of the recent extreme summers. Monitoring the major teleconnection patterns is very useful for understanding and forecasting the anomalous summer climate in East Asia.
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  • Naoya WADA, Kunio KAWADA, Ryuichi KAWAMURA, Kazuma AOKI, Atsushi KUME
    Volume 82 (2004) Issue 6 Pages 1589-1597
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Meteorological and hydrological data were analysed, over 37 years during the period from 1965 to 2001, recorded by the Kurobe Dam meteorological observatory, which is located in a nature-reserved mountainous area within a heavy-snowfall region of central Japan. The analysis demonstrated that both winter runoff and air temperature increased significantly during this period. The interannual variation of winter runoff was governed not only by precipitation, but also by air temperature. The significant increasing trends in air temperature were also found at the adjacent JMA meteorological stations, showing a high correlation with air temperature observed by the Kurobe Dam.
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Special Edition on Regional Climate Modeling for Monsoon System
  • Yuqing WANG, L. Ruby LEUNG, John L. McGREGOR, Dong-Kyou LEE, Wei-Chyun ...
    Volume 82 (2004) Issue 6 Pages 1599-1628
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Regional climate modeling using regional climate models (RCMs) has matured over the past decade to enable meaningful utilization in a broad spectrum of applications. In this paper, the latest progress in regional climate modeling studies is reviewed, including RCM development, applications of RCMs to dynamical downscaling for climate change assessment and seasonal climate predictions, climate process studies, and the study of regional climate predictability.
    Challenges and potential directions of future research in this important area are discussed, with focus on those that have received less attention previously, such as the importance of ensemble simulations, further development and improvement of the regional climate modeling approach, modeling extreme climate events and sub-daily variation of clouds and precipitation, model evaluation and diagnostics, applications of RCMs to climate process studies and seasonal predictions, and development of regional earth system models.
    It is believed that with the demonstrated credibility of RCMs in reproducing not only monthly to seasonal mean climate and interannual variability, but also the extreme climate events when driven by good quality reanalysis and continuous improvements in the skill of global general circulation models (GCMs) in simulating large-scale atmospheric circulation, regional climate modeling will remain an important dynamical downscaling tool for providing the needed information for assessing climate change impacts, and seasonal climate predictions, and a powerful tool for improving our understanding of regional climate processes. Internationally coordinated efforts can be developed to further advance regional climate modeling studies. It is also recognized that since the final quality of the results from nested RCMs depends in part on the realism of the large-scale forcing provided by GCMs, the reduction of errors and improvement in physics parameterizations in both GCMs and RCMs remain a priority for the climate modeling community.
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  • J.-H. OH, T. KIM, M.-K. KIM, S.-H. LEE, S.-K. MIN, W.-T. KWON
    Volume 82 (2004) Issue 6 Pages 1629-1643
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Recently the regional impact assessment due to global warming is one of the urgent tasks to every country in the world, under the circumstances of increasing carbon dioxide in the atmosphere. This assessment must include not only meteorological factors, such as surface air temperature and precipitation, etc., but also the response of the local ecosystem. Based on a previous study, for example, it has been known that Phyllostachys’ habitation, which is one of the bamboo species popular in Korea, is quite sensitive to temperature change, in particular during the winter season. Thus, adequate climate information is essential to derive a solid conclusion on the regional impact assessment for future climate change.
    In this study, we adopted a dynamical downscaling technique to get regional future climate information, with the regional climate model (MM5, Pennsylvania State University/National Center for Atmospheric Research mesoscale model) from the Max-Planck Institute for Meteorology Models and Data Group’s Atmosphere-Ocean General Circulation Model (AOGCM) ECAHM4, and HOPE-G (ECHO-G) simulation for future climate, based on future greenhouse gas (GHG) emission scenario of the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A2. Through this nesting process we got reasonable regional climate change information. However, we found a couple of systematic differences, such as a cold bias in the surface air temperature, simulated by MM5 compared to that by the AOGCM ECHO-G. This cold bias may cause to loose credibility on the future climate scenario to the impact assessment studies. Accordingly, we introduced a transfer function to correct the systematic bias of the dynamic model in the regional-scale, and to predict the regional climate from large-scale predictors. These transfer functions are obtained from the daily mean temperature of 17 surface observation stations in Korea for 10 years from 1992 to 2001, and 10-year simulation data obtained from regional climate model (RCM) for each mode of EOFA to correct the systematic bias of RCM data.
