Fossil climate indicators are often quite fragmental. Further, sampling points of fossil indicators do not have uniform distribution. Latitudinal distribution of the terrestrial area are changing because of plate tectonics. All these circumstances may bring about strongly biased set of sampling points, which in turn cause errors in the reconstruction of the paleo-climate during the Phanerozoic. If the number of the sample points equals 100 or less, and when there is no information on altitude of each sample point, our simulations estimate that inferred global average surface temperature is accompanied with an uncertainty of more than 2 K. Results of our simulation indicates the inferred temperature can strongly vary with the change of the land-sea distribution. This result suggests that the reconstructed climate change in precedent studies can be a result of the varying land-sea distribution under the same global climatic condition during the Phanerozoic.
In the current study, the effects of topography and urbanization on the daytime local-circulations in the area of the Osaka—Kyoto plain were investigated using a mesoscale atmospheric model. The main results obtained from the numerical and observational data analyses are as follows: 1) The stagnation of the inland penetration of the sea-breeze front that occurs in the peripheral portion of the Tokyo urban area does not clearly occur in the peripheral portion of the Osaka urban area. The stagnation is attributed to the heat-island circulation that develops over the peripheral portion of the urban area. Because the northern peripheral portion of the Osaka urban area faces mountains, the valley circulation weakens the heat-island circulation; the about 600-m mountains surrounding the Osaka and Kyoto urban areas are the most suitable to cause the weakening of the heat-island circulation. The valley circulation also induces the surface-air-temperature increase over the plain, through an adiabatic process due to its downward flow; the temperature increase strengthens the horizontal pressure gradient between the sea-breeze circulation and inland areas. 2) The surface air over the suburban region between the Osaka and Kyoto urban areas remains drier than that over the Osaka and Kyoto urban areas, until the arrival of the sea-breeze front, even though urban areas were thought to be drier than suburban areas. This dryness results from the fact that the valley circulation transports drier air downward from the upper levels over the suburban area, and transports the surface-air moisture supplied from the ground surface into mountainous areas. The valley circulation also appears to play an important role in the transport of pollutants from the suburban area into mountainous areas. 3) A chain flow, which flows downward from the upper levels over the Osaka urban area to the lower levels over the Kyoto urban area, forms ahead of the sea-breeze front. This flow can result in the transport of pollutants emitted from the Osaka urban area into the inland Kyoto area prior to the transport of pollutants by the sea-breeze circulation. Additionally, the chain flow intensifies the downward flow of the valley circulation over the suburban region between the Osaka and Kyoto urban areas.
Observations from the May—June 1998 South China Sea Monsoon Experiment (SCSMEX) have been used to determine the characteristics of the onset of the summer monsoon over the northern South China Sea (SCS). The onset occurred in mid-May with a rapid increase in deep convection over a weekto-ten-day period, followed by a ∼ten-day break, and then a resumption of deep convection for a week-toten-day period in early June. The temporal and spatial variabilities of rainfall rates from the atmospheric moisture budget, satellite, and model estimates generally agree well, although the mean rainfall rate from the models for the 47-day Intensive Observing Period (IOP) is about 25% greater than the other estimates. The SST gradually increased over the northern SCS during the IOP, interrupted by slight cooling following monsoon onset until early June when the warming resumed. Surface sensible and latent heat fluxes increased after onset, but then decreased in June as a result of warm, moist air advecting over cooler water near the south China coast. The mean apparent heat source Q1 during convective periods resembled that determined for other tropical oceanic regions with a peak near 400 hPa. The apparent moisture sink Q2 also resembled profiles for other tropical regions, except for larger values below 900 hPa. The heating and moistening rates and vertical eddy flux of total heat during the early-June active period were greater than those observed during the May monsoon onset active period, indicating more vigorous deep convection as the monsoon ensued, a finding supported by Tropical Rainfall Measuring Mission (TRMM) precipitation radar data.
