Marine aerosol particles were sampled over the coastal Pacific and Indian Oceans in 1992. By means of X-ray spectrometry, we have analyzed the composition of the individual particles with radii between ∼0.1 and ∼2μm. Many sea salts exhibit chlorine deficiencies. The relative weight ratios of Na, S and Cl favor the modification mainly by sulfuric acid. One sample, taken near Borneo, contains several particles that are enriched in potassium and thus are attributable to the combustion of vegetation. Contrasting to the other cases, sea-salt particles in this sample appear to have been modified by both acidic sulfate and yet another species, possibly acidic nitrate as a by-product of biomass burning.
Airflow structures in two snow bands, one in 1990 and the other in 1992, over the Sea of Japan have been studied by dual-Doppler radar and supplementary observations. These snow bands appeared in a wind-speed-increase zone, which moved into the experiment area several hours before the outbreak of the winter monsoon. In the 1990 case, dropsonde observations showed that the stratification of low-level air behind the trailing edge of the band was convectively unstable, and that a cold pool was situated beneath the band. A cold air presence was confirmed, also for the 1992 case, from surface temperature analyses. The mass transport budget analyses revealed the relative importance of a band-normal wind component to a mean airflow structure in the both snow bands. Their airflow structures in the band-transverse vertical cross-sections were, however, completely different from each other. The circulation in the 1990 case was characterized by the ascending inflow from the trailing edge and the descending outflow from the leading edge. For the 1992 case, the circulation was the reverse of the 1990 case, the ascending inflow being from the leading edge and the downdraft occurring in the very narrow region around the trailing edge. The different circulations in the two snow bands were explained by vertical wind shear in the band-transverse components of the horizontal winds.
A new over-ocean precipitation retrieval algorithm is presented in this study using multi-channel brightness temperatures (TBs) of Special Sensor Microwave Imager (SSM/I) measurements. The basic idea of this algorithm is to find an optimal 2-dimensional precipitation field which gives radiative transfer model-generated temperatures that fit best with the observed TBs. The algorithm utilizes vertically-polarized TBs in order to minimize the sea-surface wind effects. The radiative transfer model of Liu and Curry (1993) is used for computing field-of-views (FOV)-averaged TBs at 19.35, 37 and 85.5 GHz from precipitation fields with a spatial resolution of 25 km. Statistical characteristics of sub-pixel-scale variation and vertical profile of precipitation from GARP Tropical Experiment (GATE) data sets are employed as precipitating cloud models in the radiative transfer calculation to simulate FOV-averaged brightness temperatures. The agreement between the multichannel radiative calculated TBs and the observed TBs is expressed as a cost function which is a sum of the squares of the statistically-normalized differences between the above 2 TB fields. The optimal precipitation field with 25 km resolution is obtained by resolving the gradient equation of the cost function. Precipitation retrievals using this algorithm are executed for two meteorological cases (Typhoon Flo (T9019) at 21 UTC 17 September 1990 and warm frontal rain at 21 UTC 28 April 1988). The retrieved precipitation is compared with the precipitation data observed by the operational radar network of the Japan Meteorological Agency. The results indicate that the algorithm amplifies mesoscale precipitation patterns within the large-scale rain areas. The results also suggest that this amplification improves the linear correlation between the retrieved precipitation and the precipitation obtained from the radar observation. This improvement is ascribable to the effectiveness of the algorithm in utilizing high-frequency channel measurements to redistribute precipitation within large-scale rain areas. Statistical comparisons are also made between the precipitation retrievals using the algorithm and shipboard radar data during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP). An IOP-averaged ratio of the retrieved precipitation to the radar rain intensity is 0.944 for this period. This suggests that the characteristics of sub-pixel-scale variation and the vertical profile of precipitation used in the algorithm is appropriate since the ratio largely depends on these parameters.
Separating the impact of urbanization (urban effect temperature trends) from observational data at urban monitoring sites is an important problem in the detection of global warming impacts. A statistical method using principal component analysis of the temperature data in an area was developed to separate the urban effect temperature trend from the observed temperature record at urban stations, including those without nearby rural reference stations. After the mathematical description of the method, an example of the analysis for model data on which some trends are superimposed (given) at any stations was shown for an explanation of the method. This example not only clarified the performance (impact, weight) of the superimposed trend in the trend for each principal component score (Z-score) but also showed how all the superimposed trends were separated from the model data by this method. This method was applied to monthly mean temperature data for the past 73 years (1920-1992) from 51 meteorological stations in Japan, and the urban effect temperature trend at each station was separated from the observed temperature trend. As a result of the analysis, it was clarified that in Japan the maximum urban effect temperature trends at stations with a population of over 100 thousand in 1993 were 1.0-2.5°C/100 years, which were almost the same as those in the United States and China. The urban effect temperature trends in big cities are larger in the cold season than in the warm season, being maximum in winter or autumn and minimum in summer. After removing the urban effect temperature trend, the annual mean temperature trends at stations in Japan over the period analyzed ranged from 0.5°C/100 years in northern Japan to 1.1°C/100 years in western Japan with the areal mean for Japan of 0.8°C/100 years. The positive trends are most notable in winter and spring (December to May), and show areal mean values of 1.0-1.6°C/100 years with the maximum in March. On the other hand, they are not marked in summer and autumn. In particular, in the northern part of Japan, negative trends appear from July to November. The area with negative trends below -1.0°C/100 years extends over most of the northeastern half of Japan in July, but does not reach the southern part of Japan, which has a slight positive trend. As a result of these regional differences in temperature trends, the north-south temperature gradient increases in summer and autumn with the maximum trend difference of 2.0°C/100 years in July and October.
