A one-dimensional land surface model has been developed to estimate water and energy fluxes between the atmosphere and ground surfaces. The model includes three submodels; vegetation, snow cover, and soil. In order to adapt the model to intensely cold regions, the snow cover submodel can consider depth hoar by means of effective temperature gradient. This submodel can calculate profiles of snow temperature, density, and liquid water content using meteorological data inputs; the model structure is simple having snow layers with the same thickness. The ratio of fallen snow amount to rain amount is given as a function of wet-bulb temperature. The model has been applied to a GAME-Siberia site in the Central Yakutia, Russia. Since we do not have complete data sets in midwinter, the model has been validated through whole winter simulations at several routine stations first. Snow depth and snow-cover period are simulated reasonably. Next, fluxes have been estimated using 1998 data at the GAME plain taiga site, compared with the observed fluxes. Diurnal and seasonal changes of fluxes are simulated reasonably. The calculated snow cover disappearance date is earlier than the observed day.
Using the NCEP-NCAR reanalysis datasets, the anomalous activities of East Asian summer monsoon for 1999 have been studied, with the following characteristic features identified: 1) the South China Sea (SCS) summer monsoon began at the end of May, which was much later than the normal year (mid-May), with three active and two break phases observed, 2) the northward advance of the summer monsoonal southwesterlies or southerlies in East China was markedly weak with the northernmost position of 32°N, and the monsoonal airflow could not establish itself in the areas north of the Yangtze River, thus leading to a very weak moisture transport and the condition of prolonged drought in North China, 3) correspondingly, the stationary position of seasonal heavy rainfall-belt in East China was considerably farther south than normal, also with the northernmost position being near 32°N, never going beyond the Yangtze River for the whole summer, 4) the low-level southwesterly (SW) airflow from the Bay of Bengal and the tropical Indian Ocean played a very important role in the abnormal activities of the SCS summer monsoon. Three active periods of the SCS monsoon were almost concurrent with the anomalous eastward extension of this airflow, and 5) after the beginning of July, as the Western North Pacific subtropical high (WNPH)(110-150°E, 10-30°N) rapidly shifted eastward up to the region east of Japan, the low-level southwesterly airflow assumed a marked eastward migration with its leading front reaching the longitudes of 160-170°E, thus bringing about the major moisture transport belt to shift eastward, and anomalous heavy rainfalls to occur around Japan for a long time. The reasons behind these anomalous activities of the 1999 summer monsoon have been examined. From the viewpoint of thermal conditions, the late onset of the summer monsoon this year was likely to be related to late development and reduced intensity of heat source over the Tibetan Plateau, which resulted in the smaller thermal contrast between the atmosphere over the plateau and the atmosphere over its surrounding areas. This could lead to prevalence of the anomalous easterlies in the eastern and southeastern flanks of the plateau, which was not favorable to the normal development of the East Asian summer monsoon (EASM). The continuous northward advance of summer monsoonal southwesterlies was blocked by the anomalous easterlies, so that the SW airflow was confined in lower latitudes for a long time.
Distributions of aerosol concentrations, optical properties, and wet deposition fluxes are simulated for the next fifty years using an aerosol transport model coupled with an atmospheric general circulation model. Treated species are sulfur dioxide, and all the main tropospheric aerosols, i.e., carbonaceous (black and organic carbons), sulfate, soil dust, and sea salt. We especially pay attention to distributions of anthropogenic carbonaceous aerosols, sulfate aerosols, and sulfur dioxide. The simulation uses the Special Report on Emissions Scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC) as the future emission scenarios of anthropogenic pollutants. Simulated results suggest that carbonaceous aerosols continue to increase over industrial and densely populated regions for the next five decades, whereas sulfate aerosols decrease around Europe and North America. The aerosol single scattering albedo in the future is, therefore, calculated to become small gradually in the mid- and high-latitudes of the Northern Hemisphere. Sulfate aerosols and sulfur wet deposition fluxes are, on the other hand, simulated to increase only over East Asia. Black carbon and sulfate aerosols around Japan in 2050 are simulated to be two or three times as large as those in 2000 with one of the SRES scenarios. Hence this suggests that pollutants originating from the East Asian continent can seriously affect the atmospheric quality in Japan in the next several decades.