    With these transfer functions, we can correct the RMS error of the daily mean temperature in RCM as much as 47.6% in winter and 86.5% in summer. After dynamical downscaling and statistical adjustment, we may provide adequate climate change information for regional assessment studies.
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  • Johnny C.L. CHAN, Yiming LIU, K.C. CHOW, Yihui DING, William K.M. LAU, ...
    Volume 82 (2004) Issue 6 Pages 1645-1665
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    The objective of this study is to modify a regional climate model (RCM)—the Regional Climate Model of the National Climate Center of China—to simulate the summer monsoon rainfall over South China and the South China Sea. Such a modification is necessary because this RCM was designed for studying the climate over central and north China where precipitation processes are very different from those occurring further south so that simulations using the basic parameters of the original model give precipitation amounts much less than those observed.
    Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, many sensitivity experiments have been carried out for the months of April to June 1998, which include modifying the cumulus parameterization scheme, the large-scale precipitation scheme, the radiation scheme, the surface exchange processes, and the size of the buffer zone. The design of all the experiments is either to enhance the moisture provision to the atmosphere over South China and the South China Sea, or to help the model atmosphere realize the moisture to form precipitation. It is found that indeed modifications to the various parameters can produce rainfall amounts much closer to those observed.
    Based on the results of these experiments, an “optimal” design of the RCM is reached and tested for its effectiveness using the NCEP reanalyses for two wet years (1994 and 1997) and two dry years (1996 and 1999) during which rainfall in May and June over South China were above/below normal respectively. The physical processes in this design include the Kuo scheme for convective precipitation, the Pal scheme for large-scale precipitation, the Holtslag scheme for the planetary boundary layer, the radiation transfer scheme of the NCAR Community Climate Model Version 3, and the Biosphere-Atmosphere Transfer Scheme for the land surface process. The width of the buffer zone is reduced and the effective cloud droplet radius is fixed at different values over land and ocean. The neutral drag coefficient is prescribed as a function of surface wind speed, and the heat and moisture exchange coefficients are set at values larger than that of momentum. The simulations based on this design for all the five years are found to be much closer to observations than those from the control.
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  • Wen-Yih SUN, Jiun-Dar CHERN, Michael BOSILOVICH
    Volume 82 (2004) Issue 6 Pages 1667-1678
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    During the summer of 1988, a severe drought caused extensive loss of crops in the Midwest and Gulf States of the United States. The Purdue Regional Model (PRM), with comprehensive atmospheric physics and soil-vegetation surface package, has been integrated continuously for thirty days of June 1988 to study circulation and surface hydrology. The results show that the PRM is capable of reproducing the evolution of the observed weather and monthly mean fields of June 1988, when an extensive ridge located over the hot, dry North America suppressed the moisture supply from the oceans. Sensitivity test shows that under the observed SST and large-scale wave pattern, the model can generate more precipitation using wet soil as the initial condition, but a strong stationary anticyclone still exists in the US. The wet soil also enhances anticyclonic circulation centered over the Rocky Mountains, and the offshore flow in Texas and Gulf States, soil becomes dry eventually. Hence, the dry soil over the central US alone is insufficient to instigate the anomalous warm ridge, but could have had a role in its persistent drought due to the weakening of the low-level jet in Texas and Oklahoma region and less evaporation from the land surface.
