Observations of fogs at Misawa Fishing Port in the Aomori district were made with a tethered balloon system and a doppler sodar system from 15 June to 11 July in 1993. Minute analyses were made of time changes of fogs for a long period from the evening on 7 July to the next noon and for a short period from 6 to 13JST on July. The results show that the atmosphere at the cloud top becomes stable from the view point of CTEI (cloud-top entrainment instability) about 2 hours before the intrusion of the fog and becomes neutral (or unstable) just before fog dissipation.
Using a combination of data from various remote sensing devices, the investigation reveals the characteristics of one case of a Cloud Band. Data from AVHRR/NOAA and VISSR/GMS are used for the cloud top structure, while data from SSM/I 85GHz channels and Keifu-maru radar are used for the internal signature of the Cloud Band. The analyses of two scenes reveal the asymmetric structure of the Cloud Band in the east-west direction: the western part includes a large amount of cloud water and the precipitation area is in the central part. The eastern part comprises less cloud water and precipitation. These suggest that the updraft region is located in the western part of the Cloud Band. While these internal signatures in the east-west direction are common to both scenes, the appearance of the cloud top structure differs between the two. In addition, the internal signatures differ quantitatively in the north-south direction. The variation in the structure seems to reflect the effect of a meso-α cyclone. This implies that the Cloud Band is a front-like feature in a meso-α cyclone, which develops from a simple discontinuous line between a northwesterly monsoon and a northeasterly wind in the lower troposphere.
Using a simple model, we reexamine the mechanism of overreflection (OR) of a barotropic Rossby wave proposed by Lindzen and his colleagues. Though our model is somewhat artificial, it is advantageous to treat essential characters of a Rossby wave concerning the OR. It is clarified that the reversal of the relation between the direction of momentum flux and group velocity across an inflection point in the region where the WKB approximation is valid is essential for the occurrence of OR. As was demonstrated for the gravity wave case by Lindzen and Barker (1985), it is found that reflection layers behind the critical level is not important for the OR of a Rossby wave.
We focus on two rainfall measurement algorithms which will be used with a spaceborne radar for the Tropical Rainfall Measuring Mission. One is the conventional rainfall measurement algorithm (Z method), which uses the radar reflectivity factor, and the other is the surface reference technique (SRT method), which uses rain attenuation. Non-uniform beam filling (NUBF) is one source of error in the above methods. This paper investigates the error caused by NUBF using a computer simulation that employs rain data measured by a ground-based radar. The simulation shows that the Z method always overestimates the rainfall rate, and the SRT method almost always underestimates the rainfalll rate. The simulation takes into consideration the effect of the signal fluctuation. The results show that the effect of the signal fluctuation is larger than that of the NUBF, in both the Z method and the SRT method. In particular, the signal fluctuation causes serious errors in rainfall rate estimation by the SRT method in light rain.
The interannual variability of the South Asian summer monsoon and associated land-surface processes over the Eurasian continent in a ten-year integration (1979-88) of an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures (SSTs) is examined. The AGCM has been developed jointly by the Center for Climate System Research (CCSR), the University of Tokyo, and the National Institute for Environmental Studies (NIES). A monsoon intensity index, based on the magnitude of summer-mean vertical shear of zonal wind over the south Asian monsoon region, is used to classify weak and strong monsoon years. It is found that the simulated interannual variability of broad-scale summer monsoon shows a good correlation with observations. Furthermore, distinct precursory signals, including the Eurasian snow in winter and soil moisture anomalies in spring, have been found in the pre-monsoon seasons of weak and strong monsoon years. There is a sharp contrast between weak and strong monsoon years; excessive snow over Eurasia south of 50°N in winter and the increased soil moisture in spring are found prior to weak summer monsoon. These results are consistent with evidence found in observational data analyses and some model experiments. A detailed analysis of surface heat budget shows that snow-albedo feedback dominates over the Tibetan Plateau. On the other hand, to its west in the central Asia, the relatively lower land, the effective cloud albedo anomalies due to excessive rainfall and surface evaporation influence the surface conditions. A numerical experiment with the Eurasian land surface initial conditions in spring, interchanged between weak and strong monsoon years, indicates positive roles played by the land surface processes in influencing the subsequent summer monsoon circulations during the 10-year period. However, such land surface feedbacks are not strong enough to change the sign of the monsoon circulation anomalies. The direct influence of the El Niño/Southern Oscillation through the changes in Walker circulation appears to predominate.