Since the adoption of the barotropic adaptive-grid typhoon simulation model in the KMA numerical weather prediction system in 1997, the model has been increasingly recognized to be quite useful in forecasting tropical cyclone tracks. This paper presents the description of the model and history of the model upgrades and the corresponding performance in track predictions of tropical cyclones. There have been two major model upgrades. The first upgrade is on the modification of the steering current. The model is run for 12 hours to catch the initial tendency of the vortex motion in the model and evaluate its departure from the observed one. The vector difference is added to the original steering current and then the model is rerun using the modified steering current. The improvement is mainly for the short-term forecast. Secondly, we let the circular bogus area vary with respect to the 30 kt wind radius (referred to as the storm size) reported from the RSMC Tokyo. For tropical cyclones whose size is larger than 440 km we let the circular bogus area of 1.2 times the 30 kt wind radius from the RSMC tropical cyclone bulletin. For storms smaller than 440 km, we use a constant bogus area of 600 km radius. This provides us a great improvement over the incumbent operational version of the model. The improvement ranges from at least 10% and up to 15% during the entire forecast period of 60 hours.
A thermal property parameter for expressing the ground heat flux (cρλ; product of the heat capacity and the thermal conductivity) of urban complex terrain was estimated. The surface temperature time series was observed during nocturnal radiative cooling, and employed in the estimation. The surface temperature was obtained using the airborne-measured upward longwave radiation in order to consider the directional anisotropy of radiometric surface temperature. The effective thermal property parameter for a town-scale urban area was found to be two to four times larger than that of the surface material component. The explanation for this, shown by several model simulations and other radiometric observations, was total surface area increased due to urban canyon structure. The parameter cρλ on the town-scale was expressed by the canyon shape, and the cρλ of its component material.
Atmospheric seasonal predictability is investigated using the Japan Meteorological Agency (JMA) Atmospheric General Circulation Model (AGCM) which is a global spectral model of T63 resolution used for former operational one-month farecasts. Four-month ensemble integrations were performed from nine consecutive days of initial condition preceding the target season. All four seasons in the 15-year period from 1979 to 1993 are chosen as the target seasons. The model was forced with observed sea surface temperature (SST) during the time integrations. Verification by the anomaly correlation of seasonal averaged 500 hPa height are summarized as follows. (1) In terms of a 15-year mean, ensmble average forecasts tend to have higher skill than means of individual forecasts, persistence forecasts and climatological forecasts over the whole globe. (2) In the Northern Hemisphere, skill is high in spring and winter, and low in summer and autumn. (3) Skill is higher over East Asia and North America, than over Europe and Atlantic regions. In winter, the relatively higher skill over East Asia and North America can be attributed to reasonable reproducibility of Western Pacific (WP) and Pacific/North American (PNA) teleconnection patterns by the model. (4) A larger ensemble size improves the skill of the model both for extratropical regions and the tropics, and for all four seasons. (5) The skill of one-month lead time forecasts is lower than that of no lead time forecasts for all regions and all seasons. Focusing on the Northern Hemisphere, the relatively large skill degradation with respect to lead time is striking in spring, which suggests that the model is more sensitive to initial conditions in spring than in summer and autumn. (6) Skill enhancement for strong El Niño and the Southern Oscillation (ENSO) years is generally evident over the whole globe, but is not so striking in the Northern Hemisphere. Skill over North America increases for ENSO years, but there is no clear skill enhancement over the Europe and Atlantic regions. The model’s ability to reproduce the interannual variability of precipitation was also investigated. Geographycal distributions of interannual temporal correlation coefficients between observed precipitation, and model ensemble average forecast precipitation show that correlations are generally higher in the tropics than in the extratropics for all seasons. In the tropics, skill is relatively higher in the equatorial eastern Pacific ocean and lower over India. The model has difficulty in simulating the interannual variability of precipitation for the Indian summer monsoon season and for the East Asian rainy season in summer, which may originate from the model’s poor climatology over these regions in summer.
Seasonal variation of diurnal cloud activity (abbreviated DCA) over the Tibetan Plateau throughout the year is examined using 3-hourly geostationary meteorological satellite (GMS) data for 6-years (1989-1994). The DCA shows two distinct variance maxima in the seasonal cycle. One is in spring (pre-monsoon season), and the other is in the summer monsoon season. The DCA begins in late January, and reaches its maximum from March through April. The active DCA extends over almost the whole of the plateau, especially over the southern part (around 30°N, 90°E) and the zonally oriented belt between 35°N, 80°E and 31°N, 102°E. A short interval between the two active DCAs is found around late May to early June. The DCA starts again over the southeastern region of the plateau (centered around 30°N, 101°E) in June and moves up to the southern region. From July to August, the DCA is most active over the southern region (around 30°N, 86°E). After September, the active DCA retreats to the southeastern region. In both spring and summer, the amplitude of the DCA fluctuates on intra-seasonal time scales. In the active period of the DCA in spring, corresponding to the meandering of the upper-level mid-latitude westerly to the south of the plateau with a trough, the cold air mass at the upper-level is clarly seen over the plateau and weak wind speed is observed through the plateau troposphere. This atmospheric structure and a heating of the lower atmosphere during daytime are likely to be responsible for the enhancement of the DCA in spring. In contrast, during summer monsoon season, the increase of humidity and temperature are identified at the lower atmosphere over the plateau, associated with a humid and warm air intrusion from the South Asian monsoon area into the lower atmosphere over the plateau and precipitation due to active convection. This indicates an increase of the instability for moist convection. These features of the atmospheric circulation and the surface heating during daytime induce active moist convection. Corresponding to the enhanced DCA over the southern part of the plateau, the center of the Tibetan high is located over there. A possible mechanism for the intraseasonal variability of the DCA associated with that of the upper-level atmospheric circulation is also discussed.
