When a problem without viscosity is considered as an ideal case, a small dissipation term is necessary in many numerical calculations. Quasi-geostrophic turbulence, which is described by the barotropic version of quasi-geostrophic potential vorticity equation, (also called Charney-Hasegawa-Mima equation as an equivalent) is one of these situations: a dissipation term must be added in order to prevent the energy from piling up in the large wavenumber region. Consequently, the total energy decreases gradually, which should be conserved in an ideal situation. In this article, the total energy dissipation rate in quasi-geostrophic turbulence is estimated, based on the assumption that the energy dissipated in this system is equal to the energy transported to the large wavenumber region. This estimation complements the dynamic scaling laws for the quasi-geostrophic turbulence developed by Watanabe et al. (1998): the parameter expressing the total energy dissipation, which was determined from the result of the numerical calculation in their study, is derived from the parameters of the setting of the numerical calculation. This estimation also suggests the means to determine the appropriate artificial hyperviscosity coefficient used in numerical simulations.
Using the MRI global atmosphere-ocean coupled general circulation model, we had six simulations with different mountain heights, i.e., 0% (M0), 20% (M2), 40% (M4), 60% (M6), 80% (M8), and 100% (M, control run) of the present global orography, respectively, to study climate changes due to progressive mountain uplift. The changes of the Asian summer monsoon, with progressive mountain uplift is studied in this paper. An active convection region extends with mountain uplift to form a moist climate in South and East Asia. Monsoon circulation such as low-level westerly, and upper-level anticyclonic circulation, is also enhanced with mountain uplift. The increase in precipitation, and the enhancement of southwesterly, in the later stages of the mountain uplift, appear only over India and the south and southeastern slope of the Tibetan Plateau. Over the coastal region of Southeast and East Asia, where the maximum precipitation appears in M0, precipitation decreases gradually with mountain uplift, and the southwesterly in the later stages becomes weaker. In the connection with these changes, surface heat flux changes remarkably over moist Asia in the earlier stages of mountain uplift, compared with that in the later stages. The intensity of the Indian, Southeast Asian, and East Asian monsoon was investigated with indices which are defined by area mean precipitation. The Indian monsoon becomes strong gradually with mountain uplift; particularly, in the later stages, the remarkable enhancement is found. The intensity of the South Asian monsoon is the strongest in M4. Thus, in the later stages of mountain uplift, that becomes weaker in association with the northwestward migration of the convective activity. Although the East Asian monsoon is enhanced gradually with mountain uplift, the enhancement in the earlier stages is larger than that in the later stages. In the equatorial Indian Ocean, SST also increases with mountain uplift, resulting in the increase in precipitation. The increase in SST results from the change of the ocean surface dynamics due to the enhanced monsoon circulation. This result could not be obtained if CGCM was not used in this study.
Two nonhydrostatic effects that have been neglected traditionally in the hydrostatic primitiveequation models are studied in this article. One such effect is due to the vertical acceleration in the vertical equation of motion. The other is due to a Coriolis term involving 2Ω cos (latitude), where Ω is the rate of earth’s rotation, and is referred to here as a cos (latitude) Coriolis term. Cos (latitude) Coriolis terms appear in the vertical and zonal equations of motion. The questions to be investigated are: (1) what are the dynamical consequences of these two nonhydrostatic effects, (2) how the roles of cos (latitude) Coriolis terms can be compared with sin (latitude) Coriolis terms, (3) which nonhydrostatic effect is likely more important, and (4) should these effects be included in atmospheric modeling for describing what kind of motions? These questions are studied quantitatively through a normal mode analysis of compressible, and stratified atmosphere with rotation on a tangent plane in a three-dimensional space that is open horizontally, but bounded by two rigid horizontal planes in the vertical. Numerical results are presented for an isothermal model. Considering the current trend of numerical modeling in permitting to use finer resolutions and to extend the top of model atmosphere higher, it is prudent to include both nonhydrostatic effects in the dynamical core of next generation atmospheric models for all scales of motions.
Baroclinic waves in a rotating fluid annulus with a downward sloping bottom in the radial direction and with a free upper surface have been studied experimentally. Dispersive wave flows due to the effect of the depth variation (the topographic β-effect) have been observed and analyzed with special attention to the interactions among the wave components. The internal structure, the upper transition, the drift angular speed and the radial heat transport, are compared with those in our previous experiments with a horizontal bottom. The similarity between the wave packet formed in the dispersive wave flow, and the wave packets in the mid- and upper troposphere, is also mentioned.