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  • Omer Lutfi SEN, Bin WANG, Yuqing WANG
    Volume 82 (2004) Issue 6 Pages 1679-1693
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    This paper describes a study that investigates the local and regional effects of vegetation restoration in northern China via regional climate model simulations, and reports implications for the sustainability of vegetation under the altered rainfall regime. Ensemble simulations with the current vegetation cover and an idealized re-greening scenario for a test area in northwestern China (90°-110°E and 36°-42°N) were performed using large scale boundary forcing derived from 1998. The results indicate that such re-greening has both significant local and regional effects on the atmospheric circulation and rainfall distribution.
    Replacing desert and semi-desert areas with grass in the test area increases net radiation at the surface, and hence total heat flux from the surface to the atmosphere. This results in enhanced ascending motions and moisture supply to the atmosphere over the test area. Consequently, rainfall increases in the whole test area. However, the increase in rainfall largely occurs due to an increase in intensity rather than an increase in frequency. Lack of frequent rainfall, especially in the lowlands of the test area, makes it very difficult to maintain a vegetated surface. This implies that the current vegetation restoration activities will be largely limited to areas where water resources are relatively abundant, or they will depend heavily on irrigation. The increased runoff at higher elevations could be important for providing water for irrigation in the lowlands. The increase in rainfall in the highlands and far eastern parts of the test area, which already receive more frequent rainfall, may help support a restored vegetation cover in these regions.
    The enhanced ascending motions over the test area are compensated by increased subsidence to the east, centered over Yellow River Delta and Shandong Peninsula, resulting in a higher-pressure and anticyclonic circulation anomaly there. Consequently, rainfall decreases at these areas. This anticyclonic anomaly provides significant northeasterly low-level anomalous winds that enhance cyclonic shear vorticity in the Yangtze River Basin and South China when they meet southwesterly monsoonal flow. This causes strong ascending anomalies over southern China and the Sichuan Basin, and increases rainfall in these regions.
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  • L. Ruby LEUNG, Shiyuan ZHONG, Yun QIAN, Yiming LIU
    Volume 82 (2004) Issue 6 Pages 1695-1713
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    A regional climate model based on the Penn State/NCAR Mesoscale Model (MM5) was used to simulate the 1998 and 1999 East Asian summer monsoon conditions. Simulations were performed for 1 April-31 August of each year, with initial and lateral boundary conditions provided by the ECMWF analysis. Observations from the 1998 and 1999 GAME/HUBEX experiments were used to evaluate the regional climate simulations. Based on observations, large differences can be found between the 1998 and 1999 meteorological conditions and surface energy budgets at the Shouxian station during the IOPs, with much higher rain intensity but only slightly higher rain frequency in 1998 than 1999. For 1998, although the regional climate model was able to reproduce the general spatial distribution of monthly mean rainfall quite well during the summer monsoon season, large discrepancies can be found in comparing the observed and simulated surface climate and energy fluxes in the HUBEX region. By using Four Dimensional Data Assimilation (FDDA) technique, which constrains the simulated large-scale circulation with observations from 21 soundings in the HUBEX α-scale region, both the root mean square error and mean bias in rainfall were greatly reduced. The improvements in simulating rainfall were related to both reduction in errors of precipitation amount and timing. In the control simulation, a mean bias of −63 W/m² (−36%) was found in the simulated surface net radiation at Shouxian, which suggest large errors in simulating clouds in the region. With FDDA, the bias was significantly reduced to −23 W/m² (−13%), with corresponding reduction of bias in the latent heat flux. This suggests that at least part of the model bias in simulating net radiation is related to errors in simulating the large-scale circulation, which can affect cloud amount and vertical distribution.
    Comparing the 1998 and 1999 simulations, both without FDDA, smaller biases were found in the surface fluxes during 1999. Percentage biases in the net radiation and latent heat flux were −18% and −33% in 1999 and −36% and −50% in 1998 respectively. Based on observations, large differences in the net surface radiation, and small differences in cloud fraction between the two years suggest that cloud optical depth and/or vertical distribution were very different, with more cloudy conditions observed during 1999. Although the 1999 simulations were sensitive to the cumulus convective parameterizations (Grell scheme versus Kain-Fritsch scheme) as shown by the sensitivity experiments, the large differences in simulation skill between the 1998 and 1999 cases, regardless of the convection schemes used, suggest possible dependence of model errors on cloud properties that deserve further investigations.
