Dimethylsulfide was measured in the atmosphere and the ocean water from 28 December 1990 to 13 March 1991 in the western Pacific Ocean. Mean DMS concentration in the atmosphere was low west of 180° longitude on the equator (11pptv) as well as the temperate North Pacific Ocean (9pptv). Atmospheric DMS concentration along 160°W was nearly uniform at about 42pptv from 15°N to the equator. Sharp increases of atmospheric DMS concentration appeared south of the equator along 160°W and 180°, and the atmospheric DMS concentration reached approximately 95pptv around (5°S, 160°W). Concentrations of DMS in the surface seawater were also high south of the equator, and the maximum value was 1.85nmol L-1 at (5°S, 180°). Simultaneous measurement of atmospheric and oceanic DMS was made at an interval of 6 hours on the track from (6°S, 179°W) to (11°S, 175.5°W). While the concentration of DMS in the surface seawater was quite uniform at 1.26nmol L-1, the atmospheric DMS concentration varied diurnally with a mean value of 76pptv. Observed DMS concentrations in the atmosphere and the ocean, together with model simulations, demonstrate that the main processes controlling the concentration and diurnal variation of atmospheric DMS are the magnitude of DMS flux from the ocean to the atmosphere and the oxidation reaction with the hydroxyl radical (OH) in the daytime. The reaction with the nitrate radical (NO3) at night is also important, because it decreases the amplitude of the DMS diurnal variation. In addition, wind speed seems to be an important factor regulating the DMS flux. Another measurement of atmospheric DMS was conducted during 3-12 December 1990 in the southern Indian Ocean. North of 50°S, the atmospheric DMS concentration was approximately 60pptv, but south of 50°S it drastically increased up to 460pptv poleward. High DMS concentration in the ocean and low OH concentration due to stormy weather are considered as possible causes of the high atmospheric DMS concentration.
In the present work, winter thunderclouds and active convective clouds were observed by means of radar with CAPPI (Constant Altitude Plan Position Indicator). The lightning activity is monitored by a local network of sferics direction finders around Komatsu Airport. In midwinter, from January to early March, active convective clouds, 30 dBZ echo tops of which develop up to the -20°C temperature level, exhibit very weak lightning activity called “Single-Flash Storm”, or sometimes fail to generate even a single lightning discharge. The altitude of the -10°C temperature level is between 1.8 to 1.4km in the former case. On the other hand, it is lower than 1.4km in the latter case. These significantly low altitudes of the -10°C temperature level are the main reason for keeping such clouds in a weak or non-lightning situation. Concerning the classification of the lightning activity, the author has proposed the following criteria: (a) When the altitude of the -10°C temperature level is higher than 1.8km, clouds exhibit strong lightning activity. (b) When the altitude of the -10°C temperature level is between 1.8 to 1.4km, clouds exhibit very weak or no lightning activity. (c) When the altitude of the -10°C temperature level is lower than 1.4km, clouds never generate a lightning discharge. Along with the results of thundercloud investigation stated in Part I of the present article (Michimoto, 1991), it is concluded that the necessary conditions for convective clouds to generate lightning discharge are as follows: (1) The 30 dBZ echo of the clouds has to develop at a level higher than the -20°C temperature level. (2) The altitude of the -10°C temperature level has to be higher than 1.4km. (3) The clouds have to involve rapid development of graupel particle precipitation; specifically, they have to involve formation and rapidly vertical movement of 40-to-50 dBZ echo cells.
An effort is made in this study to explore the intraseasonal (30-60 day) oscillation of the lowertropospheric circulation over the western tropical Pacific. It is found that during the 1979 northern summer, the eastward propagation of the global-scale intraseasonal divergent mode induces quasiperiodic alternations of intraseasonal local divergent and convergent centers over the western Pacific. These alternations of intraseasonal local divergent and convergent centers result in the intraseasonal north-south migrations of the North Pacific Convergence Zone (NPCZ), i. e., the Mei Yü and Baiu fronts combined, and the South Pacific Convergence Zone (SPCZ). The regional circulations associated with the NPCZ and SPCZ also exhibit a coherent intraseasonal oscillation. With the intraseasonal local Hadley circulations and the intraseasonal streamfunction budget analysis of the lower-tropospheric circulation, it is shown that the aforementioned coherent intraseasonal oscillation is a response of the regional lower-tropospheric circulation to the eastward propagation of the global-scale intraseasonal oscillation.
Convection-associated disturbance systems that propagate westward over the equatorial Pacific with a period of 3-5 days are studied utilizing the GMS IR equivalent blackbody temperature (TBB) data and the ECMWF global analysis data. The period of analysis is JJA 1980-1989 excluding 1984. Spectral analysis, lag-correlation analysis and composite analysis are utilized for the study. The dominance of 3-5 day variations with convective activity is statistically confirmed in the equatorial Pacific ITCZ region for the boreal summer season. It is shown, in a climatological sense, that these convective variations are associated with mixed Rossby-gravity (MRG)-wave-type disturbances in the central Pacific near the dateline, while they are associated with tropical depression (TD)-type disturbances in the tropical western north Pacific. It is suggested that the changes of dominant disturbance types are attributed to variations in large-scale environmental conditions, such as the mean vertical wind shear, the speed of mean zonal wind as well as the SST distribution. This hypothesis is supported by comparing disturbance structures in an El Niño year with those in a La Niña year at the same longitude. Three-dimensional structures and characteristic values are examined for these two types of disturbances associated with organized convection. The characteristics of MRG-wave-type disturbances correspond well with MRG waves with the equivalent depth of ∼30m. A fairly large amplitude is observed in a limited longitudinal region of about a half-wavelength width (∼4000km) near the dateline. The convection and wave convergence exhibit relatively “loose” coupling. The vertical phase structure for MRG waves coincides with the result of Yanai et al. (1968) and what was proposed by Hayashi (1970). However, it is not clear that the system can be described in the framework of wave-CISK. The characteristics of TD-type disturbances, on the other hand, coincide with the classical “easterly waves” as described by Reed & Recker (1971) and exhibit “tight” coupling with convection. They do not correspond to any linear equatorial waves. It is shown that both types of disturbances obtain the kinetic energy through energy conversion from the available potential energy.