An unusually prolonged foehn was observed at Toyama in the Hokuriku district, located on the coast of the Sea of Japan of central Japan, from 30 July to 3 August 1999. A 5-day foehn is extraordinarily persistent and was not observed during the 24-year period 1975—1998, which makes the 1999 summer quite exceptional. While an anticyclone east of Japan is intensified over the 5-day period, a combination of the anticyclone with a rapidly developing cyclone over southeastern Siberia and a typhoon migrating northward into the East China Sea induces a definite intensification of southerly geostrophic winds over central Japan, a favorable condition for the occurrence of unusually prolonged foehn. Observational and model results show that on a monthly mean basis, a noticeable east-west pressure gradient around Japan is present due to the prominence of the Pacific and Japan (PJ) teleconnection pattern in response to tropical convective heating, resulting in reinforcing southerly geostrophic winds across central Japan. Once large-scale circulation anomalies are initiated and sustained as a result of such an extratropical response, the development and movement of adjacent synoptic-scale disturbances are largely regulated by those anomalies. A combined effect of the excitation of the PJ pattern and associated synoptic-scale disturbances is crucial for the extraordinary persistence of foehn along the coast of the Sea of Japan. The PJ pattern that appeared in July 1999 has an unusual geographical location and configuration because enhanced cumulus convection over the warm pool region of the western North Pacific is significantly displaced about 20° westward and 2—3° northward, compared to the typical hot summer that Japan experiences. This displacement is presumably attributed to a similar shift of distinctive warm SST anomalies over the warm pool region.
Numerical experiments are performed to simulate and understand cloud clusters associated with a Baiu front observed over Kyushu and the East China Sea on 16 July 1993. The clusters treated in this study are those that are not associated with any synoptic-scale low. A numerical model used is a mesoscale-convection-resolving model (MCRM) that resolves mesoscale organized convection by a grid, and treats cumulus convection as the subgrid-scale (Yamasaki 2001). The primary objective of this study is to investigate to what degree the MCRM can describe cloud clusters, and mesoscale organized convection that constitutes the clusters. In this study the grid size is taken to be 5/36 degrees (about 15 km) in the finest grid area of the triply-nested grid model. A global objective analysis data of JMA is used as the initial condition (00UTC, 16 July). A numerical experiment is performed by use of the MCRM referred above. For comparison, an old version of the MCRM (Yamasaki 1986) is also used, and an additional experiment is made for the case without parameterization of cumulus convection. The rainfall distributions at 12 hours after the initial time are compared with AMeDAS data, and with those from Peng and Tsuboki (1997) in which four cloud parameterization schemes are used. It is emphasized that the most important factor to prediction and understanding of the cloud cluster and rainfall over Kyushu at 12 hours is the eastward movement of a latently unstable area that exists at the initial time. A comparison of the results from the MCRM with those from the case without cumulusscale parameterization shows that the effects of cumulus convection are not essential to the eastward movement of the unstable area and rainfall over Kyushu at 12 hours, but time evolution (behavior) of mesoscale organized convection and cloud clusters are quite different, depending on the inclusion of the cumulus-scale effects. The performance of the model as to how cloud clusters and mesoscale organized convection behave realistically under the given initial condition is discussed, based on the studies in the past and physical considerations.
Verification of local climatic features produced by a regional climate model (JSM-BAIM) that includesa Biosphere-Atmosphere Interaction Model (BAIM) in the Japan Spectral Model (JSM) was performed.The data used in the verification were the results of a six-year integration calculated by the JSM-BAIM.In the time integration of the JSM-BAIM, the spectral boundary coupling (SBC) method was used. Thereproducibility of horizontally distributed features, and local and seasonal changes in the principal climaticelements of the model, were investigated in finer detail than that of our previous study.Comparisons of the model results with JMA objective analysis data were performed with respect togeopotential heights, temperatures, and winds at the 850-hPa and 500-hPa levels. Statistically signifi-cant differences appeared, mainly in the summer temperature field, and in the summer and the winterwind fields. It was considered that the statistically significant differences in the summer temperaturefield were due to the effects of land surface processes, and the summer convection process over thesouthern ocean area. The statistically significant differences in the wind field were mainly due to theinfluence of differences in topography between the regional model field and the analysis field, especiallywhen interacting with the strong wind flow during the winter season. Comparisons of the model results with the data of dense surface meteorological observation network(AMeDAS) were performed with respect to precipitation, surface air temperature, and radiation at theland surface over four typical climate areas in the Japanese Islands. The model reproduced reasonablywell the features of the seasonal and interannual variations of each variable, in each area. There were,however, some seasonal and regional features in the differences between the model results and theAMeDAS data.The JSM-BAIM, with the use of the spectral boundary coupling (SBC) method in a long period timeintegration, had sufficient accuracy for use in investigations of the interaction mechanisms between theterrestrial ecosystems and climate; temporally at least on the level of the seasonal and interannual variations,and spatially at least on the level of the climatic classification of the Japanese Islands. There is, however,the necessity for further verification using observed data, especially with respect to the near landsurface air temperature in a natural vegetation environment, and the atmospheric variables over the ocean.