Evolution of a cloud-topped boundary layer in the East China Sea area in a situation of cold-air outbreak was examined for a case during 12-13 January 1991 based on the operational rawinsonde observation data and the NOAA AVHRR data received by Nagoya University. The analysis was made by comparing with that in the downstream region in the central area of the Japan Sea on 2 February 1991. (1) According to the brief analysis during the period from 11 January to 3 February 1991, the stratification which satisfies the condition for Cloud Top Entrainment Instability (CTEI) was observed at Naze (about 1000 km downstream from the continent over the East China Sea) in most cases of cold-air outbreak situations. (2) According to a case study for 12-13 January 1991, it was suggested that the release of CTEI seems to occur and this might affect the change in characteristics of low-level clouds in the downstream of Naze, even in situations of cold-air outbreak over the East China Sea. (3) On the other hand, the unstable stratification satisfying the CTEI condition was formed on passing from Saishuh-toh (about 500 km distant from the continent) to Naze in that case. Besides, the horizontal scales of cloud areas became larger and the cloud tops became somewhat flatter, so that the cloud layer became favorable for release of CTEI in downstream of Naze. (4) The large positive value of the total apparent source just below the top of the mixed layer and its rapid decrease with height above that layer contributed primarily to generating the stratification which satisfies the CTEI condition in the upstream of Naze. The much higher "degree of unsaturation" (large value of q*-q) above the mixed layer in the East China Sea area than in the Japan Sea area, as well as the huge supply of heat and moisture from the underlying sea, seems to be one of the favorable factors for generating the CTEI condition there, even in situations of cold-air outbreak.
In cold seasons, a low-level mesoscale cloud system associated with an orographically-induced shear line often develops off the south coast of central Japan. Numerical experiments using high resolution limited-area models based on Japan Spectral Model (JSM) were performed for the 14 October 1991 case, where active development of the cloud system and associated cyclogenesis occurred. The model reproduced the evolution of the disturbance well, which is described as follows. In the initial stage of the cloud formation, a strong low-level northerly wind in the upwind side exhibited a splitting pattern around the Chubu Mountains, while the wind in the lee side was weak. High θe air that was formerly brought by a typhoon was trapped in the south coast area. Then an easterly wind gradually intensified along the south coast of Kanto, which caused the westward migration of the cloud system, and a NW-SE oriented shear line with strong positive vorticity in the lee of the Chubu Mts. A band-shaped precipitation area was simulated along the northeast side of the shear line, whereas on the southwest side, there was a dry air mass that descended the south slope. At the 18-h forecast, a shallow, mesoscale cyclone appeared on the shear line, which was nearly in geostrophic balance with the strong cyclonic circulation. The mesoscale cyclone then combined with the middle-level trough that progressed along the south coast and developed further as a synoptic-scale, extratropical cyclone. Simulated results show that the initial growth of the disturbance is triggered by the orographic effects, and that the later development is strongly influenced by the approach of the middle-level trough.
Application of trajectory uncentering is an effective remedy for ensuring noise-free solutions in semi-implicit semi-Lagrangian (SISL) models. In this study we have employed a one-dimensional SISL linear shallow-water model to investigate the impact of second-order trajectory uncentering on the meteorologically significant Rossby wave solutions. This study is an extension of the results reported in Semazzi and Dekker (1994) where first-order semi-Lagrangian trajectory uncentering was investigated. We show that for typical resolutions, in this case Δx=O(100 km) and Δt<1 hr, application of first-order uncentering results in large truncation errors for planetary-scale waves of the atmosphere. Our main result is that application of second-order uncentering drastically reduces these truncation errors.
The effect of 3-dimensionality of a mountain profile on downslope windstorms is analytically investigated. The mountain is immersed in a stably stratified uniform environmental flow. The case of an almost-2-dimensional mountain is considered. The mountain profile has a maximum and is nearly uniform in the cross-stream direction. The downslope wind speed in the along-stream vertical section, in which the mountain profile has its maximum, is calculated. In comparison with the 2-dimensional case in which the profile is strictly uniform in the cross-stream direction, the following results are obtained. When an atmospheric heightis prescribed, both the downslope wind speed and the mountain height, which is required for the douwnslope windstorm to occur, increase due to the 3-dimensional deviation of the profile. On the other hand, when a mountain height is prescribed, both the downslop wind speed and the required atmospheric height decrease due to the deviation.
Strengthening of the wintertime mid-latitude westerlies over the North Pacific since the mid-1970s is examined in wind velocity and sea level pressure (SLP) fields computed from the ship-report data archived in the Comprehensive Ocean-Atmosphere Data Set. Changes in the observed wind fields between 1966-75 and 1976-85 are consistent with geostrophic winds based on the difference of the SLP fields between the two decades. This fact confirms that the strengthening of wintertime mid-latitude westerlies in the latter decade over the North Pacific occurred in the real world. Wind fields of the annual mean and the other three seasons are also examined by the same method, and the ship-report wind velocity fields for these averages also have features consistent with the wind fields estimated from SLP.