A set of independent prognostic variables, based on a survey of the microphysical timescales in clouds, is proposed for long-term cloud-resolving model simulations. Two of the variables are the moist entropy and the total mixing ratio of airborne water with no contributions from precipitating particles. Non-prognostic variables such as air temperature can be diagnosed from the prognostic variables easily. In this proposed modeling framework, moist thermodynamics is separated (or modularized) from cloud dynamics and microphysics. Numerical results are compared with analytic solutions to show that the proposed prognostic variables work well when a large time step (e.g., 10 s) is used for numerical integration.
In this study we document the abrupt seasonal migration of the East Asian upper-tropospheric jet stream (EAJS) in boreal mid-summer over the East Asian coast, based on the NCEP/NCAR reanalysis data from 1958 to 2002. In climatology, an abrupt northward jump of the EAJS is identified in late July. By examining each of the total 45 years there are 27 years in which the EAJS exhibits an abrupt northward jump in mid-summer, and about half of the jump cases occur in late July, suggesting the characteristic of phase locking with the calendar year. A cluster analysis on the meridional variation of the upper-tropospheric zonal wind anomalies along the eastern Asian coast associated with the northward jump of the EAJS reveals that the northward jump of the EAJS is dominated by two categories, in which the intensity of the EAJS is enhanced and weakened, respectively. For both categories, there is a pair of westerly and easterly anomalies which are located to the north and south of the EAJS’s axis. However, these westerly and easterly anomalies exhibit distinct temporal variations between the two categories: The pair of westerly and easterly anomalies shifts southward from the high latitudes to the EAJS region for the first category, but appears as a quasi-stationary meridional dipole pattern for the second category.
Using the Hilbert Singular Value Decomposition (HSVD) and the Nonlinear Canonical Correlation Analysis (NLCCA), we analyzed the statistical relationship of MJO (Madden-Julian Oscillation) and ENSO (El Niño and Southern Oscillation). It was found that while a linear analysis produced no significant relationship between MJO and ENSO, a low-order nonlinear analysis based on the quadratic function of HSVD led to statistically significant lagged correlations. When their nonlinear relationship was further extracted by the NLCCA, stronger correlation was obtained, with the maximum correlation coefficient appearing while the MJO signals preceding the ENSO signals by around 2 months and 5.6 months respectively. The time lags producing the maximum correlation are respectively consistent with the characteristic time of MJO influencing ENSO dominated by two physical processes: the equatorial Kelvin waves and air-sea feedback. Corresponding to the two different physical processes, ENSO shows different features of development. In the former scenario, the westerly winds in the western Pacific excite the equatorial Kelvin waves which propagate eastward and deepen the thermocline in the eastern Pacific, resulting in the sea surface warming at the far eastern Pacific near the coast. In the latter scenario, the westerly anomalies in the western Pacific precede the development of El Niño through bringing surface warm water into the central and eastern Pacific, thus the sea surface warming occurs across the whole eastern Pacific ocean.
In this paper an attempt has been made to elucidate the relationship between the formation of photochemical oxidants and the emission rate of the primary pollutants such as nitrogen oxides (NOX) and reactive hydrocarbons (RH) over the Osaka Bay and its surrounding areas of Japan, by using a three-dimensional grid model. In Harima area, reduction in the primary pollutant emission led to simple decrease in the peak O3 concentration, and OH and HO2 radical concentrations. In Osaka area, unlike the Harima area, reduction in NOX emission led to an increase in OH and HO2 radicals concentrations, and conversely to an increase in O3 concentration. It has also been found that the time of peak O3 concentration would appear earlier with reduction in NOX emission, and later with reduction in RH emission. Under meteorological calm condition, the variations of the time of appearance of peak O3 due to the emissions reduction scenario are small. Sea breeze may be one of the important meteorological phenomena responsible for the transport of pollutants such as NOX, RH, and also O3 in the polluted coastal areas. These important results suggest that a method for a well-balanced reduction in both NOX and RH emissions is essential for the improvement of air quality from photochemical pollution over the Osaka Bay and its surrounding areas of Japan.
Size spectra of fog drops are investigated at Misawa Air Base to determine which sizes of drops are mainly responsible for degrading visibility in fogs. It is found fog drops smaller than 5 µm in diameter contribute less than 2% to the extinction coefficient. Degraded visibility is caused by smaller fog drops than 60 µm under thinner fog layers than 130 m in this study. Drop size distributions spread and the representative drop size for degrading visibility increases with fog thickness under fog layers thicker than 280 m. So, not only the maximum drop size increases with fog thickness but also the influences of large drops on the extinction coefficient in fogs increase with fog thickness. For evolutions of visibility, increase of small drops is important for rapid degrading visibility irrespective of fog thickness and increase of large drops corresponds to slow changes of visibility under thick fogs.
The diurnal cycle of convection over the northern South China Sea (SCS) during the onset of the summer monsoon is documented using data from the May.June 1998 South China Sea Monsoon Experiment (SCSMEX) and from ten years of observations from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar. Results both during SCSMEX and from TRMM observations show a prominent signal of propagating convection over the northern SCS. In particular, convection is found to initiate just off the southern coast of mainland China near sunrise and propagate southeastward at about 10-15 m s-1 to the central part of the northern SCS the early afternoon, finally dissipating about 500 km offshore in the late evening. Both indirect measures, such as infrared brightness temperatures and areal cold (< 208 K) convective cloud coverage, and direct measures, i.e., radar reflectivities from the Bureau of Meteorology Research Centre (BMRC) and TRMM radars, show this behavior. The radar data indicate an increasing stratiform contribution to total precipitation as the convective systems move farther offshore, indicative of maturing mesoscale convective systems. Neither advection nor gravity current processes appear to explain the offshore propagation of the envelope of convection, suggesting gravity wave dynamics, possibly associated with discrete propagation, as a factor in the system motion.