Using an aqua-planet version of an atmospheric general circulation model (AGCM), the dependence of the tropical intraseasonal oscillation (ISO) simulation on the cumulus parameterization was examined with three different cumulus schemes—simplified Arakawa-Schubert, Kuo, and moist convective adjustment. The simulated intensity and propagation characteristics of the ISO depend significantly on the choice of cumulus scheme, in which more constrained convection scheme produces stronger intraseasonal variability in tropics. Mean thermodynamic state and the Intertropical Convergence Zone (ITCZ) structure also vary among the simulations, demonstrating that the ISO variability and the mean states are mutually dependent. Following Tokioka et al. (1988), the simplified Arakawa-Schubert scheme was modified by posing a minimum entrainment rate constraint for cumuli, and the relationship between tropical intraseasonal variability and zonal mean rainfall structure was examined. More constraining deep convections, the tropical ISO variability becomes stronger with narrower ITCZ structures. Vertical and horizontal structures of eastward propagating waves appeared in the aqua-planet experiments were further investigated. The vertical structures of propagating waves are consistent with observations of the Madden-Julian Oscillation, but the vertical profile of ISO-modulated heating exhibits a middle-heavy structure and the simulated waves show relatively faster propagations compared with the observed. The horizontal composite structures show the boundary-layer moisture frictional convergence to the east, and divergence to the west of the convective region, and this suggests that a frictional Kelvin wave-CISK mechanism is important to these eastward propagating waves.
A heavy rainfall band was observed over southern Kyushu on 23 June 2001. This heavy rainfall was not predicted by a nonhydrostatic model (NHM) that was operated twice a day to support the field experiment of X-BAIU-01. The reason for this failure was examined in comparison with X-BAIU-01 special observation and satellite data. The maritime wind convergence zone was retrieved over the sea southwest of Kyushu Island from NASA QuickSCAT data, before the heavy rainfall system formed. This wind convergence was also found in the initial field of NHM. The lower atmosphere observed by aerosondes was very humid, while that simulated by NHM was considerably drier. The existence of this low-level humid air was also supported by total precipitable water (TPW) data derived from Tropical Rainfall Measuring Mission Microwave Imager (TMI) and Defense Meteorological Satellite Program Special Sensor Microwave/Imager (SSM/I). These comparisons indicate that the fact that the lower atmosphere was drier than it is in reality, was the reason that the NHM failed to simulate the heavy rainfall. This indication was ascertained by humidifying the lower atmosphere of NHM to correspond to the observation (MOD), and applying the four-dimensional variational data-assimilation technique to TMIderived TPW data (4DV). For MOD, the band-shaped rainfall was reproduced successfully. For 4DV, the simulated precipitation intensity was not strong enough to forecast a heavy rainfall, because 4DV could not retrieve the vertical profile of water vapor by using only the TMI-derived TPW data. Humid air concentrated in the lower atmosphere is necessary to induce heavy rainfall over the Baiu frontal zone.
El Niño and the Southern Oscillation (ENSO), and the South Asian (SA) summer monsoon interact with each other. In a previous paper, the process from the SA-monsoon to ENSO was discussed. In this paper, the process from ENSO to the monsoon is described. As the consequence of ENSO events, a set of characteristic distributions of the temperature, wind and moisture is formed over the central and eastern Pacific Ocean. The anomalies of these fields are characterized by a “butterfly pattern” above the 500 hPa level, and a “horseshoe pattern” below 500 hPa. The patterns begin to appear in the winter, and influence the SA-monsoon in the following spring and summer. The distinct pattern of the butterfly shape may be explained by the extended Matsuno-Gill type dynamics, because the Pacific sector is dominated by the ENSO heating. On the other hand, the Indian sector is characterized by the land-sea monsoon circulation. It is important to note that the anomaly components of this local circulation are controlled largely by ENSO. The air temperature anomaly of the layer between 200 and 500 hPa moves westward in the latitude belt of 20°-35°N over the Asian sector, which is associated with the butterfly pattern in the Pacific sector. During this migration, the signal provides a precursory background over the Tibetan Plateau in April-May-June at its peak, setting the stage for the initiation of the SA-monsoon. Another signal emanates also from the tropical Pacific to the Indian sector, in May-June-July at its peak, in conjunction with the horseshoe pattern of the sea surface temperature in particular. These signals emanate from the eastern part of the ENSO region, contribute to the establishment of the thermal contrast anomalies between land and ocean, and the anomalous wind system over the Indian sector, including the upward motion over the Indian subcontinent. The EOF (empirical orthogonal function) analysis of the sea surface temperature in the broader NINO3 region (15°N-15°S, 150°W-90°W) indicates that the two leading modes well represent the fields of horizontal wind and temperature for the Pacific sector (the first mode) as well as the Indian sector (the second mode). Finally, the features associated with the 1976 climate shift are discussed. In the process from the SAmonsoon to ENSO, the mode of connection changed dramatically circa 1976; however, in the process from ENSO to the monsoon, the mode is almost the same before and after 1976. The reason for this asymmetry, in the change of the relationship, is discussed.