The 1986-1989 Asian summer monsoons have been simulated using a state-of-the-art atmospheric General Circulation Model (GCM), spectrally truncated at 63, 42, 31 and 21 total wavenumbers, corresponding to horizontal resolutions ranging from 200-600km. The mean June-September Asian monsoons have been analyzed and compared to the observed mean monsoon behaviour. Large-scale features such as the lower tropospheric westerly jet, the upper tropospheric tropical easterlies, the Tibetan anticyclone, and copious rainfall over continental Asia are captured by the model at all resolutions. As the resolution is increased, the core of the low-level westerly jet moves towards Somalia and becomes more realistic. The model, however, produces excessive precipitation over the equatorial Indian Ocean and over the southern slopes of the Tibetan plateau, and these errors become accentuated at higher resolution. Furthermore, the monsoon is displaced southward at higher resolutions as is clearly evidenced by a shift in the position of the Tibetan anticyclone. A budget analysis of the upper-level vorticity suggests that this may be related to the excessive ascent over the southern slopes of the Tibetan plateau. A smooth orography test has been made at T63 truncation using T21 truncated orography, in order to assess the contribution due to orographic changes. Smooth orography alleviates the excessive precipitation over the southern slopes of the Tibetan plateau and has a strong and generally beneficial impact on the monsoon over land. It also gives large-scale dynamical monsoon indices in better agreement with observations, yet does not alleviate the excessive precipitation over the equatorial Indian Ocean. The excessive oceanic precipitation is partly due to a systematic intensification of the equatorial convergence zones with increased resolution, yet this appears to be only weakly coupled to the dynamics of the monsoon circulation.
In this study, we conducted time series analyses of barotropic energy of the atmosphere using a ten-year period of ECMWF global analysis. The spectral features are compared with the idealized model simulation spectrum of Tanaka et al. (1996). We find a regime transition over the frequency domain from red to white noise in eddy barotropic energy. The red noise spectral slope at high-frequency is approximately -3. The red noise spectrum changes to white noise at low-frequency as predicted by the model simulation. The transition from red to white noise occurs, however, at the frequency 1/20 (day-1) in the observed atmosphere, compared with 1/50 (day-1) in the model atmosphere. The difference results from the higher energy level of white noise in the model atmosphere.
In the spring of 1991, individual aerosol particles were collected on electron microscopic grids in Beijing (China) and Nagasaki (Japan). In order to study the change in the chemical composition of Asian duststorm particles during transport, the dust particles were examined using an electron microscope equipped with an energy-dispersive X-ray (EDX) analyzer. An isentropic backward-trajectory analysis was carried out in order to obtain the transport path of the dust-storm particles. Spatial distributions of clouds by the Geostationary Meteorological Satellite (GMS) were also used in order to examine the presence of clouds along the transport path. Asian dust-storm particles mixed internally with sea salt (mixed particles) ranged from 16 to 100% among the dust particles in the air over Nagasaki. For the two cases that the same dust-storm events were observed in Beijing and Nagasaki, dust-storm particles containing sea salt were present abundantly in Nagasaki in a case that the air would have been influenced largely by clouds in the maritime atmosphere during the transport. It is suggested that these mixed dust-storm particles were formed by cloud processes.