The passing of a synoptic trough is expected to contribute strongly to water vapor (WV) transport processes from the Indian Ocean to the Tibetan Plateau (TP) during the monsoon season. However, the pathway of the WV into the plateau associated with passing a trough and its contribution to the WV budget over the TP are not clear. In this study, the processes of WV transport in the case of a passing trough in 1998 were analyzed using GAME reanalysis data and the numerical model, focusing especially on the WV transportation pattern in the Indian Monsoon region and the diurnal variation of WV intrusion from south of the Himalayas to the TP. WV advection into the TP was larger in the case of a passing synoptic trough than in the case of a prevailing Tibetan High. Sub-continental scale circulation for the two synoptic types corresponded with active/break phase of the Indian monsoon. In case of prevailing Tibetan High, a cyclonic circulation with a low-pressure area over India, associated with active Indian monsoon phase, prevented the WV intrusion into the TP in the middle troposphere. However, in the case of the trough that corresponded with break phase of Indian Monsoon, WV was transported directly from Arabian Sea to the southern foot of the Himalayas with a northward shift of the low-level monsoon westerly and it intruded into the southeastern TP. Numerical experiments showed that the WV transport process was composed of multiple steps in the case of the passing trough. WV was transported by the monsoon westerly to the southern foot of the Himalayas at 1500 m above sea level (a.s.l.). The moist air mass reached south of the TP with at about 5500 m a.s.l. during the noon to evening because of the development of a mixing layer and the enhancement of an upslope wind in southern slope of the Himalayas. A moist southwesterly flow was converged latitudinally in the southeastern TP because of the dry northwesterly flow prevailing in the rear of the passing trough over the TP. This convergence area expanded northward from mid-afternoon into the night and disappeared early next morning.
This paper studies an evaluation of climatological reproducibility and one of future climate projections of extreme precipitation in the Baiu season (from mid-May to July) around Japan using data of numerical experiments. The models are a non-hydrostatic cloud-system-resolving atmospheric model with a horizontal grid size of 5 km (5km-NHM) utilized as a regional climate model (RCM), and an atmospheric general circulation model (AGCM) with a horizontal resolution of about 20 km (20km-AGCM) which provided the boundary condition of 5km-NHM. Ten-year runs were performed by the time-slice method. The mean precipitation amount of the three heaviest precipitation events that ranked in order of precipitation amount in every year (PTop3) at each grid point was used as an index to examine quantitatively extreme precipitation within a specific term with various precipitation accumulation periods (APs). The 5km-NHM experiment reproduced PTop3 much better than that of 20km-AGCM, especially for shorter APs. In the projection of PTop3 in the future climate with 5km-NHM, ratio in two climate states of PTop3 for longer APs and that of precipitation amount show the roughly same pattern. The increase areas of those were localized around Kyushu, Japan. In particular, the PTop3 for longer APs was projected to increase 30.70% in the mountainous areas along the Pacific coast of western Japan. This increase was due to a cyclonic circulation of an incremental low-pressure generated on the western side of Kyushu. On the other hand, large increases of PTop3 for shorter APs spread widely with a scattered pattern to the area where precipitation decreases. This characteristic was associated with the intensification of precipitation due to global warming.
We investigated the existence of multiple equilibrium states in the Venusian atmospheric general circulation suggested by Matsuda (1980, 1982) using a Venus-like atmospheric general circulation model. We ran the model from two initial conditions: a state with large zonal wind increasing with height and a motionless state. For the large zonal wind initial state, a strong zonal wind with weak meridional circulation, i.e., super-rotation, appears. However for the motionless initial condition, slow zonal wind with strong meridional circulation relative to the farmer state appears. Each circulation reached a quasi-steady state. These results have the same features suggested by Matsuda. The presence of multiple equilibrium states was sensitive to the horizontal eddy viscosity parameter. For the strong zonal wind state, the acceleration of the zonal mean zonal wind results from the horizontal EP flux divergence from the wavenumber one component on the equator, which is mainly maintained by the Gierasch mechanism (1975). The presence of multiple equilibrium states suggests that an alternative slow zonal wind state could appear in the Venusian atmosphere if an appropriate initial condition or drastic fluctuation is assigned.
Changes in the arid climate over North China were investigated by applying the Koppen climate classification to Chinese station measurements and global grid data sets compiled over the period of 1951. 2000. The use of the Koppen scheme enables us to detect arid climate on a longer time-scale than is attainable through satellite remote sensing. A climatic shift toward a warmer and dryer condition is seen to be robust and substantial enough to cause conspicuous changes in the arid climate in North China from the decades 1951.1970 to the recent decades 1981.2000. The areas of semiarid (BS) and desert (BW) climate exhibit an overall expansion at the periphery of the existing arid zones in the studied period; a significance test shows that the increase in BS area is significant at the 99% confidence level, while that in BW area is relatively marginal. These results are indicative of the meteorological stress due to climatic transformation in North China, which is likely to be an impediment to sustainable re-greening in that region. Therefore, careful consideration of the effects of long-term climate change is required to support on-going and planned vegetation restoration in North China.