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  • Huang-Hsiung HSU, Yi-Chiang YU, Wen-Shung KAU, Wu-Ron HSU, Wen-Yih SUN ...
    Volume 82 (2004) Issue 6 Pages 1715-1733
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    This study used the Purdue Regional Model to simulate the 1998 EASM to assess the model performance. The results indicated that the model was capable of simulating the overall characteristics of the 1998 EASM on the seasonal, intraseasonal, onset, and daily time scales. On the seasonal time scale, the model tended to produce more precipitation over the land and less precipitation over the ocean. This land-sea contrast in the model performance was consistent with the stronger-than-observed Pacific anticyclone in the simulation. It appears that the over simulated anticyclone was also found in other models, and is not a unique problem to the PRM. Future studies are needed to tackle this seemingly fundamental problem in several regional models.
    The model simulated the seasonal march of the EASM well, characterized by northward-propagating rain bands. Intraseasonal oscillation events propagating northward were also well reproduced. These results confirm that the model is capable of producing realistic sub-seasonal variability in the inner domain, once the proper lateral boundary forcing is provided.
    The model correctly simulated the onset timing and dramatic changes before and after the onset. However, the model incorrectly simulated the circulation and precipitation during the onset, because it failed to simulate the rapid development of a weak trough in the northern South China Sea. After the onset period, the model performance became reliable again. This indicates that the model is capable of fixing the existing large biases, with the proper lateral boundary forcing.
    This model was able to simulate the gross fluctuation in the regional-averaged daily precipitation, although it missed some extreme events that resulted in flooding in China. The incorrect simulation of these extreme events was partially responsible for the biases in the simulated seasonal precipitation. It is suggested that a regional model should be able to simulate the multi-scale features from the daily to seasonal time scales and their mutual interaction to correctly simulate the monthly and seasonal mean fields.
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  • Dong-Kyou LEE, Dong-Hyun CHA, Hyun-Suk KANG
    Volume 82 (2004) Issue 6 Pages 1735-1753
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    In this study, the severe flood case over East Asia during the 1998 summer was simulated using a regional climate model (SNURCM) with 60 km horizontal resolution (EX60), and the model performance in reproducing the extreme climate events was evaluated. An experiment with higher horizontal resolution of 20 km (EX20) was also performed in order to assess the impact of increased resolution on precipitation simulation of the severe flood.
    The model reproduced the severe precipitation events occurring in central China in June. In EX60, the temporal and spatial variations of the abnormal Meiyu monsoon fronts, which were well observed were also simulated reasonably except in southern China. The area-averaged daily precipitation and surface air temperatures were underestimated, but their temporal evolutions were in good agreement with observation. In the higher resolution experiment (EX20), simulated downward solar radiation, latent heat flux and convective rain were increased in the major severe rain area over the Yangtze River Basin. The increased precipitation in EX20, which was attributed mainly to the increase of convective rain, resulted in the enhanced precipitation intensity, but only slightly affected total precipitation amounts. The improvement in the higher horizontal resolution simulation appeared in precipitation resulting, in particular, from increased convective activity due to increased latent heat flux at the surface. Nevertheless, the model had significant precipitation bias in some areas with disagreement between the simulated precipitation patterns and distribution, and the observations. The model also had surface air temperature bias resulting from cold biases of the land surface model. With horizontal resolution increased to 20 km, the convective and non-convective precipitation was increased for the late afternoon and early evening time, increasing the total precipitation slightly.