The basic features of the North Pacific subtropical anticyclone in summer are studied using the European Centre for Medium Range Weather Forecasts (ECMWF) Re-Analysis data for the period (1979-1993). It has been shown that the North Pacific subtropical high displays substantial interannual variability at its western edge. Further analysis indicates a significant relationship between the zonal variation of the subtropical high, and the variation in the intensity of atmospheric convection over the warm pool. The stronger (weaker) convection over the warm pool is associated with more eastward (westward) position of the western Pacific subtropical high. The UK Universities’ Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM) is used to examine the influence of the local SST anomalies, and the associated anomalous convection over the tropical western Pacific warm pool on the zonal displacement of the North Pacific subtropical high. The model results show that the suppressed atmospheric convection caused by lower SSTs in the warm pool results in anomalous anticyclonic circulation in lower level over the subtropical western Pacific, and westward extension of the subtropical high. The results are consistent with the composite results based on the reanalysis data, and imply the SST anomalies off-equator over the western Pacific play an important role in the zonal shift of the North Pacific subtropical high in summer. Despite the fact of good agreements in many aspects between the model simulations and composite analyses based on observation, there are some differences between the two especially in terms of upper-level extratropical circulation anomalies over East Asia.
The statistical relationship between ISCCP (International Satellite Cloud Climatology Project) cloud type, and the vertical relative humidity profile observed by radiosonde, was studied during the period from August, 1992 to July, 1994. The ISCCP-DX data of GMS (Geostationary Meteorological Satellite) were used to obtain cloud type information over the 1.5 degree grid area centered at the radiosonde station. To obtain the reliable cloud type classification, the data at 00 UTC—when the visible data were available—were used. The radiosonde observations when the 1.5 degree grid area is covered by single cloud type were used to compare the vertical relative humidity profile. Four cloud types of low-level, middle-level, cirrus-type and cumulonimbus-type cloud, and cloud-free case were selected for the comparison. As was expected, the mean relative humidity was largest at all levels when the cumulonimbus-type cloud existed, and was smallest at all levels when no cloud existed. The relative humidity was larger than that of a cloud-free case at middle- and low-level (low-level) when middle-cloud (low-cloud) existed. When cirrus cloud existed, the high level relative humidity was larger than that when low- and middle-cloud existed, and showed local maximum at 400-500 hPa. The feasibility for the use of the statistical relationship in this study in estimating relative humidity profile was shown as a case study by comparing with the current JMA statistical relationship derived from the GMS single infrared data. Cirrus clouds are difficult to be identified objectively from single infrared measurement and are often misclassified as middle- or low-level cloud. The misclassification between cirrus-type cloud and middle-/low- level cloud leads to a different vertical profile of water vapor. However, cirrus-type cloud is reasonably identified in the ISCCP with the use of visible and infrared measurements.
Shortly before midnight of 28 November 1991, the Tokyo (Haneda) airport was hit by two wind bursts in short succession. These gusty winds were responsible for some aviation hazards the airport suffered from. This paper investigates the nature and cause(s) of this event, mainly through the analysis of autographic records from the meteorological observatories and AMeDAS (Automated Meteorological Data Acquisition System) data. It is found that abrupt changes in wind direction and speed occurred at the airport, when an isolated, mesoscale migrating disturbance impinged into a strong low-level southwesterly flow associated with an extratropical cyclone. It is further revealed that the disturbance traveled across the central part of the Kanto Plain toward the east or southeast with the phase speed of 13-16 ms-1. Its horizontal dimension was roughly 30 km, and the disturbance was accompanied by not only abrupt changes in wind, but also abrupt increases in pressure and temperature at the passage time of the disturbance. All these characteristics are common in the Morning-Glory-like bore, which is known to develop when a stable layer underlying a neutral layer is disturbed by an external disturbance. Thus, it is concluded that the disturbance detected in this case is a type of the internal bore. Its generating mechanism remains unclear, however. To the best knowledge of the authors, this is the first paper that documents a bore-like disturbance observed in Japan.