Using the Meteorological Research Institute of Japan Meteorological Agency, atmospheric global model (MJ98), the potential predictability is investigated (maximum possible predictability when sea surface temperature (SST) is perfectly predicted) of the seasonal mean fields of four water resource variables, i.e. precipitation minus evaporation (P - E), the terrestrial water storage, snow water equivalence, and runoff. 50-year (1949-1998) ensemble integrations are performed from six different initial conditions, forced with the same SST and sea ice cover. The seasonal mean field variance ratios of the SST-forced variability, to the total variability, are computed to measure potential predictability of the four variables. The variance ratios of P - E are generally high in the tropics, but low in the extratropics. The geographical pattern of potential predictability of the total terrestrial water storage is similar to that of P - E, since it is the net water flux into the ground surface. The variance ratios of snow water equivalence were as low in DJF as those of precipitation at high latitudes. However, the values in the coastal area of the Gulf of Alaska is high in MAM, although the other regions remained low. The geographical distribution of the variance ratio of runoff, has similar features to that of the total terrestrial water storage, but the values of the former are slightly lower than those of the latter. High variance ratios are found in some areas of the extratropics. Singular value decomposition (SVD) analysis suggests that the ratios of P - E, and snow water equivalence, are due to the teleconnection forced by the tropical SST, while the higher variance ratio of the total terrestrial water storage is due to its persistence.
Heat transfer coefficients, and relative importance of factors affecting surface turbulent heat flux in sea-ice covered ocean, were investigated using data obtained by rawinsonde observations over, and around, the southwestern region of the Sea of Okhotsk from Jan. to Feb. in 1998, 1999 and 2000. The range of the fluxes estimated by an atmospheric heat budget analysis was large, associated with the ice concentration and synoptic situation. The turbulent heat fluxes from the open water surrounded by the sea ice are always positive (upward). However, the heat flux through sea-ice surface tends to be negative (downward) over sea-ice area associated with the relationship between the air and ice-surface temperature. Using the obtained turbulent heat fluxes and a bulk formula, relationships of turbulent heat fluxes with ice concentrations and atmospheric parameters were investigated, and a representative value of the bulk heat transfer coefficient was obtained. The estimated conditional bulk heat transfer coefficient (1:33 × 10-3) in the Sea of Okhotsk shows a smaller value than that in the Arctic region, reflecting the difference of the ice conditions, and thermal conditions in the boundary layer. Through an idea of the insulating coefficient, which measures degrees of the insulating effect of sea ice on the heat transfer, we explained that the sea ice works as a more effective insulator in weaker cold-air outbreaks. From a regression analysis, we found that the primary contribution to the amount of turbulent heat flux is the ice concentration, and second is the air temperature in the boundary layer.