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  • Dodla Venkata BHASKAR RAO, Karumuri ASHOK, Toshio YAMAGATA
    Volume 82 (2004) Issue 6 Pages 1755-1775
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    In this study, the National Center for Atmospheric Research Mesoscale Model, NCAR MM5 adopted for the Indian region has been integrated for a four month period from 1 May 1994, to study the Indian summer monsoon during the months of June, July and August. This version of the MM5 mesoscale model has a horizontal resolution of 30 km, and 23 vertical levels. The initial and boundary conditions are taken from NCEP/NCAR reanalysis data available at 2.5° grid interval, and interpolated to the model domain.
    The results indicate that the NCAR MM5 model simulates many observed features of the Indian summer monsoon on a regional scale, which otherwise cannot be simulated using a global general circulation model. The simulated features of sea level pressure, 925 hPa temperature, low level wind flow are compared with NCEP and precipitation fields with GPCP. Significant features of the monsoon circulation, such as the monsoon trough, heat low over northwest India, and mesoscale precipitation patterns are well simulated. The model precipitation is also well simulated with the strengths and locations of the maxima and minima agreeing with the observations. The advancement of the monsoon current during the onset phase, as simulated by the model, is evaluated by comparing the pentad rainfall during June, 1994, with the advancement of monsoon reported by the India Meteorological Department. Model simulated area averaged time series of precipitation, and 850 hPa geopotential for five different zones covering the Indian subcontinent are evaluated through correlation, root mean square error, bias and threat score.
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  • Kun YANG, Toshio KOIKE, Hideyuki FUJII, Toru TAMURA, Xiangde XU, Linge ...
    Volume 82 (2004) Issue 6 Pages 1777-1792
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    Based on field observations, theoretical analyses, and numerical simulations, this study investigates the structure and the evolution of the atmospheric boundary layer (ABL) and convection over the Tibetan Plateau during the dry season. Both field observations and numerical simulations show that the convection over the plateau evolves from dry shallow convection in the morning to wet deep convection in the afternoon. The shallow convection is organized, and its major wavelength is controlled by mesoscale hills. The deep convection is not very regular. Both nonlinear scale interactions and latent heat release from convection may play significant roles in the development of the deep convection. However, the deep convection near mountains is related to an interactive process between mountain-valley circulations and rain evaporative cooling. The mountain-valley circulations in the afternoon can be either upslope or downslope.
    The plateau ABL can extend to heights of almost 3 km above the ground surface, and is characterized by a well-mixed layer of potential temperature. The energy budget in the ABL indicates that the sensible heat is the dominant energy for sustaining the ABL growth, and radiations also play a significant role, but the rain evaporative cooling below the wet convection suppresses ABL development. The ABL evolution is strongly associated with the convective activities. The convection not only efficiently exchanges the quantities between the near-surface layer and the upper layer, but also enhances the air entrainment near the top of the ABL.
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  • Gang FU, Yihong DUAN, Xudong LIANG, Shang-Ping XIE
    Volume 82 (2004) Issue 6 Pages 1793-1803
    Released: March 02, 2005
    JOURNALS FREE ACCESS
    An extremely heavy rainfall event was observed in Shanghai on 5 August 2001. It had the maximum 24-h accumulated precipitation of 275.2 mm and caused serious floods. This paper documents the detailed evolution of this event by using a suite of observational data, including Geostationary Meteorological Satellite (GMS)-5 satellite images, Weather Surveillance Radar 88 Doppler (WSR-88D) radar data, automatic rain-gauge and sounding data, and objective reanalysis.
    The synoptic situation prior to the heavy rainfall was characterized with a north-south oriented trough at 500 hPa and a surface meso-scale low with a surface cyclonical circulation at 00 UTC 5 August 2001. A lower-level southerly jet on the eastern flank of this meso-scale low supplied humid air to fuel the heavy rainfall. The soundings indicated that the atmosphere was most unstable before and during the rainfall, with large convective available potential energy. The clockwise rotation of wind direction with height suggested the warm advection over Shanghai. WSR-88D radar reflectivity of convective systems exceeding 50 dBZ, with large vertical integrated liquid value of 23-32 kg m−2, suggested that rainfall was very intense.
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