In this study, an orthonormal wavelet expansion along a latitudinal circle is applied to decompose atmospheric data into a series of wavelet components with different spatial scales, and longitudinal locations. The application of the local spectral energetics allows the investigation of the local energy transformations for the local structures in the atmosphere. By applying the local spectral energetics analyses to 10 events of the Pacific blocking, we find that different energy conversion terms contribute to the increase of eddy kinetic energy during the development of the blocking events. For 8 out of 10 blocking events, the gain of energy through the wave-wave interaction of eddy kinetic energy is the most important process for the onset of blocking. The intensified trough of the blocking scale which is located at the western side of the blocking and the synoptic disturbances around the blocking, especially the western side, to convert eddy kinetic energy into the other components through the wave-wave interaction. The local spectral energetics analysis demonstrates that the wave-wave interaction of the eddy kinetic energy of the planetary-scale, and synoptic disturbances, at the western side of the blocking anticyclone plays an important role for the formation, and development of the atmospheric blocking. The behavior of the wave-wave interaction of the eddy kinetic energy in terms of the scale and longitudinal location is essentially consistent with the eddy straining hypothesis at the western side of the blocking.
A winter extratropical cyclone underwent rapid development along the south coast and east of Japan on 12 February 1994. The evolution and structure of this cyclone were investigated from its formation through the rapid developing stage using subjective surface re-analyses, objective analyses for upper levels, satellite images and other data. The cyclone formed over eastern China within a broad low-level baroclinic zone. A cold air mass covered the East China Sea and around Japan behind a preceding cyclone, while strong southerly winds brought warm, moist air above and near the incipient cyclone. The surface cyclone progressed eastward over the East China Sea with a moderate deepening rate. It then began developing rapidly south of Japan, over the warm Kuroshio Current. A deep mid-tropospheric trough approached the surface low from northwest and the cyclone extended horizontally as well as vertically. A subtropical jet streak and an associated upper-tropospheric PV anomaly advanced from the rear of the surface low. This cyclone had a cloud system of about 2000 km scale. Active cloud areas were found mostly to the north and east sides of the surface low. The incipient cyclone lacked well-defined surface fronts. As it advanced over the East China Sea, frontogenesis proceeded rapidly to form a warm front with appreciable temperature gradient, which extended zonally from the cyclone center. A low-level cold air flow, corresponding to a cold conveyer belt, occupied the northern side of the cyclone. Cold frontgenesis in terms of thermal fields was inactive to the rear of the cyclone center. A strong wind shear line extended in a north-south direction from the low center, between the southerly winds to the east and the westerly winds to the west. The shear line was associated with weak temperature gradient. As the cyclone developed the warm front extended westward relative to the cyclone center to form a bent-back front. The bent-back front was accompanied by significant wind shear between the northeasterlies to the north and westerlies to the south. The frontal system of the developing cyclone thus exhibited a characteristic configuration that differs from the classical conceptual cyclone model.
The simulation studies of the Meiyu-Baiu front, by global climate models, have not been performed in depth. A few experimental studies show insufficient precipitation in the Meiyu-Baiu front, without the detailed analysis on circulation systems in and around the Meiyu-Baiu front. In the present report, we study features of the Meiyu-Baiu front and associated circulation systems simulated in the climatological SST run by the T42L52 model (the maximum zonal wave-number is 42 in triangular wave transformation, and the number of the vertical level is 52) in comparison with the features described in observational studies. The Baiu front is not properly reproduced in the monthly averaged field for June and July. This is due to the alternation of relatively short “Baiu phase” and the longer “non Baiu phase”. In the “Baiu phase”, the large-scale circulation systems, such as the cut-off cyclones and blocking ridge in the northern latitudes, westward extending Pacific Subtropical anticyclone, monsoon westerly, and subtropical jet stream, are properly maintained. Only under this large-scale condition, a realistic Baiu frontal zone is formed in the model. The structure of the Baiu front, the features of precipitation, and the thermal stratification around the frontal zone are reasonably reproduced. In contrast, in the “non Baiu phase”, features in the large-scale field are significantly different from the observed features in the break period, while the continent-ocean thermal contrast is reasonably maintained. Judging from the results, the formation of the Baiu front does not only depend on the direct local effects of physical processes, and continent-ocean thermal contrast, but depends on the maintenance of large-scale environmental circulation systems favorable to sustain the Meiyu-Baiu front.