The variability of tropical cyclone (TC) frequency over the western North Pacific (WNP) was investigated using a high-resolution atmosphere-ocean Coupled General Circulation Model (CGCM) of the National Research Institute for Earth Science and Disaster Prevention. A 50-year CGCM integration simulated highly frequent occurrence periods (HFOPs), comprising periods with a greater number of TCs than the long-term mean number, and lowly frequent occurrence periods (LFOPs), comprising periods with a lower number than the mean, as the interdecadal variability of TC frequency. The variability corresponded to the interdecadal variability of the actual TC frequency over the WNP during the past 50 years, 1951-2000. This CGCM integration also showed that the oceanic and atmospheric conditions (sea surface temperature, horizontal wind and relative vorticity at the 850 hPa level, equivalent potential temperature, and precipitation rate) favorable for TC formation during the HFOPs were the same as the observed oceanic and atmospheric conditions during the observed HFOPs. In the HFOPs, the sea surface temperatures over the tropical WNP are higher than in the LFOPs. The differences in atmospheric conditions between HFOPs and LFOPs are characterized by an anomalous westerly wind and positive relative vorticity at the 850 hPa level and an increase in the convective available potential energy. These differences in the environmental conditions simulated by the CGCM are consistent with those found in the observations. The CGCM was able to simulate the interdecadal variability of model TC frequency and the oceanic and atmospheric conditions related to the variability, which were similar to the observations. However, we need to improve the CGCM to more realistically simulate TC activity and oceanic and atmospheric conditions hereafter, because the CGCM has several deficiencies.
Satellite-observed Microwave Sounding Unit (MSU) channel 1 (Ch1) brightness temperature, and General Circulation Model (GCM) reanalyses over the globe, as well as radiative transfer simulations, have been used to investigate microwave surface emissivity, and low-tropospheric hydrometeors, during the period from January 1981 to December 1993. The average model Ch1 temperature has been derived from three kinds of GCM reanalyses, based on the MSU weighting function. Since the Ch1 temperature constructed from GCM-reanalysis air temperature neglects effects from surface emissivity differences and hydrometeors, it is highest in the summer hemisphere. On the other hand, MSU temperature over land is much higher than it is over ocean, due to depressed surface emissivity over water. Over the high latitude ocean the MSU Ch1 temperature is enhanced because of ice/snow emissivity, while it is reduced over the high latitude land. The difference values of Ch1 temperature between reanalysis and MSU decrease, in the regions of the ITCZ, SPCZ and sea ice mainly because of increased MSU temperature. The values decrease by about 4-6 K in the ITCZ and SPCZ regions, due to hydrometeors. This difference is found to decrease by about 10-30 K in sea ice regions, due to change in surface emissivity. To explain these results, we have estimated the contribution of surface emissivity and hydrometeors to the MSU Ch1 temperature, utilizing radiative transfer theory. The increase of 4-6 K in the temperature over the ITCZ and SPCZ is estimated to result from hydrometeors that give a precipitation rate of 1-1.5 mm/day under the horizontal homogeneity of rain within the MSU radiometer field of view (fov, ∼110 km), and 7-11 mm/day under inhomogeneous rain distribution within the fov, consistent with TRMM observations. The increase of 10-30 K over the high latitude ocean, arises from ice emissivity of 0.6-0.9. Seasonal movements of sea ice boundary also have been discussed. This study could be valuable by providing an independent technique of computing surface emissivity and hydrometeors over the globe, and for long time periods from microwave measurements, such as MSU and AMSU.
This paper focuses on the development and application of a new One-Dimensional Variational (1DVAR) data assimilation algorithm for estimating the spatial and temporal variations of soil moisture and temperature profiles, by grid-based analysis using remote sensing and in situ observations. This algorithm employs a heuristic optimization approach, simulated annealing (SA), which is capable of minimizing the Variational cost function without using adjoint models. The present assimilation scheme assimilates passive microwave remote sensing observations of brightness temperature into the land surface scheme (LSS), Simple Biosphere Model2 (SiB2). The LSS is used as a model operator, and a Radiative Transfer Model (RTM) is used as an observational operator. The modeling system has been applied, and validated, using data from the GAME-Tibet (Global Energy and Water cycle Experiment (GEWEX) Asian Monsoon Experiment in Tibet) mesoscale field experiment. Compared to SiB2, our assimilation scheme solves the major initialization problem, and estimates the soil temperature and soil moisture at the surface layer and in the root zone with significant improvements.