Aqua planet experiments are performed in order to investigate the effects of an equatorial warm sea surface temperature (SST) area on the tropical large-scale precipitation patterns. The numerical model utilized is a spherical three-dimensional primitive system with resolution of T42L16 and with simplified hydrological processes. The warm SST area is placed at the equator of an aqua planet whose basic SST distribution is zonally uniform and symmetric about the equator. The calculated tropical precipitation distributions are characterized by the appearance of an east-west asymmetry; precipitation decreases to the west of the warm SST area, while it increases in the longitudinally wide area to the east. The east-west asymmetry appears regardless of the cumulus parameterization schemes utilized (the convective adjustment scheme and the Kuo scheme). In the western region to the warm SST area, an increase of the stability due to a temperature rise in the middle layer is observed and correspondingly downward flow is recognized. These are consistent with the decrease of precipitation. In the eastern region, although a temperature rise appears in the middle layer, the stability decreases due to the increase of water vapor in the lower layer caused by the meridional moisture convergence. This destabilization is consistent with the increase of precipitation. Experiments in which the wind velocity used in the evaluation of the surface evaporation is fixed are performed to show that WISHE (Wind Induced Surface Heat Exchange) mechanism is not a principal cause for the generation of the east-west asymmetry of the precipitation pattern.
Latitudinal and temporal distributions of CH4 emission were estimated by an iterative inverse method using a two-dimensional atmospheric transport model and the 1983-1994 CH4 concentration data from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Sampling Network. The atmospheric transport of the model was validated by simulating the concentrations of 85Kr, CFC-11, CFC-12 and CO2 observed at various locations world wide. A zonally averaged OH field for the destruction of CH4 in the atmosphere, originally derived from a three-dimensional photochemical transport model, was adjusted to simulate the observed concentration of atmospheric CH3CCl3. A calculated average latitudinal distribution of CH4 emission showed a large north-south difference, with about 75% of the total global emission residing in the northern hemisphere. The CH4 emission varied seasonally in high latitudes of the northern hemisphere, with a maximum in the summer season, while no seasonality of the CH4 release was found in the southern hemisphere. Averaged global emission of the natural and anthropogenic CH4 for the period 1984-1994 was estimated to be 559±9Tg/yr, with chemical loss of 528±10Tg/yr and the atmospheric increase of 31Tg/yr. In sensitivity experiments of the model results, the global emission of CH4 was found to be sensitive to the OH concentration and the atmospheric temperature but less to the atmospheric transport coefficients and the CH4 concentration data used. The latitudinal CH4 emission distribution was dependent largely on the specification of the horizontal transport coefficients. It was also found that the δ13C value of a bacterial source associated with a large amount of CH4 emission, as well as the soil absorption process of CH4 with a large kinetic isotopic fractionation, significantly impacts the determination of δ13C in atmospheric CH4.
The synoptic weather situations around rainfall over the acid area in Northwestern China during HEIFE are further investigated as an extension of a previous study (Itano, 1997). The analysis shows a frequent passage of the frontal troughes over this arid area and successive formation of cyclones on them at the northern edge of the Tibetan Plateau. The rainfall in this arid area is fundamentally caused by such westward moving troughes. With the passage of these frontal troughes, a significant amount of rainfall is observed at the higher mountainous area but not necessarily at the lower area. Especially at the lowest desert area, rainfall is observed only when the relative humidity within the whole cloud base exceeds 60 percent. This fact seems to indicate the evaporation of rain drops occurs within the dry boundary layer over the desert, which prevents the rain drops from reaching lower altitude until whole boundary layer is moistened to some extent. This may be one of the principal reasons for scarce rainfall in the lowest desert area despite the frequent passage of synoptic disturbances and large number of rain events.
A fast and accurate radiative transfer model was developed primarily for microwave remote sensing and radiance data assimilation applications. Radiances at any direction were expressed by formal solution while the scattering source term was determined by a 4-stream discrete ordinate solution using Henyey-Greenstein scattering phase function. The accuracy of this model is analyzed by comparing it to a polarized 32-stream discrete ordinate model with Mie scattering phase function, and an unpolarized 32-stream discrete ordinate model with Henyey-Greenstein scattering phase function. It shows that this model is fast and accurate enough for practical use in microwave remote sending and data assimilation, the error being less than 3 K comparing to the accurate polarized 32-stream model. It is also compared with a 2-stream Eddington model. The results show that it is more accurate than the Eddington model, especially for ice cloud conditions.