In the present study it is attempted to explain the mechanism of blocking formation in the Atlantic in connection with Rossby wave propagation from the upstream side. For this purpose, assuming a realistic monthly-mean non-blocking basic flow, the nondivergent vorticity equation on the sphere is numerically time-integrated in the linear and nonlinear models, with steady and non-steady (traveling) forcings located in the southern side of the North Atlantic jet. Our numerical experiment can reproduce a blocking formation as follows. When the Rossby wave induced by traveling forcing propagates along a North Atlantic jet and the portion of the anticyclonic vorticity in the Rossby wavetrain reaches high latitudes (near 60°N), its amplitude is increased in the jet exit region. This process can be understood as a linear evolution, since it can be simulated by the linear model also. After this process, the meridional separation of induced anomalies takes place and the blocking flow is formed as a distinct dipole structure with positive anomaly (i.e., negative vorticity) northward and negative anomaly (i.e., positive vorticity) south of it, so that the blocking flow becomes quasi-stationary. This process cannot be simulated by the linear model. Therefore, nonlinear effects are considered to play a critical role in this process.
The NCEP/NCAR pentad mean reanalysis data from 1980 to 1994 are employed to examine the activities of the 100 hPa South Asia High (SAH) during the boreal summer. The results show that there exists bimodality in the longitude location of the SAH. According to the two preferred regions for the SAH, the SAH is classified into the Tibetan Mode (TM) and the Iranian Mode (IM), respectively. The studies on the maintenance mechanism, both from circulation structure and thermal structure, manifest that the SAH has the feature of warm preference. The diagnosis based on the thermodynamic equation further reveals that the TM is closely related to the diabatic heating of the Tibetan Plateau, whereas the IM is more associated with the adiabatic heating in the free atmosphere, as well as the diabatic heating near the surface. The statistical composites, and case studies, corresponding to the two modes show that the SAH bimodality is strongly related to the climate anomalies over East Asia. In the case of TM, the southerly airflow over the subtropics and the northerly airflow over the middle-high latitude at 850 hPa are enhanced, forming a convergence zone along 30°N and resulting in increased rainfall extending from the south Japan, Korea Peninsula, and Yangtze-Yellow river valley of China to the Tibetan Plateau. In the case of IM, at 850 hPa the mid-latitude East Asia is dominated by a huge anomalous anticyclone, thus results in the decreased rainfall over the area.
Characteristics of cloud-to-ground (CG) lightning flashes in the Chinese inland plateau have been analyzed by using the data obtained from a broadband slow antenna system, with a time resolution of 1 microsecond in 1996, and a CG lightning location network from 1997 to 1998. It has been found that lightning discharges in a plateau summer thunderstorm usually present some special characteristics compared with the typical summer thunderstorm. On the average, positive CG flashes comprised 16% of total CG flashes in the Chinese inland plateau. After comparison with radar echo, it is found that some weak thunderstorms are conducive to positive CG flashes. The first return strokes are usually preceded by a long duration preliminary breakdown process which is characterized by discrete bipolar pulses for both positive and negative CG flashes. The mean duration of electric field change prior to first return stroke is about 126.1 ms, in which 23% exceeds 200 ms for negative CG flashes, and the corresponding value is 165.1 ms for positive CG flashes. The mean interval between 152 successive analyzed pulses is 211.0±148.0 ms, and the total duration of the pulses is 32.0±20.0 ms for negative CG flashes, while the mean time interval between the 50 analyzed pulses is 165.0±120.0 ms and the total duration of pulses is 27.0±19.0 ms for positive CG flashes. Other discharge features, such as interstroke intervals, amplitude distribution of the subsequent return strokes and etc., are also analyzed in the paper.
The ECMWF 1979-93 upper-air dataset (ECM) has been evaluated for evidence of equatorial zonal circulation cells around the globe. For this it is essential not only to isolate the divergent part of the wind, but also to ascertain the continuity following the flow between the centers of upward and downward motion. The present analysis from ECM complements a recent evaluation from the NCEP-NCAR 1958-97 dataset (NCP). The searches from ECM and NCP are in overall broad agreement concerning the existence of equatorial zonal circulation cells. Over the eastern to central Pacific, a well developed cell with subsidence in the East is found all year round. In ECM this extends to the surface layer, while in NCP the westward return flow is concentrated in the mid troposphere. Both ECM and NCP yield a cell over the western Atlantic in boreal winter, characterized by subsidence in the East and divergent westward flow at the surface. In the Indian Ocean sector, both ECM and NCP capture a strong cell in boreal autumn, with subsidence over the East African coast and eastward flow in the lower troposphere. In perspective, equatorial zonal circulation cells are confined to certain seasons and longitudes over the oceans, with strong zonal flow in the lower troposphere.