A series of idealized numerical simulations of tropical cyclones is performed to enhance the knowledge on the mechanism of tropical cyclone (TC) motion in vertically sheared environments. The simulations are performed using an f-plane version of the typhoon track prediction model previously run at the Japan Meteorological Agency. A wide variety of TC tracks show up when the model is integrated repeatedly with different formulations of convective parameterization from an identical initial field. To account for the diversity of TC motion in vertically sheared environment, a steering-weight concept is introduced. The steeringweight is a set of weighting factors, mathematically derived from the surface pressure tendency equation, and can be a measure of how sensitive the motion of a TC is, to the steering flow at each vertical level. The experiments with different cumulus parameterizations show that the vertical profile of steering weight tends to strongly depend on the cumulus parameterization scheme used, but much less on the environment specified. This result implies that the axisymmetric thermal structure of modeled TCs, is mostly determined by parameterization schemes rather than environment in the model. And the longterm (say, 72 h mean) TC motion is well explained by the combination of steering weight, and large-scale flow fields. The result suggests that the differences in TC track among numerical prediction models, may be attributable to the differences in steering weight to some degree, given that there is no significant difference in the simulated large-scale flow fields among the models. Moreover, as long as the steering weight concept holds in nature, the steering-weighted deep layer mean flow, instead of the conventional and empirical pressure-weighted one, would work better in accounting for the motion of real tropical cyclones.
This study compares rainfall forecasts by four cumulus parameterization schemes (CPSs), the Anthes-Kuo, Betts-Miller, Grell, and Kain-Fritsch schemes, using the fifth-generation Pennsylvania State University—National Center for Atmospheric Research Mesoscale Model (MM5) nesting down to 15-km grid spacing. Six rainfall events over the Taiwan area are selected to investigate the CPS performance. The precipitation forecast is evaluated over the model grid points using statistical scores (threat score, equitable threat score, and bias score) for different threshold values based on island-wide rain gauge observations. The results show that except for the warm-season events (spring rainfall and summer thunderstorm cases), the 15-km MM5, using any of the four CPSs, shows good skill for predicted coverage of measurable 6-h rainfall over the Taiwan area. For rainfall-area and rainfall-amount predictions, the model performs better in cold-season events (winter cold-air outbreak and autumn cold front cases) than in warm-season events, in agreement with previous studies. None of these CPSs consistently outperforms the others in all measurements of forecast skill, and each CPS performs very differently for precipitation prediction under different synoptic forcings. For precipitation events with strong synoptic-scale forcings (like the winter cold-air outbreak and Mei-Yu front cases), the synoptic forcing and Taiwan's topography provide the primary control on the model's rainfall forecast, and the CPSs used in the model only modify precipitation prediction slightly. In the wettest 3 of 6 cases, the ensemble prediction with an arithmetic average of rainfall forecasts by four CPSs has the best threat score at 0.25-mm threshold. The 15-km MM5 generally overpredicts the area of light rainfall, and underpredicts the area of heavy rainfall, and the model tends to have better predictive skill in the lower land than over the mountainous area in Taiwan. Except for the spring rainfall case, all CPSs underestimate precipitation amount, especially for heavy rainfall cases (Mei-Yu front and Typhoon Otto). The Grell experiment has the best skill in total accumulated rainfall prediction in four out of six cases; the Betts-Miller experiment has the best performance for the rainfall maximum forecast in three out of six cases. The characteristic responses of four CPS experiments over the Taiwan area are: (i) Anthes-Kuo experiment tends to overpredict the rainfall area, especially for light precipitation events; (ii) Betts-Miller experiment is inclined to produce heavy rainfall, and it underpredicts the rainfall area; (iii) Kain-Fritsch experiment has the best skill in rainfall-area prediction for the winter cold-air outbreak case; and (iv) Grell experiment has the best predictive skill for the heavy-rainfall events of the Mei-Yu front, and Typhoon Otto cases.
A few sequential strong updrafts of magnitude about 8-10 m/s in the upper troposphere were observed by the MST radar at Gadanki (13.5°N, 79.2°E), India on 21-22 and 22-23 June 2000. On both days, convective storms with rainfall appeared over the radar site. The updrafts region shifted upward by a distance of about 3-4 km within a time-range of 8-10 minutes, and terminated around 15-16 km (the level of neutral buoyancy). The signature of the gravity wave was seen in both the upper troposphere and lower stratosphere. The main mechanism involved in the generation of the gravity waves most likely came from a vertically oriented oscillator, which triggered by convective updrafts near the neutral buoyancy level analogous to the mechanical oscillator. The resultant gravity waves had a vertical wavelength of about 2-5 km, and dominant wave periods of 10-20 minutes (above tropopause) and ∼10 minute (below tropopause). However, variations whose period is below 10 minutes in the troposphere are thought to be not due to the gravity waves, but due to oscillatory behavior of the updrafts. The horizontal wavelengths, and intrinsic group velocity corresponding to these gravity waves, in the lower stratosphere, are estimated in the range of 10 to 20 km, and 10-12 m/s, respectively. The direction of average group velocity is estimated at about 15-20 degrees from the horizontal.