Following the previous work (Saito and Ikawa, 1991a), the three dimensional effect of the orography of the Shikoku Mountains on the Yamaji-kaze is studied numerically, focusing on the effect of a col to the flow over a mountain range. The geographical characteristics of the Yamaji-kaze are explained in terms of the non-linear aspect of the three-dimensional flow over a mountain range with a col. Flow régimes of the three-dimensional flow over a mountain range with a periodic col are examined by use of a linear analytic solution for a homogeneous environmental atmosphere and numerical experiments using a non-hydrostatic model. Two parameters, a non-dimensional mountaintop height lhm, and the ratio of the amplitude of the col to the mean mountaintop height hc/hm, are used to prescribe the mountain shape. The critical values of lhm which divide the flow regimes are about 20% smaller in the numerical experiments compared to the linear theory. When the mountain range has a col, wave breaking occurs readily in the lee of the peak of the ridge, and the internal hydraulic jump and reversed flow behind the jump occurs for smaller lhm compared to the two-dimensional cases. On the other hand, in the lee of the col, a strong wind area easily extends leeward, and the appearance of the jump is unclear in many cases. The behavior of the hydraulic jump is quite sensitive to the existence of the col, while the upstream blocking is rather insensitive to the existence of the col. The response of the hydraulic jump to the existence of a col has qualitative similarities to the behavior of the hydraulic jump in a shallow water flow over a mountain range in a channel of variable width, which has been presented by Saito (1992). Numerical experiments using the real orography of Shikoku Island and the thermal stratification observed on 21 April 1987 are performed as an example of the simulation of a typical Yamaji-kaze, and the development and movement of the internal hydraulic jump is simulated under a time-changing wind profile. It is shown that the timing of the onset of the Yamaji-kaze, which is too early in the two-dimensional simulation, is ameliorated in the three-dimensional simulation. The geographical characteristics of the Yamaji-kaze can be explained by the reversed flow behind the hydraulic jump which stays in the lee of the Shikoku Mountains and a low-level strong wind area which extends in the lee of the col of the Shikoku Mountains. A further improved surface wind pattern is obtained by inclusion of the surface friction. An improved version of the conceptual model of the Yamaji-kaze is proposed on the basis of the results of the three-dimensional experiments.
From 1950 to 1988, 10 ENSO (El Niño-Southern Oscillation) events have occurred. These ENSO events are defined by using the Southern Oscillation index (SOI) and the sea surface temperature (SST) anomaly in the central equatorial Pacific. The ENSO events are classified based on the positive anomaly length of the SST time series: about a year with one northern winter, or two or more years with two northern winters. The shorter one (BO-ENSO) terminates in the following year of the occurrence, and the longer one (LF-ENSO) terminates in the two years after the occurrence. The occurrence and termination of both types have strong phase preference to the seasonal cycle of the mean field, i. e., the seasons between northern winter and northern summer. The occurrence years of each type are as follows; the years of BO-ENSO are 1951, '53, '63, '65, '72, and 1982 (6 cases) and the years of LF-ENSO are 1957, '68, '76, and 1986 (4 cases). Since the occurrence, development, and termination of the both types of ENSO have a seasonal phase preference, composite analysis is used to explain the difference between the two types of ENSO clearly. In the northern winter immediately after the BO-ENSO occurrence, the wind stress anomaly field has a specific clockwise eddy off the east coast of the Philippines. This eddy affects the ocean through Ekman pumping and plays an important role in ENSO termination. The most significant difference between a BO-ENSO and an LF-ENSO is whether this eddy appears or not.
The latitudinal and vertical distribution of stratospheric aerosols injected by eruptions of Mt. Pinatubo on June 15, 1991 was obtained in Japan by a lidar network extending from Naha (26.2°N) to Wakkanai (45.4°N). Results from June to October 1991 are reported. An increase of aerosols originating from the Pinatubo eruptions was observed first on June 28, 1991, at approximately 16km. The layer was observed continuously since then. The upper layer, above 20km, was observed first on July 15, thereafter it disappeared at all stations, but reappeared on August 6. The upper layer was sporadic, and not observed in July over Nagano, some 200km from Tsukuba, where it was observed. This indicates the non-uniform density of the cloud in the upper region.
A possible path to the teraflop performance that will be required by the next generation of GCMs (general circulation models) is the SIMD (Single Instruction stream, Multiple Data stream) massively parallel computer architecture. A critical consideration in moving a model to a SIMD architecture is the efficiency of the model's physical parameterizations on this type of machine. We have analysed the physics in a production GCM to evaluate its potential for efficient SIMD execution, on the assumption that each processor of the computer is allocated to a single vertical column of the model. This paper summarizes the results obtained for the various physics routines and compares these with the efficiencies obtained under the assumption of MIMD (Multiple Instruction stream, Multiple Data stream) execution. Overall, we found a performance penalty of only 15% to 20% for SIMD compared to MIMD execution. This is a very acceptable result, which suggests that SIMD computers should not be rejected for climate modelling (or numerical weather prediction) applications.