The mean state and year-to-year variations of the tropospheric temperature fields are analyzed in light of their relationship with the establishment of the East Asian summer monsoon (EASM), and the Indian monsoon (INM). Primary data for the analysis include the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis from 1982 to 1996 and the European Center for Medium-Range Weather Forecast (ECMWF) Reanalysis (ERA) from 1979 to 1993. The analysis reveals that, in most of the analyzed years, the meridional temperature gradient in the upper troposphere reverses at, or before the onset time of the summer monsoon in both the EASM region and the INM region. On the average, the reversal time of the meridional temperature gradient in the EASM region (INM region) is concurrent with (one pentad earlier than) the onset time of the summer monsoon. A budget analysis shows that the strong horizontal warm advection and the diabatic heating are the main contributors to the upper tropospheric warming against the strong adiabatic cooling during the pre-onset period over the EASM region. Over the INM region, however, strong adiabatic heating due to subsidence is the major warming process against diabatic cooling, and the strong horizontal cold advection related to the persistent northwestlies to the southwestern periphery of the Tibetan Plateau. The results show that the seasonal shift of the South Asian High in the upper troposphere, and the establishment of the EASM and the INM are closely related to the seasonal warming, which results in the reversal of the meridional temperature gradient in the upper troposphere over the two monsoon regions.
The development of anomalous subtropical anticyclone around East Asia, influences the regional climate of Japan and Korea in the summer. The effect of tropical sea surface temperature (SST) is studied using a regional atmospheric model (MM5 ver. 3.5) with realistic lateral, and surface boundary forcing. The following results were obtained. (1) An uniform increases of SST around the Philippine Islands generates the propagation of a Rossby wave, resulting in the formation of an anomalous subtropical anticyclone around Japan/Korea. (2) This Rossby wave propagation is linear in that the model response to negative SST anomalies reverses polarity. The linearity suggests that the response to the SST forcing around the Philippine Islands is a deterministic problem, with its magnitude depending on the size of the SST anomaly. Nonlinearity increases if there is a tropical depression, or typhoon in the model domain. (3) There are a zonally weakened upper-level jet, and a meridionally intensified low-level jet, when the Rossby wave is excited by SST changes. A positive SST anomaly intensifies the meandering of the upper-level jet. (4) The time scale for the atmospheric circulation to respond to such an SST forcing, is about 5 days. (5) In the vertical, the model response is barotropic over 40°N∼55°N, 100°E∼160°E, and is baroclinic to the south over 10°N∼40°N, 100°E∼140°E. (6) The effect of SST forcing is about four times stronger than model internal variability. (7) Only positive SST forcing affects the precipitation pattern around Japan and Korea, causing rainfall to decrease and increase in these regions, respectively. Both positive and negative SST forcing affect the precipitation around the Philippine Islands; a positive SST forcing increases precipitation, and vice versa. (8) The response of above Rossby wave is sensitive to the geographical location of SST forcing, reaching a maximum in the region (110°E∼140°E, 10°N∼20°N) among the experiments carried out. The study results of meridionally propagating Rossby wave induced anomalous subtropical anticyclone contribute to the understanding of regional climate around East Asia in summer.
A Baiu-frontal meso-a-scale depression was generated on 18 June 2001 in the mesoscale observational region, which was located in the downstream region of the Yangtze River. Based mainly on the data collected in this observation, we analyze the three-dimensional structure of a meso-a-scale convective system. We describe this structure and discuss the evolution of Baiu-frontal convective systems near the east coast of the continent. This convective system was composed of several meso-β-scale convective groups, which were bandshaped echoes located near the depression center, cellular echoes formed one after another ahead of the center, and a band-shaped echo extending more than 500 km in the rearward of the depression. There were three flows toward the convective system in the lower troposphere: moist southwesterly jet, dry west-to-northwesterly wind, and cold shallow easterly wind, which appeared near the coast. These convective groups were formed where the southwesterly wind, with convectively-unstable air, was lifted up to the free-convection level. This lifting resulted from low-level convergence, which was formed both by easterly and west-to-northwesterly wind in the central portion of the depression, by easterly wind at the forward portion, and by west-to-northwesterly wind at the rear portion. The system-relative speed of the southwesterly wind in the forward portion was larger than that in the rear. It is suggested that this strong inflow caused the formation of a deep convective structure in the forward portion, which reached the tropopause. The easterly wind played an important role in lifting such a strong inflow, and forming the deep convective groups in the meso-a-scale convective system, and then contributed to evolving the depression through the extension of the convective-updraft area. The mechanism that the easterly wind was formed locally near the coastline is discussed. The enhancement of low-level convergence, due to the formation of such a local wind system, is suggested to be one of the causes for the development of a mesoscale convective system, near the east coast of the continent during Baiu seasons.
A drastic increase in precipitable water vapor (PWV) in the evenings was repeatedly confirmed in October 2000, by an analysis of GPS observational data at Lhasa in the southeastern Tibetan Plateau, China. In order to investigate its mechanism, a numerical simulation was performed for a typical day using a regional atmospheric model. During the daytime, moisture is transported toward the summits of the Himalayas from the Hindustan plain along the southern slope of the mountain range by thermallyinduced upslope winds. The moist air mass penetrates the Tibetan Plateau, through some cols of the Himalayas. Moisture accumulates over the northern foot of the Himalayas. Then a horizontal gradient of moisture increases north of the Himalayas, and a stationary moisture front forms between the moist air mass and dry air on the Tibetan Plateau. In the evening, the frontal structure begins to gradually decay until the midnight, and a large amount of moisture bursts toward the inner Tibetan Plateau in the lower atmosphere as a gravity current, causing a rapid increase in PWV. It is speculated, therefore, that the drastic evening increase in PWV, extending widely along the Himalayas, is generated by the plateau scale diurnal wind induced by the thermal effects between the Tibetan Plateau and the Hindustan plain. During the following day, the PWV level decreases over a wide area of the inner Tibetan Plateau, since dry air advection is intensified in the lower atmosphere by the synoptic scale westerly winds, because of the growth of the mixed layer accompanied by the vertical transportation of momentum.
Observational study was made of the thermal belt during December 19-20, 2002, in the western slope of Mt. Tsukuba, central Japan. Using a spatially distributed temperature logger, 1.5 m above the land surface, it was revealed that the maximum temperature is centered around 200 ∼ 250 m above sea level (a.s.l.), especially developing in the early morning. The vertical temperature profile was observed by use of captive balloons. Strong surface inversion, in excess of 4°C, occurred in the bottom of the mountain around daybreak, while the vertical distribution of temperature at 150 m a.s.l. is uniform, roughly 3.5°C ∼ 4.5°C, up to 40 m from the terrain. The time series of air temperature toward daybreak implicates a significant role of radiation cooling, since the abrupt temperature fall was concurrent with rapid improvement of weather. Analysis of thermal image, obtained by thermography, also indicates that this relatively warmer region extends in the mid-slope of the mountain. However, zonally nonuniform temperature distributions implicate a presence of terrain-related cold air drainage.
High temperatures (H1), which appear at intervals of 4-6 years, have been noted in the summer surface air temperature (hereafter, referred to as summer temperature) in Japan. The interval between H1 years is 6 years in most cases. This fluctuation of summer temperature in Japan (quasi-six-year fluctuation, hereafter) is distinct in the northern part of the country, but it is also seen in other areas of Japan, except in the Southwest Islands. Summer temperatures for other years of the cycle, also show the following characteristics in Northern Japan: (i) cold temperatures appear in years of H1 - 2 (2 years before H1), and H1 - 1 (1 year before H1); (ii) years of extremely low temperature are included in the H1 - 1 group; (iii) warm temperatures appear in H1 + 1 (1 year after H1), and (iv) cold temperatures appear in H1+2 (2 years after H1). The period of 6 years also appears in fluctuations of sea surface temperature in the North Pacific Ocean (Saiki and Nagasaka 1986). High correlation between the quasi-6-year fluctuation of summer temperature in Japan, and fluctuation of sea surface temperature in the North Pacific Ocean, suggests that these two quantities are closely related.